ACTIVATION OF PROXIMAL TUBULAR NA+-H+ EXCHANGE BY ANGIOTENSIN-II

Stimulation of Na+-H+ exchange by angiotensin II (ANG II) was characterized in renal proximal tubular cells. Rabbit proximal nephron segments were incubated in the presence of absence of ANG II (5 x 10(-10) M), after which brush-border membrane vesicles (BBMV) were isolated and assayed for Na+-H+ antiporter activity using the acridine orange technique. Both the affinity (for sodium) and capacity of the carrier were elevated significantly (P < 0.05) within 15 min of incubation with ANG II. To determine whether the stimulation of transport capacity involved a change in Na+-H+ antiporter density in the luminal membrane, binding of tritiated 5-(N-methyl-N-isobutyl)amiloride ([H-3]MIA) was measured in BBMV derived from control and ANG II-treated nephron segments, following maximal stimulation. This demonstrated a significant (P < 0.05) increase in the maximal specific binding (B(max)) of [H-3]MIA binding in the ANG II-treated group compared with control, of a magnitude sufficient to account for the observed change in maximal velocity (V(max)). The data indicate that the V(max) effect is caused by an apparent increase in the number (density) of active Na+-H+ carriers present in the luminal membrane. Finally, to test the possibility that the observed kinetic change involves an exocytic mechanism, the effect of colchicine on ANG II-stimulated antiporter activity was examined. The increase in V(max) due to ANG II was blocked by the addition of 0.5 mM colchicine to the incubation medium, whereas colchicine alone had no significant effect on the V(max) of Na+-H+ kinetics. Similarly, colchicine prevented the ANG II-stimulated increase in B(max) of [H-3]MIA. This evidence is consistent with the interpretation that ANG II stimulates Na+-H+ exchange in the proximal nephron by causing the exocytic insertion of carriers, or an activator of transport, into the luminal, membrane of proximal tubular cells.