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 or absence of ANGII (5 × 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 ([³H]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 [³H]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 [³H]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.