INSULIN AND INSULIN-LIKE GROWTH-FACTOR-I RECEPTORS IN FISH BRAIN

We investigated insulin and insulin-like growth factor-I (IGF-I) recep tor-binding and receptor intrinsic tyrosine kinase activity in the bra in of carp (Cyprinus carpio) and trout (Salmo trutta fario). Glycoprot ein 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 bou nd, respectively (per 50 mu g glycoprotein), with 6.0 +/- 1.5% and 8.0 +/- 2.0% of I-125-labeled insulin added to the assay. Insulin binding was specific: much higher quantity of IGF-I (EC(50) 165 +/- 11 nM for carp and 88.0 +/- 6 nM for trout receptors) than insulin (EC(50) 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, IG F-I binding (52.8 +/- 6.5% in carp brain and 55.0 +/- 13.0% in trout b rain) surpassed insulin binding several fold. IGF-I bound to the brain receptors with high affinity (K-d for carp was 0.13 +/- 0.06 nM and f or trout 0.22 +/- 0.11 nM) and specificity. Although IGF-I binding cou ld be displaced with insulin, EC(50) were 660 +/- 51 nM for carp and 1 557 +/- 194 nM for trout. Both ligands stimulated phosphorylation of e xogenous substrates in a dose-dependent manner. Carp brain receptors w ere not significantly different from trout receptors with respect to b asal phosphotransferase activities (250.0 +/- 50.0 fm P/mg glycoprotei n 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 2 70.0 +/- 39%, for carp and trout, respectively) than insulin (250.0 +/ - 13.0% and 209.0 +/- 6.0%, for carp and trout, respectively).