Monovalent ions are spatially bound within the sarcomere.

The concentrations of sodium, potassium, chlorine, and phosphorus and the water content in A-band and I-band of rat heart muscle cells were measured by X-ray microanalysis. Concentrations per compartment water of both cations and anions are higher in the A-band, which is less hydrated than the I-band. It is concluded that at physiological ionic strength positive and negative fixed charges are effectively shielded against each other and ions of opposite sign are spatially fixed as counterions within one and the same compartment. By using sequence analysis data from the literature to evaluate the density of fixed charges in muscle it is estimated that differences in ionic concentrations between A- and I-band are chiefly due to ion binding in the A-band.