MAMMARY ARTERIAL AND VENOUS CONCENTRATIONS OF PROLACTIN IN LACTATING COWS AFTER MILKING OR ADMINISTRATION OF THYROTROPIN-RELEASING-HORMONE OR ERGOCRYPTINE

Concentrations of PRL were measured in samples of sera collected from a pudendal artery and a sc abdominal mammary vein of 12 lactating Holstein cows before and after milking, and before and after administration of TRH and/or ergocryptine. Concentrations of PRL before milking or TRH were 3-fold greater during early (5–12 weeks postpartum) compared with late (37–57 weeks postpartum) lactation. Before milking or TRH, PRL was not different in sera from arterial or venous blood. Milking and TRH increased PRL in sera approximately 2- to 10-fold within 6–14 min, respectively, but as lactation advanced, the quantities of PRL released to these stimuli decreased. Arterial PRL averaged 1.05 ng/ml (P-10) more than venous PRL for 20 min after milking during early lactation. Arterial PRL levels during 30 min after TRH were 2.6–7.7 ng/ml greater (P < 0.05) than concentrations in the mammary vein. Furthermore, arteriovenous differences tended to be greater during early, compared with lte, lactation. Ergocryptine or 2-bromo-a-ergocryptine reduced PRL in sera 68–71%, but did not affect milk yield. Concentrations of PRL in skim milk generally paralleled concentrations in serum; however, administration of ergots reduced PRL more in serum than in skim milk. Arteriovenous differences in PRL were not detectable in cows given the ergots. We conclude that arteriovenous differences in PRL occurred primarily when concentrations in sera were acutely increased. © 1979 by The Endocrine Society.