Response of the 23Na-NMR double-quantum filtered signal to changes in Na+ ion concentration in model biological solutions and human erythrocytes.

Double quantum filtered (DQF) 23Na-NMR signals were evaluated as a function of [Na+] at constant temperature in two model systems (bovine serum albumin (BSA) and Ficoll 400) and in human red blood cells (RBCs). In model systems, the ratio of double quantum filtered to single quantum (SQ) signal intensities was independent of [Na+], even over a wide range of Na+/K+ ratios. Varying the DQF preparation time affected only the DQF signal intensity. In contrast, in human red blood cells (RBCs) the shape and phase of the DQF intracellular Na+ signal (Na+in) varied as a function of preparation time. Similar observations in cartilage [Eliav, U., Shinar, H. and Navon, G. (1992) J. Magn. Reson. 28, 223-229] have been attributed to the generation of a second- and a third-rank tensor by the DQF pulse sequence, resulting from Na+ ion ordering. By using a DQF sequence which isolates the second-rank tensor only, this component was found to originate from the intracellular Na+ ion pool in human RBCs, as well as from interactions of Na+ ions with the extracellular face of the plasma membrane. The residual quadrupolar splitting for the signal originating from the former environment was shown to be less than the SQ linewidth, explaining its absence in SQ spectra, and this was confirmed by two-dimensional DQF 23Na-NMR experiments. By isolating the contribution from the third-rank tensor exclusively, the ratio of DQF:SQ signal intensities for Na+in in human RBCs was shown to be constant over a 4-fold change in [Na+in] produced by addition of an ionophore (nystatin). This indicates that such changes in physiological state do not alter the efficiency of DQF signal generation in human RBCs.