Enhanced functional sympatholysis through endothelial signalling in healthy young men and women

Abstract

That the endothelium governs vasodilatation in healthy humans and animals is well established. Indeed, impaired endothelium-dependent dilatation (EDD) is diagnostic of vascular dysfunction. Through the generation of diffusible autacoids, e.g. nitric oxide (NO) and metabolites of arachidonic acid, an increase in luminal shear stress leads to relaxation of vascular smooth muscle and is integral to flow-dependent dilatation. In humans, manipulating arterial blood flow (i.e. shear stress) is a non-invasive diagnostic tool for evaluating endothelial function as vessel diameter and blood flow velocity are monitored using ultrasound. A second pathway for EDD involves the initiation of hyperpolarization by endothelial cells and the transmission of electrical charge through myoendothelial gap junctions to evoke smooth muscle relaxation (Bagher & Segal, 2011). While endothelium-dependent hyperpolarization (EDH) has been well characterized in small resistance arteries and arterioles of animals, there is a dearth of evidence for EDH in governing blood flow in humans. This gap in translating findings from animal studies to humans is attributable to the invasive nature of recording membrane potential in the vascular wall after surgical exposure. Thus, elucidating EDH signalling as a pathway for EDD in humans has been difficult. In this issue of The Journal of Physiology a new study by Hearon et al. (2016), applying mechanistic insight gleaned from animal models, has coupled clever experimental design with established protocols in human subjects to shed new light on a role for EDH in functional sympatholysis, i.e. the ability of exercising skeletal muscle to attenuate sympathetic vasoconstriction. By manipulating both the intensity of rhythmic handgrip contractions and signalling pathways for vasodilatation, Hearon and co-workers tested whether stimulating EDD – independent of NO and prostaglandins – enhanced the ability of active skeletal muscle to attenuate sympathetic vasoconstriction induced by α1-adrenoreceptor (α1AR) activation. Key to these experiments was the judicious application of agonists whose actions have been well defined using isolated vessel preparations in which membrane potential and calcium signalling were rigorously evaluated (Tran et al. 2012). Thus, as shown in resistance arteries, acetylcholine (ACh) stimulates the opening of smalland intermediate-conductance calcium-activated K+ channels (SKCa and IKCa, respectively) in endothelial cells to initiate hyperpolarization, which is transmitted directly (through myoendothelial gap junctions) into surrounding smooth muscle cells to promote vasodilatation (Crane et al. 2003). As the electrical signal is conducted along the endothelium, the vasodilator response is coordinated within the resistance network (Bagher & Segal, 2011). Studies of the microcirculation in anaesthetized hamsters have shown that conducted vasodilatation can attenuate sympathetic vasoconstriction (Kurjiaka & Segal, 1995) and such reasoning was integral to the experimental design applied to humans. Pharmacological agents were infused into the brachial artery of healthy 23-year-old men and women to manipulate vascular resistance in the forearm musculature while avoiding systemic effects or the activation of cardiovascular reflexes. Thus, vasoconstriction was induced by infusing the α1AR agonist phenylephrine (PE) during passive vasodilatation of resting skeletal muscle, during mild or moderate rhythmic handgrip exercise, as well as during mild exercise in combination with endothelium-dependent or -independent vasodilators. Phenylephrine evoked robust vasoconstriction during passive vasodilatation and during mild-intensity exercise. However, vasoconstriction was blunted during moderate exercise. Remarkably, when ACh was infused during mild exercise, vasoconstriction to PE was attenuated to the same level as seen during moderate exercise. In contrast, when the endothelium-independent vasodilator sodium nitroprusside was infused to evoke vasodilatation similar to that obtained with ACh, vasoconstriction to PE was not attenuated during exercise. The key finding from these experiments is that stimulating the endothelium with ACh enhanced the ability of active skeletal muscle to overcome sympathetic (α1AR) vasoconstriction. Importantly, the ability of ACh plus mild exercise to blunt vasoconstriction persisted during inhibition of NO and prostaglandins, thereby implicating EDH in promoting the ability of active skeletal muscle to attenuate sympathetic vasoconstriction. Complementary experiments evaluated the effects of ATP or KCl infusion under respective conditions of α1AR activation and exercise intensity and data were analysed with respect to both absolute and relative changes in vascular conductance. The findings collectively led the authors to conclude that augmenting EDH-like signalling evoked during mild-intensity exercise attenuates vasoconstriction mediated through α1AR activation (Fig. 1, Hearon et al. 2016). Studies of EDH in humans (both at rest and during exercise) remain limited by the inability to directly evaluate membrane potential. Nevertheless, these new findings support the hypothesis