REGULATION BY GTP OF A CA2+-ACTIVATED K+ CHANNEL IN THE APICAL MEMBRANE OF RABBIT CORTICAL COLLECTING DUCT CELLS

Ca2+-activated K+ channels play an important role in Ca2+ signal transduction and may be regulated by mechanisms other than a direct effect of Ca2+ Inside-out patches of the apical membrane of confluent transformed rabbit cortical collecting duct cells cultured on collagen were subjected to patch clamp analysis. Two types of K+ channel, of medium and high conductance, were observed. The latter channel was characterized by a K+/Na+ permeability ratio of 10, an inwardly rectified current, a conductance of 80 pS at 0 mV, and an open probability dependent on both voltage and Ca2+. Guanosine 5'-triphosphate (GTP) but not a guanosine 5'-diphosphate (GDP) analogue, adenosine 5'-triphosphate (ATP), cytidine 5'-triphosphate (CTP), or inosine 5'-triphosphate (ITP), inhibited the activity of this Ca2+-activated K+ channel. The inhibitory effect of GTP was dose dependent, with a 50% inhibitory concentration of 10(-5) M in the absence of Mg2+. In the presence of Mg2+ (1 mM), which is required for the binding of GTP to G proteins, the 50% inhibitory concentration decreased to 3 X 10(-12) M. Pertussis toxin or cholera toxin (each at 10 ng/ml) did not prevent the inhibitory effect of GTP. After removal of GTP from the medium bathing an inhibited channel, subsequent application of Ca2+ failed to activate the channel. Ca2+- activated K+ channels of smooth muscle cells and proximal tubule cells did not respond to GTP. Thus, the Ca2+-activated K+ channel in the apical membrane of collecting duct cells is inhibited by GTP, which appears to exert its effect via a G protein that is insensitive to both cholera and pertussis toxins.