Medical Physiology, 3rd Edition

Regulation of the Microcirculation

The active contraction of vascular smooth muscle regulates precapillary resistance, which controls capillary blood flow

Smooth-muscle tone in arterioles, metarterioles, and precapillary sphincters (see pp. 459–460) determines the access resistance to the capillary beds. This resistance upstream of the capillary bed is also known as the afferent or precapillary resistance (Rpre). The overall resistance of a microcirculatory bed is the sum of Rpre, the resistance of the capillary bed itself (Rcap), and the efferent or postcapillary resistance (Rpost).

How do these resistances influence the flow of blood (Fcap) through a capillary bed? We can answer this question by rearranging the Ohm's law–like expression that we introduced as Equation 17-1:

image

(20-12)

Pa is the pressure just before the beginning of the precapillary resistance, and Pv is the pressure just after the end of the postcapillary resistance. Because the aggregate Rcap is small, and Rpost/Rpre is usually ~0.3, Rpre is usually much greater than Rcap + Rpost. Because Rpre is the principal determinant of total resistance, capillary flow is roughly inversely proportional to Rpre. Thus, modulating the contractility of VSMCs in precapillary vessels is the main mechanism for adjusting perfusion of a particular tissue.

Smooth-muscle cells can function as a syncytium when they are coupled through gap junctions (unitary smooth muscle), or they can function independently of one another as do skeletal muscle fibers (multiunit smooth muscle; see p. 243). Most vascular smooth muscle has a multiunit organization. In contrast to skeletal muscle, VSMCs receive multiple excitatory as well as inhibitory inputs (see p. 251). Moreover, these inputs come not only from chemical synapses (i.e., neural control) but also from circulating chemicals (i.e., humoral control). The actual contraction of VSMCs may follow smooth-muscle electrical activity in the form of action potentials, slow waves of depolarization, or graded depolarizations without spikes. VSMCs can show spontaneous rhythmic variations in tension leading to periodic changes in vascular resistance and microcirculatory flow in a process called vasomotion. These spontaneous, rhythmic smooth-muscle contractions result either from pacemaker currents or from slow waves of depolarization and associated [Ca2+]i increases in the VSMCs (see p. 244). Humoral agents can also directly trigger contraction of VSMCs via increases in [Ca2+]i without measurable fluctuations in membrane potential (pharmacomechanical coupling; see p. 247).

VSMCs rely on a different molecular mechanism of contraction than skeletal muscle does, although an increase in [Ca2+]i is the principal trigger of contraction in both cases. Whereas an increase in [Ca2+]i in skeletal muscle elicits contraction by interacting with troponin C, an increase in [Ca2+]i in VSMCs elicits contraction by activating calmodulin (see p. 60). The Ca2+-calmodulin (Ca2+-CaM) complex activates myosin light-chain kinase (MLCK; see p. 247), which in turn phosphorylates the regulatory myosin light chain (MLC) on each myosin head (see p. 247). Phosphorylation of MLC allows the myosin to interact with actin, producing contraction. Relaxation occurs when MLC phosphatase dephosphorylates the MLC. In addition to changes in [Ca2+]i, changes in the activity of MLCK itself can modulate the contraction of VSMCs. Phosphorylation of MLCK by cAMP-dependent protein kinase (protein kinase A, or PKA) or cGMP-dependent protein kinase (protein kinase G, or PKG) inactivates the enzyme and thus prevents contraction. Thus, intracellular Ca2+, cAMP, and cGMP are the principal second messengers responsible for modulating vascular tone.

Contraction of Vascular Smooth Muscle

The following changes promote contraction:

• ↑[Ca2+]i → ↑Ca2+-CaM → ↑MLCK activity → ↑phosphorylation of MLC → VSMC contraction.

• ↓[cAMP]i → ↓PKA → ↓phosphorylation of MLCK → ↑MLCK activity → ↑phosphorylation of MLC → VSMC contraction.

• ↓[cGMP]i → ↓PKG → ↓phosphorylation of MLCK → ↑MLCK activity → ↑phosphorylation of MLC → VSMC contraction.

Relaxation of Vascular Smooth Muscle

The following changes promote relaxation:

• ↓[Ca2+]i → ↓Ca2+-CaM → ↓MLCK activity → ↓phosphorylation of MLC → VSMC relaxation.

• ↑[cAMP]i → ↑PKA → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC → VSMC relaxation.

• ↑[cGMP]i → ↑PKG → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC → VSMC relaxation.

Various membrane proteins (channels, transporters, and receptors) play a role in controlling the tone of VSMCs. Together with their associated signal-transduction pathways, these membrane proteins lead to either contraction (i.e., vasoconstriction) or relaxation (i.e., vasodilation). Table 20-7 lists neural mechanisms of VSMC regulation. Table 20-8 lists paracrine/endocrine mechanisms of VSMC regulation.

TABLE 20-7

Neural Mechanisms Underlying the Contraction and Relaxation of Vascular Smooth Muscle

Vasoconstriction

Vasoconstriction in Most Blood Vessels (e.g., skin)

Sympathetic release of NE → α1-AR on VSMC → Gαq/11 → ↑PLC → ↑[Ca2+]i → VSMC contraction

Vasoconstriction in Some Blood Vessels

Sympathetic release of NE → α2-AR on VSMC → Gαi/o → ↓AC → ↓[cAMP]i → ↓PKA → ↓MLCK phosphorylation → VSMC contraction

Vasoconstriction in Some Blood Vessels

Sympathetic release (cotransmission) of NE + other neurotransmitters (e.g., ATP and NPY) → multiple receptors: α1-AR + P2X + Y1R

Vasodilation

Vasodilation in Most Blood Vessels (e.g., muscle)

Adrenal medulla release of epinephrine → β2-AR on VSMC → Gαs → ↑AC → ↑[cAMP]i → ↑PKA → ↑MLCK phosphorylation → VSMC relaxation

Vasodilation in Blood Vessels of Erectile Tissue

Parasympathetic co-release of multiple neurotransmitters

a) ACh → presynaptic M2 muscarinic receptor on noradrenergic neurons → Gαi → ↓AC → ↓[cAMP]i in neuron → reduced adrenergic activity

b) ACh → M3 muscarinic receptor on endothelial cell → Gαq → PLC → ↑Ca2+ → ↑NOS → ↑[NO] → NO diffusion to VSMC → NO receptor inside VSMC → ↑sGC → ↑[cGMP]i → ↑PKG → ↑MLCK phosphorylation → VSMC relaxation

c) NO co-release → ↑sGC inside VSMC → ↑[cGMP]i

d) VIP co-release → VIPR1 and VIPR2 on VSMC → Gαs → ↑AC → ↑[cAMP]i → ↑PKA → ↑MLCK phosphorylation → relaxation

Vasodilation in Blood Vessels of Salivary Gland

Parasympathetic release of ACh → M3 receptor on gland cell → Gαq/11 → ↑[kallikrein] → ↑[bradykinin] → B2 receptor on endothelial cell → NO release

Vasodilation in Blood Vessels of Sweat Gland

Sympathetic cholinergic release of ACh → M3 receptor on gland cell → Gαq/11 → ↑[kallikrein] → ↑[bradykinin] → B2 receptor on endothelial cell → NO release

Vasodilation in Blood Vessels of Muscle (Anticipatory Response)

Sympathetic cholinergic release (cotransmission) of ACh + other neurotransmitters

a) ACh → presynaptic M2 muscarinic receptor on noradrenergic neurons → Gαi → ↓AC → ↓[cAMP]i in neuron → ↓NE release by neuron → ↓vasoconstriction

b) mNO, NPY, VIP, CGRP → receptors on VSMC

AC, adenylyl cyclase; ACh, acetylcholine; AR, adrenoceptor; CGRP, calcitonin gene–related peptide; NE, norepinephrine; NPY, neuropeptide Y; P2X, purinergic ligand-gated cation channel; PLC, phospholipase C; sGC, soluble guanylyl cyclase; VIP, vasoactive intestinal peptide; VIPR, vasoactive intestinal peptide receptor; Y1R, neuropeptide Y receptor 1.

TABLE 20-8

Endocrine/Paracrine Mechanisms Underlying the Contraction and Relaxation of Vascular Smooth Muscle

Vasoconstriction

Angiotensin II (ANG II)

→ AT1 receptor → ↑Gαq/11 → ↑PLC → ↑[IP3]i → IP3 receptor in SR → ↑Ca2+ release → ↑[Ca2+]i

Arginine Vasopressin (AVP) = Antidiuretic Hormone (ADH)

→ V1A receptor → ↑Gαq/11 → ↑PLC → ↑[IP3]i → IP3 receptor in SR → ↑Ca2+ release → ↑[Ca2+]

Serotonin = 5-hydroxytryptamine (5-HT)

→ 5-HT2A or 5-HT2B receptor → ↑Gαq/11 → ↑PLC → ↑[IP3]i → IP3 receptor in SR → ↑Ca2+ release → ↑[Ca2+]i

Neuropeptide Y (NPY)

→ Y1R receptor → ↑Gαi/o → ↓AC → ↓[cAMP]i → ↓PKA → ↓phosphorylation of MLCK → ↑MLCK activity → ↑phosphorylation of MLC

Endothelin (ET)

→ ET receptor ETA on VSMC → ↑Gαq/11 → ↑PLC → ↑[IP3]i → IP3 receptor in SR → ↑Ca2+ release → ↑[Ca2+]i

Thromboxane A2 (TXA2)

→ TP receptor

a) → Ca2+ channels open → ↑Ca2+ entry → ↑[Ca2+]i

b) → ↑superoxide ion image → ↓NO

[ATP]o

→ P2X receptor (ligand-gated Ca2+ channel = receptor-operated Ca2+ channel = ROC) → ↑Ca2+ entry → ↑[Ca2+]i

[Adenosine]o

→ A1 receptor on VSMC → ↑Gαi/o → ↓AC → ↓[cAMP]i → ↓PKA

→ A3 receptor on VSMC → ↑Gαq/11 → ↑PLC → ↑[IP3]i → IP3 receptor in SR → ↑Ca2+ release → ↑[Ca2+]i

Vasodilation

Histamine

→ H2 receptor → ↑Gαs → ↑AC → ↑[cAMP]i → ↑PKA → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC

Vasoactive Intestinal Peptide (VIP)

→ VIPR1 and VIPR2 receptors → ↑Gαs → ↑AC → ↑[cAMP]i → ↑PKA

a) → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC

b) → Ca2+-dependent and voltage-gated K+ channels open → hyperpolarization → Ca2+ channels close → ↓[Ca2+]i

Atrial Natriuretic Peptide (ANP)

→ NPR1 receptor → ↑guanylyl cyclase → ↑[cGMP] → ↑PKG

a) → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC

b) → ↑SERCA2 in SR → ↓[Ca2+]i

NO Released by Endothelial Cells

→ enters VSMC → ↑soluble guanylyl cyclase → ↑[cGMP] → ↑PKG

a) → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC

b) → ↑SERCA2 in SR → ↓[Ca2+]i

Prostacyclin (PGI2) and Prostaglandin E2 (PGE2)

PGI2 → IP receptor

PGE2 → EP2 or EP4 receptor }

→ Gαs → ↑AC → ↑[cAMP]i → ↑PKA → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC

Bradykinin

→ B2R receptor on endothelial cells → ↑Gαq/11

a) → ↑PLC → ↑[IP3]i → ↑[Ca2+]i → ↑eNOS → ↑NO release

b) → ↑PLA2 → ↑PGI2 and PGE2 release

[ATP]o

→ P2Y metabotropic receptor → ↑Gαq/11 → ↑PLC → ↑[IP3]i → ↑[Ca2+]i → ↑eNOS → ↑NO release

[Adenosine]o

→ A2A and A2B receptors on VSMC → ↑Gαs → ↑AC → ↑[cAMP]i → ↑PKA

a) → KATP channels open → hyperpolarization → Ca2+ channels close → ↓[Ca2+]i

b) → ↑phosphorylation of MLCK → ↓MLCK activity → ↓phosphorylation of MLC


→ A1 receptor on endothelial cell → ↑eNOS → ↑NO release

AC, adenylyl cyclase; IP3, inositol trisphosphate; KATP channel, ATP-sensitive K+ channel; P2Y, purinergic G protein–coupled receptor; PLA2, phospholipase A2; PLC, phospholipase C; SERCA2, sarcoplasmic and endoplasmic reticulum Ca ATPase 2; SR, sarcoplasmic reticulum.

Tissue metabolites regulate local blood flow in specific vascular beds, independently of the systemic regulation

VSMCs not only control the resistance of arterioles (i.e., Rpre) and thus local blood flow, they also control the resistance of small terminal arteries and thereby play an important role in regulating systemic arterial blood pressure. In Chapter 23, we discuss this control of arterial blood pressure (a whole-body function) via VSMCs of small arteries and arterioles, both of which are under the control of central mechanisms—the autonomic nervous system and systemic humoral agents (e.g., angiotensin II). However, the subject of the present discussion is local regulatory mechanisms that use the arterioles to regulate blood flow through specific vascular beds. These local control mechanisms can override any of the neural or systemic humoral influences.

Mechanisms of local control involve (1) myogenic activity, and (2) local chemical and humoral factors. Myogenic regulation refers to an intrinsic mode of control of activity in which stretch of the VSMC membrane activates stretch-sensitive nonselective cation channels. The result is a depolarization that affects pacemaker activity, thereby eliciting contraction of the VSMC.

The most prominent chemical factors are interstitial imageimage, and pH as well as local concentrations of K+, lactic acid, ATP, ADP, and adenosine (Table 20-9). Total osmolality may also make a contribution. The local regulation of VSMCs by interstitial imageimage, and pH is distinct from the regulation of systemic blood pressure by the peripheral chemoreceptors, which respond to changes in arterial imageimage, and pH (see pp. 710–713) and initiate a complex neural reflex that modulates VSMC activity (see p. 544). In the case of local control, chemical changes in interstitial fluid act directly on the VSMCs through one of the three aforementioned principal second-messenger systems (i.e., intracellular Ca2+, cAMP, cGMP). Changes that typically accompany increased metabolism (e.g., low image, high image, and low pH) vasodilate vessels in the systemic circulation. Such local changes in imageimage, and pH have opposite effects in the pulmonary circulation (see p. 687).

TABLE 20-9

Local Metabolic Changes That Cause Vasodilation in the Systemic Circulation

CHANGE

MECHANISM

↓ image

↓ [ATP]i, ↑ adenosine release, ↑ PGI2 release, ↑ NO release

↑ image

↓ pHo

↓ pH

↓ pHo

↑ [K+]oimageN20-17

Transient hyperpolarization → closes voltage-gated Ca2+ channels

↑ [Lactic acid]o

Probably ↓ pHo

↓ [ATP]i

Opens KATP channels

↑ [ATP]o

Activates purinergic receptors P2Y

↑ [ADP]o

Activates purinergic receptors P2Y

↑ [Adenosine]o

Activates adenosine receptor A2

Note: The subscript i refers to intracellular levels, and the subscript o refers to interstitial levels.

KATP channel, ATP-sensitive K+ channel.

N20-17

Vasodilation Caused by Increases in [K+]o

Contributed by Emile Boulpaep

Why does a transient increase in [K+]o cause a transient paradoxical hyperpolarization, rather than the depolarization that one might expect from the Nernst equation (see Equation 6-5)?

First of all, the effect is transient because the increase in [K+]o is short-lived—the ensuing vasodilation will wash away the excess extracellular K+.

Second, the rise in [K+]o causes Vm (membrane potential) to become more negative (a hyperpolarization) even though EK (the equilibrium potential for K+) becomes more positive (see Equation 6-5). The reason is that the K+ conductance of VSMCs depends largely on inwardly rectifying K+ channels (Kir channels; see Fig. 7-20). A peculiar property of Kir channels is that an increase in [K+]o not only causes EK to shift to more positive values, but also increases the slope conductance (i.e., the slope of the current voltage relationship at EK). VSMCs normally do not live at EK, but at more positive voltages (−30 to −40 mV), reflecting the contributions from other conductances (e.g., Na+) with more positive equilibrium potentials. In the text, we introduced Equation 6-12 (shown here as Equation NE 20-19):

image

(NE 20-19)

Here, GKGNaGCaGCl, etc. represent membrane conductances for each ion, whereas Gm represents the total membrane conductance. Thus, GK/Gm represents the fractional conductance for K+. Therefore, the equation tells us that Vm not only depends on the various equilibrium potentials, but also on their respective fractional conductances. Thus, if an increase in [K+]o simultaneously causes (1) a slight decrease in the absolute value of EK and (2) a larger increase GK, the absolute value of the product (GK/Gm)EK will be larger. Because (GK/Gm)EK is a negative number, the net effect is that the computed value of Vm is more negative (i.e., a hyperpolarization).

In principle, a second phenomenon can contribute to the hyperpolarization. The increase in [K+]o will enhance the activity of the electrogenic Na-K pump, resulting an increase in the pump's outward current, and therefore a hyperpolarization.

Because blood flow itself can wash out the metabolic intermediates listed above, vasomotion (see p. 475) can arise from a local feedback system. For example, if interstitial image falls as a result of increased local O2 consumption, the ensuing vasodilation will increase O2 delivery to the metabolizing cells and in turn will tend to cause the local interstitial image to increase. As the image now increases, vascular tone will increase. The timing of release and washout of the chemical factors determines the frequency of the vasomotion. The interstitial fluid volume around the active cells—the volume in which vasoactive metabolites are distributed—also affects this periodicity because it affects the time lag for the concentration of vasoactive substances to rise or to fall. Finally, spontaneous fluctuations in metabolism may confer an additional periodicity to vasomotion.

The endothelium of capillary beds is the source of several vasoactive compounds, including nitric oxide, endothelium-derived hyperpolarizing factor, and endothelin

Table 20-10 lists several endothelial factors that act on blood vessels.

TABLE 20-10

Vasoactive Agents Produced by Endothelial Cells

VASODILATORS

VASOCONSTRICTORS

Nitric oxide (NO)

Endothelin (ET)

Endothelium-derived hyperpolarizing factor (EDHF)

Endothelium-derived constricting factor 1 (EDCF1)

Prostacyclin (PGI2)

Endothelium-derived constricting factor 2 (EDCF2)

Nitric Oxide

Originally called endothelium-derived relaxing factor (EDRF), nitric oxide (NO) is a potent vasodilator. NO also inhibits platelet aggregation, induces platelet disaggregation, and inhibits platelet adhesion. Bradykinin and acetylcholine both stimulate the NO synthase III (NOS III, or eNOS) isoform of NOS (see p. 66) that is constitutively present in endothelial cells. Increases in shear stress—the force acting on the endothelial cell along the axis of blood flow—can also stimulate the enzyme. NOS III, which depends on both Ca2+ and CaM for its activity, catalyzes the formation of NO from arginine. NO, a lipophilic gas with a short half-life, exits the endothelial cells, diffuses locally, and enters VSMCs. Inside the VSMC is the “receptor” for NO, a soluble guanylyl cyclase (see pp. 66–67) that converts GTP to cyclic GMP (cGMP). cGMP-dependent protein kinase (i.e., PKG) then phosphorylates MLCK (see p. 247), SERCA Ca pumps (see p. 118), and BKCa K+ channels (see Table 6-2, family No. 2). Phosphorylation inhibits the MLCK, thus leading to a net decrease in the phosphorylation of MLC and a decrease in the interaction between myosin and actin. Phosphorylation activates SERCA, thereby decreasing [Ca2+]i. Finally, phosphorylation activates BKCa, causing hyperpolarization. By these three complementary effects, the NO released by endothelial cells relaxes VSMCs, producing vasodilation.

The NO-mediated cascade is one of the most important mechanisms for vasodilation in the circulatory system. Physicians have used exogenous organic nitrates (e.g., nitroglycerin) for decades to dilate peripheral vessels for relief of the pain of angina pectoris. These powerful vasodilators exert their activity by breaking down chemically, thereby releasing NO near VSMCs. A similar pathway of NO-mediated smooth-muscle relaxation is involved in the physiology of penile erection (see p. 1106) as demonstrated by the action of drugs for erectile dysfunction such as Viagra (sildenafil), which inhibit the phosphodiesterase 5 enzyme to decrease hydrolysis of cGMP and thus raise [cGMP]i.

Endothelium-Derived Hyperpolarizing Factor

In addition to releasing NO, endothelial cells release another relaxing factor in response to acetylcholine, endothelium-derived hyperpolarizing factor (EDHF). EDHF causes VSMC relaxation by making the membrane potential more negative.

Prostacyclin (Prostaglandin I2)

Prostacyclin synthase (see Fig. 3-11) metabolizes arachidonic acid to the vasodilator prostacyclin (prostaglandin I2, or PGI2). This agent acts by increasing [cAMP]i and promoting the phosphorylation of MLCK, which ultimately decreases the phosphorylation of MLCs. PGI2 is especially important for dilation of pulmonary vessels at birth (see p. 1162).

Endothelins

Endothelial cells produce 21-residue peptides that cause an extremely potent and long-lasting vasoconstriction in most VSMCs. Many acute and chronic pathological conditions, including hypoxia, promote the release of endothelin (ET), which exists as three isopeptides: ET-1, ET-2, and ET-3. The precursor of ET-1 is preproendothelin, which the endothelial cell converts first to proendothelin and then to the mature endothelin, which it releases. The ET receptor subtype for vasoconstriction is ETA. Other ET receptors also exist. ETB1 mediates vasodilation, ETB2 mediates vasoconstriction, and ETC has as yet no clearly defined function. ETA receptors predominate in high-pressure parts of the circulation, whereas ETB receptors predominate in low-pressure parts of the circulation.

The binding of an ET to any ET receptor subtype ultimately results in an increase in [Ca2+]i. In the vasoconstriction response, ET-1 binding to ETA receptors acts through the phospholipase C pathway to generate inositol trisphosphate, to release Ca2+ from intracellular stores, and to raise [Ca2+]i (see p. 60). In a second, delayed phase, which is not well understood, Ca2+ entering from the outside contributes to the increase in [Ca2+]i. imageN20-14 The increased [Ca2+]i activates Ca2+-CaM, stimulating MLCK to phosphorylate MLCs and culminating in contraction.

N20-14

Delayed [Ca2+]i Increase in Response to Endothelin

Contributed by Emile Boulpaep

Endothelin may mediate the second, delayed phase of [Ca2+]i increase through multiple pathways, including Ca2+ channels, nonselective cation channels, kinases, and other second-messenger systems.

Thromboxane A2

Endothelial cells and platelets metabolize arachidonic acid via the cyclooxygenase pathway to produce thromboxane A2 (TXA2; see p. 64). This agent activates TXA2/prostaglandin H2 (TP) receptors, which leads to opening of L-type Ca channels and a consequent increase in [Ca2+]i. In addition, TP activation increases the levels of superoxide anion radical image (see p. 1238) in VSMCs. In turn, image reacts with NO, thereby reducing the vasodilating effect of NO.

Other Endothelial Factors

In some systemic arteries of the dog, anoxia produces an unexpected effect: an endothelium-dependent increase in tension mediated by a putative factor, EDCF1 (endothelium-derived constricting factor 1). In some dog arteries, rapid stretch evokes a contraction that is also endothelium dependent. This putative factor, EDCF2, could be a superoxide anion because superoxide dismutase prevents the contractions.

Autoregulation stabilizes blood flow despite large fluctuations in systemic arterial pressure

As we saw in Chapter 17, the pressure-flow relationship of an idealized rigid vessel is linear (Fig. 20-14, gray line). In most real (i.e., elastic) vessels, however, increases in pressure cause a dilation that reduces resistance and leads to a steeper-than-linear flow (see Fig. 20-14, red curve). However, some vascular beds behave very differently. Despite large changes in the systemic arterial pressure—and thus large changes in the driving pressure—these special vascular beds maintain local blood flow within a narrow range. This phenomenon is called autoregulation. These vascular beds behave more or less like rigid tubes at very low and at very high perfusion pressures (see Fig. 20-14, purple curve). However, in the physiological pressure range over which autoregulation occurs, changes in perfusion pressure have little effect on flow. Instead, increases in pressure lead to increases in resistance that keep blood flow within a carefully controlled range.

image

FIGURE 20-14 Autoregulation of blood flow.

Autoregulatory behavior takes time to develop and is due to an active process. If the perfusion pressure were to increase abruptly, we would see that immediately after the pressure increase, the pressure-flow diagram would look much like the one for the rigid tube in Figure 20-14. However, the vascular arteriolar tone then slowly adjusts itself to produce the characteristic autoregulatory pressure-flow diagram. The contraction of VSMCs that underlies autoregulation is autonomous; that is, it is entirely local and independent of neural and endocrine mechanisms. Both myogenic and metabolic mechanisms play an important role in the adjustments of smooth-muscle tone during autoregulation. For example, the stretch of VSMCs that accompanies the increased perfusion pressure triggers a myogenic contraction that reduces blood flow. Also, the increase in image (or decrease in image, or increase in pH) that accompanies increased perfusion pressure triggers a metabolic vasoconstriction that reduces blood flow (see Table 20-9).

Autoregulation is useful for at least two reasons. First, with an increase in perfusion pressure, autoregulation avoids a waste of perfusion in organs in which the flow is already sufficient. Second, with a decrease in perfusion pressure, autoregulation maintains capillary flow and capillary pressure. Autoregulation is very important under these conditions for organs that are very sensitive to ischemia or hypoxia (particularly the heart, brain, and kidneys) and for organs whose job it is to filter the blood (again, the kidney).

Blood vessels proliferate in response to growth factors by a process known as angiogenesis

In adults, the anatomy of the microcirculation remains rather constant. Notable exceptions are the growth of new vessels during wound healing, inflammation, and tumor growth and in the endometrium during the menstrual cycle. Increased capillary density is important in physical training (see pp. 1220–1222) and in acclimatization to altitude (see p. 1232).

The development of new vessels is called angiogenesis. The first step is dissolution of the venular basement membrane at a specific site, followed by activation and proliferation of previously quiescent endothelial cells. The new cells, attracted by growth factors, migrate to form a tube. Eventually, the budding tubes connect with each other, allowing the flow of blood and the development of vascular smooth muscle as the new microvascular network establishes itself. Angiogenesis relies on a balance between positive and negative regulation. The body normally produces some factors that promote angiogenesis and others that inhibit it (Table 20-11).

TABLE 20-11

Agents That Affect Vascular Growth

PROMOTERS

INHIBITORS

Vascular endothelial growth factor (VEGF)

Endostatin

Fibroblast growth factors (FGFs)

Angiostatin

Angiopoietin 1 (ANGPT1)

Angiopoietin 2 (ANGPT2)

Promoters of Vessel Growth

The principal peptides that induce angiogenesis are two polypeptides: vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). Both interact with endothelium-specific receptor tyrosine kinases (see pp. 68–70). VEGF—related to platelet-derived growth factor (PDGF) and a mitogen for vascular endothelial cells—is produced by fibroblasts and, frequently, by cancer cells. Activated coagulation factor VII (FVIIa; see p. 442) promotes VEGF production.

FGF mediates many cellular responses during embryonic, fetal, and postnatal development. At least 22 different FGFs exist in humans. FGF2 (also known as basic fibroblast growth factor or bFGF) has particular angiogenic activity.

VEGF and FGF2 promote expression of NOS. The resulting NO promotes proliferation and migration of endothelial cells as well as differentiation of vascular tubes.

The difficulty in achieving targeted delivery of these growth factors is a major obstacle in their therapeutic use. One approach has been to link the growth factor to small beads delivered into the coronary circulation. Clinical trials with local or systemic administration of FGF2 to patients with ischemic heart disease have shown mixed efficacy. A recombinant, humanized monoclonal antibody against VEGF (Avastin) is being used in patients with advanced non–small-cell lung cancer.

Other growth factors have indirect angiogenic effects that are distinct from those of VEGF or FGF. Angiopoietins (ANGPT1 and ANGPT2) are proteins that act through a receptor tyrosine kinase (Tie2) expressed almost exclusively in endothelial cells. ANGPT1 is required for embryonic vascular development, and ANGPT2—normally an antagonist of ANGPT1 at the Tie2 receptor—is required for postnatal angiogenic remodeling. Angiogenin, imageN20-15 a member of the ribonuclease (RNase) family, is normally present in plasma, but at levels too low to produce proliferative effects. Plasma angiogenin levels rise in cancer patients. Regulated surface receptors on endothelial cells bind angiogenin, which after endocytosis translocates to the nucleus, where its RNase activity is essential for its angiogenic effect.

N20-15

Angiogenin

Contributed by Emile Boulpaep

Although initially identified in the media of cultured tumor cells, angiogenin is present in normal plasma and it is a mitogen for normal endothelial cells. Angiogenin—a soluble 14-kDa protein—mediates a number of functions in addition to angiogenesis. For example, angiogenin is a microbicidal agent that plays a role in innate immunity.

Angiogenin belongs to a superfamily of ribonucleases (RNases; see p. 98). RNase A is the prototype of that family, and angiogenin has been classified as RNase 5. Critical structural differences between angiogenin and the other RNases are apparent in the ribonucleolytic site and the receptor-binding site. The ribonucleolytic activity and angiogenic activity of angiogenin can be separated because the protein can be modified and as a result retain its ribonucleolytic activity but lose its angiogenic activity.

Angiogenin-responsive endothelial cells express a specific receptor on the cell membrane. After binding to the receptor, some angiogenin is rapidly endocytosed and translocated to the nucleus. Indeed, angiogenin contains a specific nuclear-localization sequence. In addition, another portion of the angiogenin bound to its receptor may trigger a number of intracellular signaling cascades. Both pathways lead to cell growth and neovascularization. Angiogenin plays a role in vascularization not only in malignancies, but also in nonmalignant pathologic conditions (e.g., in diabetic retinopathy).

Inhibitors of Vessel Growth

The concept of antiangiogenesis was first advanced by Judah Folkman as a strategy to stop the growth of tumors. He and his colleagues have described two peptides, angiostatin and endostatin, that are inhibitors of angiogenesis.

Angiostatin is a kringle-containing fragment of plasminogen, a key fibrinolytic protein (see p. 446). Angiostatin arises by proteolytic cleavage of plasminogen by connective tissue enzymes, such as matrix metalloproteinases and elastase. Angiostatin inhibits angiogenesis by enhancing apoptosis of endothelial cells and inhibiting migration and tube formation, rather than by affecting proliferation. Recombinant angiostatin is being tested in patients with advanced lung cancer.

Endostatin is a peptide breakdown product of collagen XVIII. It is produced by the extracellular matrix of tumors.

We can illustrate the importance of angiogenesis by highlighting three clinical situations in which angiogenesis plays an important role. First, enhancement of vessel growth is important during coronary artery disease, when chronic ischemia of the heart leads to the development of new vessels and thus collateral circulation. Second, angiogenesis enhances the blood supply to a tumor, thereby promoting its growth and opening the principal route by which tumor cells exit the primary tumor during metastasis. Oncologists are exploring the use of angiogenesis inhibitors to treat cancer. Third, angiogenesis may also be important in diabetic retinopathy, in which blood vessel proliferation can cause blindness.