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.

Dichotomous effects of cadmium on the cone photoreceptor response to light.

The effects of cadmium (1.0 to 400.0 microM CdCl2) on the cone photoreceptor's response to light were studied in the superfused bullfrog retina. The effects of cadmium were complex, characterized by a purely excitatory component at the lower levels and an inhibitory component that became evident with 75.0 microM CdCl2. The maximum increase in cone response amplitude due to the excitatory component was about 60% at 100.0 microM CdCl2. At 400.0 microM CdCl2, the highest concentration examined in this study, cone response amplitude was suppressed by about 35%. Both components were dose-dependent. The excitatory component was reversible only at CdCl2 concentrations below 50.0 microM, and irreversible at levels at and above 50.0 microM. The inhibitory component was always reversible. The slope of the dose-response curve describing the excitatory component (0.60) was about twice that describing the inhibitory component (0.31). The effects of cadmium on cone response amplitude are discussed in terms of the most probable sensitive sites.

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.