Insulin and Insulin-like Growth Factor 1 (IGF-1) Modulate Cytoplasmic Glucose and Glycogen Levels but Not Glucose Transport across the Membrane in Astrocytes*

Background: Astrocytes contain glycogen, an energy buffer in the brain. Results: Stimulation with insulin and IGF-1 decreases cytosolic glucose concentration in astrocytes without affecting glucose entry across the astrocyte plasma membrane. Conclusion: Insulin and IGF-1 boost the process of glycogen formation. Significance: This is the first high temporal resolution measurement of the decrease of glucose levels in astrocytes resulting from insulin and IGF-1 stimulation. Astrocytes contain glycogen, an energy buffer, which can bridge local short term energy requirements in the brain. Glycogen levels reflect a dynamic equilibrium between glycogen synthesis and glycogenolysis. Many factors that include hormones and neuropeptides, such as insulin and insulin-like growth factor 1 (IGF-1) likely modulate glycogen stores in astrocytes, but detailed mechanisms at the cellular level are sparse. We used a glucose nanosensor based on Förster resonance energy transfer to monitor cytosolic glucose concentration with high temporal resolution and a cytochemical approach to determine glycogen stores in single cells. The results show that after glucose depletion, glycogen stores are replenished. Insulin and IGF-1 boost the process of glycogen formation. Although astrocytes appear to express glucose transporter GLUT4, glucose entry across the astrocyte plasma membrane is not affected by insulin. Stimulation of cells with insulin and IGF-1 decreased cytosolic glucose concentration, likely because of elevated glucose utilization for glycogen synthesis.