Stimulation of steroidogenesis by forskolin in rat adrenal zona glomerulosa cell preparations

Abstract

ABSTRACT The actions of forskolin have been investigated to determine to what extent its effects on steroidogenesis in rat adrenal preparations are dependent on activation of adenylate cyclase. In zona glomerulosa preparations, stimulation of both aldosterone and corticosterone production was obtained at concentrations of forskolin between 1 and 10 μmol/l. The effects of 10 μmol forskolin/l were additive with those of low doses (1 pmol/l) of corticotrophin (ACTH), but not with those of high doses (1 nmol/l) of ACTH. In contrast, in zona fasciculata/reticularis cells, doses of forskolin up to 10 μmol/l produced no significant stimulation of corticosterone production either alone or in the presence of ACTH (1 pmol/l and 1 nmol/l). The response to 1 nmol ACTH/l was attenuated in the presence of forskolin (10 μmol/l) in both zona glomerulosa and zona fasciculata/reticularis cell preparations. Cyclic AMP production increased progressively with dose up to 100 μmol forskolin/l in zona glomerulosa cells, whereas corticosterone production was maximal between 10 and 30 μmol forskolin/l and decreased at 100 μmol forskolin/l. In zona fasciculata/reticularis cells, cyclic AMP production was also increased by forskolin (1 and 10 μmol/l). The stimulation of zona glomerulosa steroido-genesis by forskolin (1–10 μmol/l) and ACTH (1–100 pmol/l) were both reduced by the adenylate cyclase inhibitor, N6-phenylisopropyladenosine (100 μmol/l). The calcium channel inhibitor, nifedipine, only reduced the steroidogenic response to forskolin (3 μmol/l) at doses of 300 μmol/l whereas the response to 8·4 mmol K+/l was inhibited at 10 μmol nifedipine/1. Although there is some dissociation between the effects of forskolin on cyclic AMP and steroidogenesis, the results are generally consistent with the view that the effects of forskolin in rat zona glomerulosa cells are mainly dependent on activation of adenylate cyclase. This contrasts with the effects of forskolin in bovine fasciculata cells which are reported to be mediated by activation of voltage-regulated calcium channels. Journal of Endocrinology (1991) 129, 391–397