Different channel-gating properties of two classes of cyclic GMP-activated channel in vertebrate photoreceptors

We report that two types of cGMP-activated channel coexist in the photoreceptor plasma membrane, with the most commonly encountered class appearing broadly similar to the channel reported in previous patch-pipette experiments. However, we find that flickering of this channel between the open and closed states is so rapid that a discrete single-channel conductance cannot unequivocally be resolved; the occurrence of flickering is largely independent of membrane voltage and of the presence of cytoplasmic Ca²⁺ or Mg²⁺. In recordings from the inner segment we occasionally find a second class of cGMP-gated channel, with activity resembling that reported for cloned channels. This channel does not flicker, but instead exhibits distinct open-close transitions. Our results suggest that the predominant form of channel in vivo differs significantly from cloned channels, and that its gating properties are not as simple as reported previously.