ROLE OF TROPONIN-I PROTEOLYSIS IN THE PATHOGENESIS OF STUNNED MYOCARDIUM

Myocardial stunning is characterized by decreased myofilament Ca2+ res ponsiveness. To investigate the molecular basis of stunned myocardium, we performed PAGE and Western immunoblot analysis of the contractile proteins. Isolated rat hearts were retrogradely perfused at 37 degrees C for either 50 minutes (control group) or for 10 minutes, followed b y 20-minute global ischemia and 20-minute reperfusion (stunned group), or for 20-minute ischemia without reflow. Another group consisted of hearts subjected to 20-minute ischemia in which stunning was mitigated by 10-minute reperfusion with low Ca2+/low pH solution. Myocardial ti ssue samples subjected to PAGE revealed no obvious differences among g roups. Western immunoblots for actin, tropomyosin, troponin C, troponi n T, myosin light chain-1, and myosin light chain-2 showed highly sele ctive recognition of the appropriate full-length molecular weight band s in all groups. Troponin I (TnI) Western blots revealed an additional band (approximate to 26 kD, compared with 32 kD for the full-length p rotein) in stunned myocardial samples only. In parallel experiments, s kinned trabeculae were treated with calpain I for 20 minutes; Western blots showed a TnI degradation pattern similar to that observed in stu nned myocardium. Such TnI degradation was prevented by calpastatin, a naturally occurring calpain inhibitor. The results show that (1) TnI i s partially and selectively degraded in stunned myocardium; (2) this d egradation could be prevented by low Ca2+/low pH reperfusion, which al so prevented the contractile dysfunction of stunning; and (3) calpain I could similarly degrade TnI, supporting the idea that Ca2+-dependent myofilament proteolysis underlies myocardial stunning.