D-Lys-GHRP-6 does not modify the endocrine response to acylated ghrelin or hexarelin in humans

Acylated ghrelin exerts numerous endocrine and non-endocrine activities via the GH Secretagogue receptor type 1a (GHS-R1a). D-Lys-GHRP-6 has been widely studied in vitro and in vivo in animal studies as GHS-R1a antagonist;, its action in humans has, however, never been tested so far. Aim of our study was to verify the antagonistic action Of D-Lys-GHRP-6 on the endocrine responses to acylated ghrelin and hexarelin, a peptidyl synthetic GHS, in humans. The effects of different doses of D-Lys-GHRP-6 (2.0 mu g/kg iv as bolus or 2.0 mu g/kg/h iv as infusion) on both spontaneous and acylated ghrelin- or hexarelin (1.0 mu g/kg iv as bolus)-stimulated GH, PRL, ACTH and cortisol levels were studied in six normal volunteers (age [mean +/- SEM]: 25.4 +/- 1.2 yr; BMI: 22.3 +/- 1.0 kg/m(2)). The effects of D-Lys-GHRP-6 (2.0 mu g/kg iv as bolus + 4.0 mu g/kg/h iv) on the GH response to 0.25 mu g/kg iv as bolus acylated ghrelin was also studied. During saline, spontaneous ACTH and cortisol decrease was observed while non changes occurred in GH and PRL levels. Acylated ghrelin and hexarelin stimulated (p < 0.05) GH, PRL, ACTH and cortisol secretions. D-Lys-GHRP-6 administered either as bolus or a continuous infusion did not modify both spontaneous and acylated ghrelin- or hexarelin-stimulated GH, PRL, ACTH and cortisol secretion. D-Lys-GHRP-6 did not modify even the GH response to 0.25 mu g/kg iv acylated ghrelin. In conclusion, D-Lys-GHRP-6 does not affect the neuroendocrine response to both ghrelin and hexarelin. These findings question D-Lys-GHRP-6 as an effective GHS-R1a antagonist for human studies. (c) 2006 Elsevier Ltd. All rights reserved.