TROPONIN SUBUNITS CONTRIBUTE TO ALTERED MYOSIN ATPASE ACTIVITY IN DIABETIC CARDIOMYOPATHY

Our group has documented that myocardial performance is impaired in th e hearts of chronically diabetic rats and rabbits. Abnormalities in th e contractile proteins and regulatory proteins may be responsible for the mechanical defects in the streptozotocin (STZ)-diabetic hearts. Pr eviously, the major focus of our research on contractile proteins in a bnormal states has concentrated on myosin ATPase and its isoenzymes. O ur present study is based on the overall hypothesis that regulatory pr oteins, in addition to contractile protein, myosin contribute to alter ed cardiac contractile performance in the rat model of diabetic cardio myopathy. The purpose of our research was to define the role of cardia c regulatory proteins (troponin-tropomyosin) in the regulation of acto myosin system in diabetic cardiomyopathy. For baseline data, myofibril lar ATPase studies were conducted in the myofibrils from control and d iabetic rats. To focus on the regulatory proteins (troponin and tropom yosin), individual proteins of the cardiac system were reconstituted u nder controlled conditions. By this approach, myosin plus actin and tr oponin-tropomyosin from the normal and diabetic animals could be studi ed enzymatically. The proteins were isolated from the cardiac muscle o f control and STZ-diabetic (4 weeks) rats. Sodium dodecyl sulfate gel electrophoretic patterns demonstrate differences in the cardiac TnT an d TnI regions of diabetic animals suggesting the different amounts of TnT and/or TnI or possibly different cardiac isozymes in the regulator y protein complex. Myofibrils probed with a monoclonal antibody TnI-1 (specific for adult cardiac TnI) show a downregulation of cardiac TnI in diabetics when compared to its controls. Enzymatic data confirm a d iminished calcium sensitivity in the regulation of the cardiac actomyo sin system when regulatory protein(s) complex was recombined from diab etic hearts. Actomyosin ATPase activity in the hearts of diabetic anim als was partially reversed when myosin from diabetic rats was regulate d with the regulatory protein complex isolated from control hearts. To our knowledge, this is the first study which demonstrates that the re gulatory proteins from normal hearts can upregulate cardiac myosin iso lated from a pathologic rat model of diabetes. This diminished calcium sensitivity along with shifts in cardiac myosin heavy chain (V1-->V3) may be partially responsible for the impaired cardiac function in the hearts of chronic diabetic rats.