Glucose-6-phosphatase activity in the retina of the awake rat

In the 2-deoxyglucose technique, the rate of glucose utilization in small areas of the central nervous system is measured using a standard operational equation that assumes negligible glucose-6-phosphatase activity. Hoping to apply this technique in the mammalian retina, we sought to identify the extent of this enzyme's activity in vivo. [2-3H] glucose but not [U-14C] glucose loses its label during metabolism and returns to the glucose precursor pool in the presence of glucose-6-phosphatase. Accordingly, a decline of 3H/14C in the retinal glucose pool with time indicates glucose-6-phosphatase activity. We injected a mixture of [2-3H] glucose and [U-14C] glucose into the internal carotid artery of 10 awake rats via a previously inserted catheter. Plasma samples were collected and the eyes enucleated at timed intervals. The eyes were immediately frozen, freeze-dried and dissected to obtain retina. Radiolabeled glucose was separated using ion exchange and paper chromatography prior to scintillation counting. The 3H/14C ratio was found to decline at a statistically significant rate of about 2.5% per minute, indicating glucose-6-phosphatase activity. However, an estimate of the turnover of retinal glucose suggests that glucose-6-phosphatase dephosphorylates a minimal percentage of the glucose entering the glycolytic pathway, allowing application of the standard operational equation to the mammalian retina in vivo.