Plasma levels of oxytocin and vasopressin before, during and after parturition in cows.

In cows, plasma oxytocin and vasopressin were determined by specific and sensitive radioimmunoassays before, during and after parturition. In no case, oxytocin is elevated until the forelimbs of the foetus were distending the vagina and presenting at the vulva, supporting the view that endogenous maternal oxytocin does not primarily induce parturition. This finding is confirmed by simultaneous recording of myometrial electrical activity in 3 animals. Maximal oxytocin levels of 60.4 to 116 pg/ml plasma were measured at delivery of the foetus. The elevation in plasma vasopressin (peak values at delivery: 4.74 to 41.5 pg/ml) might be due, at least partially, to the increase in plasma osmolality during parturition.