Postprandial and Fasting Hepatic Glucose Fluxes in Long-Standing Type 1 Diabetes
Postprandial and Fasting Hepatic Glucose Fluxes in Long-Standing Type 1 Diabetes
Objective Intravenous insulin infusion partly improves liver glucose fluxes in type 1 diabetes (T1D). This study tests the hypothesis that continuous subcutaneous insulin infusion (CSII) normalizes hepatic glycogen metabolism.
Research Design and Methods T1D with poor glycemic control (T1Dp; HbA1c: 8.5 ± 0.4%), T1D with improved glycemic control on CSII (T1Di; 7.0 ± 0.3%), and healthy humans (control subjects [CON]; 5.2 ± 0.4%) were studied. Net hepatic glycogen synthesis and glycogenolysis were measured with in vivo C magnetic resonance spectroscopy. Endogenous glucose production (EGP) and gluconeogenesis (GNG) were assessed with [6,6-H2]glucose, glycogen phosphorylase (GP) flux, and gluconeogenic fluxes with H2O/paracetamol.
Results When compared with CON, net glycogen synthesis was 70% lower in T1Dp (P = 0.038) but not different in T1Di. During fasting, T1Dp had 25 and 42% higher EGP than T1Di (P = 0.004) and CON (P < 0.001; T1Di vs. CON: P = NS). GNG was 74 and 67% higher in T1Dp than in T1Di (P = 0.002) and CON (P = 0.001). In T1Dp, GP flux (7.0 ± 1.6 μmol · kg · min) was twofold higher than net glycogenolysis, but comparable in T1Di and CON (3.7 ± 0.8 and 4.9 ± 1.0 μmol · kg · min). Thus T1Dp exhibited glycogen cycling (3.5 ± 2.0 μmol · kg · min), which accounted for 47% of GP flux.
Conclusions Poorly controlled T1D not only exhibits augmented fasting gluconeogenesis but also increased glycogen cycling. Intensified subcutaneous insulin treatment restores these abnormalities, indicating that hepatic glucose metabolism is not irreversibly altered even in long-standing T1D.
The liver is responsible for raising endogenous glucose production (EGP) to maintain constant plasma glucose concentrations mainly via gluconeogenesis during fasting or via glycogenolysis as first-line response to hypoglycemia. Patients with type 1 diabetes (T1D) not only have blunted glycogen synthesis but also impaired glycogenolysis during hypoglycemia. This, in concert with impaired counterregulatory hormonal responses, leads to a diminished defense against hypoglycemia, one of the major concerns of insulin treatment.
Short-term normoglycemia, induced by investigator-controlled variable intravenous insulin infusion, improves nocturnal hepatic glycogen synthesis in well-controlled T1D. In everyday life, patients with T1D attempt to achieve near-normoglycemia by intensified insulin therapy using frequent insulin injections or continuous subcutaneous insulin infusion (CSII) pumps. However, the effects on pathways of hepatic glucose metabolism during the critical nocturnal fed-to-fasting transition have hitherto not been described under real life conditions.
The postprandial fluxes of hepatic glucose metabolism during the first 6 h after meal ingestion can be described by a metabolic model (Fig. 1) and were measured in this study in all groups. In addition to net substrate fluxes such as conversion of glycogen to glucose, the model includes the futile exchange between hepatic glycogen and glucose-6-phosphate (G6P) that is driven by simultaneous glycogen synthase (GS) and glycogen phosphorylase (GP) fluxes, a process known as glycogen cycling. Although net glycogenolytic fluxes have been previously characterized in T1D, no measurements of glycogen cycling have been reported in these patients. Aside from its possible relevance to metabolic alterations in T1D, glycogen cycling dilutes the enrichment of hepatic G6P from gluconeogenic tracers independently of net glycogenolytic flux. Thus, with such methodologies, the contribution of glycogenolysis to EGP may be overestimated, whereas that of gluconeogenesis is correspondingly underestimated. Resolution of glycogen cycling from net glycogenolytic and gluconeogenic contributions requires that the gluconeogenic tracer assay be supplemented by an independent measurement of either net glycogenolysis (in vivo C magnetic resonance spectroscopy [MRS]) or absolute GS flux (isotope dilution of labeled galactose at the level of uridine diphosphate UDP-glucose).
(Enlarge Image)
Figure 1.
Metabolic model representing fluxes between G6P, glycogen, glucose, and the parameters of glycogenolytic flux derived by H2O and C MR methods. Component fluxes include GS flux, GP, GNG, and EGP. The in vivo C MRS assay measures the net loss of hexose from the pool of glycogen metabolites (i.e., net glycogenolysis), and GNG is calculated as EGP − net glycogenolysis. Net glycogenolysis represents the difference between GP and GS, hence the fraction of EGP derived from net glycogenolytic flux is equal to (GP − GS)/EGP. The H2O method measures the fractional contribution of GP to EGP flux. When GS is zero, net glycogenolysis and GP are equal. During glycogen cycling, where GS is active, GP is higher than net glycogenolysis.
Glycogen cycling and net substrate fluxes were resolved for humans with type 2 diabetes (T2D) or liver cirrhosis by integrating direct in vivo C MRS measurements of net glycogen depletion with isotopic tracer measurements of EGP, gluconeogenesis, and GP fluxes. Here, we tested the hypothesis that patients with CSII-treated T1D should have normal hepatic glycogen metabolism by applying an integrated approach comprising direct C MRS measurements of liver glycogen and simultaneous assessment of fluxes through GP, glycogen cycling, EGP, and GNG using [6,6-H2]glucose and H2O/glucuronide.
Abstract and Introduction
Abstract
Objective Intravenous insulin infusion partly improves liver glucose fluxes in type 1 diabetes (T1D). This study tests the hypothesis that continuous subcutaneous insulin infusion (CSII) normalizes hepatic glycogen metabolism.
Research Design and Methods T1D with poor glycemic control (T1Dp; HbA1c: 8.5 ± 0.4%), T1D with improved glycemic control on CSII (T1Di; 7.0 ± 0.3%), and healthy humans (control subjects [CON]; 5.2 ± 0.4%) were studied. Net hepatic glycogen synthesis and glycogenolysis were measured with in vivo C magnetic resonance spectroscopy. Endogenous glucose production (EGP) and gluconeogenesis (GNG) were assessed with [6,6-H2]glucose, glycogen phosphorylase (GP) flux, and gluconeogenic fluxes with H2O/paracetamol.
Results When compared with CON, net glycogen synthesis was 70% lower in T1Dp (P = 0.038) but not different in T1Di. During fasting, T1Dp had 25 and 42% higher EGP than T1Di (P = 0.004) and CON (P < 0.001; T1Di vs. CON: P = NS). GNG was 74 and 67% higher in T1Dp than in T1Di (P = 0.002) and CON (P = 0.001). In T1Dp, GP flux (7.0 ± 1.6 μmol · kg · min) was twofold higher than net glycogenolysis, but comparable in T1Di and CON (3.7 ± 0.8 and 4.9 ± 1.0 μmol · kg · min). Thus T1Dp exhibited glycogen cycling (3.5 ± 2.0 μmol · kg · min), which accounted for 47% of GP flux.
Conclusions Poorly controlled T1D not only exhibits augmented fasting gluconeogenesis but also increased glycogen cycling. Intensified subcutaneous insulin treatment restores these abnormalities, indicating that hepatic glucose metabolism is not irreversibly altered even in long-standing T1D.
Introduction
The liver is responsible for raising endogenous glucose production (EGP) to maintain constant plasma glucose concentrations mainly via gluconeogenesis during fasting or via glycogenolysis as first-line response to hypoglycemia. Patients with type 1 diabetes (T1D) not only have blunted glycogen synthesis but also impaired glycogenolysis during hypoglycemia. This, in concert with impaired counterregulatory hormonal responses, leads to a diminished defense against hypoglycemia, one of the major concerns of insulin treatment.
Short-term normoglycemia, induced by investigator-controlled variable intravenous insulin infusion, improves nocturnal hepatic glycogen synthesis in well-controlled T1D. In everyday life, patients with T1D attempt to achieve near-normoglycemia by intensified insulin therapy using frequent insulin injections or continuous subcutaneous insulin infusion (CSII) pumps. However, the effects on pathways of hepatic glucose metabolism during the critical nocturnal fed-to-fasting transition have hitherto not been described under real life conditions.
The postprandial fluxes of hepatic glucose metabolism during the first 6 h after meal ingestion can be described by a metabolic model (Fig. 1) and were measured in this study in all groups. In addition to net substrate fluxes such as conversion of glycogen to glucose, the model includes the futile exchange between hepatic glycogen and glucose-6-phosphate (G6P) that is driven by simultaneous glycogen synthase (GS) and glycogen phosphorylase (GP) fluxes, a process known as glycogen cycling. Although net glycogenolytic fluxes have been previously characterized in T1D, no measurements of glycogen cycling have been reported in these patients. Aside from its possible relevance to metabolic alterations in T1D, glycogen cycling dilutes the enrichment of hepatic G6P from gluconeogenic tracers independently of net glycogenolytic flux. Thus, with such methodologies, the contribution of glycogenolysis to EGP may be overestimated, whereas that of gluconeogenesis is correspondingly underestimated. Resolution of glycogen cycling from net glycogenolytic and gluconeogenic contributions requires that the gluconeogenic tracer assay be supplemented by an independent measurement of either net glycogenolysis (in vivo C magnetic resonance spectroscopy [MRS]) or absolute GS flux (isotope dilution of labeled galactose at the level of uridine diphosphate UDP-glucose).
(Enlarge Image)
Figure 1.
Metabolic model representing fluxes between G6P, glycogen, glucose, and the parameters of glycogenolytic flux derived by H2O and C MR methods. Component fluxes include GS flux, GP, GNG, and EGP. The in vivo C MRS assay measures the net loss of hexose from the pool of glycogen metabolites (i.e., net glycogenolysis), and GNG is calculated as EGP − net glycogenolysis. Net glycogenolysis represents the difference between GP and GS, hence the fraction of EGP derived from net glycogenolytic flux is equal to (GP − GS)/EGP. The H2O method measures the fractional contribution of GP to EGP flux. When GS is zero, net glycogenolysis and GP are equal. During glycogen cycling, where GS is active, GP is higher than net glycogenolysis.
Glycogen cycling and net substrate fluxes were resolved for humans with type 2 diabetes (T2D) or liver cirrhosis by integrating direct in vivo C MRS measurements of net glycogen depletion with isotopic tracer measurements of EGP, gluconeogenesis, and GP fluxes. Here, we tested the hypothesis that patients with CSII-treated T1D should have normal hepatic glycogen metabolism by applying an integrated approach comprising direct C MRS measurements of liver glycogen and simultaneous assessment of fluxes through GP, glycogen cycling, EGP, and GNG using [6,6-H2]glucose and H2O/glucuronide.