Nongenomic Effects of Aldosterone on Intracellular Ca 2+ in Vascular Smooth Muscle Cells

Abstract

Abstract Genomic mechanisms of steroid action have been increasingly elucidated over the past four decades. In contrast, rapid steroid actions have been widely recognized only recently, and detailed analysis of the mechanisms involved are still lacking. The present article describes rapid effects of mineralocorticoid hormones on free intracellular calcium in vascular smooth muscle cells as determined by fura 2 spectrofluorometry in single cultured cells from rat aorta. These effects are almost immediate and reach a plateau after only 3 to 5 minutes and are characterized by high specificity for mineralocorticoids versus glucocorticoids. The potent mineralocorticoids aldosterone and fludrocortisone are agonists with estimated apparent EC 50 values of ≈0.1 to 0.5 nmol/L; deoxycorticosterone acetate is an agonist with an EC 50 of ≈5 nmol/L; and progesterone, cortisol, corticosterone, and estradiol have much lower potency (EC 50 values of ≈0.5 to 5 μmol/L). The effect of aldosterone is blocked by neomycin and short-term treatment with phorbol esters but augmented by staurosporine, indicating an involvement of phospholipase C and protein kinase C. The Ca 2+ effect appears to involve the release of intracellular Ca 2+ , as shown by the inhibitory effect of thapsigargin; intriguingly, a relatively small maximum effect (≈40 nmol/L increase) is consistently seen. This mechanism operates at physiological subnanomolar aldosterone concentrations and appears to be a likely candidate for rapid fine tuning of cardiovascular responsivity. It may also contribute to known clinical features of mineralocorticoid action that are difficult to explain by the traditional genomic mechanism alone.