Effects of D- and L-Glucose and Mannitol on Retinal Capillary Cells: Inhibition by Nanomolar Aminoguanidine

Hyperglycemia may contribute directly to pericyte loss and capillary leakage in early diabetic retinopathy. To elucidate relative contributions of glycation, glycoxidation, sugar autoxidation, osmotic stress and metabolic effects in glucose-mediated capillary damage, we tested the effects of D-glucose, L-glucose, mannitol and the potentially protective effects of aminoguanidine on cultured bovine retinal capillary pericytes and endothelial cells. Media (containing 5 mM D-glucose) were supplemented to increase the concentration of each sugar by 5, 10, or 20 mM. Subconfluent pericytes and endothelial cells were exposed to the supplemented media in the presence or absence of aminoguanidine (1 nM-100 µM) for three days. Cell counts, viability and protein were determined. For both cell types, all three sugars produced concentration-dependent decreases in cell counts and protein content (p<0.001), with D-glucose being the most toxic (p<0.001). The effects of L-glucose and mannitol were similar, suggesting an osmotic contribution to the observed toxicity. Very low concentrations of aminoguanidine, in the range 0.1-1 µM, abolished the toxicity of all three sugars towards both cell types. The data suggest that D-glucose-mediated toxicity has both intra- and extracellular components and that both can be blocked by aminoguanidine. The efficacy of aminoguanidine at nanomolar concentrations suggests an action through scavenging reactive carbonyls (whether generated by oxidative or metabolic processes) and/or by enzyme inhibition. In addition, aminoguanidine may protect against the consequences of osmotic stress.