PITUITARY-TESTICULAR RESPONSES IN RAMS TO PROLONGED INFUSIONS OF LUTEINIZING HORMONE-RELEASING HORMONE (LHRH)

LHRH was administered to adult rams as constant i.v. infusions lasting 4 to 6 h. Dosages of 0.05, 0.1, 0.5 and 1.0 .mu.g/min were used. All studies were undertaken during the period of decreasing day length (Jan. to June). Plasma luteinizing hormone (LH) values during administration of the 2 highest dosages revealed a biphasic pattern of increase during the 1st 2 h of the infusion, as was found in similar studies in humans. LH values were maximal at approximately 21/2 h of the infusions, then decreased in spite of continued LHRH administration, thereby demonstrating the development of pituitary refractoriness during prolonged LHRH administration. A similar refractory period was not noted during analogous studies in men. Plasma follicle stimulating hormone (FSH) values generally increased gradually during LHRH infusions, with no evidence of a biphasic pattern. No FSH increase was noted in some studies, even at the highest LHRH dosage used. Plasma testosterone (T) levels increased sharply 15-30 min after the initial rise in LH values. Maximal levels of T obtained were 3 to 5-fold greater than basal values and were considerably higher than those found during similar studies in men. T production from the ram testis is more sensitive to gonadotropin stimulation than is that from the human testis. In spite of the prolonged elevations in gonadotropins and the marked increases in plasma T levels, plasma estradiol levels were consistently close to or below assay sensitivity (15 pg/ml).