Insulin and IGF-I binding and tyrosine kinase activity in fish heart

A study is presented of the binding of insulin and IGF-I to their respective receptors in the heart muscle of carp (Cyprinus carpio), coho salmon (Oncorhynchus kisutch), brown trout (Salmo trutta fario) and Sprague-Dawley rats. Receptor preparations were partially purified by wheat germ agglutinin affinity chromatography. Specific binding of insulin/100 mg cardiac muscle was much lower in fish (from 5.6 to 9.2%) than in rat (52.0 +/- 5.0%). In both carp and trout, insulin binding to the cardiac muscle receptor preparation was significantly higher than in the white skeletal muscle (3.4 +/- 0.3%, carp and 0.9 +/- 0.3%, trout) or in the red skeletal muscle of carp (5.5 +/- 0.8%). Specific binding of IGF-I/100 mg fish heart preparation ranged between 44 and 68%, surpassing IGF-I binding in the rat heart (20 +/- 6%). The affinity of IGF-I receptors in fish heart (Kd 0.17-0.19 nM) was higher than that in rat heart (Kd 0.40 +/- 0.05 nM) or insulin receptors in fish and rat heart preparations (0.25-0.72 nM). The IGF-I receptor binding was highly specific and required at least 100 nM insulin to cause any displacement of the bound ligand. Receptor tyrosine kinase activity could be stimulated in a dose-dependent manner by insulin and IGF-I, although in equimolar doses IGF-I was more potent than insulin. Maximum stimulation of tyrosine kinase activity (210-230%) in fish heart was in the same range as in other piscine tissues (150-260%) or in rat cardiac muscle (200-250%).