Abundant Insulin-like Growth Factor-1 (IGF-1) Receptor Binding in Fish Skeletal Muscle

Abstract

Insulin and insulin-like growth factor-1 (IGF-1) receptors were quantified in glycoprotein fractions prepared by wheat germ agglutinin-agarose affinity chromatography from skeletal muscle of several species of salmonid fish and carp. Insulin binding in fish skeletal muscle was lower than insulin binding found in rat. IGF-1-specific binding was two- to sixfold higher than insulin binding (16.0 ± 3.0 versus 5.8 ± 0.75%/50 μg glycoprotein in brown trout, 15.5 ± 0.9 versus 2.2 ± 0.5%/50 μg in coho salmon, and 39.7 ± 7.6 versus 16.0 ± 3.0%/50 μg in carp muscle). Specific IGF-1 binding in fish skeletal muscle presented values similar to those in rat muscle. IGF-1 receptor binding was, in addition, highly specific. Cold IGF-1 displaced radiolabeled IGF-1 binding in doses 100-to 1000-fold lower than cold insulin (ED 50 of IGF-1 binding to carp receptor preparations was 0.24 ± 0.04 nM when displaced with cold IGF-1 and 368 ± 83 nM when displaced with insulin). On the other hand, cold insulin displaced radiolabeled insulin binding at concentrations 10- to 100-fold lower than cold IGF-1. Receptor tyrosine kinase activity was stimulated over basal values in a dose-dependent manner by both insulin and IGF-1, although IGF-1 was more potent than insulin. Basal rates of phosphorus transferred to the artificial exogenous substrate poly(Glu 4;Tyr 1) were similar in all salmonid species (200-320 fmol P/μg protein) and higher in carp (1840 ± 300 fmol P/μg protein). Maximum percentage of stimulation of kinase activity by insulin and IGF-1 was in the same range in all salmonid species and carp (130-200% for insulin, 160-232% for IGF-1). Abundance of IGF-1 receptors in fish skeletal muscle contrasts with the pattern observed in higher vertebrates, in which insulin receptors prevail over IGF-1 receptors.