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Pituitary and Blood Plasma Oxytocic Activity and Oxytocin Disappearance Rates in Cattle¹

Department of Dairy Husbandry, University of Missouri—Columbia, Columbia 65201

ABSTRACT

Oxytocic activity in posterior pituitary glands and blood plasma and rate of disappearance of oxytocin from jugular vein plasma were measured by the in vitro rat mammary cube assay of Van Dongen and Hays. Mean oxytocic activity in 35 posterior pituitary glands with average wet weight of 193.8 ± 12.2 mg was 101.7 ± 29.9 IU. Oxytocic activity in jugular vein blood plasma sampled through a cannula varied from 3.3 /µU at five minutes prior to placement of teat cups on the cows to 824 µU when teat cups were placed. It then declined to .02 µU at five minutes after the teat cups were first placed on cows. Blood samples taken 30 to 60 seconds after washing the udder were used to relate peak oxytocic activity to milk production. Forty-one observations in 16 cows resulted in a correlation coefficient between milk production and oxytocic activity of .01 (not significant). Determination of the biological half-life (t_1/2) of oxytocin by sampling jugular vein blood plasma after injecting 50 IU oxytocin resulted in a mean t_1/2 of 1.03 ± .10 minutes in 6 cows.

¹ Contribution from the Missouri Agricultural Experiment Station Journal Series 7131. Approved by the Director.