Insulin and insulin-like growth factor-I receptors in fish brain

We investigated insulin and insulin-like growth factor-I (IGF-I) receptor-binding and receptor intrinsic tyrosine kinase activity in the brain of carp (Cyprinus carpio) and trout (Salmo trutta fario). Glycoprotein fractions of semi-purified receptors were prepared by WGA-agarose affinity chromatography. Insulin receptors were found in the brains of both fish species investigated. Carp and trout brain preparations bound, respectively (per 50 micrograms glycoprotein), with 6.0 +/- 1.5% and 8.0 +/- 2.0% of 125I-labeled insulin added to the assay. Insulin binding was specific: much higher quantity of IGF-I (EC50 165 +/- 11 nM for carp and 88.0 +/- 6 nM for trout receptors) than insulin (EC50 0.26 +/- 0.04 nM for carp and 0.25 +/- 0.02 nM for trout) was necessary to displace bound insulin tracer. In preparations of brain receptors, IGF-I binding (52.8 +/- 6.5% in carp brain and 55.0 +/- 13.0% in trout brain) surpassed insulin binding several fold. IGF-I bound to the brain receptors with high affinity (Kd for carp was 0.13 +/- 0.06 nM and for trout 0.22 +/- 0.11 nM) and specificity. Although IGF-I binding could be displaced with insulin, EC50 were 660 +/- 51 nM for carp and 1557 +/- 194 nM for trout. Both ligands stimulated phosphorylation of exogenous substrates in a dose-dependent manner. Carp brain receptors were not significantly different from trout receptors with respect to basal phosphotransferase activities (250.0 +/- 50.0 fm P/mg glycoprotein in carp and 330.0 +/- 120.0 fm P/mg glycoprotein in trout). In both species IGF-I caused higher maximal stimulation (308.0 +/- 36.0% and 270.0 +/- 39%, for carp and trout, respectively) than insulin (250.0 +/- 13.0% and 209.0 +/- 6.0%, for carp and trout, respectively).