Extensive autolytic fragmentation of membranous versus cytosolic calpain following myocardial ischemia–reperfusion

We investigated calpain activation in the heart during ischemia–reperfusion (I–R) by immunologically mapping the fragmentation patterns of calpain and selected calpain substrates. Western blots showed the intact 78 kDa large subunit of membrane-associated calpain was autolytically fragmented to 56 and 43 kDa signature immunopeptides following I–R. Under these conditions, the 78 kDa calpain large subunit from crude cytosolic fractions was markedly less fragmented, with only weakly stained autolytic peptides detected at higher molecular weights (70 and 64 kDa). Western blots also showed corresponding calpain-like degradation products (150 and 145 kDa) of membrane-associated α-fodrin (240 kDa) following I–R, but in crude myofibrils α-fodrin degradation occurred in a manner uncharacteristic of calpain. For control hearts perfused in the absence of ischemia, autolytic fragmentation of calpain and calpain-like α-fodrin degradation were completely absent from most subcellular fractions. The exception was sarcolemma-enriched membranes, where significant calpain autolysis and calpain-like α-fodrin degradation were detected. In purified sarcoplasmic reticulum membranes, RyR2 and SERCA2 proteins were also highly degraded, but for RyR2 this did not occur in a manner characteristic of calpain. When I–R-treated hearts were perfused with peptidyl calpain inhibitors (ALLN or ALLM; 25 µmol/L), calpain autolysis and calpain-like degradation of α-fodrin were equally attenuated by each inhibitor. However, only ALLN protected against early loss of developed pressure in hearts following I–R, with no functionally protective effect of ALLM observed. Our studies suggest calpain is preferentially activated at membranes following I–R, possibly contributing to impaired ion channel function implicated by others in I–R injury.