The role of calcium as a regulator of light adaptation in rod photoreceptors was examined by manipulation of the intracellular Ca2+ concentration through the use of the calcium ionophore A 23187 and external Ca2+ buffers. These studies utilized suspensions of isolated and purified frog rod outer segments that retain their mitochondria-rich inner segments (OS-IS). Three criteria of the dark- and light-adapted flash response were characterized as a function of the Ca2+ concentration: (a) the time to peak, (b) the rate of recovery, and (c) the response amplitude or sensitivity. For all Ca2+ concentrations examined, the time to peak of the flash response was accelerated in the presence of background illumination, suggesting that mechanisms controlling this aspect of adaptation are independent of the Ca2+ concentration. The recovery kinetics of the flash response appeared to depend on the Ca2+ concentration. In 1 mM Ca2+-Ringer's and 300 nM Ca2+-Ringer's + A23187, background illumination enhanced the recovery rate of the response; however, in 10 and 100 nM Ca2+-Ringer's + A23187, the recovery rates were the same for dark- and light-adapted responses. This result implies that a critical level of Ca2+ may be necessary for background illumination to accelerate the recovery of the flash response. The sensitivity of the flash response in darkness (S(F)(D)) was dependent on the Ca2+ concentration. In 1 mM Ca2+-Ringer's, S(F)(D) was 0.481 pA per bleached rhodopsin (Rh*); a background of four Rh*/s decreased S(F)(D) by half (I(o)). At 300 nM Ca2+ + A23187, S(F)(D) was reduced to 0.0307 pA/Rh* and I(o) increased to 60 Rh*/s. At 100 nM Ca2+ + A23187, S(F)(D) was reduced further to 0.0025 pA/Rh* and I(o) increased to 220 Rh*/s. In 10 nM Ca2+ + A23187, S(F)(D) was lowered to 0.00045 pA/Rh* and I(o) raised to 760 Rh*/s. Using these values of S(F)(D) and I(o) for each respective Ca2+ concentration, the dependence of the flash sensitivity on background intensity could be described by the Weber-Fechner relation. Under low Ca2+ conditions + A 23187, bright background illumination could desensitize the flash response. These results are consistent with the idea that the concentration of Ca2+ may set the absolute magnitude of response sensitivity in darkness, and that there exist mechanisms capable of adapting the photoresponse in the absence of significant changes in cytoplasmic Ca2+ concentration.
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