Both insulin-like growth factor I (IGF-I) and somatostatin (SRIH) have been shown to directly inhibit GH release and the total GH content of cultured pituitary cells. In the present study we evaluated the interrelationship between the effects of a recombinant human IGF-I analog ([Thr59]IGF-I) and SRIH on GH release by cultured normal rat pituitary cells together with the effects of glucocorticoids. In all experiments anterior pituitary cells were preincubated for 24 h without or with IGF-I, SRIH, and/or dexamethasone. Thereafter, 24-h incubations without or with IGF-I, dexamethasone, SRIH, and GHRH were performed. Both IGF-I and SRIH inhibited basal and GHRH-stimulated GH release in a dose-dependent manner; the maximal inhibitory concentrations were 5 nM IGF-I and 10 nM SRIH. These concentrations inhibited basal and GHRH-stimulated GH release by 23% and 40% (IGF-I) and 80% and 85% (SRIH), respectively. The combination of IGF-I and low concentrations of SRIH exerted an additive inhibitory effect on GHRH-stimulated GH release; IGF-I (1 nM) and SRIH (10 pM) together inhibited GH release by 50%, while the maximal inhibitory concentrations of 5 nM IGF-I and 10 nM SRIH virtually completely inhibited GH release (by 93%). Preincubation with 5 and 100 nM dexamethasone attenuated the sensitivity of somatotrophs to SRIH and completely abolished the inhibitory effects of IGF-I. This effect of dexamethasone could be reversed by coincubation with the glucocorticoid receptor antagonist RU 38486. High concentrations of 5-10 nM of the recombinant human IGF-I analog stimulated PRL cell content (5 and 10 nM) and release (10 nM), while a purified IGF-I preparation extracted from human blood exerted a parallel inhibitory effect on GH and PRL release. We conclude that 1) IGF-I and SRIH exert an additive direct inhibitory effect on basal and GHRH-stimulated GH secretion by normal cultured pituitary cells; 2) glucocorticoids directly attenuate the sensitivity of somatotrophs to SRIH, but completely prevent the inhibitory effects of IGF-I on GH secretion; and 3) in contrast to a purified IGF-I preparation extracted from human blood (which inhibits GH and PRL release) high concentrations of the recombinant IGF-I preparation (which inhibit GH release) stimulate PRL production.