FACTORS AFFECTING ESTROGEN-INDUCED LH-RELEASE IN THE COW

Six experiments were conducted to study the mechanisms controlling estrogen-induced luteinizing hormone (LH) release in the cow. Estradiol-17β (E2) induced a release of LH in ovariectomized cows at doses ranging from 0.25-10.00 mg and estrone (E1) was less effective than E2. Exogenous progesterone (P4), administered s.c. in a silicone rubber implant for periods of 2-9 days prior to E2 injections, did not inhibit the E2-induced LH release even when a very low dose (0.25 mg) of E2 was given. However, when a corpus luteum (CL) was present and endogenous P4 was more than 1 ng/ml, E2 did not induce an LH release. Possible mechanisms for this discrepancy between the effects of exogenous and endogenous P4 are discussed. Suckling and/or lactation inhibited the E2-induced LH release for at least 2 weeks postpartum. This inhibition may partially explain postpartum anestrus.