Characterization of the Na+/H+ exchanger in the luminal membrane of the distal nephron.

In the rabbit as well as the rat, a Na+/H+ exchanger is expressed in the apical membrane of both the proximal and distal tubules of the renal cortex. Whereas the isoform derived from the proximal tubule has been extensively studied, little information is available concerning the distal luminal membrane isoform. To better characterize the latter isoform, we purified rabbit proximal and distal tubules, and examined the ethylpropylamiloride (EIPA)-sensitive 22Na uptake by the luminal membrane vesicles from the two segments. The presence of 100 micron EIPA in the membrane suspension decreased the 15 sec Na+ uptake to 75.70 +/- 4.70% and 50.30 +/- 2.23% of the control values in vesicles from proximal and distal tubules, respectively. The effect of EIPA on 35 mM Na+ uptake was concentration dependent, with a IC50 of 700 micron and 75 micron for the proximal and distal luminal membranes. Whereas the proximal tubule membrane isoform was insensitive to cimetidine and clonidine up to a concentration of 2 mM, the 35 mM Na+ uptake by the distal membrane was strongly inhibited by cimetidine (IC50 700 micron) and modestly inhibited by clonidine (IC50 1.6 mM). The incubation of proximal tubule suspensions with 1 mM (Bu2) cAMP decreased the 15-sec EIPA-sensitive Na+ uptake by the brush border membranes to 24.1 +/- 2.38% of the control values. Unexpectedly, the same treatment of distal tubules enhanced this uptake by 46.5 +/- 10.3%. Finally, incubation of tubule suspensions with 100 nm phorbol 12-myristate 13-acetate (PMA) decreased the exchanger activity to 58.6 +/- 3.04% and 79.7 +/- 3.21% of the control values in the proximal and distal luminal membranes, respectively. In conclusion, the high sensitivity of the distal luminal membrane exchanger to various inhibitors, and its stimulation by cAMP-dependent protein kinase A, indicate that this isoform differs from that of the proximal tubule and probably corresponds to isoform 1.