Pulsatile Release of Follicle-Stimulating Hormone in Ovariectomized Rats Is Inhibited by Porcine Follicular Fluid (Inhibin)*

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Abstract
While the pulsatile release of LH [luteinizing hormone] in ovariectomized animals has been well established, little information exists concerning the pulsatile release of FSH and the factors involved in its regulation. The characteristics of episodic FSH secretion were analyzed and the effects of ovarian feedback signals on that pattern were examined. From conscious unrestrained ovariectomized rats, blood was collected at 10 min intervals for 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/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 < 0.005). In the 3rd h of blood sampling, PFF also significantly lowered the average FSH peak amplitude (P < 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 .mu.g, s.c., 24 h before bleeding) significantly reduced LH peak frequency, mean trough value and average mean value; 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. Apparently the release of FSH occurs in a pulsatile fashion and this pattern of release is regulated by PFF (inhibin). An intriguing synergism between estradiol and inhibin exists to suppress LH release.