Effects of alpha-melanocyte-stimulating hormone on the cyclic AMP and phospholipid metabolism of rat adrenocortical cells.

Results on the effects of peptides on the phospholipid metabolism and steroid and cyclic AMP (cAMP) outputs of rat adrenal capsular cells (96% zona glomerulosa, 4% zona fasciculata) were obtained in a series of three batch experiments. Their significance was examined by analysis of variance. Incorporation of [32P] into phosphatidylcholine, phosphatidic acid and phosphatidylinositol was measured. Production of [3H]inositol-1 monophosphate, inositol-1,4 bisphosphate and inositol-1,4,5 tris-phosphate was estimated after prelabelling with [3H]inositol followed by 1 min incubation with a steroidogenic stimulus. Angiotensin II (0.25 nmol/l to 0.25 mumol/l) highly significantly (P less than 0.01) stimulated aldosterone and corticosterone outputs, [32P] incorporation into phosphatidic acid and phosphatidylinositol (but not into phosphatidylcholine) and the production of the three [3H]inositol phosphates. Aldosterone and corticosterone outputs were stimulated by alpha-MSH (above 0.1 nmol/l). However, incorporation of [32P] was not significantly increased until 10 mumol alpha-MSH/l but, unlike with angiotensin II, incorporation into phosphatidylcholine was also then stimulated. Also, the production of the inositol phosphates was not increased significantly (P greater than 0.05) by any dose of alpha-MSH (10 nmol/l, 1 mumol/l and 0.1 mmol/l) used. Therefore, it can be concluded that alpha-MSH does not stimulate phospholipase C in rat zona glomerulosa cells. In further experiments, it was also found that there were significant increases in cAMP as well as in steroid outputs above 1 nmol alpha MSH/l (highly significant above 10 nmol alpha-MSH/l). There were plateaux of the outputs of both steroids and cAMP from 0.1 to 1 mumol alpha-MSH/l. However, there were further increases in steroid and cAMP outputs of the capsular cells at higher doses. Concomitant results on the stimulation of corticosterone output by zona fasciculata-reticularis cells indicate that this additional increase was mostly due to the stimulation of the contaminating zona fasciculata cells. It was also confirmed that alpha-MSH preferentially stimulates steroidogenesis by the zona glomerulosa. However, under our conditions, alpha-MSH highly significantly increased the output of cAMP by both zona fasciculata and glomerulosa cells.