Onset of changes in glucose transport across ocular barriers in streptozotocin-induced diabetes.

In a previous study, measurements were made of facilitated and passive transport of glucose, using [3H]-3-O-methyl-D-glucose and [14C]-L-glucose, respectively, across blood-aqueous and blood-vitreous barriers in long-term streptozotocin-diabetic rats. It was found that passive transport was increased, while facilitated transport was decreased, possibly due to saturation of the transport system. The present study examines the appearance of these changes in glucose transport at various times following streptozotocin (STZ) injection. Passive transport, as indicated by the L-glucose rate constant, began to increase at about 10 days following induction of diabetes, stabilized at the elevated rates at 50-60 days and persisted during the 170-day period of observation. Rate constants for [3H]-3-O-methyl-D-glucose transport decreased within 1 day following induction of diabetes. Prevention of hyperglycemia by insulin treatment upon onset of diabetes prevented the latter changes ruling out a direct effect of STZ. Glucose infusion into normal rats produced a similar decrease in 3-O-methylglucose transport constants suggesting that hyperglycemia was responsible for the early decrease in facilitated transport found in the diabetic rats. It is speculated that increased passive transport of glucose may reflect an early loss in ocular barrier integrity. The later decrease in carrier facilitated transport cannot be explained by hyperglycemia alone and, thus, a loss in carrier function is suggested. Despite a decrease in facilitated transport, absolute glucose entry rates are increased in the diabetic due to elevated plasma glucose, which serves as an inward driving force, due to the significantly increased entry of glucose by the passive route.