While the role of endoplasmic reticulum stress (ERS) in myocardial ischemia/reperfusion (I/R) injury has been extensively investigated, the precise mechanism by which inhibition of ERS induces cardioprotection remains unclear. We aimed to explore the mechanism of ERS inhibition-induced cardioprotection against I/R injury, focusing on the role of Zn
2+
and the mitochondrial permeability transition pore (mPTP). Exposure of H9c2 cells to 800 μM H
2
O
2
for 20 min increased GRP78 and GRP94 expressions (296.2 ± 41.0 and 150.6 ± 13.5 %, respectively), suggesting that H
2
O
2
can induce ERS. Cells treated with H
2
O
2
showed a significant decrease (40.6 ± 7.4 %) in TMRE fluorescence compared to the normal group (92.6 ± 0.1 %), indicating that H
2
O
2
can induce the mPTP opening. In contrast, ERS inhibitor TUDCA (30 μM) prevented the loss of TMRE fluorescence (77.8 ± 6.8 %), implying that inhibition of ERS can prevent the mPTP opening. This effect of TUDCA was blocked by zinc chelator TPEN (37.7 ± 13.0 %), indicating a role of Zn
2+
in the action of TUDCA on the mPTP opening. In support, TUDCA increased intracellular free zinc, as indicated by a marked increase in Newport Green DCF fluorescence intensity. In isolated rat hearts, GRP78 expression was not increased during ischemia but was increased upon reperfusion (1.3, 1.5, 1.9, and 1.6-fold increases at 10, 30, 60, and 120 min of reperfusion). Hearts treated with TUDCA showed a significant reduction of GRP78 expression 30 and 60 min after the onset of reperfusion, an effect that was reversed by TPEN. The immunofluorescence study also showed that the effect of TUDCA on GRP78 expression was reversed by TPEN. TUDCA reduced infarct size, and this was reversed by the mPTP opener atractyloside, indicating that ERS inhibition may protect the heart by modulating the mPTP opening. Experiments with transmission electron microscopy and hematoxylin-eosin staining revealed that TUDCA prevented endoplasmic reticulum and mitochondrial damages at reperfusion, which was blocked by TPEN. In conclusion, reperfusion but not ischemia initiates ERS and inhibition of ERS protects the heart from reperfusion injury through prevention of the mPTP opening. Increased intracellular free Zn
2+
accounts for the cardioprotective effect of ERS inhibition.
Sevoflurane Postconditioning Protects Chronically-Infarcted Rat Hearts against Ischemia-Reperfusion Injury by Activation of Pro-survival Kinases and Inhibition of Mitochondrial Permeability Transition Pore Opening upon Reperfusion
