Depending on the time of administration, dexamethasone potentiates or blocks growth hormone-releasing hormone-induced growth hormone release in man

Abstract

In humans, corticoids suppress growth hormone (GH) secretion elicited by a variety of stimuli, while in vitro they potentiate GH release. To further study this problem, the effect of two doses of dexamethasone on GH secretion elicited by GH-releasing hormone (GHRH) in 6 normal volunteers was studied. Each subject underwent three tests, on 3 separate days with GHRH 1-29 (1 microgram/kg i.v. at 12.00 h). On the control day, only GHRH was given, on the second day dexamethasone 4 mg i.v. was administered at 09.00 h (3 h before GHRH) and on the third day dexamethasone 8 mg p.o. was given 12 h before GHRH (at 00.00 h). The GHRH-induced GH peak was 9.9 +/- 2.0 ng/ml, while 4 mg dexamethasone significantly (p less than 0.05) potentiated GH secretion elicited by GHRH (29.2 +/- 5.7 ng/ml). When dexamethasone 8 mg was given 12 h before, GHRH-induced GH secretion was completely blocked (3.0 +/- 1.1 ng/ml) (p less than 0.05). These results indicate that corticoids have two different actions: an acute potentiating activity on GHRH, and a delayed blocking action on GHRH-induced GH secretion.