Contributions of net hepatic glycogenolysis and gluconeogenesis to glucose production in cirrhosis

Sarcopenia defined with L3-SMI is an independent predictor of survival in male patients with ARLD in mainland China

Background

The burden of alcohol-related liver disease (ARLD) is increasing in China. Patients with ARLD are more likely to have comorbid sarcopenia, which may impair their survival. This study aimed to evaluate the relationship between the prognoses of patients with ARLD and sarcopenia, identified using the skeletal muscle index at the third lumbar vertebra level (L3-SMI).

Methods

Hospitalized patients with ARLD were retrospectively enrolled between 2015 and 2018 and followed up for 24 months to evaluate their survival profiles. Cox proportional hazards regression models were used to estimate patient survival factors. A receiver operating characteristic curve was created to identify the cut-off point of the L3-SMI for predicting the prognoses of Chinese patients with ARLD.

Results

The study enrolled 168 male patients with ARLD who were followed-up for 24 months or until a study endpoint was met. The overall L3-SMI in patients with ARLD was 42.61 ± 9.15 cm2/m2, and 42.86% (72/168) of patients with ARLD were comorbid with sarcopenia. The overall survival in patients with ARLD was 77.38% at 24 months. The survival rate of patients with sarcopenia was lower than that of patients without sarcopenia (66.67% vs. 85.42%, p = 0.004). Multiple Cox regression analysis showed that sarcopenia, abstinence, and baseline creatinine level were independent prognostic factors of 24-month survival with hazard ratios (95% confidence intervals) of 2.022 (1.025-3.991), 0.275 (0.122-0.617), and 1.018 (1.008-1.027), respectively. The cut-off value of the L3-SMI for predicting 24-month survival was 40.0 cm2/m2 for male patients with ARLD.

Conclusion

Sarcopenia is an independent mortality risk factor in male patients with ARLD in mainland China. Early diagnosis and intervention of sarcopenia are important for optimizing the management of patients with ARLD.

Postprandial Glycogen Content Is Increased in the Hepatocytes of Human and Rat Cirrhotic Liver

Chronic hepatitises of various etiologies are widespread liver diseases in humans. Their final stage, liver cirrhosis (LC), is considered to be one of the main causes of hepatocellular carcinoma (HCC). About 80-90% of all HCC cases develop in LC patients, which suggests that cirrhotic conditions play a crucial role in the process of hepatocarcinogenesis. Carbohydrate metabolism in LC undergoes profound disturbances characterized by altered glycogen metabolism. Unfortunately, data on the glycogen content in LC are few and contradictory. In this study, the material was obtained from liver biopsies of patients with LC of viral and alcohol etiology and from the liver tissue of rats with CCl₄-induced LC. The activity of glycogen phosphorylase (GP), glycogen synthase (GS), and glucose-6-phosphatase (G6Pase) was investigated in human and rat liver tissue by biochemical methods. Total glycogen and its labile and stable fractions were measured in isolated individual hepatocytes, using the cytofluorometry technique of PAS reaction in situ. The development of LC in human and rat liver was accompanied by an increase in fibrous tissue (20- and 8.8-fold), an increase in the dry mass of hepatocytes (by 25.6% and 23.7%), and a decrease in the number of hepatocytes (by 50% and 28%), respectively. The rearrangement of the liver parenchyma was combined with changes in glycogen metabolism. The present study showed a significant increase in the glycogen content in the hepatocytes of the human and the rat cirrhotic liver, by 255% and 210%, respectively. An increased glycogen content in cells of the cirrhotic liver can be explained by a decrease in glycogenolysis due to a decreased activity of G6Pase and GP.

Malnutrition in Patients with Liver Cirrhosis

Liver cirrhosis is an increasing public health threat worldwide. Malnutrition is a serious complication of cirrhosis and is associated with worse outcomes. With this review, we aim to describe the prevalence of malnutrition, pathophysiological mechanisms, diagnostic tools and therapeutic targets to treat malnutrition. Malnutrition is frequently underdiagnosed and occurs-depending on the screening methods used and patient populations studied-in 5-92% of patients. Decreased energy and protein intake, inflammation, malabsorption, altered nutrient metabolism, hypermetabolism, hormonal disturbances and gut microbiome dysbiosis can contribute to malnutrition. The stepwise diagnostic approach includes a rapid prescreen, the use of a specific screening tool, such as the Royal Free Hospital Nutritional Prioritizing Tool and a nutritional assessment by dieticians. General dietary measures-especially the timing of meals-oral nutritional supplements, micronutrient supplementation and the role of amino acids are discussed. In summary malnutrition in cirrhosis is common and needs more attention by health care professionals involved in the care of patients with cirrhosis. Screening and assessment for malnutrition should be carried out regularly in cirrhotic patients, ideally by a multidisciplinary team. Further research is needed to better clarify pathogenic mechanisms such as the role of the gut-liver-axis and to develop targeted therapeutic strategies.

Metabolic Signature of Hepatic Fibrosis: From Individual Pathways to Systems Biology

Hepatic fibrosis is a major cause of morbidity and mortality worldwide, as it ultimately leads to cirrhosis, which is estimated to affect up to 2% of the global population. Hepatic fibrosis is confirmed by liver biopsy, and the erroneous nature of this technique necessitates the search for noninvasive alternatives. However, current biomarker algorithms for hepatic fibrosis have many limitations. Given that the liver is the largest organ and a major metabolic hub in the body, probing the metabolic signature of hepatic fibrosis holds promise for the discovery of new markers and therapeutic targets. Regarding individual metabolic pathways, accumulating evidence shows that hepatic fibrosis leads to alterations in carbohydrate metabolism, as aerobic glycolysis is aggravated in activated hepatic stellate cells (HSCs) and the whole fibrotic liver; in amino acid metabolism, as Fischer’s ratio (branched-chain amino acids/aromatic amino acids) decreases in patients with hepatic fibrosis; and in lipid metabolism, as HSCs lose vitamin A-containing lipid droplets during transdifferentiation, and cirrhotic patients have decreased serum lipids. The current review also summarizes recent findings of metabolic alterations relevant to hepatic fibrosis based on systems biology approaches, including transcriptomics, proteomics, and metabolomics in vitro, in animal models and in humans.

Sarcopenia in cirrhosis: from pathogenesis to interventions

Sarcopenia (severe muscle depletion) is a prevalent muscle abnormality in patients with cirrhosis that confers poor prognosis both pre- and post-liver transplantation. The pathogenesis of sarcopenia is multifactorial and results from an imbalance between protein synthesis and breakdown. Nutritional, metabolic, and biochemical abnormalities seen in chronic liver disease alter whole body protein homeostasis. Hyperammonemia, increased autophagy, proteasomal activity, lower protein synthesis, and impaired mitochondrial function play an important role in muscle depletion in cirrhosis. Factors including cellular energy status, availability of metabolic substrates (e.g., branched-chain amino acids), alterations in the endocrine system (insulin resistance, circulating levels of insulin, insulin-like growth factor-1, corticosteroids, and testosterone), cytokines, myostatin, and exercise are involved in regulating muscle mass. A favored atrophy of type II fast-twitch glycolytic fibers seems to occur in patients with cirrhosis and sarcopenia. Identification of muscle biological abnormalities and underlying mechanisms is required to plan clinical trials to reverse sarcopenia through modulation of specific mechanisms. Accordingly, a combination of nutritional, physical, and pharmacological interventions might be necessary to reverse sarcopenia in cirrhosis. Moderate exercise should be combined with appropriate energy and protein intake, in accordance with clinical guidelines. Interventions with branched chain amino acids, testosterone, carnitine, or ammonia-lowering therapies should be considered individually. Various factors such as dose, type, duration of supplementations, etiology of cirrhosis, amount of dietary protein intake, and compliance with supplementation and exercise should be the focus of future large randomized controlled trials investigating both prevention and treatment of sarcopenia in this patient population.

Nutrition in Pediatric Chronic Liver Disease

More than one-half of children with chronic liver disease suffer from malnutrition, which leads not only to a poor quality of life and even possibly catastrophic complications, but also to poor outcomes after a liver transplantation. These children have increased metabolic demands but often decreased intake with malabsorption and altered nutrient utilization, all of which make it difficult to keep up with nutritional demands. Assessment of a patient's nutritional status should be timely, and it should be performed routinely and proactively. When specific nutritional needs are identified, these should be addressed with a multidisciplinary team approach and with the close guidance of an experienced pediatric dietician. The assessment includes anthropometric and laboratory assessments, in addition to a careful physical examination and a detailed patient history. The specific nutritional needs vary, but generally dietary intervention focuses on increasing caloric intake, supplementation with medium-chain triglycerides, and prevention of essential fatty acid and fat-soluble vitamin deficiencies. [Pediatr Ann. 2018;47(11):e445-e451.].

Metabolomic Profiling of Portal Blood and Bile Reveals Metabolic Signatures of Primary Sclerosing Cholangitis

Primary sclerosing cholangitis (PSC) is a pathogenically complex, chronic, fibroinflammatory disorder of the bile ducts without known etiology or effective pharmacotherapy. Emerging in vitro and in vivo evidence support fundamental pathophysiologic mechanisms in PSC centered on enterohepatic circulation. To date, no studies have specifically interrogated the chemical footprint of enterohepatic circulation in PSC. Herein, we evaluated the metabolome and lipidome of portal venous blood and bile obtained at the time of liver transplantation in patients with PSC (n = 7) as compared to individuals with noncholestatic, end-stage liver disease (viral, metabolic, etc. (disease control, DC, n = 19)) and to nondisease controls (NC, living donors, n = 12). Global metabolomic and lipidomic profiling was performed on serum derived from portal venous blood (portal serum) and bile using ultraperformance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) and differential mobility spectroscopy-mass spectroscopy (DMS-MS; complex lipid platform). The Mann–Whitney U test was used to identify metabolites that significantly differed between groups. Principal-component analysis (PCA) showed significant separation of both PSC and DC from NC for both portal serum and bile. Metabolite set enrichment analysis of portal serum and bile demonstrated that the liver-disease cohorts (PSC and DC) exhibited similar enrichment in several metabolite categories compared to NC. Interestingly, the bile in PSC was uniquely enriched for dipeptide and polyamine metabolites. Finally, analysis of patient-matched portal serum and biliary metabolome revealed that these biological fluids were more homogeneous in PSC than in DC or NC, suggesting aberrant bile formation and enterohepatic circulation. In summary, PSC and DC patients exhibited alterations in several metabolites in portal serum and bile, while PSC patients exhibited a unique bile metabolome. These specific alterations in PSC are amenable to hypothesis testing and, potentially, therapeutic pharmacologic manipulation.

Should Sarcopenia Increase Priority for Transplant or Is It a Contraindication?

PURPOSE OF REVIEW

The purpose of this review is to discuss the current evidence regarding the impact of sarcopenia on patients with cirrhosis awaiting liver transplantation and to determine if its presence should be considered a criterion for expedited transplantation or a contraindication for transplantation.

RECENT FINDINGS

Sarcopenia is a negative predictor of survival in patients on a waiting list and after liver transplant. The gut-liver axis and the liver-muscle axis have been explored to understand the complex pathophysiology of sarcopenia. Sarcopenia is a frequent finding in patients with cirrhosis. The diagnosis is ideally based on cross-sectional image analysis (CT or MRI) and treatment consists of optimization of caloric and protein intake. To date, prioritizing tools for liver transplantation have not included nutrition or sarcopenia parameters. Patients with a low Model for End-Stage Liver Disease (MELD) or MELD-Na score and sarcopenia would benefit from prioritization for transplant in order to reduce time on waiting list and therefore mortality.

Re-evaluation of glycated hemoglobin and glycated albumin with continuous glucose monitoring system as markers of glycemia in patients with liver cirrhosis

Liver cirrhosis (LC) is frequently accompanied by glucose intolerance. The present study was designed to determine whether glycated hemoglobin A1c (HbA1c) and glycated albumin (GA) were predictive markers of glycemia, as determined by a continuous glucose monitoring system (CGMS), in patients with LC. A total of 30 patients with LC, including 3, 19, 5, 2 and 1 with LC due to hepatitis B virus, hepatitis C virus, non-alcoholic steatohepatitis, alcohol and unknown causes, respectively, were assessed by CGMS. The average, maximum and minimum blood glucose (BG) levels were measured by CGMS, and correlated with HbA1c and GA. The average, maximum and minimum BG in these individuals were 142±38.7, 209.3±65.7 and 85.1±25.4 mg/dl, respectively. HbA1c was significantly correlated with average BG (r=0.447, P=0.015) and maximum BG (r=0.523, P=0.004). In addition, GA was significantly correlated with average BG (r=0.687, P<0.001) and maximum BG (r=0.648, P<0.001). Neither HbA1c nor GA was significantly correlated with minimum BG. Correlation analysis yielded formulas by which HbA1c and GA were predictive of average BG in individuals with LC: Average BG=19.2 × HbA1c (%) + 36.5 and average BG=6.6 × GA (%) + 13.0, respectively. In conclusion, HbA1c and GA showed significant correlations with average and maximum BG, as determined by CGMS. The derived formulas allow for estimates of average BG based on HbA1c and GA, and may contribute to the control of glycemia in patients with LC.

Identification of recurrence-related serum microRNAs in hepatocellular carcinoma following hepatectomy

Hepatocellular carcinoma (HCC) is one of the most deadly tumors. Prognosis of patients with HCC is generally poor due to the high recurrence rate. In the present study, TaqMan Real-time PCR microRNA Array was used to identify differentially expressed miRNAs from 10 tumor tissue samples (5 from recurrence group vs. 5 from non-recurrence group) and the matched serum samples. Four differentially expressed miRNAs (miR-486–5p, miR-422a, miR-125b and miR-139–5p) were further quantified in 20 tumor tissues and 116 HCC patients' serum before they received hepatectomy. Univariate analysis revealed that miR-486–5p, miR-422a and miR-125b were significantly associated with patients' relapse free survival (RFS). Multivariate analysis demonstrated that miR-486–5p, AFP and microvascular invasion (MVI) were the independent prognostic factors associated with RFS in this cohort (p = 0.000, 0.043, 0.000, respectively). Besides, the expression levels of miR-486–5p were positively correlated in tumor tissues and the paired serum samples, so was miR-422a. The probability of the prognostic accuracy of miR-486–5p in predicting postoperative recurrence of HCC within the first year was 76.79% (65.38% specificity and 81.58% sensitivity), which was almost equal to the classifier established by combination of AFP and MVI (75.98% probability, 63.13% specificity and 85.90% sensitivity). Furthermore, the combination of AFP, MVI and miR-486–5p yielded a ROC curve area of 88.02% (69.20% specificity and 92.10% sensitivity). Our study was the first to identify that serum miR-486–5p could be used to stratify the patients with higher recurrence risk before hepatic resection and potentially guide more effective surveillance strategies for them.

Controlled-release mitochondrial protonophore reverses diabetes and steatohepatitis in rats

Nonalcoholic fatty liver disease (NAFLD) is a major factor in the pathogenesis of type 2 diabetes (T2D) and nonalcoholic steatohepatitis (NASH). The mitochondrial protonophore 2,4 dinitrophenol (DNP) has beneficial effects on NAFLD, insulin resistance, and obesity in preclinical models but is too toxic for clinical use. We developed a controlled-release oral formulation of DNP, called CRMP (controlled-release mitochondrial protonophore), that produces mild hepatic mitochondrial uncoupling. In rat models, CRMP reduced hypertriglyceridemia, insulin resistance, hepatic steatosis, and diabetes. It also normalized plasma transaminase concentrations, ameliorated liver fibrosis, and improved hepatic protein synthetic function in a methionine/choline-deficient rat model of NASH. Chronic treatment with CRMP was not associated with any systemic toxicity. These data offer proof of concept that mild hepatic mitochondrial uncoupling may be a safe and effective therapy for the related epidemics of metabolic syndrome, T2D, and NASH.

Treatment to improve nutrition and functional capacity evaluation in liver transplant candidates

OPINION STATEMENT

Liver transplantation is the definitive therapy for cirrhosis, and malnutrition is the most frequent complication in these patients. Sarcopenia or loss of muscle mass is the major component of malnutrition in cirrhotics and adversely affects their outcome. In addition to the metabolic consequences, functional consequences of sarcopenia include reduced muscle strength and deconditioning. Despite nearly universal occurrence of sarcopenia and its attendant complications, there are no established therapies to prevent or reverse the same. Major reasons for this deficiency include the lack of established standardized definitions or measures to quantify muscle mass, as well as paucity of mechanistic studies or identified molecular targets to develop specific therapeutic interventions. Anthropometric evaluation, bioelectrical impedance analysis, and DEXA scans are relatively imprecise measures of muscle mass, and recent data on imaging measures to determine muscle mass accurately are likely to allow well-defined outcome responses to treatments. Resurgence of interest in the mechanisms of muscle loss in liver disease has been directly related to the rapid advances in the field of muscle biology. Metabolic tracer studies on whole body kinetics have been complemented by direct studies on the skeletal muscle of cirrhotics. Hypermetabolism and anabolic resistance contribute to sarcopenia. Reduced protein synthesis and increased autophagy have been reported in cirrhotic skeletal muscle, while the contribution of the ubiquitin-proteasome pathway is controversial. Increased plasma concentration and skeletal muscle expression of myostatin, a TGFβ superfamily member that causes reduction in muscle mass, have been reported in cirrhosis. Hyperammonemia and TNFα have been reported to increase myostatin expression and may be responsible for sarcopenia in cirrhosis. Nutriceutical interventions with leucine enriched amino acid mixtures, myostatin antagonists and physical activity hold promise as measures to reverse sarcopenia. There is even less data on muscle function and deconditioning in cirrhosis and studies in this area are urgently needed. Even though macronutrient replacement is a major therapeutic goal, micronutrient supplementation, specifically vitamin D, is expected to improve outcomes.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Hepatocytes of cirrhotic rat liver accumulate glycogen more slowly than normal ones

PURPOSE

To investigate the accumulation of glycogen in cirrhotic rat liver at several time intervals after per os administration of glucose to fasted animals.

METHODS

Liver cirrhosis was produced by inhalation of the hepatotropic poison CCl4. Glycogen concentration in the liver was determined biochemically. Glycogen content in hepatocytes was measured cytofluorimetrically in the smears stained with a fluorescent PAS reaction. Glycogen content in the hepatocytes of the portal and the central zone of the liver lobule was determined by absorption cytophotometry.

RESULTS

Rats poisoned with CCl4 for 6 months developed typical liver cirrhosis characterized by a fourfold (p < 0.001) increase in the proportion of the connective tissue. In the cirrhotic rats fasted for 48 h, glycogen concentration in the liver and glycogen content in hepatocytes were lower as compared with the control by 36 and 27 % (p < 0.01), respectively. According to data obtained by different methods, the control animals accumulated glycogen at a high rate. In particular, the glycogen content in hepatocytes increased by 34 % after 10 min (p < 0.01). In the cirrhotic rats, glycogen content remained at the same level for 20 min. In both groups of animals, hepatocytes of the portal zone accumulated more glycogen than those of the central zone.

CONCLUSIONS

Glycogen accumulation in cirrhotic rats starts after a delay and proceeds at a lower rate than in the norm.

Sarcopenia and a physiologically low respiratory quotient in patients with cirrhosis: a prospective controlled study

Patients with cirrhosis have increased gluconeogenesis and fatty acid oxidation that may contribute to a low respiratory quotient (RQ), and this may be linked to sarcopenia and metabolic decompensation when these patients are hospitalized. Therefore, we conducted a prospective study to measure RQ and its impact on skeletal muscle mass, survival, and related complications in hospitalized cirrhotic patients. Fasting RQ and resting energy expenditure (REE) were determined by indirect calorimetry in cirrhotic patients (n = 25), and age, sex, and weight-matched healthy controls (n = 25). Abdominal muscle area was quantified by computed tomography scanning. In cirrhotic patients we also examined the impact of RQ on mortality, repeat hospitalizations, and liver transplantation. Mean RQ in patients with cirrhosis (0.63 ± 0.05) was significantly lower (P < 0.0001) than healthy matched controls (0.84 ± 0.06). Psoas muscle area in cirrhosis (24.0 ± 6.6 cm(2)) was significantly (P < 0.001) lower than in controls (35.9 ± 9.5 cm(2)). RQ correlated with the reduction in psoas muscle area (r(2) = 0.41; P = 0.01). However, in patients with cirrhosis a reduced RQ did not predict short-term survival or risk of developing complications. When REE was normalized to psoas area, energy expenditure was significantly higher (P < 0.001) in patients with cirrhosis (66.7 ± 17.8 kcal/cm(2)) compared with controls (47.7 ± 7.9 kcal/cm(2)). We conclude that hospitalized patients with cirrhosis have RQs well below the traditional lowest physiological value of 0.69, and this metabolic state is accompanied by reduced skeletal muscle area. Although low RQ does not predict short-term mortality in these patients, it may reflect a decompensated metabolic state that requires careful nutritional management with appropriate consideration for preservation of skeletal muscle mass.

Use of (2)H(2)O for estimating rates of gluconeogenesis: determination and correction of error due to transaldolase exchange

The use of deuterated water as a method to measure gluconeogenesis has previously been well validated and is reflective of normal human physiology. However, there has been concern since the method was first introduced that transaldolase exchange may lead to the overestimation of gluconeogenesis. We examined the impact of transaldolase exchange on the estimation of gluconenogenesis using the deuterated water method under a variety of physiological conditions in humans by using the gluconeogenic tracer [U-(13)C]propionate, (2)H(2)O, and (2)H/(13)C nuclear magnetic resonance (NMR) spectroscopy. When [U-(13)C]propionate was used, (13)C labeling inequality occurred between the top and bottom halves of glucose in individuals fasted for 12-24 h who were weight stable (n = 18) or had lost weight via calorie restriction (n = 7), consistent with transaldolase exchange. Similar analysis of glucose standards revealed no significant difference in the total (13)C enrichment between the top and bottom halves of glucose, indicating that the differences detected were biological, not analytical, in origin. This labeling inequality was attenuated by extending the fasting period to 48 h (n = 12) as well as by dietary carbohydrate restriction (n = 7), both conditions associated with decreased glycogenolysis. These findings were consistent with a transaldolase effect; however, the resultant overestimation of gluconeogenesis in the overnight-fasted state was modest (7-12%), leading to an error of 14-24% that was easily correctable by using either a simultaneous (13)C gluconeogenic tracer or a correction nomogram generated from data in the present study.

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; HbA(1c): 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 (13)C magnetic resonance spectroscopy. Endogenous glucose production (EGP) and gluconeogenesis (GNG) were assessed with [6,6-(2)H(2)]glucose, glycogen phosphorylase (GP) flux, and gluconeogenic fluxes with (2)H(2)O/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(-1) ⋅ min(-1)) was twofold higher than net glycogenolysis, but comparable in T1Di and CON (3.7 ± 0.8 and 4.9 ± 1.0 μmol ⋅ kg(-1) ⋅ min(-1)). Thus T1Dp exhibited glycogen cycling (3.5 ± 2.0 μmol ⋅ kg(-1) ⋅ min(-1)), 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.

Impaired gluconeogenesis in a porcine model of paracetamol induced acute liver failure

AIM: To investigate glucose homeostasis and in particular gluconeogenesis in a large animal model of acute liver failure (ALF).

METHODS: Six pigs with paracetamol induced ALF under general anaesthesia were studied over 25 h. Plasma samples were withdrawn every five hours from a central vein. Three animals were used as controls and were maintained under anaesthesia only. Using ¹H NMR spectroscopy we identified most gluconeogenic amino acids along with lactate and pyruvate in the animal plasma samples.

RESULTS: No significant changes were observed in the concentrations of the amino acids studied in the animals maintained under anaesthesia only. If we look at the ALF animals, we observed a statistically significant rise of lactate (P < 0.003) and pyruvate (P < 0.018) at the end of the experiments. We also observed statistically significant rises in the concentrations of alanine (P < 0.002), glycine (P < 0.005), threonine (P < 0.048), tyrosine (P < 0.000), phenylalanine (P < 0.000) and isoleucine (P < 0.01). Valine levels decreased significantly (P < 0.05).

CONCLUSION: Our pig model of ALF is characterized by an altered gluconeogenetic capacity, an impaired tricarboxylic acid (TCA) cycle and a glycolytic state.

Measurement of gluconeogenesis by deuterated water: the effect of equilibration time and fasting period

Fasting gluconeogenesis (GNG) is often quantified using the 2H2O technique, which is based on plasma 2H2O enrichment and ensuing enrichment of plasma glucose at the C5 and C2 positions. Fractional (fr)GNG can be calculated using the ratio of C5 to C2 enrichment or the ratio of C5 to plasma 2H2O enrichment. For the latter, equilibration of 2H2O and C2 is required. The optimal equilibration period of 2H2O and C2 remains to be elucidated. In six healthy male subjects fasted for 18 h, we studied the effects of 3-, 5-, and 15-h 2H2O incubation periods on 1) the equilibration of plasma 2H2O and C2 glucose enrichment, 2) the measurement of frGNG, and 3) C5 labeling of hepatic glycogen after 1 mg of glucagon administration. After 3-h 2H2O incubation, plasma 2H2O and C2 were not equilibrated, frGNG C5/2H2O and C5/C2 were also different as was gluconeogenesis calculated with C5/2H2O and C5/C2. After 5- and 15-h 2H2O incubation, plasma 2H2O and C2 were equilibrated, and frGNG C5/2H2O and C5/C2 were similar, as was GNG calculated with C5/2H2O and C5/C2. After glucagon administration, no difference of C5 enrichment was found between 3, 5, and 15 h of 2H2O incubation. In conclusion, for reliable measurement of GNG in healthy subjects with C5/2H2O incubation periods longer than 3 h are required. After 5- and 15-h 2H2O incubation, GNG can be reliably measured with C5/2H2O. Gluconeogenetic labeling of glycogen did not affect the results after 3, 5, or 15 h of 2H2O incubation.

Chronic ethanol consumption decreases mitochondrial and glycolytic production of ATP in liver

AIMS

The synthesis of ATP in the liver of the chronic ethanol consumer is suppressed, particularly if the tissue becomes hypoxic. Moreover, the perivenous region of the liver lobule becomes even more oxygen deficient as a result of ethanol consumption. Synthesis of ATP in the perivenous region of the lobule may be depressed in the chronic ethanol consumer due to decreases in both mitochondrial and glycolytic activities. In this study the effects of hypoxia on hepatic ATP levels derived from synthesis by both oxidative phosphorylation and the glycolytic mechanisms were investigated.

METHODS

Rats were pair-fed liquid diets containing 36% of calories as ethanol or an isocaloric control diet. The contributions of glycolysis and mitochondria to ATP production were assessed employing oligomycin, an inhibitor of oxidative phosphorylation. In order to localize the ethanol-elicited lesion in the glycolytic pathway, the metabolism of [3-(3)H] D-glucose was followed in hepatocytes from ethanol-fed and control animals.

RESULTS

Under both hypoxic and normoxic conditions ATP losses were due to decreases in both glycolytic and mitochondrial ATP production. The rate of production of tritiated water from [3-(3)H] D-glucose was significantly decreased in hepatocytes from ethanol-fed animals, which indicates there is an ethanol-elicited lesion in glycolysis between glucose and glyceraldehyde-3-phosphate.

Changes in hepatic glycogen cycling during a glucose load in healthy humans

AIMS/HYPOTHESIS

Glycogen cycling, i.e. simultaneous glycogen synthesis and glycogenolysis, affects estimates of glucose fluxes using tracer techniques and may contribute to hyperglycaemia in diabetic conditions. This study presents a new method for quantifying hepatic glycogen cycling in the fed state. Glycogen is synthesised from glucose by the direct and indirect (gluconeogenic) pathways. Since glycogen is also synthesised from glycogen, i.e. glycogen-->glucose 1-phosphate-->glycogen, that synthesised through the direct and indirect pathways does not account for 100% of glycogen synthesis. The percentage contribution of glycogen cycling to glycogen synthesis then equals the difference between the sum of the percentage contributions of the direct and indirect pathways and 100.

MATERIALS AND METHODS

The indirect and direct pathways were measured independently in nine healthy volunteers who had fasted overnight. They ingested (2)H(2)O (5 ml/kg body water) and were infused with [5-(3)H]glucose and acetaminophen (paracetamol; 1 g) during hyperglycaemic clamps (7.8 mmol/l) lasting 8 h. The percentage contribution of the indirect pathway was calculated from the ratio of (2)H enrichments at carbon 5 to that at carbon 2, and the contribution of the direct pathway was determined from the (3)H-specific activity, relative to plasma glucose, of the urinary glucuronide excreted between 2 and 4, 4 and 6, and 6 and 8 h.

RESULTS

Glucose infusion rates increased (p<0.01) to approximately 50 mumol kg(-1) min(-1). Plasma insulin and the insulin : glucagon ratio rose approximately 3.6- and approximately 8.3-fold (p<0.001), respectively. From the difference between 100% and the sum of the direct (2-4 h, 54+/-6%; 4-6 h, 59+/-5%; 6-8 h, 63+/-4%) and indirect (32+/-3, 38+/-4, 36+/-3%) pathways, glycogen cycling was seen to be decreased (p<0.05) from 14+/-4% (2-4 h) to 4+/-3% (4-6 h) and 1+/-3% (6-8 h).

CONCLUSIONS/INTERPRETATION

This method allows measurement of hepatic glycogen cycling in the fed state and demonstrates that glycogen cycling occurs most in the early hours after glucose loading subsequent to a fast.

Mitochondrial dysfunction and type 2 diabetes

Insulin resistance plays a major role in the pathogenesis of the metabolic syndrome and type 2 diabetes, and yet the mechanisms responsible for it remain poorly understood. Magnetic resonance spectroscopy studies in humans suggest that a defect in insulin-stimulated glucose transport in skeletal muscle is the primary metabolic abnormality in insulin-resistant patients with type 2 diabetes. Fatty acids appear to cause this defect in glucose transport by inhibiting insulin-stimulated tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and IRS-1-associated phosphatidylinositol 3-kinase activity. A number of different metabolic abnormalities may increase intramyocellular and intrahepatic fatty acid metabolites; these include increased fat delivery to muscle and liver as a consequence of either excess energy intake or defects in adipocyte fat metabolism, and acquired or inherited defects in mitochondrial fatty acid oxidation. Understanding the molecular and biochemical defects responsible for insulin resistance is beginning to unveil novel therapeutic targets for the treatment of the metabolic syndrome and type 2 diabetes.

Contribution of defects in glucose uptake to carbohydrate intolerance in liver cirrhosis: assessment during physiological glucose and insulin concentrations

It is well established that subjects with liver cirrhosis are insulin resistant, but the contribution of defects in insulin secretion and/or action to glucose intolerance remains unresolved. Healthy individuals and subjects with liver cirrhosis were studied on two occasions: 1) an oral glucose tolerance test was performed, and 2) insulin secretion was inhibited and glucose was infused in a pattern and amount mimicking the systemic delivery rate of glucose after a carbohydrate meal. Insulin was concurrently infused to mimic a healthy postprandial insulin profile. Postabsorptive glucose concentrations were equal (5.36 +/- 0.12 vs. 5.40 +/- 0.25 mmol/l, P = 0.89), despite higher insulin (P < 0.01), C-peptide (P < 0.01), and free fatty acid (P = 0.05) concentrations in cirrhotic than in control subjects. Endogenous glucose release (EGR; 11.50 +/- 0.50 vs. 11.73 +/- 1.00 mumol.kg(-1).min(-1), P = 0.84) and the contribution of gluconeogenesis to EGR (6.60 +/- 0.47 vs. 6.28 +/- 0.64 mumol.kg(-1).min(-1), P = 0.70) were unaltered by cirrhosis. A minimal model recently developed for the oral glucose tolerance test demonstrated an impaired insulin sensitivity index (P < 0.05), whereas the beta-cell response to glucose was unaltered (P = 0.72). During prandial glucose and insulin infusions, the integrated glycemic response was greater in cirrhotic than in control subjects (P < 0.05). EGR decreased promptly and comparably in both groups, but glucose disappearance was insufficient at the prevailing glucose concentration (P < 0.05). Moreover, identical rates of [3-(3)H]glucose infusion produced higher tracer concentrations in cirrhotic than in control subjects (P < 0.05), implying a defect in glucose uptake. In conclusion, carbohydrate intolerance in liver cirrhosis is determined by insulin resistance and the ability of glucose to stimulate insulin secretion. During prandial glucose and insulin concentrations, EGR suppression was unaltered, but glucose uptake was impaired, which demonstrates that intolerance can be ascribed to a defect in glucose uptake, rather than abnormalities in glucose production or beta-cell function. Although insulin secretion ameliorates glucose intolerance, impaired glucose uptake during physiological glucose and insulin concentrations produces marked and sustained hyperglycemia, despite concurrent abnormalities in glucose production or insulin secretion.

Regulation of net hepatic glycogenolysis and gluconeogenesis during exercise: impact of type 1 diabetes

The effects of type 1 diabetes on the contributions of net hepatic glycogenolysis and gluconeogenesis to glucose production (GP) at rest and during moderate (MOD) and high (HI) intensity running were examined in healthy control (n = 6) and type 1 diabetic (n = 5) subjects matched for age, weight, and maximum aerobic capacity by combined noninvasive measurements of hepatic glycogen content using (13)C nuclear magnetic resonance spectroscopy and determination of GP using [6,6-(2)H(2)]glucose. In the control subjects, GP increased in proportion to the intensity of the exercise [at rest (REST), 14.3 +/- 0.5; MOD, 18.1 +/- 0.9; HI, 28.8 +/- 1.3 micromol/(kg-min); P = 0.001, three-way comparison], and this was accounted for by an increase in the percent contribution of net hepatic glycogenolysis to GP (REST, 32 +/- 1%; MOD, 49 +/- 5%; HI, 57 +/- 5%; P = 0.006). In the diabetic subjects, resting rates of GP were 60% higher than those in the control subjects (P < 0.0001) and increased in proportion to the workload. In contrast, the contributions of net hepatic glycogenolysis to GP were consistently lower than those in the control subjects (REST, 20 +/- 6%; MOD, 32 +/- 13%; HI, 32 +/- 3%; P = 0.006 vs. control), and the exaggerated rates of GP could be entirely accounted for by increased rates of gluconeogenesis. In conclusion, 1) increases in GP in healthy control subjects with exercise intensity can be entirely attributed to increases in net hepatic glycogenolysis. 2) In contrast, moderately controlled type 1 diabetic subjects exhibit increased rates of GP both at rest and during exercise, which can be entirely accounted for by increased gluconeogenesis.

Measuring gluconeogenesis using a low dose of 2H2O: advantage of isotope fractionation during gas chromatography

The contribution of gluconeogenesis to glucose production can be measured by enriching body water with (2)H(2)O to approximately 0.5% (2)H and determining the ratio of (2)H that is bound to carbon-5 vs. carbon-2 of blood glucose. This labeling ratio can be measured using gas chromatography-mass spectrometry after the corresponding glucose carbons are converted to formaldehyde and then to hexamethylenetetramine (HMT). We present a technique for integrating ion chromatograms that allows one to use only 0.05% (2)H in body water (i.e., 10 times less than the current dose). This technique takes advantage of the difference in gas chromatographic retention times of naturally labeled HMT and [(2)H]HMT. We discuss the advantage(s) of using a low dose of (2)H(2)O to quantify the contribution of gluconeogenesis.

Effects of acute insulin excess and deficiency on gluconeogenesis and glycogenolysis in type 1 diabetes

To determine whether insulin induces acute changes in endogenous glucose production (EGP) via changes in gluconeogenesis (GNG), glycogenolysis (GL), or both, we measured GNG (with (2)H(2)O) and GL (EGP-GNG) in nine patients with type 1 diabetes during acute insulin excess produced by subcutaneous injection of insulin and during insulin deficiency which developed between 5 and 8 h after insulin injection. During insulin excess, free insulin concentration rose fivefold (from 36 to 180 pmol/l). Plasma glucose was maintained between 6.2 and 6.7 mmol/l for approximately 4 h with IV glucose. EGP (with 6,6-(2)H glucose) decreased from 17.1 to 9.8 micro mol. kg(-1). min(-1) after 1 h. This decrease was almost completely accounted for by a decrease in GL (from 10.7 to 4.6 micro mol. kg(-1). min(-1)). During insulin deficiency, plasma glucose rose from 6.2 to 10.5 mmol/l and EGP from 9.5 to 14.3 micro mol/kg min. The increase in EGP again was accounted for by an increase in GL. We conclude that in type 1 diabetes acute regulation of EGP by insulin is mainly via changes in GL while GNG changes little during the early hours of acute insulin excess or deficiency.

Effects of fasting and glucocorticoids on hepatic gluconeogenesis assessed using two independent methods in vivo

The purpose of this study was to compare the assessment of gluconeogenesis (GNG) in the overnight- and prolonged-fasted states and during chronic hypercortisolemia using the arteriovenous difference and [14C]phosphoenolpyruvate-liver biopsy techniques as well as a combination of the two. Two weeks before a study, catheters and flow probes were implanted in the hepatic and portal veins and femoral artery of dogs. Animals were studied after an 18-h fast (n = 8), a 42- or 66-h fast (n = 7), and an 18-h fast plus a continuous infusion of cortisol (3.0 microg. kg(-1). min(-1)) for 72 h (n = 7). Each experiment consisted of an 80-min tracer ([3-(3)H]glucose and [U-(14)C]alanine) and dye equilibration period (-80 to 0 min) and a 45-min sampling period. In the cortisol-treated group, plasma cortisol increased fivefold. In the overnight-fasted group, total GNG flux rate (GNG(flux)), conversion of glucose 6-phosphate to glucose (GNG(G-6-P-->Glc)), glucose cycling, and maximal GNG flux rate (GNG(max)) were 0.95 +/- 0.14, 0.65 +/- 0.06, 0.62 +/- 0.06, and 0.70 +/- 0.09 mg. kg(-1). min(-1), respectively. In the prolonged-fasted group, they were 1.50 +/- 0.18, 1.18 +/- 0.13, 0.40 +/- 0.07, and 1.28 +/- 0.10 mg. kg(-1). min(-1), whereas in the cortisol-treated group they were 1.64 +/- 0.33, 0.99 +/- 0.29, 1.32 +/- 0.24, and 0.91 +/- 0.13 mg. kg(-1). min(-1). These results demonstrate that GNG(G-6-P-->Glc) and GNG(max) were almost identical. However, these rates were 15-38% lower than GNG(flux) generated by a combination of the two methods. This difference was most apparent in the steroid-treated group, where the combination of the two methods (GNG(flux)) detected a significant increase in gluconeogenic flux.

An integrated (2)H and (13)C NMR study of gluconeogenesis and TCA cycle flux in humans

Hepatic glucose synthesis from glycogen, glycerol, and the tricarboxylic acid (TCA) cycle was measured in five overnight-fasted subjects by (1)H, (2)H, and (13)C NMR analysis of blood glucose, urinary acetaminophen glucuronide, and urinary phenylacetylglutamine after administration of [1,6-(13)C(2)]glucose, (2)H(2)O, and [U-(13)C(3)]propionate. This combination of tracers allows three separate elements of hepatic glucose production (GP) to be probed simultaneously in a single study: 1) endogenous GP, 2) the contribution of glycogen, phosphoenolpyruvate (PEP), and glycerol to GP, and 3) flux through PEP carboxykinase, pyruvate recycling, and the TCA cycle. Isotope-dilution measurements of [1,6-(13)C(2)] glucose by (1)H and (13)C NMR indicated that GP in 16-h-fasted humans was 10.7 +/- 0.9 micromol.kg(-1).min(-1). (2)H NMR spectra of monoacetone glucose (derived from plasma glucose) provided the relative (2)H enrichment at glucose H-2, H-5, and H-6S, which, in turn, reflects the contribution of glycogen, PEP, and glycerol to total GP (5.5 +/- 0.7, 4.8 +/- 1.0, and 0.4 +/- 0.3 micromol.kg(-1).min(-1), respectively). Interestingly, (13)C NMR isotopomer analysis of phenylacetylglutamine and acetaminophen glucuronide reported different values for PEP carboxykinase flux (68.8 +/- 9.8 vs. 37.5 +/- 7.9 micromol.kg(-1).min(-1)), PEP recycling flux (59.1 +/- 9.8 vs. 27.8 +/- 6.8 micromol.kg(-1).min(-1)), and TCA cycle flux (10.9 +/- 1.4 vs. 5.4 +/- 1.4 micromol.kg(-1).min(-1)). These differences may reflect zonation of propionate metabolism in the liver.

Methods for measuring glycogen cycling

Simultaneous synthesis and breakdown of glycogen is called glycogen cycling. The extent of hyperglycemia and decreased glycogen stores in diabetes mellitus may relate in part to the extent cycling occurs. Four methods have been introduced to estimate its extent in liver in humans. 1) In the fasted state, the rate of net hepatic glycogenolysis, i.e., glycogen breakdown minus synthesis, is estimated using NMR, and the rate of glycogenolysis is estimated from deuterium labeling of blood glucose on (2)H(2)O ingestion. 2) The rate of glycogen synthesis is estimated from the rate of labeling of carbon 1 of glycogen on [1-(13)C]glucose infusion, monitored by NMR, and the rate of breakdown from the rate of disappearance of that labeling on unlabeled glucose infusion. 3) The rate of synthesis from glucose-1-P, formed by glycogenolysis, is measured by the decrease in the (3)H/(14)C ratio in acetaminophen glucuronide on acetaminophen and [2-(3)H,6-(14)C]galactose administration. 4) The rate of synthesis is estimated from the dilution of label from labeled galactose in its conversion to the acetaminophen glucuronide, and the rate of glycogenolysis is estimated from the amount of label in blood glucose. In the first method, the fate of glucose-6-P is assumed to be only to glycogen and glucose. In the second, only glucose-6-P molecules formed by breakdown that are not cycled back to glycogen are measured. In the third, (3)H is assumed to be removed completely during cycling, and only the molecules cycled back to glycogen are measured. In the fourth, galactose conversion to glucose is assumed to be via glycogen. Quantitations in all four methods depend on assuming the order in which the molecules deposited in glycogen are released.

Effect of physiological hyperinsulinemia on gluconeogenesis in nondiabetic subjects and in type 2 diabetic patients

Gluconeogenesis (GNG) is enhanced in type 2 diabetes. In experimental animals, insulin at high doses decreases the incorporation of labeled GNG precursors into plasma glucose. Whether physiological hyperinsulinemia has any effect on total GNG in humans has not been determined. We combined the insulin clamp with the (2)H(2)O technique to measure total GNG in 33 subjects with type 2 diabetes (BMI 29.0 +/- 0.6 kg/m(2), fasting plasma glucose 8.1 +/- 0.3 mmol/l) and in 9 nondiabetic BMI-matched subjects after 16 h of fasting and after euglycemic hyperinsulinemia. A primed-constant infusion of 6,6-(2)H-glucose was used to monitor endogenous glucose output (EGO); insulin (40 mU. min(-1). m(-2)) was then infused while clamping plasma glucose for 2 h (at 5.8 +/- 0.1 and 4.9 +/- 0.2 mmol/l for diabetic and control subjects, respectively). In the fasting state, EGO averaged 15.2 +/- 0.4 micromol. min(-1). kg(-1)(ffm) (62% from GNG) in diabetic subjects and 12.2 +/- 0.7 micromol. min(-1). kg(-1)(ffm) (55% from GNG) in control subjects (P < 0.05 or less for both fluxes). Glycogenolysis (EGO - GNG) was similar in the two groups (P = NS). During the last 40 min of the clamp, both EGO and GNG were significantly (P < 0.01 or less, compared with fasting) inhibited (EGO 7.1 +/- 0.9 and 3.6 +/- 0.5 and GNG 7.9 +/- 0.5 and 4.5 +/- 1.0 respectively) but remained significantly (P < 0.05) higher in diabetic subjects, whereas glycogenolysis was suppressed completely and equally in both groups. During hyperinsulinemia, GNG micromol. min(-1). kg(-1)(ffm) in diabetic and control subjects, was reciprocally related to plasma glucose clearance. In conclusion, physiological hyperinsulinemia suppresses GNG by approximately 20%, while completely blocking glycogenolysis. Resistance of GNG (to insulin suppression) and resistance of glucose uptake (to insulin stimulation) are coupled phenomena. In type 2 diabetes, the excess GNG of the fasting state is carried over to the insulinized state, thereby contributing to glucose overproduction under both conditions.

Effects of short-term improvement of insulin treatment and glycemia on hepatic glycogen metabolism in type 1 diabetes

Insufficiently treated type 1 diabetic patients exhibit inappropriate postprandial hyperglycemia and reduction in liver glycogen stores. To examine the effect of acute improvement of metabolic control on hepatic glycogen metabolism, lean young type 1 diabetic (HbA1c 8.8 +/- 0.3%) and matched nondiabetic subjects (HbA1c 5.4 +/- 0.1%) were studied during the course of a day with three isocaloric mixed meals. Hepatic glycogen concentrations were determined noninvasively using in vivo 13C nuclear magnetic resonance spectroscopy. Rates of net glycogen synthesis and breakdown were calculated from linear regression of the glycogen concentration time curves from 7:30-10:30 P.M. and from 10:30 P.M. to 8:00 A.M., respectively. The mean plasma glucose concentration was approximately 2.4-fold higher in diabetic than in nondiabetic subjects (13.6 +/- 0.4 vs. 5.8 +/- 0.1 mmol/l, P < 0.001). Rates of net glycogen synthesis and net glycogen breakdown were reduced by approximately 74% (0.11 +/- 0.02 vs. 0.43 +/- 0.04 mmol/l liver/min, P < 0.001) and by approximately 47% (0.10 +/- 0.01 vs. 0.19 +/- 0.01 mmol/l liver/min, P < 0.001) in diabetic patients, respectively. During short-term (24-h) intensified insulin treatment, the mean plasma glucose level was not different between diabetic and nondiabetic subjects (6.4 +/- 0.1 mmol/l). Net glycogen synthesis and breakdown increased by approximately 92% (0.23 +/- 0.04 mmol/l liver/min, P = 0.017) and by approximately 40% (0.14 approximately 0.01 mmol/l liver/min, P = 0.011), respectively. In conclusion, poorly controlled type 1 diabetic patients present with marked reduction in both hepatic glycogen synthesis and breakdown. Both defects in glycogen metabolism are improved but not normalized by short-term restoration of insulinemia and glycemia.

Gluconeogenesis in moderately and severely hyperglycemic patients with type 2 diabetes mellitus

We tested the generally accepted concept that increased gluconeogenesis (GNG) and endogenous glucose production (EGP) are the main reasons for postabsorptive hyperglycemia in patients with type 2 diabetes mellitus (T2DM). GNG was measured with the (2)H(2)O method by use of both the C5-to-C2 ratio (C5/C2, with gas chromatography-mass spectrometry) and the C5-to-(2)H(2)O ratio (C5/(2)H(2)O, with isotope ratio mass spectrometry), and EGP was measured with 3-[(3)H]glucose in 27 patients with T2DM [13 with fasting plasma glucose (FPG) >10 mM and 14 with FPG <10 mM] and in 7 weight- and age-matched nondiabetic controls. The results showed 1) that GNG could be determined accurately with (2)H(2)O by using either C5/C2 or C5/(2)H(2)O; 2) that whereas after an overnight fast of 16 h, GNG was higher in the entire group of patients with T2DM than in controls (6.4 vs. 5.0 micromol. kg(-1). min(-1) or 60.4 vs. 51.4% of EGP, P < 0.02), GNG was within normal limits (less than the mean +/- 2 SD of controls or <65.3%) in 11/14 (79%) patients with mild to moderate hyperglycemia (FPG <10 mM) and in 5/13 (38%) of patients with severe hyperglycemia (FPG 10-20 mM); 3) that elevated GNG in T2DM was associated with a 43% decrease in prehepatic insulin secretion, i.e., with hepatic insulin deficiency; and 4) that FPG correlated significantly with glucose clearance (insulin resistance) (r = 0.70) and with GNG (r = 0.50) or EGP (r = 0.45). We conclude 1) that peripheral insulin resistance is at least as important as GNG (and EGP) as a cause of postabsorptive hyperglycemia in T2DM and 2) that GNG and EGP in T2DM are increased under conditions of significant hepatic insulin deficiency and thus probably represent a late event in the course of T2DM.

Mechanism by which metformin reduces glucose production in type 2 diabetes

To examine the mechanism by which metformin lowers endogenous glucose production in type 2 diabetic patients, we studied seven type 2 diabetic subjects, with fasting hyperglycemia (15.5 +/- 1.3 mmol/l), before and after 3 months of metformin treatment. Seven healthy subjects, matched for sex, age, and BMI, served as control subjects. Rates of net hepatic glycogenolysis, estimated by 13C nuclear magnetic resonance spectroscopy, were combined with estimates of contributions to glucose production of gluconeogenesis and glycogenolysis, measured by labeling of blood glucose by 2H from ingested 2H2O. Glucose production was measured using [6,6-2H2]glucose. The rate of glucose production was twice as high in the diabetic subjects as in control subjects (0.70 +/- 0.05 vs. 0.36 +/- 0.03 mmol x m(-2) min(-1), P < 0.0001). Metformin reduced that rate by 24% (to 0.53 +/- 0.03 mmol x m(-2) x min(-1), P = 0.0009) and fasting plasma glucose concentration by 30% (to 10.8 +/- 0.9 mmol/l, P = 0.0002). The rate of gluconeogenesis was three times higher in the diabetic subjects than in the control subjects (0.59 +/- 0.03 vs. 0.18 +/- 0.03 mmol x m(-2) min(-1) and metformin reduced that rate by 36% (to 0.38 +/- 0.03 mmol x m(-2) x min(-1), P = 0.01). By the 2H2O method, there was a twofold increase in rates of gluconeogenesis in diabetic subjects (0.42 +/- 0.04 mmol m(-2) x min(-1), which decreased by 33% after metformin treatment (0.28 +/- 0.03 mmol x m(-2) x min(-1), P = 0.0002). There was no glycogen cycling in the control subjects, but in the diabetic subjects, glycogen cycling contributed to 25% of glucose production and explains the differences between the two methods used. In conclusion, patients with poorly controlled type 2 diabetes have increased rates of endogenous glucose production, which can be attributed to increased rates of gluconeogenesis. Metformin lowered the rate of glucose production in these patients through a reduction in gluconeogenesis.