Changes in the sarcoplasmic proteins of skeletal muscles induced by experimental steroid myopathy.

The protein content and the relative distribution of cell compartments was investigated after 14 days of dexamethasone treatment in rabbits. The cell fractions were further analysed by SDS-PAGE. It was found that the loss of myofibrillar fraction was greater than that of the sarcoplasmic one. The susceptibility of fast-twitch fibres to steroid myopathy was more marked compared to the slow fibres. The sarcoplasmic proteins showed the following changes: myoglobin and myokinase decreased, while alpha-GPDH and LDH increased in both muscle types. On the other hand, the alterations of TPI, PGK, PGI, PK, ADA, phosphorylase kinase and phosphorylase b, were opposite in the two types of muscle. The results are in agreement with observations of others indicating that glucocorticoid treatment causes a shift in the equilibrium of synthesis and degradation of myofibrillar and sarcoplasmic proteins. The different susceptibility of fast versus slow muscles to steroid myopathy could be due to the activity-dependent regulatory mechanism of metabolic pathways.