APPEARANCE OF INSULIN AND INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) RECEPTORS THROUGHOUT THE ONTOGENY OF BROWN TROUT (SALMO-TRUTTA FARIO)

Insulin and IGF-I receptors were characterized in glycoprotein fractio ns prepared by affinity chromatography from different developmental st ages of brown trout. The specificity of insulin and IGF-I binding was demonstrated by crossed-competition assays: unlabelled insulin displac ed bound radiolabelled insulin at concentrations 45-fold lower than un labelled IGF-I, whilst unlabelled IGF-I displaced bound radiolabelled IGF-I at concentrations 2000-fold lower than unlabelled insulin. The a ffinity of these receptors did not change significantly during trout d evelopment. Insulin-specific binding was detectable 3 weeks after spaw ning, after which it increased to a maximum in fry weighing 0.4 g, and decreased progressively to adult levels. IGF-I specific binding was d etectable in newly laid eggs and increased to a maximum during organog enesis in eyed eggs. It then decreased progressively during subsequent stages of development to adult levels. The apparent molecular weight (M-r) of the alpha-subunit of brown trout insulin and IGF-I receptors was smaller than that of the alpha-subunit of the rat insulin receptor . Receptor tyrosine kinase activity was stimulated in a dose-dependent manner by insulin and IGF-I. Insulin and IGF-I stimulated tyrosine ki nase activity and reached a maximum of 201 +/- 17.6 and 240 +/- 29.6% of basal phosphorylation, respectively. (C) 1998 Churchill Livingstone .