Larson, Dennis G.

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

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.