Potassium channels in epithelial cells.

The broad array of K- channels in epithelial cells includes voltage-dependent (mainly outward) and Ca(2+)-activated channels, and K+ channels modulated by adenosine triphosphate (ATP). Voltage-dependent K+ channels mediate Na+/K+ absorption and secretion; typically, they are found in the basolateral membrane and exhibit burst activity. Ca(2+)-activated K+ (Ca2+/K+) channels regulate activity by decreasing Ca2+ influx via voltage-gated Ca2+ channels. Ca2+/K+ channels exhibit conductances of 4-300 pS, and have a low open probability (< 10(-7)) at the level of the resting membrane potential. ATP-sensitive channels have been observed mainly in insulin-secreting pancreatic beta-cells and in the urinary tract, where the open state is rapidly closed by ATP. The channels are voltage-dependent, exhibit burst activity, and, in the case of the urinary-tract cells, are Ca2+ dependent. Chemical compounds that selectively open or block K+ channels have been exploited to characterize channels found in different cells, but no opener or blocker has been found that specifically affects only one type of K+ channel. Specialized model systems and recombinant techniques have led to a general understanding of the structure of K+ channels, but many important details remain to be determined.