Protein metabolism in skeletal muscle, diaphragm, and heart of diabetic rats.

Rates of protein synthesis were measured in several muscle types in young streptozotocin-diabetic rats at different times after withdrawal of insulin therapy. Protein synthesis decreased in all types of muscle studied. In muscles with a substantial proportion of fast-twitch fibers (gastrocnemius and extensor digitorum longus), this decline was associated with falls in both protein synthesis-to-RNA and RNA-to-protein ratio, but in soleus and heart the effect could be explained entirely in terms of decreased ribosome concentration. Rates of protein degradation were calculated for heart, diaphragm, and gastrocnemius muscle from the changes in protein synthesis and protein content and were found in all cases to be elevated by 4 days of insulin withdrawal. However, measurements of protein synthesis and protein mass in the gastrocnemius muscle of a group of chronically diabetic rats indicated that in this condition, unlike in acute diabetes, the rate of protein degradation was depressed.