Membrane assembly in photoreceptor outer segments: Progressive increase in ‘open’ basal discs with increased temperature

Abstract

Rod outer segment renewal rates of 0·8, 2·1 and 2·5 μm/day were calculated from 3H-band displacement in autoradiographs following 3H-leucine injection into Xenopus laevis tadpoles maintained in cyclic lighting (12 hr light: 12 hr darkness) at 16°C, 21°C and 26°C, respectively. Based on disc packing density in rod outer segments, these displacement rates reflect the addition of 29, 76 and 91 discs/day at these temperatures. The accumulation of newly forming ‘open’ membrane discs at the outer segment base was also established for animals maintained at each of the above temperatures under the same lighting schedule. Open discs, though absent at the time of light onset, increase in number following light stimulation to reach a maximum after 6 hr light exposure. The numbers of open discs present at this time were 3·3 ± 0·2; 6·2 ± 0·1; and 9·8 ± 0·7 per rod (mean ± s.d.) at 16°C, 21°C and 26°C, respectively. Open discs decrease in number thereafter and are rarely observed 10 hr into the light cycle or during the dark phase of the diurnal cycle. Although the maximum number of open discs accumulating at 6 hr after light exposure at each temperature represents only 10% of the total number of new discs added each day, the similarity in the proportional increase in open discs to the increase in renewal rate over the temperature range utilized indicates that the number of open discs provides a useful morphological index for the relative rates of membrane assembly occurring in rod photoreceptors.