Evaluation of multiple-quantum-filtered Na-23 NMR in monitoring intracellular Na content in the isolated perfused rat heart in the absence of a chemical-shift reagent

The feasibility of employing triple-quantum-filtered (TQF) or double-quantum-filtered (DQF) Na-23 NMR spectra to monitor intracellular Na (Na-in) content in isolated rat hearts perfused in the absence of a chemical-shift reagent (SR) was investigated, This necessitated characterization of the following: first, the pool of Na-in represented by the intracellular TQF (TQF(in)) spectrum; second, the maximum extent to which altered transverse relaxation times affect TQF(in) spectral amplitudes; and finally, the situations for which the SR-free method can reliably be applied. The rates of increase in peak amplitudes of both intracellular TQF spectra, adjusted for changes in both fast (T-2f) and slow (T-2s) transverse relaxation times, and intracellular single-quantum (SQ(in)) spectra were identical during no-flow ischemia, indicating that TQF(in) and SQ(in) spectra represent the same Na-in population. Addition of an Na/K ATPase inhibitor, ouabain (greater than or equal to 500 mu M), and no-how ischemia induced similar rates of increase of Na-in content. However, the Na-in level for which the T-2 values started to increase was lower for ischemic (<140% of preischemic values) than for ouabain-exposed (>165%) hearts, which is consistent with the known earlier onset of intracellular swelling in ischemic hearts, Exposure of hearts to hyperosmotic perfusate (200 mM sucrose) increased [Na-in], due to a decreased cell volume and an unchanged Na-in content, but caused a decrease in T-2 values, a trend opposite to that observed with exposure of hearts to ouabain or ischemia, T-2 values therefore consistently correlated only with cell volume, not with Na-in content or concentration, indicating an important role for intracellular macromolecule concentration in modulating transverse relaxation behavior, The combined effect of ischemia-induced increases in T-2 values and their inhomogeneous broadened forms was an similar to 6% overestimation of Na-in content from amplitudes of SR-aided TQF(in) spectra, indicating negligible effect of transverse relaxation-dependent alterations on TQF(in) spectral amplitudes, Thus, Na-in content may be reliably determined from SR-free TQF spectra when the contribution from extracellular Na does not appreciably vary, such as during constant pressure perfusion. Following complete reduction in perfusion pressure, both SR-free TQF and DQF spectra respond to increases in Na-in content. However, SR-free DQF NMR provides an estimate of Na-in content much closer to that provided by the SR-aided method, due to the appreciable decrease of the extracellular DQF signal resulting from destructive interference between second- and third-rank tensors, (C) 1997 Academic Press.