Objective: Tumor necrosis factor alpha (TNFalpha) is known to mimic ischemic preconditioning (IP). However, it is not known whether TNFalpha-preconditioning is mediated by 'established' preconditioning signaling or via novel signaling cascades. Moreover, whether TNFalpha is required to induce the ischemic preconditioning phenotype has not been determined.
Methods: To evaluate the role of TNFalpha, we determined the infarct-sparing effect of IP comparing TNFalpha null (TNFalpha-/-) and wild-type mice. The IP protocol included 4x5 min ischemia/reperfusion (I/R) prior to the index 35 min of global ischemia followed by 45 min of reperfusion in isolated perfused murine hearts. Infarct size was measured as a percentage of cardiac volume. To evoke particular signaling pathways numerous pharmacologic studies were performed.
Results: Following IP, infarct size was significantly reduced by 43% in wild-type mice. In contrast, infarct size was not attenuated by IP in the TNFalpha-/- group versus I/R controls (Infarct size-36+/-3%). Interestingly, pharmacologic preconditioning with adenosine (100 microM) and diazoxide (30 microM) mimicked IP in both the wild-type (infarct size-11+/-4% and 18+/-2%) and in TNFalpha-/- mice (infarct size-15+/-4% and 23+/-3%) versus respective I/R controls. Recombinant TNFalpha (0.5 ng/ml) administered for 7 min followed by a 10-min washout mimicked IP in wild-type mice but not in the TNFalpha deficient mouse hearts. The cardioprotective effects of IP, adenosine and TNFalpha were abolished by the co-administration of the putative mitochondrial K(ATP) blocker 5-hydroxydecanoate.
Conclusions: We demonstrate that cardiac TNFalpha production is required for ischemic preconditioning-induced cardioprotection but not necessary in pharmacologic preconditioning with adenosine or diazoxide in TNFalpha-/- mice. Moreover, TNFalpha administration is sufficient to activate preconditioning in wild-type mice. Finally, as 5-hydroxydecanoate abrogates ischemic, adenosine and TNFalpha induced preconditioning, this suggests that diverse signaling pathways converge at the level of mitochondrial K(ATP) channel activation to mediate this cardioprotection.