ATP-sensitive inward rectifier and voltage- and calcium-activated K+ channels in cultured pancreatic islet cells.

K+ channels in cultured rat pancreatic islet cells have been studied using patch-clamp single-channel recording techniques in cell-attached and excised inside-out and outside-out membrane patches. Three different K+-selective channels have been found. Two inward rectifier K+ channels with slope conductances of about 4 and 17 pS recorded under quasi-physiological cation gradients (Na+ outside, K+ inside) and maximal conductances recorded in symmetrical K+-rich solutions of about 30 and 75 pS, respectively. A voltage- and calcium-activated K+ channel was recorded with a slope conductance of about 90 pS under the same conditions and a maximal conductance recorded in symmetrical K+-rich solutions of about 250 pS. Single-channel current recording in the cell-attached conformation revealed a continuous low level of activity in an apparently small number of both the inward rectifier K+ channels. But when membrane patches were excised from the intact cell a much larger number of inward rectifier K+ channels became transiently activated before showing an irreversible decline. In excised patches opening and closing of both the inward rectifier K+ channels were unaffected by voltage, internal Ca2+ or externally applied tetraethylammonium (TEA) but the probability of opening of both inward rectifier K+ channels was reduced by internally applied 1-5 mM adenosine-5'-triphosphate (ATP). The large K+ channel was not operational in cell-attached membrane patches, but in excised patches it could be activated at negative membrane potentials by 10(-7) to 10(-6) M internal Ca2+ and blocked by 5-10 mM external TEA.