Matthew Racine

Inhibition of Na+/K+‐ATPase and KIR channels abolishes hypoxic hyperaemia in resting but not contracting skeletal muscle of humans

Increasing blood flow (hyperaemia) to exercising muscle helps match oxygen delivery and metabolic demand. During exercise in hypoxia, there is a compensatory increase in muscle hyperaemia that maintains oxygen delivery and tissue oxygen consumption. Nitric oxide (NO) and prostaglandins (PGs) contribute to around half of the augmented hyperaemia during hypoxic exercise, although the contributors to the remaining response are unknown. In the present study, inhibiting NO, PGs, Na+/K+‐ATPase and inwardly rectifying potassium (KIR) channels did not blunt augmented hyperaemia during hypoxic exercise beyond previous observations with NO/PG block alone. Furthermore, although inhibition of only Na+/K+‐ATPase and KIR channels abolished hyperaemia during hypoxia at rest, it had no effect on augmented hyperaemia during hypoxic exercise. This is the first study in humans to demonstrate that Na+/K+‐ATPase and KIR channel activation is required for augmented muscle hyperaemia during hypoxia at rest but not during hypoxic exercise, thus providing new insight into vascular control.

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.

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