The anti-anginal drug fendiline elevates cytosolic Ca²⁺ in rabbit corneal epithelial cells

The effect of the anti-anginal drug fendiline on intracellular free Ca²⁺ levels ([Ca²⁺]_i) in a rabbit corneal epithelial cell line (SIRC) was explored using fura-2 as a fluorescent Ca²⁺ indicator. At a concentration above 1 μM, fendiline increased [Ca²⁺]_i in a concentration-dependent manner with an EC₅₀ value of 7 μM. The [Ca²⁺]_i response consisted of an immediate rise and an elevated phase. Extracellular Ca²⁺ removal decreased half of the [Ca²⁺]_i signal. Fendiline induced quench of fura-2 fluorescence by Mn²⁺ (50 μM), suggesting the presence of Ca²⁺ influx across the plasma membrane. This Ca²⁺ influx was abolished by La³⁺ (50 μM), but was insensitive to dihydropyridines, verapamil and diltiazem. Fendiline (10 μM)-induced store Ca²⁺ release was largely reduced by pretreatment with thapsigargin (1 μM) (an endoplasmic reticulum Ca²⁺ pump inhibitor) to deplete the endoplasmic reticulum Ca²⁺. Conversely, pretreatment with 10 μM fendiline abolished thapsigargin-induced Ca²⁺ release. Fendiline (10 μM)-induced Ca²⁺ release was not altered by inhibiting phospholipase C with 2 μM 1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H- pyrrole-2,5-dione (U73122). Cumulatively, this study shows that fendiline induced concentration-dependent [Ca²⁺]_i increases in corneal epithelial cells by releasing the endoplasmic reticulum Ca²⁺ in a phospholipase C-independent manner, and by causing Ca²⁺ influx.