Growth hormone kinetics during adaptation to a hyperosmotic environment in rainbow trout

To clarify the role of growth hormone (GH) in salmonids during seawater (SW) adaptation, we examined GH kinetics in chronically cannulated rainbow trout, weighing about 1 kg. When trout were transferred from fresh water (FW) to 75% SW, plasma chloride concentration was normalized within 1 week. Plasma GH concentration increased significantly 2 days after transfer and decreased to the initial level thereafter. Metabolic clearance rate (MCR) and secretion rate (SR) of GH were calculated from the plasma levels of GH measured by radioimmunoassay after intra-arterial injection of recombinant chum salmon GH. 4 days after transfer to 75% SW, both MCR and SR increased to levels five times higher than those in FW, and returned to the FW levels after 3-4 weeks. In rainbow trout GH seems to be involved in the development of hypoosmoregulatory mechanisms, especially during the early phase of adaptation.