Endocrinology
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GH-releasing hexapeptide (GHRP-6) is a synthetic secretagogue that stimulates the release of GH by acting at both hypothalamic and pituitary sites. GHRPs also consistently elicit small, but significant, increases in plasma concentrations of ACTH and adrenal steroids. As these secretagogues do not release ACTH directly, they probably interact with the hypothalamic peptidergic systems controlling ACTH release, such as CRH and arginine vasopressin (AVP). ⋯ CRH alone significantly inhibited GH release (pre- vs. 40 min post-CRH, 11.9 +/- 3.8 vs. 1.7 +/- 0.4 ng/ml; P < 0.05), whereas AVP alone had no effect on GH levels. Neither CRH nor AVP had any effect on the GH response to GHRP-6. We suggest that GHRP-6 acts via the hypothalamus to mediate the release of ACTH, and that these effects are probably mediated at least in part via the release of endogenous CRH and are subject to regulation by circulating glucocorticoids.