Development of a Homologous Equine Relaxin Radioimmunoassay*

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Abstract
Equine relaxin (eRlx) immunoactivity has previously been measured in the mare during pregnancy using the porcine relaxin (pRlx) RIA (pRlx-RIA). This was not the optimal system for measurement of eRlx because the dose-response curve obtained with equine plasma was not parallel to the pRlx standard curve. A homologous eRlx-RIA has been developed and used to measure relaxin immunoactivity during pregnancy and parturition in the mare. Highly purified eTlx was used for the generation of antiserum in rabbits, preparation of tracer, and as assay standards. A double antibody eRlx RIA (eRlx-RIA) was developed which was highly specific and sensitive (0.023 ng relaxin/tube). The dose-response curve obtained with eRlx plasma was parallel to the equine standard curve while there was no parallelism noted between the pRlx and eRlx standard curves. This assay was utilized to measure eRlx in samples collected during pregnancy which were previously measured for relaxin content in the pRlx-RIA. It was found that the two assay systems gave almost identical patterns of secretion throughout pregnancy. The two assays differed in the amount of relaxin measured, with the eRlx-RIA measuring considerably higher amounts during gestation than the pRlx assay. In oxytocin-induced foalings, the differences became greater with the equine assay measuring as much as 10-fold greater concentrations.