The rate of disk addition to rod outer segments (ROS) varies widely in Xenopus laevis tadpoles kept in cyclic light (12L:12D). When measured as radioactive band (3H-band) displacement during the 2nd day after injection of [3H]leucine, 75% of the daily increment of displacement occurred during the first 8 h of light. During the same interval, the number of open disks at the ROS base increased more than threefold. During the last 8 h of darkness, 3H-band displacement was undetectable and the number of open disks was reduced. These observations suggest the possibility that disk addition may occur discontinuously. During the 3rd and 4th days after injection of [3H]leucine, maximal displacement of the 3H-band occurred later in the day than on the 2nd day, its movement no longer corresponding to the increase in open disks. This delay in 3H-band displacement may reflect a time delay as a result of propagation of compressive stress in an elastic ROS system. Maximal disk loss from ROS as reflected in counts of phagosomes in the pigment epithelium occurred within 1 h of light exposure, and phagosome counts remained high for 4 h before declining to a low level in darkness. Modified lighting regimes affected the daily rhythms of shedding and disk addition differently, suggesting that control mechanisms for the two processes are not directly coupled. During 3 days in darkness, disk addition was reduced 50% compared to controls (12L:12D), whereas shedding was reduced by about 40%. Although reduced in level, shedding occurred as a free-running circadian rhythm. There was no evidence of rhythmicity of disk addition in darkness. In constant light, the rate of disk addition was not different from controls, but shedding was reduced by about 80% after the 1st day. This resulted in a 21% increase in ROS length. Among animals kept on a 2.5L:21.5D cycle, the rate of disk addition was reduced by 40% while shedding was maintained near control levels, resulting in a slight decrease in ROS length. These observations indicate that normal shedding requires alternating light and darkness, and that the daily rhythm of disk addition is due primarily to daily stimulation by light.
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