Signaling mechanisms underlying the vascular myogenic response

Excessive microvascular adaptation to changes in blood flow in mice lacking gene encoding for desmin

OBJECTIVE

Desmin, an intermediate filament, has a key role in the integrity of myocytes, and its absence induces cardiomyopathies. Mice lacking desmin (Des-/- group) exhibit microvascular dysfunction leading to smooth muscle hyporeactivity. We investigated the effect of the absence of desmin in mice (Des-/- mice versus Des+/+ mice) on the adaptation of mesenteric arteries to changes in blood flow.

METHODS AND RESULTS

With the use of selective ligations of second-order mesenteric arteries, blood flow was either diminished (low flow [LF]) or elevated (high flow [HF]); respective LF to HF values were 136+/-18 to 206+/-29 microL/min for Des+/+ mice and 119+/-14 to 189+/-24 microL/min for Des-/-mice in daughter arteries. Two weeks after ligation, arteries were mounted in an arteriograph, allowing the measurement of diameter under controlled conditions of pressure and flow. In HF arteries, diameter changes in response to increases in pressure were higher in Des-/- mice than in Des+/+ mice. Conversely, in LF arteries, diameter was lower in Des-/- mice. Flow-dependent dilation was higher in HF arteries and lower in LF arteries than in control arteries. This adaptation was lower in Des-/- mice than in Des+/+ mice (11.6+/-3.1% versus 25.5+/-4.8% dilation, respectively). Endothelial NO synthase expression increased in HF arteries in both strains.

CONCLUSIONS

These findings provide a demonstration of the role of the intermediate filament desmin in microvascular remodeling. This dysfunction might take place in desmin-related myopathies.

Axial stretching of extremity artery induces reversible hyperpolarization of smooth muscle cell membrane in vivo

Circumferential stretch due to increases in pressure induces vascular smooth muscle cell depolarization and contraction known as the myogenic response. The aim of this study was to determine the in vivo effects of axial-longitudinal stretch of the rat saphenous artery (SA) on smooth muscle membrane potential (Em) and on external diameter. Consecutive elongations of the SA were carried out from resting length (L0) in 10% increments up to 140% L0 while changes in membrane potential and diameter were determined in intact and de-endothelized vessels. Axial stretching resulted in a small initial depolarization at 120% of L0 followed by a progressive 20 to 33% hyperpolarizaion of vascular smooth muscle between 130% and 140% of L0. At 140%, an average maximal 10.6 mV reversible hyperpolarization was measured compared to -41.2 +/- 0.49 mV Em at 100% L0. De-endothelialization completely eliminated the hyperpolarization to axial stretching and augmented the reduction of diameter beyond 120% L0. These results indicate that arteries have a mechanism to protect them from vasospasm that could otherwise occur with movements of the extremities.

17 beta-Estradiol induces a rapid, endothelium-dependent, sex-specific vasodilatation in spontaneous constricted rat arterioles

OBJECTIVE

Our purpose was to resolve the apparent contradiction between the endothelium-dependent and endothelium-independent vasodilator effects of 17 beta-estradiol reported in different studies.

STUDY DESIGN

The inner diameters of isolated pressurized spontaneously constricted muscle arterioles (diameter = 63 microm) from Wistar rats (n = 21) were measured during exposure to 17 beta-estradiol, and the role of the endothelium and the influence of sex were assessed.

RESULTS

A dose-dependent dilatation was observed during exposure to 17 beta-estradiol concentrations from 10(-10) to 10(-4) mol/L. Arterioles of female rats displayed significantly more dilatation than vessels from male rats. The dilatation was significantly less in endothelium-denuded arterioles or after pretreatment with and in the presence of a nitric oxide synthase inhibitor.

CONCLUSIONS

These results provide strong evidence that, in addition to an endothelium-independent effect, 17 beta-estradiol has a dose-dependent, endothelium-mediated, rapid vasodilatory effect on muscle arterioles from the rat, which is stronger in female rats than in male rats.

Cellular signalling in arteriolar myogenic constriction: involvement of tyrosine phosphorylation pathways

1. An increase in transmural pressure in arterioles results in a shortening of vascular smooth muscle cells, with subsequent constriction of the vessel. The mechanisms underlying this myogenic contraction are not fully understood; however, the obligatory role of increases in intracellular [Ca(2+)] and myosin light chain phosphorylation have been demonstrated. 2. The myogenic response shows a relationship with smooth muscle cell membrane potential and influx of extracellular Ca(2+) through voltage-operated Ca(2+) channels (VOCC). Mechanically sensitive channels and possibly release of Ca(2+) from intracellular stores may play a role. However, there are other components of myogenic contraction that cannot be explained by a Ca(2+)-MLCK mechanism, for example the initial sensing of alterations in transmural pressure, whether sustained myogenic constriction involves myofilament Ca(2+) sensitization or remodelling of the vessel wall in response to a maintained increase in transmural pressure. 3. In an attempt to investigate these areas, recent studies have examined a role for tyrosine phosphorylation pathways in pressure-induced contraction of arterioles. In rat pressurized cremaster arterioles, tyrosine kinase inhibitors dilated vessels showing spontaneous myogenic tone and tyrosine phosphatase inhibitors caused vasoconstriction. However, pressure-induced myogenic constriction of vessels persisted in the presence of these agents. Biochemical studies revealed that phosphotyrosine formed at a relatively slow rate (significant after 5 min, with maximal increase after approximately 15 min) in response to increased vessel transmural pressure, in contrast with myosin light chain phosphorylation or the time-course of myogenic constriction itself (maximum within 1 min). 4. Taken together, these observations support the idea of a role for tyrosine phosphorylation pathways in longer-term responses to increased transmural pressure rather than acute myogenic constriction. Phosphotyrosine formation was also more closely correlated to vessel wall tension (pressure x diameter) than the diameter of the arterioles alone. The identity of the tyrosine-phosphorylated proteins requires further investigation; however, there is some evidence supporting roles for cSrc-type tyrosine kinases and p44 mitogen-activated protein kinase. The longer-term responses of blood vessels to increased transmural pressure that may involve tyrosine phosphorylation pathways include maintenance of myogenic constriction and vessel wall remodelling.

Chronic hydralazine improves flow (shear stress)-induced endothelium-dependent dilation in mouse mesenteric resistance arteries in vitro

Flow (shear stress)-mediated dilation (FMD) plays a key role in the local control of vascular diameter and blood flow supply. Although vasodilator treatments improve FMD in diverse models of hypertension, FMD may also change in situations where systemic blood pressure is not affected. In pathological situations such as ischemia, local blood flow and vascular density are increased by vasodilators not affecting systemic blood pressure. As the mechanisms involved remain obscure, we studied FMD in resistance arteries from mice treated chronically (1 month) with hydralazine (200 mg/L in drinking water). Blood flow in mesenteric arteries of mice treated with hydralazine was significantly increased (130 +/- 15 to 169 +/- 27 microl/min, n = 10/group), whereas mean arterial blood pressure was not affected (79 +/- 5 vs 82 +/- 3 mm Hg in controls). Mesenteric resistance arteries (90 microm internal diameter, 75 mm Hg) were isolated and mounted in vitro in an arteriograph. Pressure (myogenic tone)-, phenylephrine-, and KCl-induced contractions, as well as acetylcholine- and sodium nitroprusside-induced dilations, were unaffected by hydralazine. Flow-mediated dilation in arteries from hydralazine-treated mice was significantly increased, especially for low flow values (up to sevenfold). L-NAME-sensitive and indomethacin-sensitive FMD were both increased by hydralazine. Passive arterial diameter increased and arterial wall thickness decreased after chronic hydralazine. This is the first functional evidence that flow (shear stress)-mediated dilation in resistance arteries is improved by a chronic treatment with a nonselective vasodilator. This arteriolar adaptation to a chronic increase in blood flow might be of importance in the pathophysiology of ischemic diseases.

Effects of aging on vasoconstrictor and mechanical properties of rat skeletal muscle arterioles

Exercise capacity and skeletal muscle blood flow during exercise are reduced with advancing age. This reduction in blood flow capacity may be related to increased reactivity of skeletal muscle resistance vessels to vasoconstrictor stimuli. The purpose of this study was to test the hypothesis that aging results in increased vasoconstrictor responses of skeletal muscle resistance arterioles. First-order (1A) arterioles (90-220 microm) from the gastrocnemius and soleus muscles of young (4 mo) and aged (24 mo) Fischer-344 rats were isolated, cannulated, and pressurized via hydrostatic reservoirs. Vasoconstriction in response to increases in norepinephrine (NE; 1 x 10(-9)-1 x 10(-4) M) and KCl (20-100 mM) concentrations and increases in intraluminal pressure (10-130 cmH(2)O) were evaluated in the absence of flow. Responses to NE and KCl were similar in both soleus and gastrocnemius muscle arterioles from young and aged rats. In contrast, active myogenic responses to changes in intraluminal pressure were diminished in soleus and gastrocnemius arterioles from aged rats. To assess whether alterations in the mechanical properties of resistance arterioles underlie altered myogenic responsiveness, passive diameter responses to pressure and mechanical stiffness were evaluated. There was no effect of age on the structural behavior (passive pressure-diameter relationship) or stiffness of arterioles from either the soleus or gastrocnemius muscles. These results suggest that aging does not result in a nonspecific decrease in vasoconstrictor responsiveness of skeletal muscle arterioles. Rather, aging-induced adaptations of vasoreactivity of resistance arterioles appear to be limited to mechanisms that are uniquely involved in the signaling of the myogenic response.

Chronic hypoxia increases MCA contractile response to U-46619 by reducing NO production and/or activity

Chronic hypoxia alters contractile sensitivity of isolated arteries to alpha-adrenergic stimulation and other agonists. However, most studies have been performed in thoracic aortas or other large vessels making little contribution to vascular resistance in their respective circulations. To determine the effect of chronic hypoxia on the vasoconstrictor response in a small, resistance-sized vessel, we studied second and third generation middle cerebral arteries (MCA; approximately 75-microm internal diameter before mounting). MCA were isolated from normoxic (inspired oxygen = 125 Torr) and hypoxic (8 wk at 3,960 m; inspired oxygen = 90 Torr) guinea pigs, and their vasoconstrictor responses were determined to the thromboxane mimetic U-46619 by using dual-pipette video microscopy. Arteries from hypoxic animals had greater contractile sensitivity to U-46619 compared with those of the normoxic animals (-log EC50 = 7.86 +/- 0.11 vs. 7.62 +/- 0.06, respectively, P < 0.05). Addition of the nitric oxide (NO) inhibitor nitro-L-arginine (200 microM) to the vessel bath eliminated the differences in contractile sensitivity between the MCA from the normoxic and chronically hypoxic groups. Supplementation with L-arginine in the drinking water sufficient to raise plasma L-arginine levels 41% reduced MCA contractile sensitivity to U-46619 in the normoxic group (-log EC50 = 7.22 +/- 0.31, P < 0.05 compared with the nonsupplemented normoxic group) but not in the chronically hypoxic group. These results show that chronic hypoxia increases the sensitivity of the MCA to the vasoconstrictor U-46619, likely because of a reduction in NO production and/or activity.

Smooth muscle dysfunction in resistance arteries of the staggerer mouse, a mutant of the nuclear receptor RORalpha

Retinoic acid receptor-related orphan receptor alpha (RORalpha) is a member of the nuclear receptor superfamily. The mouse mutant staggerer (sg/sg) carries a deletion within the RORalpha gene. RORalpha plays a major role in cellular differentiation during development and growth. In the present study, we found a lower mean arterial blood pressure in sg/sg than in +/+ mice (80.1+/-1.2 and 87.0+/-0.9 mm Hg, respectively; P<0.0002) and a smaller increase in blood pressure after in vivo injections of phenylephrine. To elucidate the mechanisms responsible for this phenotype, we investigated the vascular reactivity of large vessels (aorta and carotid arteries) and small resistance mesenteric arteries in response to mechanical forces or vasoactive agents. Arteries from sg/sg and +/+ mice were studied in vitro in arteriographs. Vascular responses of large vessels to all stimuli were similar in both groups. However, we found a markedly altered vascular function in mesenteric arteries from sg/sg mice. Flow-induced dilation, pressure-induced myogenic tone, responses to endothelium-dependent or -independent vasodilators, and responses to vasoconstrictors were significantly reduced in sg/sg compared with +/+ mice. We also determined by Western blot analysis the expression of smooth muscle (SM)-myosin, calponin, and heavy (h)-caldesmon, in large and small arteries of sg/sg and +/+ mice, and found a marked decrease in the expression of these contractile proteins in mesenteric arteries of sg/sg mice. Our findings provide the first evidence that functional RORalpha is required for normal contractile phenotype of smooth muscle cells (SMCs) in small resistance arteries and suggest that RORalpha might be involved in the differentiation of SMCs in mesenteric arteries.

Actin cytoskeletal modulation of pressure-induced depolarization and Ca(2+) influx in cerebral arteries

The objective of this study was to examine the role of the actin cytoskeleton in the development of pressure-induced membrane depolarization and Ca(2+) influx underlying myogenic constriction in cerebral arteries. Elevating intraluminal pressure from 10 to 60 mmHg induced membrane depolarization, increased intracellular cytosolic Ca(2+) concentration ([Ca(2+)](i)) and elicited myogenic constriction in both intact and denuded rat posterior cerebral arteries. Pretreatment with cytochalasin D (5 microM) or latrunculin A (3 microM) abolished constriction but enhanced the [Ca(2+)](i) response; similarly, acute application of cytochalasin D to vessels with tone, or in the presence of 60 mM K(+), elicited relaxation accompanied by an increase in [Ca(2+)](i). The effects of cytochalasin D were inhibited by nifedipine (3 microM), demonstrating that actin cytoskeletal disruption augments Ca(2+) influx through voltage-sensitive L-type Ca(2+) channels. Finally, pressure-induced depolarization was enhanced in the presence of cytochalasin D, further substantiating a role for the actin cytoskeleton in the modulation of ion channel function. Together, these results implicate vascular smooth muscle actin cytoskeletal dynamics in the control of cerebral artery diameter through their influence on membrane potential as well as via a direct effect on L-type Ca(2+) channels.

Newborn intestinal circulation. Physiology and pathophysiology

The physiologic characteristics of the newborn intestinal circulation are unique when compared with the adult condition. Most important, intestinal vascular resistance across newborn intestine is exceptionally low and this transient reduction is mediated by an increased constitutive and stimulated production of NO. The low vascular resistance characteristic of newborn intestine alters the capacity of this vasculature to respond to systemic circulatory perturbations, such as hypotension and arterial hypoxemia. The essential role of endothelial production of NO in maintaining newborn intestinal hemodynamics might be important in the pathogenesis of NEC, because endothelial dysfunction would limit, or possibly eliminate, NO production, leading to substantial intestinal ischemia.

Mechanotransduction through the cytoskeleton

We constructed a model cytoskeleton to investigate the proposal that this interconnected filamentous structure can act as a mechano- and signal transducer. The model cytoskeleton is composed of rigid rods representing actin filaments, which are connected with springs representing cross-linker molecules. The entire mesh is placed in viscous cytoplasm. The model eukaryotic cell is submitted to either shock wave-like or periodic mechanical perturbations at its membrane. We calculated the efficiency of this network to transmit energy to the nuclear wall as a function of cross-linker stiffness, cytoplasmic viscosity, and external stimulation frequency. We found that the cytoskeleton behaves as a tunable band filter: for given linker molecules, energy transmission peaks in a narrow range of stimulation frequencies. Most of the normal modes of the network are spread over the same frequency range. Outside this range, signals are practically unable to reach their destination. Changing the cellular ratios of linker molecules with different elastic characteristics can control the allowable frequency range and, with it, the efficiency of mechanotransduction.

Maturation alters the contribution of potassium channels to resting and 5HT-induced tone in small cerebral arteries of the sheep

To address the hypothesis that maturation alters the contribution of K-channels to resting and agonist-induced tone in small cerebral arteries, second branch middle cerebral arteries (approximately 200 microm) were taken from term fetal (139-141 days gestation) and adult sheep, denuded of endothelium, and mounted in myographs. After determination of length-tension relations, the arteries were stretched to 55, 100, and 145% of optimum length. At each level of stretch, contractile responses to 5 mM 4-aminopyridine (4-AP, voltage-sensitive K-channel blocker), 100 nM iberiotoxin (calcium-sensitive K-channel blocker), 10 microM glibenclamide (ATP-sensitive K-channel blocker), or 10 microM Ba(2+) (inward rectifier K-channel blocker) were recorded. In separate experiments, concentration--response relations were determined for 5-HT in the presence and absence of each of the four K-channel blockers at the same concentrations. Both 4-AP and iberiotoxin produced stretch-dependent contractions of greater magnitude in adult (37% for 4-AP and 43% for iberiotoxin at 100% optimum) than in fetal (5% for 4-AP and 7% for iberiotoxin at 100% optimum) arteries. 4-AP also enhanced the pD(2) for 5-HT in adult (from 7.15 to 7.49), but not in fetal, arteries. Conversely, glibenclamide attenuated the pD(2) for 5-HT in fetal (from 7.02 to 6.71), but not in adult, arteries. Iberiotoxin enhanced the pD(2) for 5-HT in both fetal (from 7.05 to 7.51) and adult (from 7.15 to 7.75) arteries. In addition, iberiotoxin enhanced maximum responses to 5-HT (from 59 to 82%) in adult but not fetal arteries. Finally, 4-AP enhanced the maximum responses to 5-HT in both fetal (from 67 to 85%) and adult (from 59 to 79%) arteries. These results indicate that maturation modulates the contribution of K(V), K(Ca), and K(ATP), but not K(IR) channels to basal and/or 5HT-induced cerebrovascular tone, and demonstrate that K(V) and K(Ca) channels are coupled to stretch-sensitive receptors, and that K(V) and K(Ca) limit contractile responses to 5-HT. To the extent that changes in pD(2) values reflect changes in agonist--ligand interactions, the data also suggest that K(V), K(Ca), and K(ATP) channels may possibly influence ligand--receptor binding for 5-HT.

Genetic determinants of vascular reactivity

Blood pressure is controlled by a complex combination of processes that influence cardiac output and peripheral vascular resistance. Multiple genes potentially influence each parameter involved in the control of blood pressure, and individuals with the same blood pressor level do not necessarily have the same genotype at relevant loci, nor do individuals with the same genotype at particular loci necessarily have the same blood pressure. Nevertheless, pharmacogenetic studies of vascular reactivity will certainly allow the analysis of the mechanisms affected by genes, and lead to a better understanding of the epidemiologic observations seen in large groups of patients. Polymorphisms in the genes of the renin-angiotensin system allow definition of the "genetic profile" associated with a higher risk of cardiovascular disease, and can also be linked to significant changes in vascular reactivity in arteries isolated from patients carrying the polymorphisms.

Abnormal vascular function and hypertension in mice deficient in estrogen receptor beta

Blood vessels express estrogen receptors, but their role in cardiovascular physiology is not well understood. We show that vascular smooth muscle cells and blood vessels from estrogen receptor beta (ERbeta)-deficient mice exhibit multiple functional abnormalities. In wild-type mouse blood vessels, estrogen attenuates vasoconstriction by an ERbeta-mediated increase in inducible nitric oxide synthase expression. In contrast, estrogen augments vasoconstriction in blood vessels from ERbeta-deficient mice. Vascular smooth muscle cells isolated from ERbeta-deficient mice show multiple abnormalities of ion channel function. Furthermore, ERbeta-deficient mice develop sustained systolic and diastolic hypertension as they age. These data support an essential role for ERbeta in the regulation of vascular function and blood pressure.

Evidence that the human cutaneous venoarteriolar response is not mediated by adrenergic mechanisms

The venoarteriolar response causes vasoconstriction to skin and muscle via local mechanisms secondary to venous congestion. The purpose of this project was to investigate whether this response occurs through alpha-adrenergic mechanisms. In supine individuals, forearm skin blood flow was monitored via laser-Doppler flowmetry over sites following local administration of terazosin (alpha(1)-antagonist), yohimbine (alpha(2)-antagonist), phentolamine (non-selective alpha-antagonist) and bretylium tosylate (inhibits neurotransmission of adrenergic nerves) via intradermal microdialysis or intradermal injection. In addition, skin blood flow was monitored over an area of forearm skin that was locally anaesthetized via application of EMLA (2.5 % lidocaine (lignocaine) and 2.5 % prilocaine) cream. Skin blood flow was also monitored over adjacent sites that received the vehicle for the specified drug. Each trial was performed on a minimum of seven subjects and on separate days. The venoarteriolar response was engaged by lowering the subject's arm from heart level such that the sites of skin blood flow measurement were 34 +/- 1 cm below the heart. The arm remained in this position for 2 min. Selective and non-selective alpha-adrenoceptor antagonism and presynaptic inhibition of adrenergic neurotransmission did not abolish the venoarteriolar response. However, local anaesthesia blocked the venoarteriolar response without altering alpha-adrenergic mediated vasoconstriction. These data suggest that the venoarteriolar response does not occur through adrenergic mechanisms as previously reported. Rather, the venoarteriolar response may due to myogenic mechanisms associated with changes in vascular pressure or is mediated by a non-adrenergic, but neurally mediated, local mechanism.

Force-velocity relationship of myogenically active arterioles

We compared the shortening velocity of smooth muscle in arterioles that had low or high levels of myogenic tone or norepinephrine (NE)-induced tone. We hypothesized that enhanced myogenic tone of arterioles reflects an enhanced maximum velocity of shortening of arteriolar smooth muscle in a way that is different from that produced by NE. These concepts are untested assumptions of arteriolar mechanics. Second-order arterioles from hamster cheek pouch (passive diameter at 40 mmHg = 42 microm) were isolated and cannulated for in vitro study. In the absence of flow, pressure was controlled by hydraulic pumps so that servo control of wall tension could be achieved from measurement of internal diameter and pressure. Isotonic quick-release protocols were used to measure the initial velocity of shortening following release from control wall tension (afterload) to a series of fractional afterloads. After release, the initial rates of shortening were fit to the Hill equation to obtain coefficients for a hyperbolic fit of the velocity-afterload relationship. The maximal unloaded shortening velocity for partially activated arterioles (V'(max)) was determined from the y-intercept of each plot. Using this procedure, we compared V'(max) from two groups of arterioles equilibrated at low or high pressure, i.e., with low or high myogenic tone. Arterioles with higher myogenic tone had higher values of V'(max) than arterioles with lower myogenic tone. V'(max) for arterioles partially activated with NE at low pressure was comparable to V'(max) for arterioles with high myogenic tone, but NE produced high velocities at low force, whereas enhanced myogenic tone produced roughly parallel shifts in velocity and force. The results suggest that increased myogenic tone does indeed reflect enhanced activation of arteriolar smooth muscle, and this effect is mechanically different from that produced by NE.

Effects of age, gender, and blood pressure on myogenic responses of mesenteric arteries from C57BL/6 mice

The myogenic response (MR) may represent an important physiological parameter underlying arterial blood pressure (BP). We studied the effects of age, gender, and BP on the MR of mesenteric arteries from 8- to 52-wk-old mice. Increasing age and BP are associated with an increase in the perfusion pressure at which tone develops (myogenic set point). An inverse correlation exists between age and extent (magnitude) of the MR in male (r(2) = 0.93, P = 0.0087) and female mice (r(2) = 0.90, P = 0.013) as well as between BP and extent of the MR in male (r(2) = 0.96, P = 0.0036) and female (r(2) = 0.90, P = 0.014) mice. In contrast, the strength of the MR (slope of active diameter-pressure relationship) and phenylephrine-mediated constriction did not differ among these groups. Although gender had no effect on MR at any perfusion pressure or age, only male mice showed significant salt-induced hypertension and an associated increase in the set point and reduction in the extent of the MR. The set point and extent of the MR is linked to the in vivo pressure during development and experimental hypertension.

Chronic hypertension impairs flow-induced vasodilation and augments the myogenic response in fetal lung

We hypothesized that altered vasoreactivity in perinatal pulmonary hypertension (PH) is characterized by abnormal responses to hemodynamic stress, including the loss of flow-induced vasodilation and an augmented myogenic response. Therefore, we studied the acute hemodynamic effects of brief compression of the ductus arteriosus (DA) in control fetal lambs and in lambs during exposure to chronic PH. In both groups, acute DA compression decreased pulmonary vascular resistance (PVR) by 20% at baseline (day 0). After 2 days of hypertension, acute DA compression paradoxically increased PVR by 50% in PH lambs, whereas PVR decreased by 25% in controls. During the 8-day study period, PVR increased during acute DA compression in PH lambs, whereas acute DA compression continued to cause vasodilation in controls. Brief treatment with the nitric oxide (NO) synthase inhibitor nitro-L-arginine (L-NA) increased basal PVR in control but not PH lambs, suggesting decreased NO production in PH lambs. Chronic hypertension increased the myogenic response after L-NA in PH lambs, whereas the myogenic response remained unchanged in controls. The myogenic response was inhibited by nifedipine in PH lambs, suggesting that the myogenic response is dependent upon the influx of extracellular calcium. We conclude that chronic PH impairs flow-induced vasodilation and increases the myogenic response in fetal lung. We speculate that decreased NO signaling and an augmented myogenic response contributes to abnormal vasoreactivity in PH.

P-450 metabolites of arachidonic acid in the control of cardiovascular function

Recent studies have indicated that arachidonic acid is primarily metabolized by cytochrome P-450 (CYP) enzymes in the brain, lung, kidney, and peripheral vasculature to 20-hydroxyeicosatetraenoic acid (20-HETE) and epoxyeicosatrienoic acids (EETs) and that these compounds play critical roles in the regulation of renal, pulmonary, and cardiac function and vascular tone. EETs are endothelium-derived vasodilators that hyperpolarize vascular smooth muscle (VSM) cells by activating K(+) channels. 20-HETE is a vasoconstrictor produced in VSM cells that reduces the open-state probability of Ca(2+)-activated K(+) channels. Inhibitors of the formation of 20-HETE block the myogenic response of renal, cerebral, and skeletal muscle arterioles in vitro and autoregulation of renal and cerebral blood flow in vivo. They also block tubuloglomerular feedback responses in vivo and the vasoconstrictor response to elevations in tissue PO(2) both in vivo and in vitro. The formation of 20-HETE in VSM is stimulated by angiotensin II and endothelin and is inhibited by nitric oxide (NO) and carbon monoxide (CO). Blockade of the formation of 20-HETE attenuates the vascular responses to angiotensin II, endothelin, norepinephrine, NO, and CO. In the kidney, EETs and 20-HETE are produced in the proximal tubule and the thick ascending loop of Henle. They regulate Na(+) transport in these nephron segments. 20-HETE also contributes to the mitogenic effects of a variety of growth factors in VSM, renal epithelial, and mesangial cells. The production of EETs and 20-HETE is altered in experimental and genetic models of hypertension, diabetes, uremia, toxemia of pregnancy, and hepatorenal syndrome. Given the importance of this pathway in the control of cardiovascular function, it is likely that CYP metabolites of arachidonic acid contribute to the changes in renal function and vascular tone associated with some of these conditions and that drugs that modify the formation and/or actions of EETs and 20-HETE may have therapeutic benefits.

Physiological and pathophysiological functions of the AT(2) subtype receptor of angiotensin II: from large arteries to the microcirculation

Angiotensin II exerts a potent role in the control of hemodynamic and renal homeostasis. Angiotensin II is also a local and biologically active mediator involved in both endothelial and smooth muscle cell function acting on 2 receptor subtypes: type 1 (AT(1)R) and type 2 (AT(2)R). Whereas the key role of AT(2)R in the development of the embryo has been extensively studied, the role of AT(2)R in the adult remains more questionable, especially in humans. In vitro studies in cultured cells and in isolated segments of aorta have shown that AT(2)R stimulation could lead to the production of vasoactive substances, among which NO is certainly the most cited, suggesting that acute AT(2)R stimulation will produce vasodilation. However, in different organs or in small arteries isolated from different type of tissues, other vasoactive substances may also mediate AT(2)R-dependent dilation. Sometimes, such as in large renal arteries, AT(2)R stimulation may lead to vasoconstriction, although it is not always seen. In isolated arteries submitted to physiological conditions of pressure and flow, AT(2)R stimulation may also have a role in shear stress-induced dilation through a endothelial production of NO. Thus, when acutely stimulated, the most probable response expected from AT(2)R stimulation will be a vasodilation. Therefore, in the perspective of a chronic AT(1)R blockade in patients, overstimulation of AT(2)R might be beneficial, given their potential vasodilator effect. Concerning the possible role of AT(2)R in cardiovascular remodeling, the situation is more controversial. In vitro AT(2)R stimulation clearly inhibits cardiac and vascular smooth muscle growth and proliferation, stimulates apoptosis, and promotes extra cellular matrix synthesis. In vivo, the situation might be less beneficial if not deleterious; indeed, if chronic AT(2)R overstimulation would lead to cardiovascular hypertrophy and fibrosis, then the long-term consequences of chronic AT(1)R blockade, and thus AT(2)R overstimulation, require more in-depth analysis.

Initial and sustained phases of myogenic response of rat mesenteric small arteries

A possible role for a metabolite of cytochrome P-450 omega-hydroxylase in the initial and sustained phases of the myogenic response in cannulated rat mesenteric small arteries was studied. With slight preconstriction (norepinephrine and neuropeptide Y), pressure was raised from 60 to 100 mmHg, and both initial (within 2 min) and sustained phases (at 10 min) of the myogenic response were quantified. The myogenic response was fully inhibited by D600 (methoxyverapamil). Ketoconazole and 17-octadecanoic acid did not affect the initial phase but inhibited the sustained phase. In contrast, miconazole did not affect either phase. Charybdotoxin and iberiotoxin potentiated the initial phase but eliminated the sustained phase. Apamin, glibenclamide, 4-aminopyridine, and barium had no effect on either phase. The results demonstrate different mechanisms for the initial and sustained phases of the myogenic response of rat mesenteric small arteries. Only the sustained phase appears mediated through a cytochrome P-450 omega-hydroxylase metabolite and calcium-activated K+ channels. However, both phases of the response are dependent on calcium influx through voltage-dependent calcium channels.

Role of protein kinase C in myogenic calcium-contraction coupling of rat cannulated mesenteric small arteries

1. The present study was designed to determine the role of protein kinase C (PKC) in the myogenic response of small arteries. In particular, we tested whether inhibition of PKC reverses the previously found pressure-induced elevation of contractile element calcium sensitivity. 2. Rat mesenteric small arteries were cannulated and pressurized. The internal diameter was continuously monitored with a video camera and intracellular calcium levels were measured by means of fura-2. Myogenic responses were observed when the pressure was raised stepwise from 20 to 60 and then to 100 mmHg in physiological saline solution and during application of phenylephrine (0.1 or 1 micromol/L) or potassium (36 mmol/L). 3. The PKC inhibitors H-7 (20 micromol/L), staurosporine (100 nmol/L) and calphostin C (10 nmol/L) all completely abolished the myogenic response. Whereas staurosporine caused an ongoing reduction in intracellular calcium, pressure-induced calcium transients were not affected by either H-7 or calphostin C. In particular, the slope of the wall tension-calcium relationship remained similar in the presence of both H-7 and calphostin C, despite an upward shift of this relationship to higher calcium levels in the case of calphostin C. 4. These results show that activity of PKC isoform(s) is essential for myogenic calcium-contraction coupling.

An endothelium-derived hyperpolarizing factor-like factor moderates myogenic constriction of mesenteric resistance arteries in the absence of endothelial nitric oxide synthase-derived nitric oxide

Myogenic tone is an important determinant of vascular tone and blood flow in small resistance arteries of certain vascular beds. The role of the endothelium in myogenic responses is unclear. We hypothesized that endothelium-derived NO release modulates myogenic constriction in small resistance arteries and that mesenteric small arteries from mice with targeted disruption of the gene for endothelial NO synthase (eNOS) (knockout mice) demonstrate greater myogenic tone than do wild-type mice. Third-order mesenteric arteries (approximately 200 micrometer) were isolated and mounted in a pressure myograph. Internal diameter was recorded over a pressure range of 10 to 80 mm Hg. Removal of the endothelium significantly (P<0.05) enhanced the magnitude of myogenic constriction in wild-type mice. Similarly, pretreatment of arteries with N(G)-nitro-L-arginine methyl ester (L-NAME; 300 micromol/L) produced a comparable significant (P<0.05) increase in myogenic tone, whereas indomethacin (5 micromol/L) had no effect. eNOS knockout arteries also exhibited myogenic constriction. Neither L-NAME nor indomethacin had any effect on myogenic tone in the arteries of eNOS knockout mice. However, blockade of potential endothelium-derived hyperpolarizing factor-like mechanisms via inhibition of K(+) flux using either apamin (100 nmol/L) with charybdotoxin (100 nmol/L), Ba(2+) (30 micromol/L) with ouabain (1 mmol/L), or 18alpha-glycyrrhetinic acid (100 micromol/L) significantly (P<0.01) enhanced myogenic constriction. This study demonstrates that basal endothelium-derived NO modulates myogenic tone in mesenteric small arteries of wild-type mice. However, eNOS knockout arteries display normal myogenic responsiveness despite the absence of basal NO activity. The data suggest that this compensatory effect is due to the activity of an endothelium-derived hyperpolarizing factor to normalize vascular tone.

Tyrosine phosphorylation following alterations in arteriolar intraluminal pressure and wall tension

Arterioles respond to increased transmural pressure with myogenic constriction. The present study investigated the role of tyrosine phosphorylation in myogenic activity. Cannulated segments of a rat cremaster arteriole were fixed under pressure, followed by incubation with fluorescein isothiocyanate (FITC)-conjugated anti-phosphotyrosine. Smooth muscle cell fluorescence intensity was measured with the use of confocal laser-scanning microscopy. Anti-phosphotyrosine fluorescence intensity in muscle cells of arterioles maintained at 100 mmHg was reduced by the tyrosine kinase inhibitor tyrphostin A47 (30 microM) and increased by the tyrosine phosphatase inhibitor pervanadate (100 microM). In time-course experiments, anti-phosphotyrosine fluorescence increased slowly (over 5 min) after an acute increase in intraluminal pressure, and was dissociated from myogenic contraction (within 1 min). In contrast, angiotensin II (0.1 microM) caused rapid constriction and increased tyrosine phosphorylation. Anti-phosphotyrosine fluorescence was also pressure dependent (10-100 mmHg). Abolition of myogenic activity, either through removal of extracellular Ca2+, or exposure to verapamil (5 microM) or forskolin (0.1 microM) caused a further increase in anti-phosphotyrosine fluorescence. We conclude that transmural pressure and/or wall tension in arterioles causes increased tyrosine phosphorylation; however, this is not involved in the acute phase of myogenic constriction but may be involved in later responses, such as sustained myogenic tone or mechanisms possibly related to growth.

Pharmacological evidence for capacitative Ca(2+) entry in cannulated and pressurized skeletal muscle arterioles

Arteriolar myogenic tone shows a marked dependency on extracellular Ca(2+). The contribution played by mechanisms such as intracellular Ca(2+) release and capacitative entry, however, are less certain. The present studies aimed to demonstrate functional evidence for involvement of such mechanisms in myogenic tone and reactivity. Single cremaster arterioles were denuded of endothelium, pressurized under no-flow conditions and loaded with fura 2-AM for measurement of changes in intracellular Ca(2+) [Ca(2+)](i). The cell permeable, putative, IP(3) receptor antagonist 2APB (2 aminoethoxydiphenyl borate) was used to determine the possible role of IP(3) receptor-mediated mechanisms in arteriolar myogenic tone and reactivity. Arterioles dilated in response to increasing concentrations of 2APB (1 - 300 microM) without a concomitant change in global [Ca(2+)](i). Also 2APB (50 microM) completely inhibited the myogenic constriction in response to a step change in luminal pressure (50 - 120 mmHg) with no apparent effect on pressure-mediated increases in [Ca(2+)](i). 2APB markedly attenuated the constrictor response and [Ca(2+)](i) increase stimulated by phenylephrine but not KCl. Capacitative Ca(2+) influx in arterioles was demonstrated either by re-addition of extracellular [Ca(2+)] following pre-treatment with 1 or 10 microM nifedipine in Ca(2+) free buffer or exposure of vessels to thapsigargin (1 microM) to induce store depletion. In both cases 2APB inhibited the increase in [Ca(2+)](i). Capacitative Ca(2+) entry showed an inverse relationship with intraluminal pressure over the range 10 - 120 mmHg. Consistent with an effect on a Ca(2+) entry pathway, 2APB had no effect on intracellular (caffeine releasable) Ca(2+) stores while decreasing the rate of Mn(2+) quench of fura 2 fluorescence. The results provide functional evidence for capacitative Ca(2+) entry in intact arteriolar smooth muscle. The effectiveness of 2APB in inhibiting both non-voltage gated Ca(2+) entry and responsiveness to an acute pressure step is consistent with the involvement of an axis involving IP(3)-mediated and or capacitative Ca(2+) entry mechanisms in myogenic reactivity. Given the lack of effect of 2APB on pressure-induced changes in global [Ca(2+)](i) it is suggested that such mechanisms participate on a localized level to couple the myogenic stimulus to contraction.

Invited review: mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca(2+) regulates the contractile state of smooth muscle cells and tissues. Elevations in global cytoplasmic Ca(2+) resulting in contraction are accomplished by Ca(2+) entry and release from intracellular stores. Pathways for Ca(2+) entry include dihydropyridine-sensitive and -insensitive Ca(2+) channels and receptor and store-operated nonselective channels permeable to Ca(2+). Intracellular release from the sarcoplasmic reticulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors. The impact of Ca(2+) entry and release on cytoplasmic concentration is modulated by Ca(2+) reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca(2+) transients (i.e., sparks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca(2+) entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca(2+) handling in smooth muscles.

Intravascular pressure regulates local and global Ca(2+) signaling in cerebral artery smooth muscle cells

The regulation of intracellular Ca(2+) signals in smooth muscle cells and arterial diameter by intravascular pressure was investigated in rat cerebral arteries (approximately 150 microm) using a laser scanning confocal microscope and the fluorescent Ca(2+) indicator fluo 3. Elevation of pressure from 10 to 60 mmHg increased Ca(2+) spark frequency 2.6-fold, Ca(2+) wave frequency 1.9-fold, and global intracellular Ca(2+) concentration ([Ca(2+)](i)) 1.4-fold in smooth muscle cells, and constricted arteries. Ryanodine (10 microM), an inhibitor of ryanodine-sensitive Ca(2+) release channels, or thapsigargin (100 nM), an inhibitor of the sarcoplasmic reticulum Ca(2+)-ATPase, abolished sparks and waves, elevated global [Ca(2+)](i), and constricted pressurized (60 mmHg) arteries. Diltiazem (25 microM), a voltage-dependent Ca(2+) channel (VDCC) blocker, significantly reduced sparks, waves, and global [Ca(2+)](i), and dilated pressurized (60 mmHg) arteries. Steady membrane depolarization elevated Ca(2+) signaling similar to pressure and increased transient Ca(2+)-sensitive K(+) channel current frequency e-fold for approximately 7 mV, and these effects were prevented by VDCC blockers. Data are consistent with the hypothesis that pressure induces a steady membrane depolarization that activates VDCCs, leading to an elevation of spark frequency, wave frequency, and global [Ca(2+)](i). In addition, pressure induces contraction via an elevation of global [Ca(2+)](i), whereas the net effect of sparks and waves, which do not significantly contribute to global [Ca(2+)](i) in arteries pressurized to between 10 and 60 mmHg, is to oppose contraction.

The mechanism of phenylephrine-mediated [Ca(2+)](i) oscillations underlying tonic contraction in the rabbit inferior vena cava

1. We characterized the mechanisms in vascular smooth muscle cells (VSMCs) that produce asynchronous, wave-like Ca(2+) oscillations in response to phenylephrine (PE). Confocal imaging was used to observe [Ca(2+)](i) in individual VSMCs of intact inferior vena cava (IVC) from rabbits. 2. It was found that the Ca(2+) waves were initiated by Ca(2+) release from the sarcoplasmic reticulum (SR) via inositol 1,4,5-trisphosphate-sensitive SR Ca(2+) release channels (IP(3)R channels) and that refilling of the SR Ca(2+) store through the sarcoplasmic-endoplasmic reticulum Ca(2+)-ATPase (SERCA) was required for maintained generation of the repetitive Ca(2+) waves. 3. Blockade of L-type voltage-gated Ca(2+) channels (L-type VGCCs) with nifedipine reduced the frequency of PE-stimulated [Ca(2+)](i) oscillations, while additional blockade of receptor-operated channels/store-operated channels (ROCs/SOCs) with SKF96365 abolished the remaining oscillations. Parallel force measurements showed that nifedipine inhibited PE-induced tonic contraction by 27 % while SKF96365 abolished it. This indicates that stimulated Ca(2+) entry refills the SR to support the recurrent waves of SR Ca(2+) release and that both L-type VGCCs and ROCs/SOCs contribute to this process. 4. Application of the Na(+)-Ca(2+) exchanger (NCX) inhibitors 2',4'-dichlorobenzamil (forward- and reverse-mode inhibitor) and KB-R7943 (reverse-mode inhibitor) completely abolished the nifedipine-resistant component of [Ca(2+)](i) oscillations and markedly reduced PE-induced tone. 5. Thus, we conclude that each Ca(2+) wave depends on initial SR Ca(2+) release via IP(3)R channels followed by SR Ca(2+) refilling through SERCA. Na(+) entry through ROCs/SOCs facilitates Ca(2+) entry through the NCX operating in the reverse mode, which refills the SR and maintains PE-induced [Ca(2+)](i) oscillations. In addition some Ca(2+) entry through L-type VGCCs and ROCs/SOCs serves to modulate the frequency of the oscillations and the magnitude of force development.

Invited review: arteriolar smooth muscle mechanotransduction: Ca(2+) signaling pathways underlying myogenic reactivity

The smooth muscle of arterioles responds to an increase in intraluminal pressure with vasoconstriction and with vasodilation when pressure is decreased. Such myogenic vasoconstriction provides a level of basal tone that enables arterioles to appropriately adjust diameter in response to neurohumoral stimuli. Key in this process of mechanotransduction is the role of changes in intracellular Ca(2+). However, it is becoming clear that considerable complexity exists in the spatiotemporal characteristics of the Ca(2+) signal and that changes in intracellular Ca(2+) may play roles other than direct effects on the contractile process via activation of myosin light-chain phosphorylation. The involvement of Ca(2+) may extend to modulation of ion channels and release of Ca(2+) from the sarcoplasmic reticulum, alterations in Ca(2+) sensitivity, and coupling between cells within the vessel wall. The purpose of this brief review is to summarize the current literature relating to Ca(2+) and the arteriolar myogenic response. Consideration is given to coupling of Ca(2+) changes to the mechanical stimuli, sources of Ca(2+), involvement of ion channels, and spatiotemporal aspects of intracellular Ca(2+) signaling.

Redox signaling of the arteriolar myogenic response

Arteriolar vascular smooth muscle cells (VSMCs) are mechanosensitive, constricting to elevations in transmural pressure (P(TM)). The goal of the present study was to determine using mouse isolated tail arterioles and arteries whether oxidant signaling regulates this myogenic response. In response to P(TM) elevation, VSMCs of arterioles but not arteries generated constriction and increased reactive oxygen species (ROS) activity (using the H(2)O(2)-sensitive probe dichlorodihydrofluorescein). Arterioles had increased expression of NADPH oxidase components compared with arteries. Inhibition of NADPH oxidase, using mice with targeted impairment of enzyme components (p47(phox) or rac1) or diphenyleneiodonium, prevented the pressure-induced generation of ROS. When ROS activity was inhibited, either by inhibiting NADPH oxidase or with N-acetylcysteine, the myogenic constriction was abolished. The myogenic constriction was also inhibited by catalase, which inactivates H(2)O(2), but was unaffected by a cell-permeant mimic of superoxide dismutase (MnTMPyP). alpha(1)-Adrenergic constriction was not associated with altered ROS activity and was not affected by inhibition of NADPH oxidase or ROS. Exogenous H(2)O(2) constricted VSMCs of arterioles but not arteries. Thus, NADPH oxidase and ROS, in particular H(2)O(2), contribute to the myogenic response of arteriolar VSMCs.

L-type Ca(2+) channel regulation by pituitary adenylate cyclase-activating polypeptide in vascular myocytes from spontaneously hypertensive rats

Pituitary adenylate cyclase-activating polypeptide (PACAP), a vasoactive peptide, modulates the L-type Ca(2+) channel current (L channel current) in vascular smooth muscle cells (VSMC) through activation and integration of two intracellular pathways, protein kinase A and protein kinase C (PKC). In the present study we compared the effects of PACAP on the L channel current in VSMC from the spontaneously hypertensive rats (SHR) and normotensive controls, Wistar Kyoto rats (WKY). We found that compared with WKY, VSMC from SHR had a higher L channel current density. Stimulation by PACAP (10 nM) caused an increase in the amplitude of the whole cell current and prolonged open time in VSMC from SHR and WKY, with the increase greater in SHR. These effects of PACAP on the L channel current was mimicked by an activator of PKC. In contrast, PACAP caused a smaller increase in cAMP accumulation in VSMC from SHR than WKY, and there was no difference in the inhibitory effect of 8-bromo-cAMP on the L channel current from both type of cells. The greater increase in amplitude of the L channel current by PACAP in VSMC from SHR persisted in the presence of adenosine cyclic 3',5'-monophosphothioate, Rp-isomer, a cAMP antagonist, but not calphostin C, a PKC inhibitor. Taken together, our results show an increase in L channel current density and an enhanced PACAP effect on the L channel current in VSMC from SHR compared with WKY. This difference in PACAP response appears to be predominately secondary to an increased PKC sensitivity.

Calcium channel blockers, postural vasoconstriction and dependent oedema in essential hypertension

Treatment with calcium channel blocker (CCB)s, dihydropyridines and others, is frequently complicated by dependent oedema in the absence of sodium retention or cardiac failure, a bothersome side effect of unclear aetiology. The present paper reviews our own and other work dealing with the antagonism exerted by such drugs on postural vasoconstriction, a mechanism triggered by limb venous congestion during orthostasis and controlled through a local sympathetic axo-axonic reflex and increased myogenic tone in response to changes in transmural pressure. By stabilising capillary pressure, postural vasoconstriction counteracts fluid hyperfiltration consequent to gravitational stimuli, and consistent evidence shows attenuation of this response by L-type calcium channel blockers. Interference with the postural reflex control of skin blood flow may therefore contribute to dependent oedema, although cannot entirely explain its development. Attenuation of postural vasoconstriction may amplify the fluid hyperfiltration induced by CCBs through other mechanisms, such as imbalanced intracapillary pressure or enhanced vascular permeability, which are the main factors determining net fluid filtration into the interstitial compartment.

Capacitative Ca(2+) entry in agonist-induced pulmonary vasoconstriction

Agonist-induced increases in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery (PA) smooth muscle cells (SMCs) consist of a transient Ca(2+) release from intracellular stores followed by a sustained Ca(2+) influx. Depletion of intracellular Ca(2+) stores triggers capacitative Ca(2+) entry (CCE), which contributes to the sustained increase in [Ca(2+)](cyt) and the refilling of Ca(2+) into the stores. In isolated PAs superfused with Ca(2+)-free solution, phenylephrine induced a transient contraction, apparently by a rise in [Ca(2+)](cyt) due to Ca(2+) release from the intracellular stores. The transient contraction lasted for 3-4 min until the Ca(2+) store was depleted. Restoration of extracellular Ca(2+) in the presence of phentolamine produced a contraction potentially due to a rise in [Ca(2+)](cyt) via CCE. The store-operated Ca(2+) channel blocker Ni(2+) reduced the store depletion-activated Ca(2+) currents, decreased CCE, and inhibited the CCE-mediated contraction. In single PASMCs, we identified, using RT-PCR, five transient receptor potential gene transcripts. These results suggest that CCE, potentially through transient receptor potential-encoded Ca(2+) channels, plays an important role in agonist-mediated PA contraction.

Evidence that Rho-kinase activity contributes to cerebral vascular tone in vivo and is enhanced during chronic hypertension: comparison with protein kinase C

The small G protein Rho and its target Rho-kinase may participate in the mechanisms underlying vascular contractile tone via inhibition of myosin light chain phosphatase. The present study has tested the hypothesis that Rho-kinase activity normally contributes to cerebral vascular tone in vivo, and that this effect is augmented during chronic hypertension. Comparative studies also examined the role of protein kinase C (PKC) in regulation of cerebral artery tone. Two Rho-kinase inhibitors, Y-27632 (0.1 to 100 micromol/L) and HA1077 (1 to 10 micromol/L), caused marked concentration-dependent increases in basilar artery diameter of anesthetized normotensive rats (Sprague-Dawley and Wistar-Kyoto [WKY] strains), as measured using a cranial window approach. By comparison, the selective PKC inhibitors calphostin C (0.01 to 0.5 micromol/L) and Ro 31-8220 (5 micromol/L) had little or no effect on basilar artery diameter. Vasodilator responses to Y-27632 were unaffected by PKC inhibition or activation. In two models of chronic hypertension (spontaneously hypertensive rats and WKY rats treated with N-nitro-L-arginine methyl ester for 4 weeks), Y-27632 elicited cerebral vasodilator responses that were significantly greater than in control WKY rats (P<0.05), indicating that the chronically hypertensive state and not genetic factors contributed to the increased responses to Rho-kinase inhibition. PKC inhibition had no significant effect on basilar artery diameter in chronically hypertensive rats. These data suggest that Rho-kinase, but not PKC, activity contributes substantially to cerebral artery tone in vivo, and this effect is augmented in the cerebral circulation during chronic hypertension.

Integrins and mechanotransduction of the vascular myogenic response

This review summarizes what is currently known about the role of integrins in the vascular myogenic response. The myogenic response is the rapid and maintained constriction of a blood vessel in response to pressure elevation. A role for integrins in this process has been suggested because these molecules form an important mechanical link between the extracellular matrix and the vascular smooth muscle cytoskeleton. We briefly summarize evidence for a general role of integrins in mechanotransduction. We then describe the integrin subunit combinations known to exist in smooth muscle and the vascular wall matrix proteins that may interact with these integrins. We then discuss the effects of integrin-specific peptides and antibodies on vascular tone and on calcium entry mechanisms in vascular smooth muscle. Because integrin function is linked to the cytoskeleton, we discuss evidence for the role of the cytoskeleton in determining myogenic responsiveness. Finally, we analyze evidence that integrin-linked signaling pathways, such as those involving protein tyrosine phosphorylation cascades and mitogen-activated protein kinases, are required for myogenic tone.

Characterization of stretch-activated cation current in coronary smooth muscle cells

Stretch-activated ion currents were recorded from vascular smooth muscle (VSM) after enzymatic isolation of single cells from porcine coronary arterioles. Patch pipettes were used to record whole cell current and control cell length. Under voltage clamp in physiological saline solution, an inward cation current (I(CAT)) was activated by 105--135% longitudinal stretch. I(CAT) coincided with an increase in intracellular Ca(2+) concentration. Under current clamp, membrane depolarization was induced by stretch. The magnitude of I(CAT) varied from -0.8 to -6.9 pA/pF at a holding potential of -60 mV. I(CAT) was graded with stretch, inactivated on release, and could be repeatedly induced. A potassium current (I(K)) activated in unstretched cells by depolarization was also enhanced by stretch. In Ca(2+)-free bath solution, stretch-induced enhancement of I(K) was blocked, but I(CAT) was still present. Hexamethyleneamiloride (50 microM), a reputed inhibitor of mechanosensitive channels, blocked I(CAT) and the stretch-induced increase in I(K) but not basal I(K). Grammostolla spatulata venom (1:100,000) blocked basal I(K), blocked stretch-induced increases in I(K), and blocked I(CAT). Iberiotoxin, a specific Ca(2+)-activated K(+) channel blocker, did not alter I(CAT) but blocked the stretch-induced increase in I(K) and increased the magnitude of stretch-induced depolarization. We concluded that longitudinal stretch directly activates a cation current and secondarily activates a Ca(2+)-activated K(+) current in isolated coronary myocytes. Although these two currents would partially counteract each other, the predominance of I(CAT) at physiological potentials is likely to explain the depolarization and contraction observed in intact coronary VSM during pressure elevation.

20-HETE modulates myogenic response of skeletal muscle resistance arteries from hypertensive Dahl-SS rats

The present study determined the role of 20-hydroxyeicosatetraenoic acid [20-HETE; produced by omega-hydroxylation of arachidonic acid via cytochrome P-450 (CP450) 4A enzymes] in regulating myogenic activation of skeletal muscle resistance arteries from normotensive (NT) and hypertensive (HT) Dahl salt-sensitive (SS) rats. Gracilis arteries (GA) were isolated from each rat and viewed via television microscopy, and changes in vessel diameter with altered transmural pressure were measured with a video micrometer. Under control conditions, GA from both groups exhibited strong, endothelium-independent myogenic activation. Treatment of GA with 17-octadecynoic acid (17-ODYA; inhibitor of CP450 4A enzymes) did not alter myogenic activation in NT rats, but impaired this response in HT animals. Treatment of GA from HT rats with dibromo-dodecynyl-methylsulfimide (DDMS; inhibitor of 20-HETE production) impaired myogenic activation, as did application of 20-hydroxyeicosa-6(Z),15(Z)-dienoic acid, an antagonist for 20-HETE receptors. Application of iberiotoxin, a Ca(2+)-activated potassium (K(Ca)) channel inhibitor, restored myogenic activation from HT rats treated with DDMS. These results suggest that myogenic activation of skeletal muscle resistance arteries from NT Dahl-SS rats does not depend on CP450, whereas myogenic activation of these vessels in HT Dahl-SS rats is partly a function of 20-HETE production, inhibiting K(Ca) channels through a receptor-mediated process.

Flow (shear stress)-induced endothelium-dependent dilation is altered in mice lacking the gene encoding for dystrophin

BACKGROUND

Dystrophin has a key role in striated muscle mechanotransduction of physical forces. Although cytoskeletal elements play a major role in the mechanotransduction of pressure and flow in vascular cells, the role of dystrophin in vascular function has not yet been investigated. Thus, we studied endothelial and muscular responses of arteries isolated from mice lacking dystrophin (mdx mice).

METHODS AND RESULTS

Carotid and mesenteric resistance arteries 120 micrometer in diameter were isolated and mounted in vitro in an arteriograph to control intraluminal pressure and flow. Blood pressure was not affected by the absence of dystrophin. Pressure-induced (myogenic), phenylephrine-induced, and KCl-induced forms of tone were unchanged. Flow (shear stress)-induced dilation in arteries isolated from mdx mice was decreased by 50% to 60%, whereas dilation to acetylcholine or sodium nitroprusside was unaffected. NG-nitro-L-arginine methyl ester-sensitive flow dilation was also decreased in arteries from mdx mice. Thus, the absence of dystrophin was associated with a defect in signal transduction of shear stress. Dystrophin was present in vascular endothelial and smooth muscle cells, as shown by immunolocalization, and localized at the level of the plasma membrane, as seen by confocal microscopy of perfused isolated arteries.

CONCLUSIONS

-This is the first functional study of arteries lacking the gene for dystrophin. Vascular reactivity was normal, with the exception of flow-induced dilation. Thus, dystrophin could play a specific role in shear-stress mechanotransduction in arterial endothelial cells. Organ damage in such diseases as Duchenne dystrophy might be aggravated by such a defective arterial response to flow.

Role of calcium-sensitive K(+) channels and nitric oxide in in vivo coronary vasodilation from enhanced perfusion pulsatility

BACKGROUND

In vitro studies support K(+)(Ca) channel-induced smooth muscle hyperpolarization as underlying acetylcholine-mediated (or bradykinin-mediated) vasodilation that persists despite combined nitric oxide (NO) and PGI(2) inhibition. We tested the hypothesis that these channels are activated by enhanced pulsatile perfusion in vivo and contribute substantially to vasodilation from this stimulus.

METHODS AND RESULTS

The canine left descending coronary artery was perfused with whole blood at constant mean pressure, and physiological flow pulsatility was set at 40 or 100 mm Hg by computer servo-pump. Cyclooxygenase was inhibited by indomethacin. Mean flow increased +18+/-2% (P:<0.0001) with enhanced pulsatility. This response declined approximately 50% by blocking NO synthase (L-NMMA) or K(+)(Ca) [charybdotoxin (CbTX)+apamin (AP)]. Combining both inhibitors virtually eliminated the flow rise. Inhibiting either or both pathways minimally altered basal coronary flow, whereas agonist-stimulated flow was blocked. Bradykinin-induced dilation declined more with CbTX+AP than with L-NMMA (-66% versus -46%, P:=0.03) and was fully blocked by their combination. In contrast, acetylcholine-induced dilation was more blunted by L-NMMA than by CbTX+AP (-71% versus -44%, P:<0.002) and was not fully prevented by the combination. Substituting iberiotoxin (IbTX) for CbTX greatly diminished inhibition of pulse pressure and agonist flow responses (with or without NOS inhibition). Furthermore, blockade by IbTX+AP was identical to that by AP alone, supporting a minimal role of IbTX-sensitive large-conductance K(+)(Ca) channels.

CONCLUSIONS

K(+)(Ca) activation and NO comodulate in vivo pulsatility-stimulated coronary flow, supporting an important role of a hyperpolarization pathway in enhanced mechanovascular signaling. Small- and intermediate-conductance K(+)(Ca) channels are the dominant species involved in modulating both pulse pressure- and bradykinin-induced in vivo coronary dilation.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Control of vascular tone in the syndromes of Bartter and Gitelman

Bartter's and Gitelman's syndromes can be used as models to gain insight into the mechanisms responsible for maintaining/controlling vascular tone. In fact, the study of patients with these syndromes provides important insights into mechanistic details of the most relevant pathways of vascular tone. So far, several experimental findings in patients with these syndromes point to G protein abnormalities and suggest that the intracellular signaling systems that involve the G protein complex transducing components may be defective, leading to altered vascular reactivity. These results are also of particular interest because the derangements found in Bartter's and Gitelman's syndromes are the mirror images of those involved in the pathophysiology of hypertension.

Response of arterial smooth muscle to length perturbation

The ability of arterial smooth muscle to generate tension is influenced by muscle length. An unsettled question is whether the length-tension relationship is a simple reflection of the contractile filament overlap, as it is in skeletal muscle. There are several factors that could potentially affect tension generation in arterial smooth muscle; these include stretch-induced myogenic response and length-oscillation-induced disruption of the contractile filament organization. In this study, in which rabbit carotid arterial preparations were used, we found that different length-tension curves could be obtained at different times after a length change. In addition, length oscillation at a frequency of normal pulse rate and with small to moderate oscillation amplitude was found to potentiate tension generation but reduced tension at large amplitudes. The observed response could be attributed to adaptation of the muscle to length change over time and to myogenic potentiation associated with stretching of the muscle.

Effects of mibefradil and nifedipine on arteriolar myogenic responsiveness and intracellular Ca(2+)

1. Ca(2+) entry mechanisms underlying spontaneous arteriolar tone and acute myogenic reactivity remain uncertain. These studies aimed to compare the effects of nifedipine and the putative T-channel blocker, mibefradil, on arteriolar myogenic responsiveness and intracellular Ca(2+) (Ca(2+)(i)). 2. First order cremaster muscle arterioles (1A) were isolated from rats, cannulated, pressurized to 70 mmHg in the absence of intraluminal flow, and mechanical responses studied by video microscopy. The Ca(2+)(i) was measured using fluorescence imaging of Fura 2 loaded arterioles. 3. Both nifedipine and mibefradil showed dose-dependent inhibition of spontaneous myogenic tone (at 70 mmHg; pEC(50) 7.04+/-0.17 vs 6.65+/-0.20 respectively, n=6 for both, n.s.) and KCl-induced vasoconstriction (at 70 mmHg; pEC(50) 6.93+/-0. 38 vs 6.45+/-0.27 respectively, n=6 for both, n.s.). 4. In arterioles maintained at 50 mmHg, nifedipine (10(-7) and 10(-5) M) caused a concentration dependent reduction in Ca(2+)(i), however, mibefradil (10(-7) and 10(-5) M) had no effect. Furthermore nifedipine significantly attenuated the increase in Ca(2+)(i) associated with an acute pressure step (50 - 120 mmHg) whereas mibefradil was considerably less effective. 5. Mibefradil (10(-7) M) significantly attenuated contractile responses to 60 mM KCl without altering the KCl-induced increase in Ca(2+)(i), in contrast to nifedipine (10(-7) M) which reduced both Ca(2+)(i) and contraction. 6. Membrane potential of arterioles with spontaneous myogenic tone (70 mmHg) was -41.5+/-1. 0 mV. Nifedipine (10(-7) or 10(-5) M) had no effect on membrane potential, however mibefradil (10(-5) M) caused significant depolarization. 7. In summary, both mibefradil and nifedipine inhibit arteriolar spontaneous tone and acute myogenic reactivity. While there may be overlap in the mechanisms by which these agents inhibit tone, differences in effects on membrane potential and intracellular Ca(2+) levels suggest mibefradil exhibits actions other than blockade of Ca(2+) entry in skeletal muscle arterioles.

Evidence for involvement of the PKC-alpha isoform in myogenic contractions of the coronary microcirculation

The role of protein kinase C (PKC) isoforms in myogenic tone of the ferret coronary microcirculation was investigated by measuring fura 2 Ca(2+) signals, PKC immunoblots, contractile responses, and confocal microscopy of PKC translocation. Phorbol ester-evoked contractions were completely abolished in the absence of extracellular Ca(2+) but involved a Ca(2+) sensitization relative to KCl contractions. Immunoblotting using isoform-specific antibodies showed the presence of PKC-alpha and -iota and traces of PKC-epsilon and -mu in the ferret coronary microcirculation. PKC-beta was not detectable. When intraluminal pressure (40 to 60 and 80 mmHg) was increased, ferret coronary arterioles showed a transient increase in fura 2 Ca(2+) signals, whereas the myogenic tone remained sustained. The increase in Ca(2+) and tone was sustained at 100 mmHg. Isolated ferret coronary arterioles were fixed and immunostained for PKC-alpha at 40 and 100 mmHg intraluminal pressure. PKC translocation was determined by confocal microscopy. Increased PKC translocation was observed when vessels were exposed to 100 mmHg relative to that at resting pressure (40 mmHg). These results suggest a link between the Ca(2+) sensitization that occurs during the myogenic contraction and activation of the alpha-isoform of PKC.

Gonadal hormones affect diameter of male rat cerebral arteries through endothelium-dependent mechanisms

Gender is known to influence the incidence and severity of cerebrovascular disease. In the present study, luminal diameter was measured in vitro in pressurized middle cerebral artery segments from male rats that were either untreated, orchiectomized (ORX), ORX with testosterone treatment (ORX+TEST), or ORX with estrogen treatment (ORX+EST). The maximal passive diameters (0 Ca(2+) + 3 mM EDTA) of arteries from all four groups were similar. In endothelium-intact arteries, myogenic tone was significantly greater in arteries from untreated and ORX+TEST compared with arteries from either ORX or ORX+EST. During exposure to N(G)-nitro-L-arginine-methyl ester (L-NAME), an NO synthase (NOS) inhibitor, myogenic tone significantly increased in all groups. The effect of L-NAME was significantly greater in arteries from untreated and ORX+EST compared with arteries from ORX and ORX+TEST rats. Differences in myogenic tone between ORX and ORX+TEST persisted after inhibition of NOS. After endothelium removal or inhibition of the cyclooxygenase pathway combined with K(+) channel blockers, myogenic tone differences between ORX and ORX+TEST were abolished. Wall thickness and forced dilation were not significantly different between arteries from ORX and ORX+TEST. Our data show that gonadal hormones affect myogenic tone in male rat cerebral arteries through NOS- and/or endothelium-dependent mechanisms.

Reinforcement of arteriolar myogenic activity by endogenous ANG II: susceptibility to dietary salt

The purpose of this study was to determine whether endogenous ANG II augments arteriolar myogenic behavior in striated muscle. Because circulating ANG II is decreased during high salt intake, we also investigated whether dietary salt could alter any influence of ANG II on myogenic behavior. Normotensive rats fed low-salt (0.45%, LS) or high-salt (7%, HS) diets were enclosed in a ventilated box with the spinotrapezius muscle exteriorized for intravital microscopy. Dietary salt did not affect resting arteriolar diameters. Microvascular pressure elevation by box pressurization caused greater arteriolar constriction in LS rats (up to 12 microm) than in HS rats (up to 4 microm). The ANG II-receptor antagonists saralasin and losartan attenuated myogenic responsiveness in LS rats but not HS rats. The bradykinin-receptor antagonist HOE-140 had no effect on myogenic responsiveness in LS rats but augmented myogenic responsiveness in HS rats. HOE-140 with the angiotensin-converting enzyme inhibitor captopril attenuated myogenic responsiveness to a greater extent in LS rats than in HS rats. We conclude that endogenous ANG II normally reinforces arteriolar myogenic behavior in striated muscle and that attenuated myogenic behavior associated with high salt intake is due to decreased circulating ANG II and increased local kinin levels.

Tissue angiotensin II and endothelin-1 modulate differently the response to flow in mesenteric resistance arteries of normotensive and spontaneously hypertensive rats

In resistance arteries pressure-induced (myogenic) tone (MT) and flow (shear stress)-induced dilation (FD) are potent determinant of vascular resistance. We investigated the role of angiotensin II and endothelin-1 in FD and MT in resistance arteries and their potential change in hypertension. Flow - diameter - pressure relationship was established in situ, under anaesthesia, in two daughter branches of a mesenteric resistance artery (180 microM, n=7 per group) from spontaneously hypertensive (SHR) or normotensive (WKY) rats. One artery was ligated distally, so that it was submitted to pressure only, while the other was submitted to pressure and flow. Drugs were added to the preparation and external diameter, pressure and flow measured continuously. External diameter (with flow) ranged from 150+/-3 to 191+/-7 microM in WKY (n=28) rats and from 168+/-6 to 186+/-6 microM in SHR (n=28). Flow induced a dilation of the non-ligated arteries which was lower in SHR (13+/-5 - 31+/-4 microM vs WKY: 5+/-5 - 44+/-4 microM). In the ligated artery, the diameter did not significantly change, due to MT. In the vessels submitted to flow angiotensin converting enzyme inhibition (perindopril, 10 micromol L(-1)) increased the diameter in SHR (+11+/-2 microM) significantly more than in WKY (+2+/-1 microM). Angiotensin type 1 receptor (AT(1)R) blockade (losartan, 10 micromol L(-1)) increased the diameter in the vessels with flow in SHR only (+6+/-1 microM). Angiotensin type 2 receptor (AT(2)R) blockade (PD 123319, 1 micromol L(-1)) decreased arterial diameter in WKY only (9+/-2). Endothelin-1 type A receptor (ET(A)R) blockade (LU135252, 0.1 micromol L(-1)) increased the diameter only in SHR in the artery submitted to flow (by 6+/-1 microM). Thus FD was counteracted by a flow-dependent AT(1) and ET(A) receptors-activation in SHR whereas in WKY FD AT(2)-dependent dilation is involved.