Mechanisms of Na+ uptake into renal brush border membrane vesicles

Brush border membrane vesicles were used to investigate the pathways for Na+ uptake across the apical membrane of the renal proximal tubular cell. The kinetics of uptake in the absence of organic solutes were consistent with parallel saturable and nonsaturable pathways. At pH equilibrium (pHin = pHout = 7.5), the Jmax and Kt for saturable uptake were 41 +/- 15 (+/- SE) nmol X mg-1 X min-1 and 33 +/- 9, respectively, and the apparent permeability coefficient, P'Na, was 0.27 +/- 0.02 microliters X mg-1 X min-1. As the equilibrium pH was varied between 6.0 and 8.0, no consistent trend for Kt or P'Na was observed; Jmax varied up to twofold. In contrast, in the presence of an outward H+ gradient (pHin = 6.0 vs. pHout = 7.5), the Kt decreased by an order of magnitude, with little change in Jmax. At low sodium concentrations (1 mM) external Li+ and NH+4, and to a lesser extent K+, Rb+, and Cs+, inhibited Na+ uptake. Amiloride (10(-3) M) inhibited 1 mM Na+ uptake by 80% even in the absence of a H+ gradient. Uptake also varied with the anion composition at high sodium concentrations (100 mM), as predicted from the anion permeabilities. Sodium uptake was more sensitive to variations in membrane potential at high sodium concentrations than at low concentrations. On the basis of these experiments we suggest that the saturable Na+ uptake occurs via an electroneutral Na+-H+ antiporter and that the diffusive flux occurs through a conductive pathway.