GLUCOSE DEPENDENCE OF GLYCOLYSIS, HEXOSE-MONOPHOSPHATE SHUNT ACTIVITY, ENERGY STATUS, AND THE POLYOL PATHWAY IN RETINAS ISOLATED FROM NORMAL (NONDIABETIC) RATS

Purpose. To measure glucose-dependent metabolic activities and selecte d parameters of the polyol pathway in retinas isolated from normal rat s to test the hypothesis recently proposed by Van den Enden et al that incubation of whole retinas for 2 hours with elevated concentrations of glucose results in activation of the polyol pathway, which is the c ause of a redox imbalance, as measured by an increase in the retinal c ytosolic lactate-pyruvate ratio and a diabetic-like state. Methods. Re tinas obtained from nondiabetic mts and separated from other ocular ti ssues were incubated for several hours in incubation medium containing glucose at concentrations ranging from 5 to 30 mM. Measurements were made under aerobic and anaerobic conditions of lactic acid production, retinal adenosine triphosphate (ATP), lactic acid content, the hexose monophosphate shunt pathway, aldose reductase activity, and levels of sorbitol and galactitol. Morphology was examined by light microscopy at the end of the incubations. Results. Incubation of isolated rat ret inas with 20 mM glucose increased lactic acid production by approximat ely 25% in comparison to the rate observed in 5 mM glucose under aerob ic and anaerobic conditions. The content of ATP and lactate in the ret inas after a 2-hour incubation in the presence of oxygen and 20 mM glu cose was equal to the amounts found in fresh tissues, whereas these me tabolites declined, respectively, by 25% and 45% when 5 mM glucose was used. The activity of the hexose monophosphate shunt pathway in isola ted rat retinas was not increased significantly when the concentration of glucose was raised from 5 to 30 mM. Aldose reductase activity and polyols were below our limits of detection, 0.5 nmol/minute mg protein and 3.5 nmol/retina, respectively, under all conditions tested. The m orphologic appearance of the retina was similar in the presence of nor mal and high concentrations of glucose. Conclusions. These results sho w that incubation of isolated rat retinas, obtained from nondiabetic r ats, with elevated concentrations of glucose for 2 hours leads to incr eases in glycolysis and a higher tissue content of lactic acid and ATP in comparison to values obtained with 5 mM glucose. However, the magn itude of the glucose-dependent increase in the retinal level of lactat e in the current study and in that of Van den Enden er al is six to se ven times greater than the calculated flux of glucose through the poly ol pathway. These results, therefore, do not support the hypothesis of Van den Enden et al. Rather, it is suggested that supranormal concent rations of glucose yield more lactate and ATP in a whole retina becaus e they optimize the supply of this essential nutrient to cells through out the tissue by overcoming diffusional limitations that result when the retina is separated from its normal choroidal and intraretinal blo od supplies.