The characteristics of 86Rb+ fluxes through conductive channels in basolateral-membrane vesicles isolated from pars convoluta of rabbit proximal tubule were investigated. In KCl loaded vesicles a transient accumulation of 86Rb+ was observed which was inhibited by BaCl2. The accumulation was driven by an electrical diffusion potential, as shown in experiments using membrane vesicles loaded with Li2SO4 and an outwardly directed Li+ gradient established with a Li+-ionophore. The vesicles containing the channel showed a cation selectivity with the order K+ = Rb+ < < Li+ ⩾ Na+ < choline+. The 86Rb+ flux was dependent on intravesicular Ca2+. Increasing concentrations of Ca2+ gradually decreased the 86Rb+ uptake.
