Ion flux studies were performed on Langendorff-perfused rat hearts using 87Rb, 7Li and 23Na NMR at 36, 20 and 10 degreesC, 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 degreesC (k=0.044+/-0.015 min-1), our measurements showed incomplete Rb+ efflux with a dramatically (5-fold) increased rate constant, k, at 20 degreesC, k=0.238+/-0.080 min-1. 5 microM 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 degreesC also showed an increase (3-fold) in efflux rate constant: k=0.090+/-0.003 min-1 relative to its value at 36 degreesC. At 10 degreesC, both Rb+ and Li+ showed efflux rate constants similar to those observed at 36 degreesC, 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 degreesC 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 degreesC 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.