Key points• ATP is a substance in the blood vessels that can cause vasodilatation and increase blood flow and oxygen delivery in humans.• The exact signalling pathways that ATP stimulates to cause vasodilatation are not well known.• We show that a large portion of ATP-mediated vasodilatation occurs through the activation of inwardly rectifying potassium channels (K IR ).• Our results lend insight into the vasodilator mechanisms of ATP, a substance that is important for vascular control.• Further, our results may stimulate additional investigations in humans regarding the activation of K IR channels and subsequent vascular hyperpolarization during other physiologically relevant conditions.Abstract Circulating ATP possesses unique vasomotor properties in humans and has been hypothesized to play a role in vascular control under a variety of physiological conditions. However, the primary downstream signalling mechanisms underlying ATP-mediated vasodilatation remain unclear. The purpose of the present experiment was to determine whether ATP-mediated vasodilatation is independent of nitric oxide (NO) and prostaglandin (PG) synthesis and occurs primarily via the activation of Na + /K + -ATPase and inwardly rectifying potassium (K IR ) channels in humans. In all protocols, young healthy adults were studied and forearm vascular conductance (FVC) was calculated from forearm blood flow (measured via venous occlusion plethysmography) and intra-arterial blood pressure to quantify local vasodilatation. Vasodilator responses (% FVC) during intra-arterial ATP infusions were unchanged following combined inhibition of NO and PGs (n = 8; P > 0.05) whereas the responses to KCl were greater (P < 0.05). Combined infusion of ouabain (to inhibit Na + /K + -ATPase) and barium chloride (BaCl 2 ; to inhibit K IR channels) abolished KCl-mediated vasodilatation (n = 6; % FVC = 134 ± 13 vs. 4 ± 5%; P < 0.05), demonstrating effective blockade of direct vascular hyperpolarization. The vasodilator responses to three different doses of ATP were inhibited on average 56 ± 5% (n = 16) following combined ouabain plus BaCl 2 infusion. In follow-up studies, BaCl 2 alone inhibited the vasodilator responses to ATP on average 51 ± 3% (n = 6), which was not different than that observed for combined ouabain plus BaCl 2 administration. Our novel results indicate that the primary mechanism of ATP-mediated vasodilatation is vascular hyperpolarization via activation of K IR channels. These observations translate in vitro findings to humans in vivo and may help explain the unique vasomotor properties of intravascular ATP in the human circulation.

Crecelius AR, Kirby BS, Luckasen GJ, Larson DG, Dinenno FA. Mechanisms of rapid vasodilation after a brief contraction in human skeletal muscle. Am J Physiol Heart Circ Physiol 305: H29-H40, 2013. First published May 3, 2013 doi:10.1152/ajpheart.00298.2013.-A monophasic increase in skeletal muscle blood flow is observed after a brief single forearm contraction in humans, yet the underlying vascular signaling pathways remain largely undetermined. Evidence from experimental animals indicates an obligatory role of vasodilation via K ϩ -mediated smooth muscle hyperpolarization, and human data suggest little to no independent role for nitric oxide (NO) or vasodilating prostaglandins (PGs). We tested the hypothesis that K ϩ -mediated vascular hyperpolarization underlies the rapid vasodilation in humans and that combined inhibition of NO and PGs would have a minimal effect on this response. We measured forearm blood flow (Doppler ultrasound) and calculated vascular conductance 10 s before and for 30 s after a single 1-s dynamic forearm contraction at 10%, 20%, and 40% maximum voluntary contraction in 16 young adults. To inhibit K ϩ -mediated vasodilation, BaCl2 and ouabain were infused intraarterially to inhibit inwardly rectifying K ϩ channels and Na ϩ -K ϩ -ATPase, respectively. Combined enzymatic inhibition of NO and PG synthesis occurred via N G -monomethyl-L-arginine (L-NMMA; NO synthase) and ketorolac (cyclooxygenase), respectively. In protocol 1 (n ϭ 8), BaCl 2 ϩ ouabain reduced peak vasodilation (range: 30 -45%, P Ͻ 0.05) and total postcontraction vasodilation (area under the curve, ϳ55-75% from control) at all intensities. Contrary to our hypothesis, L-NMMA ϩ ketorolac had a further impact (peak: ϳ60% and area under the curve: ϳ80% from control). In protocol 2 (n ϭ 8), the order of inhibitors was reversed, and the findings were remarkably similar. We conclude that K ϩ -mediated hyperpolarization and NO and PGs, in combination, significantly contribute to contraction-induced rapid vasodilation and that inhibition of these signaling pathways nearly abolishes this phenomenon in humans.hyperemia; exercise; potassium THE REGULATION of skeletal muscle hyperemia during muscle contractions is complex and involves a variety of signals that control both the arteriovenous perfusion pressure gradient and arteriolar caliber (10, 13). In an attempt to isolate the local mechanisms underlying exercise hyperemia, early experiments used a single brief muscle contraction to allow for contractioninduced hyperemia without the continuous interruption of the blood flow response or further stimulus for hyperemia, as occurs with repeated contractions (16). In this regard, the single contraction model can serve as a tool to examine feedforward mechanisms of hyperemia that are largely independent of changes in tissue oxidative metabolism (48). The typical response is characterized by an intensity-dependent, rapid, monophasic increase in blood flow that occurs immediately (within one cardiac cycle) after contraction, achieves full magni...

Rationale Reactive hyperemia (RH) in the forearm circulation is an important marker of cardiovascular health yet the underlying vasodilator signaling pathways are controversial and thus remain unclear. Objective We hypothesized RH occurs via activation of inwardly-rectifying potassium (KIR) channels and Na+/K+-ATPase and is largely independent of the combined production of the endothelial autocoidsnitric oxide (NO) and prostaglandins (PGs) in young healthy humans. Methods and Results In 24 (23±1 years) subjects, we performed RH trials by measuring forearm blood flow (FBF; venous occlusion plethysmography) following 5 minutes of arterial occlusion. In Protocol 1, we studied 2 groups of 8 subjects and assessed RH in the following conditions; Group 1:control (saline), KIR channel inhibition (barium chloride; BaCl2), combined inhibition of KIR channels and Na+/K+-ATPase (BaCl2+ouabain, respectively), and combined inhibition of KIR channels, Na+/K+-ATPase, NO and PGs (BaCl2+ouabain+L-NMMA+ketorolac, respectively). Group 2 received ouabain rather than BaCl2 in the 2nd trial. In Protocol 2 (n=8), 3 RH trials were performed: control, L-NMMA+ketorolac, and L-NMMA+ketorolac+BaCl2+ouabain. All infusions were intra-arterial (brachial). Compared to control, BaCl2 significantly reduced peak FBF (-50±6%; P<0.05) whereas ouabain and L-NMMA+ketorolac did not. Total FBF (area under curve) was attenuated by BaCl2 (-61±3%) and ouabain (-44±12%) alone and this effect was enhanced when combined (-87±4%), nearly abolishing RH. L-NMMA+ketorolacdid not impact total RH FBF prior to or after administration of BaCl2+ouabain. Conclusions Activation of KIR channels is the primary determinant of peak RH, whereas activation of both KIR channels and Na+/K+-ATPase explains nearly all of total RH in humans.

We tested the hypothesis that activation of inwardly rectifying potassium (KIR) channels and Na(+)-K(+)-ATPase, two pathways that lead to hyperpolarization of vascular cells, contributes to both the onset and steady-state hyperemic response to exercise. We also determined whether after inhibiting these pathways nitric oxide (NO) and prostaglandins (PGs) are involved in the hyperemic response. Forearm blood flow (FBF; Doppler ultrasound) was determined during rhythmic handgrip exercise at 10% maximal voluntary contraction for 5 min in the following conditions: control [saline; trial 1 (T1)]; with combined inhibition of KIR channels and Na(+)-K(+)-ATPase alone [via barium chloride (BaCl2) and ouabain, respectively; trial 2 (T2)]; and with additional combined nitric oxide synthase (N(G)-monomethyl-l-arginine) and cyclooxygenase inhibition [ketorolac; trial 3 (T3)]. In T2, the total hyperemic responses were attenuated ~50% from control (P < 0.05) at exercise onset, and there was minimal further effect in T3 (protocol 1; n = 11). In protocol 2 (n = 8), steady-state FBF was significantly reduced during T2 vs. T1 (133 ± 15 vs. 167 ± 17 ml/min; Δ from control: -20 ± 3%; P < 0.05) and further reduced during T3 (120 ± 15 ml/min; -29 ± 3%; P < 0.05 vs. T2). In protocol 3 (n = 8), BaCl2 alone reduced FBF during onset (~50%) and steady-state exercise (~30%) as observed in protocols 1 and 2, respectively, and addition of ouabain had no further impact. Our data implicate activation of KIR channels as a novel contributing pathway to exercise hyperemia in humans.