Role of Endothelial K+ Channels in NO-Dependent Vasorelaxant Responses to Acetylcholine in Rat Aorta.
Endothelium-dependent vasorelaxant responses to acetylcholine (Ach) in rat aorta are mediated solely by NO. Rings precontracted with U46619 were used to investigate the role of endothelial K+channels. Thus, any effect of K+ channel inhibitors on Ach responses in the absence of an effect on those to nitroprusside (NP) can be attributed to interference with Ach-induced stimulation of NO. Vasorelaxant responses to Ach (log EC50 -7.29M) were abolished by removal of the endothelium or inhibition of NO synthesis with nitroarginine (100μM) which potentiated responses to NP (log EC50 -9.41M vs -8.47M for control). In the presence of TEA (10mM) to inhibit K+ channels, the dose-response curve for Ach, but not NP, was shifted to the right (log EC50 -6.06). Elevation of extracellular K+ (25mM KCl)also shifted the dose-response curve for Ach to the right. Inhibitors of specific types of K+ channels: BaCl2(30μM), apamin (100nM), glibenclamide (10μM), charybdotoxin (50nM) and iberiotoxin (100nM) were without effect on dose-response curves to either Ach or NP. However, the combination of apamin (100nM) and charybdotoxin (50nM) but not apamin plus iberiotoxin, reduced relaxant responses to Ach (log EC50 -6.95M) without affecting those to NP.These results confirm that Ach-induced relaxation of rat aorta is mediated entirely by endothelium-derived NO, the release of which apparently involves hyperpolarization of the endothelium. This effect is dependent on activation of a K+ channel that is blocked by a combination of apamin/charybdotoxin but neither agent alone, possibly indicating characteristics of both Ca2+- activated and voltage-dependent K+ channels.