EFFECTS OF GLUCOSE DEPRIVATION, CHEMICAL HYPOXIA, AND SIMULATED ISCHEMIA ON NA-CORD ASTROCYTES( HOMEOSTASIS IN RAT SPINAL)

A steep inwardly directed Na+ gradient is essential for glial function s such as glutamate reuptake and regulation of intracellular ion conce ntrations, We investigated the effects of glucose deprivation, chemica l hypoxia, and simulated ischemia on intracellular Na+ concentration ( [Na+](i)) in cultured spinal cord astrocytes using fluorescence ratio imaging with sodium binding benzofuran isophthalate (SBFI) AM. Glucose removal or chemical hypoxia (induced by 10 mM NaN3) for 60 min increa sed [Na+](i) from a baseline of 8.3 to 11 mM. Combined glycolytic and respiratory blockage by NaN3 and 0 glucose saline caused [Na+](i) to i ncrease by 20 mM, similar to the [Na+](i) increases elicited by blocki ng the Na+/K+-ATPase with ouabain. Recovery from large [Na+](i) increa ses (>15 mM) induced by the glutamatergic agonist kainate was attenuat ed during glucose deprivation or NaN3 application and was blocked in N aN3 and 0 glucose. To mimic in vivo ischemia, we exposed astrocytes to NaN3 and 0 glucose saline containing L-lactate and glutamate with inc reased [K+] and decreased [Na+], [Ca2+], and pH. This induced an [Na+] (i) decrease followed by an [Na+](i) rise and a further [Na+](i) incre ase after reperfusion with standard saline, Similar multiphasic [Na+]( i) changes were observed after NaN3 and 0 glucose saline with only red uced [Na+](e). Our results suggest that the ability to maintain a low [Na+](i) enables spinal cord astrocytes to continue uptake of K+ and/o r glutamate at the onset of energy failure. With prolonged energy fail ure, however, astrocytic [Na+](i) rises with loss of their steep trans membrane Na+ gradient, astrocytes may aggravate metabolic insults by c arrier reversal and release of acid, K+, and/or glutamate into the ext racellular space.