Activities of potassium and sodium ions in rabbit heart muscle

Abstract

Activities (a) of intracellular K and Na in rabbit ventricular papillary muscles were determined with cation-selective glass microelectrodes and concentrations (C) were estimated with flame photometry. The CK and a: of the muscles were 134.9 ± 3.1 mM (mean value ± SE) and 82.6 mM, respectively, at 25°C. The corresponding CNa and aN, were 32.7 ± 2.7 and 5.7, respectively. The apparent intraceUular activity coefficients for K (γK) and Na (γNa) were 0.612 and 0.175, respectively. Similar results were obtained at 35 ± 1°C. γK was substantially lower than the activity coefficient (0.745) of extracellular fluid (Tyrode’s solution), which might be expected on the basis of a different intracellular ionic strength. γNa, was much lower than that of extracellular fluid, and suggests that much of the Na was compartmentalized or sequestered. For external K concentrations greater than 5 raM, the resting membrane potentials agreed well with the potential differences calculated from the K activity gradients across the cell membrane as a potassium electrode. These results emphasize that potassium equilibrium potentials in heart muscle should be calculated by activities rather than concentrations. © 1975, Rockefeller University Press., All rights reserved.