Growth hormone (GH) releasing peptide-6 requires endogenous hypothalamic GH-releasing hormone for maximal GH stimulation

GH-releasing peptide-6 (GHRP-6) is a potent GH secretagogue that releases GH by uncertain mechanisms. To assess whether GHRH is required for GH release by GHRP-6 in humans, are used the specific antagonist to GHRH (N-Ac-Tyr(1),D-Arg(2))GHRH(1-29)NH2 (GHRH Anti. We have previously shown that GHRH-Ant (400 mu g/kg) blocked the GH response to 0.33 and 3.3 mu g/kg boluses of GHRH by 95% and 81%, respectively, Nine healthy men between the ages of 20 and 30 yr were studied on two occasions. They received either saline or GHRH-Ant (400 mu g/kg, iv) at 0840 h, followed by GHRP-6 (1 mu g/kg, iv bolus) at 0900 h. Blood was sampled every 10 min from 0800-1100 h. GH responses were measured as the maximal increase over the baseline GH concentration and as the area under the curve. GHRH-Ant eliminated most of the GK response to GHRP-6 [maximal increase over the baseline GH concentration, 33.8 +/- 4.8 vs. 6.2 +/- 1.8 mu g/L (mean +/- SEM; P < 0.0001); area under the curve, 1701 +/- 275 us. 376 +/- 113 mu g/min.L (P < 0.001)]. These data show that endogenous GHRH is necessary for most of the GH response to GHRP-6 in humans.