Depending on the Time of Administration, Dexamethasone Potentiates or Blocks Growth Hormone-Releasing Hormone-Induced Growth Hormone Release in Man

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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 µg/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 < 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 < 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.