Insights into the mechanism by which inhibition of Na,K-ATPase stimulates aldosterone production.

Inhibition of Na,K-adenosine triphosphatase (Na,K-ATPase) activity by ouabain has been shown to increase the release of aldosterone from rat glomerulosa cells, but the mechanism by which this elevation of aldosterone production occurs has not been established. Small changes in membrane potential can significantly affect aldosterone release. Consequently, inhibition of Na,K-ATPase in glomerulosa cells may stimulate aldosterone production by membrane depolarization. If so, ouabain-stimulated production should be dependent on calcium influx through voltage-gated calcium channels. It has previously been shown that ouabain induces a moderately rapid increase in cytosolic calcium in rat glomerulosa cells. Therefore, in this study, we test whether ouabain stimulates aldosterone production with a time course consistent with early membrane depolarization as suggested by the previously reported early increase in cytosolic calcium. To study the time course of aldosterone production, we developed a perfusion technique that allows an examination of the initial effects of ouabain on aldosterone production. The results show that ouabain rapidly stimulates aldosterone production. Continuous perfusion with 0.25 or 1 mmol/L ouabain induced a brisk, robust increase in aldosterone production, followed by a decrease to near baseline over 60 minutes. Ouabain-stimulated aldosterone production was dependent on the presence of extracellular calcium and calcium influx through voltage-gated calcium channels. Our results support the hypothesis that the inhibition of Na,K-ATPase in rat adrenal glomerulosa cells immediately depolarizes the membrane potential and opens voltage-gated calcium channels.