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Physiological Laboratory, Cambridge, UK.

1. In order to study the possible involvement of Ca2+ in the bleaching adaptation of cones isolated from the retina of the salamander Ambystoma tigrinum, changes in cytoplasmic calcium concentration ([Ca2+]i were opposed by exposing the outer segment to a low-Ca(2+)-O Na+ solution designed to minimize Ca2+ fluxes across the outer segment membrane. 2. When a cone was exposed in normal Ringer solution to bright light bleaching a significant fraction of the photopigment, the circulating current was initially suppressed completely and then recovered to a maintained value less than the value in darkness before the bleach. When the outer segment of the cone was stepped to low-Ca(2+)-O Na+ solution before the bleach was delivered, the circulating current recovered more slowly or (for large bleaches) remained completely suppressed for the duration of the solution exposure. 3. If, during the period for which the current was suppressed in low-Ca(2+)-O Na+ solution, the cone outer segment was exposed to the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX), the circulating current was restored. The dim flash response recorded under these conditions exhibited kinetics and integration times similar to those recorded in low-Ca(2+)-O Na+ solution in darkness before the bleach. If, instead, the outer segment was returned to Ringer solution after the bleach, thereby allowing [Ca2+]i to fall from its dark-adapted level to the appropriate bleach-adapted level, the kinetics of the response in low-Ca(2+)-O Na+ solution were greatly accelerated, and the integration time considerably reduced. This was true regardless of whether or not the low-Ca(2+)-O Na+ solution included IBMX. 4. The role of Ca2+ in bleaching adaptation appeared to resemble its role in background adaptation, since in both cases exposure to low-Ca(2+)-O Na+ solution suppressed the acceleration of response kinetics. Responses recorded from cones in low-Ca(2+)-O Na+ solution were nearly identical in waveform and sensitivity during background light or after bleaches, provided that IBMX was used to restore sufficient photocurrent so that responses to flashes could be recorded, and sensitivity was corrected for loss in quantum catch. 5. These results indicate that the fall in [Ca2+]i in cones after a bleach is necessary both for the acceleration of the flash response and the adaptational decrease in sensitivity, as is the case for adaptation by background light.