Distinct effects of intracellular vs. extracellular acidic pH on the cardiac metabolome during ischemia and reperfusion

Tissue ischemia results in intracellular pH (pH(IN)) acidification, and while metabolism is a known driver of acidic pH(IN), less is known about how acidic pH(IN) regulates metabolism. Furthermore, acidic extracellular (pH(EX)) during early reperfusion confers cardioprotection, but how this impacts metabolism is unclear. Herein we employed LCMS based targeted metabolomics to analyze perfused mouse hearts exposed to: (i) control perfusion, (ii) hypoxia, (iii) ischemia, (iv) enforced acidic pH(IN), (v) control reperfusion, and (vi) acidic pH(EX) (6.8) reperfusion. Surprisingly little overlap was seen between metabolic changes induced by hypoxia, ischemia, and acidic pH(IN). Acidic pH(IN) elevated metabolites in the top half of glycolysis, and enhanced glutathione redox state. Meanwhile, acidic pH(EX) reperfusion induced substantial metabolic changes in addition to those seen in control reperfusion. This included elevated metabolites in the top half of glycolysis, prevention of purine nucleotide loss, and an enhancement in glutathione redox state. These data led to hypotheses regarding potential roles for methylglyoxal inhibiting the mitochondrial permeability transition pore, and for acidic inhibition of ecto-5 '-nucleotidase, as potential mediators of cardioprotection by acidic pH(EX) reperfusion. However, neither hypothesis was supported by subsequent experiments. In contrast, analysis of cardiac effluents revealed complex effects of pH(EX) on metabolite transport, suggesting that mildly acidic pH(EX) may enhance succinate release during reperfusion. Overall, each intervention had distinct and overlapping metabolic effects, suggesting acidic pH is an independent metabolic regulator regardless which side of the cell membrane it is imposed.

Automated summary

Tissue ischemia leads to acidic intracellular pH and the effect of acidic pH on metabolism is not well understood. This study used LCMS based targeted metabolomics to analyze the metabolic changes in perfused mouse hearts exposed to various conditions. It was found that acidic pH increased glycolysis metabolites and enhanced the glutathione redox state. Acidic pH reperfusion also induced significant metabolic changes, including elevated glycolysis metabolites, preservation of purine nucleotides, and enhancement of glutathione redox state. However, hypotheses regarding the mechanisms of cardioprotection by acidic pH reperfusion were not supported. In contrast, analysis of cardiac effluents suggested that mildly acidic pH may enhance succinate release during reperfusion. Overall, acidic pH is an independent metabolic regulator regardless of its location in the cell membrane.