Terwoord, Janée D.

During muscle contractions, an increase in interstitial potassium (K + ) and subsequent activation of inwardly‐rectifying K + (K IR ) channels is thought to be a functional vasodilatory signal contributing to exercise hyperemia. Indeed, our laboratory has recently demonstrated that inhibition of K IR channels blunts exercise hyperemia ~30% in humans. In theory, the vasodilatory response to changes in interstitial K + during muscle contraction could be minimized by artificially elevating the concentration of K + perfusing the vasculature prior to the onset of muscle contractions, thus limiting further recruitment of this pathway and blunting the dilatory response to exercise. We tested the hypothesis that infusion of potassium chloride (KCl) to elevate extracellular potassium concentrations and induce hyperpolarization prior to the onset of muscle contractions would reduce the contribution of this mechanism to the dilatory response to exercise. In young adults (n = 10), we measured forearm blood flow (FBF; Doppler ultrasound) and calculated changes in vascular conductance (FVC; FBF/intra‐arterial blood pressure) during 4 minutes of rhythmic handgrip exercise at 10% of maximal voluntary contraction during 1) control conditions (CTRL), 2) local, intra‐arterial infusion of KCl to elevate the concentration of K + prior to the initiation of exercise, and 3) intra‐arterial infusion of sodium nitroprusside (SNP) as a high flow control to match the K + ‐induced vasodilation observed at rest. As intended, infusion of KCl or SNP elevated resting FVC similarly prior to the onset of exercise (CTRL: 40 ± 7 vs. KCl: 84 ± 13 and SNP: 85 ± 14 ml·min −1 ·100 mmHg −1 ; both P < 0.01 vs. CTRL). Infusion of KCl diminished the subsequent vasodilatory response to handgrip exercise by ~31% when compared to control conditions (ΔFVC: KCl: 92 ± 14 vs. CTRL: 133 ± 16 ml·min −1 ·100 mmHg −1 ; P < 0.01), whereas elevation of resting FVC with SNP prior to contractions did not change the vasodilatory response to exercise (ΔFVC, SNP: 117 ± 15 vs . CTRL: 133 ± 16 ml·min −1 ·100 mmHg −1 ; P = NS). These findings indicate that K + ‐mediated hyperpolarization prior to the onset of muscle contractions blunts the normal vasodilatory response to exercise, which supports a role for elevated K + signaling in the hyperemic response to exercise in humans. Support or Funding Information HL119337, HL095573

We have recently demonstrated that vasodilation to the endothelium‐dependent agonist acetylcholine is amplified in contracting compared with quiescent skeletal muscle, highlighting the importance of the vascular endothelium in integrating vasomotor signals to regulate vascular tone in humans. In the present study, we tested the hypothesis that sensitivity to the endothelium‐dependent vasodilator ATP is augmented during exercise and systemic hypoxia, whereas sensitivity to the nitric oxide donor and endothelium‐independent dilator sodium nitroprusside (SNP) is not affected by these physiological conditions. In 12 young adults (7 M, 5 F), we measured forearm blood flow (FBF; Doppler ultrasound) and calculated changes in vascular conductance (FVC: quotient of FBF and mean arterial pressure) in response to local, intra‐arterial infusion of ATP or SNP during 1) control resting conditions (CON), 2) dynamic handgrip exercise (EX, 20 contractions per minute at 10% of maximal voluntary contraction), or 3) systemic hypoxia (HYP, 80% O 2 saturation). Three doses of each vasodilator were infused via a brachial artery catheter during each condition (ATP: 1.25, 2.50, and 5.00 μg·dl forearm volume −1 ·min −1 ; SNP: 0.25, 0.50, and 1.00 μg·dl forearm volume −1 ·min −1 ). The order of drugs and conditions was randomized. Within each trial, infusions began after 3 min of rest, steady‐state exercise, or hypoxia, and each dose was infused for 2 min while the exercise or hypoxic stimulus was sustained. Compared to control conditions, vasodilatory sensitivity to the high dose of ATP was enhanced by 53 ± 18% during mild handgrip exercise (Δ FVC, CON: 120 ± 16 vs. EX: 176 ± 28 ml·min −1 ·100 mmHg −1 ; P < 0.05), whereas vasodilatory sensitivity was not affected at lower doses of ATP ( P = NS; Dose × Condition interaction P < 0.05). In contrast, exercise had no effect on vascular sensitivity to any dose of SNP ( P = NS). Similarly, during systemic hypoxia, vasodilation to ATP was enhanced by 18 ± 10% at the medium dose (Δ FVC, CON: 100 ± 15 vs. HYP: 125 ± 24 ml·min −1 ·100 mmHg −1 ; P < 0.05) and 28 ± 11% at the high dose (Δ FVC, CON: 120 ± 16 vs. HYP: 158 ± 27 ml·min −1 ·100 mmHg −1 ; P < 0.05; Dose × Condition interaction P < 0.05), whereas vasodilation to SNP was not different from control conditions at any dose ( P = NS). These findings provide evidence that muscle contractions and hypoxia augment ATP‐mediated, endothelium‐dependent vasodilation in human skeletal muscle. Moreover, given that intravascular ATP increases under these physiological conditions, the results indicate that small changes in plasma ATP may result in augmented vasodilation and contribute to blood flow and oxygen delivery in humans. Support or Funding Information HL119337 This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal .

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.