Alterations in skeletal muscle proteins in obese and nonobese rats.

Skeletal muscle proteins (myofibrillar, sarcoplasmic, and stromal) were assayed in the gastrocnemius muscles from Zucker obese rats (fa/fa) at 3, 8, and 52 weeks of age. Total muscle weights were similar in obese and nonobese rats at 3 weeks, but reduced in the obese rats by 8 weeks of age. When the muscle proteins were expressed per 100 mg of muscle, the obese rats had lower concentrations of all three muscle proteins, with a greater loss of sarcoplasmic proteins than the nonobese rats at 8 and 52 weeks of age. By 8 weeks of age total muscle proteins were also reduced in obese compared with nonobese rats. With increasing age, the amount of total proteins continued to rise in both obese and nonobese rats with concomitant increases in myofibrillar, sarcoplasmic, and stromal proteins. The increase in muscle proteins was greater between 3 and 8 weeks than between 8 and 52 weeks of age. It is suggested that the inactivity of the obese rat may contribute to the reduction in muscle proteins observed during their early development.