Changes in oestrogen-synthesizing ability of preovulatory bovine follicles relative to the peak of LH

Abstract

Preovulatory bovine follicles (n = 28) were collected at different times after the onset of standing estrus until shortly before ovulation. In vitro conversion of tritiated androstenedione in the presence of NADPH by homogenates of the follicular wall was compared in phases relative to the LH [luteinizing hormone] peak. During phase 0 (before the LH surge) conversion into estradiol-17.beta. was high and production of estrone was about 8-fold lower. During phases 1 (0-6 h after the LH peak) and 2A (6-14 h after the LH peak) the production of estradiol and estrone remained constant; the percentage of remaining androstenedion increased. In phase 2B (14-20 h after the LH peak) conversion into estradiol and estrone had decreased to about one-third correlating with a higher percentage of remaining androstenedione. In phase 3 (20 h after the LH peak until ovulation) conversion into estradiol and estrone remained constant. The ratio between the production of estrone and estradiol remained constant throughout the phases of preovulatory development (0.13), indicating a concurrent inhibition of aromatase and 17.beta.-hydroxysteroid dehydrogenase activities. Conversion into 19-hydroxyandrostenedione showed a pattern similar to that of estradiol, and testosterone was produced in minute quantities. In preovulatory bovine follicles eventual inhibition of aromatization takes place at about 14 h after the preovulatory LH peak.