Dulor, B. Barenton

We have examined the regulation of GH secretion from monolayer cultures of prepubertal male lamb anterior pituitary cells. Growth hormone-releasing factor (GRF 1-44) stimulated GH release in a dose-related manner : the maximal effective dose was 10IO M, which caused a 500 % increase in basal GH secretion, while the half-maximal effect was reached with a dose of 2.5 x 10-&dquo;M (ED 5Q ). Thyrotropin-releasing hormone (TRH) also elicited a dose-dependent stimulation of GH secretion, although it was approximately 1000 times less potent than GRF. GRF and TRH did not have additive or synergistic effects on GH secretion. Somatostatin (SRIF) at a concentration of 10-!M maximally inhibited basal GH release to 40 % of that of the control ; the ED 50 was 2.0 x 10-’M. Moreover, 107M SRIF blocked the stimulation of GH secretion induced by 108M GRF. However, when the cells were incubated with these two peptides at an identical concentration (10’!M), GH secretion was stimulated significantly above control values. When added at the same concentration (10-’M), TRH and SRIF nullified their respective effects. A dose of 100 ng/ml of synthetic IGF-I was without effect on basal GH release, but significantly decreased 109M GRFinduced stimulation of GH secretion. These data indicate that in prepubertal male lambs : (1) the stimulatory effect of GRF is predominant over the inhibitory effect of SRIF, (2) somatostatin inhibits TRH stimulation of GH secretion in vitro, and (3) IGF-I may control GH secretion by modulating GRF effects at the pituitary level.