The recombinant human TRPV6 channel functions as Ca2+ sensor in human embryonic kidney and rat basophilic leukemia cells

The activation mechanism of the recently cloned human transient receptor potential vanilloid type 6 (TRPV6) channel, originally termed C2+ transporter-like protein and Ca2+ transporter type 1, was investigated in whole-cell patch-clamp experiments using transiently transfected human embryonic kidney and rat basophilic leukemia cells. The TRPV6-mediated currents are highly Ca2+-selective, show a strong inward rectification, and reverse at positive potentials, which is similar to store-operated Ca2+ entry in electrically non-excitable cells. The gating of TRPV6 channels is strongly dependent on the cytosolic free Ca2+ concentration; lowering the intracellular free Ca2+ concentration results in Ca2+ influx, and current amplitude correlates with the intracellular EGTA or BAPTA concentration. This is also the case for TRPV6-mediated currents in the absence of extracellular divalent cations; compared with endogenous currents in nontransfected rat basophilic leukemia cells, these TRPV6-mediated monovalent currents reveal differences in reversal potential, inward rectification, and slope at very negative potentials. Release of stored Ca2+ by inositol 1,4,5-trisphosphate and/or the sarco/endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin appears not to be involved in TRPV6 channel gating in both cell lines but, in rat basophilic leukemia cells, readily activates the endogenous Ca2+ release-activated Ca2+ current. In conclusion, TRPV6, expressed in human embryonic kidney cells and in rat basophilic leukemia cells, functions as a Ca2+- sensing Ca2+ channel independently of procedures known to deplete Ca2+ stores.