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 [1-¹³C]pyruvate, coupled with ¹³C magnetic resonance spectroscopy (MRS), to measure pH_i in the healthy and diseased heart.

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 pH_i. We tested the validity of this approach by comparing ¹³C-based pH_i measurements with ³¹P MRS measurements of pH_i. There was good agreement between the pH_i 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 pH_i 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 pH_i by 0.80 pH units. Mathematical modelling predicted that the validity of measuring pH_i from the H¹³CO₃⁻/¹³CO₂ 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 [1-¹³C]pyruvate was also infused into healthy living rats, where in vivo pH_i from the H¹³CO₃⁻/¹³CO₂ ratio was measured to be 7.20 ± 0.03.

Metabolically generated ¹³CO₂ and H¹³CO₃⁻ can be used as a marker of cardiac pH_iin vivo, provided that CA activity is at normal levels.