Photopigment quenching is Ca2+ dependent and controls response duration in salamander L-cone photoreceptors

Abstract

The time scale of the photoresponse in photoreceptor cells is set by the slowest of the steps that quench the lightinduced 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 Caa+-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 [Ca2+], pigment quenching rate limits recovery and provides an additional mechanism for modulating the cone response during light adaptation. © 2010 Matthews and Sampath.