Abstract
The level of intracellular sodium (Na~i~) is maintained at approximately 14 m M in healthy myocytes. When myocytes are damaged, Na~i~ increases and therefore the level of Na~i~ may be a means of evaluating myocardial cell integrity. A particularly useful method to monitor Na~i~ levels is ^23^Na NMR spectroscopy. However, because of the isochronous nature of the extracellular sodium (Na~o~) and Na~i~ NMR signals, paramagnetic lanthanide shift reagents (LSR), such as dysprosium triphosphate, Dy(PPP)^7^~2~, have been used to shift the Na~o~ signal. This reveals the unshifted Na~i~ signal and allows the NMR monitoring of Na~i~ in isolated perfused hearts and other systems. A major shortcoming of this method (the “shift‐only” method) is in the need to minimize the Na~o~ signal by not submerging the perfused hearts in Na^+^‐containing buffer. An equally undesirable alternative is the utilization of relatively high concentrations of LSR to shift a large Na~o~ signal sufficiently to enable reasonable resolution and quantitation of Na~i~. We present here a method, the “shiftrelaxation” method, which is a combination of using a mixture of Dy(PPP)^7^~2~, a shift reagent, and gadolinium triphosphate, Gd(PPP)^7^~2~, a relaxation agent, with data acquisition using an inversion‐recovery (IR) pulse sequence. This combination allows differentiation between Na~o~ and Na~i~ by the difference in their respective T~1~ values in addition to the shift between them. With this technique we can selectively minimize the extracellular signal and therefore minimize the need for a large Dy‐induced shift, as well as allow data acquisition on a heart submerged in Na^+^ ‐containing perfusate. The resulting improved discrimination between Na~i~ and Na~o~ at relatively low levels of LSR should be helpful for ultimate in vivo application and potential clinical applications, where a lower dose of LSR also means a decreased possibility of physiologically deleterious effects. Also included in this paper is a method for the quick determination of an accurate 180° pulse which is required for the optimization of the IR method. © 1991 Academic Press, Inc.