EFFECT OF PROTEIN NUTRITION ON INSULIN-LIKE GROWTH-FACTOR-I (IGF-I) RECEPTOR IN VARIOUS TISSUES OF RATS

The effects of protein nutrition on insulin-like growth factor-I (IGF- I) receptor in various tissues of rats were investigated. Northern blo t and dot blot hybridization analyses were performed using RNA from te stis, heart, lung, intestine, stomach, kidney, and brain of rats fed o n a 12% casein diet, on a 12% gluten diet which was marginally deficie nt in lysine and threonine, or on a protein-free diet. The mRNA conten t of IGF-I receptor in the testis and heart of the rats fed on the 12% gluten and protein-free diets was significantly larger than those of the rats fed on the 12% casein diet. Whereas in other tissues examined , IGF-I receptor mRNA content did not change significantly under the d ifferent nutritional conditions. The amount of IGF-I receptor in these tissues and the affinity to IGF-I were determined by measuring the am ount of I-125-labelled-IGF-I bound to solubilized IGF-I receptor. The affinity of IGF-I receptor to IGF-I in each tissue under the various n utritional conditions did not show any marked differences. The number of receptors did not change in the testis, lung, intestine, brain or k idney; possibly increased in stomach of the rats fed on the 12% gluten or protein-free diet; and slightly decreased in heart of the rats fed on the 12% gluten diet compared with that of the rats fed on the 12% casein diet. These results indicate that the synthesis of IGF-I recept or is regulated in a distinctive way in each tissue in response to pro tein nutrition, and suggest that the regulation may have some physiolo gical meaning in signal transmission of IGF-I. The amount of IGF-I rec eptor, however, was relatively constant in most tissues. Because the p lasma IGF-I and IGF-binding protein concentrations change dramatically under different nutritional conditions, we conclude that concentratio ns of plasma IGF-I and IGF-binding proteins may mainly regulate the IG F-I action in tissues in response to protein nutrition.