Exogenous growth hormone administration does not inhibit the growth hormone response to hexarelin in normal men

Administration of exogenous human growth hormone (GH) blunts the GH response to physiological as well as pharmacological stimuli, including GH-releasing hormone (GHRH). Hexarelin (Hex) is a new synthetic GH-releasing peptide (GHRP) similar to GHRP-6 with potent GH-releasing activity in animals and men. To determine whether the short-term administration of GH inhibits the Hex-induced GH release, we measured the GH response to Hex (2 micrograms/kg iv) in five normal adult males (age 26-32 yr) three h after an iv bolus of rhGH (2 IU) or saline. Mean incremental change of serum GH from value at time 0 was 47.5 +/- 5.5 and 41.5 +/- 4.1 micrograms/l after saline + Hex and GH + Hex, respectively. Mean incremental area under the curve over baseline was 3216 +/- 586 and 3735 +/- 506 micrograms.min.1 after saline + Hex and GH + Hex, respectively. One of the proposed mechanism of action of GHRPs is to serve as functional somatostatin (SRIH) antagonists, and it is known that GH feeds backs on the hypothalamus to stimulate SRIH release. Therefore, we speculate that antagonisms of SRIH function by Hex prevented the inhibitory effect of exogenous GH, thus lending further support to the hypotheses that SRIH is involved in the feedback regulation of GH secretion, and that GHRPs action involves inhibition of SRIH function.