Endothelial biology and ATP-sensitive potassium channels

ATP sensitive potassium (KATP) channels are metabolic sensors with channel activity promoted by decreases in ATP andnor increases in MgADP. The most studied are channels present in cardiac myocytes and pancreatic b cells but KATP currents also exist more widely in a range of tissues and organs [1,2]. The channel is a hetero-octamer comprising four sulphonylurea receptors and four pore-forming inwardly rectifying subunits (either Kir6.2 or Kir6.1). Kir6.2 underlies many of the traditional KATP channel populations. Kir6.1 is less studied and thought to underlie the current in vascular smooth muscle cells, however it appears to be ubiquitously expressed [1,3]. KATP currents are also present in endothelial cells but defining their physiological role is difficult; pharmacological tools and global knockout mice will also lead to effects on vascular smooth muscle and nerve endings [4]. To overcome this limitation we have used crenloxP technology to selectively delete the KATP channel subunit Kir6.1 in endothelium (eKO) and studied the phenotype of the mice [5]. The deletion of this subunit was indeed sufficient to abolish expression of KATP currents in endothelial cells when studied using patch-clamp [5]. Intracellular calcium plays an important role in endothelial function by being linked to mediator release and is also important for angiogenesis. Unlike muscle cells the pathway for membrane calcium entry is not mediated by voltage-dependent calcium channels but by TRP channels (including TRPC1/3/4/6 and TRPV4) and the OrainSTIM1 complex [6]. Thus calcium entry into endothelial cells is determined by the electrochemical gradient and as result, hyperpolarisation increases calcium entry. A number of pathways, in addition to membrane potential can increase calcium entry, and these include store and receptor-operated calcium entry. The receptors are coupled to the Gqn11 family of G-protein and include muscarinic, protease-activated and bradykinin receptors. Cytosolic endothelial calcium is also significantly influenced by IP3 mediated release from intracellular stores. Thus opening KATP channels could potentially promote calcium entry as illustrated in Figure 1. Indeed this was the case when measuring the effect of potassium channel openers and KATP channel inhibitors on calcium concentration in endothelial cells present on aortic valve leaflets. These drug effects were abolished in the eKO mice. We then examined the role of the endothelial KATP channel in coronary artery reactivity. In eKO mice basal coronary perfusion pressure was not, elevated but vasorelaxation was impaired during hypoxia. Furthermore, cardiac ischaemia-reperfusion injury was increased in eKO mice compared to littermate controls. Thus it is clear that the endothelial channel contributes to the vascular response to hypoxia. In fact when compared to a murine line in which KATP channels were deleted from vascular smooth cells the magnitude of the fall in coronary perfusion pressure during hypoxia and that of infarct size after ischaemia reperfusion injury were at least comparable and possibly larger. A critical question is how is the metabolic signal sensed and transduced by the endothelial cells. We explored one possibility, namely that release of adenosine from ischaemic tissues may bind to the adenosine family of G-protein coupled receptor. It has been