EFFECTS OF PULSATILE INFUSION OF LUTEINIZING-HORMONE-RELEASING HORMONE ON LUTEINIZING-HORMONE SECRETION AND OVARIAN-FUNCTION IN HYPOPHYSEAL STALK-TRANSECTED BEEF HEIFERS

Hypothalamic regulation of luteinizing hormone (LH) secretion and ovarian function were investigated in beef heifers by infusing LHRH in a pulsatile manner (1 .mu.g/ml; 1 ml during 1 min every h) into the external jugular vein of 10 hypophysial stalk-transected (HST) animals. The heifers were HST .apprx. 30 mo. earlier. All heifers had increased ovarian size during the LHRH infusion. The maximum ovarian size (16 .+-. 2.7 cm3) was greater (P < 0.01) than the initial ovarian size (8 .+-. 1.4 cm3). Ovarian follicular growth occurred in 4 of 10 HST heifers in response to pulsatile LHRH infusion. In 2 heifers, an ovarian follicle developed to preovulatory size, but ovulation occurred in only 1 animal after the frequency of LHRH was increased (1 .mu.g every 20 min during 8 h). In blood samples obtained at 20-min intervals every 5th day, LH concentrations in peripheral serum remained consistently low (0.9 ng/ml) and nonepisodic in the 10 HST heifers during infusion of vehicle on the day before beginning LHRH. In 7 of 10 HST animals, episodic LH secretion occurred in response to pulsatile infusion of LHRH. In 3 of these long-term HST heifers, however, serum LH remained at basal levels and the isolated pituitary seemingly was unresponsive to pulsatile infusion of LHRH as indicated by sequential patterns of gonadotropin secretion obtained at 5-day intervals. Pulsatile infusion of LHRH apparently induces LH release in HST beef heifers.