Cyclic GMP can increase rod outer-segment light-sensitive current 10-fold without delay of excitation.
To test the hypothesis that cyclic GMP is the internal messenger coupling rhodopsin activation to membrane excitation in vertebrate rod photoreceptors, we used a novel technique combining measurement of membrane currents of isolated salamander rods with a suction electrode and the introduction of cyclic GMP through a whole-cell recording patch pipette. Rupture of an attached patch was followed by a rapid (approximately 10 s), approximately 10-fold increase in outer-segment membrane current, all of which was light-sensitive. There was little change in the rising phase of the response to a saturating flash, but the duration of the saturated phase of the response increased approximately 10-fold. The effects reversed completely within 3-4 min after withdrawal of the cyclic GMP-containing patch pipette. A formal kinetic analysis shows that the first two observations are inconsistent with the postulate that cyclic GMP opens the light-sensitive conductance by simple binding to channels, unless free cyclic GMP in the outer segment is assumed to be much lower than published estimates, and most of the outer-segment cyclic GMP is bound and inexchangeable on the timescale of 200 ms. Furthermore, our results suggest that rod cyclic GMP is not involved solely in keeping the light-sensitive conductance open, but may also affect the activity of the phosphodiesterase that mediates cyclic GMP hydrolysis.
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D015106:3',5'-Cyclic-GMP Phosphodiesterases / Q000378:metabolism; D000818:Animals; D006152:Cyclic GMP / Q000494:pharmacology*; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects; D008959:Models, Neurological; D010786:Photoreceptor Cells / Q000502:physiology*; D012243:Rhodopsin / Q000528:radiation effects; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000502:physiology*; D014562:Caudata
D012243:Rhodopsin; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D006152:Cyclic GMP