Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors.
The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the light-induced activity of the phototransduction cascade. In vertebrate photoreceptor cells, this rate-limiting reaction is thought to be either shutoff of catalytic activity in the photopigment or shutoff of the pigment's effector, the transducin-GTP-phosphodiesterase complex. In suction pipette recordings from isolated salamander L-cones, we found that preventing changes in internal [Ca(2+)] delayed the recovery of the light response and prolonged the dominant time constant for recovery. Evidence that the Ca(2+)-sensitive step involved the pigment itself was provided by the observation that removal of Cl(-) from the pigment's anion-binding site accelerated the dominant time constant for response recovery. Collectively, these observations indicate that in L-cones, unlike amphibian rods where the dominant time constant is insensitive to [Ca(2+)], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation.
Matthews
Hugh R
HR
Department of Physiology, Development and Neuroscience, Physiological Laboratory, University of Cambridge, Cambridge, England, UK. hrm1@cam.ac.uk
Sampath
Alapakkam P
AP
D000221:Adaptation, Ocular / Q000502:physiology* / Q000528:radiation effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000502:physiology* / Q000528:radiation effects; D002478:Cells, Cultured; D025461:Feedback, Physiological / Q000502:physiology / Q000528:radiation effects; D017949:Retinal Cone Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects; D012168:Retinal Pigments / Q000502:physiology* / Q000528:radiation effects; D014562:Caudata / Q000502:physiology*
D012168:Retinal Pigments; D002118:Calcium