Kinetic studies of growth hormone and prolactin during adaptation of coho salmon, Oncorhynchus kisutch, to different salinities.

The kinetics of growth hormone (GH) and prolactin (PRL) in coho salmon (Oncorhynchus kisutch) transferred from fresh water (FW) to seawater (SW) and vice versa were examined to help clarify the osmoregulatory roles of the two hormones during periods of migration to different salinities. Chum salmon GH or PRL was administered by a single injection intraarterially, and metabolic clearance rate (MCR) and secretion rate (SR) of injected hormones were calculated from the disappearance of the hormones from the plasma. When coho salmon smolts were acclimated to SW, MCR, SR, and plasma level of GH in SW-adapted (2-3 weeks) fish were twice as great as those in fish in FW. On the other hand, there was no difference in the kinetics of GH between the adult coho salmon in SW and those adapted to FW (2-3 weeks). The transfer of the adult coho salmon from SW to FW was followed after 2 days by a rise in plasma level and SR of PRL, which tended to stay at high levels after 2-3 weeks. The MCR of PRL increased significantly after 2-3 weeks in FW. These results support the likelihood of an important role of GH in SW adaptation and of PRL in FW adaptation in coho salmon.