Changes in androgen secretion and luteinizing hormone pulse amplitude are associated with the recruitment and growth of ovarian follicles during the luteal phase of the bovine estrous cycle.

Abstract

In cattle the development of large antral follicles occurs in two or three successive waves during the estrous cycle, with one follicle per wave selected for dominance. To test the hypothesis that negative feedback effects of steroids secreted by the dominant follicle are critical to the regulation of follicular waves, we examined temporal relationships among ovarian follicular growth, steroid secretion, and gonadotropin secretion. Follicular growth was monitored by ultrasonography. In the first experiment, blood was collected from 5 Holstein heifers every 8 h between Days 0 and 14 of the estrous cycle from both a jugular vein and the vena cava (to collect ovarian blood). Jugular samples were also collected every 12 min for 8 h during three periods (Days 3 or 4, 7 or 8, and 11, 12, or 13; n = 6) to characterize the pulsatile pattern of LH secretion. Both estradiol and testosterone concentrations in the vena cava increased as pre-wave elevations in FSH concentrations decreased (p < 0.05) between Days 1 and 4 (first follicular wave) and between Days 9 and 12 (second follicular wave). LH pulse amplitude was greater during the second period of frequent blood collection (Day 7 or 8, end of the growth phase of the first dominant follicle) compared to the other two periods (p < 0.05), suggesting that increased LH pulse amplitude may be important for the later stages of dominant follicle growth. In the second experiment, to determine whether ovarian steroids are secreted primarily by dominant follicles, blood samples were collected from the utero-ovarian veins draining ovaries with (n = 4) and without (n = 4) a dominant follicle during the first follicular wave. Testosterone, androstenedione, and estradiol concentrations in the utero-ovarian veins fluctuated in relation to the pattern of follicular growth (p < 0.05), and secretion was much greater from ovaries with a dominant follicle. In blood collected both from the vena cava and from the utero-ovarian veins, estradiol secretion reached a peak and started to decline before androgen concentrations peaked (p < 0.05), suggesting that the initial decrease in estradiol secretion from the dominant follicle is not due to a lack of androgen precursors. The results suggest that 1) a transient increase in LH pulse amplitude during the early-midluteal phase may be important for supporting the final stages of dominant follicle growth; 2) ovarian androgens, as well as estradiol, may play a critical role in the control of FSH secretion during waves of follicular development; and 3) the dominant follicle is responsible for fluctuations in circulating estradiol and androgens during follicular waves.