Evaluation of Multiple-Quantum-Filtered 23 Na NMR in Monitoring Intracellular Na Content in the Isolated Perfused Rat Heart in the Absence of a Chemical-Shift Reagent

Author： Tauskela J.S. Dizon J.M. Whang J. Katz J.

Publisher： Academic Press

ISSN： 1090-7807

Source： Journal of Magnetic Resonance, Vol.127, Iss.1, 1997-07, pp. : 115-127

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Abstract

The feasibility of employing triple-quantum-filtered (TQF) or double-quantum-filtered (DQF) 23 Na 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 (>500 mu M ), and no-flow 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 m M 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 =~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.