Pituitary and gonadal secretory responses of rams following intravenous infusion or injection of graded doses of GnRH

Four adult Romney rams were utilized in a study of LH and testosterone secretory responses following intravenous administration of GnRH by continuous infusions over 8 h (total doses were 12.5, 50 and 200 μg) or by single rapid injections (doses were 3.1, 12.5, 50 and 200 μg). In the former case infusions of sterile saline were made in control experiments. Blood samples were collected via jugular catheters at intervals during and for 7 h after GnRH infusion, and for 4 h following GnRH injection. Plasma LH and testosterone concentrations were measured by specific radioimmunoassays.Each infusion of GnRH resulted in the secretion of LH with peak levels being reached within 1 – 3 h of commencing the experiment, then levels decreased slowly despite continued infusion. Plasma testosterone levels rose subsequent to the LH elevation and continued to be elevated after completion of the GnRH infusion. Each GnRH injection resulted in a rapid and marked elevation of plasma LH concentrations to a peak within 15 – 20 min. Higher GnRH doses (50 and 200 μg) generally resulted in a second peak occurring approximately 1.5 – 2 h later. Testosterone levels rose subsequent to each LH elevation.