Multiple quantum filtered 23Na NMR spectroscopy in the perfused heart

It has been proposed that multiple quantum filtered (MQF) 23Na NMR spectroscopy may distinguish between sodium in the intra- and extracellular spaces, in vivo, and without the requirement for toxic shift reagents. We have investigated the origin of such signals in perfused rat hearts, using shift reagents to discriminate between NMR signals from the two compartments. We find that the observed MQF signal arises predominantly from the extracellular space in the normal heart, and from the intracellular space in the ischaemic heart. However, the quenching effect of the shift reagent may hide some of the extracellular MQF signal, especially during ischaemia. Estimates of intracellular and extracellular relaxation times are obtained and possible mechanisms of the interaction discussed.