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 d eclines with age. However, the precise mechanisms causing this impairm ent in GH release are unknown. His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (GHRP-6 ) is a synthetic compound that releases GH in a dose related and speci fic manner in several species, including man. In order to gain a furth er insight into disrupted GH secretion in late adulthood, we evaluated GH responses to GHRP-6 or GHRH, administered either alone or in combi nation, in healthy young and late adulthood groups of subjects. DESIGN All subjects underwent three different tests carried out in random or der and one week. Tests were performed at 0900 h after an overnight fa st. GHRH (100 mu g), GHRP-6 (90 mu g) either alone or in combination w ere 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 m easured by radioimmunoassay. RESULTS In the group of young adult subje cts 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 a lso 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. GH res ponses to GHRH were greater in younger than in older subjects (F = 3.4 5, P = 0.03). In contrast, GH responses to either GHRP-6 (F = 0.71, P = NS) or combined GHRH plus GHRP-6 administration (F = 0.68, P = NS) w ere not significantly different between the two groups. CONCLUSIONS Th ese data show that GH responses to GHRP-6 are much greater than to GHR H in late adulthood. The marked increase of plasma GH levels observed after administration of GHRP-6 alone or in combination with GHRH indic ates that impaired GH secretion in late adulthood Is a functional and potentially reversible state.