Temperature dependence of monovalent cation fluxes in isolated rat hearts: a magnetic resonance study

AbstractIon flux studies were performed on Langendorff-perfused rat hearts using 87Rb, 7Li and 23Na NMR at 36, 20 and 10°C, and at constant extracellular pH (7.40). Using 31P NMR, the intracellular pH was estimated and the high energy phosphate content monitored. Compared to 36°C (k=0.044±0.015 min−1), our measurements showed incomplete Rb+ efflux with a dramatically (5-fold) increased rate constant, k, at 20°C, k=0.238±0.080 min−1. 5 μM glibenclamide, a KATP-channel inhibitor, completely depressed the hypothermia-activated Rb+ efflux at this temperature (k=0.052±0.018 min−1). 7Li NMR efflux studies on KCl-arrested hearts at 20°C also showed an increase (3-fold) in efflux rate constant: k=0.090±0.003 min−1 relative to its value at 36°C. At 10°C, both Rb+ and Li+ showed efflux rate constants similar to those observed at 36°C, k=0.071±0.016 min−1 and k=0.050±0.005 min−1, respectively, and the washout was complete. 31P NMR at 36, 20 and 10°C indicated cytosolic alkalinization at pH values of 7.05, 7.21 and 7.40, respectively. The ion transport data could be interpreted in terms of a myocyte model allowing for temperature-dependent changes in transport coefficients. The incomplete efflux of Rb+ at 20°C may indicate the existence of a mitochondrial Rb+-pool with a very low Rb+ permeability for efflux. These findings correlate with previously observed membrane phase transitions in these systems