Bioactivity of growth hormone releasing hormone (1-29) analogues after SC injection in man

The bioactivity of growth hormone releasing hormone 1-29 [GHRH(1-29)NH2] has been compared with that of an agonist analogue [Ac-D-Tyr1,D-Ala2]-GHRH(1-29)NH2, in normal male volunteers. Using a submaximal dose of 3 micrograms/kg administered subcutaneously, peak growth hormone (GH) response and area under the GH curve were similar for the native and agonist analogue. In addition, no significant differences were found in peak GHRH(1-29) immunoreactivity, area under the GHRH(1-29) curves or plasma disappearance rates of the two peptides. The results suggest that, in keeping with the relative activities of other "superactive" analogues tested so far, the greatly enhanced activity of [Ac-D-Tyr1,D-Ala2]-GHRH(1-29)NH2 observed in the rat is not found in humans. It is possible that this species difference is due to differences in the interaction of GHRH peptides with the rat and the human somatotroph GHRH receptor.