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

Hidde H. Huidekoper 0 1 3 4 Gepke Visser 0 1 3 4 Maritte T. Ackermans 0 1 3 4 Hans P. Sauerwein 0 1 3 4 Frits A. Wijburg 0 1 3 4 0 M. T. Ackermans Department of Clinical Chemistry, Laboratory of Endocrinology, University of Amsterdam , Amsterdam, The Netherlands 1 Communicated by: Jean-Marie Saudubray 2 ) Department of Pediatrics (G8-205) Academic Medical Center, University Hospital of Amsterdam , PO Box 22660 NL-1100 DD( Amsterdam, The Netherlands 3 G. Visser Wilhelmina Children's Hospital, University Medical Center Utrecht , Utrecht, The Netherlands 4 H. P. Sauerwein Department of Endocrinology & Metabolism, Academic Medical Center, University of Amsterdam , Amsterdam, The Netherlands Background A potential role for muscle in glucose homeostasis was recently suggested based on characterization of extrahepatic and extrarenal glucose-6-phosphatase (glucose6-phosphatase-). 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 Competing interest: None declared. - 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 mol/kg per min (30% of normal EGP after an overnight fast), GGL 3.09 mol/kg per min, and GNG 0.75 mol/kg per min. The patient with FBPase deficiency had EGP 8.53 mol/kg per min (62% of normal EGP after an overnight fast), GGL 6.89 mol/kg per min GGL, and GNG 1.64 mol/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. Endogenous glucose production (EGP) during fasting is predominantly derived from hepatic gluconeogenesis (GNG) and glycogenolysis (GGL), with a minor contribution from renal GNG (Ekberg et al. 1999). Recently, a potential additional role for muscle in EGP has been suggested based on characterization of an isoform of glucose-6-phosphatase, glucose-6-phosphatase- (Glc-6Pase-) expressed in muscle and other extrahepatic tissue (Martin et al. 2002; Shieh et al. 2003). Gl-6-Pase- has been shown to have structural and functional properties in muscle comparable with glucose-6-phosphatase- expressed in liver, kidney, and intestine (EC 3.1.3.9; Glc6-Pase-) (Shieh et al. 2004). As patients with glycogen storage disease 1a (GSD-1a; OMIM #232200) are deficient for Glc-6-Pase-, resulting in defective hepatic and renal GNG and GGL, Gl-6-Pase- activity in muscle might explain the residual EGP previously observed in these patients (Kalhan et al. 1982; Schwenk et al. 1986; Tsalikian et al. 1984; Weghuber et al. 2007). In order to investigate the potential role of extrahepatic and extrarenal tissue in glucose homeostasis during fasting in vivo, we performed whole-body kinetic studies in a patient with GSD-1a and a patient with fructose-1,6-bisphosphatase (FBPase) deficiency (OMIM #229700), an inborn error of hepatic and renal GNG. For the first time, differential contributions of GGL and GNG to EGP during fasting were quantified in these disorders using the [6,6-2H2]glucose isotope dilution method combined with the deuterated water method (Landau et al. 1996; Wolfe et al. 2005). Materials and methods Patient 1 presented with severe hypoglycemia (plasma glucose 0.3 mmol/L) and hepatomegaly at the age of 4 months. GSD-1a was diagnosed on the the basis of a complete deficiency of glucose-6-phosphatase activity in a fresh liver biopsy. This diagnosis was later confirmed by mutation analysis revealing two mutations known to completely abolish Glc-6-Pase- activity (Table 1) (Rake et al. 2000). Patient 2 was admitted at 11 months because of convulsions due to hypoglycemia. At this time, she exhibited severe metabolic acidosis with hyperlactatemia and a marked hepatomegaly. She was diagnosed with FBPase deficiency by repeated demonstration of undetectable enzyme activity in leucocytes (Table 1) (Baker et al. 1970). The in vivo stable isotope studies were approved by the Institutional Review Board. Both patients and their parents gave informed consent prior to the studies. Fasting tests were performed at the age of 17.9 and of 16.7 years, respectively. Both patients were admitted 1 day before the test. An intravenous catheter was inserted into antecubital veins of both arms after topical application of lidocaine cream. One catheter was used to administer [6,6-2H2]glucose and the other for blood sampling. At baseline, a blood sample was collected to determine background enrichment of deuterated water in plasma. Fasting was started at a time considered safe based on previous experience with fasting in the patients. Prior to fasting, both patients consumed their regular evening meal. Patient 1 received nocturnal nasogastric drip feeding without glucose polymers. This drip feeding was discontinued 2 h prior to initiation of [6,6-2H2]glucose infusion and substituted by an unlabeled glucose infusion at a rate of 5 mg/kg per min, which was continued until the start of the [6,6-2H2]glucose infusion. Both patients remained fasted throughout the test and maintained bed rest (Fig. 1). Twelve hours prior to [6,6-2H2]glucose infusion, both patients drank deuterium-enriched water (99% pure; Cambridge Isotope Laboratories, Cambridge, MA, USA) at a dose of 5 g/kg body water divided in five doses within 120 min (Ackermans et al. 2001). The total amount of body water (kg) was estimated as 60% of body weight (kg) (Friis-Hansen 1961). Thereafter, patients were only allowed to drink tap water enriched to 0.5% with deuterated water until the end of the test. At the start of the fasting test in patient 1, after 10 h of fasting in patient 2, and after collection of a blood sample to determine background enrichment of Table 1 Patient characteristics Sex Age (years) Height (m) Inborn error of metabolism Enzyme activity (normal range) 1.76 (-1 SD) 75.0 (+1.5 SD) Glucose-6-phosphatase deficiency (GSD Ia) 1.50 (-2 SD) 60.0 ( +2 SD) Fructose-1,6bisphosphatase deficiency 0.0 (1030) nmol/min/mg proteina R170X F327b <0.1 (320) nmol/min/mg proteinc ND Patient 1 (GSD-1a) fasting time (h) fasting time (h) Patient 2 (FBPase deficiency) Fig. 1 Study protocols in patients 1 [glycogen storage disease type 1a ( (...truncated)