Insulin dependent and independent actions of dietary protein on in vitro protein synthesis in skeletal muscle of rats.

Insulin dependency of the effect of dietary protein on in vitro protein synthesis in skeletal muscle was studied in rats. Re-feeding fasted rats with an adequate protein diet caused an increase in protein synthetic activity of skeletal muscle within only 5 hours with a concomitant increase in concentration of polyribosomes. During re-feeding of a protein-free diet muscle protein synthesis increased significantly but it was somewhat lower than in rats fed an adequate protein diet. The concentration of insulin in plasma increased promptly and markedly when the rats were re-fed either of these diets. However, the degree of increase was slightly less in rats fed a protein-containing meal. Re-feeding normal rats with egg albumin alone, after fasting 1 day, produced a marked increase in protein synthetic activity of skeletal muscle with a parallel increase in proportion of polyribosomes in the tissue. The stimulatory action of dietary protein on muscle protein synthesis was not abolished even in streptozotocin-diabetic rats. However, there was no appreciable change in ribosomal profile in the diabetic rats. These results indicate that the function of dietary protein in regulating muscle protein synthesis is not mediated entirely by insulin.