Effect of insulin on protein synthesis and degradation in skeletal muscle after exercise

This study examined whether insulin stimulation of protein synthesis and inhibition of protein degradation is enhanced after exercise. The isolated perfused rat hindquarter preparation was used to evaluate net protein breakdown, myofibrillar protein degradation, and protein synthesis. Thirty minutes after treadmill exercise of high and moderate intensity, rates of tyrosine release were increased by 58 and 25%, respectively. Insulin at 75 microU/ml had no effect on these increases after intense exercise; however, 20,000 microU/ml of insulin totally inhibited this increase. Cycloheximide increased the tyrosine release in both control and exercised rat muscle. It also abolished the difference between them, suggesting that the increase in tyrosine release after exercise is caused by an inhibition of protein synthesis. Phenylalanine incorporation into protein was marginally depressed (22%, P = NS) in the white gastrocnemius muscle after intense exercise. Insulin at 200 microU/ml stimulated protein synthesis in these rats, but no more than it did in a nonexercised control group. Failure to observe a greater effect of insulin on protein metabolism was also noted when rat muscle was studied 150 min after intense exercise and after contractions induced by electrical stimulation of the sciatic nerve. These findings suggest that after exercise or electrically induced contractions the enhanced ability of insulin to stimulate hexose and amino acid transport is not paralleled by an increase in its ability to stimulate protein synthesis or inhibit protein degradation.