Biological Activity of Luteinizing Hormone in the Peripartum Cow: Least Activity at Parturition with an Increase throughout the Postpartum Interval1

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Abstract
The biological (B) and immunological (I) activities of luteinizing hormone (LH) were determined during four peripartum periods in the cow. The sampling periods were PRE-5 (approximately 5 days prior to parturition), POST-3 (Day 2 or 3 postpartum), POST-12 (Day 11 or 12 postpartum) and LUTEAL (Day 11 or 12 following the first observed postpartum estrus). Blood samples were collected at 10-min intervals for either 8 h (PRE-5, POST-3, and POST-12) and 6 h (LUTEAL). Four hours prior to the end of each sampling period, 100 .mu.g of luteinizing hormone-releasing hormone (LHRH) was injected i.v. Plasma concentrations of LH were analyzed by a validated bioassay (rat interstitial cell testosterone) and a radioimmunoassay. Area under the LHRH-induced LH release curve was calculated by the trapezoidal rule, and endogenous LH was determined by averaging the pre-LHRH treatment vales. Paired t-analyses were used to compare LH release between periods. The periods of lowest (p<0.05) bioactive LH (BLH) were PRE-5 and POST-3. During POST-12, BLH was maximal. Endogenous immunoactive LH (ILH) was maximal during the POST-12 period, with PRE-5, POST-3, and LUTEAL being the least (p<0.005) by Day 12 postpartum. The ratio then increased (p<0.05) again by the LUTEAL period. These data suggest that the biological quality of LH changes throughout the puerperium. We infer from these data that the low biological activity of LH (subquality LH) may contribute to the postpartum infertility in the cow.

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