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Release of GH from perifused duckling hemipituitaries was stimulated, in a biphasic manner, by synthetic TRH and human pancreatic GH-releasing factor (GRF). At all effective concentrations, the level of GH release was increased within 5 min of TRH or GRF perifusion and was maximal after 10 min of TRH perifusion and after 20 min of GRF perifusion. Although TRH was perifused for 20 min the level of GH release declined during the last 10 min. The most effective dose of TRH (1·0 µg/ml; 2·7 µmol/l) and GRF (0·5 µg/ml; 110 nmol/l) provoked similar (250– 300%) increases in the level of GH release. However, since the effect of TRH was only of short duration, the total release of GH induced by GRF was higher than that elicited by TRH, especially with the low dose. The increase in release of GH induced by TRH or GRF was blunted when pituitaries from adult ducks were used. As in young ducks, the GH response to GRF was higher, whereas the response to TRH was very low. The GH response of perifused adult pituitaries to GRF was, however, potentiated when TRH was perifused simultaneously. The basal release of GH from both young and adult pituitary glands was unaffected by perifusion with somatostatin-14 (SRIF-14) at doses of 1 and 2 µg/ml. The perifusion of hemipituitary glands with similar doses of SRIF-14 was also unable to suppress the stimulation of GH release induced by prior perifusion with GRF, although when SRIF-14 and TRH were simultaneously perifused TRH-induced GH release was markedly suppressed.

These results demonstrate direct effects and interactions of TRH, GRF and SRIF on the release of GH from duck pituitary glands. GRF is the most potent releasing factor for GH in both young and adult ducks although in adult ducks it is less effective. These results also provide evidence that the age-related decline in the in-vivo GH response to TRH is due to a desensitization of pituitary somatotrophs.

J. Endocr. (1988) 119, 421–429