Measurement of relative Ca2+ permeability during sustained activation of TRPV1 receptors

Some cation permeable ligand-gated ion channels, including the capsaicin-sensitive TRPV1, have been reported to exhibit a time-dependent increase in permeability to large inorganic cations during sustained activation, a phenomenon termed “pore dilation.” TRPV1 conducts substantial Ca2+ entry, and it has been suggested that this channel undergoes a time-dependent change in Ca2+ permeability relative to Na+ (PCa/PNa) that parallels pore dilation. However, our experiments employing whole cell patch clamp photometry and single channel recordings to directly measure relative Ca2+ current in TRPV1 expressing HEK293 cells show that relative Ca2+ influx remains constant for the duration of capsaicin-evoked channel activation. Further, we present evidence from patch clamp photometry experiments suggesting that sustained activation of Ca2+ permeable ion channels in the voltage-clamp configuration leads to rapid saturation of the pipette Ca2+ chelator, and that subsequent observed shifts in the current reversal potentials in the presence of extracellular Ca2+ are likely due to intracellular accumulation of this ion and a movement of the Ca2+ equilibrium potential (ECa) towards zero. Finally, using an adapted reversal potential-based protocol in which cells are only exposed to Ca2+ after sustained capsaicin exposure in the absence of added extracellular Ca2+, we demonstrate that the calculated PCa/PNa is unaffected by duration of TRPV1 activation. In conclusion, we find no evidence in support of a time-dependent change in PCa/PNa for TRPV1. Our data further urges caution in estimating relative Ca2+ permeability using reversal potentials, as there is a limited time window in which the cytosolic Ca2+ chelator included in the patch pipette can prevent localised elevations in cytosolic free Ca2+ and thus allow for an accurate estimate of this important channel permeability parameter.