Possible mode of seawater-adapting actions of growth hormone in salmonids

Growth hormone has both growth-promoting and seawater-adapting actions in salmonids. The growth influence of growth hormone is mediated largely by hepatic production of insulin-like growth factor-I (IGF-I). Transfer of several salmonids to seawater evoked rises in plasma concentration and turnover of growth hormone. A single class of growth hormone receptors was present not only in the liver but also in the gill, intestine, and kidney of rainbow trout. Growth hormone binding in the liver decreased after seawater transfer. Binding in the gill and kidney was not altered significantly. MgCl sub(2) treatment to remove bound hormone from the receptors indicated that the reduction in the liver binding is probably due to receptor occupancy by increased endogenous growth hormone. The occupied and the total (MgCl sub(2)-treated) receptors in the liver increased after seawater transfer, indicating the likelihood of at least partial mediation by the liver of the seawater-adapting action of growth hormone. IGF-I mRNA was detectable in the liver, gill and kidney of the trout and coho salmon by Northern blot analysis. The levels of IGF-I mRNA in these tissues increased significantly after growth hormone-injection. IGF-I mRNA was not altered significantly in the liver after seawater transfer, but increased in the gill and kidney after 1 day and 8 days, respectively. Thus, growth hormone may stimulate hypo-osmoregulatory ability in part by inducing local expression of IGF-I in the osmoregulatory organs in salmonids.