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

Abstract

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.