Persistent activation of transducin by bleached rhodopsin in salamander rods.
The hydrolysis-resistant GTP analogue GTP-gamma-S was introduced into rods isolated from the retina of the salamander Ambystoma tigrinum to study the origin of the persistent excitation induced by intense bleaching illumination. Dialysis of a dark-adapted rod with a whole-cell patch pipette containing 2 mM GTP-gamma-S resulted in a gradual decrease in circulating current. If the rod was first bleached and its sensitivity allowed to stabilize for at least 30 min, then dialysis with GTP-gamma-S produced a much faster current decay. The circulating current could be restored by superfusion with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, suggesting that the decay in current originated from persistent excitation of the phosphodiesterase by transducin bound to GTP-gamma-S. We conclude that the persistent excitation which follows bleaching is likely to involve the GTP-binding protein transducin, which mediates the normal photoresponse. This observation suggests that a form of rhodopsin which persists long after bleaching can activate transducin much as does photoisomerized rhodopsin, although with considerably lower gain.
Matthews
H R
HR
Physiological Laboratory, University of Cambridge, United Kingdom. hrml@cam.ac.uk
Cornwall
M C
MC
Fain
G L
GL
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000221:Adaptation, Ocular / Q000502:physiology*; D000557:Ambystoma / Q000502:physiology*; D000818:Animals; D003623:Dark Adaptation / Q000502:physiology; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D010775:Photic Stimulation; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000502:physiology*; D012243:Rhodopsin / Q000502:physiology*; D013997:Time Factors; D015293:Transducin / Q000502:physiology*
D010726:Phosphodiesterase Inhibitors; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D012243:Rhodopsin; D019204:GTP-Binding Proteins; D015293:Transducin; D015056:1-Methyl-3-isobutylxanthine