Specific radioimmunoassay of oxytocin in rat plasma

Sensitive, specific and reproducible radioimmunoassay (RIA) was developed for the measurement of oxytocin (OXT) in rat blood plasma after various extraction methods. The assay is based on an antiserum raised against OXT in rabbit. The sensitivity, affinity constant, and cross-reactivity of the antiserum were determined. The 125I-labelled OXT for RIA was produced by chloramine-T method and purified with high pressure liquid chromatography (HPLC). Two extraction procedures were employed: 1. adsorption to an artificial silicate, Lichroprep Si 60 (Merck); 2. immunoextraction of the hormone applying a magnetic bearer covered with purified antibodies against OXT. The specificity of the extraction methods was characterized in comparative HPLC/RIA studies of specimens extracted from blood plasma in different ways. The basal level of the peptide measured after the extraction with thermally activated Lichroprep Si 60 or after the immunoextraction method was found to be 9.6 +/- 2.3 pg/ml (mean +/- S.E.) and 15.3 +/- 0.9 pg/ml (mean +/- S.E.), respectively. Various well known factors (ether exposure, hyperosmotic stress and suckling) appeared to be potent peripheral stimuli of OXT release, and thus indicated the suitability of the RIA method for the measurement of OXT in blood plasma.