Effect of dietary proteins on the plasma immunoreactive insulin-like growth factor-1/somatomedin C concentration in the rat.

Plasma immunoreactive insulin-like growth factor-1/somatomedin C (IR-IGF-1) was determined in rats fed for 1 week on a protein-free diet, or diets containing gluten, gluten supplemented with lysine and threonine, maize-gluten meal (with arginine), maize-gluten meal (with arginine) supplemented with tryptophan and lysine, or casein. IR-IGF-1 concentration was higher in the arterial plasma of rats fed on a diet containing casein at 120 g/kg diet (4-8 U/ml) than in rats fed on a protein-free or a low-casein (50 g/kg diet) diet (1.5-2 U/ml). The plasma of rats fed on gluten or maize-gluten meal as the protein source showed intermediate values. However, giving a diet containing an amino acid mixture as recommended by the National Research Council (1978) but deprived of lysine or tryptophan did not affect significantly the plasma IR-IGF-1 concentration. Total IGF-1 concentration (which was measured immunologically after extraction of the plasma with acid-ethanol) was also lower in the rats fed on the protein-free diet than in those fed on the casein (120 g/kg diet) diet. The ratio IR-IGF-1:total IGF-1 was higher in the rats fed on the casein diet (120 g casein/kg diet) than in those fed on the protein-free diet. The results suggest an important influence of IR-IGF-1 or IR-IGF-1:total IGF-1 ratio on protein anabolism and nutrition. IR-IGF-1 and total IGF-1 were found in the fractions of molecular weights 40 kDa and 150 kDa after gel filtration of rat plasma. A larger amount of IGF-1 was recovered in the fraction of 150 kDa in the rats fed on the casein diet. 125I-IGF-1 added to the plasma of rats fed on the protein-free diet was found mainly in the fraction of 40 kDa after gel-filtration. On the other hand, 125I-IGF-1 added to the plasma of rats fed on the gluten or casein diets was mainly recovered in the free IGF-1 fraction. The results suggest that IGF-binding protein(s) of molecular weight about 40 kDa was not saturated with IGF-1 in the rats fed on the protein-free diet. The results indicate the important role of IGF-1 and its binding proteins in the regulation of protein metabolism in rats.