Relationship between L-glutamate-regulated intracellular Na⁺ dynamics and ATP hydrolysis in astrocytes

Glutamate uptake into astrocytes and the resulting increase in intracellular Na⁺ (Na⁺_i) have been identified as a key signal coupling excitatory neuronal activity to increased glucose utilization. Arguments based mostly on mathematical modeling led to the conclusion that physiological concentrations of glutamate more than double astrocytic Na⁺/K⁺-ATPase activity, which should proportionally increase its ATP hydrolysis rate. This hypothesis was tested in the present study by fluorescence monitoring of free Mg²⁺ (Mg ²⁺i), a parameter that inversely correlates with ATP levels. Glutamate application measurably increased Mg²⁺_i (i.e. decreased ATP), which was reversible after glutamate washout. Na ⁺_i and ATP changes were then directly compared by simultaneous Na⁺_i and Mg²⁺ imaging. Glutamate increased both parameters with different rates and blocking the Na ⁺/K⁺-ATPase during the glutamate-evoked Na ⁺_i response, resulted in a drop of Mg²⁺ _i levels (i.e. increased ATP). Taken together, this study demonstrates the tight correlation between glutamate transport, Na⁺ homeostasis and ATP levels in astrocytes.