Light Adaptation in Toad Rods: Requirement for an Internal Messenger Which is Not Calcium

1. The mechanism of light adaptation was investigated by recording intracellularly from single rods in the isolated, superfused retina of the toad, Bufo marinus. Steady background lights produce decreases in rod sensitivity and changes in response wave form similar to those previously observed in the toad eyecup. 2. The sensitivity of a dark-adapted rod is halved by a background light which bleaches about 4 rhodopsins per rod per second. Since a toad rod contains over 2000 disks, a rhodopsin bleached in one disk must alter the effectiveness of rhodopsins bleached in others. This could occur if the state of adaptation in the rod were regulated by the concentration of some diffusable substance. 3. This diffusable substance cannot be Ca2+. Increases in intracellular Ca2+, produced experimentally either by increasing extracellular Ca2+ or by facilitating Ca2+ permeability into the rod with the ionophore X537A, cause a hyperpolarization of membrane potential and a decrease in response amplitude; but they do not produce changes in sensitivity and response wave form like those produced by background light. 4. Either Ca2+ is not the internal transmitter released from the disks during excitation, or the disks release or otherwise alter the concentration of a second diffusable substance, in addition to Ca2+, which regulates the state of adaptation.