Glucose metabolism

Schematic representation of the major regulatory steps in hepatic gluconeogenesis (arrows that point upward) and glycolysis (arrows that point downward). The circles with positive or negative signs represent the reactions that are stimulated or inhibited by glucagon. Glucagon promotes gluconeogenesis and blocks glycolysis by decreasing the activity of phosphofructokinase (PFK) and pyruvate kinase (PK). The sites of entry of the gluconeogenetic precursors (glycerol, alanine, and lactate) are shown. Muscle cells lack glucose-6-phosphatase (G6Pase); they can therefore use glycogen for energy but cannot convert it to glucose for release into the systemic circulation. In pancreatic beta cells, glucokinase (GK) acts as the glucose sensor that regulates insulin release.