EFFECT OF GHRP-6 AND GHRH ON GH SECRETION IN RATS FOLLOWING CHRONIC GLUCOCORTICOID TREATMENT

The aim of our study was to investigate the effects of His-DTrp-Ala-Tr p-Phe-Lys-NH2 (GHRP-6) on baseline and growth hormone-releasing hormon e (GHRH) stimulated growth hormone (GH) release in conscious, freely-m oving rats receiving chronic glucocorticoid treatment. Animals were tr eated daily for seven days with either vehicle or dexamethasone (dex, 40 mu g/day). On the day of experimentation, rats received an iv injec tion of saline or GHRP-6 followed 15 min later by an iv injection of s aline or rat GHRH. Three doses of GHRP-6 were evaluated, 1 mu g, 4 mu g and 25 mu g/kg; one dose of GHRH was evaluated, 500 ng/kg. GHRP-6 in creased plasma GH levels over baseline concentrations in a dose-depend ant fashion both in vehicle- and dex-treated rats. The GH response to GHRP-6 and GHRH was significantly less in dex-treated rats as compared to vehicle-treated rats. The combined administration of GHRP-6 and GH RH did not result in any change in plasma GH levels which could not be predicted from the administration of either peptide alone. Our result s show that GHRP-6 is able to stimulate GH secretion in glucocorticoid -treated rats but it is unable to counteract the glucocorticoid-induce d inhibition of GH secretion.