Determination of chloride potential in perfused rat hearts by nuclear magnetic resonance spectroscopy

Isolated beating rat hearts were perfused with trifluoroacetamide (TFM) and trifluoroacetate (TFA) and monitored by ¹⁹F-nuclear magnetic resonance (NMR). The average membrane TFA potential in spontaneously beating rat hearts, calculated according to standard principles assuming that TFA is distributed in its anionic form, was found to be -36.2 ± 3.2 mV (n = 9) under normoxic conditions. In separate experiments, the chloride and potassium potentials were determined to be -38.5 ± 3.6 mV (n = 7) and -85.3 ± 3.3 mV (n = 7), respectively, from freeze-clamped heart tissue. In the presence of the anion-exchange inhibitor, 4-acetamido-4'- isothiocyanostilbene-2,2'-disulfonic acid (SITS), TFA uptake into heart was significantly reduced, suggesting that TFA uptake occurs partly via the Cl^- -HCO₃/^- exchanger. Based on these results and the results of R. E. London and S. A. Gabel (Biochemistry 28:2378-2382, 1989), we conclude that the distribution of TFA in hearts reflects the chloride potential (E(Cl)) and not the membrane potential. A time-dependent change in the E(Cl) occurs during global ischemia, and changes in E(Cl) were also observed when the hearts were perfused with high concentrations of KCl. These results demonstrate that ¹⁹F-NMR may be utilized to monitor the E(Cl) of perfused hearts under a variety of conditions.