Flow-induced dilation of skeletal muscle feed arteries: relevance to exercise hyperemia

During exercise, an increase in blood flow to working skeletal muscle is accomplished by dilation of arteries and arterioles supplying the muscle. Arterioles, located within contracting muscle, are exposed to dilatory metabolites released by the muscle; however, the mechanism by which feed arteries, located external to the muscle, dilate is still unknown. One potential mechanism for feed artery dilation is flow-induced dilation, occurring when arteries dilate in response to increased vascular wall shear stress. Shear stress is the frictional force between blood and the arterial wall, which increases when blood flow velocity increases. Data from previous in vitro experiments (8) indicate that flow-induced dilation in rat soleus feed arteries occurs at blood flow levels that are far less than normal resting blood flow in conscious rats. This data led to the conclusion that flow-induced dilation was not a plausible mechanism to explain the increase in blood flow during exercise. Furthermore, the soleus muscle is primarily composed of slow-oxidative fibers and used in maintaining posture; thus, it receives a substantial amount of blood flow at rest. We sought to test whether flow-induced dilation could contribute to exercise hyperemia in rat extensor digitorum longus muscle, primarily composed of fast-glycolytic fibers, and rat gastrocnemius, a muscle of mixed fiber type (4). The differences in fiber type of each muscle may be a factor in how the feed arteries dilate during exercise. The purpose of this study was to determine if flow-induced dilation potentially contributes to exercise hyperemia in rat extensor digitorum longus and gastrocnemius muscle feed arteries, EDLFA and GFA, respectively. In this study, blood flow was induced through the arteries and corresponding flow measurements (µl/min) were collected. The flow values were used to calculate intraluminal wall shear stress in the arteries and then compared to calculated in vivo shear stress values from previously published studies (1,2,3,7,10,11,12,13,14,15). We hypothesized that flow-induced dilation in GFA and EDLFA occurs at shear stress values lower than the shear stress normally present in non-exercising rats. This would preclude flow-induced dilation from causing the dilation of feed arteries to gastrocnemius and EDL muscles in exercise

Effect of Shear Stress on ecNOS Expression and Dilation in Soleus Feed Arteries

Shear stress causes artery dilation and increased expression of endothelial cell nitric oxide synthase (ecNOS) in coronary and placental arteries. We sought to determine the importance of shear stress in maintaining normal dilation and normal levels of ecNOS in rat soleus feed arteries (SFA). SFA were isolated from male Sprague-Dawley rats and cannulated for in vitro microscopy (Fig. 6). SFA were exposed to no shear stress, low shear stress, or high shear stress conditions for 4 hours. After 4 hours, endothelium-dependent dilation (acetylcholine: ACh) and endothelium-independent dilation (sodium nitroprusside: SNP) were tested. Arteries were then uncannulated, mRNA was isolated, and RT-PCR for ecNOS mRNA was performed to determine whether shear stress altered ecNOS gene expression. Shear stress did not alter dilation to ACh, but dilation to SNP was greater in the high shear stress arteries. ecNOS mRNA content was greater in high shear stress arteries than low shear stress arteries. These data indicate that altered wall shear stress conditions alter ecNOS gene expression and vascular smooth muscle cell function

Positional differences in reactive hyperemia provide insight into initial phase of exercise hyperemia.

Studies have reported a greater blood flow response to muscle contractions when the limb is below the heart compared with above the heart, and these results have been interpreted as evidence for a skeletal muscle pump contribution to exercise hyperemia. If limb position affects the blood flow response to other vascular challenges such as reactive hyperemia, this interpretation may not be correct. We hypothesized that the magnitude of reactive hyperemia would be greater with the limb below the heart. Brachial artery blood flow (Doppler ultrasound) and blood pressure (finger-cuff plethysmography) were measured in 10 healthy volunteers. Subjects lay supine with one arm supported in two different positions: above or below the heart. Reactive hyperemia was produced by occlusion of arterial inflow for varying durations: 0.5 min, 1 min, 2 min, or 5 min in randomized order. Peak increases in blood flow were 77 ± 11, 178 ± 24, 291 ± 25, and 398 ± 33 ml/min above the heart and 96 ± 19, 279 ± 62, 550 ± 60, and 711 ± 69 ml/min below the heart (P \u3c 0.05). Thus a standard stimulus (vascular occlusion) elicited different responses depending on limb position. To determine whether these differences were due to mechanisms intrinsic to the arterial wall, a second set of experiments was performed in which acute intraluminal pressure reduction for 0.5 min, 1 min, 2 min, or 5 min was performed in isolated rat soleus feed arteries (n = 12). The magnitude of dilation upon pressure restoration was greater when acute pressure reduction occurred from 85 mmHg (mimicking pressure in the arm below the heart; 28.3 ± 7.9, 37.5 ± 5.9, 55.1 ± 9.9, and 68.9 ± 8.6% dilation) than from 48 mmHg (mimicking pressure in the arm above the heart; 20.8 ± 4.8, 22.6 ± 4.4, 31.2 ± 5.8, and 49.2 ± 7.1% dilation). These data support the hypothesis that arm position differences in reactive hyperemia are at least partially mediated by mechanisms intrinsic to the arterial wall. Overall, these results suggest the need to reevaluate studies employing positional changes to examine muscle pump influences on exercise hyperemia

The effect of aerobic exercise training on endothelium-dependent dilation

The vascular endothelium is important for the regulation of blood pressure and blood flow, and impaired endothelial function is associated with increased risk of heart attack and stroke. Numerous studies have examined whether exercise training improves endothelium-dependent dilation, but the results have been inconclusive, with studies showing both improvement and no effect. Therefore, we undertook a systematic review of the literature and a meta-analysis to test the hypothesis that aerobic exercise training improves endothelium-dependent dilation in different subject populations. The Embase, Medline, Scopus, and Cochrane databases were searched using search phrases that included the terms “endothelium”, “nitric oxide”, “exercise training”, and “physical activity”. These search phrases returned 9,709 articles. Of these resultant articles, 832 titles were selected for abstract screening. Articles were rejected at this point if they did not mention aerobic exercise or clearly did not pertain to endothelium-dependent dilation. In addition, 41 more titles were selected for abstract screening after searching the references of selected reviews, resulting in 873 total abstracts. During abstract screening, papers were discarded if they lacked a control group, did not specifically measure endothelium-dependent dilation, or did not include aerobic exercise training. Following abstract screening, 331 papers remained that met all inclusion criteria. During our literature search, all population types were accepted. This included humans, animals, young, old, healthy, and those with pathologies such as diabetes, heart disease, stroke, hyperlipidemia, hypercholesterolemia, or hypertension. We intend to eventually address all of these different conditions. However, as this comprehensive population is too large for a single meta-analysis, we are presently aiming to determine the effects of training on endothelium-dependent dilation by applying meta-analysis to a single subgroup

Do ATP and hydrogen peroxide cause sympatholysis in rat soleus feed arteries?

Arteries and arterioles constrict less to sympathetic stimulation in contracting compared to resting skeletal muscle (sympatholysis). There is some uncertainty regarding the specific agents causing sympatholysis. We have shown that acidosis, but not shear stress, potassium, or adenosine, is sympatholytic in feed arteries from the predominantly slow twitch soleus muscle. Interstitial fluid concentrations of both adenosine triphosphate (ATP) and hydrogen peroxide (H2O2) increase in contracting skeletal muscle, and we hypothesized that ATP and H2O2 are sympatholytic. Soleus feed arteries (n = 6 per group) were isolated from male Sprague-Dawley rats and cannulated on two glass micropipettes for in vitro videomicroscopy. We measured the constriction response to the α-1 adrenergic receptor agonist phenylephrine (PE; 10-9 M to 10-4 M, 0.5 log increments) in the presence of varying physiological levels of ATP (0 uM, 1 uM, 10 uM, and 100 uM) and H2O2 (0 uM, 1 uM, 10 uM, and 100 uM). Our data indicate no significant difference in PE-induced constriction between levels of ATP (maximum constriction 64.1 10.4 % vs. 74.4 8.1 %, 68.9 8.7 %, and 66.1 11.4 %) or H2O2 (maximum constriction 76.9 9.2 % vs. 77.5 5.7 %, 79.7 1.8 %, and 76.5 8.0 %). We conclude that neither ATP nor H2O2 independently cause sympatholysis of α-1 adrenergic receptors in rat soleus feed arteries

Arteriolar vasodilation involves actin depolymerization

It is generally assumed that relaxation of arteriolar vascular smooth muscle occurs through hyperpolarization of the cell membrane, reduction in intracellular Ca concentration, and activation of myosin light chain phosphatase/ inactivation of myosin light chain kinase. We hypothesized that vasodilation is related to depolymerization of F-actin. Cremaster muscles were dissected in rats under pentobarbital sodium anesthesia (50 mg/kg). First-order arterioles were dissected, cannulated on glass micropipettes, pressurized, and warmed to 34°C. Internal diameter was monitored with an electronic video caliper. The concentration of G-actin was determined in flash-frozen intact segments of arterioles by ultracentrifugation and Western blot analyses. Arterioles dilated by ~40% of initial diameter in response to pinacidil (1 × 10 mM) and sodium nitroprusside (5 × 10 mM). The G-actin-to-smooth muscle 22α ratio was 0.67 ± 0.09 in arterioles with myogenic tone and increased significantly to 1.32 ± 0.34 (P \u3c 0.01) when arterioles were dilated with pinacidil and 1.14 ± 0.18 (P \u3c 0.01) with sodium nitroprusside, indicating actin depolymerization. Compared with control vessels (49 ± 5%), the percentage of phosphorylated myosin light chain was significantly reduced by pinacidil (24 ± 2%, P \u3c 0.01) but not sodium nitroprusside (42 ± 4%). These findings suggest that actin depolymerization is an important mechanism for vasodilation of resistance arterioles to external agonists. Furthermore, pinacidil produces smooth muscle relaxation via both decreases in myosin light chain phosphorylation and actin depolymerization, whereas sodium nitroprusside produces smooth muscle relaxation primarily via actin depolymerization. NEW & NOTEWORTHY This article adds to the accumulating evidence on the contribution of the actin cytoskeleton to the regulation of vascular smooth muscle tone in resistance arterioles. Actin depolymerization appears to be an important mechanism for vasodilation of resistance arterioles to pharmacological agonists. Dilation to the K channel opener pinacidil is produced by decreases in myosin light chain phosphorylation and actin depolymerization, whereas dilation to the nitric oxide donor sodium nitroprusside occurs primarily via actin depolymerization. 2+ -6 -5

Positional differences in reactive hyperemia provide insight into initial phase of exercise hyperemia

© 2015 the American Physiological Society. Studies have reported a greater blood flow response to muscle contractions when the limb is below the heart compared with above the heart, and these results have been interpreted as evidence for a skeletal muscle pump contribution to exercise hyperemia. If limb position affects the blood flow response to other vascular challenges such as reactive hyperemia, this interpretation may not be correct. We hypothesized that the magnitude of reactive hyperemia would be greater with the limb below the heart. Brachial artery blood flow (Doppler ultrasound) and blood pressure (finger-cuff plethysmography) were measured in 0 healthy volunteers. Subjects lay supine with one arm supported in two different positions: above or below the heart. Reactive hyperemia was produced by occlusion of arterial inflow for varying durations: 0.5 min, 1 min, 2 min, or 5 min in randomized order. Peak increases in blood flow were 77 ±11, 178 ±24, 291 ±25, and 398 ±33 ml/min above the heart and 96 ±19, 279 ±62, 550 ±60, and 711 ±69 ml/min below the heart (P \u3c0.05). Thus a standard stimulus (vascular occlusion) elicited different responses depending on limb position. To determine whether these differences were due to mechanisms intrinsic to the arterial wall, a second set of experiments was performed in which acute intraluminal pressure reduction for 0.5 min, 1 min, 2 min, or 5 min was performed in isolated rat soleus feed arteries (n=12). he magnitude of dilation upon pressure restoration was greater when acute pressure reduction occurred from 85 mmHg (mimicking pressure in the arm below the heart; 28.3 ±7.9, 37.5 ±5.9, 55.1 ±9.9, and 68.9 ±8.6% dilation) than from 48 mmHg (mimicking pressure in the arm above the heart; 20.8 ±4.8, 22.6 ±4.4, 31.2 ±5.8, and 49.2 ±7.1% dilation). These data support the hypothesis that arm position differences in reactive hyperemia are at least partially mediated by mechanisms intrinsic to the arterial wall. Overall, these results suggest the need to reevaluate studies employing positional changes to examine muscle pump influences on exercise hyperemia