Synthesis and degradation of retinal proteins in darkness and during photic stimulation

Rabbit retinas were maintained in a physiological state in vitro and exposed to 0.5-h pulses of labeled leucine. Protein synthesis was determined from incorporation of the label, and degradation from its subsequent release. The retinas were treated as test-control pairs. The control retina remained in darkness while the test retina was subjected to photic stimulation, either during labeling to determine the effect on synthesis or after labeling to determine the effect on degradation. 3H and 14C alternated as test and control labels. The two retinas were combined for solubilization. Their proteins were separated according to size by gel electrophoresis. Each gel was cut into 95 slices and each slice was differentially counted. The isolated retinas synthesized new protein rapidly and reproducibly. The average S.D. of the isotope ratios measured on gel slices from replicate experiments was 2.2% of the mean. Retinas driven by flashing light of constant intensity exhibited a marked increase in ganglion cell firing and accumulated 38% more (p = 0.005) 2-deoxyglucose than their controls kept in darkness. However, they did not differ from the controls in the incorporation of labeled leucine into total protein or into any of the protein fractions separated on the gel, the largest deviation from unity in the 95 test-control ratios being 1.7%. Continuous light and flashing light of increasing intensity also markedly affected function and energy metabolism, but had no significant effects on leucine incorporation. None of the stimuli affected degradation, though the experiments would have been less sensitive to a change in degradation that to a change in synthesis. These results indicate that the synthesis and degradation of proteins are little affected by a marked increase in functional activity.