Development of a homologous radioimmunoassay for eel prolactin

A highly specific and homologous radioimmunoassay (RIA) for the measurement of prolactin (PRL) in the plasma and the pituitary of the eel was developed using a rabbit antiserum to eel PRL. PRL was purified from the pituitary of Japanese eel (Anguilla japonica). Pituitary extracts and plasma from the Japanese and European eels exhibited displacement curves parallel to the eel PRL standard. Plasma and pituitary extracts from chum salmon, rainbow trout, Japanese charr, tilapia, goldfish, and carp, as well as plasma from hypophysectomized eel, showed negligible cross-reactivity. PRL and growth hormone (GH) preparations from chum salmon, tilapia, and sheep, carp PRL, and eel GH did not cross-react with the antibody. The RIA sensitivity was less than 0.1 ng eel PRL per milliliter. Intra- and interassay coefficients of variations were 2.4 and 11.8%, respectively. The immunoreactive PRL levels in plasma and pituitary of the eel adapted to 50% seawater were significantly lower than those of the eel in fresh water. Plasma PRL levels increased maximally 2 days after transfer from seawater to fresh water, as would be expected from the well-established role of PRL in freshwater adaptation in several euryhaline teleosts.