Sequential restoring O2 then unloading CO2 is beneficial against reperfusion injury: role of CO2 in metabolism

CO2 is one of main byproducts during mitochondrial oxidation. Under the acute occlusion of coronary artery situation, the intra-tissue pCO2 of heart could be extremely high. This CO2 accumulation will be acutely unloaded and discharged by blood reperfusion. However, the effect of this intra-tissue CO2 accumulation then unloading process on cardiac ischemic reperfusion injury has not been well investigated yet. In the present study, we show that the perfusion with a high level of pCO2 and normal pO2 in the initial 30min followed by a 30min normal pCO2 and normal pO2 is better than the perfusion with 1h normal pCO2 and normal pO2 simultaneously during the reperfusion after a 45min global ischemia in isolated rat hearts. To observe the effect of high pCO2 on cellular metabolism, we exposed C2C12 cells under about 370mmHg pCO2 to observe the mitochondrial substrate switch and TCA cycle flux change, by using 13C tracers. We show that a short time exposure to the extremely high level of pCO2 is not completely destructive for cellular metabolism but has specific effects. The high pCO2 inhibits pyruvate transport into mitochondria and the next oxidation, switching to more reliance on fatty acid oxidation and enhancing the glutamine oxidation to maintain the TCA cycle. Intriguingly, the high pCO2 significantly activates the reductive carboxylation from glutamine, fixation of mitochondrial excessive CO2. The mechanism under the beneficial effect of the high-then-low CO2 sequential reperfusion strategy is discussed further.