Effect of nortriptyline on Ca ²⁺ handling in SIRC rabbit corneal epithelial cells

To explore the effect of nortriptyline, a tricyclic antidepressant, on cytosolic free Ca ²⁺ concentrations ([Ca ²⁺ ] _i ) in corneal epithelial cells, [Ca ²⁺ ] _i levels in suspended SIRC rabbit corneal epithelial cells were measured by using fura-2 as a Ca ²⁺ -sensitive fluorescent dye. Nortriptyline at concentrations between 20-200 μM increased [Ca ²⁺ ] _i in a concentration-dependent manner. The Ca ²⁺ signal was reduced partly by removing extracellular Ca ²⁺ . Nortriptyline-induced Ca ²⁺ influx was inhibited by the store-operated Ca ²⁺ channel blockers econazole and SK&F96365, the phospholipase A2 inhibitor aristolochic acid, and alteration of activity of protein kinase C. In Ca ²⁺ -free medium, 200 μM nortriptyline pretreatment greatly inhibited the rise of [Ca ²⁺ ] _i induced by the endoplasmic reticulum Ca ²⁺ pump inhibitor thapsigargin. Conversely, pretreatment with thapsigargin or 2,5-di-tert-butylhydroquinone (BHQ; another endoplasmic reticulum Ca ²⁺ pump inhibitor) nearly abolished nortriptyline-induced [Ca ²⁺ ] _i rise. Inhibition of phospholipase C with U73122 decreased nortriptyline-induced [Ca ²⁺ ] _i rise by 75%. Taken together, nortriptyline induced [Ca ²⁺ ] _i rises in SIRC cells by causing phospholipase C-dependent Ca ²⁺ release from the endoplasmic reticulum and Ca ²⁺ influx via store-operated Ca ²⁺ channels.