UCLA

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.