Suppression of the Pituitary Follicle-Stimulating Hormone Secretion during Proestrus and Estrus in Rats by Porcine Follicular Fluid: Possible Site of Action*

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Abstract
These studies were designed to investigate the action of porcine follicular fluid (PFF) on the secretion of FSH and LH in cyclic fen lie rats. PFF was obtained from medium sized follicles and was charcoal treated to remove steroids. Control animals received pig serum (PS). Proestrous rats were injected ip with 1 ml PFF or PS at 1100 or 1800 h, or at both times. Sequential blood collections were obtained at 1300, 1600, and 1900 h proestrus and at 0800, 1100, and 1400 h on estrus. When PFF was injected at 1100 h, plasma FSH levels were significantly depressed on proestrous afternoon but not on estrous morning. In rats treated with PFF at 1800 h, FSH was significantly lower only at 0800 h on estrous morning. When two injections of PFF were given to rats at 1100 and 1800 h, plasma FSH was markedly reduced at 1600 and 1900 h proestrus and at 0800 and 1100 h estrus. In contrast, plasma LH and progesterone in proestrous rats were only altered when two PFF injections were given. All PFF-treated rats ovulated and oocyte maturation and ovarian morphology were not altered. Experiments were conducted to determine the site of action of PFF. Phenobarbital-blocked proestrous rats (1215 h) were pretreated with PS or PFF at 1100 h and infused with LHRH at a rate of 50 ng/h for 3 h beginning at 1300 h. Blood collections were taken at hourly intervals from 1300–1800 h. In animals receiving PFF, peak FSH levels achieved by infusion of LHRH were only 43% of those levels observed in PS-treated controls at 1600 h. Plasma LH concentrations were unaffected by PFF except at 1400 h. These results demonstrate that PFF can inhibit the spontaneous FSH surge occurring on proestrous afternoon and/or on estrous morning without appreciably altering the preovulatory LH surge. The onset of action of PFF seems to be 2–5 h after ip administration and the duration of action is 10–14 h. The inhibition of pituitary FSH secretion by PFF during proestrous afternoon can be adequately explained by a pituitary gland site of action, since the effect of PFF on LHRH-induced FSH secretion was strikingly similar to that seen on the spontaneous release of FSH.