Development of a heterologous radioimmunoassay for eel (Anguilla anguilla) gonadotropin

A heterologous radioimmunoassay (RIA) for eel gonadotropin (GTH) was developed, using an antiserum to the beta subunit of carp GTH (cGTHbeta). Partially purified eel GTH preparations or crude eel pituitary extracts produced partial cross-reactions: the inhibition curves reached a plateau before complete inhibition of the binding of the tracer and the slopes were lower than those for cGTH. However, the parallelism between the inhibition curves produced by all the eel samples allowed estimation of their GTH activity relative to an eel GTH standard; under these conditions, the RIA exhibited good reproducibility within and between assays. Three different RIA systems for cGTHbeta using different antisera gave similar estimations of immunoreactive GTH (irGTH) content of eel GTH preparations and pituitary extracts. The specificity of teh heterologous RIA for eel GTH was first assessed by comparing RIA and bioassay data: as with carp pituitary extracts, no significant difference was found between immunological and biological estimations of GTH activity for partially purified or crude pituitary extracts from normal female eel. Further validation of the specificity of the RIA for eel GTH was provided by the variation of the eel pituitary irGTH content under different hormonal treatments: a significant increase in female eel pituitary irGTH content was produced by treatment with estradiol-17 beta or with carp pituitary extract, a result in agreement with previous histological reports of Olivereau and collaborators. The heterologous RIA for eel GTH, in view of its sensitivity, reproducibility, and specificity, is a useful tool for physiological studies in the eel.