Ovary

Plasma Concentrations of Inhibin A and Follicle-Stimulating Hormone Differ Between Cows with Two or Three Waves of Ovarian Follicular Development in a Single Estrous Cycle

Abstract

Patterns of ovarian follicle development were monitored daily in Holstein-Friesian cows which had 2 (n=4) or 3 waves (n=4) of ovarian follicle development during a single estrous cycle. The plasma from daily blood samples was used in assays for inhibin A, FSH, progesterone and estradiol-17ß. Mean cycle lengths for cows with 2 and 3 waves were 21.8 and 25.3 days respectively (p<0.02). Although the average number of follicles of >3 mm diameter on each pair of ovaries was similar for 2- and 3-wave cows on Days 2, 3, and 4 (Day 0 = day of ovulation; 8.6 vs 9.6 follicles), there were more follicles of >6mm diameter on the ovaries of cows with 2 waves on Days 3 and 4. This difference was associated with a shorter interval from wave emergence to peak concentrations of inhibin A during the first wave in 2-wave cows (2.0 vs 3.8 days; p=0.03) and with higher peak concentrations (474 vs 332 pg/ml; p=0.03). Differences in peak FSH concentrations were less significant (1.7 vs 1.3 ng/ml; p=0.10). They had an inverse relationship on a daily basis with inhibin A concentrations. The peak concentrations of inhibin A and FSH in the second non-ovulatory wave in the 3-wave cows were similar to the low concentrations measured in the first wave (292 vs 332 pg/ml of inhibin A; 1.3 vs 1.3 ng/ml of FSH; p>0.20). Average peak concentrations of inhibin A and FSH were similar during the ovulatory wave for cows with either 2 or 3 waves in a cycle (432 vs 464 pg/ml of inhibin A; 2.3 vs 2.1 ng/ml of FSH; p>0.3). The lower concentrations of FSH during the emergence of the first follicular wave in cows with 3-wave cycles may have reduced the rate of development of some of the follicles as well as reducing the concentrations of inhibin A. This pattern of lower concentrations of FSH and inhibin A was repeated in the second non-ovulatory wave, but not in the ovulatory wave. Subtle differences in the concentrations of these two hormones may underlie the mechanism which influences the number of waves of ovarian follicle development which occur during the bovine estrous cycle.