ATP-mediated vasodilatation: all thanks to potassium?

Emerging evidence suggests circulating adenosine triphosphate (ATP) plays an important role in local blood flow control. ATP is primarily thought to bind to P2Y receptors on the endothelium, and activate downstream vasodilator pathways. Specific mechanisms behind ATP-mediated vasodilatation are currently unknown in humans; however, research supports the contribution of nitric oxide (NO), prostaglandins (PGs) and endothelial-derived hyperpolarization (EDH) via one or several potassium (K+) channels. Despite consistent findings in vitro, the contribution of NO and PGs to ATP-mediated vasodilatation in vivo is contradictory. Specifically, Crecelius et al. (2011) demonstrated NO and PGs to contribute to only ∼20–30% of the vasodilatory response to exogenous ATP in healthy humans, suggesting other mechanisms must be largely responsible for the increase in blood flow. Using evidence from in vitro research identifying a role for K+ channel activation in ATP-mediated vasodilation, Crecelius et al. (2012) continued to advance our understanding of local blood flow control in a recent article published in The Journal of Physiology. K+ efflux from small and intermediate-conductance Ca2+-activated K+ channels increases myoendothelial K+ concentration. This increase in K+ concentration results in activation of Na+/K+-ATPase and KIR channels leading to hyperpolarization of vascular smooth muscle cells. In vitro research has shown that application of ATP evokes local and conducted vasodilatation via these pathways. Thus, Crecelius et al. thought outside the NO/PG box by testing the hypotheses that ATP-mediated vasodilatation in the forearm is largely independent of NO and PG synthesis and occurs via Na+/K+ -ATPase and KIR channel activation in humans. To test their hypotheses, the authors assessed forearm vascular conductance (FVC) to intra-arterial infusions using standard venous occlusion plethysmography (VOP) procedures in 33 young, healthy adults. To confirm that ATP-mediated vasodilatation is largely independent of NO and PGs, and to focus on the role of K+ channels, Crecelius et al. used three separate but related protocols. First, the group tested the effect of combined NO synthase (NOS) and cyclooxygenase (COX) inhibition (l-NMMA and ketorolac, respectively) on ATP-mediated vasodilatation (n = 8). Confirming previous findings (Crecelius et al. 2011), combined NOS and COX inhibition significantly reduced FVC at rest, but not during ATP infusion. Thus, ATP vasodilatation appears to be largely independent of NO and PGs. The authors also examined K+-mediated vasodilatation using KCl. As expected, KCl infusion increased FVC; yet surprisingly, the relative response of KCl infusion was greater after NOS–COX inhibition. The second protocol examined the contribution of alternative downstream vascular control mechanisms to ATP-mediated vasodilatation via Na+/K+-ATPase and KIR channel inhibition. ATP (n = 8) or KCl (n = 6) were infused with and without ouabain and BaCl2 (Na+/K+-ATPase and KIR channel inhibitors, respectively). Co-infusion abolished forearm vasodilatation to KCl, and, as the authors hypothesized, oubain and BaCl2 attenuated vasodilatation in response to ATP (56% reduction). To test the independent effect of KIR channels on ATP-mediated smooth muscle relaxation, BaCl2 was administered alone in six additional subjects. The authors observed a significant reduction (51%) in the vasodilatory response to ATP. Taken together, inhibition of KIR had relatively the same effect as combined Na+/K+-ATPase and KIR inhibition, indicating KIR channels are a key player in ATP-mediated vasodilatation in the resting forearm circulation. However, the authors chose to compare double blockade of Na+/K+-ATPase and KIR channels vs. single blockade of KIR channels in separate research cohorts. Future studies might aim to examine the effect of single and double blockade in the same subset of subjects in order to control for inter-subject variability in blood flow responses to different pharmacological interventions. Additionally, figures directly comparing the two trials were not provided. A helpful approach to compare data from multiple protocols might be providing figures comparing percentage inhibition across doses and between various drug conditions. Such data presentation could provide readers with key information regarding the efficacy of inhibition and might allow insight into the relative importance of various K+ channels. In summary, Crecelius and colleagues provided novel and exciting research regarding specific mechanisms of ATP-mediated dilatation. They verified previous results (Crecelius et al. 2011) that blood flow responses to ATP in the forearm are largely independent of NO and PG synthesis. In addition, this research group was the first to identify vascular hyperpolarization in the forearm via KIR channel activation as one of the primary pathways underlying the vasodilator mechanisms of intravascular ATP in humans. Interestingly, blockade of KIR, with or without Na+/K+-ATPase blockade, did not completely abolish ATP-induced vasodilatation. One possible explanation for these findings is that ATP might have the ability to promote dilatation via auxiliary down-stream pathways. For example, ATP has been shown to stimulate epoxyeicosatrienoic acid (EETs) release from erythrocytes (Jiang et al. 2007), BKCa channel hyperpolarization of the smooth muscle, and subsequent vasodilatation independent of KIR channels. Therefore, a protocol utilizing blockade of KIR channels with and without blockade of cytochrome P450 epoxygenase (using miconazole or sulfaphenazole) could be considered in follow-up studies. This protocol would demonstrate the relative contribution of BKCa channel vasodilatation in response to ATP. The results from the current study have potential to provide insight to other applications regarding ATP-mediated vasodilatation such as exercise and disease. One exciting area to consider is the role of ATP dilatation and its associated pathways during steady-state exercise. ATP is thought to be released from red blood cells in response to a decrease in blood oxygen content and mechanical deformation and, thus, can contribute to the blood flow response seen with exercise. Future studies could test whether KIR channels are also a primary player in ATP-mediated dilatation during exercise. This study holds potential to provide clinical insight into patient populations such as those with hypertension. Adults with hypertension have been shown to exhibit endothelial dysfunction due, in part, to impairments in NO bioavailability, but other mechanisms contributing to the endothelial dysfunction have yet to be fully explored. Research in animal models suggests KIR channel function is impaired in the cerebral and mesenteric circulation during hypertension (Haddy et al. 2006). It is interesting to consider the potential for KIR channel impairments in other vascular beds, which could be explored in hypertensive humans using a protocol similar to that of the Crecelius group. If KIR channel function in response to ATP is indeed impaired in hypertensive humans, it might be interesting to consider the idea that expression and/or activity of BKCa channels is enhanced in this population. This could be in order to compensate for a reduction in KATP, Kv and KIR channel function in an attempt to ultimately restrict increases in vascular tone (Sobey, 2001). In conclusion, novel studies by Crecelius et al. uncovered one of the major players (KIR) of ATP-mediated vasodilatation. Their research allows for new avenues to test alternative downstream vasodilatory mechanisms and novel interactions between NO, PGs, EETs and KIR channels in a variety of vascular beds and cardiovascular conditions.