The Role of the Ca ²⁺ -activated Cl ⁻ Conductance in the Membrane Potential and Light Response of Mouse Rods

Abstract

To characterize the function of the Ca 2+ -activated Cl − current I Cl(Ca) in mammalian rod photoreceptors, we made patch-clamp recordings from retinal slices of mice ( Mus musculus ) of both sexes that lack Ano2 (TMEM16B). Depolarizing voltage ramps in solutions blocking K + currents elicited a large outward current inhibited by the Cl − channel blocker niflumic acid; this current was absent in Ano2 −/− rods. The membrane potential of Ano2 −/− rods was 10–15 mV more depolarized in darkness than WT or Cx36 −/− rods, indicating a substantial resting Cl − permeability. Rod outer-segment photocurrents were similar in waveform and amplitude in Ano2 −/− and Cx36 −/− rods, but photovoltages in Ano2 −/− rods were nearly doubled. Measurements of light-response reversal potentials in rods with and without Ano2 suggest that the outer-segment conductance is nearly linear with a reversal potential of −9 mV and that Δ g Cl increases during the light response. Using these results, we estimated E Cl from permeabilized patch recordings of reversal potentials of Cx36 −/− rods to have a mean value of −35 mV near the rod resting potential, but other evidence suggests that E Cl may be more positive by as much as 10–15 mV. Thus activation of I Cl(Ca) during the light response would be depolarizing. At dim intensities, the photocurrents of downstream rod bipolar cells were larger and about twice as sensitive in Ano2 −/− retinas with reduced nonlinearity. These experiments show that Ca 2+ -activated Cl − currents in mammalian rods have more important roles in photoreceptor physiology than previously appreciated.