Astrocytes respond to energetic demands by upregulating glycolysis, lactate production, and respiration. This study addresses the role of respiration and calcium regulation of respiration as part of the astrocyte response to the workloads caused by extracellular ATP and glutamate. Extracellular ATP (100 μM to 1 mM) causes a Ca2+ -dependent workload and fall of the cytosolic ATP/ADP ratio which acutely increases astrocytes respiration. Part of this increase is related to a Ca2+ -dependent upregulation of cytosolic pyruvate production. Conversely, glutamate (200 μM) causes a Na+ , but not Ca2+ , dependent workload even though glutamate-induced Ca2+ signals readily reach mitochondria. The glutamate workload triggers a rapid fall in the cytosolic ATP/ADP ratio and stimulation of respiration. These effects are mimicked by D-aspartate a nonmetabolized agonist of the glutamate transporter, but not by a metabotropic glutamate receptor agonist, indicating a major role of Na+ -dependent workload in stimulated respiration. Glutamate-induced increase in respiration is linked to a rapid increase in glycolytic pyruvate production, suggesting that both glutamate and extracellular ATP cause an increase in astrocyte respiration fueled by workload-induced increase in pyruvate production. However, glutamate-induced pyruvate production is partly resistant to glycolysis blockers (iodoacetate), indicating that oxidative consumption of glutamate also contributes to stimulated respiration. As stimulation of respiration by ATP and glutamate are similar and pyruvate production smaller in the first case, the results suggest that the response to extracellular ATP is a Ca2+ -dependent upregulation of respiration added to glycolysis upregulation. The global contribution of astrocyte respiratory responses to brain oxygen consumption is an open question.
Keywords: ATP; astrocytes; calcium; glutamate; pyruvate; respiration; workload.
© 2019 Wiley Periodicals, Inc.