Capsazepine elevates intracellular Ca ²⁺ in human osteosarcoma cells, questioning its selectivity as a vanilloid receptor antagonist

Capsazepine is thought to be a selective antagonist of vanilloid type 1 receptors; however, its other in vitro effect on different cell types is unclear. In human MG63 osteosarcoma cells, the effect of capsazepine on intracellular Ca ²⁺ concentrations ([Ca ²⁺] _i) and cytotoxicity was explored by using fura-2 and tetrazolium, respectively. Capsazepine caused a rapid rise in [Ca ²⁺] _i in a concentration-dependent manner with an EC ₅₀ value of 100 μM. Capsazepine-induced [Ca ²⁺] _i rise was partly reduced by removal of extracellular Ca ²⁺, suggesting that the capsazepine-induced [Ca ²⁺] _i rise was composed of extracellular Ca ²⁺ influx and intracellular Ca ²⁺. In Ca ²⁺-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca ²⁺-ATPase, caused a monophasic [Ca ²⁺] _i rise, after which the increasing effect of capsazepine on [Ca ²⁺] _i was inhibited by 75%. Conversely, pretreatment with capsazepine to deplete intracellular Ca ²⁺ stores totally prevented thapsigargin from releasing more Ca ²⁺. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca ²⁺ mobilizer)-induced, but not capsazepine-induced, [Ca ²⁺] _i rise. Overnight treatment with 1-100 μM capsazepine inhibited cell proliferation in a concentration-dependent manner. These findings suggest that in human MG63 osteosarcoma cells, capsazepine increases [Ca ²⁺] _i by stimulating extracellular Ca ²⁺ influx and also by causing intracellular Ca ²⁺ release from the endoplasmic reticulum via a phospholiase C-independent manner. Capsazepine may be mildly cytotoxic.