The effect of medium composition and in vitro stimuli on the conversion of corticosterone to aldosterone in rat glomerulosa tissue.

Abstract

A late pathway of aldosterone biosynthesis, i.e., the conversion of H3-corticosterone to H3-aldosterone and H3-18-hydroxycorticosterone, was studied in rat glomerulosa (capsular) tissue. With capsular tissue incubated in Krebs- Ringer-bicarbonate (KRB) solution (potassium = 5.6 DIM) without bovine serum albumin (BSA), the percent conversion of H3-corticosterone to H3-aldosterone was 9% with no response to ACTH. With 4% BSA added, the percent conversion fell to a control value of 2.9%, and under these conditions there was a doubling of the percent conversion when 2.56 mU ACTH was added. Increasing the potassium content of the medium to 8.6 HIM likewise caused a doubling in the percent conversion. Both potassium and ACTH produced an increase in aldosterone and corticosterone production. The increase in corticosterone production paralleled the increase in percent conversion. When exogenous corticosterone (4 μg/ml) was added, it caused maximally a 50% increase in the percent conversion implying that the increase in the activity of the late pathway may be at least in part secondary to the increase in corticosterone production. The possibility that the corticosterone was producing its effect by substrate enzyme induction was investigated. A timed repetitive sampling of the incubation medium was carried out before and after the addition of 2.56 mU ACTH/ ml, 4 μg corticosterone/ml or an increase in potassium concentration from 3.6 to 8.56 HIM. There was a significant increase in the percent conversion at the earliest possible sampling period (IS min) with all alterations of the control situation. When cycloheximide (0.11 mil) and chloramphenicol (0.9 DIM) blocked the effect of ACTH and potassium on corticosterone production, they also inhibited the conversion of corticosterone to aldosterone. They had no effect on the increase in percent conversion produced by the addition of corticosterone to the incubation medium. These results together with the lack of effect of dexamethasone (0.4 μg and 4.0 μg/ml) suggest that the effect on the late pathway is not secondary to substrate enzyme induction. The conversion of H3-corticosterone to H3-18-hydroxycorticosterone was not altered by the addition of corticosterone, ACTH or by increasing the potassium content of the medium. (Endocrinology91: 948, 1972)