Procainamide transport in rabbit renal cortical brush border membrane vesicles

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Abstract
Previous studies in canine and rat renal cortical brush border membrane vesicles (BBMV) and some results in isolated perfused rabbit proximal tubules indicate that organic cations may be transported across the apical cell membrane by an organic cation/proton exchange process. To determine more directly whether organic cations are transported across the apical cell membrane of rabbit proximal tubules, [3H]procainamide uptake in BBMV was studied. Procainamide uptake was linearly related to the inverse of the media osmolarity, indicating uptake into an intravesicular space. A proton gradient directed from vesicle interior outwardly stimulated and an opposite gradient inhibited procainamide uptake. pH-stimulated uptake was inhibited by the proton ionophore carbonyl cyanide p-trifluoromethoxyphenyl hydrazone (FCCP) and was also reduced by an inwardly directed sodium gradient. pH-stimulated procainamide uptake was inhibited by other organic cations including the quarternary ammonium ion tetramethylammonium, indicating that the effect of proton gradients was not due to changes in nonionic diffusion. pH-stimulated procainamide uptake at 10 s was saturable with an apparent Km of 5.4 X 10(-4) M and Vmax of 4.7 X 10(-10) mol X mg protein-1. Uptake of [3H]procainamide was enhanced when BBMV were preloaded with nonradioactive procainamide but this was prevented by FCCP and valinomycin. Finally, an outwardly directed potassium gradient in the presence of valinomycin failed to significantly stimulate procainamide uptake. These results are consistent with a mechanism of secretion that involves electroneutral exchange of procainamide for protons across the apical cell membrane of rabbit proximal tubules.