Rod outer segment (ROS) renewal as a mechanism for adaptation to a new intensity environment. II. Rhodopsin synthesis and packing density.

Using intraocular injection of a 3H-phe and 3H-leu mixture, we found that the net synthesis of rhodopsin is light-intensity-dependent and can adjust when an animal encounters a new lighting environment. Rhodopsin net synthesis dropped dramatically in day 1, 200-lx immigrants; however, preliminary studies show that the opsin mRNA levels in these animal were the same as that found in the 200-lx-native group. This suggests that the changes in the net synthesis of rhodopsin found when an animal is moved to a higher intensity light may be controlled at some point post-transcriptionally. Microspectrophotometric measurements on single rod cells revealed that the differences in whole-eye rhodopsin levels between the two cyclic intensity groups, 3 lx and 200 lx, were also present at the single cell level. This supports previous studies suggesting that the density of rhodopsin per disc varies according to the intensity of light to which the animal is exposed. Individual rods also had differences in packing density of rhodopsin at the base compared to the tip of the same outer segment when the animal had been switched to a new intensity for one half of a turnover period (5 days). This indicates the amount of rhodopsin per disc is regulated and suggests that renewal is the process responsible for creating a modified outer segment. Animals, when switched to a new intensity, can adjust the synthesis of rhodopsin and the number of molecules added per disk to ensure the appropriate packing density of rhodopsin in the rod outer segment (ROS) disk membranes for that level of light intensity.