Abstract
KEY POINTS: In humans, the vasodilatory response to skeletal muscle contraction is mediated in part by activation of inwardly rectifying potassium (K(IR) ) channels. Evidence from animal models suggest that K(IR) channels serve as electrical amplifiers of endothelium-dependent hyperpolarization (EDH). We found that skeletal muscle contraction amplifies vasodilatation to the endothelium-dependent agonist ACh, whereas there was no change in the vasodilatory response to sodium nitroprusside, an endothelium-independent nitric oxide donor. Blockade of K(IR) channels reduced the exercise-induced amplification of ACh-mediated vasodilatation. Conversely, pharmacological activation of K(IR) channels in quiescent muscle via intra-arterial infusion of KCl independently amplified the vasodilatory response to ACh. This study is the first in humans to demonstrate that specific endothelium-dependent vasodilatory signalling is amplified in the vasculature of contracting skeletal muscle and that K(IR) channels may serve as amplifiers of EDH-like vasodilatory signalling in humans. ABSTRACT: The local vasodilatory response to muscle contraction is due in part to the activation of inwardly rectifying potassium (K(IR) ) channels. Evidence from animal models suggest that K(IR) channels function as 'amplifiers' of endothelium-dependent vasodilators. We tested the hypothesis that contracting muscle selectively amplifies endothelium-dependent vasodilatation via activation of K(IR) channels. We measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to local intra-arterial infusion of ACh (endothelium-dependent dilator) during resting conditions, handgrip exercise (5% maximum voluntary contraction) or sodium nitroprusside (SNP; endothelium-independent dilator) which served as a high-flow control condition (n = 7, young healthy men and women). Trials were performed before and after blockade of K(IR) channels via infusion of barium chloride. Exercise augmented peak ACh-mediated vasodilatation (ΔFVC saline: 117 ± 14; exercise: 236 ± 21 ml min(-1) (100 mmHg)(-1) ; P < 0.05), whereas SNP did not impact ACh-mediated vasodilatation. Blockade of K(IR) channels attenuated the exercise-induced augmentation of ACh. In eight additional subjects, SNP was administered as the experimental dilator. In contrast to ACh, exercise did not alter SNP-mediated vasodilatation (ΔFVC saline: 158 ± 35; exercise: 121 ± 22 ml min(-1) (100 mmHg)(-1) ; n.s.). Finally, in a subset of six subjects, direct pharmacological activation of K(IR) channels in quiescent muscle via infusion of KCl amplified peak ACh-mediated vasodilatation (ΔFVC saline: 97 ± 15, KCl: 142 ± 16 ml min(-1) (100 mmHg)(-1) ; respectively; P < 0.05). These findings indicate that skeletal muscle contractions selectively amplify endothelium-dependent vasodilatory signalling via activation of K(IR) channels, and this may be an important mechanism contributing to the normal vasodilatory response to exercise in humans.