Ca2+-dependent K+ efflux regulates deoxycholate-induced apoptosis of BHK-21 and Caco-2 cells

BACKGROUND & AIMS: Deoxycholate (DC) has proapoptotic and tumorigenic effects in different cell types of the gastrointestinal tract. Exposure of BHK-21 (stromal) cells to DC induces Ca2 entry at the plasma membrane, which affects intracellular Ca2 signaling. We assessed whether DC-induced increases in [Ca2] can impinge on plasma membrane properties (eg, ionic conductances) involved in cell apoptosis. METHODS: Single- and doublebarreled microelectrodes were used to measure membrane potential (Vm) and extracellular [K] in BHK-21 fibroblasts and Caco-2 colon carcinoma cells. Apoptosis was assessed by Hoechst labeling, propidium iodide staining, and caspase-3 and caspase-7 assays. RESULTS: DC-induced cell membrane hyperpolarization was directly measured with intracellular microelectrodes in both cell lines. Diverse Ca2 mobilizing agents, such as membrane receptor agonists, an inhibitor of the sarco/endoplasmic reticulum Ca2 adenosine triphosphatase and a Ca2 ionophore, also induced increases in Vm. Removal of extracellular Ca2 reduced the agonist- and DC-induced membrane hyperpolarization by approximately 15% and 60%, respectively. These findings indicate a prominent role for Ca2 entry at the plasma membrane in the action of this bile salt. Blockade of Ca2- activated K conductances by charybdotoxin and apamin reduced DC-induced hyperpolarization by 75% and 64% in BHK-21 and Caco-2 cells, respectively. These inhibitors also reduced the DC-induced increase in extracellular [K] by 75% and cell apoptosis by approximately 50% in both cell lines. CONCLUSIONS: Ca2-dependent K conductance is an important regulator of DC-induced apoptosis in stromal and colon cancer cells.