Prolongation of Actions of Ca²+ Early in Phototransduction by 9-Demethylretinal

During adaptation Ca²+ acts on a step early in phototransduction, which is normally available for only a brief period after excitation. To investigate the identity of this step, we studied the effect of the light-induced decline in intracellular Ca²+ concentration on the response to a bright flash in normal rods, and in rods bleached and regenerated with 11-cis 9-demethylretinal, which forms a photopigment with a prolonged photoactivated lifetime. Changes in cytoplasmic Ca²+ were opposed by rapid superfusion of the outer segment with a 0Na⁺/0Ca²+ solution designed to minimize Ca²+ fluxes across the surface membrane. After regeneration of a bleached rod with 9-demethlyretinal, the response in Ringer's to a 440-nm bright flash was prolonged in comparison with the unbleached control, and the response remained in saturation for 10–15s. If the dynamic fall in Ca²+_i induced by the flash was delayed by stepping the outer segment to 0Na⁺/0Ca²+ solution just before the flash and returning it to Ringer's shortly before recovery, then the response saturation was prolonged further, increasing linearly by 0.41 ± 0.01 of the time spent in this solution. In contrast, even long exposures to 0Na⁺/0Ca²+ solution of rods containing native photopigment evoked only a modest response prolongation on the return to Ringer's. Furthermore, if the rod was preexposed to steady subsaturating light, thereby reducing the cytoplasmic calcium concentration, then the prolongation of the bright flash response evoked by 0Na⁺/0Ca²+ solution was reduced in a graded manner with increasing background intensity. These results indicate that altering the chromophore of rhodopsin prolongs the time course of the Ca²+-dependent step early in the transduction cascade so that it dominates response recovery, and suggest that it is associated with photopigment quenching by phosphorylation.