Turnover of rod photoreceptor outer segments. I. Membrane addition and loss in relationship to temperature

Membrane turnover in outer segments of Rana pipiens red rods (ROS) was studied in tadpoles maintained under cyclic lighting (12L:12D) at 23 degrees, 28 degrees, and 33 degrees C. Large fragments (greater than 2 microns in diameter or length) were shed from the ROS tips shortly after the onset of light. These were phagocytized by the pigment epithelium (PE) which caused an increase in the number of phagosomes greater than 2 microns in size (large phagosomes). Large phagosomes were present in highest numbers 2-4 h after light exposure and were degraded by 8-12 h. The proportion of ROS that shed each day after the onset of the light cycle increased with increment increases in temperatures (23 degrees C-18%, 28 degrees C-33%, 33 degrees C-42% per day), resulting, in a reduction in the average interval of time between repeated sheddings (23 degrees C-5.6 days, 28 degrees C-3 days, 33 degrees C-2.4 days) though the average numbers of disks shed from ROS at the various temperatures were not significantly different (23 degrees C-139.5 +/- 5.7, 28 degrees C-129.4 +/- 7.6, 33 degrees C-129.9 +/- 4.8 disks/shed packet). Phagosomes in the PE that were less than 2 microns in diameter (small phagosomes) were present in relatively constant numbers throughout the day, and their numbers increased at higher temperatures. The absence of a concomitant increase in small phagosomes as large phagosomes were degraded indicates that large phagosomes were not the major source of small phagosomes. When the PE was isolated to culture in the absence of the retina, these small phagosomes were degraded. The rate of disk addition to the ROS base was determined by autoradiography after [3H]leucine injection. The number of disks added per day increased with elevations of temperature (23 degrees C-32.4; 28 degrees C-55.9; 33 degrees C-65.5). The average number of disks added to the ROS between repeated sheddings (23 degrees C-181.4; 28 degrees C-167.7; 33 degrees C-157.2) was greater than the number of disks shed after light exposure. Inasmuch as the ROS show no net increase in length during the tadpole stages utilized, the remaining disks must be lost at some other time. Electron microscope analysis revealed the presence of small groups of disks in curled configurations at the tips of ROS, suggesting possible stages of detachment.(ABSTRACT TRUNCATED AT 400 WORDS)

Turnover of rod photoreceptor outer segments. II. Membrane addition and loss in relationship to light

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.

Cyclic GMP accumulation causes degeneration of photoreceptor cells: simulation of an inherited disease

Guanosine 3',5'-monophosphate (cyclic GMP) metabolism in developing eye rudiments of Xenopus laevis embryos in culture is disrupted by the phosphodiesterase inhibitor isobutylmethylxanthine. At low concentrations of inhibitor the rudiments develop normally, but at higher concentrations of the inhibitor, cyclic GMP accumulates in the rudiments and the retinal photoreceptor cells degenerate selectively. The isobutylmethylxanthine-induced photoreceptor degeneration is associated with an accumulation of cyclic GMP and, in this respect, it stimulates an early biochemical defect in the inherited degenerative disease of rd mice.

Photoreceptor outer segments: accelerated membrane renewal in rods after exposure to light

The rate of rod outer segment renewal in Rana pipiens tadpoles under constant light and under diurnal conditions of 12 or 2 hours light per day is significantly increased compared to that in animals in darkness. Furthermore, during 24 hours in light after 6 days in darkness the rate of renewal is three to four times that in darkness. In Xenopus laevis tadpoles the rate of renewal is more than five times greater during the first 8 hours of a normal diurnal cycle than during the following 16 hours. These observations demonstrate that bursts of renewal activity occur as a response to light, and suggest that a normal pattern of light alternating with darness plays a fundamental role in the regulation of rod outer segment turnover.

Renewal of normal and degenerating photoreceptor outer segments in the Ozark cave salamander

To determine whether photoreceptor degeneration in the Ozark cave salamander is associated with cessation or changes in the kinetics of outer segment (OS) renewal, an autoradiographic study of 3H-leucine incorporation in photoreceptors was carried out. Six days after isotope injection rods and cones showed labeling in both inner and outer segments. Cone OS were diffusely labeled whereas rods contained a band of radioactivity at the base of the OS. At 13 and 21 days the radioactive band in rods was located progressively nearer the distal tip of the OS. The rate of rod OS renewal ranged from 0.30 to 0.38 mu of OS length per day at 18 degrees C. L-thyroxin induced metamorphosis and light increased the renewal rate compared to larvae in darkness, and adults with photoreceptors in an early stage of degeneration had a slightly higher renewal rate than larvae. Light and electron microscope autoradiographs of degenerate photoreceptors revealed that even in the final stages of degeneration when OS are reduced to small, irregular whorls of membrane, 3H-leucine labeling was present in inner segments and OS membranes. These observations demonstrate that OS renewal occurs in both larvae and adults, and suggest that photoreceptor degeneration may be due to disruption of some aspect of the OS disposal process.

Photoreceptor thresholds and visual pigment levels in normal and vitamin A-deprived Xenopus tadpoles

1. Measurements of the aspartate-isolated PIII component of the electroretinogram (ERG) were used to estimate photoreceptor threshold in dark-adapted tadpoles of the clawed toad, Xenopus laevis raised on a normal diet. Spectral sensitivity functions established that the rod mechanism governed absolute dark thresholds from stage 40 to the end of premetamorphic development. 2. Parallel measures of rod outer-segment dimensions and visual pigment levels demonstrated a) that visual pigment concentration remained constant at all tadpole stages, and b) that the fall in threshold over the course of premetamorphic development depended exclusively on the increased probability of quantal absorption that accompanied the growth of the rod outer segments. 3. When tadpoles were obtained from vitamin A-deficient females and raised subsequently on a vitamin A-free diet, the first appearance of the ERG was delayed and its absolute threshold raised, with respect to controls. 4. Histological examination of vitamin A-deprived retinas indicated that rod outer segments retained their structural integrity in spite of up to a 75% loss of visual pigment. 5. The threshold rise which accompanied a fall in visual pigment levels, whether effected by dietary deficiency or by bleaching, was greater than could be attributed solely to a reduction in the probability of quantal absorption.