Chronic treatment with bromocryptine during the oestrous cycle of the cow: Evidence that prolactin is not involved in preovulatory follicular growth

Abstract

Nine heifers received an intramuscular injection of 15 mg of bromocryptine twice daily from Day 1 (the day of first standing oestrus defined as Day 0) until 50 h after the onset of luteal regression. At this time the ovaries were collected and the number and quality of follicles > 2 mm were determined. Binding of 125I-labelled hCG and 125I-labelled ovine prolactin to follicles > 5 mm and concentrations of progesterone, oestradiol and androstenedione in the follicular fluid were estimated. Plasma concentrations of prolactin, LH and oestradiol were studied from the onset of luteal regression until ovariectomy. Seven heifers served as a control group. Prolactin was effectively suppressed during bromocryptine treatment (0.21 ± 0.01 μg/ l, mean ± SEM; n = 9) as compared to control heifers (16.8 ± 3.6 μg/ l; n = 7). Concentrations of LH were not affected by the low level of prolactin (0.99 ± 0.08 μg/ l vs. 0.95 ± 0.11 μg/1). Both groups showed a similar time-course of the oestradiol concentration in the peripheral blood during the 50-h period. No specific binding of prolactin to granulosa cells and thecal tissue of follicles > 5 mm could be demonstrated, regardless of their quality or diameter. Preovulatory follicles of bromocryptine-treated and control heifers did not differ with regard to the number of specific binding sites for hCG and the concentrations of oestradiol, progesterone and androstenedione in the follicular fluid. It is concluded that prolactin is not involved in the growth and development of antral follicles during the normal oestrous cycle of the cow.