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Influence of Calf Removal on the Serum Luteinizing Hormone Response to Naloxone in the Postpartum Beef Cow¹

University of Georgia, Athens 30602

Abstract

Twelve anestrous, postpartum beef cows were used to determine the effect of calf removal on the effect of naloxone on serum luteinizing hormone (LH) concentrations. On d 1, six cows were injected iv with saline and six with 200 mg naloxone dissolved in saline. Blood samples were taken at 15-min intervals for 2 h before and 2 h after naloxone or saline administration. At the beginning of blood sampling, calves were removed from three cows in each treatment. At 48 h after calf removal (d 3), all cows were injected iv with 200 mg naloxone and blood samples were collected as on d 1. On d 1, naloxone treatment increased (P<.01) serum LH concentrations from 1.2 ± .3 ng/ml at time 0 to 4.3 ± .6 ng/ml and 4.7 ± .8 ng/ml at 15 and 30 min, respectively. Injection of saline had no effect on serum LH concentrations. Forty-eight-hour calf removal increased (P<.01) serum LH concentrations in five of six cows (1.7 ± .8 vs 4.4 ± 1.2 ng/ml). Naloxone treatment failed to increase serum LH concentrations in these cows. Injection of naloxone increased (P<.01) serum LH concentrations in the one cow that did not exhibit an LH increase after calf removal and in six cows whose calves were not removed (1.4 ± .2 vs 4.4 ± .5 ng/ml). The present study provides additional evidence that endogenous opioids regulate LH in the postpartum beef cow. We hypothesize that suckling stimulates an opioid inhibition of LH secretion and removal of the suckling stimulus removes the opioid inhibitory tone.

¹ This research was conducted with State, Hatch, and USDA funds. LH for iodination was donated by Dr. L. E. Reichert, Jr. Albany Med. College. The authors wish to thank Dr. J. W. Crim and Dr. G. B. Rampacek, Univ. of Georgia and Dr. R. R. Kraeling, USDA-ARS for their advice in conducting this research.

² Dept. of Anim. and Dairy Sci. College of Agr.

³ Dept. of Physiol. and Pharmacol. College of Vet. Med.

⁴ USDA-ARS, Russell Res. Center, Athens, GA 30613.