Thapsigargin binds to and inhibits the cloned vanilloid receptor-1

We investigated the effect of thapsigargin, a well-known sarcoplasmic reticulum ATPase (SERCA) inhibitor, on the non-specific Ca2+ channel vanilloid receptor-1 (VR1) in CHO-VR1 cells. We found that thapsigargin inhibited the VR-1 mediated (45)Ca2+ uptake of CHO-VR1 cells (IC50=6.4+/-1.9 microM) and the [3H]RTX binding to VR1 (IC50=4.0+/-1.3 microM). Further analysis revealed that thapsigargin is a mixed-type inhibitor, suggesting both direct and indirect interactions between thapsigargin and the capsaicin binding site of VR1. Thapsigargin alone transiently elevated the [Ca2+]i in CHO-VR1 cells (EC50=44 nM). However, 45Ca2+ uptake was not detected after thapsigargin treatment, indicating that the emptying of the thapsigargin sensitive intracellular pools of Ca2+ was responsible for the elevated [Ca2+]i level rather than the activation of VR-1. We conclude that thapsigargin represents a new prototype of a VR1 inhibitor and that caution should be exercised in interpreting the effects of thapsigargin, especially when it is used in the micromolar range to inhibit SERCA activity.