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

Abstract

An extracellular Ca²⁺-activated K channel (K_R channel), having a conductance of 30 pS, was identified in isolated single smooth muscle cells from porcine coronary artery. The K_R channel was active at > 10⁻⁵ M Ca²⁺, and was blocked by 4-aminopyridine (4AP). At < 10⁻⁶ M Ca²⁺, the K_R 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 K_R channel activity and cell contraction: cells contracted under conditions in which the KR channel became inactive, but were relaxed when the K_R channel was active. As the K_R 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.