Determination of cell membrane resistance in cultured renal epithelioid (MDCK) cells: effects of cadmium and mercury ions

Previous studies have indicated that the cell membrane of Madin Darby Canine Kidney (MDCK) cells is hyperpolarized by a number of hormones and trace elements, in parallel with an enhancement of potassium selectivity. Without knowledge of the cell membrane resistance (R_m), however, any translation of potassium selectivity into potassium conductance remains equivocal. The present study was performed to determine the R_m of MDCK cells by cellular cable analysis. To this end, three microelectrodes were impaled into three different cells of a cell cluster; current was injected via one microelectrode and the corresponding voltage deflections measured by the other two microelectrodes. In order to extract the required specific resistances, the experimental data were analysed mathematically in terms of an electrodynamical model derived from Maxwell's equations. As a result, a mean R_m of 2.0±0.2 kcm² and an intercellular coupling resistance (R_c) of 6.1±0.8 M were obtained at a mean potential difference across the cell membrane of -47.0±0.6 mV. An increase of the extracellular K⁺ concentration from 5.4 to 20 mmol/l depolarized the cell membrane by 16.2±0.5 mV and decreased R_m by 30.6±3.0%; 1 mmol/l barium depolarized the cell membrane by 20.1±1.1 mV and increased R_m by 75.9±14.3%. Omission of extracellular bicarbonate and carbon dioxide at constant extracellular pH caused a transient hyperpolarization (up to –60.4±1.4 mV), a decrease of R_m (by 75±4.5%) and a decrease of R_c (by 23.1±8.4%). The changes in R_m and R_c were probably the result of intracellular alkalosis. Cadmium ions (1 mol/l) led to a sustained, reversible hyperpolarization (to –64.8±1.3 mV) and to a decrease of R_m (by 77.0±2.7%); mercury ions (1 mol/l) cause a sustained hyperpolarization (to –60.1±1.2 mV) and a decrease of R_m (by 76.3±3.9%). Neither manoeuvre significantly altered R_c. We have previously shown that both cadmium and mercury hyperpolarize the cell membrane potential and increase its potassium selectivity; the decrease of the R_m observed in the present study indicates that these effects are due to an increase of the potassium-selective conductance of the cell membrane.