Inhibition of TRP3 channels by lanthanides -: Block from the cytosolic side of the plasma membrane

The lanthanide ions La3+ and Gd3+ block Ca2+-permeable cation channels and have been used as important tools to characterize channels of the transient receptor potential (TRP) family, However, widely different concentrations of La3+ and Gd3+ have reportedly been required for block of TRP3 channels in Various expression systems. The present study provides a possible explanation for this discrepancy. After overexpression of TRP3 in Chinese hamster ovary cells, whole-cell currents through TRP3 were reversibly inhibited by La3+ with an EC50 of 4 mum. For comparison, the organic blocker SKF96365 required an EC50 of 8 mum. Gd3+ blocked with an EC50 of 0.1 mum, but this block was slow in onset and was not reversible after wash-out. When the two lanthanides were added to the cytosolic side of inside-out patches, block was achieved with considerably lower concentrations (EC50 for La3+, 0.02 mum; EC50 for Gd3+, 0.02 mum). Uptake of La3+ into the cytosol of Chinese hamster ovary cells was demonstrated with intracellular fura a. We conclude that lanthanides block TRP3 more potently from the cytosolic than from the extracellular side of the plasma membrane and that uptake of lanthanides will largely affect the apparent EC50 values after extracellular application.