Decline in circulating estradiol during the periovulatory period is correlated with decreases in estradiol and androgen, and in messenger RNA for P450 aromatase and P450 17 alpha-hydroxylase, in bovine preovulatory follicles

The preovulatory surge of gonadotropins induces meiotic maturation of the o ocyte, the follicular/luteal phase shift in hormone production, and ovulati on. This complex and rapid series of developmental changes is difficult to study in large mammals, such as primates and ruminants, because variability in the length of individual reproductive cycles makes it virtually impossi ble to predict the time of the LH surge. We have validated an experimental model for inducing the LH surge and ovulation in cattle and used it to stud y the sequence of changes in hormone secretion and some of the mechanisms o f these changes. Luteolysis and a follicular phase were induced by injectio n of prostaglandin F-2 alpha; injection of a GnRH analogue 36 h later induc ed an LH surge and ovulation. The LH surge peaked 2 h after GnRH and ovulat ion followed 22-31 h after the surge, consistent with the periovulatory int erval in natural cycles. The ensuing luteal phase was normal, both in lengt h and in concentrations of circulating progesterone. In experiment I, the u teroovarian effluent was collected, via cannulation of the vena cava, at fr equent intervals relative to GnRH injection. Circulating estradiol declined progressively after GnRH, reaching a nadir by 8-10 h before ovulation, whe reas concentrations of androstenedione and testosterone remained constant. In experiment II, preovulatory follicles were obtained at 0, 3.5, 6, 12, 18 , or 24 h after GnRH. Concentrations of androgens and estradiol were measur ed in follicular fluid and medium from cultures of follicle wall (theca + g ranulosa cells); steady-state levels of mRNA for 17 alpha -hydroxylase (17 alpha OH) and P450 aromatase were measured in follicular tissue. Shortly af ter the LH surge (3.5 h post-GnRH) there was an acute increase in the capac ity of follicular tissue to secrete androstenedione, but not estradiol, in vitro. Thereafter, both androgens and estradiol declined, both in follicula r fluid and in medium collected from cultures of follicle wall. Levels of m RNA for 17aOH and aromatase in follicle wall decreased significantly by 6 h after GnRH, suggesting that declining levels of these enzymes underlie the decreases in steroid production by follicular cells. These results show th at in cattle the preovulatory decrease in follicular estradiol production i s mediated by redundant mechanisms, because androgen production and the cap acity of granulosa cells to convert androgens to estradiol decline coordina tely, in concert with decreases in mRNA for 77 alpha OH and P450 aromatase.