DIFFERENTIAL RESPONSE OF PLASMA PROLACTIN TO FRESH-WATER TRANSFER OF SMOLTS AND POSTSMOLTS OF SEAWATER-ADAPTED COHO SALMON (ONCORHYNCHUS-KISUTCH)

Prolactin (PRL) was purified from chum salmon, Oncorhynchus keta, pituitary glands and was used to develop a homologous radioimmunoassay for the measurement of PRL from salmon. The plasma PRL response to freshwater (FW) transfer differed in seawater (SW)-adapted postsmolt (250 g) and smolts (15 g) of coho salmon. Postsmolts had a pronounced and prolonged elevation of plasma titers of PRL with hypercalcemia and stable plasma sodium levels. The FW-transferred postsmolts had significantly lower pituitary gland PRL only at 0.5 and 2 hr post-transfer as compared to SW-SW. Smaller smolts showed stable plasma PRL levels after FW transfer, hypocalcemia 48 post-transfer, depressed plasma sodium concentrations, and lowered plasma osmotic pressure. This different response may be due to an increased osmoionic regulatory challenge encountered by the smaller smolts or possibly due to some other developmental change between the two different age classes. © 1990.