Plasma somatotropin response to exogenous growth hormone releasing factor in lambs

Two experiments were performed to examine the ability of human pancreatic growth hormone releasing factor (hGRF) administration to stimulate endogenous growth hormone (GH) secretion in lambs. Each study utilized eight Dorset wether lambs in replicated 4 X 4 Latin square experiments. Growth hormone response (integrated area under the curve for 150 min post-injection) for 0, 1, 5 and 10 micrograms hGRF/kg body weight averaged 13, 23, 92 and 134 units, respectively. While the 1-microgram hGRF dose was not different (P greater than .05) than the response to saline injection, there was an increased (P less than .01) GH response to 5 or 10 micrograms hGRF. Overall the GH response increased in a log dose-response fashion. There was distinct variation between lambs in their response to hGRF. Study II examined the optimal method to administer 40 micrograms hGRF/kg body weight to maximize GH concentration over 24 h. Continuous infusion (CI) was compared with eight (8X), four (4X), or two (2X) injections/d. Hourly blood samples were obtained from all lambs. Growth hormone response (area under the curve for 24 h) was 162, 305, 306 and 220 units for CI, 8X, 4X and 2X, respectively. Growth hormone response to CI was inferior to discrete injections, and the GH response to 4X or 8X was superior to 2X/d. Results demonstrate that, in spite of lamb-to-lamb variation, one can utilize exogenous hGRF to enhance GH secretion in lambs. Thus, the ability of exogenous hGRF to enhance growth performance merits further study.