The effect of lead on photoreceptor response amplitude--influence of removing external calcium and bleaching rhodopsin.

The relationship between the effect of lead on the amplitude of the photoreceptor response to light and the ability of the photoreceptor response to be enhanced by a light stimulus was studied in the excised, superfused, bullfrog retina. Photoreceptor responses were isolated by treatment of the retina with sodium aspartate and were recorded with extracellular electrodes. Rod and cone responses were monitored separately by employing a stimulus regimen which took advantage of differing rates of rapid dark adaptation. When the ability of the rod response to undergo enhancement by light was severely reduced by bleaching 80% of the rhodopsin in the retina, the usual depressive effect of 12.5 microM lead was not observed. When light enhancement of the rod response was completely eliminated by removing calcium from the superfusing solution, the effect of 12.5 microM lead was actually to increase rather than decrease rod response amplitude. We conclude from this and previous studies that the effect of lead on the rod photoreceptor is complex, that it can involve both inhibitory and excitatory components, and that the inhibitory effect is exerted on the same mechanism which is responsible for the ability of the rod response to be enhanced by exposure of the retina to light. The cone response never exhibited enhancement by light and was never depressed by exposure of the retina to lead, but the ability of lead to increase cone response amplitude appeared to be greater in the absence of calcium.