Growth hormone receptors in the liver and osmoregulatory organs of rainbow trout: Characterization and dynamics during adaptation to seawater

Specific binding sites for chum salmon growth hormone (sGH) were identified in the membranes obtained from tissues of rainbow trout. Specific binding of ¹²⁵I-labelled sGH (% per mg protein) was found in the liver (37%), ovary (6%), brain (6%), gill (4%), intestine (4%) and posterior body kidney (4%). Specific binding was not significant in head kidney, anterior body kidney, spleen, heart, skeletal muscle or skin. Scatchard analyses demonstrated the presence of a single class of high-affinity low-capacity receptors in the liver, gill, intestine and kidney. The association constants for the membranes from liver, gill, intestine and kidney were of the same order (1 litre/nmol). Chum salmon prolactin did not inhibit the binding of ¹²⁵I-labelled sGH to receptors in the liver, gill, intestine and kidney. Transfer of rainbow trout from fresh water to 80% seawater evoked a rise in plasma concentration of GH and a significant decrease in the GH binding to the liver membranes after 1 day. Binding in the gill and kidney was not altered significantly. Membranes were treated with 4 mol MgCl₂/l to remove bound GH from the receptors, and results indicated that the reduction in binding in the liver after transfer to seawater was probably due to receptor occupancy by increased endogenous GH. The occupancy of liver GH-binding sites was maximal 4 days after transfer. Total (MgCl₂-treated) binding sites in the liver increased significantly 14 days after transfer. Scatchard analysis indicated that receptors were altered in capacity without changes in binding affinity. Although GH may also directly affect osmoregulatory organs through their GH receptors, the present results indicate the likelihood of at least partial mediation by the liver of the seawater-adapting action of GH in the rainbow trout.