We previously developed a new approach for studying trans-membrane ion gradients in the intact brain utilizing the different NMR relaxation times of intracellular and extracellular ions. Double quantum NMR spectra are more sensitive than conventional single quantum NMR spectra to changes in relaxation times. Double quantum and single quantum 23Na and 39K NMR spectra were obtained from rat brain in vivo. Upon death, the double quantum 23Na NMR signal increased by a factor of three, while the single quantum signal decreased by 20%. The results are consistent with the well-known influx of sodium ions into the cell, and suggest that double quantum sodium and potassium NMR may be useful in visualizing compromised regions of the brain. We have extended this approach in a number of directions. First we extended the approach to study the cat brain in vivo, and the perfused rat liver. Second, we used double-quantum 23Na NMR to study compartmentation of sodium in the intact brain. Third, we combined multiple-quantum 23Na NMR with 31P NMR to study the correlation between changes in the multiple-quantum sodium NMR signals and changes in cellular energetics. In the last year we performed a comprehensive double quantum 23 Na study of perfused liver using shift reagents. The results indicate that, in this organ, most of the double-quantum signal arises from intracellular sodium ions.
