Abstract
The hyperpolarizing light response of vertebrate photoreceptors depends on cation-selective ionic channels in the cell membrane, which open in darkness and close in light. Little is known about these channels even though they are crucial elements in the transduction of light into a change in receptor potential. Here we report our attempts to study the light-sensitive channel using a patch pipette technique for recording ionic currents from a small area of outer segment membrane. The electrode was sealed against the membrane with a sealing resistance higher than a gigaohm, permitting good resolution of membrane currents1,2. Working at low external calcium concentrations this allowed us to observe in response to light small outward currents (the difference currents observed when the light-sensitive channel closes, thus switching-off an inward dark current), which reversed polarity with membrane depolarization. We were unable to recognize discrete current steps that might represent single channel openings and closings modulated by light. However, the light-induced outward current was accompanied by a decrease in membrane current noise. Analysis showed that the elementary event in 0.1 mM external calcium is ∼12 fA. If it is assumed that the elementary event is due to the closure of a single light-sensitive channel and that the potentiating effect of low external calcium on the photocurrent is due solely to an increase in single-channel current, one can calculate that in normal calcium the value of the elementary event is 2 fA, corresponding to a single channel conductance of 50 fS. A conductance of this size is consistent with ion permeation via either a mobile carrier or a pore.
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Detwiler, P., Conner, J. & Bodoia, R. Gigaseal patch clamp recordings from outer segments of intact retinal rods. Nature 300, 59–61 (1982). https://doi.org/10.1038/300059a0
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DOI: https://doi.org/10.1038/300059a0
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