Development of a biotin-streptavidin amplified enzyme immunoassay for oxytocin and its application during milk ejection and the reproductive cycle in the mithun (Bos frontalis).

Oxytocin is a key hormone involved in milk ejection. It plays a key role in regulation of reproductive cyclicity in female mammals by taking part in the process of luteolysis. Determination of oxytocin is, therefore, important for studying the control of its secretion and its role in reproduction of the mithun. A simple and sufficiently sensitive enzyme immunoassay (EIA) for oxytocin determination in mithun plasma using the biotin-streptavidin amplification system and second antibody coating technique was therefore developed. Biotin was coupled to oxytocin and used to bridge between streptavidin-peroxidase and the immobilized oxytocin antiserum in a competitive assay. The EIA was conducted directly in 200 microl of unknown mithun plasma. Standards prepared in hormone-free plasma were used. The lowest detection limit was 0.5 pg/ml plasma. Plasma volumes for the EIA (50, 100, and 200 microl) did not influence the shape of standard curve, even though a drop in OD450 was seen with higher plasma volumes. A parallelism test was carried out to compare endogenous mithun oxytocin with a bovine oxytocin standard. The former showed good parallelism with the bovine standard curve. For biological validation of the assay, plasma oxytocin was measured in the blood samples collected before, during, and after milking in three mithun cows and in six non-lactating cyclic mithuns during the entire estrous cycle. A sharp release of oxytocin shortly after udder stimulation was observed. A high level of oxytocin was maintained during milking, falling sharply thereafter. The mean plasma oxytocin concentration was different on different days of the estrous cycle (P < 0.001). Two peaks of oxytocin were recorded, one at day 6 and another at day 18 of the estrous cycle. In conclusion, a simple, sufficiently sensitive and direct EIA procedure has been developed for the first time to determine plasma oxytocin levels in mithuns. Apart from being non-radioactive, the EIA procedure described here also utilizes a highly stable biotinalyted hormone which has a shelf life of several years, unlike the short shelf life of iodinated tracer used in RIA procedures.