Functional relationship between Ca^<2+>-dependent cation channel and Ca^<2+>-permeable channel

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Ca^<2+> controls diverse cellular processes, which include muscle contraction, neurotransmitter release and other forms of secretion, gene expression, and cell proliferation. To evoke these cellular responses, Ca^<2+> influx across the plasma membrane makes a major contribution to augmenting the cytosolic free Ca^<2+> concentration ([Ca^<2+>]_i). In addition to the well-characterized voltage-gated Ca^<2+> channels and Ca^<2+>-permeable ligand-gated channels, Ca^<2+>-permeable cation channels, activated by downstream of GTP binding protein (G-protein) coupled receptors, have been recognized to be one of the major transmembrane pathways. An important clue to understand the molecular basis of these mammalian Ca^<2+>-permeable cation channels came from the findings of Drosophila visual transduction cascade, since some of the Ca^<2+>-permeable cation channels may be formed by the mammalian homologues of the Drosophila cation channel TRP (the gene product of the transient receptor potential, trp).Currents from HEK cells expressing the TRP3 or the TRP5 were recorded at room temperature using the conventional patch-clamp technique of the whole-cell mode with an Axopatch 200B amplifier. ATP 0.1 mM induced the TRP3 and the TRP5 channel currents at a holding potential of -50 mV in HEK cells expressing the TRP3 and the TRP5, respectively. In the HEK cells expressing the TRP5, extracellular Ca^<2+> at 10 mM induced currents without application of ATP. The current-voltage (I-V) relationships were linear for the TRP3 channel, and outwardly rectified at negative potentials for the TRP5 channel. Ca^<2+> is permeable to TRP5 channel and regulates TRP5 channel.