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The characteristics of 86Rb+ fluxes through conductive channels in basolateral-membrane vesicles isolated from pars recta of rabbit kidney proximal tubule were investigated. In RbCl-, KCl-and NaCl-loaded vesicles a transient and almost equal accumulation of 86Rb+ was observed. The uptakes of 86Rb+ were inhibited to the same extent by 10 mM-BaCl2 in all loadings. The accumulation was driven by an electrical diffusion potential. The 86Rb+ flux was dependent on intravesicular Ca2 . Increasing concentrations of Ca2+ gradually decreased the 86Rb+ uptake. At 10 /sM-Ca2 the radioisotope flux was below 20 % of control. The vesicles containing the channel showed very low selectivity among the univalent cations K+, Rb+, Li+, Na+ and choline+.

The characteristics of "Rb+ fluxes through conductive channels in basolateral-membrane vesicles isolated from pars convoluta of rabbit proximal tubule were investigated. In KC1 loaded vesicles a transient accmnulation of "Rb+ was observed which was inhibited by BaC4. The accumulation was driven by an electrical diffusion potential, as shown in experiments using membrane vesicles loaded with LixS04 and an outwardly directed Li+ gradient established with a Li+-ionophote. The vesicles containing the channel showed a cation selectivity with the order K+ = Rb+ > > Li+ 3 Na+ > choline+. The wRb+ flux was dependent on intravesicular Ca 2+ . Increasing concentrations of Ca2+ gradually decreased the =Rb+ uptake.

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