Pentoxifylline inhibits Ca2+-dependent and ATP proteasome-dependent proteolysis in skeletal muscle from acutely diabetic rats

Previous studies from this laboratory have shown that catecholamines exert an inhibitory effect on muscle protein degradation through a pathway involving the cAMP cascade. The present work investigated the systemic effect of pentoxifylline ( PTX; cAMP- phosphodiesterase inhibitor) treatment on the rate of overall proteolysis, the activity of proteolytic systems, and the process of protein synthesis in extensor digitorum longus muscles from normal and acutely diabetic rats. The direct in vitro effect of this drug on the rates of muscle protein degradation was also investigated. Muscles from diabetic rats treated with PTX showed an increase ( 22%) in the cAMP content and reduction in total rates of protein breakdown and in activity of Ca2+- dependent ( 47%) and ATP proteasome- dependent ( 23%) proteolytic pathways. The high content of m- calpain observed in muscles from diabetic rats was abolished by PTX treatment. The addition of PTX ( 10(-3) M) to the incubation medium increased the cAMP content in muscles from normal ( 22%) and diabetic ( 51%) rats and induced a reduction in the rates of overall proteolysis that was accompanied by decreased activity of the Ca2+-dependent and ATP proteasome- dependent proteolytic systems, in both groups. The in vitro addition of H-89, an inhibitor of protein kinase A ( PKA), completely blocked the effect of PTX on the reduction of proteolysis in muscles from normal and diabetic rats. The present data suggest that PTX exerts a direct inhibitory effect on protein degradative systems in muscles from acutely diabetic rats, probably involving the participation of cAMP intracellular pathways and activation of PKA, independently of tumor necrosis factor-alpha inhibition.