Luteinizing Hormone and Estradiol in Peripheral Blood of Mares during Estrous Cycle1

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Abstract
Serum LH and estradiol were measured concurrently in peripheral blood of six mares during eight estrous cycles, and rectal palpations were performed to determine the day of ovulation in relation to those measurements. LH was measured by radioimmunoassay using ovine LH antiserum known to cross-react with LH in several mammalian species. Estradiol was also determined by radioimmunoassay. Serum LH increased gradually from a low of 20.0 ± 0.4 ng/ml approximately 5 days prior to ovulation to a peak of 129.34 ± 7.1 ng/ml the day of ovulation. Concurrently, serum estradiol increased from 20.0 ± 0.2 pg/ml to a peak of 141.00 ± 9.8 pg/ml 2 days before LH peaked. Both LH and estradiol decreased in the serum gradually to diestrus levels in 5 days, suggesting that a positive feedback mechanism in estrogen induces ovulatory LH secretion in mares.

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