Christopher Hearon

Regulation of skeletal muscle blood flow during exercise in ageing humans

The regulation of skeletal muscle blood flow and oxygen delivery to contracting skeletal muscle is complex and involves the mechanical effects of muscle contraction; local metabolic, red blood cell and endothelium‐derived substances; and the sympathetic nervous system (SNS). With advancing age in humans, skeletal muscle blood flow is typically reduced during dynamic exercise and this is due to a lower vascular conductance, which could ultimately contribute to age‐associated reductions in aerobic exercise capacity, a primary predictor of mortality in both healthy and diseased ageing populations. Recent findings have highlighted the contribution of endothelium‐derived substances to blood flow control in contracting muscle of older adults. With advancing age, impaired nitric oxide availability due to scavenging by reactive oxygen species, in conjunction with elevated vasoconstrictor signalling via endothelin‐1, reduces the local vasodilatory response to muscle contraction. Additionally, ageing impairs the ability of contracting skeletal muscle to blunt sympathetic vasoconstriction (i.e. ‘functional sympatholysis’), which is critical for the proper regulation of tissue blood flow distribution and oxygen delivery, and could further reduce skeletal muscle perfusion during high intensity and/or large muscle mass exercise in older adults. We propose that initiation of endothelium‐dependent hyperpolarization is the underlying signalling event necessary to properly modulate sympathetic vasoconstriction in contracting muscle, and that age‐associated impairments in red blood cell adenosine triphosphate release and stimulation of endothelium‐dependent vasodilatation may explain impairments in both local vasodilatation and functional sympatholysis with advancing age in humans.

Contracting human skeletal muscle maintains the ability to blunt α1-adrenergic vasoconstriction during KIR channel and Na+/K+-ATPase inhibition

During exercise there is a balance between vasoactive factors that facilitate increases in blood flow and oxygen delivery to the active tissue and the sympathetic nervous system, which acts to limit muscle blood flow for the purpose of blood pressure regulation. Functional sympatholysis describes the ability of contracting skeletal muscle to blunt the stimulus for vasoconstriction, yet the underlying signalling of this response in humans is not well understood. We tested the hypothesis that activation of inwardly rectifying potassium channels and the sodium–potassium ATPase pump, two potential vasodilator pathways within blood vessels, contributes to the ability to blunt α1‐adrenergic vasoconstriction. Our results show preserved blunting of α1‐adrenergic vasconstriction despite blockade of these vasoactive factors. Understanding this complex phenomenon is important as it is impaired in a variety of clinical populations.

Endothelium-dependent vasodilatory signalling modulates α1 -adrenergic vasoconstriction in contracting skeletal muscle of humans.

‘Functional sympatholysis’ describes the ability of contracting skeletal muscle to attenuate sympathetic vasoconstriction, and is critical to ensure proper blood flow and oxygen delivery to metabolically active skeletal muscle. The signalling mechanism responsible for sympatholysis in healthy humans is unknown. Evidence from animal models has identified endothelium‐derived hyperpolarization (EDH) as a potential mechanism capable of attenuating sympathetic vasoconstriction. In this study, increasing endothelium‐dependent signalling during exercise significantly enhanced the ability of contracting skeletal muscle to attenuate sympathetic vasoconstriction in humans. This is the first study in humans to identify endothelium‐dependent regulation of sympathetic vasoconstriction in contracting skeletal muscle, and specifically supports a role for EDH‐like vasodilatory signalling. Impaired functional sympatholysis is a common feature of cardiovascular ageing, hypertension and heart failure, and thus identifying fundamental mechanisms responsible for sympatholysis is clinically relevant.

Acute ingestion of dietary nitrate increases muscle blood flow via local vasodilation during handgrip exercise in young adults

Dietary nitrate ( NO3− ) is converted to nitrite ( NO2− ) and can be further reduced to the vasodilator nitric oxide (NO) amid a low O2 environment. Accordingly, dietary NO3− increases hind limb blood flow in rats during treadmill exercise; however, the evidence of such an effect in humans is unclear. We tested the hypothesis that acute dietary NO3− (via beetroot [BR] juice) increases forearm blood flow (FBF) via local vasodilation during handgrip exercise in young adults (n = 11; 25 ± 2 years). FBF (Doppler ultrasound) and blood pressure (Finapres) were measured at rest and during graded handgrip exercise at 5%, 15%, and 25% maximal voluntary contraction (MVC) lasting 4 min each. At the highest workload (25% MVC), systemic hypoxia (80% SaO2) was induced and exercise continued for three additional minutes. Subjects ingested concentrated BR (12.6 mmol nitrate (n = 5) or 16.8 mmol nitrate (n = 6) and repeated the exercise bout either 2 (12.6 mmol) or 3 h (16.8 mmol) postconsumption. Compared to control, BR significantly increased FBF at 15% MVC (184 ± 15 vs. 164 ± 15 mL/min), 25% MVC (323 ± 27 vs. 286 ± 28 mL/min), and 25% + hypoxia (373 ± 39 vs. 343 ± 32 mL/min) and this was due to increases in vascular conductance (i.e., vasodilation). The effect of BR on hemodynamics was not different between the two doses of BR ingested. Forearm VO2 was also elevated during exercise at 15% and 25% MVC. We conclude that acute increases in circulating NO3− and NO2− via BR increases muscle blood flow during moderate‐ to high‐intensity handgrip exercise via local vasodilation. These findings may have important implications for aging and diseased populations that demonstrate impaired muscle perfusion and exercise intolerance.

Elevated extracellular potassium prior to muscle contraction reduces onset and steady-state exercise hyperemia in humans

The increase in interstitial potassium (K+) during muscle contractions is thought to be a vasodilatory signal that contributes to exercise hyperemia. To determine the role of extracellular K+ in exercise hyperemia, we perfused skeletal muscle with K+ before contractions, such that the effect of any endogenously-released K+ would be minimized. We tested the hypothesis that local, intra-arterial infusion of potassium chloride (KCl) at rest would impair vasodilation in response to subsequent rhythmic handgrip exercise in humans. In 11 young adults, we determined forearm blood flow (FBF) (Doppler ultrasound) and forearm vascular conductance (FVC) (FBF/mean arterial pressure) during 4 min of rhythmic handgrip exercise at 10% of maximal voluntary contraction during 1) control conditions, 2) infusion of KCl before the initiation of exercise, and 3) infusion of sodium nitroprusside (SNP) as a control vasodilator. Infusion of KCl or SNP elevated resting FVC similarly before the onset of exercise (control: 39 ± 6 vs. KCl: 81 ± 12 and SNP: 82 ± 13 ml·min-1·100 mmHg-1; both P < 0.05 vs. control). Infusion of KCl at rest diminished the hyperemic (ΔFBF) and vasodilatory (ΔFVC) response to subsequent exercise by 22 ± 5% and 30 ± 5%, respectively (both P < 0.05 vs. control), whereas SNP did not affect the change in FBF ( P = 0.74 vs. control) or FVC ( P = 0.61 vs. control) from rest to steady-state exercise. These findings implicate the K+ ion as an essential vasodilator substance contributing to exercise hyperemia in humans. NEW & NOTEWORTHY Our findings support a significant and obligatory role for potassium signaling in the local vasodilatory and hyperemic response to exercise in humans.

Integration of vasodilatory stimuli in skeletal muscle vasculature: subtraction by addition?

Regulating blood flow to match oxygen delivery and metabolic demand is a fundamental physiological principle that is critical to ensure the proper functioning of the majority of tissues in the human body, particularly those that experience profound and dynamic changes in metabolic demand such as cardiac and skeletal muscle. In contracting skeletal muscle, a number of putative vasodilatory signals have been identified, but evidence for a primary dilatory signal responsible for mediating exercise hyperaemia is lacking. As such, it has been suggested that interactions between multiple vasodilators provide redundancy, whereby the loss of one signal can be compensated for by the upregulation of another. While there is evidence of redundancy in humans (Schrage et al. 2004), little work has been done to characterize the exact nature and kinetics of vasodilator interactions in skeletal muscle. In the 1 December 2015 issue of The Journal of Physiology, Lamb and colleagues attempt to directly test assumptions surrounding the redundant dilator hypothesis. Specifically, they aimed to identify inhibitory interactions between vasodilators where the presence of an interacting vasodilator can limit the action of others (Lamb & Murrant, 2015). If this scenario were to exist, blocking the action of the interacting vasodilator would remove inhibition of the secondary vasodilatory pathways thus allowing for compensatory vasodilatation. To address this question, they used an in situ hamster cremaster muscle preparation and intravital microscopy to measure microcirculatory responses to the physiologically relevant vasodilators: potassium (K+), nitric oxide (NO) and adenosine (ADO). In the first protocol, an initial dose response to K+, NO, or ADO was conducted; following drug washout and return to baseline conditions, a second ‘interacting’ vasodilator (either K+, NO, or ADO) was added to the superfusion, and the initial dose response was repeated. In support of the hypothesis that the presence of an interacting vasodilator may limit the action of secondary vasodilators, K+ significantly attenuated the peak vasodilatory response to both NO and ADO across a range of concentrations, while neither NO nor ADO altered the peak vasodilatory response to other vasodilators. Interestingly, administration of K+, NO, or ADO as the interacting vasodilator resulted in a transient vasodilatation ( 2 min) that returned to baseline prior to repeating the initial dose response (more on this later); because of this, the effects of K+ on NO and ADO are due to specific interactions between vasoactive signalling pathways as opposed to a general effect of adding an additional vasodilatory stimulus to an already dilated vasculature. Protocol 2 tested similar interactions between vasodilators when added simultaneously, in an attempt to mimic physiological conditions at the onset of exercise. In this protocol, the same concentration of NO and ADO that induced a transient vasodilatation in protocol 1 elicited a sustained vasodilatation when given alone, whereas the vasodilatory response to 10 mM KCl remained transient. However, when NO and ADO were administered simultaneously with K+, the dilatory response once again became transient such that the peak vasodilatory responses matched those of NO or ADO alone, but the sustained vasodilatation was abolished in the latter half of the response. These findings clearly demonstrate that two independent vasodilatory stimuli in combination do not necessarily summate, and may support the assertion that K+ modulates the action of other vasodilators. However, there are a few important considerations for the latter point. First, as seen in protocol 1, both NO and ADO can exhibit the tachyphylaxis observed in protocol 2 after prior exposure to other vasodilators. Thus, it is unclear if the results from protocol 2 represent an effect of repeated exposure to vasodilators or a specific effect of K+. However, it is important to note that this effect was not seen when NO and ADO were added in tandem, supporting a potential specific effect of K+. Further studies employing appropriate time and repeated exposure controls will be necessary to properly characterize these interactions. Second, the effect of K+ on NOand ADO-mediated dilatation is time dependent in this protocol, such that the effect of K+ was observed only after 6–10 min of vasodilator administration. Why were these interactions not observed within the first 5 min of superfusion? The reasons for this are unclear, especially considering that the activated signalling pathways following superfusion should be engaged throughout this time period. Interpreting the data during the first 5 min alters the simple narrative that K+ limits NOand ADO-mediated dilatation, and reinforces the concept of a more complicated integration of vasodilator stimuli in the vasculature. In the context of exercise hyperaemia, these results lend support to the redundancy hypothesis and suggest that K+-related signalling pathways may contribute to exercise hyperaemia by interacting with or potentially limiting the contributions of other vasodilators (i.e. NO and ADO). Thus, it would stand to reason that inhibition of K+-mediated vasodilatation during exercise would remove inhibition of NO or ADO and uncover a greater role for these pathways in maintaining hyperaemia. Indeed, findings from our laboratory during handgrip exercise in humans support this concept. While blockade of NO and prostaglandin (PG) production typically (though not exclusively) has a minimal impact on forearm exercise hyperaemia in humans, Crecelius et al. (2014) demonstrated that local intra-arterial infusion of barium chloride and ouabain (to inhibit inwardly rectifying K+ (KIR) channels and Na+/K+-ATPase, respectively) reduced exercise hyperaemia by 30%, and subsequent inhibition of NO and PG production further reduced vascular conductance during exercise, thereby revealing a role for NO and PGs only after the vasodilatory pathways for extracellular K+ were inhibited (Dawes, 2002). Thus, the findings of Lamb and colleagues offer a potential explanation for observations