Melittin-induced [Ca2+]i increases and subsequent death in canine renal tubular cells

The effect of melittin on cytosolic free Ca(2+) concentration (Ca(2+)](i)) and viability is largely unknown. This study examined whether melittin alters Ca(2+) levels and causes Ca(2+)-dependent cell death in Madin-Darby canine kidney (MDCK) cells. Ca(2+)](i) and cell death were measured using the fluorescent dyes fura-2 and WST-1 respectively. Melittin at concentrations above 0.5 microM increased Ca(2+)](i) in a concentration-dependent manner. The Ca(2+) signal was reduced by 75% by removing extracellular Ca(2+). The melittin-induced Ca(2+) influx was also implicated by melittin-caused Mn(2+) influx. After pretreatment with 1 microM thapsigargin (an endoplasmic reticulum Ca(2+) pump inhibitor), melittin-induced Ca(2+) release was inhibited; and conversely, melittin pretreatment abolished thapsigargin-induced Ca(2+) release. At concentrations of 0.5-20 microM, melittin killed cells in a concentration-dependent manner. The cytotoxic effect of 0.5 microM melittin was nearly completely reversed by prechelating cytosolic Ca(2+) with BAPTA. Melittin at 0.5-2 microM caused apoptosis as assessed by flow cytometry of propidium iodide staining. Collectively, in MDCK cells, melittin induced a Ca(2+)](i) rise by causing Ca(2+) release from endoplasmic reticulum and Ca(2+) influx from extracellular space. Furthermore, melittin can cause Ca(2+)-dependent cytotoxicity in a concentration-dependent manner.