Elevated glucose levels increase retinal glycolysis and sorbitol pathway metabolism. Implications for diabetic retinopathy.

PURPOSE: To assess effects of elevated glucose levels on retinal glycolysis and sorbitol pathway metabolism.
METHODS: Freshly isolated retinas from normal male Sprague-Dawley rats were incubated for 2 hours at 37 degrees C, pH 7.45, in Krebs bicarbonate-Hepes buffer containing 5, 10, 20, or 30 mM glucose. Glycolytic metabolites, sorbitol, and fructose were measured in extracts of retina and medium.
RESULTS: Elevated glucose levels increased retinal levels of sorbitol and triose phosphates, decreased sn-glycerol-3-phosphate levels, increased lactate and fructose production, and increased the retinal lactate-pyruvate ratio (indicative of an increased cytosolic ratio of free NADH-NAD+ like that induced by hypoxia). An inhibitor of aldose reductase (AL 4114) normalized sorbitol, fructose, triose phosphates, and the lactate-pyruvate ratio without affecting lactate production or sn-glycerol 3-phosphate levels.
CONCLUSIONS: Elevation of retinal glucose levels causes a hypoxia-like redox imbalance "pseudohypoxia" that results from increased oxidation of sorbitol to fructose in the second step of the sorbitol pathway. This redox imbalance provides a plausible explanation for impaired regulation of retinal blood flow (in the absence of vascular structural changes) in humans with diabetes and in nondiabetic acutely hyperglycemic animals. These findings, together with other observations, suggest that this redox imbalance precedes, and may contribute to, hypoxic and ischemic retinopathy associated with diabetes.