Thirty-four gilts in two experiments were fed
altrenogest for 18 d to block spontaneous growth of ovulatory follicles after luteolysis. They were injected with
estradiol or
charcoal-extracted porcine follicular fluid (pFF) to determine 1) whether
gonadotropin secretion could be depressed and 2) whether exposure to reduced levels of
gonadotropins would result in decreased numbers of medium follicles (3 to 6 mm in diameter). Gilts in Exp. 1 received treatments in a 2 X 2 X 2 factorial arrangement starting 48 h before the last feeding of
altrenogest.
Corn oil or
estradiol (2 micrograms/kg
body weight), 5 ml of
charcoal-extracted porcine serum (pS) or pFF were injected im four times at 8-h intervals and gilts were sacrificed 24 or 96 h after last feeding of
altrenogest. In Exp. 2, gilts received one of four treatments consisting of 1) pS, injected iv nine times at 8-h intervals starting 48 h before the last feeding of
altrenogest; 2) pFF, with injection protocol the same as for pS; 3)
estradiol injected im three times and 4) four times at 8-h intervals starting 0 and 24 h, respectively, before the last feeding of
altrenogest. Compared with pS or
corn oil,
estradiol increased (P less than .001) plasma
estrogen and decreased (P less than .05) plasma
luteinizing hormone (LH) without a significant effect on plasma
follicle stimulating hormone (FSH).
Estradiol, compared with
corn oil, decreased (P less than .01) the number of medium follicles from 24.8 to 0/gilt and decreased (P less than .05) the weight of ovarian follicular fluid from 4.2 to 2.1 g/gilt at 72 h after the first injection. Five milliliters of pFF had no significant effect on plasma
gonadotropins or number of medium follicles. However, 20 ml of pFF, compared with pS, decreased (P less than .05) plasma FSH from 45 ng/ml to 9 ng/ml 32 h after the first injection, had no effect on plasma LH, decreased (P less than .01) the number of medium follicles from 29.2 to 2.2/gilt and decreased (P less than .01) follicular fluid weight from 3.9 to 1.6 g/gilt by 72 h after the first injection. These results indicate that
estradiol or a non-steroidal component of follicular origin can decrease secretion of
gonadotropins and suppress recruitment of medium follicles in the pig.