Effects of Ouabain and Potassium on Protein Synthesis and Angiotensin-Stimulated Aldosterone Synthesis in Bovine Adrenal Glomerulosa Cells*

Abstract

Ouabain (1 .mu.M and below) inhibited both basal and angiotensin II-stimulated aldosterone synthesis in bovine adrenal glomerulosa cells. Ouabain had no effect on binding of 125I-labeled angiotensin, on angiotensin''s effects on 45Ca2+ fluxes, or on 32PO4 incorporation into phosphatidylinositol. This spectrum of activities resembles that of the protein synthesis inhibitor cycloheximide, which also blocks aldosterone synthesis. Ouabain was, therefore, tested for its effect on protein synthesis, as measured by uptake of [3H]leucine into acid-precipitable material. Ouabain inhibited protein synthesis at concentrations similar to those that depressed aldosterone synthesis, but did not block uptake of the nonmetabolized amino acid [carboxyl-14C]aminocyclopentane-1-carboxylic acid, nor the entrance of [3H]leucine into cells. When cells previously loaded with 86Rb+ were treated with 1 .mu.M ouabain, they lost approximately half of the accumulated radioactivity in 30 min. When cells were incubated in potassium-free buffer, both protein and aldosterone synthesis were severely inhibited. Increased extracellular potassium reversed ouabain''s inhibition of protein and aldosterone synthesis in parallel. Pregnenolone synthesis was inhibited by ouabain, and elevated potassium overcame that blockade. Ouabain did not block aldosterone synthesis from exogenous progesterone. These data fit a model in which ouabain causes loss of cell potassium which, in turn, depresses protein synthesis. Since protein synthesis is necessary for angiotensin II stimulation of cholesterol side-chain cleavage, ouabain depresses that step, pregnenolone synthesis, and thus, aldosterone synthesis.