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.