Transitory Changes in Plasma Progestins, Estradiol, and Luteinizing Hormone Approaching Ovulation in the Bovine1,2

Abstract

Progestins, estradiol, and luteinizing hormone were measured by radioimmunoassay in bovine plasma samples collected from indwelling jugular catheters daily on days −6 to −4, every 6h on days −3 and −2, and every 2h from day −1 to ovulation. Least squares analyses characterized time trends within animals and interrelationships between these hormones. Plasma progestins decreased from 5.7ng/ml at day −6 to .07ng at the peak of luteinizing hormone. No preovulatory peripheral increase in plasma progestins was detected. Estradiol increased from 2 pg/ml at day −4 to 6 pg at −12h and then increased abruptly to 7.4 pg at the peak of luteinizing hormone. This latter increase in estradiol was synchronous with the acute preovulatory surge of luteinizing hormone. Association was positive between estradiol and luteinizing hormone, but negative between progestins and estradiol. Estradiol decreased 50% by 5h following the peak of luteinizing hormone with a return to base (2 pg) at 14h. An increase of .1ng luteinizing hormone/ml plasma per day was linear from day −6 to 8h prior to the luteinizing hormone peak. Luteinizing hormone increased to a peak of 13.5ng/ml and remained elevated for 10.5h. These results are consistent with the hypothesis that an increase at proestrus in estradiol rather than progestins triggers the preovulatory surge of luteinizing hormone in the bovine.