Mechanism of action of a diterpene alkaloid hypaconitine on cytotoxicity and inhibitory effect of BAPTA‐AM in HCN‐2 neuronal cells

Hypaconitine, a neuromuscular blocker, is a diterpene alkaloid found in the root of Aconitum carmichaelii. Although hypaconitine was shown to affect various physiological responses in neurological models, the effect of hypaconitine on cell viability and the mechanism of its action of Ca2+ handling is elusive in cortical neurons. This study examined whether hypaconitine altered viability and Ca2+ signalling in HCN‐2 neuronal cell lines. Cell viability was measured by the cell proliferation reagent (WST‐1). Cytosolic Ca2+ concentrations [Ca2+]i was measured by the Ca2+‐sensitive fluorescent dye fura‐2. In HCN‐2 cells, hypaconitine (10–50 μmol/L) induced cytotoxicity and [Ca2+]i rises in a concentration‐dependent manner. Removal of extracellular Ca2+ partially reduced the hypaconitine's effect on [Ca2+]i rises. Furthermore, chelation of cytosolic Ca2+ with BAPTA‐AM reduced hypaconitine's cytotoxicity. In Ca2+‐containing medium, hypaconitine‐induced Ca2+ entry was inhibited by modulators (2‐APB and SKF96365) of store‐operated Ca2+ channels and a protein kinase C (PKC) inhibitor (GF109203X). Hypaconitine induced Mn2+ influx indirectly suggesting that hypaconitine evoked Ca2+ entry. In Ca2+‐free medium, treatment with the endoplasmic reticulum Ca2+ pump inhibitor thapsigargin abolished hypaconitine‐induced [Ca2+]i rises. Conversely, treatment with hypaconitine inhibited thapsigargin‐induced [Ca2+]i rises. However, inhibition of phospholipase C (PLC) with U73122 did not inhibit hypaconitine‐induced [Ca2+]i rises. Together, hypaconitine caused cytotoxicity that was linked to preceding [Ca2+]i rises by Ca2+ influx via store‐operated Ca2+ entry involved PKC regulation and evoking PLC‐independent Ca2+ release from the endoplasmic reticulum. Because BAPTA‐AM loading only partially reversed hypaconitine‐induced cell death, it suggests that hypaconitine induced a second Ca2+‐independent cytotoxicity in HCN‐2 cells.