Growth hormone secretion after the administration of GHRP-6 or GHRH combined with GHRP-6 does not decline in late adulthood

OBJECTIVE Growth hormone (GH) secretion in middle and late adulthood declines with age. However, the precise mechanisms causing this impairment in OH release are unknown. HIs-D-Trp-Ala-Trp-D-Phe-Lys-NH₂(GHRP-6) Is a synthetic compound that releases OH in a dose related and specific manner in several species, including man. In order to gain a further Insight Into disrupted GH secretion in late adulthood, we evaluated GH responses to GHRP-6 or GHRH, administered either alone or in combination, in healthy young and late adulthood groups of subjects. DESIGN All subjects underwent three different tests carried out in random order and separated by at least one week. Tests were performed at 0900 h after an overnight fast. GHRH (100 μg), GHRP-6 (90 μg) either alone or In combination were administered as an I.v. bolus. SUBJECTS Groups of healthy young (mean ± SEM 22 ± 1·1 years, n= 9) and older adult subjects (59·5 ± 1·7 years, n= 9) were studied. MEASUREMENTS Serum GH levels were measured by radioimmunoassay. RESULTS In the group of young adult subjects the combined administration of GHRH and GHRP-6 elicited a greater GH increase than GHRH alone (F= 21·9, P < 0·001) or GHRP-6 alone (F= 6·2, P= 0·01). Similarly, the response to the combined stimuli was also greater than with GHRH alone (F= 21·8, P < 0·001) or GHRP-6 alone (F= 23·9, P < 0·001) In the late adulthood group of subjects. OH responses to GHRH were greater in younger than in older subjects (F= 3·45, P= 0·03). In contrast, OH responses to either GHRP-6 (F= 0·71, P= NS) or combined GHRH plus GHRP-6 administration (F= 0·68, P= NS) were not significantly different between the two groups. CONCLUSIONS These data show that OH responses to GHRP-6 are much greater than to GHRH In late adulthood. The marked Increase of plasma GH levels observed after administration of GHRP-6 alone or in combination with GHRH Indicates that Impaired GH secretion in late adulthood is a functional and potentially reversible state.