Ca2+ and pH regulation of K+ channels in membrane vesicles of rabbit proximal tubule.

The effect of Ca2+ and pH on the renal epithelial K+ channel was investigated by measuring the Ba2(+)-sensitive 86Rb+ fluxes in membrane vesicles from pars convoluta of rabbit proximal tubule. It was found that the presence of nanomolar concentrations of Ca2+ in the internal compartment (cytoplasmic) of the vesicles ([Ca2+]i) substantially lowered the channel-mediated flux. Ba2(+)-sensitive 86Rb+ uptake was completely blocked by 10 microM [Ca2+]i. This inhibitory effect of Ca2+ was strongly dependent on pH. Thus 0.1 microM [Ca2+]i produced a maximal inhibition of 86Rb+ uptake at pH greater than 7.4 but had no effect at pH less than 7.0. The tracer fluxes measured in the absence of Ca2+ were pH independent over this range. The data are compatible with the model that Ca2+ blocks K+ channels by binding to a site composed of one or several deprotonated groups. The protonation of any one of these groups prevents Ca2+ from binding to this site but does not by itself block transport.