The late receptor potential of the excised, perfused bullfrog retina was isolated with sodium aspartate. By employing a three-flash technique, cone responses were monitored without interference from ...rods. In cones barium ions were found to delay the onset of rapid dark adaptation, but the rate of recovery, once begun, was unaffected. We propose that barium ions act directly upon the enzyme system postulated to govern the onset of rapid dark adaptation of cones. In addition, barium was found to affect the amplitude of the rod receptor potential differently from that of cones, increasing the former but decreasing the latter. The effect of barium upon photoreceptor potential amplitude is discussed in terms of a reduction in the potassium conductance of the photoreceptors and the mechanisms postulated for photoreceptor excitation and rapid dark adaptation.
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 aspartate-isolated receptor potential was studied in the excised, perfused bullfrog retina. Cones were monitored without interference from rods by employing conditioning and test stimuli in a ...manner previously described (23,24). Lowering extracellular calcium by switching from a perfusate containing 0.4mM CaCl2 to one having no added calcium resulted in an increase in cone response amplitude. Conversely, elevating extracellular calcium by perfusing with a Ringer containing 0.8mM CaCl2 resulted in a decrease in cone response amplitude. These changes were sustained and fully reversible. In contrast, perfusing the retina with a Ringer solution containing EGTA resulted in a transient increase in cone response amplitude. Decreasing external calcium by simple depletion also shortened the delay prior to onset of rapid dark adaptation of the cones, thereby hastening the entire process of recovery. Increasing external calcium had little effect on rapid dark adaptation. Decreasing external calcium with EGTA led to extremely rapid response recovery, but the effect was not reversible. In no case did EGTA lead to a complete suppression of the response. The results of this study are interpreted as being inconsistent with the view that calcium is the internal transmitter responsible for the generation of the receptor potential in cones. They are consistent with the view that calcium functions to modulate recovery of the cones' ability to generate a response following a stimulus, perhaps by affecting the activity of a cyclic nucleotide.
