Mitochondria have emerged as key participants in and regulators of myocardial injury during ischemia and reperfusion. This review examines the sites of damage to cardiac mitochondria during ischemia and focuses on the impact of these defects. The concept that mitochondrial damage during ischemia leads to cardiac injury during reperfusion is addressed. The mechanisms that translate ischemic mitochondrial injury into cellular damage, during both ischemia and early reperfusion, are examined. Next, we discuss strategies that modulate and counteract these mechanisms of mitochondrial-driven injury. The new concept that mitochondria are not merely stochastic sites of oxidative and calcium-mediated injury but that they activate cellular responses of mitochondrial remodeling and cellular reactions that modulate the balance between cell death and recovery is reviewed, and the therapeutic implications of this concept are discussed.

Calpain 1 (CPN1) is a ubiquitous cysteine protease that exists in both cytosol and cardiac mitochondria. Mitochondrial CPN1 (mit-CPN1) is located in the intermembrane space and matrix. Activation of mit-CPN1 within the intermembrane space increases cardiac injury by releasing apoptosis-inducing factor from mitochondria during ischemia-reperfusion (IR). We asked if activation of mit-CPN1 is involved in mitochondrial injury during IR. MDL-28170 (MDL) was used to inhibit CPN1 in buffer-perfused hearts following 25-min ischemia and 30-min reperfusion. MDL treatment decreased the release of lactate dehydrogenase into coronary effluent compared with untreated hearts, indicating that inhibition of CPN1 decreases cardiac injury. MDL also prevented the cleavage of spectrin (a substrate of CPN1) in cytosol during IR, supporting that MDL treatment decreased cytosolic calpain activation. In addition, MDL markedly improved calcium retention capacity compared with untreated heart, suggesting that MDL treatment decreases mitochondrial permeability transition pore opening. In addition, we found that IR led to decreased complex I activity, whereas inhibition of mit-CPN1 using MDL protected complex I. Pyruvate dehydrogenase content was decreased following IR. However, pyruvate dehydrogenase content was preserved in MDL-treated mitochondria. Taken together, MDL treatment decreased cardiac injury during IR by inhibiting both cytosolic and mit-CPN1. Activation of mit-CPN1 increases cardiac injury during IR by sensitizing mitochondrial permeability transition pore opening and impairing mitochondrial metabolism through damage of complex I.

Activation of calpain 1 (CPN1) and calpain 2 (CPN2) contributes to cardiac injury during ischemia (ISC) and reperfusion (REP). Complex I activity is decreased in heart mitochondria following ISC-REP. CPN1 and CPN2 are ubiquitous calpains that exist in both cytosol (cs)-CPN1 and 2 and mitochondria (mit)-CPN1 and 2. Recent work shows that the complex I subunit (NDUFS7) is a potential substrate of the mit-CPN1. We asked whether ISC-REP led to decreased complex I activity via proteolysis of the NDUFS7 subunit via activation of mit-CPN1 and -2. Activation of cs-CPN1 and -2 decreases mitophagy in hepatocytes following ISC-REP. We asked whether activation of cs-CPN1 and -2 impaired mitophagy in the heart following ISC-REP. Buffer-perfused rat hearts underwent 25 min of global ISC and 30 min of REP. MDL-28170 (MDL; 10 µM) was used to inhibit CPN1 and -2. Cytosol, subsarcolemmal mitochondria (SSM), and interfibrillar mitochondria (IFM) were isolated at the end of heart perfusion. Cardiac ISC-REP led to decreased complex I activity with a decrease in the content of NDUFS7 in both SSM and IFM. ISC-REP also resulted in a decrease in cytosolic beclin-1 content, a key component of the autophagy pathway required to form autophagosomes. MDL treatment protected the contents of cytosolic beclin-1 and mitochondrial NDUFS7 in hearts following ISC-REP. These results support that activation of both cytosolic and mitochondrial calpains impairs mitochondria during cardiac ISC-REP. Mitochondria-localized calpains impair complex I via cleavage of a key subunit. Activation of cytosolic calpains contributes to mitochondrial dysfunction by impairing removal of the impaired mitochondria through depletion of a key component of the mitophagy process.