Luckasen, Gary J.

Skeletal muscle blood flow during exercise is typically reduced in older compared with young adults, which may be mediated in part by age‐related reductions in local vasodilator molecules such as nitric oxide (NO) and ATP. Despite this, muscle blood flow does increase substantially to contracting muscle of older adults. We questioned whether reductions in muscle blood flow with age, particularly at the onset of exercise, leads to greater accumulation of local metabolites that could play a more prominent role in contraction‐induced vasodilation compared with young adults. Thus, in the present study we tested the hypothesis that the contribution of adenosine (ADO) to contraction‐induced vasodilation is greater in older vs. young adults. In young (25 ± 2 years; n=8) and older (65 ± 2 years; n=8) adults, forearm vascular conductance (FVC) responses to a single 20% maximum voluntary contraction (MVC) handgrip contraction and to 5 minutes of 10% MVC rhythmic handgrip exercise were measured using Doppler ultrasound of the brachial artery before and after brachial artery infusion of the ADO receptor blocker aminophylline (APH). In order to determine the effect of APH infusion that was due specifically to the blockade of ADO receptors as opposed to the elevated baseline FVC that often occurs as an off‐target effect of APH, FVC responses to contraction were also measured after sodium nitroprusside (SNP) infusion to match the APH‐induced increase in baseline FVC. In response to a single contraction, both total FVC area under the curve (AUC) and the absolute change in FVC from baseline to peak were lower in older vs. young adults (−61.2% and −56.1%, respectively; p < 0.05) under control conditions. There was no significant effect of APH on total FVC AUC in young (−5.4 ± 7.8%) or older (16.6 ± 12.4%) adults vs. control, whereas the absolute change in FVC from baseline to peak was significantly lower in young adults (−22.5 ± 7.7%; p < 0.05), but unaffected in older adults (4.1 ± 7.8%) vs. control. During rhythmic handgrip exercise, FVC was significantly lower in older adults in the first 60 sec of exercise (~45–50%) as well as during steady‐state (~25%; 110.0 ± 14.9 vs. 149.5 ±12.6 mL/min/100mmHg; both p < 0.05) under control conditions. APH had no effect on the FVC response throughout the first 3 min of exercise onset in young (range: 0.8–6.2%) or older (range: −3.9 to 10.5%) adults vs. control (p > 0.05), and did not alter the absolute change in FVC from baseline to steady‐state exercise in either group (p > 0.05). SNP had no effect on these FVC responses during single contractions or rhythmic handgrip exercise in either age group (p > 0.05). We conclude that that ADO contributes to the peak vasodilatory response to a single contraction in young but not older adults, and that advancing age is not associated with a greater reliance on ADO for local vasodilation during skeletal muscle contractions. Support or Funding Information HL095573, HL119337

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

The vasoactive properties of exogenous adenosine triphosphate (ATP) are unique because it has the dual ability to induce robust vasodilation as well as blunt sympathetic vasoconstriction, similar to what is observed in contracting human skeletal muscle. ATP‐mediated vasodilation and exercise hyperemia are dependent, in part, on activation of inwardly rectifying potassium (K IR ) channels and subsequent smooth muscle cell hyperpolarization, with a modest reliance on the production of nitric oxide (NO) and prostaglandins (PGs). However, recent studies from our laboratory indicate that these pathways are not obligatory for contracting skeletal muscle to blunt α 1 ‐adrenergic vasoconstriction. Given the specific role of K IR channels in ATP‐mediated vasodilation, the primary purpose of this study was to determine whether the ability of ATP to blunt α 1 ‐adrenergic vasoconstriction is dependent upon K IR channel activity in humans. A secondary purpose was to determine whether the ability of ATP to blunt α 1 ‐adrenergic vasoconstriction is dependent upon activation of K IR channels and Na + /K + ‐ATPase, as well as the synthesis of NO and PGs. In a total of 13 young subjects we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to local intra‐arterial infusion of phenylephrine (PE; α 1 ‐agonist) during infusion of ATP or a control vasodilator(s) (Protocol 1 (n=7): adenosine (ADO), and sodium nitroprusside (SNP); Protocol 2 (n=6): ADO). Vasomotor responses were assessed before and after blockade of signaling pathways that contribute to ATP‐mediated vasodilation (Protocol 1: K IR channels (barium chloride); Protocol 2: combined blockade of K IR channels, Na + /K + ‐ATPase (ouabain), NO (L‐NMMA), PGs (Ketorolac)). Protocol 1: ATP significantly attenuated PE‐mediated vasoconstriction relative to both SNP and ADO (Δ FVC: ATP: −16 ± 2; ADO: −38 ± 6; SNP: −59 ± 6%; P<0.05 vs ADO and SNP). Blockade of K IR channels significantly attenuated ATP mediated vasodilation ~40% (P<0.05 vs. control), but had no effect on the ability of ATP to blunt PE‐ mediated vasoconstriction (Δ FVC: −6 ± 5%; P>0.05 vs. control). Protocol 2: Despite combined blockade of K IR , Na + /K + ‐ATPase, NO, and PGs resulting in profound attenuation of ATP‐mediated vasodilation (−~60%; P<0.01 vs. control), ATP was still able to blunt PE‐mediated vasoconstriction (Δ FVC: −4 ± 14%; P=0.50 vs. control). Together these studies highlight that while NO, PGs, and vascular smooth muscle hyperpolarization (via K IR channel activation) are contributors to the normal vasodilatory response to exercise and exogenous ATP, these pathways are not obligatory for exercise or ATP to blunt α 1 ‐adrenergic vasoconstriction in humans. Support or Funding Information Support: 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.

Key points Intravascular ATP attenuates sympathetic vasoconstriction (sympatholysis) similar to what is observed in contracting skeletal muscle of humans, and may be an important contributor to exercise hyperaemia. Similar to exercise, ATP‐mediated vasodilatation occurs via activation of inwardly rectifying potassium channels (K IR ), and synthesis of nitric oxide (NO) and prostaglandins (PG). However, recent evidence suggests that these dilatatory pathways are not obligatory for sympatholysis during exercise; therefore, we tested the hypothesis that the ability of ATP to blunt α 1 ‐adrenergic vasoconstriction in resting skeletal muscle would be independent of K IR , NO, PGs and Na + /K + ‐ATPase activity. Blockade of K IR channels alone or in combination with NO, PGs and Na + /K + ‐ATPase significantly reduced the vasodilatatory response to ATP, although intravascular ATP maintained the ability to attenuate α 1 ‐adrenergic vasoconstriction. This study highlights similarities in the vascular response to ATP and exercise, and further supports a potential role of intravascular ATP in blood flow regulation during exercise in humans. Abstract Exercise and intravascular ATP elicit vasodilatation that is dependent on activation of inwardly rectifying potassium (K IR ) channels, with a modest reliance on nitric oxide (NO) and prostaglandin (PG) synthesis. Both exercise and intravascular ATP attenuate sympathetic α‐adrenergic vasoconstriction (sympatholysis). However, K IR channels, NO, PGs and Na + /K + ‐ATPase activity are not obligatory to observe sympatholysis during exercise. To further determine similarities between exercise and intravascular ATP, we tested the hypothesis that inhibition of K IR channels, NO and PG synthesis, and Na + /K + ‐ATPase would not alter the ability of ATP to blunt α 1 ‐adrenergic vasoconstriction. In healthy subjects, we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (FVC) to intra‐arterial infusion of phenylephrine (PE; α 1 ‐agonist) during ATP or control vasodilatator infusion, before and after K IR channel inhibition alone (barium chloride; n = 7; Protocol 1); NO ( l ‐NMMA) and PG (ketorolac) inhibition alone, or combined NO, PGs, Na + /K + ‐ATPase (ouabain) and K IR channel inhibition ( n = 6; Protocol 2). ATP attenuated PE‐mediated vasoconstriction relative to adenosine (ADO) and sodium nitroprusside (SNP) (PE‐mediated ΔFVC: ATP: −16 ± 2; ADO: −38 ± 6; SNP: −59 ± 6%; P < 0.05 vs . ADO and SNP). Blockade of K IR channels alone or combined with NO, PGs and Na + /K + ‐ATPase, attenuated ATP‐mediated vasodilatation (∼35 and ∼60% respectively; P < 0.05 vs . control). However, ATP maintained the ability to blunt PE‐mediated vasoconstriction (PE‐mediated ΔFVC: K IR blockade alone: −6 ± 5%; combined blockade:−4 ± 14%; P > 0.05 vs . control). These findings demonstrate that intravascular ATP modulates α 1 ‐adrenergic vasoconstriction via pathways independent of K IR channels, NO, PGs and Na + /K + ‐ATPase in humans, consistent with a role for endothelium‐derived hyperpolarization in functional sympatholysis.