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 T₁ and T₂ of 10-15 × 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 T₂ less than 1 × 10^-3 sec, which resembles the values of T₂ 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.