Intracellular Na⁺, K⁺ and Cl^- activities in Ehrlich ascites tumor cells

Ehrlich ascites tumor cell membrane potential (V_m) and intracellular Na⁺, K⁺ and Cl^- activities were measured under steady-state conditions in normal saline medium (Na⁺ = 154, K⁺ = 6, Cl^- = 150 mequiv./l). Membrane potential was estimated to be -23.3 ± 0.8 mV using glass microelectrodes. Intracellular ion activities were estimated with similar glass electrodes rendered ion-selective by incorporation of ion-specific ionophores. Measurements of V_m and ion-activity differences were made in the same populations of cells. Under these conditions the intracellular Na⁺, K⁺ and Cl^- activities are 4.6 ± 0.5; 68.3 ± 8.0; and 43.6 ± 2.1 mequiv./l, respectively. The apparent activity coefficients for Na⁺ and K⁺ are 0.18 ± 0.02 and 0.41 ± 0.05 respectively. These are significantly lower than the activity coefficients expected for the ions in physiological salt solutions (0.71 and 0.73, respectively). The activity coefficient for intracellular Cl^- (0.67 ± 0.03), however, is close to that of the medium (0.73), and the transmembrane electrochemical potential difference for Cl^- is not different from zero. The results establish that the energy available from the Na⁺ electrochemical gradient is much greater than previously estimated from chemical measurements.