Temperature preconditioning of isolated rat hearts – a potent cardioprotective mechanism involving a reduction in oxidative stress and inhibition of the mitochondrial permeability transition pore

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Abstract
We investigate whether temperature preconditioning (TP), induced by short‐term hypothermic perfusion and rewarming, may protect hearts against ischaemic/reperfusion injury like ischaemic preconditioning (IP). Isolated rat hearts were perfused for 40 min, followed by 25 min global ischaemia and 60 min reperfusion (37°C). During pre‐ischaemia, IP hearts underwent three cycles of 2 min global ischaemia and 3 min reperfusion at 37°C, whereas TP hearts received three cycles of 2 min hypothermic perfusion (26°C) interspersed by 3 min normothermic perfusion. Other hearts received a single 6 min hypothermic perfusion (SHP) before ischaemia. Both IP and TP protocols increased levels of high energy phosphates in the pre‐ischaemic heart. During reperfusion, TP improved haemodynamic recovery, decreased arrhythmias and reduced necrotic damage (lactate dehydrogenase release) more than IP or SHP. Measurements of tissue NAD+ levels and calcium‐induced swelling of mitochondria isolated at 3 min reperfusion were consistent with greater inhibition of the mitochondrial permeability transition at reperfusion by TP than IP; this correlated with decreased protein carbonylation, a surrogate marker for oxidative stress. TP increased protein kinase Cɛ (PKCɛ) translocation to the particulate fraction and pretreatment with chelerythrine (PKC inhibitor) blocked the protective effect of TP. TP also increased phosphorylation of AMP‐activated protein kinase (AMPK) after 5 min index ischaemia, but not before ischaemia. Compound C (AMPK inhibitor) partially blocked cardioprotection by TP, suggesting that both PKC and AMPK may mediate the effects of TP. The presence of N‐(2‐mercaptopropionyl) glycine during TP also abolished cardioprotection, indicating an involvement of free radicals in the signalling mechanism.