Effect of acute hyperglycemia on oxygen and oxidative metabolism in the intact cat retina

PURPOSE. The Crabtree effect is the phenomenon of inhibition of respiration by glycolysis, as a result of elevated glucose levels. It is not certain whether the Crabtree effect occurs in the retina, which has a high glycolytic capacity. In the current study, in vivo photoreceptor oxygen consumption was examined during the normo- and hyperglycemic states in the dark-adapted cat retina to determine whether the Crabtree effect occurs in the outer retina. METHODS. Spatial profiles of oxygen tension were obtained in the cat retina, in vivo, with the use of oxygen microelectrodes during control conditions and acute (5.19 ± 0.83 hour) episodes of hyperglycemia (blood glucose, >350 mg/dL). The outer retinal portions of the profiles were fitted to a model of oxygen diffusion to quantify photoreceptor oxygen consumption. RESULTS. Photoreceptor oxygen consumption did not significantly change during hyperglycemia compared with control conditions. Choroidal PO₂ decreased during hyperglycemia by an average of 5.8 ± 7.4 mm Hg. This led to an increase in the fraction of O₂ used by the photoreceptors that was derived from the inner retina. Choroidal PO₂ did not recover when blood glucose levels were returned to normal. Average inner retinal PO₂ was not affected by the episodes of hyperglycemia. CONCLUSIONS. The Crabtree effect does not occur to any significant degree in the outer retina, because hyperglycemia did not affect photoreceptor oxygen consumption. Choroidal PO₂ decreased during hyperglycemia, and the oxygen deficit was made up by the retinal circulation.