Pyruvate protects glucose-deprived Müller cells from nitric oxide-induced oxidative stress by radical scavenging

The cellular defense of Müller cells against oxidative and nitrosative stress was examined after the addition of the nitric oxide donor papanonoate. Glucose concentrations of > or = 550 microM efficiently protected the Müller cells from cell death by maintaining high ATP and glutathione and allowing only a moderate increase of free radicals. Fluorescence-activated cell sorting (FACS) analysis showed that 22% of the cells underwent apoptosis whereas necrosis was strongly suppressed. Under glucose deprivation, the intracellular concentration of ATP declined to 15% after 1 h; glutathione dropped to 50% within 2 h after papanonoate addition. Both the number of cells containing excess free radicals and the mean concentration of free radicals increased twofold at 0.5-2 h of incubation with papanonoate. Cell death switched from prevailing apoptosis to massive necrosis and cell viability decreased drastically. Several metabolites of glycolysis, gluconeogenesis, and the pentose phosphate pathway were tested with respect to their capability to protect the stressed Müller cells. 2 mM pyruvate was found to enhance cell viability 1.6-fold predominantly by reducing the necrotic cell demise. It could be shown that pyruvate did not act by improving the energy status of Müller cells but by scavenging excess free radicals. Inhibition of the monocarboxylate transporters in Müller cells by alpha-cyano-4-hydroxycinnamate abolished this effect. Other 2-ketoacids, like oxalacetate, 2-ketoglutarate and 2-ketobutyrate had a similar protecting effect as pyruvate. Lactate, glutamate, 2-deoxyglucose, and ribose 5-phosphate did not protect Müller cells against oxidative and nitrosative stress.