Estradiol-Luteinizing Hormone Relationship during the Proestrous Gonadotropin Surge*

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Abstract
Changes in peripheral LH and estradiol (E2) concentration were examined in sequential samples taken from nonanesthetized rats from 1320–1640 h on proestrus in order to characterize the LH-E-2 relationship during the rising phase of the LH surge. At 80 min before the peak in plasma LH, plasma E> was 67 ± 4 pg/ml; at 80 min after the LH peak, plasma E-2 had fallen to 26 ± 4 pg/ml. Expressed as the change from a reference point of 120 min before the observed LH peak, plasma E2 concentration had decreased by 24% at 40 min before the LH peak; by 40 min after the LH peak, E-2 concentration had decreased by 54%. Because of this significant decrease in E2 before the peak in plasma LH in the proestrous rat, experiments were performed in which this decrease in E2 was prevented to determine the effect on LH secretion. An E2-containing Silastic, capsule was implanted at 1240 h on proestrus; sequential blood samples were collected from 1240–1800 h. When plasma E2 was maintained throughout the afternoon at levels comparable to those found before the surge, there was a significant decrease in the peak concentration of LH compared to controls with empty implants. When plasma E2 was maintained at supraphysiological levels (approximately 150 or 300 pg/ml), there were further decreases in plasma LH concentration. These studies demonstrate a significant decrease in peripheral estradiol concentration during the rising phase of the LH surge. This work also indicates that when this decline in E2 is prevented, the magnitude of the LH surge is decreased.