Oxytocin determination by radioimmunoassay. III. Improvement to subpicogram sensitivity and application to blood levels in cyclic cattle.

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).