Radioreceptor assay for growth hormone in coho salmon (Oncorhynchus kisutch) and its application to the study of stunting.

Binding sites for native chum salmon growth hormone (sGH) in coho salmon (Oncorhynchus kisutch) hepatic membranes were analyzed by radioreceptor assay. Displaceable (specific) binding represented up to 25% of total radiolabeled sGH added. Binding was dependent on buffer pH and membrane protein concentration, and was complete by 24 hours at 15 degrees C. Specific binding was greatest in liver membranes, and was also detected in muscle, ovary, gill, kidney, and brain. Scatchard analyses indicated a single class of hepatic binding sites that were specific for sGH. In stunts, abnormal seawater salmon with elevated plasma GH levels and inhibited growth, specific binding of sGH to liver membranes was three times lower than in normal seawater smolts. The concentration of salmon GH binding sites was decreased in stunt livers by 60%, while their affinity for sGH was unchanged. Down-regulation of hepatic GH receptors by high plasma GH levels may explain in part the low sGH binding in stunts.