Effect of Exogenous Prolactin on Peripheral Luteinizing Hormone Levels in Ovariectomized Cows1

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Abstract
Peripheral levels of luteinizing hormone (LH) and prolactin (PRL) were determined in mature chronically ovariectomized beef cows during i.v. infusion of either saline (0.9% NaCl) (SAL), or 30 mg bovine PRL in SAL (PRL) over a 3 h period. Each cow was infused once in the morning and once in the afternoon with either SAL or PRL (sequence alternated) for 4 consecutive days. Jugular vein blood samples were obtained at 10 min intervals. Thirty min following the initiation of PRL infusion, plasma PRL levels were maintained above 200 ng/ml, while plasma PRL concentrations remained well below 50 ng/ml during SAL infusion. A pulsatile fluctuation in plasma LH was observed in all cows, with each pulse being characterized by a rapid increase in LH followed by a more gradual decline to basal levels. This pulsatile pattern was consistent within cows over the 4 day period but varied significantly among cows. There were no differences in the pattern of plasma LH between SAL and PRL infusions. The results suggest that short term elevations in plasma PRL do not influence the pulsatile pattern of plasma LH in ovariectomized cows.

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