The role of the Ca2+-activated Cl- conductance in the membrane potential and light response of mouse rods.

Abstract

To characterize the function of the Ca2+-activated Cl- current ICl(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 [Formula: see text] increases during the light response. Using these results, we estimated ECl 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 ECl may be more positive by as much as 10 - 15 mV. Thus activation of ICl(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 Ca2+-activated Cl- currents in mammalian rods have more important roles in photoreceptor physiology than previously appreciated.Significance Statement To characterize the function of the Ca2+-activated Cl- channel in mammalian rods, we recorded from Ano2-/- mice with a disrupted channel gene. We show that these Cl- channels make a surprisingly large contribution to the rod resting permeability. Moreover, measurements of reversal potentials indicate that light produces an increase in Cl- conductance, which can only occur if the Ca2+ concentration near the channels is increasing even as the rod membrane potential is hyperpolarizing. We describe a novel method to make the first measurement of ECl in a mammalian rod, which is near to or somewhat positive of Erest Thus, channel activation would depolarize the rod. Ca2+-activated Cl- channels have more important roles in mammalian photoreceptor physiology than previously appreciated.