Mechanisms of insulin inhibition of ACTH-stimulated steroid secretion by cultured bovine adrenocortical cells

Results of previous studies indicated that insulin at levels comparable to those in humans during hyperinsulinemia decreased ACTH-stimulated cortisol and androstenedione secretion by bovine adrenal fasciculata-reticularis cells in primary culture. In the present studies this inhibitory action was examined further by comparing the effects of insulin on ACTH-stimulated corticosteroid secretion with its effects on 8-(4-chlorophenylthio)-cAMP (cpt-cAMP), forskolin- and [ 5val]angiotensin II (Ang II)-stimulated corticosteroid secretion. Effects on corticosteroid secretion were correlated with effects on cAMP accumulation and rates of cAMP production. Monolayers were incubated for 24 h in the absence or presence of each agonist alone or in combination with insulin. Insulin (1.7 × 10 −9 or 17.5 × 10 −9 M) caused about a 50% decrease in cortisol and androstenedione secretion in response to ACTH (10 −11 or 10 −8 M). Insulin also decreased ACTH-stimulated aldosterone secretion by cultured glomerulosa cells. Cpt-cAMP (10 −4 or 10 −3 M)-stimulated increases in cortisol and androstenedione secretion were inhibited by insulin, but to a lesser extent than those in response to ACTH. The inhibition of cpt-cAMP-stimulated steroid secretion was not related to increased degradation of the cyclic nucleotide. Increases in cortisol and androstenedione secretion caused by a submaximal concentration (10 −6 M) of forskolin were decreased 50–70% by insulin. In contrast, insulin failed to significantly affect cortisol or androstenedione secretion caused by a maximal concentration (10 −5 M) of forskolin. The secretory responses to Ang II (10 −8 M) were also unaffected by insulin. The effect of insulin to inhibit ACTH-stimulated steroid secretion was accompanied by a reduction in cAMP accumulation as well as an apparent inhibition of adenylate cyclase activation. These data indicate that the effect of insulin to attenuate ACTH-stimulated corticosteroid secretion results from both an inhibition of ACTH-stimulated adenylate cyclase activity and an antagonism of the intracellular actions of cAMP.