Estrogen secretion and gonadotropin binding by individual bovine follicles during estrus.

Abstract

This study was conducted to examine the capacity of individual bovine follicles to secrete estradiol at estrus and to determine if the rate of estrogen secretion can be altered by the level of feed intake. Estrus was synchronized in 16 mature beef cows after 80 d on a high or low feeding regimen (130 or 65% of NRC requirements for total digestible nutrients). Ovaries were removed either 2 or 16 h after the second behavioral estrus following synchronization. To define the preovulatory luteinizing hormone (LH) surge, blood samples were taken at frequent intervals as estrus approached. The nine largest follicles (per cow) were isolated from the ovaries, incubated individually in medium 199 (M 199) for 4 h, and the medium then assayed for estradiol-17β (E2). Testosterone (250 ng/ml) was added to the incubation medium of one-half of the follicles for the last 2 h incubation. After incubation, binding of iodinated gonadotropins to the theca and granulosa cells of the largest follicle was quantitated. The LH profile for each cow was determined, and the time of ovary removal classified as occurring on the ascending portion of the LH peak or after (postpeak) the peak returned to baseline. The largest follicle of each cow secreted E2 at a rate two to three orders of magnitude greater than the smaller follicles. Secretion of E2 was much greater by the largest follicle in the peak group than in the postpeak group (51.6 vs 8.2 ng•ml−1• 30 min−1). Addition of testosterone to the M 199 had no effect on E2 secretion by either the largest or small follicles. Binding of human chorionic gonadotropin (hCG) to theca cells of large follicles was correlated with E2 secretion (r = .68, P<.01) and with basal LH concentrations (r = .59, P<.05) when computed across all treatment groups. When calculated within the peak and postpeak groups, these correlations were not significant. There was no effect of nutrition on E2 secretion (34.7 vs 32.9 ng•ml−1•30 min−1), serum concentration of LH at the preovulatory surge (43.4 vs 42.1 ng/ml) or serum baseline concentration of LH (2.6 vs 2.9 ng/ml); all for high vs low feed, respectively. These results indicate that the largest follicle is responsible for most of the E2 secretion of the ovary pair at estrus and that the E2 secretion by the largest follicle decreases rapidly at the time of the LH peak.