Photoreceptor light adaptation is mediated by cytoplasmic calcium concentration

The vertebrate visual system can operate over a large range of light intensities. This is possible in part because the sensitivity of photoreceptors decreases approximately in inverse proportion to the background light intensity^1-3. This process, called photoreceptor light adaptation, is known to be mediated by a diffusible intracellular messenger^4-6, but the identity of the messenger is still unclear. There has been considerable speculation that decreased cytoplasmic Ca²⁺ concentration (Ca_i²⁺) may play a role in light adaptation^7-11, and recent experiments in which Ca²⁺ buffer was incorporated into rod-cells^10,11 have supported this notion. The extent of the contribution of calcium, however, remains unresolved. We now show that light-dependent changes in sensitivity in amphibian photoreceptors can be abolished by preventing movements of Ca²⁺ across the outer-segment plasma membrane. These experiments demonstrate that light adaptation in photoreceptors is mediated in cones primarily, and in rods perhaps exclusively, by changes in Ca_i²⁺.