Regulation of in situ skeletal muscle arteriolar tone: interactions between two parameters

Regulation of Skeletal Muscle Resistance Arteriolar Tone: Integration of Multiple Mechanisms

INTRODUCTION

Physiological system complexity represents an imposing challenge to gaining insight into how arteriolar behavior emerges. Further, mechanistic complexity in arteriolar tone regulation requires that a systematic determination of how these processes interact to alter vascular diameter be undertaken.

METHODS

The present study evaluated the reactivity of ex vivo proximal and in situ distal resistance arterioles in skeletal muscle with challenges across the full range of multiple physiologically relevant stimuli and determined the stability of responses over progressive alterations to each other parameter. The five parameters chosen for examination were (1) metabolism (adenosine concentration), (2) adrenergic activation (norepinephrine concentration), (3) myogenic activation (intravascular pressure), (4) oxygen (superfusate PO2), and (5) wall shear rate (altered intraluminal flow). Vasomotor tone of both arteriole groups following challenge with individual parameters was determined; subsequently, responses were determined following all two- and three-parameter combinations to gain deeper insight into how stimuli integrate to change arteriolar tone. A hierarchical ranking of stimulus significance for establishing arteriolar tone was performed using mathematical and statistical analyses in conjunction with machine learning methods.

RESULTS

Results were consistent across methods and indicated that metabolic and adrenergic influences were most robust and stable across all conditions. While the other parameters individually impact arteriolar tone, their impact can be readily overridden by the two dominant contributors.

CONCLUSION

These data suggest that mechanisms regulating arteriolar tone are strongly affected by acute changes to the local environment and that ongoing investigation into how microvessels integrate stimuli regulating tone will provide a more thorough understanding of arteriolar behavior emergence across physiological and pathological states.

Effects of hyperoxia on vascular tone in animal models: systematic review and meta-analysis

BACKGROUND

Arterial hyperoxia may induce vasoconstriction and reduce cardiac output, which is particularly undesirable in patients who already have compromised perfusion of vital organs. Due to the inaccessibility of vital organs in humans, vasoconstrictive effects of hyperoxia have primarily been studied in animal models. However, the results of these studies vary substantially. Here, we investigate the variation in magnitude of the hyperoxia effect among studies and explore possible sources of heterogeneity, such as vascular region and animal species.

METHOD

Pubmed and Embase were searched for eligible studies up to November 2017. In vivo and ex vivo animal studies reporting on vascular tone changes induced by local or systemic normobaric hyperoxia were included. Experiments with co-interventions (e.g. disease or endothelium removal) or studies focusing on lung, brain or fetal vasculature or the ductus arteriosus were not included. We extracted data pertaining to species, vascular region, blood vessel characteristics and method of hyperoxia induction. Overall effect sizes were estimated with a standardized mean difference (SMD) random effects model.

RESULTS

We identified a total of 60 studies, which reported data on 67 in vivo and 18 ex vivo experiments. In the in vivo studies, hyperoxia caused vasoconstriction with an SMD of - 1.42 (95% CI - 1.65 to - 1.19). Ex vivo, the overall effect size was SMD - 0.56 (95% CI - 1.09 to - 0.03). Between-study heterogeneity (I2) was high for in vivo (72%, 95% CI 62 to 85%) and ex vivo studies (86%, 95% CI 78 to 98%). In vivo, in comparison to the overall effect size, hyperoxic vasoconstriction was less pronounced in the intestines and skin (P = 0.03) but enhanced in the cremaster muscle region (P < 0.001). Increased constriction was seen in vessels 15-25 μm in diameter. Hyperoxic constriction appeared to be directly proportional to oxygen concentration. For ex vivo studies, heterogeneity could not be explained with subgroup analysis.

CONCLUSION

The effect of hyperoxia on vascular tone is substantially higher in vivo than ex vivo. The magnitude of the constriction is most pronounced in vessels ~ 15-25 μm in diameter and is proportional to the level of hyperoxia. Relatively increased constriction was seen in muscle vasculature, while reduced constriction was seen in the skin and intestines.

Potassium inhibits nitric oxide and adenosine arteriolar vasodilatation via K(IR) and Na(+)/K(+) ATPase: implications for redundancy in active hyperaemia

Redundancy, in active hyperaemia, where one vasodilator can compensate for another if the first is missing, would require that one vasodilator inhibits the effects of another; therefore, if the first vasodilator is inhibited, its inhibitory influence on the second vasodilator is removed and the second vasodilator exerts a greater vasodilatory effect. We aimed to determine whether vasodilators relevant to skeletal muscle contraction [potassium chloride (KCl), adenosine (ADO) and nitric oxide] inhibit one another and, in addition, to investigate the mechanisms for this interaction. We used the hamster cremaster muscle and intravital microscopy to directly visualize 2A arterioles when exposed to a range of concentrations of one vasodilator [10(-8) to 10(-5) M S-nitroso-N-acetyl penicillamine (SNAP), 10(-8) to 10(-5) M ADO, 10 and 20 mM KCl] in the absence and then in the presence of a second vasodilator (10(-7) M ADO, 10(-7) M SNAP, 10 mM KCl). We found that KCl significantly attenuated SNAP-induced vasodilatations by ∼65.8% and vasodilatations induced by 10(-8) to 10(-6) M ADO by ∼72.8%. Furthermore, we observed that inhibition of KCl vasodilatation, by antagonizing either Na(+)/K(+) ATPase using ouabain or inward rectifying potassium channels using barium chloride, could restore the SNAP-induced vasodilatation by up to ∼53.9% and 30.6%, respectively, and also restore the ADO-induced vasodilatations by up to ∼107% and 76.7%, respectively. Our data show that vasodilators relevant to muscle contraction can interact in a way that alters the effectiveness of other vasodilators. These data suggest that active hyperaemia may be the result of complex interactions between multiple vasodilators via a redundant control paradigm.

Significance of K(ATP) channels, L-type Ca²⁺ channels and CYP450-4A enzymes in oxygen sensing in mouse cremaster muscle arterioles in vivo

Background

ATP-sensitive K⁺ channels (K_ATP channels), NO, prostaglandins, 20-HETE and L-type Ca²⁺ channels have all been suggested to be involved in oxygen sensing in skeletal muscle arterioles, but the role of the individual mechanisms remain controversial. We aimed to establish the importance of these mechanisms for oxygen sensing in arterioles in an in vivo model of metabolically active skeletal muscle. For this purpose we utilized the exteriorized cremaster muscle of anesthetized mice, in which the cremaster muscle was exposed to controlled perturbation of tissue PO₂.

Results

Change from “high” oxygen tension (PO₂ = 153.4 ± 3.4 mmHg) to “low” oxygen tension (PO₂ = 13.8 ± 1.3 mmHg) dilated cremaster muscle arterioles from 11.0 ± 0.4 μm to 32.9 ± 0.9 μm (n = 28, P < 0.05). Glibenclamide (K_ATP channel blocker) caused maximal vasoconstriction, and abolished the dilation to low oxygen, whereas the K_ATP channel opener cromakalim caused maximal dilation and prevented the constriction to high oxygen. When adding cromakalim on top of glibenclamide or vice versa, the reactivity to oxygen was gradually restored. Inhibition of L-type Ca²⁺ channels using 3 μM nifedipine did not fully block basal tone in the arterioles, but rendered them unresponsive to changes in PO₂. Inhibition of the CYP450-4A enzyme using DDMS blocked vasoconstriction to an increase in PO₂, but had no effect on dilation to low PO₂.

Conclusions

We conclude that: 1) L-type Ca²⁺ channels are central to oxygen sensing, 2) K_ATP channels are permissive for the arteriolar response to oxygen, but are not directly involved in the oxygen sensing mechanism and 3) CYP450-4A mediated 20-HETE production is involved in vasoconstriction to high PO₂.

Computational analyses of intravascular tracer washout reveal altered capillary-level flow distributions in obese Zucker rats

Intravascular tracer washout data obtained from gastrocnemius muscle of lean Zucker rats (LZRs) and obese Zucker rats (OZRs) were analysed to investigate flow distributions in the OZR, a model of non-atherosclerotic peripheral vascular disease. A computer model used to simulate the network washout curves was developed based on experimentally observed relative dispersions in large vessels and asymmetrical flow distributions at bifurcations in dichotomous microvascular networks. The model results of simulations were compared to experimental washout data of (125)I-labelled albumin, an intravascular tracer, to uncover flow distributions on the arterial-network and capillary levels. The lean and obese Zucker rats demonstrated distinct capillary-level flow distributions, with higher dispersion and significantly more low-flow capillaries in the OZRs than in the LZRs. Targeted pharmacological treatments against identified sites of vascular dysfunction in OZRs (adrenoreceptor blockade with phentolamine, antioxidant treatment with Tempol and thromboxane receptor antagonism with SQ-29548) were shown to improve the capillary-level flow distributions in treated OZRs toward distributions determined in control LZRs. Combination therapy with multiple pharmacological interventions resulted in a greater degree of recovery. This study demonstrates that the enhanced perfusion heterogeneity at arteriole bifurcations is a potential mechanism underlying perfusion-demand mismatching in OZRs, and suggests that amelioration of this dysfunction must involve a multi-faceted interventional approach.

Oxygen sensing and conducted vasomotor responses in mouse cremaster arterioles in situ

This study examines mechanisms by which changes in tissue oxygen tension elicit vasomotor responses and whether localized changes in oxygen tension initiates conducted vasomotor responses in mouse cremaster arterioles. Intravital microscopy was used to visualize the mouse cremaster microcirculation. The cremaster was superfused with Krebs' solution with different oxygen tensions, and a gas exchange chamber was used to induce localized changes in oxygen tension. In arterioles where red blood cells were removed by buffer perfusion, arterioles responded with same magnitudes of vasodilatation (DeltaD = 16.0 +/- 4.9 microm) when changing from high (PO(2) = 242.5 +/- 13.3 mm Hg) to low (PO(2) = 22.5 +/- 4.8 mm Hg) oxygen tension as seen in the intact cremaster circulation (DeltaD = 18.7 +/- 1.0 microm). Blockade of NO synthases by L: -NAME and adenosine receptors by DPCPX had no effects on vasomotor responses to low or high oxygen. Induction of localized low (PO(2) = 23.3 +/- 5.7 mmHg) or high (PO(2) = 300.0 +/- 25.7 mm Hg) oxygen tension caused vasodilatation or -constriction locally and at a site 1,000 microm upstream (distantly). Glibenclamide blocker of ATP-sensitive K(+) channels inhibited vasodilatation and -constriction to low (PO(2) = 16.0 +/- 6.4 mm Hg) and high (PO(2) = 337.4 +/- 12.8 mm Hg) oxygen tension. 1) ATP-sensitive K(+) channels seem to mediate, at least in part, vasodilatation and vasoconstriction to low and high oxygen tension; 2) Red blood cells are not necessary for inducing vasodilatation and vasoconstriction to low or high oxygen tension; 3) localized changes in the oxygen tension cause vasomotor responses, which are conducted upstream along arterioles in mouse cremaster microcirculation.

Theoretical models for regulation of blood flow

Blood-flow rate in the normal microcirculation is regulated to meet the metabolic demands of the tissues, which vary widely with position and with time, but is relatively unaffected by changes of arterial pressure over a considerable range. The regulation of blood flow is achieved by the combined effects of multiple interacting mechanisms, including sensitivity to pressure, flow rate, metabolite levels, and neural signals. The main effectors of flow regulation, the arterioles and small arteries, are located at a distance from the regions of tissue that they supply. Flow regulation requires the sensing of metabolic and hemodynamic conditions and the transfer of information about tissue metabolic status to upstream vessels. Theoretical approaches can contribute to the understanding of flow regulation by providing quantitative descriptions of the mechanisms involved, by showing how these mechanisms interact in networks of interconnected microvessels supplying metabolically active tissues, and by establishing relationships between regulatory processes occurring at the microvascular level and variations of metabolic activity and perfusion in whole tissues. Here, a review is presented of previous and current theoretical approaches for investigating the regulation of blood flow in the microcirculation.

Nitric oxide and muscle blood flow in exercise

Despite being the subject of investigation for well over 100 years, the nature of exercising muscle blood flow control remains, in many respects, poorly understood. In this review we focus on the potential role of nitric oxide in vasodilation of muscle resistance vessels during a bout of exercise. Its contribution is explored in the context of whether it contributes to steady-state exercise hyperemia, the dynamic adjustment of muscle blood flow to exercise, or the modulation of sympathetic vasoconstriction in exercising muscle. It appears that the obligatory role of nitric oxide in all three of these categories is modest at best. The elucidation of the integrated nature of exercise hyperemia control in terms of synergy and redundancy of mechanism interaction remains in its infancy, and much more remains to be learned about the role of nitric oxide in this type of integrated control.

Impaired hemorrhage tolerance in the obese Zucker rat model of metabolic syndrome

As obese Zucker rats (OZR) manifesting the metabolic syndrome exhibit enhanced vascular adrenergic constriction and potentially an enhanced adrenergic activity vs. lean Zucker rats (LZR), this study tested the hypothesis that OZR exhibit an improved tolerance to progressive hemorrhage. Preliminary experiments indicated that, corrected for body mass, total blood volume was reduced in OZR vs. LZR. Anesthetized LZR and OZR had a cremaster muscle prepared for in situ videomicroscopy and had renal, splanchnic, hindlimb, and skeletal muscle perfusion monitored with flow probes. Arterial pressure, arteriolar reactivity to norepinephrine, and tissue/organ perfusion were monitored after either infusion of phentolamine or successive withdrawals of 10% total blood volume. Phentolamine infusion indicated that regional adrenergic tone under control conditions differs substantially between LZR and OZR, whereas with hemorrhage OZR exhibit decompensation in arterial pressure before LZR. Renal, distal hindlimb, and skeletal muscle perfusion decreased more rapidly and to a greater extent in OZR vs. LZR after hemorrhage. In contrast, hemorrhage-induced reductions in splanchnic perfusion in OZR lagged behind those in LZR, although a similar maximum reduction was ultimately attained. With increasing hemorrhage, cremasteric arteriolar tone increased more in OZR than LZR, and this increase in active tone was entirely due to an elevated adrenergic contribution. Norepinephrine-induced arteriolar constriction was greater in OZR vs. LZR under control conditions and during hemorrhage, with arterioles from OZR demonstrating early closure vs. LZR. These results suggest that a combination of reduced blood volume and elevated peripheral adrenergic constriction contribute to impaired hemorrhage tolerance in OZR.

Enhanced skeletal muscle arteriolar reactivity to ANG II after recovery from ischemic acute renal failure

In addition to the long-term renal complications, previous studies suggested that after acute renal failure (ARF), rats manifest an increased pressor response to an overnight infusion of ANG II. The present study tested whether recovery from ARF results in alterations in sensitivity to the peripheral vasculature. ARF was induced in Sprague-Dawley rats by 45 min of bilateral renal ischemia and reperfusion. Animals were allowed to recover renal structure and function for 5-8 wk, after which the acute pressor responses to ANG II were evaluated either in vivo in in situ skeletal muscle arterioles or in isolated gracilis muscle arteries in vitro. Baseline arterial pressure was not different in ARF rats vs. sham-operated controls, although ARF rats exhibited an enhanced pressor response to bolus ANG II infusion compared with control rats. Steady-state plasma ANG II concentration and plasma renin activity were similar between ARF and control rats. Constrictor reactivity of in situ cremasteric arterioles from ARF rats was enhanced in response to increasing concentrations of ANG II; however, no difference was observed in arteriolar responses to elevated PO2, norepinephrine, acetylcholine, or sodium nitroprusside. Isolated gracilis muscle arteries from ARF rats also showed increased vasoconstriction in response to ANG II but not norepinephrine. In conclusion, recovery from ischemic ARF is not associated with hypertension but is associated with increased arteriolar constrictor reactivity to ANG II. Although the mechanisms of this altered responsiveness are unclear, such changes may relate, in part, to cardiovascular complications in patients with ARF and/or after renal transplant.

Oxidant stress and constrictor reactivity impair cerebral artery dilation in obese Zucker rats

This study tested the hypothesis that evolution of the metabolic syndrome in obese Zucker rats (OZR) leads to impaired dilator reactivity of cerebral resistance arteries vs. responses determined in lean Zucker rats (LZR). Middle cerebral arteries (MCA) from 17-wk-old male LZR and OZR were isolated and cannulated with glass micropipettes. Vascular reactivity was assessed in response to challenge with ACh, sodium nitroprusside (SNP), reductions and elevations in Po2, 5-HT, and increased intralumenal pressure. Vessels were treated with the free radical scavenger 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (tempol) to assess the role of superoxide production in altering reactivity, and passive vascular wall mechanics was assessed in each vessel. Vascular superoxide production was assessed in isolated arteries using fluorescence microscopy. Vessel dilation to ACh and hypoxia was impaired in OZR vs. LZR, although responses to SNP were normal. Vessel constriction to 5-HT, elevated Po2, and elevated intralumenal pressure was enhanced in OZR vs. LZR. Fluorescence microscopy demonstrated an increased superoxide production in arteries of OZR vs. LZR, correctable by incubation with tempol. Although treatment of vessels from OZR with tempol improved dilation to ACh and hypoxia, constrictor responses to 5-HT, elevated Po2, and pressure were not altered by tempol treatment. Indexes of vessel wall mechanics were comparable between groups. These results suggest that vasodilator reactivity of MCA of OZR in response to endothelium-dependent dilator stimuli is impaired vs. LZR and that this may represent a reduced bioavailability of signaling molecules due to oxidant scavenging. However, oxidative stress-independent increases in myogenic tone and constrictor reactivity may contribute to blunted dilator responses of cerebral microvessels.

Enhanced arteriolar α-adrenergic constriction impairs dilator responses and skeletal muscle perfusion in obese Zucker rats

The present study tested the hypothesis that enhanced vascular α-adrenergic constriction in obese Zucker rats (OZR) impairs arteriolar dilation and perfusion of skeletal muscle at rest and with increased metabolic demand. In lean Zucker rats (LZR) and OZR, isolated gracilis arterioles were viewed via television microscopy, and the contralateral cremaster muscle or gastrocnemius muscle was prepared for study in situ. Gracilis and cremasteric arterioles were challenged with dilator stimuli under control conditions and after blockade of α-adrenoreceptors with prazosin, phentolamine, or yohimbine. Gastrocnemius muscles performed isometric twitch contractions of increasing frequency, and perfusion was continuously monitored. In OZR, dilator responses of arterioles to hypoxia (gracilis), wall shear rate (cremaster), acetylcholine, and iloprost (both) were impaired vs. LZR. Treatment with prazosin and phentolamine (and in cremasteric arterioles only, yohimbine) improved arteriolar reactivity to these stimuli in OZR, although responses remained impaired vs. LZR. Gastrocnemius muscle blood flow was reduced at rest in OZR; this was corrected with intravenous infusion of phentolamine or prazosin. At all contraction frequencies, blood flow was reduced in OZR vs. LZR; this was improved by infusion of phentolamine or prazosin at low-moderate metabolic demand only (1 and 3 Hz). At 5 Hz, adrenoreceptor blockade did not alter blood flow in OZR from levels in untreated rats. These results suggest that enhanced α-adrenergic constriction of arterioles of OZR contributes to impaired dilator responses and reduced muscle blood flow at rest and with mild-moderate (although not with large) elevations in metabolic demand.