The effects of lead and of cadmium on the mass photoreceptor potential: the dose-response relationship.

The effects of micromolar (1.0-60.0 microM) amounts of lead and of cadmium on the light response of photoreceptors were studied using the isolated, perfused bullfrog retina. The effect of lead in depressing rod activity is readily and fully reversible. Lead is very effective in depressing the rod response at concentrations as low as 1.0 microM, but the effect of lead saturates at about 25.0 microM with about 34% depression of the rod response. At the higher concentrations some spontaneous recovery of rod response amplitude is observed shortly after exposure of the retina to lead begins. The cone response is affected by lead only rarely and then only at the higher concentrations. When affected, the cone response is enhanced rather than depressed. The effects of cadmium are generally similar to those of lead. However, saturating concentrations of cadmium depress the rod response to a greater degree than lead and the cadmium effects are not as readily reversible as the effects of lead at the higher concentrations. Cones are much more sensitive to cadmium than they are to lead and 60.0 microM cadmium always enhances cone response amplitude. The results of experiments in which lead and cadmium were both added to the retinal perfusate indicate to us that cadmium and lead both affect the same sensitive site or sites responsible for generating the rod response, but that cadmium affects an additional site that is not sensitive to lead. The depressive effects of lead and of cadmium on the rod response are discussed in terms of the postulated mechanisms of transduction in the vertebrate photoreceptor.