Spatial spread of light-induced sensitization in rod photoreceptors exposed to low external calcium

Light sensitizes rods that have been desensitized by exposure to low external calcium. Yoshikami and Hagins [Biophys. Soc. Abstr. 15, 169a (1975)] suggested that desensitization in low external calcium results from exposure of intracellular calcium binding sites subsequent to depletion of internal calcium, and that background light sensitizes in this situation by releasing calcium to occupy those binding sites. In this view, it might be expected that light-induced sensitization would be spatially restricted to the illuminated region of the outer segment. However, in the present experiments, background illumination at one end of the outer segment potentiated responses to test flashes at the other end; resensitization was global rather than local. Patch-clamp recordings from the outer segment showed that the spread of internal transmitter was longitudinally restricted. Therefore, the sensitizing effect of background light is apparently not mediated via the internal transmitter, as required in the calcium-depletion explanation described above.