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

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