Modulation of Ca2+-activated K+ channels by Mg2+ and ATP in frog oxyntic cells.

Ca2+-activated K+ channels in the basolateral plasma membrane of bullfrog oxynticopeptic cells are intimately involved in the regulation of acid secretion. Patch-clamp techniques were applied to study the regulating mechanism of these channels. In the excised inside-out configuration, intracellular Mg2+ decreased channel activity in a dose-dependent manner. In the absence of Mg2+, administration of adenosine 5'-trisphosphate (ATP) to the cytoplasmic side also inhibited channel activity. On the other hand, in the presence of Mg2+, addition of ATP markedly increased channel activity. At a fixed concentration of free Mg2+, the Mg-ATP complex caused channel activation and shifted the dose response relationship between channel activity and the intracellular Ca2+ concentration to the left. A nonhydrolysable ATP analogue, adenosine 5'-[beta,gamma-imido]triphosphate (AMP-PNP) adenylyl [beta,gamma-methylene]diphosphate (AMP-PCP), could not substitute for ATP in channel activation, but a hydrolysable ATP analogue, adenosine 5'-O-(3-thiotriphosphate) (ATP[gammaS]) could do so. Furthermore, application of alkaline phosphatase to the cytoplasmic side inhibited channel activity. These results demonstrate that Ca2+-activated K+ channels are regulated by Mg2+ and ATP, and suggest that a phosphorylation reaction may be involved in the regulation mechanism of these channels.