Calpains contribute to reperfusion-induced myocardial cell death. However, it remains controversial whether its activation occurs during
ischemia or reperfusion. We investigated the regulation and time-course of
calpain activation secondary to transient
ischemia and the efficacy of its inhibition at reperfusion as a therapeutic strategy to limit
infarct size. In isolated rat hearts (Sprague-Dawley),
ischemia induced a time-dependent translocation of
m-calpain to the membrane that was not associated with
calpain activation as assessed by proteolysis of its substrate
alpha-fodrin. Translocation of
calpain was dependent on Ca(2+) entry through reverse mode Na(+)/Ca(2+)-exchange and was independent of
acidosis.
Calpain activation occurred during reperfusion, but only after intracellular pH (pHi) normalization, and was not prevented by inhibiting its translocation during
ischemia with
methyl-beta-cyclodextrin. The
intravenous infusion of
MDL-28170 in an in vivo rat model with transient
coronary occlusion during the first minutes of reperfusion resulted in a reduction of
infarct size (43.9+/-3.9% vs. 60.2+/-4.7, P=0.046, n=18) and
alpha-fodrin degradation. These results suggest that (1) Ca(2+)-induced
calpain translocation to the membrane during
ischemia is independent of its activation, (2) intracellular
acidosis inhibits
calpain activation during
ischemia and pHi normalization allows activation upon reperfusion, and (3)
calpain inhibition at the time of reperfusion appears as a potentially useful strategy to limit
infarct size.