Characterizing the sympatholytic role of endothelial‐dependent vasodilator signalling during handgrip exercise

The dynamic metabolic range of skeletal muscle necessitates an equally dynamic blood flow requirement, to the point where the demand for muscle blood flow during intense bouts of whole body exercise can theoretically ‘outstrip’ the heart's ability to generate the cardiac output necessary for maintaining sufficient mean arterial pressure (MAP) and muscle perfusion. However, exercise that could otherwise compromise systemic perfusion is met with a paralleled increase in both muscle sympathetic nerve activity and release of the α‐adrenoceptor agonist, noradrenaline (NA), the latter of which elicits potent vasoconstriction of the resistance vasculature, ultimately safeguarding MAP. Yet, as exercising muscle requires vast quantities of blood flow, the question that arises is; ‘how does active skeletal muscle receive sufficient blood flow in the face of sympathetically mediated vasoconstriction?’ Remensnyder et al. (1962) were the first to observe that the vasculature of exercising muscle becomes less sensitive to the effects of sympathetic vasoconstriction, terming this reduction in sensitivity as ‘functional sympatholysis’. This phenomenon has been the focus of extensive research in both animal models and in humans, with recent research finding that certain vasoactive compounds, present during muscle contraction (nitric oxide (NO), adenosine (ADO), adenosine triphoshate (ATP)), can modulate sympathetic vasoconstriction. However, in the exercising human, signalling mechanisms contributing to the vasculature's reduced sensitivity to sympathetic vasoconstriction remains undetermined. With evidence from animal models demonstrating that the endothelium and endothelium‐derived hyperpolarization (EDH) play a role in attenuating sympathetic vasoconstriction (Kurjiaka & Segal, 1995), the study published in The Journal of Physiology by Hearon et al. (2016) aimed to characterize the role of the endothelium and EDH signalling in modulating α₁‐adrenoceptor‐mediated vasoconstriction in the contracting forearm muscles of humans.

Hearon et al. sought to assess the role of endothelium‐dependent and ‐independent signalling in modulating phenylephrine‐mediated vasoconstriction (PE; α₁‐adrenergic receptor agonist, mimicking elevated sympathetic vasoconstriction) during various experimental hyperaemic conditions/trials. Mild‐ to moderate‐intensity exercise, in the form of dynamic hand‐gripping was performed, blood flow was measured at the brachial artery in response to PE, and forearm vascular conductance (FVC) was calculated. Changes in FVC were assessed under the following conditions: (1) intra‐brachial arterial infusion of endothelium‐dependent (acetylcholine (ACh) and ATP) and endothelial‐independent (NO via donor sodium nitroprusside (SNP)), and potassium chloride (KCl)) vasodilators; (2) rhythmic handgrip exercise at mild‐intensity (5% maximum voluntary contraction (MVC); shown not to activate sympatholytic response); (3) moderate‐intensity (15% MVC; shown to activate a sympatholytic response); and (4) mild‐intensity handgrip exercise, in conjunction with infusion of either ACh, SNP, ATP or KCl. To account for potential changes in vascular tone elicited by experimental drug trials, the authors reported both absolute and percentage changes of FVC in response to PE from steady‐state hyperaemic conditions.

Prior to infusion of PE, all vasodilatory agents and both mild and moderate exercise intensity trials increased steady‐state FVC significantly, relative to resting conditions. Vascular responses to ACh or SNP infusion, and 5% MVC + ACh or 5% MVC + SNP infusion were matched to steady‐state FVC at 15% MVC by means of modifying vasodilator infusion concentration in an effort to remove the confounding variability of vascular tone between drug and exercise trials and to directly assay the dilatory mechanisms opposing PE‐mediated reductions in steady‐state FVC with exercise. Due to the fact that infusion of high concentration ATP can blunt sympathetic vasoconstriction directly, and the safety concerns associated with infusion of high concentration KCl, both ATP and KCl were infused at low dose concentrations. Despite the lower concentrations, ATP and KCl infusion elicited elevated FVC when infused alone and to a greater magnitude when infused in conjunction with 5% MVC forearm exercise.

Infusion of PE during application of each of the individual vasodilators resulted in the reduction of forearm blood flow and FVC. PE infusion during 5% MVC handgrip exercise also caused a reduction in FVC to a similar magnitude than that observed during PE and vasodilator infusion. While PE infusion also decreased FVC during 15% MVC handgrip exercise, the magnitude of vasoconstriction was substantially less than that observed during vasodilator infusion or 5% MVC exercise. In other words, while PE decreased FVC, handgrip exercise at 15% MVC was sympatholytic, significantly blunting the magnitude of PE‐mediated vasoconstriction to a greater extent than either vasodilator infusion or 5% MVC exercise. Interestingly, PE‐mediated reductions in FVC during the combination of 5% MVC and vasodilator infusion were not consistent across vasoactive agents. In fact, PE‐mediated vasoconstriction was nearly abolished when ACh, an endothelial‐dependent vasodilator, was infused in conjunction with 5% MVC handgrip exercise. Furthermore, when ATP, another endothelial‐dependent vasodilator, was infused in conjunction with 5% MVC exercise PE‐mediated vasoconstriction was blunted to a magnitude similar to that observed during 15% MVC handgrip exercise. Overall these data clearly demonstrate a role for endothelium‐dependent vasodilatory signalling in blunting sympathetic vasoconstriction during muscle contraction. In contrast, endothelium‐independent dilator NO‐ (via SNP) and KCl‐mediated vasodilatation did not impact PE's ability to reduce FVC during mild exercise. In fact, FVC during KCl infusion alone and together with 5% MVC were reduced in even greater magnitude upon PE infusion compared to FVC in ACh trials. Given that ACh produces vasodilatation, in part through EDH as well as NO and prostaglandins (PGs) synthesis, further experiments were required to parse out the involvement of EDH. ACh infusion and ACh + 5% MVC exercise trials were repeated in the presence of NO and PG synthesis inhibition (via N ^G‐monomethyl‐l‐arginine (l‐NMMA) and ketorolac, respectively), thereby isolating the true effect of EDH signalling. In the presence of NO and PG antagonists the magnitude of PE‐mediated vasoconstriction was attenuated during both ACh infusion alone and ACh + 5% MVC handgrip, demonstrating that the sympatholytic role of ACh is primarily mediated by EDH signalling.

The finding that non‐sympatholytic concentrations of ATP, when coupled with a mild, non‐sympatholytic exercise intensity cause a seemingly summative, sympatholytic response is intriguing and it would be of keen interest to explore the underlying mechanism(s) that mediate this apparent synergistic effect. Studies have shown that the vasodilatory effects of ATP are mediated, in part, by secondary messengers including NO, PG and endothelium‐derived hyperpolarizing factor (EDHF) (Clifford, 2008). While independently, these vasodilators may not be sympatholytic; it appears that when present in conjunction with products released during mild, non‐sympatholytic exercise, the milieu of vasodilatory agents are able to act synergistically to produce a sympatholytic response and blunt PE‐mediated vasoconstriction. While human studies have failed to identify PG as sympatholytic (Dinenno & Joyner, 2004), the role of NO and EDHF is still contentious and may, perhaps, be involved in the additive response observed during ATP+5% MVC trials. While elucidating the additive nature of non‐sympatholytic vasoactive stimuli is outside the scope of this study, it certainly is a compelling finding and merits future investigation.

The findings by Hearon et al. provide evidence that the vascular endothelium and EDH are key players in opposing sympathetic vasoconstriction. Furthermore, parsing out the relative role of endothelial‐dependent vasodilators during heightened sympathetic nerve activity begins to identify the signalling mechanisms underlying potential sympatholytic response in healthy humans. Further, there are clear translational applications of these findings to pathological states especially as elevated resting sympathetic nerve activity accompanies conditions such as; heart failure, obesity and diabetes. The findings that EDH is a potent sympatholytic stimulus, suggest treatments stimulating EDH signalling may be of therapeutic interest. Exercise is used as a therapy due to the fact that sympatholytic agents, released during muscle contraction, can reduce sympathetic nerve activity and NA‐mediated vasoconstriction, thereby improving vascular function and physical performance in the aforementioned clinical populations. Interestingly, a study by Dua et al. (2009) demonstrated the production of specific vasodilators (endothelial‐dependent or ‐independent) is determined by the stimulation parameters in which muscle is contracted. Based on the findings by both Dua et al. and Hearon et al. an interesting future study would be to investigate how the milieu of vasodilators released differ during different exercise regimes, with the aim of elucidating the exercise parameters, that stimulate a higher proportion of vasodilators that function through EDH signaling. This in turn may offer improvements in exercise therapies aiming to ‘silence’ sympathoexcitation within the aforementioned clinical populations.

In this study Hearon et al. have clearly demonstrated EDH as a powerful signalling mechanism that enhances the ability of exercising muscle to attenuate α₁‐adrenergic vasoconstriction in humans. The identification of the endothelium and endothelium‐dependent vasodilators as important in the attenuation of sympathetic vasoconstriction begins to reveal some of the mystery surrounding the signalling mechanisms that mediate functional sympatholysis in the exercising human. With this knowledge comes the opportunity to investigate how this pathway is impacted during pathological conditions and may offer new avenues for therapeutic intervention.

Additional information

Competing interests

None declared.