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-q uantum-filtered (DQF) Na-23 NMR spectra to monitor intracellular Na (N a-in) content in isolated rat hearts perfused in the absence of a chem ical-shift reagent (SR) was investigated, This necessitated characteri zation of the following: first, the pool of Na-in represented by the i ntracellular TQF (TQF(in)) spectrum; second, the maximum extent to whi ch altered transverse relaxation times affect TQF(in) spectral amplitu des; and finally, the situations for which the SR-free method can reli ably be applied. The rates of increase in peak amplitudes of both intr acellular TQF spectra, adjusted for changes in both fast (T-2f) and sl ow (T-2s) transverse relaxation times, and intracellular single-quantu m (SQ(in)) spectra were identical during no-flow ischemia, indicating that TQF(in) and SQ(in) spectra represent the same Na-in population. A ddition 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 N a-in content. However, the Na-in level for which the T-2 values starte d to increase was lower for ischemic (<140% of preischemic values) tha n for ouabain-exposed (>165%) hearts, which is consistent with the kno wn earlier onset of intracellular swelling in ischemic hearts, Exposur e of hearts to hyperosmotic perfusate (200 mM sucrose) increased [Na-i n], 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 wi th exposure of hearts to ouabain or ischemia, T-2 values therefore con sistently correlated only with cell volume, not with Na-in content or concentration, indicating an important role for intracellular macromol ecule 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 N a-in content from amplitudes of SR-aided TQF(in) spectra, indicating n egligible effect of transverse relaxation-dependent alterations on TQF (in) spectral amplitudes, Thus, Na-in content may be reliably determin ed from SR-free TQF spectra when the contribution from extracellular N a does not appreciably vary, such as during constant pressure perfusio n. Following complete reduction in perfusion pressure, both SR-free TQ F and DQF spectra respond to increases in Na-in content. However, SR-f ree 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 th e extracellular DQF signal resulting from destructive interference bet ween second- and third-rank tensors, (C) 1997 Academic Press.