CYCLIC METABOLISM OF PHOTORECEPTORS AND RETINAL PIGMENT EPITHELIUM IN THE FROG

Abstract

The diurnal patterns of 3H-uridine, 3H-leucine and 3H-mannose incorporation by red rods, single cones and pigment epithelial cells were evaluated utilizing quantitative autoradiography. Adult Rana pipiens, previously maintained under a 14L:10D lighting cycle for 3-4 months, were injected with 3H-labeled precursors 3 hrs prior to killing and were sampled throughout a diurnal cycle. In rods, the rate of 3H-uridine incorporation was highest during the middle of the light cycle and was lowest in the middle of the dark cycle, whereas 3H-leucine and 3H-mannose incorporation was more rapid during the latter hours of the dark cycle and lowest during the light cycle, out of phase by approximately 12 hrs from the pattern of 3H-uridine incorporation. In cones, 3H-uridine incorporation was highest during the middle of the light cycle and was lowest during the dark cycle, similar in pattern but reduced in amplitude from the changes observed in 3H-uridine incorporation in rods. 3H-leucine incorporation was high a few hours into the light cycle and just prior to the onset of the dark cycle. Relatively few grains were present over the cone myoids following 3H-mannose injection with no cyclic changes evident. In the pigment epithelium, 3H-uridine incorporation showed two peaks of rapid incorporation, both during the light portion of the diurnal cycle, whereas incorporation dropped to a low level in the middle of the dark cycle. 3H-leucine incorporation was most rapid a few hours into the light cycle, decreasing through the middle portion of the light cycle and then increasing prior to the onset of the dark cycle with this rate sustained throughout the remainder of the dark cycle. 3H-mannose incorporation showed little change throughout the diurnal cycle. In each of these three cell types, 3H-uridine (RNA) incorporation occurred most rapidly during the early part of the light cycle followed by rapid rates of 3H-leucine (protein) incorporation at later times in the day. These cyclic changes in the incorporation of precursors of RNA, protein and glycoprotein in the photoreceptor pigment epithelium complex are discussed in relationship to the times of membrane addition and loss from photoreceptors and times of phagocytosis by the pigment epithelium.