NMR Evidence for Complexing of Na⁺ in Muscle, Kidney, and Brain, and by Actomyosin. The Relation of Cellular Complexing of Na⁺ to Water Structure and to Transport Kinetics

The nuclear magnetic resonance (NMR) spectrum of Na⁺ is suitable for qualitative and quantitative analysis of Na⁺ in tissues. The width of the NMR spectrum is dependent upon the environment surrounding the individual Na⁺ ion. NMR spectra of fresh muscle compared with spectra of the same samples after ashing show that approximately 70% of total muscle Na⁺ gives no detectable NMR spectrum. This is probably due to complexation of Na⁺ with macromolecules, which causes the NMR spectrum to be broadened beyond detection. A similar effect has been observed when Na⁺ interacts with ion exchange resin. NMR also indicates that about 60% of Na⁺ of kidney and brain is complexed. Destruction of cell structure of muscle by homogenization little alters the per cent complexing of Na⁺. NMR studies show that Na⁺ is complexed by actomyosin, which may be the molecular site of complexation of some Na⁺ in muscle. The same studies indicate that the solubility of Na⁺ in the interstitial water of actomyosin gel is markedly reduced compared with its solubility in liquid water, which suggests that the water in the gel is organized into an icelike state by the nearby actomyosin molecules. If a major fraction of intracellular Na⁺ exists in a complexed state, then major revisions in most theoretical treatments of equilibria, diffusion, and transport of cellular Na⁺ become appropriate.