Milking–Induced Release of Endogenous Prolactin In Cows Infused with Exogenous Prolactin 1

Abstract

Summary

Exogenous prolactin (3 or 9 mg NIH–B₃/hr) or saline was infused for 2.5 hr into 6 cows. The prolactin or saline infusions were repeated on alternate days for a total of 4 days. Approximately 45 min was required to achieve a stable baseline after the prolactin infusion was started. The exogenous prolactin, which increased serum prolactin approximately 5– and 8–fold above baseline endogenous concentrations, did not prevent the release of endogenous prolactin in response to milking. When the infusion was stopped serum prolactin disappeared with a 1/2 time of 23–25 min. We conclude that the inhibitory effects of exogenous prolactin previously reported on endogenous prolactin secretion must be exerted primarily on synthesis rather than release mechanisms, or the animals may require chronic exposure to high concentrations of prolactin.

The able technical assistance of Mrs. Annemieke Ambrosier is gratefully acknowledged.