Effects of progesterone on LH release in cycling rats were studied. Doses of 25 mg of progesterone were injected sc into intact adult female rats. Forty-eight hr after the injection, the rats were anesthetized with urethane and a preparation of highly purified LH-RH was injected into the common carotid artery or infused into the pituitary. Control serum LH concentrations in these rats were undetectable both in oil-injected control and progesteronetreated rats. Intracarotid injection of 10 and 50 ng LH-RH into the untreated rats raised serum LH concentrations by 1.5 and 3.6 ng/ml, respectively, 15 min after the injection. The same materials in the progesterone-treated animals increased serum LH by 0.5 and 1.6 ng/ml, respectively. Fifteen min after intrapituitary infusion of saline, 2 ng LH-RH and 10 ng LH-RH, serum LH levels were, respectively, 0.1, 7.6 and 12.5 ng/ml in the control rats and 0, 3.8 and 6.1 ng/ml in progesterone-pretreated rats. Factorial analysis of variance indicated that progesterone significantly suppressed LH release induced by intracarotid or intrapituitary administration of LH-RH. The magnitude of suppression was approximately 50%. In conclusion, although a single large dose of progesterone did not abolish LH-RH-induced LH release in cycling rats, it suppressed it significantly, indicating a direct effect of the steroid on the pituitary.
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