title: Measuring intracellular pH in the heart using hyperpolarized carbon dioxide and bicarbonate: a 13C and 31P magnetic resonance spectroscopy study.
abstract: 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.
authors.affiliation: 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
authors.forename: Marie A
authors.initials: MA
authors.lastname: Schroeder
authors.forename: Pawel
authors.initials: P
authors.lastname: Swietach
authors.forename: Helen J
authors.initials: HJ
authors.lastname: Atherton
authors.forename: Ferdia A
authors.initials: FA
authors.lastname: Gallagher
authors.forename: Phillip
authors.initials: P
authors.lastname: Lee
authors.forename: George K
authors.initials: GK
authors.lastname: Radda
authors.forename: Kieran
authors.initials: K
authors.lastname: Clarke
authors.forename: Damian J
authors.initials: DJ
authors.lastname: Tyler
mesh_terms: 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
chemical_list: D001639:Bicarbonates; D002247:Carbon Isotopes; D010759:Phosphorus Isotopes; D002245:Carbon Dioxide; D019289:Pyruvic Acid; D002256:Carbonic Anhydrases