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 double-barreled 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.