title: Fractional Ca(2+) currents through TRP and TRPL channels in Drosophila photoreceptors.
abstract: Light responses in Drosophila photoreceptors are mediated by two Ca(2+) permeable cation channels, transient receptor potential (TRP) and TRP-like (TRPL). Although Ca(2+) influx via these channels is critical for amplification, inactivation, and light adaptation, the fractional contribution of Ca(2+) to the currents (Pf) has not been measured. We describe a slow (τ ∼ 350 ms) tail current in voltage-clamped light responses and show that it is mediated by electrogenic Na(+)/Ca(2+) exchange. Assuming a 3Na:1Ca stoichiometry, we derive empirical estimates of Pf by comparing the charge integrals of the exchanger and light-induced currents. For TRPL channels, Pf was ∼17% as predicted by Goldman-Hodgkin-Katz (GHK) theory. Pf for TRP (29%) and wild-type flies (26%) was higher, but lower than the GHK prediction (45% and 42%). As predicted by GHK theory, Pf for both channels increased with extracellular [Ca(2+)], and was largely independent of voltage between -100 and -30 mV. A model incorporating intra- and extracellular geometry, ion permeation, diffusion, extrusion, and buffering suggested that the deviation from GHK predictions was largely accounted for by extracellular ionic depletion during the light-induced currents, and the time course of the Na(+)/Ca(2+) exchange current could be used to obtain estimates of cellular Ca(2+) buffering capacities.
authors.affiliation: Department of Physiology, Development and Neuroscience, Cambridge University, Cambridge, UK.
authors.forename: Brian
authors.initials: B
authors.lastname: Chu
authors.forename: Marten
authors.initials: M
authors.lastname: Postma
authors.forename: Roger C
authors.initials: RC
authors.lastname: Hardie
mesh_terms: D000200:Action Potentials; D000818:Animals; D017920:Antiporters / Q000235:genetics / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D004331:Drosophila melanogaster / Q000378:metabolism* / Q000502:physiology; D017136:Ion Transport; D009154:Mutation; D010775:Photic Stimulation; D017956:Photoreceptor Cells, Invertebrate / Q000378:metabolism* / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D050051:Transient Receptor Potential Channels / Q000235:genetics / Q000378:metabolism*
chemical_list: D017920:Antiporters; C108396:Calx protein, Drosophila; D029721:Drosophila Proteins; D050051:Transient Receptor Potential Channels; C063169:trp protein, Drosophila; C074192:trpl protein, Drosophila; D012964:Sodium; D002118:Calcium