Fluoxetine inhibits ATP-induced [Ca²⁺]_i increase in PC12 cells by inhibiting both extracellular Ca²⁺ influx and Ca ²⁺ release from intracellular stores

Fluoxetine, a widely used antidepressant, has additional effects, including the blocking of voltage-gated ion channels. We examined whether fluoxetine affects ATP-induced calcium signaling in PC12 cells using fura-2-based digital calcium imaging, an assay for [³H]-inositol phosphates (IPs) and whole-cell patch clamping. Treatment with ATP (100 μM) for 2 min induced increases in intracellular free Ca²⁺ concentrations ([Ca ²⁺]_i). Treatment with fluoxetine (100 nM to 30 μM) for 5 min inhibited the ATP-induced [Ca²⁺]_i increases in a concentration-dependent manner (IC₅₀ = 1.85 μM). Treatment with fluoxetine (1.85 μM) for 5 min significantly inhibited the ATP-induced responses following the removal of extracellular Ca²⁺ or depletion of intracellular Ca²⁺ stores. Whereas treatment for 10 min with nimodipine (1 μM) significantly inhibited the ATP-induced [Ca ²⁺]_i increase, treatment with fluoxetine further inhibited the ATP-induced response. Treatment with fluoxetine significantly inhibited [Ca²⁺]_i increases induced by 50 mM K⁺. In addition, treatment with fluoxetine markedly inhibited ATP-induced inward currents in a concentration-dependent manner. However, treatment with fluoxetine did not inhibit ATP-induced [³H]-IPs formation. Therefore, we conclude that fluoxetine inhibits ATP-induced [Ca²⁺]_i increases in PC12 cells by inhibiting both the influx of extracellular Ca ²⁺ and the release of Ca²⁺ from intracellular stores without affecting IPs formation.