Validation of a homologous canine relaxin radioimmunoassay and application with pregnant and non-pregnant Northern fur seals (Callorhinus ursinus)

The primary objectives of this study were to validate a canine relaxin RIA for use in otariids and phocids and consider practical applications. For 6 captive Northern fur seal females, serum samples were grouped and examined according to pregnancy (n = 13), post-partum (n = 8) and non-pregnancy (n = 6), and, for 2 captive Northern fur seal males, serum samples were grouped and examined together regardless of age (2 mo–15 yrs, n = 6). Placental tissue was available for examination from one Northern fur seal, Steller sea lion and harbor seal. The validation process involved several steps using an acid-acetone extraction process to isolate a relaxin-containing fraction in pools of serum from each group of fur seals and placental tissue from each seal species. A relaxin-like substance was detected in extracts of pregnant, non-pregnant and male serum and placental tissue in a dose-responsive manner as increasing volumes of respective extracts or amounts of canine relaxin were introduced into the assay. In raw serum samples, mean immuno-reactive relaxin concentrations were higher (P < 0.05) during pregnancy than post-partum and non-pregnancy, and lower (P < 0.05) in male than female fur seals. During pregnancy, mean serum concentrations of relaxin progressively increased (P < 0.05) over Months 4–10 and, in serial samples collected from the same fur seals before and after parturition, mean concentrations were higher (P < 0.06) pre-partum than post-partum. In conclusion, validation of a homologous canine relaxin RIA for use in otariids and phocids resulted in the discovery of a relaxin-like substance in extracted and raw serum and placental tissue from Northern fur seals, a Steller sea lion and harbor seal. Distinctly higher immuno-reactive concentrations during pregnancy indicated the potential for relaxin to serve as a hormonal marker to differentiate between pregnant and non-pregnant or pseudopregnant pinnipeds.