The electrical potential difference (PD) across the membrane of the Ehrlich mouse ascites tumor cell has been measured with iniracellular microelectrodes. The mean for 111 cells in control Ringer solution was -11.2 m V ? 0.29 (SE), interior negative. When sulfate replaced chloride in the external medium the PD fell to -2.8 mV if measured as soon as possible after mixing the cells with a sulfate medium, but when nitrate replaced chloride the PD fell only to -8.5 mV. Cells equilibrated in nitrate had the same PD as those in control Ringer. These results indicate that the PD is sensitive to changes in the external chloride concentration and that nitrate can substitute for chloride electrically. However, since the PD for chloride, based on the Nernst equation and calculated on the basis of 70% exchangeability of cell chloride, is three times greater than the measured PD, it is hypothesized that sodium contributes significantly to the membrane potential in addition to chloride. On the other hand, potassium does not influence the PD to any great extent.
Ehrlich ascites tumor cells have the ability to accumulate potassium and extrude sodium against large chemical gradients (Hempling, '58; Maizels et al., '58). These gradients, once established, are maintained in a steady state. The conclusion that sodium and potassium are actively transported requires a demonstration that these cation concentration ratios are maintained against an electrical gradient.
Hempling ('58, '59), Grobecker et al. ('63), and Hempling and Kromphardt ('65) provided evidence that chloride was distributed passively across the tumor cell membrane and therefore it has been assumed that the electrical potential difference (PD) can be calculated from the Nernst equilibrium potential for chloride. Such a calculation gave a value of from 12 to 24 mV, inside negative with respect to medium. Actual electrode penetrations of 14 cells in Ringer-agar by Giebisch and Hempling (Hempling, '62) demonstrated a mean PD of -11.1 mV.
To document further the intracellular negativity of ascites tumor cells, it is necessary to measure the PD in Ringer solution, free of complications produced by the agar, such as spurious positive potentials and diffusion barriers. In addition, evidence that the membrane potential can be