Further characterization of volume regulatory decrease in cultured renal epitheloid (MDCK) cells

In Madin Darby canine kidney (MDCK) cells volume regulatory decrease (VRD) is paralleled by a variable, transient hyperpolarization followed by a sustained depolarization of the cell membrane. In the depolarized cells, the cell membrane selectivity is decreased for potassium and increased for chloride. Without knowledge of the cell membrane resistance (R_m), these changes of cell membrane selectivity cannot be translated into conductances, i.e. the observed alterations of ion selectivity could have been due to inhibition of potassium conductance or activation of anion conductance. In the present study R_m has been determined by cellular cable analysis. To this end, three microelectrodes were impaled into three different cells of a cell cluster, current (up to 3 nA) was injected into one cell and the corresponding voltage deflections determined in the other two cells. As a result, exposure of the cells to hypotonic perfusates leads to a marked, sustained reduction of R_m. In the absence of chloride and in the absence of bicarbonate and chloride, the decrease of R_m is only transient. The data indicate that cell swelling leads to a transient increase of potassium conductance followed by a sustained increase of anion conductance. As evident from BCECF fluorescence, exposure of MDCK cells to hypotonic perfusates leads to a significant decrease of intracellular pH, which may in part be due to loss of bicarbonate through the anion conductive pathway.