Myogenic and vasoconstrictor responsiveness of skeletal muscle arterioles is diminished by hindlimb unloading.

The purpose of the present study was to determine whether hindlimb unloading of rats alters vasoconstrictor and myogenic responsiveness of skeletal muscle arterioles. After either 2 wk of hindlimb unloading (HU) or cage control (C), second-order arterioles were isolated from the white portion of gastrocnemius (WG; C: n = 9, HU: n = 10) or soleus (Sol; C: n = 9, HU: n = 10) muscles and cannulated with two micropipettes connected to reservoir systems for in vitro study. Intraluminal pressure was set at 60 cmH2O. The arterioles were exposed to step changes in intraluminal pressure ranging from 20 to 140 cmH2O to determine myogenic responsiveness and to KCl (10-100 mM) and norepinephrine (10(-9)-10(-4) M) to determine vasoconstrictor responsiveness. Although maximal diameter of WG arterioles was not different between C (185 +/- 12 microm) and HU (191 +/- 14 microm) rats, WG arterioles from HU rats developed less spontaneous tone (C: 33 +/- 5%, HU 20 +/-3%), were unable to maintain myogenic tone at pressures from 140 to 100 cmH2O, and were less sensitive to the vasoconstrictor effects of KCl and norepinephrine (as indicated by a higher agonist concentration that produced 50% of maximal vasoconstrictor response). In contrast, maximal diameter of Sol arterioles from HU rats (117 +/- 12 microm) was smaller than that in C rats (148 +/- 14 microm). However, the development of spontaneous tone (C: 30 +/- 4%, HU: 36 +/- 5%), myogenic activity, and the responsiveness to vasoconstrictor agonists were not different between Sol arterioles from C and HU rats. These results indicate that hindlimb unloading diminishes the myogenic autoregulatory and contractile responsiveness of arterioles from muscle composed of type IIB fibers and suggest that the compromised ability to elevate vascular resistance after exposure to microgravity may be related to these vascular alterations. In addition, hindlimb unloading appears to induce vascular remodeling of arterioles from muscle composed of type I fibers, as indicated by the decrease in maximal diameter of arterioles from Sol muscle.     

Nonuniform changes in arteriolar myogenic tone within skeletal muscle following hindlimb unweighting.

Hindlimb unweighting (HLU) has been shown to alter myogenic tone distinctly in arterioles isolated from skeletal muscles composed predominantly of fast-twitch (white gastrocnemius) compared with slow-twitch (soleus) fibers. Based on these findings, we hypothesized that HLU would alter myogenic tone differently in arterioles isolated from distinct fiber-type regions within a single skeletal muscle. We further hypothesized that alterations in myogenic tone would be associated with alterations in voltage-gated Ca(2+) channel current (VGCC) density of arteriolar smooth muscle. After 14 days of HLU or weight bearing (control), first-order arterioles were isolated from both fast-twitch and mixed fiber-type regions of the gastrocnemius muscle, cannulated, and pressurized at 90 cmH(2)O. Mixed gastrocnemius arterioles of HLU rats demonstrated increased spontaneous tone [43 +/- 5% (HLU) vs. 27 +/- 4% (control) of possible constriction] and an approximately twofold enhanced myogenic response when exposed to step changes in intraluminal pressure (10-130 cmH(2)O) compared with control rats. In contrast, fast-twitch gastrocnemius arterioles of HLU rats demonstrated similar levels of spontaneous tone [6 +/- 2% (HLU) vs. 6 +/- 2% (control)] and myogenic reactivity to control rats. Neither KCl-induced contractile responses (10-50 mM KCl) nor VGCC density was significantly different between mixed gastrocnemius arterioles of HLU and control rats. These results suggest that HLU produces diverse adaptations in myogenic reactivity of arterioles isolated from different fiber-type regions of a single skeletal muscle. Furthermore, alterations in myogenic responses were not attributable to altered VGCC density.     

Aging impairs endothelium-dependent vasodilation in rat skeletal muscle arterioles

Blood flow capacity in skeletal muscle declines with age. Reduced blood flow capacity may be related to decline in the maximal vasodilatory capacity of the resistance vasculature. This study tested the hypothesis that aging results in impaired vasodilatory capacity of first-order (1A) arterioles isolated from rat-hindlimb locomotory muscle: 1A arterioles (90-220 microm) from gastrocnemius and soleus muscles of young (4 mo) and aged (24 mo) Fischer-144 rats were isolated, cannulated, and pressurized via hydrostatic reservoirs. Vasodilatory responses to increasing concentrations of ACh (10(-9) to 10(-4) M), adenosine (ADO, 10(-10) to 10(-4) M), and sodium nitroprusside (SNP, 10(-10) to 10(-4) M) were evaluated at a constant intraluminal pressure of 60 cmH(2)O in the absence of flow. Flow-induced vasodilation was also evaluated in the absence of pressure changes. Responses to ADO and SNP were not altered by age. Endothelium-dependent vasodilation induced by flow was significantly reduced in arterioles from both gastrocnemius and soleus muscles. In contrast, endothelium-dependent vasodilation to ACh was reduced only in soleus muscle arterioles. These results indicate that aging impairs vasodilatory responses mediated through the endothelium of resistance arterioles from locomotory muscle, whereas smooth muscle vasodilatory responses remain intact with aging. Additionally, ACh-induced vasodilation was altered by age only in soleus muscle arterioles, suggesting that the mechanism of age-related endothelial impairment differs in arterioles from soleus and gastrocnemius muscles.     

Effects of fiber composition and hindlimb unloading on the vasodilator properties of skeletal muscle arterioles.

It has been hypothesized that microgravity-induced orthostatic hypotension may result from an exaggerated vasodilatory responsiveness of arteries. The purpose of this study was to determine whether skeletal muscle arterioles exhibit enhanced vasodilation in rats after 2 wk of hindlimb unloading (HU). First-order arterioles isolated from soleus and white gastrocnemius muscles were tested in vitro for vasodilatory responses to isoproterenol (Iso), adenosine (Ado), and sodium nitroprusside (SNP). HU had no effect on responses induced by Iso but diminished maximal vasodilation to Ado and SNP in both muscles. In addition, vasodilatory responses in arterioles from control rats varied between muscle types. Maximal dilations induced by Iso (soleus: 42 +/- 6%; white gastrocnemius: 60 +/- 7%) and Ado (soleus: 51 +/- 8%; white gastrocnemius: 81 +/- 6%) were greater in arterioles from white gastrocnemius muscles. These data do not support the hypothesis that microgravity-induced orthostatic hypotension results from an enhanced vasodilatory responsiveness of skeletal muscle arterioles. Furthermore, the data support the concept that dilatory responsiveness of arterioles varies in muscle composed of different fiber types.     

Chronic intermittent hypoxia alters NE reactivity and mechanics of skeletal muscle resistance arteries.

Although arterial dilator reactivity is severely impaired during exposure of animals to chronic intermittent hypoxia (CIH), few studies have characterized vasoconstrictor responsiveness in resistance arteries of this model of sleep-disordered breathing. Sprague-Dawley rats were exposed to CIH (10% inspired O2 fraction for 1 min at 4-min intervals; 12 h/day) for 14 days. Control rats were housed under normoxic conditions. Diameters of isolated gracilis muscle resistance arteries (GA; 120-150 microm) were measured by television microscopy before and during exposure to norepinephrine (NE) and angiotensin II (ANG II) and at various intraluminal pressures between 20 and 140 mmHg in normal and Ca2+-free physiological salt solution. There was no difference in the ability of GA to constrict in response to ANG II (P = 0.42; not significant; 10(-10)-10(-7) M). However, resting tone, myogenic activation, and vasoconstrictor responses to NE (P < 0.001; 10(-9)-10(-6) M) were reduced in CIH vs. controls. Treatment of rats with the superoxide scavenger 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (tempol; 1 mM) in the drinking water restored myogenic responses and NE-induced constrictions of CIH rats, suggesting that elevated superoxide production during exposure to CIH attenuates vasoconstrictor responsiveness to NE and myogenic activation in skeletal muscle resistance arteries. CIH also leads to an increased stiffness and reduced vessel wall distensibility that were not correctable with oral tempol treatment.     

Decreased NO signaling leads to enhanced vasoconstrictor responsiveness in skeletal muscle arterioles of the ZDF rat prior to overt diabetes and hypertension

Approximately 40% of patients with type 2 diabetes present with concurrent hypertension at the time of diabetes diagnosis. Increases in peripheral vascular resistance and correspondingly enhanced vasoconstrictor capacity could have profound implications for the development of hypertension and the progression of insulin resistance to overt diabetes. The purpose of this study was to determine whether skeletal muscle arteriolar vasoconstrictor dysfunction precedes or occurs concurrently with the onset of diabetes and hypertension. Male Zucker diabetic fatty (ZDF) rats were studied at 7, 13, and 20 wk of age to represent prediabetic and short-term and long-term diabetic states, respectively. Conscious mean arterial pressure (MAP), fasted plasma insulin and glucose, vasoconstrictor responses, and passive mechanical properties of isolated skeletal muscle arterioles were measured in prediabetic, diabetic, and age-matched control rats. Elevated MAP was manifest in short-term diabetes (control 117 +/- 1, diabetic 135 +/- 3 mmHg) and persisted with long-term diabetes (control 113 +/- 2, diabetic 135 +/- 3 mmHg). This higher MAP was preceded by augmented arteriolar vasoconstrictor responses to norepinephrine and endothelin-1 and followed by diminished beta-adrenergic vasodilation and enhanced myogenic constriction in long-term diabetes. Furthermore, we demonstrate that diminished nitric oxide (NO) signaling underlies the increases in vasoconstrictor responsiveness in arterioles from prediabetic and diabetic rats. Arteriolar stiffness was not different between control and prediabetic or diabetic rats at any time point studied. Collectively, these results indicate that increases in vasoconstrictor responsiveness resulting from diminished NO signaling in skeletal muscle arterioles precede the development of diabetes and hypertension in ZDF rats.     

Time course of flow-induced vasodilation in skeletal muscle: contributions of dilator and constrictor mechanisms

The purpose of this study was to determine the time course of flow-induced vasodilation in soleus and gastrocnemius muscle arterioles and the mechanisms that underlie vasodilatory responses to an increase in intraluminal flow. Vasodilation was assessed during 20 min of continuous exposure to intraluminal flow. Both soleus and gastrocnemius muscle arterioles dilated in response to flow, although the magnitude of vasodilation was greater in arterioles from the gastrocnemius muscle. Neither blockade of nitric oxide synthase with N(G)-nitro-L-arginine methyl ester (L-NAME) nor blockade of cyclooxygenase with indomethacin inhibited the initial vasodilation (0-2 min) in arterioles from either muscle. In contrast, vasodilation to sustained exposure to flow (2-20 min) was eliminated by treatment with L-NAME in arterioles from both muscles. Both depolarization with 40 mM KCl and blockade of Ca(2+)-activated K(+) channels inhibited the initial flow-induced dilation, and the inhibition was greater in gastrocnemius muscle arterioles than soleus muscle arterioles. In the presence of L-NAME, prolonged exposure to flow resulted in constriction in soleus and gastrocnemius muscle arterioles. This constriction was abolished by endothelin receptor blockade. These results indicate that the time course and magnitude of flow-induced vasodilation differs between arterioles from soleus and gastrocnemius muscles. The immediate response to increased flow is greater in gastrocnemius muscle arterioles and involves activation of K(+) channels. In arterioles from both soleus and gastrocnemius muscles, vasodilation to sustained flow exposure occurs primarily through production of nitric oxide. In the absence of nitric oxide, sustained exposure to flow results in pronounced constriction that is mediated by endothelin.     

Diaphragm arterioles are less responsive to alpha1- adrenergic constriction than gastrocnemius arterioles.

The sympathetic nervous system has greater influence on vascular resistance in low-oxidative, fast-twitch skeletal muscle than in high-oxidative skeletal muscle (17). The purpose of this study was to test the hypothesis that arterioles isolated from low-oxidative, fast-twitch skeletal muscle [the white portion of gastrocnemius (WG)] possess greater responsiveness to adrenergic constriction than arterioles isolated from high-oxidative skeletal muscle [red portion of the gastrocnemius muscle (RG) and diaphragm (Dia)]. Second-order arterioles (2As) were isolated from WG, RG, and Dia of rats and reactivity examined in vitro. Results reveal that Dia 2As constrict less to norepinephrine (NE) (10(-9) to 10 (-4) M) than 2As from RG and WG, which exhibited similar NE-induced constrictions. This difference was not endothelium dependent, because responses of denuded 2As were similar to those of intact arterioles. The blunted NE-induced constrictor response of Dia 2As appears to be the result of differences in alpha1-receptor effects because 1) arterioles from Dia also responded less to selective alpha1-receptor stimulation with phenylephrine than RG and WG arterioles; 2) arterioles from Dia, RG, and WG dilated similarly to isoproterenol (10(-9) to 10(-4) M) and did not respond to selective alpha2-receptor stimulation with UK-14304; and 3) endothelin-1 produced similar constriction in 2As from Dia, RG, and WG. We conclude that differences in oxidative capacity and/or fiber type composition of muscle tissue do not explain different NE responsiveness of Dia 2As compared with 2As from gastrocnemius muscle. Differences in alpha1-adrenergic constrictor responsiveness among arterioles in skeletal muscle may contribute to nonuniform muscle blood flow responses observed during exercise and serve to maintain blood flow to Dia during exercise-induced increases in sympathetic nerve activity.     

Structural and functional remodeling of skeletal muscle microvasculature is induced by simulated microgravity

Hindlimb unloading of rats results in a diminished ability of skeletal muscle arterioles to constrict in vitro and elevate vascular resistance in vivo. The purpose of the present study was to determine whether alterations in the mechanical environment (i.e., reduced fluid pressure and blood flow) of the vasculature in hindlimb skeletal muscles from 2-wk hindlimb-unloaded (HU) rats induces a structural remodeling of arterial microvessels that may account for these observations. Transverse cross sections were used to determine media cross-sectional area (CSA), wall thickness, outer perimeter, number of media nuclei, and vessel luminal diameter of feed arteries and first-order (1A) arterioles from soleus and the superficial portion of gastrocnemius muscles. Endothelium-dependent dilation (ACh) was also determined. Media CSA of resistance arteries was diminished by hindlimb unloading as a result of decreased media thickness (gastrocnemius muscle) or reduced vessel diameter (soleus muscle). ACh-induced dilation was diminished by 2 wk of hindlimb unloading in soleus 1A arterioles, but not in gastrocnemius 1A arterioles. These results indicate that structural remodeling and functional adaptations of the arterial microvasculature occur in skeletal muscles of the HU rat; the data suggest that these alterations may be induced by reductions in transmural pressure (gastrocnemius muscle) and wall shear stress (soleus muscle).     

Simulated microgravity alters rat mesenteric artery vasoconstrictor dynamics through an intracellular Ca(2+) release mechanism

Previous work has shown that orthostatic hypotension associated with cardiovascular deconditioning results from inadequate peripheral vasoconstriction. We used the hindlimb-unloaded (HU) rat in this study as a model to induce cardiovascular deconditioning. The purpose of this study was to test the hypothesis that 14 days of HU diminishes vasoconstrictor responsiveness of mesenteric resistance arteries. Mesenteric resistance arteries from control (n = 43) and HU (n = 44) rats were isolated, cannulated, and pressurized to 108 cm H(2)O for in vitro experimentation. Myogenic (intralumenal pressure ranging from 30 to 180 cm H(2)O), KCl (2-100 mM), norepinephrine (NE, 10(-9)-10(-4) M) and caffeine (1-20 mM) induced vasoconstriction, as well as the temporal dynamics of vasoconstriction to NE, were determined. The active myogenic and passive pressure responses were unaltered by HU when pressures remained within physiological range. However, vasoconstrictor responses to KCl, NE, and caffeine were diminished by HU, as well as the rate of constriction to NE (C, 14.8 +/- 3.6 microm/s vs. HU 7.6 +/- 1.8 microm/s). Expression of sarcoplasmic reticulum Ca(2+)ATPase 2 and ryanodine 3 receptor mRNA was unaffected by HU, while ryanodine 2 receptor mRNA and protein expression were diminished in mesenteric arteries from HU rats. These data suggest that HU-induced and microgravity-associated orthostatic intolerance may be due, in part, to an attenuated vasoconstrictor responsiveness of mesenteric resistance arteries resulting from a diminished ryanodine 2 receptor Ca(2+) release mechanism.     

Exercise training enhances flow-induced vasodilation in skeletal muscle resistance arteries of aged rats: role of PGI2 and nitric oxide

Flow-induced vasodilation is attenuated with old age in rat skeletal muscle arterioles. The purpose of this study was to determine whether diminished cyclooxygenase (COX) signaling contributes to the age-induced attenuation of flow-induced vasodilation in gastrocnemius muscle arterioles and to determine whether, and through which mechanism(s), exercise training restores this deficit in old rats. Fischer 344 rats (3 and 22 mo old) were assigned to a sedentary or exercise-trained group. First-order arterioles were isolated from the gastrocnemius muscles, cannulated, and pressurized to 70 cm H(2)O. Diameter changes were determined in response to graded increases in intraluminal flow in the presence and absence of nitric oxide synthase (NOS) inhibition [10(-5) M N(G)-nitro-L-arginine methyl ester (L-NAME)], COX inhibition (10(-5) M indomethacin), or combination NOS (10(-5) M L-NAME) plus COX (10(-5) M indomethacin) inhibition. Aging reduced flow-induced vasodilation in gastrocnemius muscle arterioles. Exercise training restored responsiveness to flow in arterioles of aged rats and enhanced flow-induced vasodilation in arterioles from young rats. L-NAME inhibition of flow-induced vasodilation was greater in arterioles from old rats compared with those from young rats and was increased after exercise training in arterioles from both young and old rats. Although the indomethacin-sensitive portion of flow-induced dilation was not altered by age or training, both COX-1 mRNA expression and PGI(2) production increased with training in arterioles from old rats. These data demonstrate that exercise training restores flow-induced vasodilation in gastrocnemius muscle arterioles from old rats and enhances flow-induced vasodilation in gastrocnemius muscle arterioles from young rats. In arterioles from both old and young rats, the exercise training-induced enhancement of flow-induced dilation occurs primarily through a NOS mechanism.     

Time course of vasodilatory responses in skeletal muscle arterioles: role in hyperemia at onset of exercise

At the onset of dynamic exercise, muscle blood flow increases within 1-2 s. It has been postulated that local vasodilatory agents produced by the vascular endothelium or the muscle itself contribute to this response. We hypothesized that only vasodilators that act directly on the vascular smooth muscle could produce vasodilation of skeletal muscle arterioles in <2 s. To test this hypothesis, we determined the time course of the vasodilatory response of isolated skeletal muscle arterioles to direct application of potassium chloride, adenosine, acetylcholine, and sodium nitroprusside. Soleus and gastrocnemius muscles were dissected from the hindlimbs of male Sprague-Dawley rats. First-order arterioles (100-200 microm) were isolated, cannulated on micropipettes, and pressurized to 60 cmH(2)O in an organ bath. Vasodilatory agents were added directly to the bath, and diameter responses of the arterioles were recorded in real time on a videotape recorder. Frame-by-frame analysis of the diameter responses indicated that none of the vasodilator agents tested produced significant diameter increases in <4 s in either soleus or gastrocnemius muscle arterioles. These results indicate that, although these local vasodilators produce significant vasodilation of skeletal muscle resistance arterioles, these responses are not rapid enough (within 1-2 s) to contribute to the initiation of the exercise hyperemic response at the onset of dynamic exercise.     

Tyrosine phosphorylation modulates arteriolar tone but is not fundamental to myogenic response

The present study investigated the role of protein tyrosine phosphorylation in myogenic responsiveness of rat skeletal muscle arterioles. Arteriolar segments were cannulated and pressurized without intraluminal flow. All vessels studied developed spontaneous tone and demonstrated significant myogenic constriction to step changes in pressure with a resultant increase in myogenic tone over an intraluminal pressure range of 50-150 mmHg. Step increases in intraluminal pressure from 50 to 120 mmHg caused a rapid and sustained elevation in intracellular [Ca(2+)], as measured using fura 2. Vessels with myogenic tone dilated in response to tyrosine kinase inhibitors genistein (10 or 30 microM) and tyrphostin A47 (10 or 30 microM) and constricted to the tyrosine phosphatase inhibitor pervanadate (1 or 10 microM). Despite the dilator effect, myogenic reactivity was not blocked by the inhibitors. Daidzein (10 microM), a compound structurally similar to genistein but without tyrosine kinase-inhibiting activity, did not alter vessel tone or myogenic responses. Preincubation of arterioles with genistein or tyrphostin A47 did not significantly alter baseline arteriolar [Ca(2+)], and neither drug reduced the increase in [Ca(2+)] following an acute increase in intraluminal pressure. Constriction induced by pervanadate (10 microM) was not accompanied by a significant increase in intracellular [Ca(2+)], even though removal of extracellular Ca(2+) reversed the constriction. Examination of smooth muscle tyrosine phosphorylation, using a fluorescent phosphotyrosine antibody and confocal microscopy, showed that increased intraluminal pressure resulted in an increase in anti-phosphotyrosine fluorescence. Because manipulation of tyrosine kinase activity was found to alter vessel diameter, these data support a role for tyrosine phosphorylation in modulation of arteriolar tone. However, the results indicate that acute arteriolar myogenic constriction does not require tyrosine phosphorylation.     

Myogenic contraction in rat skeletal muscle arterioles: smooth muscle membrane potential and Ca(2+) signaling

The present studies examined relationships between intraluminal pressure, membrane potential (E(m)), and myogenic tone in skeletal muscle arterioles. Using pharmacological interventions targeting Ca(2+) entry/release mechanisms, these studies also determined the role of Ca(2+) pathways and E(m) in determining steady-state myogenic constriction. Studies were conducted in isolated and cannulated arterioles under zero flow. Increasing intraluminal pressure (0-150 mmHg) resulted in progressive membrane depolarization (-55.3 +/- 4.1 to -29.4 +/- 0.7 mV) that exhibited a sigmoidal relationship between extent of myogenic constriction and E(m). Thus, despite further depolarization, at pressures >70 mmHg, little additional vasoconstriction occurred. This was not due to an inability of voltage-operated Ca(2+) channels to be activated as KCl (75 mM) evoked depolarization and vasoconstriction at 120 mmHg. Nifedipine (1 microM) and cyclopiazonic acid (30 microM) significantly attenuated established myogenic tone, whereas inhibition of inositol 1,4,5-trisphosphate-mediated Ca(2+) release/entry by 2-aminoethoxydiphenylborate (50 microM) had little effect. Combinations of the Ca(2+) entry blockers with the sarcoplasmic reticulum (SR) inhibitor caused a total loss of tone, suggesting that while depolarization-mediated Ca(2+) entry makes a significant contribution to myogenic tone, an interaction between Ca(2+) entry and SR Ca(2+) release is necessary for maintenance of myogenic constriction. In contrast, none of the agents, in combination or alone, altered E(m), demonstrating the downstream role of Ca(2+) mobilization relative to changes in E(m). Large-conductance Ca(2+)-activated K(+) channels modulated E(m) to exert a small effect on myogenic tone, and consistent with this, skeletal muscle arterioles appeared to show an inherently steep relationship between E(m) and extent of myogenic tone. Collectively, skeletal muscle arterioles exhibit complex relationships between E(m), Ca(2+) availability, and myogenic constriction that impact on the tissue's physiological function.     

Increased myogenic responsiveness of skeletal muscle arterioles with juvenile growth

Previous studies from this laboratory suggest that during juvenile growth, structural changes in the arteriolar network are accompanied by changes in some of the mechanisms responsible for regulation of tissue blood flow. To test the hypothesis that arteriolar myogenic behavior is altered with growth, we studied gracilis muscle arterioles isolated from Sprague-Dawley rats at two ages: 21-28 and 42-49 days. When studied at their respective in vivo pressures, the myogenic index (instantaneous slope of the active pressure-diameter curve) of arterioles from 42-49-day-old rats was more negative than that of arterioles from 21-28-day-old rats, indicating greater myogenic responsiveness. Endothelial denudation, or prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) receptor antagonism without denudation, significantly reduced the myogenic responsiveness of arterioles from the older rats over a wide range of pressures but had no consistent effects on the myogenic responsiveness of arterioles from the younger rats. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride had no effect on the myogenic activity of arterioles from either age group. These findings indicate that microvascular growth in young animals is accompanied by an increase in the myogenic behavior of arterioles, possibly because PGH(2) or TxA(2) assumes a role in reinforcing myogenic activity over this period. As a result, the relative contribution of myogenic activity to blood flow regulation in skeletal muscle may increase during rapid juvenile growth.     

Radiotelemetric monitoring of blood pressure and mesenteric arterial bed responsiveness in rats with streptozotocin-induced diabetes

We investigated the changes in arterial blood pressure (BP) and of mesenteric arterial bed (MAB) responsiveness that accompany streptozotocin (STZ)-induced diabetes. BP was recorded by radiotelemetry in conscious animals before and during a 4-week period following induction of the diabetic state with STZ. At the end of this period, the MAB was isolated and perfused under constant flow conditions: perfusion pressure (PP, mmHg) was taken as an index of arteriolar tone. BP was lower (P < 0.05) in STZ-treated diabetic rats (82.9+/-5.0 mmHg) than in vehicle-treated rats (108.9+/-6.3 mmHg). Basal perfusion pressure of the MAB was lower in STZ-treated rats than in control rats and inhibition of nitric oxide (NO) synthesis with N(G)-nitro-L-arginine-methyl-ester and N(G)-nitro-L-arginine (100 microM each) failed to change this relationship. Increases in PP of MAB to phenylephrine (Phe), norepinephrine (NE), and potassium chloride (KCl) were reduced in STZ-treated rats compared with control rats. Inhibition of NO synthesis reduced responses to Phe, NE, and KCL in both STZ and control rats. The reduced responsiveness of STZ rats to Phe, NE, and KCl persisted after inhibition of NO synthesis. Acetylcholine (ACh) evoked relaxation of the MAB in a dose-dependent fashion. Maximal responses to ACh, but not sodium nitroprusside, were lower in STZ rats than in vehicle treated rats. Inhibition of NO synthesis reduced responses to ACh in both STZ and control rats. The reduced responsiveness of STZ rats to ACh persisted after inhibition of NO synthesis. The data demonstrate that STZ-induced diabetes is associated with a fall in blood pressure when pressure is recorded with radiotelemetry. The fall in blood pressure may be related to a non-specific decrease in responsiveness to vasoconstrictor stimuli mediated at least in part by NO-independent mechanisms. A decrease in responsiveness to endothelial dependent vasodilator mechanisms appeared insufficient to restore responsiveness to vasoconstrictor stimuli.     

Different roles of PKC and MAP kinases in arteriolar constrictions to pressure and agonists

Protein kinase C (PKC) and mitogen-activated protein (MAP) kinases have been implicated in the modulation of agonist-induced contractions of large vessels. However, their role in pressure- and agonist-induced constrictions of skeletal muscle arterioles, which have a major role in regulating peripheral resistance, is not clearly elucidated. Thus constrictions of isolated rat gracilis muscle arterioles (approximately 80 microm in diameter) to increases in intraluminal pressure and to norepinephrine (NE) or angiotensin II (ANG II) were assessed in the absence or presence of chelerythrine, PD-98058, and SB-203580 (inhibitors of PKC, p42/44 and p38 MAP kinase pathways, respectively). Arteriolar constriction to NE and ANG II were significantly reduced by chelerythrine (by approximately 90%) and unaffected by SB-203580, whereas PD-98058 decreased only ANG II-induced constrictions (by approximately 60%). Pressure-induced increases in wall tension (from 0.1 to 0.7 N/m) resulted in significant arteriolar constrictions (50% maximum) that were abolished by chelerythrine without altering smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)) (fura 2 microfluorimetry). PD-98058 and SB-203580 significantly decreased the magnitude of myogenic tone (by 20% and 60%, respectively) and reduced the sensitivity of the myogenic mechanism to wall tension, causing a significant rightward shift in the wall tension-myogenic tone relationship without affecting smooth muscle [Ca(2+)i]. MAP kinases were demonstrated with Western blotting. Thus in skeletal muscle arterioles 1) PKC is involved in both myogenic and agonist-induced constrictions, 2) PD-98058-sensitive p42/44 MAP kinases modulate both wall tension-dependent and ANG II-induced constrictions, whereas 3) a SB-203580-sensitive p38 MAP kinase pathway seems to be specifically involved in the mechanotransduction of wall tension.     

Nonuniform effects of endurance exercise training on vasodilation in rat skeletal muscle.

Endurance exercise training (Ex) has been shown to increase maximal skeletal muscle blood flow. The purpose of this study was to test the hypothesis that increased endothelium-dependent vasodilation is associated with the Ex-induced increase in muscle blood flow. Furthermore, we hypothesized that enhanced endothelium-dependent dilation is confined to vessels in high-oxidative muscles that are recruited during Ex. To test these hypotheses, sedentary (Sed) and rats that underwent Ex (30 m/min x 10% grade, 60 min/day, 5 days/wk, 8-12 wk) were studied using three experimental approaches. Training effectiveness was evidenced by increased citrate synthase activity in soleus and vastus lateralis (red section) muscles (P < 0.05). Vasodilatory responses to the endothelium-dependent agent acetylcholine (ACh) in situ tended to be augmented by training in the red section of gastrocnemius muscle (RG; Sed: control, 0.69 +/- 0.12; ACh, 1.25 +/- 0.15; Ex: control, 0.86 +/- 0.17; ACh, 1.76 +/- 0.27 ml x min(-1) x 100 g(-1) x mmHg(-1); 0.05 < P < 0.10 for Ex vs. Sed during ACh). Responses to ACh in situ did not differ between Sed and Ex for either the soleus muscle or white section of gastrocnemius muscle (WG). Dilatory responses of second-order arterioles from the RG in vitro to flow (4-8 microl/min) and sodium nitroprusside (SNP; 10(-7) through 10(-4) M), but not ACh, were augmented in Ex (vs. Sed; P < 0.05). Dilatory responses to ACh, flow, and SNP of arterioles from soleus and WG muscles did not differ between Sed and Ex. Content of the endothelial isoform of nitric oxide synthase (eNOS) was increased in second-order, fourth-order, and fifth-order arterioles from the RG of Ex; eNOS content was similar between Sed and Ex in vessels from the soleus and WG muscles. These findings indicate that Ex induces endothelial adaptations in fast-twitch, oxidative, glycolytic skeletal muscle. These adaptations may contribute to enhanced skeletal muscle blood flow in endurance-trained individuals.     

Adrenergic control of vascular resistance varies in muscles composed of different fiber types: influence of the vascular endothelium

The influence of the sympathetic nervous system (SNS) upon vascular resistance is more profound in muscles comprised predominately of low-oxidative type IIB vs. high-oxidative type I fiber types. However, within muscles containing high-oxidative type IIA and IIX fibers, the role of the SNS on vasomotor tone is not well established. The purpose of this study was to examine the influence of sympathetic neural vasoconstrictor tone in muscles composed of different fiber types. In adult male rats, blood flow to the red and white portions of the gastrocnemius (Gast(Red) and Gast(White), respectively) and the soleus muscle was measured pre- and postdenervation. Resistance arterioles from these muscles were removed, and dose responses to α?-phenylephrine or α?-clonidine adrenoreceptor agonists were determined with and without the vascular endothelium. Denervation resulted in a 2.7-fold increase in blood flow to the soleus and Gast(Red) and an 8.7-fold increase in flow to the Gast(White). In isolated arterioles, α?-mediated vasoconstriction was greatest in Gast(White) (～50%) and less in Gast(Red) (～31%) and soleus (～17%); differences among arterioles were abolished with the removal of the endothelium. There was greater sensitivity to α(1)-mediated vasoconstriction in the Gast(White) and Gast(Red) vs. the soleus, which was independent of whether the endothelium was present. These data indicate that 1) control of vascular resistance by the SNS in high-oxidative, fast-twitch muscle is intermediate to that of low-oxidative, fast-twitch and high-oxidative, slow-twitch muscles; and 2) the ability of the SNS to control blood flow to low-oxidative type IIB muscle appears to be mediated through postsynaptic α?- and α?-adrenoreceptors on the vascular smooth muscle.     

alphavbeta3- and alpha5beta1-integrin blockade inhibits myogenic constriction of skeletal muscle resistance arterioles

In isolated resistance arterioles with spontaneous tone, ligation of alpha4beta1- and alpha5beta1-integrins induces vasoconstriction whereas ligation of alphavbeta3-integrin induces vasodilation. However, whether integrins directly participate in myogenic constriction to pressure elevation is not known. To answer this question, isolated rat skeletal muscle arterioles were exposed to step increments in pressure in the absence or presence of peptides and function-blocking antibodies known to bind alpha4beta1-, alpha5beta1-, or alphavbeta3-integrins while vessel diameter was continually monitored. Myogenic constriction, as assessed by the ability of isolated arterioles to reduce their diameter in response to two consecutive increments in intraluminal pressure (90-110 and 110-130 cmH2O), was not affected by treatment with any of the control peptides (RAD, LEV), a control antibody (anti-rat major histocompatibility complex), an alpha4beta1-integrin-binding peptide (LDV), or an anti-alpha4-integrin antibody. In contrast, alpha5beta1-integrin blockade with either anti-alpha5- or anti-beta1-integrin antibody caused a significant inhibition of myogenic constriction. Also, both RGD peptide and anti-beta3-integrin antibody inhibited myogenic constriction. These results indicate that alpha5beta1- and alphavbeta3-integrins are necessary for myogenic constriction and further suggest that integrins are part of the mechanosensory apparatus responsible for the ability of vascular smooth muscle cells to detect and/or respond to changes in intraluminal pressure.