Membrane addition to photoreceptor outer segments: progressive reduction in the stimulatory effect of light with increased temperature.

Previous studies have indicated that temperature and light regimens affect the rates of membrane addition to rod outer segments in amphibia. However, it is not known how membrane renewal responds to simultaneous alterations of these two environmental factors. In the present study, the rates of membrane addition have been assessed in adult Rana pipiens maintained in three lighting conditions (constant light, constant darkness, and cyclic light) at three different temperatures by following the displacement of a radioactive band in autoradiographs after 3H-leucine injection. At 16 degrees, 21 degrees, and 26 degrees C, constant light increases the rate of membrane addition in red rods above the rates which occur in darkness by 39%, 14%, and 9%, respectively, whereas in the green rods, the rates of increase were 41%, 22%, and 8%. This progressive reduction in the stimulatory effect of light suggest that at the higher temperatures, rod outer segment renewal is proceeding at near maximal rates and can no longer be further accelerated by light stimulation.