Plasma GH responses to human GHRH-antagonist in normal subjects

The effect of GHRH-antagonist [(N-Ac-Tyr1, D-Arg2) GRF-(1-29)-NH2] on plasma GH morning and evening secretion was evaluated in 14 normal subjects (10 males, 4 females, aged 19-25 years). Plasma GH was determined using a high sensitivity IRMA kit (detection limit, 0.006 micrograms/l). After intravenous infusion of GHRH-antagonist (100 micrograms/100 ml saline over 75 min) in the morning, plasma GH remained constant during the 150 min post-infusion (N = 6). In contrast, when GHRH-antagonist was administered in the evening, plasma GH showed a clear and gradual decrease through the initial 90 min and returned to baseline levels at 150 min. Plasma GH values were also significantly lower from 75 min to 135 min when compared to physiological fluctuations in plasma GH (P < 0.05). Other anterior pituitary hormones remained unaffected by GHRH-antagonist. In conclusion, our data suggest that evening basal GH secretion, but not morning GH secretion, is maintained by endogenous GHRH.