Light cycle--dependent axial variations in frog rod outer segment structure.

Polarized light microscopy reveals that the structural parameters of Rana pipiens rod outer segments (ROS) are not uniform along the cell axis. In addition to a pronounced birefringence (delta n) gradient found in the basal half of most ROS, periodic delta n bands are seen in approximately 10% of intact ROS isolated by agitating retinas in frog Ringer's solution. These small delta n differences appear as very faint light and dark striations that have a period and width that depends on the duration of light and dark exposure. In ROS from frogs kept on a 14 hr light/10 hr dark cycle at 20 degrees to 22.5 degrees C, the band period for a light-dark band pair is 1.0 to 1.6 micron. Portions of ROS produced during total darkness or constant light are free of distinct periodic bands. Quantitative delta n measurements show that the ROS sections generated in the dark have a relatively higher delta n than those produced in light. Band contrast is irreversibly enhanced when ROS are treated with the calcium ionophore A23187 in the presence of calcium-free saline solution. These results indicate that the synthesis of some calcium-sensitive ROS component is different when the frog is in the dark than when exposed to light.