Oxytocin determination by radioimmunoassay in cattle. I. Method and preliminary physiological data

A radioimmunoassay for oxytocin in cow plasma is described. Antisera were raised in rabbits against synthetic oxytocin coupled to bovine thyroglobulin. Iodinated oxytocin free of unlabelled oxytocin and most likely also free of diiodo-oxytocin was used as radioactive tracer. The tracer showed a high degree of purity, and was stable on storage. It could be used in the assay for 2-3 months. The assay showed very little cross-reactivity with vasopressin. Acetone was used for the extraction of oxytocin from plasma as well as from standards made of synthetic oxytocin in pooled cow plasma. Inhibition curves obtained with plasma collected from cows at parturition were parallel to those obtained with the oxytocin standard preparation. The mean recovery of oxytocin added to cow plasma was 106% (SD=14). The within-assay coefficient of variation (CV) varied from 5.2 to 10.9%, and the between-assay CV was in the order of 13%. The assay sensitivity was 1 pg (0.5 μU) per tube, corresponding to 3 pg/ml plasma. Around the time of milking the plasma oxytocin profile showed a strong response to the preparation for milking, and a further effect related to the attachment of the teat cups of the milking machine. Peak concentrations were in the range of 15-50 pg/ml. During parturition there was a peak of oxytocin (65 pg/ml) coinciding with the expulsion phase. After this peak levels decreased but remained measurably elevated until the expulsion of the placenta. The plasma disappearance curve for immuno-reactive oxytocin after the infusion of 100 IU oxytocin over a period of 1 h showed two components with apparent half-lives of 7-9 and 25 min, respectively.