Voltage-dependent changes of TRPV6-mediated Ca2+ currents

The physiological role and activation mechanism for most proteins of the transient receptor potential (TRP) family are unknown. This is also the case for the highly Ca2+ selective transient receptor potential vanilloid type 6 (TRPV6) channel. Patch clamp experiments were performed on transiently transfected human embryonic kidney (HEK) cells to address this issue. Currents were recorded under various conditions of intracellular Ca2+ buffering and monitored at the same voltage throughout. No TRPV6-mediated Ca2+ entry was detected under in vivo Ca2+ buffering conditions at a slightly negative holding potential; however, moderate depolarization resulted in current activation. Very similar results were obtained with different Ca2+ chelators, either EGTA or BAPTA dialyzing the cell. TRPV6 channel activity showed a negative correlation with the intracellular free Ca2+ concentration ([Ca2+](i)) and was modulated by the membrane potential: Hyperpolarization decreases and depolarization increases TRPV6-mediated currents. Monovalent ions permeated TRPV6 channels in the absence of extracellular divalent cations. These currents were resistant to changes in the holding potential while the negative correlation to the [Ca2+](i) was conserved, indicating that the voltage-dependent current changes depend on blocking and unblocking the charge carrier Ca2+ within the pore. In summary, these results suggest that the voltage dependence of TRPV6-mediated Ca2+ influx is of physiological importance since it occurs at cytosolic Ca2+ buffering and takes place within a physiologically relevant membrane potential range.