ENERGETIC DEMANDS OF THE NA+ K+ ATPASE IN MAMMALIAN ASTROCYTES/

Cultured astrocytes and cell lines derived therefrom maintain a high e nergy level ([ATP]/[ADP]) through operation of oxidative phosphorylati on and glycolysis. The contribution from the latter to total ATP produ ction is 25-32%. A powerful Na+/K+ pump maintains potassium, sodium, a nd calcium gradients out of equilibrium. [Na+](i) is about 20 mM, [K+] (i) is 130 mM and [Ca2+](i) is less than 100 mM. Under non-stimulated conditions, the Na+/K+ ATPase consumes 20% of astrocytic ATP productio n. Inhibition of the pump by ouabain decreases energy expenditure, rai ses [creatine phosphate]/[creatine], and leads to a leakage of sodium, potassium, and calcium ions. Decrease in the pump function via a fall in [ATP] also collapses ion gradients; the rate and extent of the fal l correlates positively with cellular energy state. Under ''normal'' c onditions (i.e., when ATP production pathways are not inhibited), ther e appears to be no preferential utilization of energy produced by eith er glycolysis or oxidative phosphorylation for the support of pump fun ction. (C) 1997 Wiley-Liss, Inc.