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 ty rosine kinase activity were characterized in cardiac and skeletal musc les of several vertebrates. Specific insulin binding per unit weight o f skeletal muscle was clearly higher in pigeon and rat than in ectothe rmic vertebrates (32 +/- 5 and 25 +/- 2.7%/100 mg initial tissue in pi geon and rat, respectively, vs. 4.4 +/- 0.2%/100 mg in carp samples). Insulin binding clearly predominated over IGF-I binding in skeletal mu scle 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 o f both insulin and especially IGF-I was higher than the values found i n 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 spe cies studied presented basal phosphotransferase rates (250 - 1,600 fmo l P .mu g protein(-1). 30 min(-1)) and percentage of stimulation (150 - 520%) with clear differences between species. The present data sugge st that insulin and IGF-I binding to skeletal and cardiac muscles chan ge through the vertebrate scale in both quantity and activity.