Abstract
Multiple‐quantum filtered (MQF) NMR offers the possibility of monitoring intracellular (IC) Na content in the absence of shift reagents (SR), provided that (i) the contribution from IC Na to the MQF spectrum is substantial and responds to a change in IC Na content, and (ii) the amplitude of the extracellular (EC) MQF component remains constant during a change in IC Na content. The validity and basis for these conditions were examined in isolated perfused rat hearts using SR‐aided and SR‐free triple‐quantum filtered (TQF) ^23^NaNMR. Despite a myocardial Na content that was only ˜1/70 that of EC Na. IC Na contributed to over 25% of the total TQF spectrum acquired in the absence of SR. Transverse relaxation times (T~2~) were approximately twice as long for EC compared to IC Na, despite SR‐induced relaxation of T~2~ for the former pool. However, the efficiency of generation of the TQF signal was similar for IC and EC Na, indicating that a much greater percentage of IC relative to EC Na exhibits TQ coherence. During constant perfusion with ouabain (0.2 mM for 25 min) or with a hypoxic and aglycemic solution (50 min), the amplitude of the IC TQF spectrum increased by ˜330% and ˜280%, respectively. In contrast, the amplitude of the EC TQF spectra remained essentially constant for both interventions. The amplitude for IC Na increased ˜250% relative to baseline during no‐flow ischemia (60 min), whereas the amplitude of the EC TQF spectra decreased by ˜33% before stabilizing. In SR‐free experiments, the TQF spectral amplitude increased ˜2‐fold during the constant perfusion interventions, but did not change significantly during no‐flow ischemia. These data suggest that the change in the TQF spectral amplitude during constant perfusion interventions is from IC Na, and that TQF techniques in the absence of SR may be useful in monitoring IC Na during these interventions. The fall in the amplitude of the EC TQF spectral amplitude during no‐flow ischemia complicates the use of TQF techniques without SR during this intervention.