Insulin/IGF-I binding ratio in skeletal and cardiac muscles of vertebrates: A phylogenetic approach

Insulin and insulin-like growth factor (IGF-I) receptor binding and tyrosine kinase activity were characterized in cardiac and skeletal muscles of several vertebrates. Specific insulin binding per unit weight of skeletal muscle was clearly higher in pigeon and rat than in ectothermic vertebrates (32 +/- 5 and 25 +/- 2.7%/100 mg initial tissue in pigeon and rat, respectively, vs. 4.4 +/- 0.2%/100 mg in carp samples). Insulin binding clearly predominated over IGF-I binding in skeletal muscle of endotherms (IGF-I binding was 7.7 +/- 0.5%/100 mg in rat). In ectothermic vertebrates the situation was reversed, and IGF-I binding was higher than insulin binding. In cardiac muscle, specific binding of both insulin and especially IGF-I was higher than the values found in skeletal muscle of the same species (IGF-I binding was 60 +/- 4, 103 +/- 2, and 20 +/- 3%/100 mg in carp, turtle, and rat, respectively). The tyrosine kinase activity of insulin and IGF-I receptors of all species studied presented basal phosphotransferase rates (250 - 1,600 fmol P .mu g protein(-1). 30 min(-1)) and percentage of stimulation (150 - 520%) with clear differences between species. The present data suggest that insulin and IGF-I binding to skeletal and cardiac muscles change through the vertebrate scale in both quantity and activity.