Pharmacological Profile of U‐37883A,a Channel Blocker of Smooth Muscle‐Type ATP‐Sensitive K+ Channels

U‐37883A (PNU‐37883A, guanidine; 4‐morpholinecarboximidine‐N‐1‐adamantyl‐N′‐cyclohexyl hydrochloride) was originally developed as a potential diuretic with specific binding in kidney and vascular smooth muscle rather than in brain or pancreatic β cells. U‐37883A inhibits ATP‐sensitive K⁺ channels (K_ATP channels) in vascular smooth muscle at submicromolar concentrations whilst even at high concentrations (≥10 μM) it has no inhibitory effect at pancreatic, cardiac or skeletal K_ATP channels. Thus, it is generally thought that U‐37883A is a selective inhibitor of vascular smooth muscle K_ATP channels. Approximately one decade ago, K_ATP channels were cloned and found to consist of at least two subunits: an inwardly‐rectifying K⁺ channel six family (K_ir6.x; K_ir6.1 and K_ir6.2) which forms the ion conducting pore and a modulatory sulphonylurea receptor (SUR.x; SUR1, SUR2A, and SUR2B) that accounts for several pharmacological properties. It is generally believed that different combinations of K_ir6.x and SUR.x determine the molecular properties of K_ATP channels. Thus, K_ir6.2/SUR1 channel represents the pancreatic β‐cell K_ATP channel, K_ir6.2/SUR2A channel is thought to represent the cardiac K_ATP channel, whereas K_ir6.1/SUR2B channel is likely to represent the vascular smooth muscle K_ATP channel. Recent molecular studies have shown that U‐37883A selectively suppresses the activity of recombinant K_ATP channels which contain K_ir6.1 subunits in the channel pore unit. It was thus thought that U‐37883A was a selective pharmacological tool which could be used to investigate the activity of vascular smooth muscle K_ATP channels. However, due to its multiple pharmacological actions on several ion channels and poor tissue selectivity, U‐37883A should not be viewed as a selective blocker of smooth muscle K_ATP channels.