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 blot and dot blot hybridization analyses were performed using RNA from testis, heart, lung, intestine, stomach, kidney, and brain of rats fed on a 12% casein diet, on a 12% gluten diet which was marginally deficient in lysine and threonine, or on a protein-free diet. The mRNA content 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 different nutritional conditions. The amount of IGF-I receptor in these tissues and the affinity to IGF-I were determined by measuring the amount 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 nutritional conditions did not show any marked differences. The number of receptors did not change in the testis, lung, intestine, brain or kidney; 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 receptor is regulated in a distinctive way in each tissue in response to protein nutrition, and suggest that the regulation may have some physiological meaning in signal transmission of IGF-I. The amount of IGF-I receptor, however, was relatively constant in most tissues. Because the plasma IGF-I and IGF-binding protein concentrations change dramatically under different nutritional conditions, we conclude that concentrations of plasma IGF-I and IGF-binding proteins may mainly regulate the IGF-I action in tissues in response to protein nutrition.