Simultaneous Measurement of Luteinizing Hormone-releasing Hormone and Luteinizing Hormone during Estradiol-induced Luteinizing Hormone Surges in the Ovariectomized Ewe1

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Abstract
Sequential bleeding and push-pull perfusion of the hypothalamus were used to characterize luteinizing hormone (LH) and LH-releasing hormone (LHRH) release in ovariectomized (OVX) ewes after injection of corn oil or estradiol benzoate (EB). Push-pull cannulae were surgically implanted into the stalk median eminences of 24 OVX ewes. Seven to 14 days later each of 20 animals was given an i.m. injection of 50 .mu.g EB. Blood samples and push-pull perfusate were collected at 10-min intervals for 6-12 h beginning 12-15 h after EB injection. Four OVX ewes were given i.m. injections of corn oil 7 days after implantation of push-pull cannulae. Blood samples and push-pull perfusate were collected at 10-min intervals for 4 h between 18 and 22 h after injection of corn oil. Luteinizing hormone remained below 2 ng/ml throughout most of the sampling periods in 9 of 20 EB-treated ewes. In 5 of these 9 LHRH also was undetectable, whereas in 4 LHRH was detectable (1.84 .+-. 0.29 pg/10 min.), but did not increase with time. Preovulatory-like surges of LH occurred in 11-EB-treated ewes, but LHRH was undetectable in 5. In 4 of 6 ewes showing LH surges and detectable LHRH, sampling occurred during the onset of the LH surge. Regression analysis of data from these 4 ewes showed that concentrations of LH and LHRH output increased during the initial portion of the LH surge, but mean concentrations of LHRH during the LH surge were not different from LHRH concentrations in OVX ewes treated with corn oil (2.63 .+-. 0.89 vs. 1.82 .+-. 0.25 pg/10 min, P < 0.1). These results suggest that there is a transitory LHRH secretion during an estrogen-induced LH surge in OVX ewes. However, output of LHRH during the surge does not appear to be greater than that of oil-treated OVX ewes.