Changes of pituitary sensitivity to LH-RH during the rat estrous cycle

The plasma LH (luteinizing hormone) response to 200 ng of LH-RH (LH-releasing hormone) injected subcutaneously at different stages of the estrous cycle in normal rats under Surital anesthisia was maximal during the afternoon of proestrus and lowest on diestrus I. The area under the plasma LH curve measured at 13:00 on proestrus was approximately 7-fold higher than that obtained at 15:30 h on diestrus I. Intermediate responses were found on diestrus II, estrus and morning of proestrus. An approsimately 2.5-fold higher LH response was observed on proestrus than on diestrus I after injection of [D-Ala2, des Gly-NH2-10] LH-RH ethylamide at 15:00 h. That these marked changes of LH response are not secondary to interference by endogenous LH-RH, changes of the metabolism or transport of exogenous LH-RH or modification of plasma LH clearance is ascertanied by the finding of similar changes of pituitary sensitivity to LH-RH under in vitro conditions using pituitaries collected at the same stag-s of the estrous cycle. As measured both in vivo and in vitro, not only the amplitude but also the speed of LH response are maximal during the afternoon of proestrus and minimal on diestrus I.