A potential role for muscle in glucose homeostasis: in vivo kinetic studies in glycogen storage disease type 1a and fructose‐1,6‐bisphosphatase deficiency

Abstract

Background A potential role for muscle in glucose homeostasis was recently suggested based on characterization of extrahepatic and extrarenal glucose‐6‐phosphatase (glucose‐6‐phosphatase‐β). To study the role of extrahepatic tissue in glucose homeostasis during fasting glucose kinetics were studied in two patients with a deficient hepatic and renal glycogenolysis and/or gluconeogenesis. Design Endogenous glucose production (EGP), glycogenolysis (GGL), and gluconeogenesis (GNG) were quantified with stable isotopes in a patient with glycogen storage disease type 1a (GSD‐1a) and a patient with fructose‐1,6‐bisphosphatase (FBPase) deficiency. The [6,6‐²H₂]glucose dilution method in combination with the deuterated water method was used during individualized fasting tests. Results Both patients became hypoglycemic after 2.5 and 14.5 h fasting, respectively. At that time, the patient with GSD‐1a had EGP 3.84 μmol/kg per min (30% of normal EGP after an overnight fast), GGL 3.09 μmol/kg per min, and GNG 0.75 μmol/kg per min. The patient with FBPase deficiency had EGP 8.53 μmol/kg per min (62% of normal EGP after an overnight fast), GGL 6.89 μmol/kg per min GGL, and GNG 1.64 μmol/kg per min. Conclusion EGP was severely hampered in both patients, resulting in hypoglycemia. However, despite defective hepatic and renal GNG in both disorders and defective hepatic GGL in GSD‐1a, both patients were still able to produce glucose via both pathways. As all necessary enzymes of these pathways have now been functionally detected in muscle, a contribution of muscle to EGP during fasting via both GGL as well as GNG is suggested.