Growth hormone-releasing hormone and somatostatin concentrations in the hypophysial portal blood of conscious sheep during the infusion of growth hormone-releasing peptide-6.

The effects of growth hormone-releasing peptide-6 (GHRP-6) on peripheral plasma concentrations of growth hormone (GH) and hypophysial portal plasma concentrations of growth hormone-releasing hormone (GHRH) and somatostatin (SRIF) were investigated in conscious ewes. Paired blood samples were collected from the hypophysial portal vessels and from the jugular vein of nine ewes for at least 2 hr. The sheep were then given a bolus injection of 10 micrograms of GHRP-6 per kg followed by a 2-hr infusion of GHRP-6 (0.1 microgram/kg.hr). Blood sampling continued throughout the infusion and for 2 hr afterwards. An increase in plasma GH concentration was observed in the jugular samples of six of the nine ewes (1.4 +/- 0.3 vs 7.4 +/- 2.0 ng/ml, P < 0.05) 5-10 min after the GHRP-6 bolus injection, but in no case did we observe a significant coincident release of GHRH. During the infusion period, mean plasma GHRH levels were not significantly increased but there was a 50% increase (P < 0.05) in GHRH pulse frequency; GHRH pulse amplitude was not changed. Mean SRIF concentration, pulse frequency, and pulse amplitude were unchanged by GHRP-6 treatment. These data indicate that GHRP-6 causes a small, but significant effect on the pulsatile secretion of GHRH, indicating action at the hypothalamus or higher centers of the brain. The large initial GH secretory response to GHRP-6 injection does not appear to be the result of GHRP-6 action on GHRH or SRIF secretion.