Factors Affecting Estrogen-Induced LH Release in the Cow1

Abstract

Six experiments were conducted to study the mechanisms controlling estrogen-induced luteinizing hormone (LH) release in the cow. Estradiol-l7β (E₂) induced a release of LH in ovariiectomized 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.