Glucocorticoids may inhibit growth hormone release by enhancing beta-adrenergic responsiveness in hypothalamic somatostatin neurons.

The aim of this study was to investigate the mechanisms by which glucocorticoids inhibit GH secretion in man. In 10 normal volunteers subjects we compared the pattern of GHRH-induced GH release to that elicited by similar challenge given 60 min after a pretreatment with drugs affecting adrenergic and muscarinic cholinergic neurotransmission, both in basal situations and after having induced hypercortisolism. In a first study (P), synthetic GHRH [GRF-(1-29); 1 microgram/kg, i.v.] was administered 60 min after giving placebo. In other experiments, the administration of propranolol (PRO; 40 mg, orally), or clonidine (CLO; 0.300 mg, orally), or pyridostigmine (PD; 120 mg, orally) was followed by GHRH administration 60 min later. These experiments were repeated after giving a nocturnal dose of dexamethasone (DEX; 8 mg, orally at 2300 h). The administration of DEX significantly (P < 0.05) blunted the GH response to GHRH (peaks: 10.7 +/- 3.9 vs. 20.3 +/- 5.5 micrograms/L; DEX vs. P study, respectively). Conversely, either beta-adrenergic blockade (PRO), or alpha 2-adrenergic agonism (CLO), or the enhancement of muscarinic cholinergic tone (PD) significantly increased the GH response to GHRH (peaks: 43 +/- 4.6, 55.6 +/- 5.6 and 51.2 +/- 7, micrograms/L; PRO, CLO, and PD, respectively; P < 0.01 vs. P study). After nocturnal DEX administration, both PRO and CLO, but not PD, were able to reverse the inhibitory effect of DEX on GHRH-elicited GH release (peaks: 39 +/- 5.5, 25.9 +/- 3.9 and 12.9 +/- 3.1, micrograms/L; DEX + PRO, DEX + CLO, and DEX + PD, respectively). However, whereas the potentiating effect of PRO on GHRH-elicited GH release was still observed under hypercortisolism, it was lacking for both CLO and PD when these drugs were given in this situation. These data suggest that the inhibitory effect of glucocorticoid excess on GH release is due to increased hypothalamic somatostatin secretion which appears to be dependent on DEX-induced enhanced beta-adrenergic responsiveness. Moreover, the data further support a major role of hypothalamic alpha 2-adrenergic and beta-adrenergic activities in GH neuroregulation in man.