Uptake and localization of 3H-2 deoxy-D-glucose by retinal photoreceptors

Following dark incubation of isolated retinas of Xenopus laevis in Ringer solution supplemented with 3H-2-Deoxy-D-glucose (2DG), virtually all of the uptake of the label was by the inner segments and synaptic bases of the photoreceptor cells. Autoradiographs prepared from conventionally fixed tissue showed the same cellular distribution of label as those prepared from identically incubated, unfixed, freeze-dried retinas. However, fixation removed about 77% of the total counts. This fixation-labile, soluble fraction was identified as being primarily 2DG-6 phosphate by thin-layer chromatography. The remaining insoluble fraction corresponded in distribution to glycogen grains. In cones, glycogen is stored primarily in the paraboloid, whereas in rods it is distributed throughout the inner segment and synaptic base. EM autoradiographs illustrated that these were the sites over which fixation-resistant 2DG label was localized. Measurements of radioactivity associated with extracts of retinal glycogen following 2DG incubation demonstrated that a disproportionately high fraction of total counts were associated with the glycogen fraction. We conclude that in the amphibian retina 2DG may be incorporated into glycogen.