Blockade of Gonadotropin-Releasing Hormone-Induced Secretion of Pituitary Follicle-Stimulating Hormone by Inhibin-Containing Preparations*

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Abstract
The interaction of exogenously administered gonadotropin- releasing hormone (GnRH) and inhibin-containing gonadal preparations on the pituitary in intact male rats has been studied. The inhibin preparations were crude bovine testicular extracts (bTE) or porcine follicular fluid (pFF). Both preparations were extracted with ether and/or charcoal to remove steroids. The biological activity of both the bTE and the pFF was established in the 34-day-old acutely castrated male rat. Two experimental approaches were used. In series A, the inhibin preparation was administered first, and 6 h later, a single iv injection of GnRH (125 or 500 ng) was given. In series B, GnRH was infused continuously for 8 h, but after 2 h, the inhibin preparation was administered (sc). In series A, both bTE and pFF completely suppressed the 125-ng GnRH-induced rise in plasma FSH; however, neither was effective when 500 ng GnRH were administered. Plasma LH levels were unaffected by bTE or pFF. In series B, neither the bTE nor the pFF was effective in preventing the 22 ng/min GnRH-induced rise of either FSH or LH. However, when GnRH was infused at the rate of 11 ng/min, both the bTE and the pFF suppressed plasma FSH levels. LH levels were unaffected. Thus, both the bTE and pFF exert similar sex-independent inhibitory effects on the pituitary to control FSH secretion selectively. The data are most simply interpreted as evidence that GnRH and inhibin reversibly interact at the level of the pituitary.