Na+ reabsorption involves apical transporters or ENaCs and a basolateral Na-K pump
Along the first half of the tubule (Fig. 35-4A), a variety of cotransporters in the apical membrane couples the downhill uptake of Na+ to the uphill uptake of solutes such as glucose, amino acids, phosphate, sulfate, lactate, and citrate. Many of these Na+-driven cotransporters are electrogenic, carrying net positive charge into the cell. Thus, both the low [Na+]i and the negative apical membrane voltage fuel the secondary active uptake of these other solutes, which we discuss in Chapter 36. In addition to being coupled to the cotransporters, Na+ entry is also coupled to the extrusion of H+ through the electroneutral Na-H exchanger 3 (NHE3). We discuss the role of NHE3 in renal acid secretion on page 827.
FIGURE 35-4 Cell models of Na+ reabsorption. PCT, proximal convoluted tube.
Both cotransporters and exchangers exploit the downhill Na+ gradient across the apical cell membrane that is established by the Na-K pump in the basolateral membrane. The Na-K pump—and, to a lesser extent, the electrogenic Na/HCO3 cotransporter 1 (NBCe1)—are also responsible for the second step in Na+ reabsorption, moving Na+ from cell to blood. The presence of K+ channels in the basolateral membrane is important for two reasons. First, these channels establish the negative voltage across the basolateral membrane and establish a similar negative voltage across the apical membrane via paracellular electrical coupling. Second, these channels permit the recycling of K+ that had been transported into the cell by the Na-K pump.
Because of a lumen-negative Vte in the early proximal tubule, as well as a paracellular pathway that is permeable to Na+, approximately one third of the Na+ that is transported from lumen to blood by the transcellular pathway diffuses back to the lumen by the paracellular pathway (“backleak”).
Thin Limbs of Henle's Loop
Na+ transport by the thin descending and thin ascending limbs of Henle's loop is almost entirely passive and paracellular (see p. 811).
Thick Ascending Limb
Two major pathways contribute to Na+ reabsorption in the TAL: transcellular and paracellular (see Fig. 35-4B). The transcellular pathway includes two major mechanisms for taking up Na+ across the apical membrane. Na/K/Cl cotransporter 2 (NKCC2) couples the inward movement of 1 Na+, 1 K+, and 2 Cl− ions in an electroneutral process driven by the downhill concentration gradients of Na+ and Cl− (see p. 122). The second entry pathway for Na+ is an NHE3. As in the proximal tubule, the basolateral Na-K pump keeps [Na+]i low and moves Na+ to the blood.
Two features of the apical step of Na+ reabsorption in the TAL are noteworthy. First, the loop diuretics (e.g., furosemide and bumetanide) inhibit Na/K/Cl cotransport. Second, a large fraction of the K+ that NKCC2 brings into the cell recycles to the lumen via apical K+ channels. These channels are essential for replenishing luminal K+ and thus for maintaining adequate Na/K/Cl cotransport.
A key aspect of the paracellular pathway for Na+ reabsorption in the TAL is a lumen-positive Vte (see Fig. 35-4B). Nearly all other epithelia have a lumen-negative Vte because the apical membrane voltage is less negative than the basolateral membrane voltage (see Fig. 5-20B). The TAL is just the opposite. Its lumen-positive Vte develops because of a substantial difference in the ion permeabilities of the apical and basolateral membranes. The apical membrane is K+ selective, so that the apical membrane potential depends mainly on the cell-to-lumen [K+] gradient. In contrast, the TAL basolateral membrane is permeable to both K+ and Cl−. Hence, the basolateral membrane potential lies between the equilibrium potentials of Cl− (approximately −50 mV) and K+ (approximately −90 mV), so that it is less negative than if the basolateral membrane were permeable only to K+. Because the apical membrane potential is more negative than the basolateral membrane potential, the Vte is lumen positive. N37-9 Because the TAL has a low water permeability, removing luminal NaCl leaves the remaining tubule fluid hypo-osmotic. Hence, the TAL is sometimes referred to as the diluting segment.
The lumen-positive Vte provides the driving force for the diffusion of Na+ across the tight junctions, accounting for approximately half of the Na+ reabsorption by the TAL. The lumen-positive Vte also drives the passive reabsorption of K+ (see p. 798), Ca2+ (see p. 787), and Mg2+ (see p. 791) via the paracellular pathway.
Distal Convoluted Tubule
Na+ reabsorption in the DCT occurs almost exclusively by the transcellular route (see Fig. 35-4C). The apical step of Na+ uptake is mediated by an electroneutral Na/Cl cotransporter (NCC; see p. 123) that belongs to the same family as NKCC2 in the TAL. However, the NCC differs from NKCC2 in being independent of K+ and highly sensitive to thiazide diuretics. Although the thiazides produce less diuresis than do the loop diuretics, the thiazides are nevertheless effective in removing excess Na+ from the body. The basolateral step of Na+ reabsorption, as in other cells, is mediated by the Na-K pump. Because the DCT, like the TAL, has a low water permeability, removing luminal NaCl leaves the remaining tubule fluid even more hypo-osmotic. Hence, the DCT is also part of the “diluting segment.”
Initial and Cortical Collecting Tubules
Na+ reabsorption in the connecting tubule, initial collecting tubule (ICT), and CCT is transcellular and mediated by the majority cell type, the principal cell (see Fig. 35-4D). The neighboring β-intercalated cells are important for reabsorbing Cl−, as discussed below. Na+ crosses the apical membrane of the principal cell via the epithelial Na+ channel (ENaC; see Table 6-2, family No. 14), which is distinct from the voltage-gated Na+ channels expressed by excitable tissues (see p. 187). ENaC is a trimer comprising homologous α, β, and γ subunits, each of which has two membrane-spanning segments. This channel is unique in that low levels of the diuretic drug amiloride specifically block it. This compound is a relatively mild diuretic because Na+ reabsorption along the collecting duct is modest. N23-14 The basolateral step of Na+reabsorption is mediated by the Na-K pump, which also provides the electrochemical driving force for the apical entry of Na+.
The unique transport properties of the apical and basolateral membranes of the principal cells are also the basis for the lumen-negative Vte of approximately −40 mV in the CCT (see Table 35-1). In addition to ENaCs, the CCT has both apical and basolateral K+ channels, which play a key role in K+ transport (see p. 799). The apical entry of Na+ (which tends to make the lumen negative) and the basolateral exit of K+(which tends to make the cell negative) are, in effect, two batteries of identical sign, arranged in series. In principle, these two batteries could add up to a Vte of ~100 mV (lumen negative). However, under most conditions, K+ exit from cell to lumen partially opposes the lumen-negative potential generated by Na+ entry. The net effect of these three batteries is a Vte of approximately −40 mV (lumen negative).
The Vte of the CCT can fluctuate considerably, particularly because of changes in the apical Na+ battery owing to, for example, changes in luminal [Na+]. In addition, changing levels of aldosterone or AVP may modulate the number of ENaCs that are open in the apical membrane and thus may affect the relative contribution of this Na+ battery to apical membrane voltage.
Medullary Collecting Duct
The inner and outer medullary collecting ducts reabsorb only a minute amount of Na+, ~3% of the filtered load (see Fig. 35-2). It is likely that ENaC mediates the apical entry of Na+ in these segments and that the Na-K pump extrudes Na+ from the cell across the basolateral membrane (see Fig. 35-4D).
Cl− reabsorption involves both paracellular and transcellular pathways
Most of the filtered Na+ is reabsorbed with Cl−. However, the segmental handling of Cl− differs somewhat from that of Na+. Both transcellular and paracellular pathways participate in Cl− reabsorption.
The proximal tubule reabsorbs Cl− by both the transcellular and the paracellular routes, with the paracellular believed to be the dominant one in the early proximal tubule (Fig. 35-5A). The transcellular pathway is more appreciable in the late proximal tubule (see Fig. 35-5B), where the energetically uphill influx of Cl− across the apical membrane occurs via an exchange of luminal Cl− for cellular anions (e.g., formate, oxalate, and OH−), mediated at least in part by the anion exchanger SLC26A6. Cl-base exchange is an example of tertiary active transport: the apical NHE3, itself a secondary active transporter (see p. 115), provides the H+ that neutralizes base in the lumen, thereby sustaining the gradient for Cl-anion exchange. The basolateral exit step for transcellular Cl− movement may occur in part via a Cl− channel that is analogous in function to the cystic fibrosis transmembrane conductance regulator (CFTR; see p. 120). In addition, the basolateral membrane of the proximal tubule expresses K/Cl cotransporters (KCC1, KCC3, KCC4), which are in the same family as the NKCCs and NCCs.
FIGURE 35-5 Cell models of Cl− transport. In B, “Base” may include formate, oxalate, , and OH−. “HBase” represents the conjugate weak acid (e.g., formic acid). CA, carbonic anhydrase.
Passive Cl− reabsorption via the paracellular pathway is driven by different electrochemical Cl− gradients in the early versus the late proximal tubule. The S1 segment initially has no Cl− concentration gradient between lumen and blood. However, the lumen-negative Vte (see Table 35-1)—generated by electrogenic Na/glucose and Na/amino-acid cotransport—establishes a favorable electrical gradient for passive Cl−reabsorption. Solvent drag also makes a contribution in the S1 segment. Preferential reabsorption in the S2 and S3 portions of the proximal tubule leaves Cl− behind (see p. 825), so that [Cl−] in the lumen becomes higher than that in the blood (Fig. 35-6). This favorable lumen-to-blood chemical gradient of Cl− provides a driving force for its passive paracellular reabsorption in the late proximal tubule, and generates a lumen-positive Vte (see Table 35-1) that drives a component of passive paracellular Na+ reabsorption.
FIGURE 35-6 Changes in solute composition along the proximal tubule. In the upper panel, TF/P is the ratio of concentration or osmolality in tubule fluid to that in blood plasma. Because we assume that the proximal tubule reabsorbs half of the filtered water, TFIn/PIn rises from 1 to 2. The lower panel shows the transition of Vte from a negative to a positive value.
The upper part of Figure 35-6 shows the profile of the ratio of concentration of various solutes X in tubule fluid versus plasma (TFX/PX; see p. 733) for the major solutes in tubule fluid along the proximal tubule. Only minor changes occur in the TF/P for osmolality (TFOsm/POsm) or TFNa/PNa. Because the tubule does not reabsorb inulin, the substantial increase in TFIn/PIn indicates net fluid reabsorption. The fall in mirrors an increase in TFCl/PCl because the tubule reabsorbs more rapidly than it does Cl−. The early proximal tubule avidly reabsorbs glucose and amino acids, which leads to sharp decreases in the concentrations of these substances in the tubule fluid.
An important characteristic of the proximal tubule is that the Vte reverses polarity between the S1 and the S2 segments (see Fig. 35-6, lower panel). The early proximal tubule is lumen negative because it reabsorbs Na+ electrogenically, via both electrogenic apical Na+ transporters (e.g., Na/glucose cotransporter) and the basolateral Na-K pump. The late proximal tubule reabsorbs Na+ at a lower rate. Moreover, because TFCl is greater than PCl, the paracellular diffusion of Cl− from lumen to blood generates a lumen-positive Vte that facilitates passive Na+ reabsorption via the same paracellular route as discussed above.
Thick Ascending Limb
Cl− reabsorption in the TAL takes place largely by Na/K/Cl cotransport across the apical membrane (see Fig. 35-5C), as we already noted in our discussion of Na+ reabsorption (see Fig. 35-4B). The exit of Cl−across the basolateral cell membrane via Cl− channels of the ClC family (see Table 6-2, family No. 16) overwhelms any entry of Cl− via the Cl-HCO3 exchanger. You may recall that only half of the Na+ reabsorption by the TAL is transcellular, whereas all Cl− reabsorption is transcellular. Overall, Na+ and Cl− reabsorption are identical because the apical NKCC2 cotransporter moves two Cl− for each Na+.
Distal Convoluted Tubule
Cl− reabsorption by the DCT (see Fig. 35-5D) occurs by a mechanism that is somewhat similar to that in the TAL, except the apical step occurs via NCC, as discussed above in connection with Na+ reabsorption by the DCT (see Fig. 35-4C). Cl− channels that are probably similar to those in the TAL mediate the basolateral Cl− exit step.
The ICT and the CCT reabsorb Cl− by two mechanisms. First, the principal cell generates a Vte (~40 mV, lumen negative) that is favorable for paracellular diffusion of Cl− (see Fig. 35-5E). Second, the β-type intercalated cells reabsorb Cl− via a transcellular process in which pendrin (SLC26A4; see pp. 124–125) mediates Cl− uptake across the apical membrane in exchange for , and Cl− exits via channels in the basolateral membrane (see Fig. 35-5F). Moreover, two cycles of Cl-HCO3 exchange via pendrin may operate in parallel with one cycle of Na-dependent Cl-HCO3 exchange via NDCBE (see p. 124) to produce net electroneutral NaCl entry across the apical membrane of β-intercalated cells (see Fig. 35-5F). Neither the α-type intercalated cells nor the principal cells participates in transcellular Cl− reabsorption.
Water reabsorption is passive and secondary to solute transport
If water reabsorption by the proximal tubule were to follow solute reabsorption passively, then one would expect the osmolality inside the peritubular capillaries to be greater than that in the luminal fluid. Indeed, investigators have found that the lumen is slightly hypo-osmotic.
If, conversely, proximal-tubule water reabsorption were active, one might expect it to be independent of Na+ reabsorption. Actually, the opposite was found by Windhager and colleagues in 1959. Using the stationary microperfusion technique, N33-5 these investigators introduced solutions having various [Na+] values into the proximal-tubule lumen, and they kept luminal osmolality constant by adding mannitol (which is poorly reabsorbed). Because these experiments were performed on amphibian proximal tubules, the maximal [Na+] was only 100 mM, the same as that in blood plasma. After a known time interval, they measured the volume of fluid remaining in the tubule lumen and calculated the rate of fluid reabsorption (JV). They also measured the [Na+] of the luminal fluid and calculated the rate of Na+reabsorption (JNa). These experiments showed that both JV and JNa are directly proportional to the initial luminal [Na+] (Fig. 35-7). Moreover, the ratio JNa/JV is constant and equal to the osmolality of the lumen, which indicates that Na+ reabsorption and secretion are approximately isosmotic. N35-1
FIGURE 35-7 Isosmotic water reabsorption in the proximal tubule. JV and JNa are the rates of fluid (or volume) and Na+ reabsorption, respectively, in stationary microperfusion experiments.
Windhager and colleagues also found that at a luminal [Na+] of 100 mM, the JNa and JV were large and in the reabsorptive direction. When luminal [Na+] was only 65 mM, both JNa and JV were zero. At lower luminal [Na+] values, both JNa and JV reversed (i.e., the tubule secreted both Na+ and water). At these low luminal [Na+] values, active transcellular movement of Na+ from lumen to blood cannot overcome the increasing paracellular backleak of Na+, which results in net secretion of an isosmotic NaCl solution. This experiment shows that Na+ can move uphill from lumen to blood as long as the opposing Na+ gradient is not too steep, and also suggests that the movement of water is not active but passively follows the reabsorption of Na+.
If water movement is passive, why is the difference in osmolality so small between the proximal-tubule lumen and blood? The answer is that, because the water permeability of the proximal-tubule epithelium to water is so high, the osmolality gradient needed to generate the observed passive reabsorption of water is only 2 to 3 mOsm. A somewhat greater osmolality gradient probably exists between the lumen and an inaccessible basolateral compartment comprising the lateral intercellular space and the microscopic unstirred layer that surrounds the highly folded basolateral membrane of the proximal-tubule cell.
The pathway for water movement across the proximal-tubule epithelium appears to be a combination of transcellular and paracellular transit, with the transcellular route dominating. The reason for the high rate of water movement through the proximal-tubule cell is the presence of a high density of aquaporin 1 (AQP1) water channels in both the apical and basolateral membranes. Indeed, in the AQP1-null mouse, the rates of solute and fluid reabsorption are reduced, and the fluid that the proximal tubule reabsorbs becomes hyperosmotic compared to normal conditions, resulting in marked luminal hypo-osmolality.
Loop of Henle and Distal Nephron
Two features distinguish water and Na+ transport in the distal nephron. First, the TAL and all downstream segments have a relatively low water permeability in the absence of AVP (or antidiuretic hormone). We discuss the upregulation of this water permeability by AVP on pages 817–818. Second, the combination of NaCl reabsorption and low water permeability allows these nephron segments to generate a low luminal [Na+] and osmolality with respect to the surrounding interstitial fluid. Given this large osmotic gradient across the epithelium, the distal nephron is poised to reabsorb water passively from a hypo-osmotic luminal fluid into the isosmotic blood when AVP increases the water permeability (Box 35-1).
Normally, luminal [Na+] does not change along the proximal tubule. The only exception is osmotic diuresis, a state in which poorly permeable substances are present in the plasma and, therefore, in the glomerular filtrate. Examples are the infusion of sucrose and mannitol. Another is untreated diabetes mellitus (see Box 51-5), when the blood glucose level exceeds the capacity of renal tubules to reabsorb the highly elevated glucose load. Glucose then acts as a poorly reabsorbed substance and as an osmotic diuretic. Because the proximal tubule must reabsorb isosmotic Na+ salts from a luminal mixture of Na+ salts and poorly reabsorbable solutes (e.g., mannitol), luminal [Na+] progressively falls and luminal [mannitol] rises, but luminal osmolality does not change (Fig. 35-8). Because the net rate of Na+ and fluid reabsorption falls as the luminal [Na+] falls (see Fig. 35-7), the proximal tubule—with rising luminal [mannitol]—reabsorbs progressively less Na+ and fluid as the fluid travels along the tubule. This decrease in Na+ and water reabsorption leaves a larger volume of tubule fluid behind, thereby producing osmotic diuresis.
FIGURE 35-8 Osmotic diuresis and luminal [Na+] along the proximal tubule. In this example, blood and glomerular filtrate both contain 40 mM mannitol and have an osmolality of 300 mOsm. The isosmotic reabsorption of NaCl (but not mannitol) from proximal-tubule lumen to blood causes luminal [mannitol] to rise and [NaCl] to fall, but causes no change in luminal osmolality. Once luminal [Na+] falls sufficiently, the Na+ backleak from peritubular capillaries balances active Na+ reabsorption, and net reabsorption of NaCl and water is zero. The absence of fluid absorption after this point causes osmotic diuresis. Viewed another way, the rising luminal [mannitol], with the osmotically obligated water the mannitol holds in the lumen, produces the diuresis. TFIn/PIn, the ratio of inulin concentrations in the tubule fluid to that in plasma.
Osmotic diuresis is of clinical use in several settings. For example, mannitol osmotically draws water out of brain tissue into the vascular system, for ultimate excretion by the kidneys. For this reason, osmotic diuresis is sometimes used in treating patients with acutely increased intracranial pressure from cerebral edema (e.g., from an embolic stroke), an expanding tumor or abscess, hematoma, or hemorrhage. In patients with severe hyperglycemia due to poorly controlled diabetes mellitus, pronounced osmotic diuresis can lead to a potentially life-threatening depletion of ECF volume.
It is common to distinguish between two types of diuresis, solute and water diuresis. The foregoing example of solute or osmotic diuresis is characterized by excretion of a larger-than-normal volume of urine that is rich in solutes. In contrast, water diuresis leads to excretion of larger-than-normal volumes of urine that is poor in solutes.
The kidney's high O2 consumption reflects a high level of active Na+ transport
Because virtually all Na+ transport ultimately depends on the activity of the ATP-driven Na-K pump and, therefore, on the generation of ATP by oxidative metabolism, it is not surprising that renal O2consumption is large and parallels Na+ reabsorption. Despite their low weight (<0.5% of body weight), the kidneys are responsible for 7% to 10% of total O2 consumption. Although it would seem that the high O2consumption would necessitate a large arteriovenous difference in , the renal blood flow is so large that the arteriovenous difference is actually much smaller than that in cardiac muscle or brain.
If one varies Na+ reabsorption experimentally and measures renal O2 consumption, the result is a straight-line relationship (Fig. 35-9). However, the kidneys continue to consume a modest but significant amount of O2 even in the complete absence of net Na+ reabsorption. This transport-independent component reflects the basic metabolic needs for the maintenance of cell viability.
FIGURE 35-9 Dependence of O2 consumption on Na+ transport. Na+ reabsorption (JNa) was varied by changing GFR, administering diuretics, or imposing hypoxia. O2 consumption was computed from the arteriovenous difference.