Hexarelin (HEX) is a synthetic growth-hormone-releasing peptide (GHRP) which acts via specific receptors at both the pituitary and the hypothalamic level to stimulate GH release both in animals and in man. Like other GHRPs, HEX possesses also significant prolactin-and adrenocorticotropin (ACTH) cortisol-releasing activity, but the mechanisms underlying these effects are even less clear. To clarify the mechanisms by which HEX stimulates the pituitary-adrenal axis in man, in 7 healthy young volunteers we studied the effects of HEX (2.0 µg/kg i.v.) and/or human corticotropin-releasing hormone (hCRH; 2.0 µg/kg i.v.) and /or arginine vasopressin (AVP; 0.17 U/kg i.m.) on ACTH and cortisol secretion. The GH responses to HEX alone and combined with hCRH and/or AVP were also studied. HEX increased ACTH and cortisol secretion (peak, mean ± SEM:26.3 ± 5.1 vs. 15.8 ± 3.1 pg/ml and 145.0 ± 11.4 vs. 131.7 ± 11.7 µg/1, p < 0.01, respectively) to levels overlapping with those induced by AVP (27.9 ± 6.1 vs. 13.1 ± 3.5 pg/ml and 167.6 ± 16.2 vs. 113.3 ± 9.4 µg/lp < 0.01, respectively) and similar to those elicited by hCRH (28.1 ± 4.6 vs. 17.4 ± 3.1 pg/ml and 182.7 ± 22.8 vs. 114.8 ± 12.3 µg/lp < 0.02, respectively). The ACTH but not the cortisol response to hCRH was higher (p < 0.02) than those to HEX when evaluated as area under the curve. The co-administration of HEX and AVP had no significant interaction on ACTH and cortisol peak levels (40.7 ± 5.3 pg/ml and 168.8 ± 13.5 µg/1, respectively). On the other hand, the co-administration of HEX and hCRH had a less than additive effect on ACTH and cortisol secretion (53.3 ± 11.2 pg/ml and 204.0 ± 13.7 µg/1, respectively). CRH and AVP had a true synergistic effect on ACTH (104.9 ± 14.2 pg/ml, p < 0.01) and an additive effect on cortisol secretion (281.3 ± 10.8 µg/1, p < 0.02). HEX did not modify the effect of CRH + AVP on both ACTH (135.5 ± 22.0 pg/ml) and cortisol secretion (261.1 ± 13.2 µg/1). The GH response to HEX (55.7 ± 19.8 vs. 2.7 ± 1.9 µg/1, p < 0.005) was unaffected by the administration of CRH alone (53.5 ± 21.0 µg/1) and/or AVP co-administration (60.2 ± 21.2 and 45.9 ± 10.6 µg/1, respectively). In conclusion, the results of this study demonstrate that GHRPs, beside their well-known GH-releasing activity, possess a remarkable ACTH-releasing activity, overlapping with that of AVP and similar to that of hCRH, two neurohormones which are known to play the major role in the control of the pituitary-adrenal axis. It is noteworthy that HEX shows no synergistic effect with either AVP or hCRH which, on the other hand, truly synergize. This evidence suggests the hypothesis that the ACTH-releasing activity of GHRPs could be, at least partially, independent of both CRH- and AVP-mediated actions in humans.

Keywords:
Corticotropin-releasing hormone, Growth hormone, Growth-hormone-releasing peptides, Corticotropin, Adrenal steroids, Vasopressin
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© 1997 S. Karger AG, Basel
1997
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