Measuring intracellular pH in the heart using hyperpolarized carbon dioxide and bicarbonate: a 13C and 31P magnetic resonance spectroscopy study.
OBJECTIVE
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-(13)C]pyruvate, coupled with (13)C magnetic resonance spectroscopy (MRS), to measure pH(i) in the healthy and diseased heart.


RESULTS
Hyperpolarized [1-(13)C]pyruvate was infused into isolated rat hearts before and immediately after ischaemia, and the formation of (13)CO(2) and H(13)CO(3)(-) was monitored using (13)C MRS. The HCO(3)(-)/CO(2) ratio was used in the Henderson-Hasselbalch equation to estimate pH(i). We tested the validity of this approach by comparing (13)C-based pH(i) measurements with (31)P MRS measurements of pH(i). There was good agreement between the pH(i) measured using (13)C and (31)P MRS in control hearts, being 7.12 +/- 0.10 and 7.07 +/- 0.02, respectively. In reperfused hearts, (13)C and (31)P measurements of pH(i) also agreed, although (13)C equilibration limited observation of myocardial recovery from acidosis. In hearts pre-treated with the carbonic anhydrase (CA) inhibitor, 6-ethoxyzolamide, the (13)C measurement underestimated the (31)P-measured pH(i) by 0.80 pH units. Mathematical modelling predicted that the validity of measuring pH(i) from the H(13)CO(3)(-)/(13)CO(2) 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-(13)C]pyruvate was also infused into healthy living rats, where in vivo pH(i) from the H(13)CO(3)(-)/(13)CO(2) ratio was measured to be 7.20 +/- 0.03.


CONCLUSIONS
Metabolically generated (13)CO(2) and H(13)CO(3)(-) can be used as a marker of cardiac pH(i) in vivo, provided that CA activity is at normal levels.
Schroeder
Marie A
MA
Cardiac Metabolism Research Group, Department of Physiology, Anatomy and Genetics, University of Oxford, Sherrington Building, Parks Road, Oxford OX1 3PT, UK. marie.schroeder@dpag.ox.ac.uk
Swietach
Pawel
P
Atherton
Helen J
HJ
Gallagher
Ferdia A
FA
Lee
Phillip
P
Radda
George K
GK
Clarke
Kieran
K
Tyler
Damian J
DJ
D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism*; D002247:Carbon Isotopes; D002256:Carbonic Anhydrases / Q000378:metabolism; D004734:Energy Metabolism / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy / Q000379:methods* / Q000592:standards; D008297:Male; D008954:Models, Biological; D017202:Myocardial Ischemia / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism*; D010759:Phosphorus Isotopes; D019289:Pyruvic Acid / Q000493:pharmacokinetics; D051381:Rats; D017208:Rats, Wistar; D015203:Reproducibility of Results
D001639:Bicarbonates; D002247:Carbon Isotopes; D010759:Phosphorus Isotopes; D002245:Carbon Dioxide; D019289:Pyruvic Acid; D002256:Carbonic Anhydrases