Spin-echo nuclear magnetic resonance evidence for complexing of sodium ions in muscle, brain, and kidney.

Na(+) in muscle, brain, and kidney is shown by spin-echo nuclear magnetic resonance (NMR) to consist of two fractions with different NMR parameters. The slow fraction of Na(+) in these tissues has NMR relaxation times T(1) and T(2) of 10-15 x 10(-3) sec, which is approximately 4-5 times shorter than for Na(+) in aqueous NaCl solution. The slow fraction may represent Na(+) dissolved in structured tissue water. The fast fraction of tissue Na(+), which is shown to represent approximately 65% of the total tissue Na(+) concentration, has T(2) less than 1 x 10(-3) sec, which resembles the values of T(2) observed for Na(+) complexed by synthetic ion-exchange resins. One is drawn to the conclusion that approximately 65% of total Na(+) in muscle, brain, and kidney is complexed by tissue macromolecules.