Ionic Partition and Fine Structure in Muscle

Ion Partition in Polyelectrolyte Gels and Nanogels

Polyelectrolyte gels provide a load-bearing structural framework for many macroscopic biological tissues, along with the organelles within the cells composing tissues and the extracellular matrices linking the cells at a larger length scale than the cells. In addition, they also provide a medium for the selective transportation and sequestration of ions and molecules necessary for life. Motivated by these diverse problems, we focus on modeling ion partitioning in polyelectrolyte gels immersed in a solution with a single type of ionic valence, i.e., monovalent or divalent salts. Specifically, we investigate the distribution of ions inside the gel structure and compare it with the bulk, i.e., away from the gel structure. In this first exploratory study, we neglect solvation effects in our gel by modeling the gels without an explicit solvent description, with the understanding that such an approach may be inadequate for describing ion partitioning in real polyelectrolyte gels. We see that this type of model is nonetheless a natural reference point for considering gels with solvation. Based on our idealized polymer network model without explicit solvent, we find that the ion partition coefficients scale with the salt concentration, and the ion partition coefficient for divalent ions is higher than for monovalent ions over a wide range of Bjerrum length (lB) values. For gels having both monovalent and divalent salts, we find that divalent ions exhibit higher ion partition coefficients than monovalent salt for low divalent salt concentrations and low lB. However, we also find evidence that the neglect of an explicit solvent, and thus solvation, provides an inadequate description when compared to experimental observations. Thus, in future work, we must consider both ion and polymer solvation to obtain a more realistic description of ion partitioning in polyelectrolyte gels.

Na Extrusion by the Sartorius of Rana pipiens

Sartorius muscles of R. pipiens may be enriched in sodium and depleted of potassium by prolonged soaking in the cold in K-free or low K Ringer's solution. Such K-depleted muscles take up potassium and extrude sodium when exposed to Ringer's solution containing 10 mM K at ordinary room temperature (ca. 20°C), with no dependence on external Na ion concentration in either phase within fairly wide limits. Extrusion of excess fiber Na is demonstrated on single muscles using Na²⁴-loaded tissues. Rate of exchange of excess fiber Na does not differ significantly from the rate of exchange of Na normally present in muscle fibers.

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.