Role of troponin I proteolysis in the pathogenesis of stunned myocardium

Myocardial stunning is characterized by decreased myofilament Ca²⁺ responsiveness. 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°C for either 50 minutes (control group) or for 10 minutes, followed by 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 tissue samples subjected to PAGE revealed no obvious differences; among groups. Western immunoblots for actin, tropomyosin, troponin C. troponin T. myosin light chain-I, and myosin light chain-2 showed highly selective recognition of the appropriate full-length molecular weight hands in all groups. Troponin 1 (Tn1) Western blots revealed an additional band (≃26 kD, compared with 32 kD for the full-length protein) in stunned myocardial samples only. In parallel experiments, skinned trabeculae were treated with calpain I for 20 minutes; Western blots showed a Tn1 degradation pattern similar to that observed in stunned myocardium. Such Tn1 degradation was prevented by calpastatin, a naturally occurring calpain inhibitor. The results show that (1) Tn1 is partially and selectively degraded in stunned myocardium; (2) this degradation could be prevented by low Ca²⁺/low pH reperfusion, which also prevented the contractile dysfunction of stunning; and (3) calpain I could similarly degrade Tn1, supporting the idea that Ca²⁺-dependent myofilament proteolysis underlies myocardial stunning.