ALDOSTERONE MODULATES SODIUM KINETICS OF NA,K-ATPASE CONTAINING AN ALPHA-1 SUBUNIT IN A6 KIDNEY-CELL EPITHELIA

Short-term aldosterone (10(-6) M, 2.5 h) induces in A6-C1 cell epithel ia an increase in Na transport, which is due to the in situ activation of the apical Na channel and, presumably, the basolateral Na pump (Na ,K-ATPase). We have now directly measured the effect of aldosterone on the transport activity of endogenous Na pumps and hybrid Na pumps con taining an exogenous alpha 1 subunit by measuring the pump current (I- p) across epithelia apically permeabilized with amphotericin B. Aldost erone (2.5 h) had no significant early effect on the maximal I-p, nor on the Na concentration required for half-maximal activation. In contr ast, it increased the I-p at physiological intracellular Na concentrat ions (1.7-fold at 5 mM Na). This effect was blocked by the protein syn thesis inhibitor cycloheximide. Hybrid pumps containing the transfecte d cardiotonic steroid-resistant alpha 1 subunit of Bufo marinus were a lso stimulated by aldosterone (2.5 h). A long aldosterone treatment (4 days) increased the maximal I-p produced by the endogenous pumps 1.5 to 2.1-fold. In conclusion, aldosterone acts on Na pumps containing an alpha 1 subunit in two ways. During its early phase of action it stim ulates their transport activity by increasing their apparent Na affini ty at physiological intracellular Na concentrations. In the long term it produces an increase in the maximal transport capacity, which corre sponds to the known increase in the number of Na pumps.