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.
Torre
V
V
Dipartimento di Fisica, Università di Genova, Italy.
Straforini
M
M
Sesti
F
F
Lamb
T D
TD
D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002462:Cell Membrane / Q000502:physiology; D006152:Cyclic GMP / Q000494:pharmacology*; D015640:Ion Channel Gating / Q000187:drug effects; D007473:Ion Channels / Q000502:physiology*; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D010786:Photoreceptor Cells / Q000502:physiology*; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000502:physiology*
D007473:Ion Channels; D006152:Cyclic GMP; D008274:Magnesium; D002118:Calcium