Mechanisms of light adaptation in toad rods

A steady illumination equivalent to about 1000 or more photoisomerizations per s per rod completely abolishes the dark current and induces a total desensitization of toad rods. Under these conditions, however, both current and sensitivity recover from the initial suppression and reach a new steady level within about 30 s. Both extent and time course of recovery depend on the intensity of the conditioning light. The ability of rods to respond to light stimuli under light saturating conditions is enhanced by procedures aimed at increasing the sodium gradient across their surface membrane. The results suggest that rods can also contribute significantly to vision at photopic levels of ambient light. The mechanisms underlying this adaptational property of rods are discussed.