EFFECTS OF GLUCOSE-LOAD ON BRAIN EXTRACELLULAR LACTATE CONCENTRATION IN CONSCIOUS RATS USING A MICRODIALYSIS TECHNIQUE

Changes in the brain lactate concentration in cerebral extracellular fluid (ECF) during intravenous infusion of glucose and local administration of glucose were investigated in adult, conscious, unrestrained rats, with a microdialysis probe in the posterior hippocampus. The rats were infused intravenously with either 25 % sucrose solution or 25 % glucose solution at a rate of 16.6 mul . min-1 . 100 g-1 for three hours. The blood glucose concentration reached 17.0+/-2.6 mM at the end of the glucose infusion, and brain ECF glucose showed a parallel change with the blood glucose concentration and increased to 2.37+/-0.30 mM. However, blood and brain ECF glucose concentrations did not change in animals infused with the sucrose solution. On the other hand, the blood lactate concentration in the glucose-infused group also increased from 0.93+/-0.18 mM to 2.85+/-0.39 mM at the end of the glucose infusion, which was significantly higher than that measured in the sucrose-infused group. The blood lactate level in the glucose-infused group returned to the basal level by the end of the experiment. Brain ECF lactate concentrations increased from 1.21+/-0.06 mM to 1.69+/-0.11 mM in glucose-infused animals, but did not change in the sucrose-infused animals. The brain ECF lactate concentration showed a positive correlation with the brain ECF glucose concentration in glucose-infused animals. Another group of rats was administered glucose locally for 90 min after substitution of artificial cerebrospinal fluid. As a result, the brain ECF lactate concentration increased significantly from the control level during the glucose loading period, then gradually decreased by the end of the experiment. The present study demonstrated that the brain lactate concentration increased with the brain glucose concentration.