Ovulation was induced in estrous rabbits in response either to crude rabbit median eminence extract (MEE) or to 0.2 ng of a highly purified preparation of porcine luteinizing hormone-releasing hormone (LH-RH) when they were infused directly into the adenohypophysis through stereo taxically implanted cannulae. Pretreatment with a single injection of 2 mg progesterone (P) sc 16—19 hr before infusion blocked ovulation to MEE infusion in all cases (10/10) and to LH-RH in 14/20 cases (70%). Four of the 6 rabbits which had ovulated in spite of P subsequently ovulated to a smaller dose (0.1 μg) of LH-RH in the absence of P. The remaining 2 were not tested. On the other hand, when the dose of LH-RH was increased to 1.0 μg, 4 of the 6 animals tested, in which P had blocked ovulation to the 0.2 fig dose, now ovulated in spite of P. Endogenous P, released during pregnancy or pseudopregnancy, was completely effective in blocking ovulation in response to either MEE or LH-RH. Pretreatment with estradiol benzoate (100 μg ×2) counteracted the blocking action of P in 5/6 rabbits. These data indicate that the ability of LH-RH to overcome P blockade is directly related to the dose of LH-RH administered. They also support the hypothesis that progestagens exert at least part of their ovulation-blocking effect directly on the pituitary.
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