Effects of dexamethasone on growth hormone (GH)-releasing hormone, arginine- and dopaminergic stimulated GH secretion, and total plasma insulin-like growth factor-I concentrations in normal male volunteers.

Abstract

In man, glucocorticoid treatment and endogenous corticosteroid excess generally suppress stimulated GH release. However, such effects are not entirely consistent and depend on both the duration of pituitary exposure to steroids and the secretagogue employed. To further evaluate the effects of glucocorticoids in man, we have studied the response to four different GH stimulation tests before and after treatment with dexamethasone (DEX; 2 mg twice daily during 84 h). Twelve healthy male volunteers were divided into two groups of six subjects (groups A and B). Group A underwent stimulation tests with arginine (500 mg/kg, i.v.) and GH-releasing hormone (GHRH, 100 micrograms, i.v.) before and after DEX treatment. Group B were subjected to stimulation tests with two dopaminergic agents, a novel nonergot D2-dopamine agonist CV205-502 (CV; 10 micrograms, i.v.) and dopamine (4 micrograms/kg.min, i.v.), before and after DEX. Within each group, the effect of DEX on the different secretagogues was studied 4 weeks apart. GHRH-stimulated GH release was significantly blunted by DEX treatment [median peak GH value, 34.2 micrograms/L; 25-75th percentiles, 22.1-56.2), control, vs. 19.8 (9.7-34.5), DEX; P less than 0.05; integrated GH secretion expressed as the area under the curve (AUC) was 48% lower after DEX; P less than 0.01]. In the same group, DEX treatment significantly enhanced the response to arginine [10.6 (8.0-22.8), control, vs 26.1 (15.1-38.6), DEX; P less than 0.01; with an increase in AUC of 72%; P less than 0.01]. In group B, under control conditions before glucocorticoid administration, the GH response to CV was significantly greater than that to dopamine in terms of both peak response [25.1 (8.6-30.9), CV, vs. 11.8 (5.5-16.4), dopamine; P less than 0.05] and AUC [2406 +/- 654 (CV) vs. 658 +/- 125 (dopamine); P less than 0.01], suggesting that CV may be a useful adjunct in the diagnosis of GH deficiency. After DEX administration, responses to both dopaminergic agents were suppressed [CV, 6.7 (4.0-21.2); P less than 0.01 vs. control response; and dopamine, 5.3 (4.8-7.9); P less than 0.05 vs. control response]. When compared with the effects of dexamethasone on the GH response to arginine, the results with dopaminergic agents highlight important differences in the mechanisms of action of these indirectly acting GH secretagogues. Moreover, this may be of physiological importance, because in contrast to the inhibitory effect of glucocorticoid on GHRH-stimulated GH release, DEX treatment significantly increased basal plasma GH levels [1.4 (0.5-5.1) vs. control 0.3 (0.1-0.6) microgram/L; P less than 0.001].(ABSTRACT TRUNCATED AT 400 WORDS)