AJP - Renal Physiology, Vol 258, Issue 6 1634-F1639, Copyright © 1990 by American Physiological Society
Ca2+ and pH regulation of K+ channels in membrane vesicles of rabbit proximal tubule
C. Jacobsen, S. Mollerup and M. I. Sheikh
Institute of Medical Biochemistry, University of Aarhus, Denmark.
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.
