ATP-Sensitive Potassium Channels

ATP-sensitive potassium (KATP) channels link membrane excitability to metabolism. They are regulated by intracellular nucleotides and by other factors including membrane phospholipids, protein kinases and phosphatases. KATP channels comprise octamers of four Kir6 pore-forming subunits associated with four sulphonylurea receptor subunits. The exact subunit composition differs between the tissues in which the channels are expressed, which include pancreas, cardiac, smooth and skeletal muscle and brain. KATP channels are targets for antidiabetic sulphonylurea blockers, and for channel opening drugs that are used as antianginals and antihypertensives. This review focuses on non-pancreatic KATP channels. In vascular smooth muscle, KATP channels are extensively regulated by signalling pathways and cause vasodilation, contributing both to resting blood flow and vasodilator-induced increases in flow. Similarly, KATP channel activation relaxes smooth muscle of the bladder, gastrointestinal tract and airways. In cardiac muscle, sarcolemmal KATP channels open to protect cells under stress conditions such as ischaemia or exercise, and appear central to the protection induced by ischaemic preconditioning (IPC). Mitochondrial KATP channels are also strongly implicated in IPC, but clarification of their exact role awaits information on their molecular structure. Skeletal muscle KATP channels play roles in fatigue and recovery, K+ efflux, and glucose uptake, while neuronal channels may provide ischaemic protection and underlie the glucose-responsiveness of hypothalamic neurones. Current therapeutic considerations include the use of KATP openers to protect cardiac muscle, attempts to develop openers selective for airway or bladder, and the question of whether block of extra-pancreatic KATP channels may cause adverse cardiovascular side-effects of sulphonylureas.