ENDOCRINE CHANGES ASSOCIATED WITH LUTEAL REGRESSION IN EWE - SECRETION OF OVARIAN ESTRADIOL, PROGESTERONE AND ANDROSTENEDIONE AND UTERINE PROSTAGLANDIN-F2-ALPHA THROUGHOUT ESTROUS-CYCLE

The concentrations of estradiol, androstenedione, progesterone and prostaglandin F2.alpha. (PGF2.alpha.) were measured in utero-ovarian venous blood collected throughout 6 estrous cycles in 2 ewes with utero-ovarian autotransplants. The secretion of estradiol was closely correlated with that of androstenedione (r = 0.67, P < 0.001) indicating a common origin from the Graafian follicle. The concentration of these 2 steroids fluctuated at random throughout the luteal phase with the maximum secretion occurring about 2 days before the onset of estrus. Functional regression of the corpus luteum, as indicated by a fall in the secretion of progesterone, began on day 12 or day 13, i.e., about 4 days before the onset of estrus. In 5 of the 6 cycles the 1st significant rise in the secretion of PGF2.alpha. occurred on days 12-14 at the time of decline of progesterone secretion, although the release of PGF2.alpha. was maximal on the day before the onset of estrus. There was very little release of PGF2.alpha. from the uterus before day 12. The temporal relationship of these events suggests that the uterus will only release PGF2.alpha. after it has been primed for 7-10 days with progesterone. The initiation of luteal regression is independent of secretion of estradiol by the pre-ovulatory follicle which may stimulate the further release of PGF2.alpha. responsible for irreversible structural luteolysis on the day of proestrus.