The electrical potential difference across the cell membrane of heart muscle. Biophysical considerations.

T HE CELL MEMBRANE of mammalian heart muscle cells may be characterized by three interrelated observations. In the first place, the membrane separates an intracellular solution having a high potassium (K) and a low sodium (Na) concentration from an ex-tracellular solution with a high Na and low K concentration. Secondly, K and Na ions can be shown with radioactive tracer technics to be continuously passing into and out of the cell; to do so, these ions. must cross the cell membrane, which is therefore said to be per-mneable to them. In spite of the apparent per-meability of the membrane to both Na and K, the intracellular concentrations of these ions are observed to remain remarkably constant.