Noradrenaline‐induced l ‐lactate production requires d ‐glucose entry and transit through the glycogen shunt in single‐cultured rat astrocytes

During cognitive efforts mediated by local neuronal networks, approximately 20% of additional energy is required; this is mediated by chemical messengers such as noradrenaline (NA). NA targets astroglial aerobic glycolysis, the hallmark of which is the end product l ‐lactate, a fuel for neurons. Biochemical studies have revealed that astrocytes exhibit a prominent glycogen shunt, in which a portion of d ‐glucose molecules entering the cytoplasm is transiently incorporated into glycogen, a buffer and source of d ‐glucose during increased energy demand. Here, we studied single astrocytes by measuring cytosolic L‐lactate ([lac] i ) with the FRET nanosensor Laconic. We examined whether NA‐induced increase in [lac] i is influenced by: (a) 2‐deoxy‐ d ‐glucose (2‐DG, 3 mM), a molecule that enters the cytosol and inhibits the glycolytic pathway; (b) 1,4‐dideoxy‐1,4‐imino‐d‐arabinitol (DAB, 300 µM), a potent inhibitor of glycogen phosphorylase and glycogen degradation; and (c) 3‐nitropropionic acid (3‐NPA, 1 mM), an inhibitor of the Krebs cycle. The results of these pharmacological experiments revealed that d ‐glucose uptake is essential for the NA‐induced increase in [lac] i , and that this exclusively arises from glycogen degradation, indicating that most, if not all, d ‐glucose molecules in NA‐stimulated cells transit the glycogen shunt during glycolysis. Moreover, under the defined transmembrane d ‐glucose gradient, the glycolytic intermediates were not only used to produce l ‐lactate, but also to significantly support oxidative phosphorylation, as demonstrated by an elevation in [lac] i when Krebs cycle was inhibited. We conclude that l ‐lactate production via aerobic glycolysis is an essential energy pathway in NA‐stimulated astrocytes; however, oxidative metabolism is important at rest.