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.
Copyright 2002 Elsevier Science (USA).