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Evidence that estradiol induces the preovulatory LH surge in cattle by increasing pituitary sensitivity to LHRH and then increasing LHRH release.

The objectives of these studies were to determine 1) the influence of estradiol on the ability of the pituitary gland to release LH in response to LHRH in vivo, and 2) whether LHRH is required to cause the preovulatory LH surge in cattle. To minimize the confounding effect of endogenous LHRH or LHRH-induced LH release, we used the estradiol-treated steer as a model for the cow. The rationale for this choice is that estradiol does not normally cause an LH surge in steers, but it does exert a negative feedback effect inhibiting LHRH secretion. In the first experiment, an LH surge was induced in steers by injecting 1 μg LHRH at 20-min intervals between 12 and 20 h after estradiol. This surge resembled the preovulatory LH surge in cows with respect to both duration (10 h) and amplitude (49 ng/ml). After the peak, LH concentrations returned to baseline despite continued LHRH injections. In control steers given oil, repetitive injections of LHRH increased serum LH to only 13 ng/ml. In a second experiment, steer...