Activation of vanilloid receptor 1 by resiniferatoxin mobilizes calcium from inositol 1,4,5‐trisphosphate‐sensitive stores

Capsaicin and resiniferatoxin (RTX) stimulate Ca2+ influx by activating vanilloid receptor 1 (VR1), a ligand‐gated Ca2+ channel on sensory neurones. We investigated whether VR1 activation could also trigger Ca2+ mobilization from intracellular Ca2+ stores. Human VR1‐transfected HEK293 cells (hVR1‐HEK293) were loaded with Fluo‐3 or a mixture of Fluo‐4 and Fura Red and imaged on a fluorometric imaging plate reader (FLIPR) and confocal microscope respectively. In Ca2+‐free media, RTX caused a transient elevation in intracellular free Ca2+ concentration in hVR1‐HEK293 cells (pEC50 6.45±0.05) but not in wild type cells. Capsaicin (100 μM) did not cause Ca2+ mobilization under these conditions. RTX‐mediated Ca2+ mobilization was inhibited by the VR1 receptor antagonist capsazepine (pIC50 5.84±0.04), the Ca2+ pump inhibitor thapsigargin (pIC50 7.77±0.04), the phospholipase C inhibitor U‐73122 (pIC50 5.35±0.05) and by depletion of inositol 1,4,5‐trisphosphate‐sensitive Ca2+ stores by pretreatment with the acetylcholine‐receptor agonist carbachol (20 μM, 2 min). These data suggest that RTX causes Ca2+ mobilization from inositol 1,4,5‐trisphosphate‐sensitive Ca2+ stores in hVR1‐HEK293 cells. In the presence of extracellular Ca2+, both capsaicin‐mediated and RTX‐mediated Ca2+ rises were a ttenuated by U‐73122 (10 μM, 30 min) and thapsigargin (1 μM, 30 min). We conclude that VR1 is able to couple to Ca2+ mobilization by a Ca2+ dependent mechanism, mediated by capsaicin and RTX, and a Ca2+ independent mechanism mediated by RTX alone. British Journal of Pharmacology (2003) 138, 172–176. doi:10.1038/sj.bjp.0705003