Observation of intracellular potassium and sodium in the heart by NMR: a major fraction of potassium is "invisible".

Using 39K and 23Na NMR in conjunction with extracellularly localized shift reagents, we have determined the intracellular concentrations of NMR visible sodium and potassium in isolated, perfused rat hearts. We find this concentration to be 9.9 mM/kg cell water for sodium and 31 mM/kg cell water for potassium. Values of activity determined by ion-sensitive microelectrodes are in good agreement with our sodium value but do not agree with our potassium value. Our results mean that a major pool of intracellular potassium is, on average, significantly immobilized and that the mobile NMR visible fraction (31 mM/kg) is not in exchange with the NMR invisible pool (114 mM/kg). The immobilized fraction is characterized by T2 values which are too short to be observed by our conventional spectrometer. This fraction is, therefore, said to be "invisible" under our experimental conditions.