Medical Physiology A Cellular and Molecular Approach, Updated 2nd Ed.

THE PARATHYROID GLANDS AND VITAMIN D

Eugene J. Barrett and Paula Barrett

CALCIUM AND PHOSPHATE BALANCE

Calcium plays a critical role in many cellular processes, including hormone secretion, muscle contraction, nerve conduction, exocytosis, and the activation and inactivation of many enzymes. As described in Chapter 3 and elsewhere, Ca2+ also serves as an intracellular second messenger by carrying information from the cell membrane into the interior of the cell. It is therefore not surprising that the body very carefully regulates the plasma concentration of free ionized Ca2+ ([Ca2+]), the physiologically active form of the ion, and maintains plasma [Ca2+] within a narrow range (between 1.0 and 1.3 mM, or between 4.0 and 5.2 mg/dL).

Phosphate is no less important. Because it is part of the ATP molecule, PO3−4 plays a critical role in cellular energy metabolism. It also plays crucial roles in the activation and deactivation of enzymes. However, unlike Ca2+, the plasma PO3−4 concentration is not very strictly regulated, and its levels fluctuate throughout the day, particularly after meals.

Calcium homeostasis and PO3−4 homeostasis are intimately tied to each other for two reasons. First, Ca2+ and PO3−4 are the principal components of hydroxyapatite crystals [Ca10(PO4)6(OH)2)], which by far constitute the major portion of the mineral phase of bone. Second, they are regulated by the same hormones, primarily parathyroid hormone (PTH) and 1, 25-dihydroxyvitamin D (calcitriol) and, to a lesser extent, the hormone calcitonin. These hormones act on three organ systems—the bone, the kidneys, and the gastrointestinal (GI) tract—to control the levels of these two ions in plasma. However, the actions of these hormones on Ca2+ and PO3−4 are typically opposed in that a particular hormone may elevate the level of one ion while lowering that of the other. Figures 52-1 and 52-2 depict the overall daily balance of Ca2+ and PO3−4 for a subject in a steady state.

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Figure 52-1 Calcium distribution and balance. All values are examples for a 70-kg human, expressed in terms of elemental calcium. These values can vary depending on factors such as diet. ECF, extracellular fluid.

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Figure 52-2 Phosphate distribution and balance. All values are examples for a 70-kg human, expressed in elemental phosphorus. These values can vary depending on factors such as diet. ECF, extracellular fluid.

Calcium balance

Most Ca2+ is located within bone, ~1 kg (Fig. 52-1). The total amount of Ca2+ in the extracellular pool is only a fraction of this amount, ~1 g or 1000 mg. The typical daily dietary intake of Ca2+ is ~800 to 1200 mg. Dairy products are the major dietary source of Ca2+. Although the intestines absorb approximately one half the dietary Ca2+ (~500 mg/day), they also secrete Ca2+ for removal from the body (~325 mg/day), and, therefore, the net intestinal uptake of Ca2+is only ~175 mg/day. The second major organ governing Ca2+ homeostasis is bone, in which Ca2+ deposition of ~280 mg/day is matched by an equal amount of Ca2+ resorption in the steady state. The third organ system involved, the kidney, filters ~10 times the total extracellular pool of Ca2+ per day, ~10,000 mg/day. More than 98% of this Ca2+ is reabsorbed, and, therefore, the net renal excretion of Ca2+ is less than 2% of the filtered load (see Chapter 36). In a person in Ca2+ balance, urinary excretion (~175 mg/day) is the same as net absorption by the GI tract.

In plasma, Ca2+ exists in three physicochemical forms: (1) as a free ionized species, (2) bound to (more accurately, associated with) anionic sites on serum proteins (especially albumin), and (3) complexed with low-molecular-weight organic anions (e.g., citrate and oxalate). The total concentration of all three forms in the plasma is normally 2.2 to 2.6 mM (8.8 to 10.6 mg/dL). In healthy individuals, ~45% of Ca2+is free, 45% is bound to protein, and 10% is bound to small anions (e.g. PO3−4, citrate, and sulfate). The ionized form is the most important with regard to regulating the secretion of PTH and is involved in most of the biological actions of Ca2+.

Phosphate balance

Most PO3−4 is also present in bone, ~0.6 kg of elemental phosphorus (Fig. 52-2). A smaller amount of phosphorus (0.1 kg) exists in the soft tissues, mainly as organic phosphates such as phospholipids, phosphoproteins, nucleic acids, and nucleotides. An even smaller amount (~500 mg) is present in the extracellular fluid as inorganic phosphate (Pi). The daily dietary intake of phosphorus is typically 1400 mg, mostly as Pi. Again, dairy products are the major source. The net absorption of PO3−4 by the intestines is ~900 mg/day. In the steady state, bone has relatively small PO3−4 turnover, ~210 mg/day. The kidneys filter ~14 times the total extracellular pool of PO3−4 per day (~7000 mg/day) and reabsorb ~6100 mg/day. Hence, the net renal excretion of phosphorus is ~900 mg/day, the same as the net absorption by the GI tract.

The concentration of total PO3−4 in plasma ranges from 0.8 to 1.5 mM (or 2.5 to 4.5 mg/dL of elemental phosphorus), a variation of 80%. Between 85% and 90% of the circulating PO3−4 is filterable by the kidneys, either ionized (50%) or complexed to Na+, Ca2+ or Mg2+ (40%), and only a small proportion (10% to 15%) is protein bound.

PHYSIOLOGY OF BONE

Dense cortical bone and the more reticulated trabecular bone are the two major bone types

Bone consists largely of an extracellular matrix composed of proteins and hydroxyapatite crystals, in addition to a small population of cells. The matrix provides strength and stability. The cellular elements continually remodel bone to accommodate growth and allow bone to reshape itself in response to varying loading stresses. Basically, bone has three types of bone cells. Osteoblasts promote bone formation. Osteoclasts promote bone resorption and are found on the growth surfaces of bone. Osteocytes are found within the bony matrix and are derived from osteoblasts that have encased themselves within bone. These cells sense mechanical stress on bone and secrete growth factors that stimulate both osteoblasts and lining cells. They also appear to play a role in the transfer of mineral from the interior of bone to the growth surfaces. Bone remodeling consists of a carefully coordinated interplay of osteoblastic, osteocytic, and osteoclastic activity.

As shown in Figure 52-3, bone consists of two types of bone tissue. Cortical (also called compact) bone represents ~80% of the total bone mass. Cortical bone is the outer layer (the cortex) of all bones and forms the bulk of the interior of the long bones of the body. It is a dense tissue composed mostly of bone mineral and extracellular matrix elements, interrupted only by penetrating blood vessels and a sparse population of osteocytes nested within the bone. These osteocytes are interconnected with one another and with the osteoblasts on the surface of the bone by canaliculi, through which the osteocytes extend cellular processes. These connections permit the transfer of Ca2+ from the interior of the bone to the surface, a process called osteocytic osteolysis. Dense cortical bone provides much of the strength for weight bearing by the long bones.

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Figure 52-3 Cortical and trabecular bone. Under the periosteum is a layer of compact cortical bone that surrounds the more reticulated trabecular bone. The fundamental unit of cortical bone is the osteon, which is a tube-like structure that consists of a haversian canal, surrounded by ring-like lamellae. The inset shows a cross section through an osteon. The superficial lining cells surround the osteoblasts, which secrete osteoid, a matrix of proteins that are the organic part of bone. The lining cells are formed from osteoblasts that become quiescent. Osteocytes are osteoblasts that have become surrounded by matrix. Canaliculi allow the cellular processes of osteocytes to communicate, through gap junctions, with each other and with osteoblasts on the surface. Trabecular bone has both osteoblasts and osteoclasts on its surface; this is where most bone remodeling takes place.

Trabecular (or cancellous or medullary) bone constitutes ~20% of the total bone mass. It is found in the interior of bones and is especially prominent within the vertebral bodies. It is composed of thin spicules of bone that extend from the cortex into the medullary cavity. The lacework of bone spicules is lined in many areas by osteoblasts and osteoclasts, the cells involved in bone remodeling. Trabecular bone is constantly being synthesized and resorbed by these cellular elements. Similar bone turnover occurs in cortical bone, but the fractional rate of turnover is much lower. When the rate of bone resorption exceeds that of synthesis over time, the loss of bone mineral produces the disease osteoporosis.

The extracellular matrix forms the nidus for the nucleation of hydroxyapatite crystals

Collagen and the other extracellular matrix proteins that form the protein matrix of bone are called osteoidOsteoid provides sites for the nucleation of hydroxyapatite crystals, the mineral component of bone. Osteoid is not a single compound, but rather is a highly organized matrix of proteins synthesized principally by osteoblasts. Type I collagen accounts for ~90% of the protein mass of osteoid. It comprises a triple helix of two α1 monomers and one α2 collagen monomer. While they are still within the osteoblast, the monomers self-associate into the helical structure. After secretion from the osteoblast, the helices associate into collagen fibers; cross-linking of collagen occurs both within a fiber and between fibers. These collagen fibers are arranged in the osteoid in a highly ordered manner. The organization of collagen fibers is important for the tensile strength (i.e., the ability to resist stretch or bending) of bone. In addition to providing tensile strength, collagen also acts as a nidus for nucleation of bone mineralization. Within the collagen fibers, the crystals of hydroxyapatite are arranged with their long axis aligned with the long axis of the collagen fibers.

Several other osteoblast-derived proteins are important to the mineralization process, including osteocalcin and osteonectin. Osteocalcin is a 6-kDa protein synthesized by osteoblasts at sites of new bone formation. 1, 25-Dihydroxyvitamin D induces the synthesis of osteocalcin. Osteocalcin has an unusual structure in that it possesses three γ-carboxylated glutamic acid residues. These residues are formed by post-translation modification of glutamic acid by vitamin K–dependent enzymes. Like other proteins with γ-carboxylated glutamic acid, osteocalcin binds Ca2+ avidly. It binds hydroxyapatite, the crystalline mineral of bone, with even greater avidity. This observation has led to the suggestion that osteocalcin participates in the nucleation of bone mineralization at the crystal surface. Osteonectin, a 35-kDa protein, is another osteoblast product that binds to hydroxyapatite. It also binds to collagen fibers and facilitates the mineralization of collagen fibers in vitro. Additional proteins have been identified that appear likely to participate in the mineralization process. Some evidence indicates that the glycoproteins present extracellularly in bone may act to inhibit mineralization; removal of these glycoproteins may be necessary for bone formation to occur.

Bone remodeling depends on the closely coupled activities of osteoblasts and osteoclasts

In addition to providing the proteins in osteoid, osteoblasts promote mineralization by exporting Ca2+ and PO3−4 from intracellular vesicles that have accumulated these minerals. Exocytosis of Ca2+ and PO3−4raises the local extracellular concentration of these ions around the osteoblast to levels that are higher than in the bulk extracellular fluid, thus promoting crystal nucleation and growth (Fig. 52-4). Bone formation along spicules of trabecular bone appears to occur exclusively at sites of previous resorption by osteoclasts. The processes of bone resorption and synthesis are thus spatially coupled.

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Figure 52-4 Bone formation and resorption. PTH and vitamin D stimulate osteoblastic cells to secrete factors such as M-CSF. This and other agents induce stem cells to differentiate into osteoclast precursors, mononuclear osteoclasts, and, finally, mature, multinucleated osteoclasts. Osteoblasts also secrete Ca2+ and Pi, which nucleate on the surface of bone. PTH indirectly stimulates bone resorption by osteoclasts. Osteoclasts do not have PTH receptors. Instead, the PTH binds to receptors on osteoblasts and stimulates the release of factors, such as IL-6 and RANK ligand, and the expression of membrane-bound RANK ligand. These factors promote bone resorption by osteoclasts.

Vitamin D and PTH stimulate osteoblastic cells to secrete factors—such as macrophage colony-stimulating factor (M-CSF)—that cause osteoclast precursors to proliferate (Fig. 52-4). These precursors differentiate into mononuclear osteoclasts and then fuse to become multinucleated osteoclasts. Osteoclasts resorb bone in discrete areas in contact with the ruffled border of the cell (Fig. 52-5). The osteoclast closely attaches to the bone matrix when integrins on its membrane attach to vitronectin in the bone matrix. The osteoclast—in reality a one-cell epithelium—then secretes acid and proteases across its ruffled border membrane into a confined resorption space (the lacuna). The acid secretion is mediated by a V-type H+ pump (see Chapter 2) at the ruffled border membrane. Abundant carbonic anhydrase provides the H+. Cl-HCO3 exchangers, located in the membrane on the opposite side of the osteoclast, remove the HCO3 formed by carbonic anhydrase as a byproduct. The acidic environment beneath the osteoclast dissolves bone mineral, and acid proteases hydrolyze the matrix proteins. Having reabsorbed some of the bone in a very localized area, the osteoclast moves away from the pit or trough in the bone that it has created. Osteoblastic cells replace the osteoclast and now build new bone matrix and promote its mineralization.

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Figure 52-5 Bone resorption by the osteoclast. The osteoclast moves along the surface of bone and settles down, sealing itself to the bone through integrins that bind to vitronectins on the bone surface. The osteoclast reabsorbs bone by secreting H+ and acid proteases into the lacuna. Thus, the osteoclast behaves as a one-cell epithelium. The acid secretion is mediated by a V-type H+ pump at the ruffled border membrane facing the lacuna. Carbonic anhydrase (CA) in the cytosol supplies the H+ to the H+ pump and also produces HCO3 as a byproduct. Cl-HCO3 exchangers—located on the membrane opposite the ruffled border—remove this HCO3. AC, adenylyl cyclase; PKA, protein kinase A; TRAP, tartrate-resistant acid phosphatase.

A newly described protein called osteoprotegerin ligand or RANK ligand appears to be a major stimulator both of the differentiation of preosteoclasts to osteoclasts and of the activity of mature osteoclasts. RANK ligand is a member of the tumor necrosis factor (TNF) cytokine family and exists both as a membrane-bound form on the surface of stromal cells and osteoblasts and as a soluble protein secreted by these same cells. RANK ligand binds to and stimulates a membrane-bound receptor of the osteoclast called RANK (receptor for activation of nuclear factor-κB), a member of the TNF receptor family. Osteoblastic and stromal cells also produce a soluble member of the TNF receptor family called osteoprotegerin (from the Latin osteo [bone] + protegere [protect]). By binding RANK ligand, osteoprotegerin protects the bone from osteoclastic activity. Glucocorticoids increase the production of RANK ligand by osteoblastic cells but decrease the production of osteoprotegerin. The net result is that more free RANK ligand is available to bind to RANK and thus to promote bone loss. The precise role of these proteins in the development of various forms of osteoporosis and osteopetrosis is only beginning to be understood. However, the balance between the amounts of osteoprotegerin and RANK ligand produced by the osteoblast/stromal cell appears to be a very important factor.

PARATHYROID HORMONE

Plasma Ca2+ regulates the synthesis and secretion of PTH

We have four parathyroid glands, two located on the posterior surface of the left lobe of the thyroid and two more on the right. Combined, these four glands weigh less than 500 mg. They are composed largely of chief cells, which are responsible for the synthesis and secretion of PTH. These cells, like cells that secrete other peptide hormones, are highly specialized to synthesize, process, and secrete their product. The major regulator of PTH secretion is ionized plasma Ca2+, although vitamin D also plays a role. Both inhibit the synthesis or release of PTH. In contrast, an increase in plasma phosphorus concentration stimulates PTH release.

PTH Synthesis and Vitamin D The PTH gene possesses upstream regulatory elements in the 5′ region, including both vitamin D and vitamin A response elements (see Chapter 4). The vitamin D response element binds a vitamin D receptor (VDR) when the receptor is occupied by a vitamin D metabolite, usually 1, 25-dihydroxyvitamin D. The VDR is a member of the family of nuclear receptors, like the steroid hormone and thyroid hormone receptors. Like the thyroid hormone receptor, VDR forms a heterodimer with the retinoic acid X receptor (RXR) and acts as a transcription factor (see Table 4-2). The receptor has a very high affinity for the 1, 25-dihydroxylated form of vitamin D (Kd≅ 10−10 M), less affinity for the 25-hydroxy form (Kd ≅ 10−7 M), and little affinity for the parent vitamin (either D2 or D3; see later). Binding of the vitamin D–VDR complex to the VDR response element decreases the rate of PTH transcription.

Processing of PTH After transport of the mature PTH mRNA to the cytosol, PTH is synthesized on ribosomes of the rough endoplasmic reticulum and begins its journey through the secretory pathway. PTH is transcribed as a prepro-PTH of 115 amino acids (Fig. 52-6). The 25–amino acid “pre” fragment targets PTH for transport into the lumen of the endoplasmic reticulum. This signal sequence appears to be cleaved as the PTH enters the rough endoplasmic reticulum (see Chapter 2). During transit through the secretory pathway, the 90–amino acid pro-PTH is further processed to the mature, active 84–amino acid PTH. This cleavage appears quite efficient, and no pro-PTH appears in the storage granules. Conversely, the breakdown of 1-84 PTH or “intact” PTH into its N- and C-terminal fragments—as discussed later—already starts in the secretory granules.

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Figure 52-6 PTH synthesis. The synthesis of PTH begins with the production of pre-pro-PTH (115 amino acids) in the rough endoplasmic reticulum (ER). Cleavage of the signal sequence in the ER lumen yields pro-PTH (90 amino acids). During transit through the vesicular pathway, enzymes in the Golgi cleave the “pro” sequence, thus yielding the mature or “intact” PTH (84 amino acids), which is stored in secretory granules. Beginning in the secretory granule, enzymes cleave PTH into two fragments. The N-terminal fragment is either 33 or 36 amino acids in length and contains all the biological activity.

Metabolism of PTH Once secreted, PTH circulates free in blood plasma and is rapidly metabolized; the half-life of 1-84 PTH is ~4 minutes. Beginning in the secretory granules inside the parathyroid chief cells and continuing in the circulation—predominantly in the liver—PTH is cleaved into two principal fragments, a 33– or 36–amino acid N-terminal peptide and a larger C-terminal peptide (Fig. 52-6). Virtually all the known biological activity of PTH resides in the N-terminal fragment, which is rapidly hydrolyzed, especially in the kidney. However, the half-life of the C-terminal fragment is much longer than that of either the N-terminal peptide or the intact 84–amino acid PTH molecule. An estimated 70% to 80% of the PTH-derived peptide in the circulation is represented by the biologically inactive C-terminal fragment. The presence of a significant amount of antigenically recognized, but biologically inactive C-terminal fragments in the circulation has complicated the use of the usual radioimmunoassay methods for measuring PTH in both clinical and experimental settings. This problem has been solved by the development of sensitive enzyme-linked immunosorbent assays that use two antibodies that react with two distinct sites on the PTH molecule. This method measures only the intact PTH hormone (i.e., 84 amino acids). These assays are invaluable in the diagnosis of disorders of PTH secretion, particularly when kidney function is impaired (a circumstance that further prolongs the half-life of the inactive C-terminal metabolites).

High plasma [Ca2+] inhibits the synthesis and release of PTH

To a first approximation, and ignoring the contributions of vitamin D that are discussed later, regulation of PTH secretion by plasma Ca2+ appears to be a simple negative feedback loop. The major stimulus for PTH secretion is a decline in the concentration of Ca2+ in the blood (hypocalcemia) and extracellular fluid. Hypocalcemia also stimulates synthesis of new PTH, which is necessary because the parathyroid gland contains only enough PTH to maintain a stimulated secretory response for several hours.

The mechanism by which the parathyroid gland senses extracellular [Ca2+] and signals both the synthesis and secretion of PTH has only recently been clarified. Studies with cultured parathyroid chief cells showed that these cells respond to very small decreases in the concentration of ionized Ca2+ in the bathing medium, similar to the way the parathyroid gland acts in vivo. Using an expression cloning approach, investigators identified a Ca2+-sensing receptor (CaSR) that resides in the plasma membrane of the parathyroid cell (Fig. 52-7). This receptor binds Ca2+ in a saturable manner, with an affinity profile that is similar to the concentration dependence for PTH secretion. CaSR is a member of the G protein–coupled receptor family. Coupling of this Ca2+ receptor to Gαq activates phospholipase C, thus generating inositol 1, 4, 5-triphosphate (IP3) and diacylglycerol (DAG) and resulting in the release of Ca2+ from internal stores and the activation of protein kinase C (PKC) (see Chapter 3). Unlike most endocrine tissues, in which activation of these signaling systems promotes a secretory response, in the parathyroid, the rise in [Ca2+]i and activation of PKC inhibit hormone secretion. (Another example is the granular cell of the juxtaglomerular apparatus, in which an increase in [Ca2+]i inhibits secretion of renin; see Chapter 40.) Thus, increasing levels of plasma [Ca2+] decrease PTH secretion (Fig. 52-7B).

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Figure 52-7 PTH secretion and its dependence on ionized Ca2+ in the plasma. A, Four parathyroid glands lie on the posterior side of the thyroid. The chief cells synthesize, store, and secrete PTH. Increases in extracellular [Ca2+] inhibit PTH secretion in the following manner: Ca2+binds to a receptor that is coupled to a heterotrimeric G protein, Gαq, which activates phospholipase C (PLC). This enzyme converts phosphoinositides (PIP2) to IP3 and DAG. The IP3 causes the release of Ca2+ from internal stores, whereas the DAG stimulates PKC. Paradoxically, both the elevated [Ca2+]i and the stimulated PKC inhibit release of granules containing PTH. Increased [Ca2+]o also inhibits PTH synthesis. Thus, increased levels of plasma Ca2+ lower PTH release and thus tend to lower plasma [Ca2+]. B, Small decreases in free plasma [Ca2+] greatly increase the rate of PTH release. About half of the total plasma Ca2+ is free. In patients with familial hypocalciuric hypercalcemia (FHH), the curve is shifted to the right; that is, plasma [Ca2+] must rise to higher levels before inhibiting PTH secretion. As a result, the patients have normal PTH levels, but elevated plasma [Ca2+].

The PTH receptor couples through G proteins to either adenylyl cyclase or phospholipase C

The action of PTH to regulate plasma [Ca2+] appears to be secondary to its binding to the PTH 1R receptor. A second PTH receptor, PTH 2R, has been identified. However, its role, if any, in the regulation of plasma [Ca2+] is uncertain. Kidney and bone have the greatest abundance of PTH 1R receptors. Within the kidney, PTH 1R receptors are most abundant in the proximal and distal convoluted tubules. In bone, the osteoblast appears to be the important target cell. The PTH 1R receptor is a G protein–linked receptor (see Chapter 3). The PTH 1R receptor binds some N-terminal fragments of PTH (Fig. 52-6), as well as the 1-84 intact PTH molecule. The PTH 1R receptor also binds PTH-related peptide (PTHrP), which is discussed later. In contrast, the PTH 2R receptor is selectively activated by PTH.

The PTH 1R receptor appears to be coupled to two heterotrimeric G proteins and thus to two signal transduction systems. Binding of PTH to the receptor stimulates Gαs, which, in turn, activates adenylyl cyclase and thus releases cAMP and stimulates protein kinase A. The activated PTH receptor also stimulates Gαq, which, in turn, stimulates phospholipase C to generate IP3 and DAG (see Chapter 3). The IP3releases Ca2+ from internal stores, thus increasing [Ca2+]i and activating Ca2+-dependent kinases. In humans, the bone PTH receptor on osteoblastic cells is identical to that present in renal cortical cells. As discussed in the next two sections, the net effects of PTH on the kidney and bone are to increase plasma [Ca2+] and to lower plasma [PO3−4].

In the kidney, PTH promotes Ca2+ reabsorption, PO3−4 loss, and 1-hydroxylation of 25-hydroxyvitamin D

PTH exerts a spectrum of actions on target cells in the kidney (Fig. 52-8). In the renal proximal tubule, PTH receptors are located on the basolateral membrane of the polarized epithelial cell. Binding of PTH to its receptors activates dual intracellular signaling systems and in this manner modifies several properties of the cell related to transepithelial transport.

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Figure 52-8 Feedback loops in the control of plasma Ca2+ levels. PTH released from the parathyroid glands acts through receptors in both bone and kidney. In kidney, PTH has three effects. First, PTH promotes Ca2+ reabsorption and thus an increase in plasma [Ca2+]. Second, PTH inhibits PO3−4reabsorption. Third, PTH promotes the hydroxylation of 25-hydroxyvitamin D, thereby creating the active metabolite 1, 25-dihydroxyvitamin D. This metabolite further promotes renal Ca2+ reabsorption. In bone, PTH promotes net bone reabsorption and hence increases plasma [Ca2+]. In intestine, the 1, 25-dihydroxyvitamin D (produced indirectly as the result of PTH) enhances Ca2+ absorption. Thus, net effect on bone, kidney, and gut, as far as Ca2+ is concerned, is to increase plasma [Ca2+]. The increased plasma [Ca2+] feeds back on the parathyroid glands and inhibits the release of PTH.

Stimulation of Ca2+ Reabsorption A key action of PTH is to promote the reabsorption of Ca2+ in the thick ascending limb and distal convoluted tubule of the kidney (see Chapter 36). Most of the ~250 mmol of Ca2+filtered each day is reabsorbed in the proximal tubule (~65%) and thick ascending limb (~25%). The distal nephron is responsible for reabsorbing an additional 5% to 10% of the filtered load of Ca2+, with ~0.5% of the filtered load left in the urine. Thus, when PTH stimulates distal Ca2+ reabsorption, it greatly decreases the amount of Ca2+ excreted in the urine (usually 4 to 5 mmol/day) and tends to raise plasma [Ca2+] (Fig. 52-8).

Greater fractional Ca2+ excretion occurs when either proximal or distal Ca2+ reabsorption is impaired, as occurs with osmotic diuresis and hypoparathyroidism, respectively. As discussed later, vitamin D has a synergistic action of promoting Ca2+ reabsorption in the distal convoluted tubule.

Inhibition of PO3−4 Reabsorption Most strikingly, PTH reduces the reabsorption of PO3−4 in both the proximal and distal tubule. Because most of the PO3−4 reabsorption occurs in the proximal tubule and because the effect of PTH is greatest on the proximal tubule, PTH produces characteristic phosphaturia and tends to decrease plasma PO3−4 levels (Fig. 52-8). This phosphaturia results from a PTH-induced redistribution of the Na/PO cotransporter (NaPi) away from the apical membrane of the renal proximal tubule and into a pool of subapical vesicles (see Chapter 36). As discussed later, PTH stimulates bone to resorb Ca2+ and PO3−4 from the hydroxyapatite crystals of bone mineral. Thus, PTH stimulates both steps in the movement of PO3−4 from bone to blood to urine.

Elimination of PO3−4 through urine plays an important role when elevated levels of PTH liberate Ca2+ and PO3−4 from bone. At normal plasma Ca2+ and phosphorus concentrations, the Ca/PO ion product approaches the solubility product. Further PTH-induced increases in [Ca2+]—if accompanied by a rise in PO3−4 levels—could cause Ca/PO salts to precipitate out of the soluble phase, thereby at least partly negating the action of PTH to raise plasma [Ca2+]. Thus, PTH-induced phosphaturia diminishes the Ca/PO ion product and prevents precipitation when Ca2+ mobilization is needed. The body tightly regulates plasma [Ca2+] but allows plasma levels of PO3−4 to vary rather widely.

Familial Hypocalciuric Hypercalcemia

The Ca2+ receptor on the parathyroid cell is mutated in patients with the disorder familial hypocalciuric hypercalcemia (FHH). Serum Ca2+ in these patients is typically elevated 10% to 30% higher than the normal range. These patients generally tolerate this elevated plasma [Ca2+] very well, and the finding of a total plasma Ca2+ of 3.0 to 3.3 mM (12 to 13 mg/dL)—the normal range is 2.2 to 2.7 mM (8.8 to 10.6 mg/dL)—is frequently more alarming to the physician than to the patient. Despite a plasma [Ca2+] that would make most people symptomatic with kidney stones, fatigue, or loss of mental concentration, these patients have remarkably few symptoms. The amount of Ca2+ present in their urine is typically less than that seen in physiologically normal individuals (hence the designation hypocalciuric) and much lower than that encountered in persons with hypercalcemia of other causes. Despite the high plasma [Ca2+], the serum PTH concentrations in these patients are typically normal. The normal PTH concentration suggests that regulation of PTH secretion is intact, but the set-point at which the Ca2+ turns off PTH secretion is shifted toward higher plasma [Ca2+] (see Fig. 52-7B).

Only one allele for CaSR appears to be present. FHH is an autosomal dominant disorder, and several different point mutations have been identified in different families. Heterozygotes express both normal and mutated Ca2+ receptors. Rarely, infants are born with the homozygousdisorder (i.e., both copies of the receptor are defective). These infants have very severe hypercalcemia ([Ca2+], >15 mg/dL) and severe hyperparathyroidism. The condition is life threatening and is characterized by markedly elevated plasma [Ca2+], neuronal malfunction, demineralization of bone, and calcification of soft tissues. These infants die unless the inappropriately regulated parathyroid glands are removed.

As in the parathyroid gland, the distal convoluted tubule of the kidney has abundant plasma membrane Ca2+ receptors. Investigators have suggested that serum Ca2+ binds to this renal Ca2+ receptor and inhibits Ca2+ reabsorption (see Chapter 36). Thus, with a mutated receptor, renal Ca2+ reabsorption may not be inhibited until plasma [Ca2+] rises to abnormally high levels. The result would be the increased Ca2+ reabsorption and hypocalciuria characteristic of FHH.

The discovery of CaSR led to the development of a CaSR agonist that mimics Ca2+ (a calcimetic). This drug has now come into clinical use for treating patients with parathyroid cancer or hyperparathyroidism secondary to chronic renal disease. Calcimetics decrease the secretion of PTH and secondarily decrease plasma [Ca2+].

In addition to affecting PO3−4 reabsorption, PTH causes a decrease in proximal tubule reabsorption of HCO3−4 (see Chapter 39) and of several amino acids. These actions appear to play a relatively minor role in whole-body acid-base and nitrogen metabolism, respectively.

Stimulation of the Last Step of Synthesis of 1, 25-Dihydroxyvitamin D A third important renal action of PTH is to stimulate the 1-hydroxylation of 25-hydroxyvitamin D in the mitochondria of the proximal tubule. The resulting 1, 25-dihydroxyvitamin D is the most biologically active metabolite of dietary or endogenously produced vitamin D. Its synthesis by the kidney is highly regulated, and PTH is the primary stimulus to increase 1-hydroxylation. Hypophosphatemia, either spontaneous or induced by the phosphaturic action of PTH, also promotes the production of 1, 25-dihydroxyvitamin D. As discussed later, the 1, 25-dihydroxyvitamin D formed in the proximal tubule has three major actions: (1) enhancement of renal Ca2+ reabsorption, (2) enhancement of Ca2+ absorption by the small intestine, and (3) modulation of the movement of Ca2+ and PO3−4 in and out of bone. (See Note: Historical Assays for PTH)

In bone, PTH can promote net resorption or net deposition

The second major target tissue for PTH is bone, in which PTH promotes both bone resorption and bone synthesis.

Bone Resorption by Indirect Stimulation of Osteoclasts The net effect of persistent increases of PTH on bone is to stimulate bone resorption, thus increasing plasma [Ca2+]. Osteoblasts express abundant surface receptors for PTH; osteoclasts do not. Because osteoclasts lack PTH receptors, PTH by itself cannot regulate the coupling between osteoblasts and osteoclasts. Rather, it appears that PTH acts on osteoblasts and osteoclast precursors to induce the production of several cytokines that increase both the number and the activity of bone-resorbing osteoclasts. Precisely which cytokines are involved in the physiological signaling of osteoclasts by PTH-stimulated osteoblasts in vivo is not clear. PTH causes osteoblasts to release agents such as M-CSF and stimulates the expression of RANK ligand (i.e., osteoprotegerin ligand), actions that promote the development of osteoclasts (Fig. 52-4). In addition, PTH and vitamin D stimulate osteoblasts to release interleukin 6 (IL-6), which stimulates existing osteoclasts to resorb bone (Fig. 52-5).

One of the initial clues that cytokines are important mediators of osteoclastic bone resorption came from observations on patients with multiple myeloma—a malignancy of plasma cells, which are of B-lymphocyte lineage. The tumor cells produce several proteins that activate osteoclasts and enhance bone resorption. These proteins were initially called “osteoclast-activating factors.” We now know that certain lymphocyte-derived proteins strongly activate osteoclastic bone resorption; these proteins include lymphotoxin, IL-1, and TNF-α.

Bone Resorption by Reduction in Bone Matrix PTH also changes the behavior of osteoblasts in a manner that can promote net loss of bone matrix. For example, PTH inhibits collagen synthesis by osteoblasts and also promotes the production of proteases that digest bone matrix. Digestion of matrix is important because osteoclasts do not easily reabsorb bone mineral if the bone has an overlying layer of unmineralized osteoid.

Bone Deposition Whereas persistent increases in PTH favor net resorption, intermittent increases in plasma [PTH] have predominantly bone synthetic effects. PTH can promote bone synthesis by two mechanisms. First, PTH promotes bone synthesis directly by activating Ca2+ channels in osteocytes, a process that leads to a net transfer of Ca2+ from bone fluid to the osteocyte. The osteocyte then transfers this Ca2+ through gap junctions to the osteoblasts at the bone surface. This process is called osteocytic osteolysis. The osteoblasts then pump this Ca2+ into the extracellular matrix, thus contributing to mineralization. Second, PTH stimulates bone synthesis indirectly in that osteoclastic bone resorption leads to the release of growth factors such as insulin-like growth factor 1 (IGF-1), IGF-2, and transforming growth factor β.

The PTH–1-34 peptide is now available as a pharmacological agent for the treatment of osteoporosis. Clinical data show marked increases in bone density—particularly within the axial skeleton—in response to injections of PTH–1-34 once or twice daily. The effects on trabecular bone are striking, with less positive responses seen in cortical bone—particularly in the limbs.

VITAMIN D

The active form of vitamin D is its 1, 25-dihydroxy metabolite

By the 1920s, investigators recognized that dietary deficiency of a fat-soluble vitamin was responsible for the childhood disease rickets. This disorder is characterized clinically by hypocalcemia and multiple skeletal abnormalities. Dietary replacement of vitamin D corrects this disorder and has led to the practice of adding vitamin D to milk, bread, and other products. This practice has greatly reduced the prevalence of this previously common disorder.

Our understanding of the involvement of vitamin D in the regulation of plasma [Ca2+] and skeletal physiology has been clarified only over the past 2 decades. Vitamin D exists in the body in two forms, vitamin D3 and vitamin D2(Fig. 52-9). Vitamin D3 can be synthesized from the 7-dehydrocholesterol that is present in the skin, provided sufficient ultraviolet light is absorbed. This observation explains why nutritional rickets had been a much more prevalent problem in northern countries, where clothing covers much of the skin and where individuals remain indoors much more of the year. Vitamin D3 is also available from several natural sources, including cod and halibut liver, eggs, and fortified milk. Vitamin D2 is obtained only from the diet, largely from vegetables. Vitamins D3 (Fig. 52-9A) and vitamin D2(Fig. 52-9B) differ only in the side chains of ring D. The side chain in vitamin D3 (cholecalciferol) is characteristic of cholesterol, whereas that of vitamin D2 (ergocalciferol) is characteristic of plant sterols.

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Figure 52-9 Forms of vitamin D. (See Note: Metabolism of Vitamin D3 and D2)

Vitamin D (i.e., either D2 or D3) is fat soluble but water insoluble. Its absorption from the intestine depends on its solubilization by bile salts (see Chapter 45). In the circulation, vitamin D is found either solubilized with chylomicrons (see Chapter 46) or associated with a plasma binding protein. Most of the body stores of vitamin D are located in body fat. The body’s pools of vitamin D are large, and only 1% to 2% of the body’s vitamin D is turned over each day. Therefore, several years of very low dietary intake (as well as diminished endogenous synthesis) is required before the endogenous pools are depleted and deficiency develops.

The principal active form of vitamin D is not vitamin D2 or D3, but rather a dihydroxylated metabolite of either. Hydroxylation of vitamin D proceeds in two steps (Fig. 52-9A). When circulating levels of 25-hydroxyvitamin D are low, adipocytes release vitamin D into the blood plasma. A cytochrome P-450 mixed-function oxidase, principally in the liver, creates the first hydroxyl group at carbon 25. The 25-hydroxylation of vitamin D does not appear to be highly regulated, but rather it depends on the availability of vitamin D2 or D3. The second hydroxylation reaction occurs in the renal proximal tubule under the tight control of PTH, vitamin D itself, and PO3−4. PTH stimulates this 1-hydroxylation, whereas PO3−4 and 1, 25-dihydroxyvitamin D (the reaction product) both inhibit the process (Fig. 52-8).

In addition to vitamins D2 and D3 and their respective 25-hydroxy and 1, 25-dihydroxy metabolites, more than 15 other metabolites of vitamin D have been identified in plasma. However, the specific physiological function of these metabolites, if any, is unclear.

Although considered a vitamin because of its dietary requirement, vitamin D can also be considered a hormone, both because it is endogenously synthesized and because even the fraction that arises from the diet must be metabolized to a biologically active form.

Vitamin D and its metabolites, like the steroid hormones, circulate bound to a globulin binding protein, in this case a 52-kDa vitamin D–binding protein. This binding protein appears particularly important for the carriage in the blood of vitamins D2 and D3, which are less soluble than their hydroxylated metabolites. Vitamin D and its metabolites arrive at target tissues and, once in the cytosol, associate with the VDR, a transcription factor that is in the family of nuclear receptors (see Chapter 3). Like the thyroid hormone receptor (see Table 4-2), VDR forms a heterodimer with RXR. The VDR specifically recognizes the 1, 25-dihydroxyvitamin D with an affinity that is three orders of magnitude higher than that for 25-hydroxyvitamin D. However, because the circulating concentration of 25-hydroxyvitamin D is ~1000-fold higher than that of 1, 25-dihydroxyvitamin D, both species probably contribute to the biological actions of the hormone.

The biological actions of 1, 25-dihydroxyvitamin D appear to be expressed principally, but not exclusively, through regulation of the transcription of a variety of proteins. The VDR/RXR complex associates with a regulatory site in the promoter region of the genes coding for certain vitamin D–regulated proteins. Thus, the occupied VDR alters the synthesis of these vitamin D–dependent proteins. An example is PTH, which stimulates the formation of 1, 25-dihydroxyvitamin D. The 5′ regulatory region of the PTH gene has a VDR consensus sequence; when occupied by the VDR complex, this element diminishestranscription of the PTH gene.

Vitamin D, by acting on the small intestine and kidney, raises plasma [Ca2+] and thus promotes bone mineralization

The actions of vitamin D can be grouped into two categories: actions on classic target tissues involved in regulating body mineral and skeletal homeostasis and a more general action that regulates cell growth. The actions of vitamin D on the small intestine, bone, and kidney serve to prevent any abnormal decline or rise in plasma [Ca2+].

Small Intestine In the duodenum, 1, 25-dihydroxyvitamin D increases the production of several proteins that enhance Ca2+ absorption. Figure 52-10A summarizes the intestinal absorption of Ca2+ (see Chapter 45), which moves from the intestinal lumen to the blood by both paracellular and transcellular routes. In the paracellular route, which occurs throughout the small intestine, Ca2+ moves passively from the lumen to the blood; 1, 25-dihydroxyvitamin D does not regulate this pathway. The transcellular route, which occurs only in the duodenum, involves three steps. First, Ca2+ enters the cell across the apical membrane through Ca2+ channels and possibly endocytosis. Second, the entering Ca2+ binds to several high-affinity binding proteins, particularly calbindin. These proteins, together with the exchangeable Ca2+pools in the endoplasmic reticulum and mitochondria, effectively buffer the cytosolic Ca2+ and maintain a favorable gradient for Ca2+ entry across the apical membrane of the enterocyte. Thus, the intestinal cell solves the problem of absorbing relatively large amounts of Ca2+ while keeping its free, cytosolic [Ca2+] low. Third, the enterocyte extrudes Ca2+ across the basolateral membrane by means of both a Ca2+pump and an Na-Ca exchanger.

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Figure 52-10 Intestinal absorption of Ca2+ and PO3−4A, The small intestine absorbs Ca2+ by two mechanisms. The passive, paracellular absorption of Ca2+ occurs throughout the small intestine. This pathway predominates but is not under the control of vitamin D. The second mechanism—the active, transcellular absorption of Ca2+—occurs only in the duodenum. Ca2+ enters the cell across the apical membrane through a channel. Inside the cell, the Ca2+ is buffered by binding proteins, such as calbindin, and is also taken up into intracellular organelles, such as the endoplasmic reticulum. The enterocyte then extrudes Ca2+ across the basolateral membrane through a Ca2+ pump and an Na-Ca exchanger. Thus, the net effect is Ca2+ absorption. The active form of vitamin D—25-dihydroxyvitamin D—stimulates all three steps of transcellular Ca2+ absorption. B, Pi enters the enterocyte across the apical membrane through an Na/Pi (NaPi) cotransporter. Once inside the cell, the Pi is extruded across the basolateral membrane. Thus, the net effect is Pi absorption.

Vitamin D promotes Ca2+ absorption primarily by genomic effects that involve induction of the synthesis of epithelial Ca2+ channels and pumps and Ca2+-binding proteins, as well as other proteins (e.g., alkaline phosphatase). Although these actions probably facilitate Ca2+ transport by the intestine, not all steps involved in the action of vitamin D to enhance transcellular Ca2+ transport have been well defined experimentally. The effect of PTH to stimulate intestinal Ca2+ absorption is thought to be entirely indirect and mediated by increasing the renal formation of 1, 25-dihydroxyvitamin D (Fig. 52-8), which then enhances Ca2+ absorption.

Vitamin D also stimulates PO3−4 absorption by the small intestine (Fig. 52-10B). The initial step is mediated by the NaPi cotransporter (see Chapter 36) and appears to be rate limiting for transepithelial transport and subsequent delivery of PO3−4 to the circulation. 1, 25-Dihydroxyvitamin D stimulates the synthesis of this transport protein and thus promotes PO3−4 entry into the mucosal cell.

Kidney In the kidney, vitamin D appears to act synergistically with PTH to enhance Ca2+ reabsorption in the distal convoluted tubule (see Chapter 36). High-affinity Ca2+-binding proteins, similar to those found in the intestinal mucosa, have been specifically localized to this region of the kidney. It appears that PTH is a more potent regulator of Ca2+ reabsorption than vitamin D is (Fig. 52-8). Indeed, parathyroidectomy increases the fractional excretion of Ca2+, and even high doses of vitamin D cannot correct this effect. In addition, as in the intestine, vitamin D promotes PO3−4 reabsorption in the kidney. The effects of vitamin D on PO3−4 reabsorption, like its effects on Ca2+, are less dramatic than those of PTH. Finally, 1, 25-dihydroxyvitamin D directly inhibits the 1-hydroxylation of vitamin D.

Rickets and Osteomalacia

Deficiency of vitamin D in children produces the disease rickets, in which bone has abnormal amounts of unmineralized osteoid. Both cortical bone and trabecular bone are involved. The lack of mineralization diminishes bone rigidity and leads to a characteristic bowing of the long bones of the legs. In adults, vitamin D deficiency produces a disorder called osteomalacia. Microscopically, the bone looks very much the same in adult and childhood vitamin D deficiency. However, because the longitudinal growth of the long bones has been completed in adults, bowing of weight-bearing bones does not occur. Instead, the increased unmineralized osteoid content of bone causes a decline in bone strength. Affected individuals are more prone to the development of bone fractures. These fractures may be very small and difficult to see radiographically. As more and more of the bone surface is covered by osteoid and as recruitment of new osteoclasts is diminished, osteoclastic bone resorption is impaired, and hypocalcemia develops. Hypocalcemia causes nerves to become more sensitive to depolarization. In sensory nerves, this effect leads to sensations of numbness, tingling, or burning; in motor nerves, it leads to increased spontaneous contractions, or tetany.

Whereas rickets and osteomalacia are very uncommon in developed countries because of vitamin D supplementation, milder degrees of vitamin D deficiency are increasingly recognized, particularly in the elderly population, in whom milk consumption and sunlight exposure are frequently inadequate. The resulting fall in plasma [Ca2+] can lead to mild, secondary hyperparathyroidism. Such continuous elevations of PTH can lead to further bone resorption and worsening osteoporosis.

Rickets or osteomalacia also can occur with impaired ability of the kidney to 1-hydroxylate the 25-hydroxyvitamin D previously synthesized in the liver. An acquired version is seen in many patients with chronic renal failure, in which the activity of 1α-hydroxylase is reduced. The genetic form of the 1α-hydroxylase deficiency is a rare autosomal recessive disorder. Either form is called vitamin D–dependent rickets because it can be successfully treated with either 1, 25-dihydroxyvitamin D or higher doses of dietary vitamin D2 or vitamin D3 (~10-to 100-fold) than the 400 U/day used to prevent nutritional rickets.

Bone The actions of vitamin D on bone are complex and are the result of both indirect and direct actions. The overall effect of vitamin D replacement in animals with dietary-induced vitamin D deficiency is to increase the flux of Ca2+ into bone. However, as we see later, these major effects of vitamin D on bone are indirect: the action of vitamin D on both the small intestine and the kidneys makes more Ca2+available to mineralize previously unmineralized osteoid. The direct effect of vitamin D on bone is to mobilize Ca2+ out of bone. Both osteoblasts and osteoclast precursor cells have VDRs. In response to vitamin D, osteoblasts produce certain proteins, including alkaline phosphatase, collagenase, and plasminogen activator. In addition, as noted earlier (Fig. 52-4), vitamin D and PTH promote the development of osteoclasts from precursor cells. Thus, because vitamin D directly increases the number of mature osteoclasts, supplying vitamin D to bone obtained from vitamin D–deficient animals in in vitro experiments mobilizes Ca2+ from bone into the medium. Additional evidence that vitamin D directly promotes bone resorption comes from experiments on rachitic animals who are maintained on a Ca2+-deficient diet. Treating these animals with vitamin D causes plasma [Ca2+] to rise, an indication of net bone resorption. At the same time, however, the elevated plasma [Ca2+] promotes the mineralization of previously unmineralized osteoid—at the expense of bone resorption from other sites.

The direct effects of vitamin D on bone, which are to mobilize Ca2+, seem to be contrary to the overall effect of vitamin D on bone, which is to promote mineralization. These observations, as well as others, have led to the hypothesis, now generally accepted, that the antirachitic action of vitamin D is largely indirect. By enhancing the absorption of Ca2+ and PO3−4 from the intestine and by enhancing the reabsorption of Ca2+ and PO3−4 from the renal tubules, vitamin D raises the concentrations of both Ca2+ and PO3−4 in the blood and extracellular fluid. This increase in the Ca/PO ion product results in net bone mineralization. These indirect effects overshadow the direct effect of vitamin D to increase bone mobilization.

Ca2+ ingestion lowers levels of PTH and 1, 25-dihydroxyvitamin D, whereas PO3−4 ingestion raises levels of both PTH and 1, 25-dihydroxyvitamin D

Consider a situation in which a subject ingests a meal containing Ca2+. The rise in plasma [Ca2+] inhibits PTH secretion. The decline in PTH causes a decrease in the resorption of Ca2+ and phosphorus from bone, thus limiting the postprandial increase in plasma Ca2+ and PO3−4 levels. In addition, the decrease in PTH diminishes Ca2+ reabsorption in the kidney and thus facilitates a calciuric response. If dietary Ca2+intake remains high, the lower PTH will result in decreased 1-hydroxylation of 25-hydroxyvitamin D, which will eventually diminish the fractional absorption of Ca2+ from the GI tract.

If dietary Ca2+ intake is deficient, the body will attempt to restore Ca2+ toward normal by increasing plasma [PTH]. This response will help to mobilize Ca2+ from bone, to promote renal Ca2+ retention, and over time, to increase the level of 1, 25-dihydroxyvitamin D, which will enhance gut absorption of Ca2+.

If one ingests phosphorus much in excess of Ca2+ (e.g., after drinking several colas), the rise in plasma [PO3−4] will lower plasma [Ca2+] because the increased plasma Ca/PO ion product will promote the deposition of mineral in bone. The resultant decrease in plasma [Ca2+] will, in turn, increase PTH secretion. This rise in PTH will provoke phosphaturia that will act to restore plasma [PO3−4] toward normal while Ca2+ and PO3−4 are mobilized from bone by the action of PTH. Over longer periods, the action of PTH to modulate the 1-hydroxylation of 25-hydroxyvitamin D plays an increasingly important role in defending the plasma [Ca2+] by increasing intestinal Ca2+ absorption.

Osteoporosis

Approximately 25 million Americans, mostly elderly women, are afflicted with osteoporosis, and between 1 and 2 million of these individuals experience a fracture related to osteoporosis every year. The cost in economic and human terms is immense. Hip fractures are responsible for much of the morbidity associated with osteoporosis, but even more concerning is the observation that as many as 20% of women with osteoporotic hip fractures die within 1 year of their fracture.

The major risk factor for osteoporosis is the declining estrogen levels in aging women. Rarely, other endocrine disorders such as hyperthyroidism, hyperparathyroidism, and Cushing disease (hypercortisolism) are responsible. Other risk factors include inadequate dietary Ca2+intake, alcoholism, cigarette smoking, and a sedentary lifestyle.

Strategies to prevent the development of osteoporosis begin in the premenopausal years. High Ca2+ intake and a consistent program of weight-bearing exercises are widely recommended.

Pharmacological agents are now available for preventing or at least retarding the development of osteoporosis or for treating the disease once it has become established. These agents can be broadly classified into two groups: antiresorptive drugs and agents that are able to stimulate bone formation.

Among the former, estrogen is by far the most widely used therapy. It is most effective when started at the onset of menopause, although it may offer benefits even in patients who are 20 or more years past menopause. Calcitonin is generally offered to women who cannot or are unwilling to take estrogen. However, it is expensive and must be given parenterally; an intranasal spray is also available. Another class of drugs, the bisphosphonates, is becoming popular. These drugs are powerful inhibitors of bone resorption, but some of the first agents of this class have also been found to impair mineralization. The newer bisphosphonates can be safely given in doses that decrease bone resorption without affecting mineralization.

Among the drugs that can stimulate bone formation, vitamin D—often given as 1, 25-dihydroxyvitamin D (calcitriol)—is combined with Ca2+ therapy to increase the fractional absorption of Ca2+ and to stimulate the activity of osteoblasts. PTH, recently available as an injectable treatment for osteoporosis, potently stimulates osteoblast formation and increases bone mass. PTH also appears to decrease the rate of vertebral fractures.

Calcitriol and the bisphosphonates have also been used successfully to treat Paget disease of bone, a disorder characterized by localized regions of bone resorption and reactive sclerosis. The level of bone turnover can be extremely high. Although it remains asymptomatic in many individuals, the disease can cause pain, deformity, fractures, and vertigo and hearing loss if bony overgrowth occurs in the region of the eighth cranial nerve. The cause of Paget disease is not known.

CALCITONIN AND OTHER HORMONES

Calcitonin inhibits osteoclasts, but its effects are transitory

Calcitonin is a 32–amino acid peptide hormone made by the clear or C cells of the thyroid gland. C cells (also called parafollicular cells) are derived from neural crest cells of the fifth branchial pouch, which in humans migrate into the evolving thyroid gland. Although it is located within the thyroid, calcitonin’s major, if not sole, biological action relates to the regulation of mineral metabolism and bone turnover. The incidental nature of its relationship with the major functions of the thyroid is emphasized by the finding that in many nonhuman species, C cells are found in a body called the ultimobranchial gland and not in the thyroid at all.

Calcitonin is synthesized in the secretory pathway (see Chapter 2) by post-translational processing of a large procalcitonin. As illustrated in Figure 52-11, alternative splicing of the calcitonin gene product gives rise to several biologically active peptides. In the C cells, calcitonin is the only peptide made in biologically significant amounts. Within the central nervous system, calcitonin gene–related peptide (CGRP) is the principal gene product, and it appears to act as a neurotransmitter in peptidergic neurons (see Table 13-1). Calcitonin is stored in secretory vesicles within the C cells, and its release is triggered by raising the extracellular [Ca2+] to levels higher than normal. Conversely, lowering the extracellular [Ca2+] diminishes calcitonin secretion. The threshold [Ca2+] for enhancing calcitonin secretion is in the midphysiological range. In principle, this secretory profile would leave calcitonin well poised to regulate body Ca2+ homeostasis.

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Figure 52-11 Synthesis of calcitonin and CGRP. A common primary RNA transcript gives rise to both calcitonin and CGRP. In the thyroid gland, C cells produce a mature mRNA that they translate to procalcitonin. They then process this precursor to produce an N-terminal peptide, calcitonin (a 32–amino acid peptide), and CCP. In the brain, neurons produce a different mature mRNA and a different “pro” hormone. They process the peptide to produce an N-terminal peptide, CGRP, and a C-terminal peptide.

The precise role for calcitonin in body Ca2+ homeostasis has been difficult to define. This difficulty was first apparent from the simple clinical observation that after complete thyroidectomy with removal of all calcitonin-secreting tissue, plasma [Ca2+] remains normal (provided the parathyroid glands are not injured). Conversely, patients with a rare calcitonin-secreting tumor of the C cells frequently have plasma calcitonin concentrations that are 50 to 100 times normal, yet they maintain normal plasma levels of Ca2+, vitamin D, and PTH. Nevertheless, several lines of evidence suggest that calcitonin does have biologically important actions. First, although calcitonin appears to have a minimal role in the minute-to-minute regulation of plasma [Ca2+] in humans, it does serve an important role in many nonmammalian species. This role is particularly clear for teleost fish. Faced with the relatively high [Ca2+] in sea water (and therefore in food), calcitonin, secreted in response to a rise in plasma [Ca2+], decreases bone resorption, thus returning the plasma [Ca2+] toward normal. Salmon calcitonin, which differs from human calcitonin in 14 of its 32 amino acid residues, is roughly 10-fold more potent on a molar basis in inhibiting osteoclast function than is the human hormone.

The second line of evidence that calcitonin may have biologically important actions is the presence of calcitonin receptors. Like PTH receptors, the calcitonin receptor is a G protein–coupled receptor that, depending on the target cell, may activate either adenylyl cyclase or phospholipase C (see Chapter 3). Within bone, the osteoclast—which lacks PTH receptors—appears to be the principal target of calcitonin. Indeed, the presence of calcitonin receptors may be one of the most reliable methods of identifying osteoclasts. In the osteoclast, calcitonin appears to work by raising [cAMP]i and then presumably acting through one or more protein kinases. Calcitonin inhibits the resorptive activity of the osteoclast and slows the rate of bone turnover. It also appears to diminish osteocytic osteolysis, and this action—together with calcitonin’s effect on the osteoclast—is responsible for the hypocalcemic effect after the short-term administration of pharmacological doses of calcitonin. The hypocalcemic action of calcitonin is particularly effective in circumstances in which bone turnover is accelerated, as occurs in rapidly growing young animals and in human patients with hyperparathyroidism. The antiosteoclastic activity of calcitonin is also useful in treating Paget disease of bone (see the box titled Osteoporosis). However, within hours of exposure of osteoclasts to high concentrations of calcitonin, the antiresorptive action of calcitonin begins to wane. This “escape” from the hypocalcemic effect of calcitonin has limited the use of calcitonin in the clinical treatment of hypercalcemia. The transitory nature of the action of calcitonin appears partly to result from rapid downregulation of calcitonin receptors.

In the kidney, calcitonin, like PTH, causes mild phosphaturia by inhibiting proximal tubule PO3−4 transport. Calcitonin also causes mild natriuresis and calciuresis. These actions may contribute to the acute hypocalcemic and hypophosphatemic actions of calcitonin. However, these renal effects are of short duration and do not appear to be important in the overall renal handling of Ca2+, PO3−4, or Na+.

Sex steroid hormones promote bone deposition, whereas glucocorticoids promote resorption

Although PTH and 1, 25-dihydroxyvitamin D are the principal hormones involved in modulating bone turnover, other hormones participate in this process. For example, the sex steroids testosterone and estradiol are needed for maintaining normal bone mass in male and female subjects, respectively. The decline in estradiol that occurs postmenopausally exposes women to the risk of osteoporosis, that is, a decreased mass of both cortical and trabecular bone caused by a decrease in bone matrix (see the box titled Osteoporosis). Osteoporosis is less common in men because their skeletal mass tends to be greater throughout adult life and because testosterone levels in men decline slowly as they age, unlike the abrupt menopausal decline of estradiol in women.

Glucocorticoids also modulate bone mass. This action is most evident in circumstances of glucocorticoid excess, which leads to osteoporosis, as suggested by the effects of glucocorticoids on the production of osteoprotegerin and osteoprotegerin ligand.

The precise cellular mechanisms that mediate the action of androgens, estrogens, or glucocorticoids on bone have not been well defined. Despite the loss of bone that occurs with androgen or estrogen deficiency or glucocorticoid excess, in each case, the coupling of bone synthesis to degradation is qualitatively preserved. Synthesis of new bone continues to occur at sites of previous bone resorption, and no excess of unmineralized osteoid is present. Presumably, the decline in bone mass reflects a quantitative shift whereby the amount of new bone formed at any site is less than what was resorbed. Because this shift occurs at multiple sites, the result is a decline in overall bone mass.

PTH-related peptide, encoded by a gene that is entirely distinct from that for PTH, can cause hypercalcemia in certain malignancies

Unlike PTH, which is synthesized exclusively by the parathyroid gland, a peptide called PTHrP appears to be made in many different normal and malignant tissues. The PTH 1R receptor in kidney and bone recognizes PTHrP with an affinity similar to that for intact PTH. PTHrP mimics each of the actions of PTH on kidney and bone. Thus, when present in sufficient concentrations, PTHrP causes hypercalcemia. PTHrP exists in three alternatively spliced isoforms of a single gene product. The gene encoding PTHrP is completely distinct from that for PTH. The similar actions of PTHrP and PTH arise from homology within the first 13 amino acids of PTHrP with native PTH. Only weak homology is seen between amino acids 14 and 34 (three amino acids are identical) and essentially no homology beyond amino acid 34. This situation is an unusual example of mimicry among peptides that are structurally quite diverse.

The normal physiological function of PTHrP is still largely undefined. The lactating breast secretes PTHrP, and this hormone is present in very high concentrations in milk. PTHrP may promote the mobilization of Ca2+ from maternal bone during milk production. In nonlactating humans, the plasma PTHrP concentration is very low, and PTHrP does not appear to be involved in the day-to-day regulation of plasma [Ca2+]. It appears likely that under most circumstances, PTHrP acts in a paracrine or autocrine, rather than an endocrine, regulatory fashion.

Many tumors are capable of manufacturing and secreting PTHrP, among them the following: squamous cell tumors of the lung, head, and neck; renal and bladder carcinomas; adenocarcinomas; and lymphomas. Patients with any of these tumors are subject to severe hypercalcemia of fairly abrupt onset. (See Note: Parathyroid Hormone-Related Peptide (PTHrP))

REFERENCES

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Murer H, Forster I, Hilfiker H, et al: Cellular/molecular control of renal Na/Pi-cotransport. Kidney Int Suppl 1998; 65:2-10.

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Brown EM, Gamba G, Riccardi D, et al: Cloning and characterization of an extracellular Ca2+-sensing receptor from bovine parathyroid. Nature 1993; 366:575-580.

Burgess TL, Qian Y, Kaufman S, et al: The ligand for osteoprotegerin (OPGL) directly activates mature osteoclasts. J Cell Biol 1999; 145:527-538.