Oxytocin determination by radioimmunoassay. III

An improved RIA for measurement of oxytocin in blood is described by using an extraction method with SEP-PAK C18 cartridges, which allows also concentration of the sample, a new antiserum with a higher sensitivity to standard oxytocin and preparation of the standard curve in buffer. The lower limit of assay sensitivity was 0.25 pg/tube, corresponding to 0.25-1.0 pg/ml plasma depending on the amount of plasma extracted. Hence, it was no problem to measure oxytocin basal concentrations in peripheral blood in the range of 0.6-4 pg/ml plasma depending on the stage of the oestrous cycle. The highest oxytocin concentrations occurred during the early and mid-luteal phase. The method has been applied also for samples from women, sheep, pigs and horses. Mean (+-SD) recovery of oxytocin added to plasma or only buffer after extraction was 71.3+-8,1%, and the coefficient of variation (CV) = 11.4% (n = 27 assays). The intra-assay CV of two control samples was 7.9+-2.8 and 7.8+-2.4% (n = 17 assays). The inter-assay CV of 5 control samples with low and high oxytocin concentrations varied between 10.8+-17.3% (n = 25 assays). The 50% intercept was 2.5+-0.3 pg, CV = 11.3% (n = 29 assays). (author)