Abstract

A method for extracting and concentrating oxytocin from blood, combined with a sensitive bioassay for oxytocin performed on the isolated rat mammary strip, was utilized to determine the effect of vaginal distention on the level of oxytocin in the plasma of lactating, pregnant and normally cycling ewes. Before distention, plasma of lactating and cycling ewes contained about 6 μU oxytocin/ml plasma. The plasma oxytocin concentration increased by 133 ± 29 % (mean ±SE) and 123 ±16%, respectively, in those 2 groups of animals during the first minute of stimulation. In pregnant animals, by contrast, the plasma oxytocin titer decreased 48 ± 5 % from a control level of 11 ±2 μU/ml plasma. The conclusion that some factor peculiar to pregnancy interferes with the reflex release of oxytocin in response to vaginal stimulation is discussed. (Endocrinology 83: 272, 1968)

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