A model system for studying inhibin.

We report here a sensitive model for studying inhibin-like activity in crude ovine testicular extracts (CoTE). The administration of CoTE in 25-100 mg amounts to 35-day-old immature male rats, orchidectomized just prior to use, resulted in the prevention of a rise in plasma FSH levels, seen 10 h post-treatment. In a second model, CoTE was injected at 1000 h of day 35 to a group of rats that was castrated 12-24 h prior to injection, and the animals were sacrificed 6 h later; plasma FSH levels were found to be significantly suppressed. CoTE, administered subcutaneously, both in multiple doses and as a single injection, was found to be equally effective. A single injection suppressed FSH levels within 3-6 h while LH levels were unaffected. The suppressive effect was dose-dependent, reaching a maximum value at doses of 100 mg CoTE and above. FSH levels could not be suppressed below the tonic level, either by the administration of a single large dose of CoTE, or by repetitive injections of maximal doses of CoTE at 3 h intervals. The suppression in FSH levels caused by a single injection of CoTE waned with time and totally disappeared by 36h. CoTE was prepared by the heat treatment (55 C for 30 min) of an aqueous extract of ovine testis, followed by centrifugation and ether extraction of the supernatant. The aqueous layer was then dialyzed and lyophilized. The lyophilized material was found to be free of testosterone, as measured by a specific radioimmunoassay, indicating that the active factor is a heat-stable, lyophilizable, nondialyzable material, free of contamination by testosterone and probably by other steroids.