MUSCLE INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) RECEPTORS IN CHICKENS WITH HIGH OR LOW BODY-WEIGHT - EFFECTS OF AGE AND MUSCLE-FIBER TYPE

IGF-I receptors were characterized in leg and breast muscles from 1- a nd 7-week-old chickens selected for high (HG) or low (LG) growth rate. Following whole muscle solubilization, receptors were partially purif ied by wheat germ agglutinin (WGA) chromatography. IGF-I and insulin b inding did not differ between genotypes, but significantly decreased w ith age, without any change in receptor affinity. In the older birds, insulin binding was lower in breast than in leg muscle, whereas IGF-I binding was similar. Using the artificial substrate poly(Glu-Tyr) 4.1, the IGF-I-stimulated tyrosine kinase activity of the receptors was me asured and compared between genotypes (at 1 week), and in HG chickens between muscle types (1 and 7 weeks) and ages tin leg muscle). It was not modified by any of these factors. We conclude that: (1) IGF-I and insulin receptor number do not differ between genotypes, (2) the numbe r of both receptors decreases with age, and (3) in 7-week-old birds, b reast muscle could become less responsive to insulin than leg muscles, following a decrease in receptor number. (C) 1998 Churchill Livingsto ne.