ROLE OF CYTOPLASMIC CALCIUM-CONCENTRATION IN THE BLEACHING ADAPTATIONOF SALAMANDER CONE PHOTORECEPTORS

1. In order to study the possible involvement of Ca2+ in the bleaching adaptation of cones isolated from the retina of the salamander Ambyst oma tigrinum, changes in cytoplasmic calcium concentration ([Ca2+](i) were opposed by exposing the outer segment to a low-Ca2+-0 Na+ solutio n designed to minimize Ca2+ fluxes across the outer segment membrane. 2. When a cone was exposed in normal Ringer solution to bright light b leaching a significant fraction of the photopigment, the circulating c ur rent was initially suppressed completely and then recovered to a ma intained value less than tile value in darkness before the bleach. Whe n the outer segment of the cone was stepped to low-Ca2+-O Na+ solution before tile bleach was delivered, the circulating current recovered m ore slowly or (for large bleaches) remained completely suppressed for the duration of the solution exposure. 3. If, during the period for wh ich the current was suppressed in low-Ca2+-0 Na+ solution, the cone ou ter 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-Ca2+-0 Na+ solutio n in darkness before the bleach. If, instead, the outer segment was re turned 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-Ca2+-0Na(+) solution were greatly accelerated, and the integration time considerably reduced. Th is was true regardless of whether or not the low-Ca2+-0Na(+) solution included IBMX. 4. The role of Ca2+ in bleaching adaptation appeared to resemble its role in background adaptation, since in both cases expos ure to low-Ca2+-0Na(+) solution suppressed the acceleration of respons e kinetics. Responses recorded from cones in low-Ca2+-0Na solution wer e 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 sensi tivity was corrected for loss in quantum catch. 5. These results indic ate that the fall in [Ca2+](i) in cones after a bleach is necessary bo th for the acceleration of the flash response and the adaptational dec rease in sensitivity as is the case for adaptation by background light .