Pulsatile release of follicle-stimulating hormone in ovariectomized rats is inhibited by porcine follicular fluid (inhibin).

While the pulsatile release of LH in ovariectomized animals has been well established, little information exists concerning the pulsatile release of FSH and the factors involved in its regulation. The present studies analyzed the characteristics of episodic FSH secretion and examined the effects of ovarian feedback signals on that pattern. From conscious unrestrained ovariectomized rats, blood was collected at 10-min intervals for a period of 3 h. A pulsatile pattern of FSH was observed in the plasma of control rats, with an average frequency of 4 peaks/3, h, while LH oscillations occurred at an average of almost 6 peaks per 3 h. The injection of porcine follicular fluid (PFF; 0.5 ml), a source of inhibin, reduced FSH peak frequency to 2.5 peaks/3 h (P less than 0.005). In the third hour of blood sampling, PFF also significantly lowered the average FSH peak amplitude (P less than 0.05), the trough value, and the average mean value for that hour. In contrast, none of these parameters for LH was altered by PFF. Estradiol benzoate (EB; 20 micrograms, sc, 24 h before bleeding) significantly reduced LH peak frequency, mean trough value, and average mean value, whereas it only reduced the mean FSH average value and mean trough value. Combined treatment with EB and PFF synergized to decrease the mean value, trough value, peak frequency, and peak amplitude of episodic LH release. Pituitary responsiveness to LHRH was evaluated after the treatments described above. PFF inhibited the response of FSH to LHRH, but not that of LH. EB by itself potentiated the release of both hormones. The combined EB/PFF treatment resulted in a decreased pituitary response in terms of FSH and LH compared to that of the EB-treated control group. We conclude that the release of FSH occurs in a pulsatile fashion and, further, that this pattern of release is regulated by PFF (inhibin). It was also shown that an intriguing synergism between estradiol and inhibin exists to suppress LH release.