Measuring intracellular pH in the heart using hyperpolarized carbon dioxide and bicarbonate: a 13C and 31P magnetic resonance spectroscopy study.

Aims Technological limitations have restricted in vivo assessment of intracellular pH (pHi) in the myocardium. The aim of this study was to evaluate the potential of hyperpolarized [1-13C]pyruvate, coupled with 13C magnetic resonance spectroscopy (MRS), to measure pHi in the healthy and diseased heart. Methods and results Hyperpolarized [1-13C]pyruvate was infused into isolated rat hearts before and immediately after ischaemia, and the formation of 13CO2 and H13CO3− was monitored using 13C MRS. The HCO3−/CO2 ratio was used in the Henderson–Hasselbalch equation to estimate pHi. We tested the validity of this approach by comparing 13C-based pHi measurements with 31P MRS measurements of pHi. There was good agreement between the pHi measured using 13C and 31P MRS in control hearts, being 7.12 ± 0.10 and 7.07 ± 0.02, respectively. In reperfused hearts, 13C and 31P measurements of pHi also agreed, although 13C equilibration limited observation of myocardial recovery from acidosis. In hearts pre-treated with the carbonic anhydrase (CA) inhibitor, 6-ethoxyzolamide, the 13C measurement underestimated the 31P-measured pHi by 0.80 pH units. Mathematical modelling predicted that the validity of measuring pHi from the H13CO3−/13CO2 ratio depended on CA activity, and may give an incorrect measure of pHi under conditions in which CA was inhibited, such as in acidosis. Hyperpolarized [1-13C]pyruvate was also infused into healthy living rats, where in vivo pHi from the H13CO3−/13CO2 ratio was measured to be 7.20 ± 0.03. Conclusion Metabolically generated 13CO2 and H13CO3− can be used as a marker of cardiac pHi in vivo , provided that CA activity is at normal levels.