ACUTE EFFECTS OF 2 NATURAL CORTICOSTEROIDS ON THE KINETICS OF THE NA+H+ EXCHANGER IN RAT RENAL BRUSH-BORDER MEMBRANES/

We studied the effect of adrenalectomy and acute hormone replacement, using physiological doses of natural corticosteroids, on the kinetics of the Na+/H+ exchanger in brush border membrane vesicles. We collecte d the data using the acridine orange uptake technique. Adrenalectomize d (ADX) rats presented a decreased maximal rate (V-max) when compared with sham-operated animals (30 000 versus 41 000 fluorescent units/min , respectively). Administration of corticosterone (B) to ADX rats rest ored V-max to values above control (up to 66 000 fluorescent units/min ). Smaller doses of 18-OH-B led to similar results. K-m (16 mM) remain ed the same for all the groups. Amiloride behaved as a pure competitiv e inhibitor, with a K-i = 0.02 mM and an I-50 = 98 mu M (in the presen ce of 50 mM sodium gluconate). The presence of sodium in the external buffer, before adding the vesicles, inhibited the exchange, with an I- 50 = 2 mM. We observed, a significant decrease in the Na+/H+ exchanger under non-acidotic conditions in response to adrenalectomy. Acute adm inistration of physiological doses of natural occurring corticosteroid s reversed the effect.