In muscle cells such as cardiac, skeletal, vascular and nonvascular smooth muscle, ATP sensitive K+ channels assemble as octamers of four Kir 6.x subunits and four low-affinity sulfonyl urea receptor 2 subunits (SUR2). The human gene encoding SUR2 gives rise to two splice variants, SUR2A and SUR2B. These channels are blocked by excess intracellular levels of ATP. When the ATP is low, ATP dissociates and the channel opens to allow K+ efflux (Krenz et al. 2002).