Na+(Li+)-H+ exchange in rat renal cortical vesicles with endosomal characteristics

Abstract

Preparations of rat renal cortical endocytic vesicles, loaded with fluorescein isothiocyanate-labeled dextran (FITC-dextran) in vivo had an ATP-dependent N-ethylmaleimide (NEM)-sensitive H+ pump and, in addition, showed a limited activity of an electroneutral Na(+)-H+ and Li(+)-H+ exchanger. However, the majority of FITC-dextran-containing vesicles possessed only the H+ pump. The H+ pump and the electroneutral Na+(Li+)-H+ antiporter colocalized in only a small proportion of FITC-dextran-containing vesicles. These vesicles also exhibited marked electrically coupled Na+, Li+, and H+ movement due to conductances in the vesicle membrane for these cations. As measured by the acridine orange fluorescence quench method, the electroneutral Na(+)-H+ exchange in endosomal preparations obeyed hyperbolic kinetics with an apparent Michaelis constant (Km) for Na+ of 5.9 mM and resembled that of brush-border membrane vesicles (BBMV) isolated from the same tissue (Km for Na+, 3.7 mM). The endosomal Na+(Li+)-H+ exchange was not affected by the presence or absence of ATP or NEM. However, Li+, added before or simultaneously with Na+, inhibited the endosomal Na(+)-H+ antiporter. The Na+(Li+)-H+ antiporter was insensitive to extravesicular amiloride but was partially inhibited by intravesicular amiloride. Acidic buffers stimulated the activity of the electroneutral exchanger. We found that vesicles with similar characteristics contaminate preparations of right-side-out BBMV. We conclude that the vesicles with endosomal characteristics, which contain both a vacuolar-type H+ pump and the Na+(Li+)-H+ antiporter, are either a special pool of endocytic vesicles responsible for the trafficking of the exchanger between intracellular and luminal membranes or are inside-out BBMV that form during the homogenization of the tissue.