Capsazepine elevates intracellular Ca2+ 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 Ca2+ concentrations ([Ca2+](i)) and cytotoxicity was explored by using fura-2 and tetrazolium, respectively. Capsazepine caused a rapid rise in [Ca2+]i in a concentration-dependent manner with an EC50 value of 100 muM. Capsazepine-induced [Ca2+](i) rise was partly reduced by removal of extracellular Ca2+ suggesting that the capsazepine-induced [Ca2+](i) rise was composed of extracellular Ca2+ influx and intracellular Ca2+. In Ca2+-free medium, thapsigargin, an inhibitor of the endoplasmic reticulum Ca2+-ATPase, caused a monophasic [Ca2+](i) rise, after which the increasing effect of capsazepine on [Ca2+](i) was inhibited by 75%. Conversely, pretreatment with capsazepine to deplete intracellular Ca2+ Stores totally prevented thapsigargin from releasing more Ca2+. U73122, an inhibitor of phospholipase C, abolished histamine (an inositol 1,4,5-trisphosphate-dependent Ca2+ mobilizer)induced, but not capsazepine-induced, [Ca2+](i) rise. Overnight treatment with 1-100 muM capsazepine inhibited cell proliferation in a concentration-dependent manner. These findings suggest that in human MG63 osteosarcoma cells, capsazepine increases [Ca2+](i) by stimulating extracellular Ca2+ influx and also by causing intracellular Ca2+ release from the endoplasmic reticulum via a phospholiase C-independent manner. Capsazepine may be mildly cytotoxic. (C) 2004 Elsevier Inc. All rights reserved.