Fractional Ca²⁺ currents through TRP and TRPL channels in Drosophila photoreceptors

Light responses in Drosophila photoreceptors are mediated by two Ca ²⁺ permeable cation channels, transient receptor potential (TRP) and TRP-like (TRPL). Although Ca²⁺ influx via these channels is critical for amplification, inactivation, and light adaptation, the fractional contribution of Ca²⁺ to the currents (P_f) 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⁺/Ca²⁺ exchange. Assuming a 3Na:1Ca stoichiometry, we derive empirical estimates of P_f by comparing the charge integrals of the exchanger and light-induced currents. For TRPL channels, P_f was ∼17% as predicted by Goldman-Hodgkin-Katz (GHK) theory. P_f for TRP (29%) and wild-type flies (26%) was higher, but lower than the GHK prediction (45% and 42%). As predicted by GHK theory, P_f for both channels increased with extracellular [Ca²⁺], 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⁺/Ca²⁺ exchange current could be used to obtain estimates of cellular Ca²⁺ buffering capacities.