Modulation of VEGF production by pH and glucose in retinal Muller cells

Purpose. To investigate the influence of pH and glucose concentration, both of which represent significant biochemical variables in tissue ischemia, on the production of VEGF protein by retinal Muller cells and C6 glioma cells, under normoxic and hypoxic conditions. Methods. Rat retinal Muller cells and C6 glioma cells grown in tissue culture monolayers were studied. The effect of pH (range 7.0-8.0) and glucose concentration (0.6-25 mmol/L) on VEGF protein production, under both normoxic and hypoxic conditions, were evaluated by ELISA analysis of the conditioned media. Establishment of significant cell hypoxia was verified by measurement of lactate release into the conditioned media. Results. Hypoxia caused a 7.9-fold increase in VEGF production in C6 cells at 24 h, and a 3.4-fold increase in Muller cells after 48 h. Under hypoxic conditions, VEGF protein production was increased further by increasing pH and increasing glucose, and decreased by low pH and low glucose. Varying the glucose concentration or pH of the medium did not result in significant induction of VEGF protein production by either cell type under normoxic conditions. Conclusions. Both glucose and pH significantly affected VEGF production induced by low oxygen. However, neither exerted a measurable stimulatory effect on VEGF production in normoxic conditions. Coexisting hypoxia and acidosis or hypoglycemia, as might occur in severe tissue ischemia, may render glial cells incapable of effectively upregulating VEGF synthesis, while alkalosis or hyperglycemia may augment hypoxia-induced VEGF production.