Segmental localization of the rabbit renal proximal tubular Na+-H+ exchange system

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Abstract
The activity of the Na+-H+ exchanger in rabbit proximal tubule was investigated by using luminal membrane vesicles prepared from "outer cortex" (proximal convoluted tubule) or "outer medulla" (proximal straight tubule). The purity of the preparations was examined by measuring the activity of several marker enzymes, and the degree of cross-contamination and the functional state of the membrane vesicles were assessed by studying Na+-dependent uptake of D-glucose. The Na+ uptake by pars convoluta membrane vesicles exhibited an overshoot in the presence of an intravesicular greater than extravesicular H+ gradient. The overshoot was eliminated by omitting or reversing the transmembranal H+ gradient or by adding amiloride. In contrast, Na+ uptake by pars recta membrane vesicles did not show an overshoot and was independent of H+ gradients and of amiloride. However, Na+ uptake by pars recta membrane vesicles pretreated with monensin exhibited an overshoot. This overshoot apparently was amiloride insensitive. The findings propose that the Na+-H+ exchanger is predominantly operative in the proximal convoluted tubule and is either lacking or of minor significance in the proximal straight tubule.