Extracellular Ca2+-activated K channel in coronary artery smooth muscle cells and its role in vasodilation.

An extracellular Ca2+-activated K channel (KR channel), having a conductance of 30 pS, was identified in isolated single smooth muscle cells from porcine coronary artery. The KR channel was active at greater than 10(-5) M Ca2+, and was blocked by 4-aminopyridine (4AP). At less than 10(-6) M Ca2+, the KR channel became inactive, but could be activated by 2-nicotinamidethyl nitrate (nicorandil), or by 4-acetamide-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) applied to the pipette solution. It was found that there is a close correlation between the KR channel activity and cell contraction: cells contracted under conditions in which the KR channel became inactive, but were relaxed when the KR channel was active. As the KR channel is highly active in cells in physiological saline, we suggest that it controls the tonus of the coronary artery, as an endogenous dilating factor.