Effects of diabetes on protein turnover in cardiac muscle

Effects of alloxan diabetes of 10-day duration on protein turnover were investigated in hearts perfused with buffers simulating control and diabetic sera. Diabetes produced a 30% inhibition of protein synthesis in hearts perfused as Langendorff or working preparations. This reduction was attributable to a 20% fall in RNA concentration and a 10% decrease in efficiency of protein synthesis. Determination of RNA in ribosomal subunits indicated that the reduction in efficiency that was observed with diabetes may be due to an inhibition of polypeptide chain elongation/termination. Pharmacological levels of insulin (25 mU/ml) and cardiac work stimulated protein synthesis in both control and diabetic hearts. Effects of diabetes and insulin on protein synthesis in isolated heart muscle cells were similar to those found in whole heart. Diabetes increased protein degradation in hearts perfused with buffer similating diabetic serum and under conditions of cardiac work. Insulin (25 mU/ml) decreased protein degradation in both control and diabetic hearts. These studies indicate that long-term diabetes produces a greater negative nitrogen balance that, in contrast to control hearts, cannot be normalized by pharmacological levels of insulin or by cardiac work.