GROWTH HORMONE-RELEASING HORMONE AND SOMATOSTATIN CONCENTRATIONS IN THE HYPOPHYSEAL 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 peripher al 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 th e jugular vein of nine ewes for at least 2 hr. The sheep were then giv en a bolus injection of 10 mu g of GHRP-6 per kg followed by a 2-hr in fusion of GHRP-6 (0.1 mu g/kg . hr). Blood sampling continued througho ut the infusion and for 2 hr afterwards. An increase in plasma GH conc entration 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 coincide nt release of GHRH. During the infusion period, mean plasma GHRH level s were not significantly increased but there was a 50% increase (P < 0 .05) in GHRH pulse frequency; GHRH pulse amplitude was not changed. Me an SRIF concentration, pulse frequency, and pulse amplitude were uncha nged by GHRP-6 treatment. These data indicate that GHRP-6 causes a sma ll, but significant effect on the pulsatile secretion of GHRH, indicat ing action at the hypothalamus or higher centers of the brain. The lar ge initial GH secretory response to GHRP-6 injection does not appear t o be the result of GHRP-6 action on GHRH or SRIF secretion.