Quantitation of prolactin-dependent responses in porcine mammary explants.

A system was developed to quantitate prolactin-dependent responses in porcine mammary tissue obtained from pregnant gilts. Metabolic responses to prolactin (Prl) and cortisol (C) in the presence of varying doses of insulin (I) were examined in mammary explants cultured on the surface of the medium or submerged in medium, under an atmosphere of humidified air. Explants suspended on grids at the surface of medium oxidized 45% more glucose (P less than .05) and incorporated 67% more glucose into lipids (P less than .05) than explants submerged in culture medium. In explants cultured on grids, both 100 and 1,000 ng I/ml increased glucose oxidation (by 50%) and glucose incorporation into lipids (by 150%) compared with 10 ng/ml (P less than .05), but responses to 100 and 1,000 ng I/ml were not different. Therefore, in subsequent studies, explants were cultured on grids with 100 ng I/ml. Rates of glucose metabolism for mammary explants cultured with I + C for 48 or 72 h were not different from those in fresh tissue. However, addition of Prl (200 or 1,000 ng/ml) increased oxidation rate 130% and fat synthesis 400% compared with I + C (P less than .05). Addition of triiodothyronine to I + C + Prl further increased rate of fat synthesis by 87%. Dose-dependent responses to Prl were demonstrated and were within the concentrations of Prl found in blood of gestating gilts. These studies demonstrate that the lactogenic complex of I, C and Prl induces metabolic activity in porcine mammary tissue from late pregnancy.

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.