Studies on the mechanism of secretion of rat adrenal steroids in vitro

Abstract

The possibility that in the rat adrenal cortex synthesis and secretion of steroid hormones may be distinct. processes has been studied using in vitro methods. The uptake and subsequent release of radioactive corticosterone and 18-OH-DOC added to incubation media showed little difference between the two steroids. Both were taken up rapidly by the tissue, and after exchange of medium, 50% was released within 15 min. The properties of muscle and adrenal tissue were qualitatively similar in these respects, and were consistent with steroid movement by diffusion. Different results were obtained when steroids formed from the tissues' endogenous precursor supply were studied. Following incubation for 2h tissue and medium steroid cohtent were measured, and tissue/medium ratios of steroid were used as an index of secretion. With varying volumes of incubation medium (1.4–10 ml), secretion of corticosterone was related to volume, although synthesis was constant. With 18-hydroxydeoxycorticosterone (18-OH-DOC) on the other hand neither synthesis nor secretion was affected by medium volume. The effects of ouabain (10 −4 M) on synthesis and secretion were also studied, using a 5 ml volume of medium. Under these conditions, tissue levels of corticosterone were very low, while 18-OH-DOC was extractable from tissues in amounts up to 30% of the total synthesised. In capsules, steroid synthesis was decreased by ouabain, or by a lower potassium medium (3.6 mM) or both together. Secretion of corticosterone was not affected by these treatements, whereas secretion of 18-OH-DOC and aldosterone was greatly inhibited: maximally 50% of the synthesised 18-oxygenated steroid was retained within the tissue. These treatments also affected inner zone function, ouabain inhibited steroid synthesis, but the low potassium medium did not. Secretion of corticosterone was unaffected, whereas secretion of 18-OH-DOC was again inhibited both by low potassium medium and by ouabain. Maximally as in the capsules, 50% of the synthesised 18-OH-DOC was retained within the tissue. The results suggest that the secretion of 18-OH-DOC formed from endogenous precursors by rat adrenal tissue is controlled by a mechanism involving a ouabain sensitive process in the cell membrane. Corticosterone is released by a different mechanism, quite possibly passive diffusion.