Fractional Ca(2+) currents through TRP and TRPL channels in Drosophila photoreceptors

Biophys J. 2013 May 7;104(9):1905-16. doi: 10.1016/j.bpj.2013.03.047.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Action Potentials*
  • Animals
  • Antiporters / genetics
  • Antiporters / metabolism
  • Calcium / metabolism*
  • Drosophila Proteins / genetics
  • Drosophila Proteins / metabolism*
  • Drosophila melanogaster / metabolism*
  • Drosophila melanogaster / physiology
  • Ion Transport
  • Mutation
  • Photic Stimulation
  • Photoreceptor Cells, Invertebrate / metabolism*
  • Photoreceptor Cells, Invertebrate / physiology
  • Sodium / metabolism
  • Transient Receptor Potential Channels / genetics
  • Transient Receptor Potential Channels / metabolism*


  • Antiporters
  • Calx protein, Drosophila
  • Drosophila Proteins
  • Transient Receptor Potential Channels
  • trp protein, Drosophila
  • trpl protein, Drosophila
  • Sodium
  • Calcium