Calcium and light adaptation in retinal rods and cones

Retinal rods and cones respond to light with a membrane hyperpolarization. This hyperpolarization is mediated by an ionic conductance (the light-regulated conductance) that is kept open in darkness by cyclic GMP acting as a ligand, and which closes in the light as a result of an increase in cGMP hydrolysis triggered by illumination^1-3. Calcium ions appear to have a role in this phototransduction process: they provide negative feedback between the conductance, which is permeable to Ca²⁺ (refs 4, 5), and the concentration of cGMP, which is sensitive to Ca²⁺ (refs 6-8). This feedback down-regulates the sensitivity to light of a photoreceptor and probably contributes to the important phenomenon of light adaptation in vision^9-11. It is still not clear, however, how much of the light adaptation is actually attributable to this Ca²⁺ feedback. We have examined the responses of amphibian rods and cones to light with the Ca²⁺ feedback removed. Normally, the response of a cell to a step of light rises transiently to a peak, but rapidly relaxes to a lower level, indicative of light adaptation. When the feedback is removed, however, the relaxation of the response is completely absent; furthermore, the steady response levels at different light-step intensities are well predicted by a statistical superposition of invariant single-photon responses. We therefore conclude that the Ca²⁺ feedback underlies essentially all light adaptation in rods and cones.