Physiological roles of ATP-sensitive K+ channels in smooth muscle

Potassium channels that are inhibited by intracellular ATP (ATP(i)) were first identified in ventricular myocytes, and are referred to as ATP-sensitive K+ channels (i.e. K-ATP channels). Subsequently, K+ channels with similar characteristics have been demonstrated in many other tissues (pancreatic beta-cells, skeletal muscle, central neurones, smooth muscle). Approximately one decade ago, K-ATP channels were cloned and were found to be composed of at least two subunits: an inwardly rectifying K+ channel six family (Kir6.x) that forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR) that accounts for several pharmacological properties. Various types of native K-ATP channels have been identified in a number of visceral and vascular smooth muscles in single-channel recordings. However, little attention has been paid to the molecular properties of the subunits in K-ATP channels and it is important to determine the relative expression of K-ATP channel components which give rise to native K-ATP channels in smooth muscle. The aim of this review is to briefly discuss the current knowledge available for K-ATP channels with the main interest in the molecular basis of native K-ATP channels, and to discuss their possible linkage with physiological functions in smooth muscle.