EXTRACELLULAR GLUCOSE-CONCENTRATION IN MAMMALIAN BRAIN - CONTINUOUS MONITORING OF CHANGES DURING INCREASED NEURONAL-ACTIVITY AND UPON LIMITATION IN OXYGEN-SUPPLY IN NORMOGLYCEMIC, HYPOGLYCEMIC, AND HYPERGLYCEMIC ANIMALS

The concentration of extracellular glucose in anesthetized rat brain w as measured continuously with two types of substrate-specific microele ctrodes in a number of physiological and pathological conditions. Extr acellular glucose level increased in hyperglycemia and decreased in hy poglycemia, paralleling the changes in blood sugar. Increased neuronal activity and in particular spreading depression, evoked triphasic alt erations in extracellular glucose concentration: an initial rapid fall was followed by an equally swift overshoot above the baseline and a s ubsequent return to it. Limitation in O-2 supply led to a decline in e xtracellular content of glucose: respiration with 5% O-2 reduced the l evel by 7-20% and that with 3% O-2 by 75-85%. Decreases to undetectabl e concentrations were seen in ischemia despite the use of an oxygen-in sensitive microglucose sensor. Restoration of oxygen supply to the bra in was accompanied by increases in extracellular glucose content above the original normoxic level, which returned to baseline values after 10-15 min. In hyperglycemic animals ischemia-induced leakage of K+ was delayed while the rate of recovery to control levels after restitutio n of blood flow was enhanced. It is concluded that continuous monitori ng of glucose with glucose-specific microelectrodes provides a new and important insight into brain energy metabolism.