A Ca-activated K channel from rabbit renal brush-border membrane vesicles in planar lipid bilayers

Rabbit renal brush-border membranes were fused to planar lipid bilayers to gain insight into the nature and properties of ion channels from the luminal membrane of the proximal tubule. Fusion was obtained using osmotic gradients. A large conductance channel was commonly observed. Measurements of reversal potentials indicated that the channel was selective for K over Rb, Na, and Cl. Channel open probability was increased by membrane depolarization and by increased Ca activity on the intracellular face of the channel. The channel was inhibited by charybdotoxin (CTX), a protein from leiurus venom, from the external side of the channel. The channel was also blocked by Ba and quinidine added to the intracellular bathing solution. Na added to the intracellular bathing solution reduced current amplitude in a voltage-dependent fashion. In addition, methylisobutyl amiloride, an analogue of the K-sparing diuretic amiloride, inhibited channel activity when added to the external solution. The possible physiological role of the channel is discussed. The usefulness to the study of renal ion channels of the technique of fusing membrane vesicles to planar lipid bilayers is evaluated.