Relationship between LH receptor concentrations in thecal and granulosa cells and in-vivo and in-vitro steroid secretion by ovine follicles during the preovulatory period

Abstract

Ewes were ovariectomized before (Group 1, n = 5) or after (Group 2, n = 6) the peak of the preovulatory gonadotropin surge. Ovarian secretion rates of estradiol and testosterone were significantly higher in Group 1 than in Group 2. The presence of high levels of LH [luteinizing hormone] receptors in both thecal and granulosa cells was used to identify ovulatory from non-ovulatory follicles. There was a significant fall in the LH receptor concentration in the thecal and granulosa cells of ovulatory follicles after the peak of the preovulatory gonadotropin surge. Ovulatory follicles in Group 1 produced significantly more estradiol and testosterone in vitro than did those in Group 2. In both groups ovulatory follicles secreted significantly more estradiol in vitro than did non-ovulatory follicles. Follicular fluid estradiol concentrations were similar in pattern to the in vitro estradiol secretion activity in ovulatory and non-ovulatory follicles. Follicular fluid testosterone concentrations were significantly higher in non-ovulatory follicles than in ovulatory follicles. Incubation of follicles with 250 ng testosterone/ml did not significantly alter the in vitro estradiol secretion rate in any of the groups of follicles except for Group 2 non-ovulatory follicles in which estradiol accumulation increased. The number of thecal and granulosa cell LH receptors was significantly correlated with follicular fluid estradiol concentrations in ovulatory follicles and with in vitro estradiol production by Group 1 ovulatory follicles. Apparently the fall in estradiol secretion rates, which occurs after the peak of the preovulatory gonadotropin surge, may be due to a decrease of aromatase activity associated with a fall in the concentration of LH receptors and is not due to a lack of the estrogen precursor testosterone. The elevated concentration of testosterone and low estradiol concentrations in non-ovulatory follicles compared with ovulatory follicles are probably due to an inactive aromatase system, perhaps associated with the lack of granulosa cell LH receptors.