The influence of calcium on the aspartate-isolated mass receptor potential of bullfrog cones

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Abstract
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.