Pituitary function, ovarian follicular growth, and plasma concentrations of 17 beta-estradiol and progesterone in prepubertal heifers during and after treatment with the luteinizing hormone-releasing hormone agonist deslorelin.

Pituitary function, ovarian follicle growth, and plasma steroid concentrations were determined in prepubertal heifers during and after treatment with the LHRH agonist deslorelin. Brahman heifers, 13 mo of age and 204 +/- 5 kg, were allocated to 4 groups with treatments as follows: group C (n = 6), control, received no treatment; group C + LHRH (n = 6), control, received LHRH tests at the same times as group D + LHRH below; group D (n = 6), received deslorelin (approximately 300 micrograms/day) for 28 days; group D + LHRH (n = 6), received deslorelin for 28 days were given LHRH tests (50 micrograms LHRH i.m.) on Day 28 of treatment and on Days 2, 4, 8, 12, 16, and 20 after treatment. Number and size of ovarian follicles were determined by rectal ultrasonography. Deslorelin induced an increase in plasma concentrations of LH within 2 h (p < 0.001); LH remained greater (p < 0.05) for heifers treated with deslorelin for 48 h, but from Day 4 to 28 of treatment, LH in treated and control heifers did not differ. Ovarian follicles of heifers treated with deslorelin were characterized by increased (p < 0.05) maximum size, and also tended to have a greater rate of growth, compared with those of control heifers. Heifers treated with agonist had greater (p < 0.05) plasma concentrations of 17 beta-estradiol from Day 0 to 28 of treatment, and 17 beta-estradiol in these heifers showed cyclical fluctuations that appeared to be related to cycles of growth and regression of ovarian follicles. After cessation of deslorelin treatment, plasma LH was similar for heifers in group C and group D. Heifers in group C + LHRH had significant (p < 0.05) LH responses to exogenous LHRH at all times. In contrast, heifers in group D + LHRH did not show an LH release in response to exogenous LHRH on Day 28 of treatment and also failed to have a significant (p < 0.05) release of LH until Day 12 after treatment. A significant increase in plasma 17 beta-estradiol after LHRH injection also occurred on Day 12 after treatment in heifers in group D + LHRH. The findings indicated that LHRH agonist treatment in prepubertal heifers is associated with an acute LH response followed by a return of LH to concentrations similar to those of control heifers. Size of the largest ovarian follicle is greater in heifers treated with LHRH agonist and is associated with increased plasma concentrations of 17 beta-estradiol. After cessation of agonist treatment, the pituitary remains desensitized to LHRH for approximately 12 days. This study is the first demonstration of enhanced ovarian follicular growth and increased 17 beta-estradiol secretion during treatment with an LHRH agonist.