Voltage Dependence of the Cation-Nonselective TRP Channels Expressed in HEK293 Cells
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As many as 20 mammalian homologues to the Drosophila melanogaster transient receptor potential (TRP) gene have been identified and, along with others in the Cenhabordis elegans genome, constitute the TRP gene family. Despite continuous and careful research for the past two decades, the mechanism by which the TRP family of ion channels is activated has evaded scientists. Intracellular Ca2+ store depletion is one hypothesis for the gating of TRP channels, and some researchers believe that TRP is responsible for the calcium-release-activated current, Icrac. Although TRP proteins were first thought to assemble as homomeric cation channels, it has been shown that different TRP proteins coassemble to form functional cation channels. To determine the electrophysiological properties of TRP channels, we expressed human TRP1 and urine TRP4 in human embryonic kidney cells. Co-expression of TRP1 and TRP4 also revealed that TRP4 is an integral part of the resulting channels, and we confirmed that cationic currents can be carried by such heteromers. Further recordings revealed that the arrangement of TRP subunits in the channel tetramer might be responsible for the voltage-dependence/independence of Ca2+ currents detected by voltage clamp technique. In addition, we demonstrate by comparison of the TRP1/4 current-voltage relationship with that of Icrac that the heteromer TRP1/4 is not responsible for Icrac. Although TRP1/4 channels are not responsible for the endogenous Icrac, the electrophysiological recordings in this study show that TRP channels may be responsible for endogenous currents recorded from the pyramidal cells of the prefrontal cortex of rats.