Simultaneous intracellular and extracelular p H measurement in the heart by 19 F NMR of 6‐fluoropyridoxol

6‐Fluoropyridoxol (6‐FPOL) was evaluated as a simultaneous indicator of intracellular and extracellular pH and, hence, pH gradient in perfused rat hearts. After infusion, 19 F NMR spectra rapidly showed two well‐resolved peaks assigned to the intracellular and extracellular compartments, and pH was calculated on the basis of chemical shift with respect to a sodium trifluoroacetate standard. To demonstrate use of this molecule, dynamic changes in myocardial pH were assessed with a time resolution of 2 min during respiratory and metabolic alkalosis or acidosis and ischemia. For a typical heart, intracellular pH (pHi) = 7.14 ± 0.01 and extracellular pH (pHe) = 7.52 ± 0.02. In response to metabolic alkalosis, pHi remained relatively constant and the pH gradient increased. In contrast, respiratory challenge caused a significant increase in pHi. Independent measurements using pH electrodes and 31 P NMR confirmed validity of the 19 F NMR results.