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 t he apical membrane of both the proximal and distal tubules of the rena l cortex. Whereas the isoform derived from the proximal tubule has bee n extensively studied, little information is available concerning the distal luminal membrane isoform. To better characterize the latter iso form, we purified rabbit proximal and distal tubules, and examined the ethylpropylamiloride (EIPA)-sensitive Na-22 uptake by the luminal mem brane vesicles from the two segments. The presence of 100 mu M 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 proxi mal and distal tubules, respectively. The effect of EIPA on 35 mM Nauptake was concentration dependent, with a IC50 of 700 mu M and 75 mu M 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 membra ne was strongly inhibited by cimetidine (IC50 700 mu M) and modestly i nhibited by clonidine (IC50 1.6 mM). The incubation of proximal tubule suspensions with 1 mM (Bu-2) cAMP decreased the 15-sec EIPA-sensitive Na+ uptake by the brush border membranes to 24.1 +/- 2.38% of the con trol values. Unexpectedly, the same treatment of distal tubules enhanc ed this uptake by 46.5 +/- 10.3%. Finally, incubation of tubule suspen sions 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 excha nger to various inhibitors, and its stimulation by cAMP-dependent prot ein kinase A, indicate that this isoform differs from that of the prox imal tubule and probably corresponds to isoform 1.