GROWTH-HORMONE (GH)-RELEASING HORMONE-SECRETION IS STIMULATED BY A NEW GH-RELEASING HEXAPEPTIDE IN SHEEP

The acute effect of a new GH-releasing peptide, hexarelin (1 mg, iv), on GH secretion and the mechanisms involved in its changes were invest igated in conscious sheep. Peripheral GH levels and GH-releasing hormo ne (GHRH) and somatostatin concentrations in hypophysial portal blood were measured in six rams. An increase in jugular GH levels was observ ed 15 min after hexarelin injection (9.1 +/- 1.8 vs. 3.9 +/- 0.8 ng/ml ; P < 0.05). This was associated with a stimulation of GHRH release in to hypophysial portal blood (145.4 +/- 19.9 vs. 59.2 +/- 10.8 pg/ml; P < 0.01) without a change in somatostatin secretion. Our data indicate that GH-releasing peptide-induced GH stimulation in the sheep involve s an activation of GHRH neurons in addition to the previously demonstr ated direct effect on the pituitary cells.