The measurement of cell membrane potential (V _m) is important for understanding ion channel function. V _m plays a role in several routine cellular functions and diseases, particularly in excitable cells such as muscle and nerve. However, measuring V _m is difficult, relying either on labour-intensive direct measurement of single cells (intracellular electrodes, patch clamp) or indirect measurement of fluorescence intensity, using V _m-sensitive labels. Here we demonstrate a direct measurement technique based on determination of the cell’s ζ-potential, the electrical potential at the hydrodynamic shear plane, approximately 1 nm beyond the cell surface. We demonstrate this principle using excitable H9c2 cardiomyoblasts, measured in both polarised and depolarised states, before and after extracellular intervention to alter cell ion concentration. Given widespread availability of ζ-potential measurement apparatus (most typically in chemistry and materials science settings), this offers a new method of measuring V _m without the need for fluorescence measurements or calibration curves.

To survive, cells are able to adapt to a wide range of adverse conditions, such as varying pH from optimal (~7.4). They do this through mechanisms including acid-sensing ion channels, which alter cytosolic ion content and thus the cell’s electrophysiological profile. However, the impact of this adaptation on cellular electrophysiology remains unexplored. We investigated the effects of culture at a range of extracellular pH on the electrophysiological features of breast cancer cell lines MDA-MB-231 and MCF-7. Cells were subject to an acid–neutral–base pH from 3.0 to 9.2, after which their membrane potential (V_m), cytoplasm conductivity σ_cyto, effective membrane conductance G_eff, and ζ-potential were measured. Cells were also analyzed after permeabilization, to examine whether observed changes were due to cell surface chemistry, or to V_m. Both cell lines exhibited different electrophysiological phenotypes in acidic environments (pH < 6.7); MDA-MB-231 exhibited statistically significant differences in ζ-potential, V_m, G_eff and σ_cyto; MCF-7 only exhibited significant differences in σ_cyto. These findings suggest cells adapt to acidic microenvironments by altering V_m and potentially ζ-potential, reducing the extracellular potential, and hence potentially lowering proton concentration at the extracellular membrane surface. This offers new insights into potential therapeutic avenues to target the pH-dependent adaptations of cancer cells.