Knockdown of the gene encoding Drosophila tribbles homologue 3 (Trib3) improves insulin sensitivity through peroxisome proliferator-activated receptor-γ (PPAR-γ) activation in a rat model of insulin resistance.

AIMS/HYPOTHESIS: Insulin action is purportedly modulated by Drosophila tribbles homologue 3 (TRIB3), which in vitro prevents thymoma viral proto-oncogene (AKT) and peroxisome proliferator-activated receptor-γ (PPAR-γ) activation. However, the physiological impact of TRIB3 action in vivo remains controversial. METHODS: We investigated the role of TRIB3 in rats treated with either a control or Trib3 antisense oligonucleotide (ASO). Tissue-specific insulin sensitivity was assessed in vivo using a euglycaemic-hyperinsulinaemic clamp. A separate group was treated with the PPAR-γ antagonist bisphenol-A-diglycidyl ether (BADGE) to assess the role of PPAR-γ in mediating the response to Trib3 ASO. RESULTS: Trib3 ASO treatment specifically reduced Trib3 expression by 70% to 80% in liver and white adipose tissue. Fasting plasma glucose, insulin concentrations and basal rate of endogenous glucose production were unchanged. However, Trib3 ASO increased insulin-stimulated whole-body glucose uptake by ~50% during the euglycaemic-hyperinsulinaemic clamp. This was attributable to improved skeletal muscle glucose uptake. Despite the reduction of Trib3 expression, AKT2 activity was not increased. Trib3 ASO increased white adipose tissue mass by 70% and expression of Ppar-γ and its key target genes, raising the possibility that Trib3 ASO improves insulin sensitivity primarily in a PPAR-γ-dependent manner. Co-treatment with BADGE blunted the expansion of white adipose tissue and abrogated the insulin-sensitising effects of Trib3 ASO. Finally, Trib3 ASO also increased plasma HDL-cholesterol, a change that persisted with BADGE co-treatment. CONCLUSIONS/INTERPRETATION: These data suggest that TRIB3 inhibition improves insulin sensitivity in vivo primarily in a PPAR-γ-dependent manner and without any change in AKT2 activity.

Impaired insulin sensitivity among nondiabetic patients with a recent TIA or ischemic stroke.

OBJECTIVES: To determine the prevalence of impaired insulin sensitivity among nondiabetic patients with a recent TIA or nondisabling ischemic stroke. METHODS: Eligible subjects were nondiabetic men and women over age 45 years who were hospitalized with a TIA or ischemic stroke. To measure insulin sensitivity, subjects underwent an oral glucose tolerance test between 2 and 6 months after their event. Impaired insulin sensitivity was defined by a value of < or =2.5 on the Composite Insulin Sensitivity Index derived from insulin and glucose values during the test. RESULTS: Between July 2000 and June 2001, we identified 177 eligible patients, among whom 105 declined to participate and 72 enrolled. The median age of participants was 71 years and 46 (64%) were men. The baseline event was stroke for 57 subjects (79%). A history of myocardial infarction (MI) was reported by 14 subjects (19%), and 16 (22%) were obese (body mass index > 30). Fasting glucose was normal (<110 mg/dL) for 58 (80%) participants and impaired (110 to 125 mg/dL) for 14 (20%). Among 72 participants, the median insulin sensitivity index value was 2.6 (range 0.9 to 10.2). The prevalence of impaired insulin sensitivity was 36 of 72 (50%, 95% CI 38% to 62%). Impaired insulin sensitivity was more prevalent among younger patients and patients with obesity, lacunar stroke etiology, and disability (Rankin grade >1). CONCLUSION: Impaired insulin sensitivity is highly prevalent among nondiabetic patients with a recent TIA or nondisabling ischemic stroke. This finding has important therapeutic implications if treatment to improve insulin sensitivity is shown to reduce risk for subsequent stroke and heart disease.

Cellular mechanism of insulin resistance: potential links with inflammation.

Insulin resistance is a pivotal feature in the pathogenesis of type 2 diabetes, and it may be detected 10-20 y before the clinical onset of hyperglycemia. Insulin resistance is due to the reduced ability of peripheral target tissues to respond properly to insulin stimulation. In particular, impaired insulin-stimulated muscle glycogen synthesis plays a significant role in insulin resistance. Glucose transport (GLUT4), phosphorylation (hexokinase) and storage (glycogen synthase) are the three potential rate-controlling steps regulating insulin-stimulated muscle glucose metabolism, and all three have been implicated as being the major defects responsible for causing insulin resistance in patients with type 2 diabetes. Using (13)C/(31)P magnetic resonance spectroscopy (MRS), we demonstrate that a defect in insulin-stimulated muscle glucose transport activity is the rate-controlling defect. Using a similar (13)C/(31)P MRS approach, we have also demonstrated that fatty acids cause insulin resistance in humans due to a decrease in insulin-stimulated muscle glucose transport activity, which could be attributed to reduced insulin-stimulated IRS-1-associated phosphatidylinositol 3-kinase activity, a required step in insulin-stimulated glucose transport into muscle. Furthermore, we have recently proposed that this defect in insulin-stimulated muscle glucose transport activity may be due to the activation of a serine kinase cascade involving protein kinase C theta and IKK-beta, which are key downstream mediators of tissue inflammation. Finally, we propose that any perturbation that leads to an increase in intramyocellular lipid (fatty acid metabolites) content such as acquired or inherited defects in mitochondrial fatty acid oxidation, defects in adipocyte fat metabolism or simply increased fat delivery to muscle/liver due to increased energy intake will lead to insulin resistance through this final common pathway. Understanding these key cellular mechanisms of insulin resistance should help elucidate new targets for treating type 2 diabetes.

AICAR and phlorizin reverse the hypoglycemia-specific defect in glucagon secretion in the diabetic BB rat.

Individuals with type 1 diabetes demonstrate a hypoglycemia-specific defect in glucagon secretion. To determine whether intraislet hyperinsulinemia plays a role in the genesis of this defect, glucagon-secretory responses to moderate hypoglycemia induced by either insulin or a novel combination of the noninsulin glucose-lowering agents 5-aminoimidazole-4-carboxamide (AICAR) and phlorizin were compared in diabetic BB rats (an animal model of type 1 diabetes) and nondiabetic BB rats. The phlorizin-AICAR combination was able to induce moderate and equivalent hypoglycemia in both diabetic and nondiabetic BB rats in the absence of marked hyperinsulinemia. Diabetic BB rats demonstrated impaired glucagon and epinephrine responses during insulin-induced hypoglycemia compared with nondiabetic rats. In contrast, both glucagon (9- to 10-fold increase) and epinephrine (5- to 6-fold increase) responses were markedly improved during phlorizin-AICAR hypoglycemia. Combining phlorizin, AICAR, and insulin attenuated the glucagon response to hypoglycemia by 70% in the diabetic BB rat. Phlorizin plus AICAR had no effect on counterregulatory hormones under euglycemic conditions. We conclude that alpha-cell glucagon secretion in response to hypoglycemia is not defective if intraislet hyperinsulinemia is prevented. This suggests that exogenous insulin plays a pivotal role in the etiology of this defect.

Free fatty acids in obesity and type 2 diabetes: defining their role in the development of insulin resistance and beta-cell dysfunction.

Plasma free fatty acids (FFA) play important physiological roles in skeletal muscle, heart, liver and pancreas. However, chronically elevated plasma FFA appear to have pathophysiological consequences. Elevated FFA concentrations are linked with the onset of peripheral and hepatic insulin resistance and, while the precise action in the liver remains unclear, a model to explain the role of raised FFA in the development of skeletal muscle insulin resistance has recently been put forward. Over 30 years ago, Randle proposed that FFA compete with glucose as the major energy substrate in cardiac muscle, leading to decreased glucose oxidation when FFA are elevated. Recent data indicate that high plasma FFA also have a significant role in contributing to insulin resistance. Elevated FFA and intracellular lipid appear to inhibit insulin signalling, leading to a reduction in insulin-stimulated muscle glucose transport that may be mediated by a decrease in GLUT-4 translocation. The resulting suppression of muscle glucose transport leads to reduced muscle glycogen synthesis and glycolysis. In the liver, elevated FFA may contribute to hyperglycaemia by antagonizing the effects of insulin on endogenous glucose production. FFA also affect insulin secretion, although the nature of this relationship remains a subject for debate. Finally, evidence is discussed that FFA represent a crucial link between insulin resistance and beta-cell dysfunction and, as such, a reduction in elevated plasma FFA should be an important therapeutic target in obesity and type 2 diabetes.

Chronic activation of AMP kinase results in NRF-1 activation and mitochondrial biogenesis.

The underlying mechanism by which skeletal muscle adapts to exercise training or chronic energy deprivation is largely unknown. To examine this question, rats were fed for 9 wk either with or without beta-guanadinopropionic acid (beta-GPA; 1% enriched diet), a creatine analog that is known to induce muscle adaptations similar to those induced by exercise training. Muscle phosphocreatine, ATP, and ATP/AMP ratios were all markedly decreased and led to the activation of AMP-activated protein kinase (AMPK) in the beta-GPA-fed rats compared with control rats. Under these conditions, nuclear respiratory factor-1 (NRF-1) binding activity, measured using a cDNA probe containing a sequence encoding for the promoter of delta-aminolevulinate (ALA) synthase, was increased by about eightfold in the muscle of beta-GPA-fed rats compared with the control group. Concomitantly, muscle ALA synthase mRNA and cytochrome c content were also increased. Mitochondrial density in both extensor digitorum longus and epitrochlearis from beta-GPA-fed rats was also increased by more than twofold compared with the control group. In conclusion, chronic phosphocreatine depletion during beta-GPA supplementation led to the activation of muscle AMPK that was associated with increased NRF-1 binding activity, increased cytochrome c content, and increased muscle mitochondrial density. Our data suggest that AMPK may play an important role in muscle adaptations to chronic energy stress and that it promotes mitochondrial biogenesis and expression of respiratory proteins through activation of NRF-1.

Effect of triiodothyronine on mitochondrial energy coupling in human skeletal muscle.

The mechanism underlying the regulation of basal metabolic rate by thyroid hormone remains unclear. Although it has been suggested that thyroid hormone might uncouple substrate oxidation from ATP synthesis, there are no data from studies on humans to support this hypothesis. To examine this possibility, we used a novel combined (13)C/(31)P nuclear magnetic resonance (NMR) approach to assess mitochondrial energy coupling in skeletal muscle of seven healthy adults before and after three days of triiodothyronine (T(3)) treatment. Rates of ATP synthesis and tricarboxylic acid (TCA) cycle fluxes were measured by (31)P and (13)C NMR spectroscopy, respectively, and mitochondrial energy coupling was assessed as the ratio. Muscle TCA cycle flux increased by approximately 70% following T(3) treatment. In contrast, the rate of ATP synthesis remained unchanged. Given the disproportionate increase in TCA cycle flux compared with ATP synthesis, these data suggest that T(3) promotes increased thermogenesis in part by promoting mitochondrial energy uncoupling in skeletal muscle.

Functional inactivation of the IGF-I and insulin receptors in skeletal muscle causes type 2 diabetes.

Peripheral insulin resistance and impaired insulin action are the primary characteristics of type 2 diabetes. The first observable defect in this major disorder occurs in muscle, where glucose disposal in response to insulin is impaired. We have developed a transgenic mouse with a dominant-negative insulin-like growth factor-I receptor (KR-IGF-IR) specifically targeted to the skeletal muscle. Expression of KR-IGF-IR resulted in the formation of hybrid receptors between the mutant and the endogenous IGF-I and insulin receptors, thereby abrogating the normal function of these receptors and leading to insulin resistance. Pancreatic beta-cell dysfunction developed at a relative early age, resulting in diabetes. These mice provide an excellent model to study the molecular mechanisms underlying the development of human type 2 diabetes.

Prevention of fat-induced insulin resistance by salicylate.

Insulin resistance is a major factor in the pathogenesis of type 2 diabetes and may involve fat-induced activation of a serine kinase cascade involving IKK-beta. To test this hypothesis, we first examined insulin action and signaling in awake rats during hyperinsulinemic-euglycemic clamps after a lipid infusion with or without pretreatment with salicylate, a known inhibitor of IKK-beta. Whole-body glucose uptake and metabolism were estimated using [3-(3)H]glucose infusion, and glucose uptake in individual tissues was estimated using [1-(14)C]2-deoxyglucose injection during the clamp. Here we show that lipid infusion decreased insulin-stimulated glucose uptake and activation of IRS-1-associated PI 3-kinase in skeletal muscle but that salicylate pretreatment prevented these lipid-induced effects. To examine the mechanism of salicylate action, we studied the effects of lipid infusion on insulin action and signaling during the clamp in awake mice lacking IKK-beta. Unlike the response in wild-type mice, IKK-beta knockout mice did not exhibit altered skeletal muscle insulin signaling and action following lipid infusion. In summary, high-dose salicylate and inactivation of IKK-beta prevent fat-induced insulin resistance in skeletal muscle by blocking fat-induced defects in insulin signaling and action and represent a potentially novel class of therapeutic agents for type 2 diabetes.

Uncoupling protein-2 negatively regulates insulin secretion and is a major link between obesity, beta cell dysfunction, and type 2 diabetes.

beta cells sense glucose through its metabolism and the resulting increase in ATP, which subsequently stimulates insulin secretion. Uncoupling protein-2 (UCP2) mediates mitochondrial proton leak, decreasing ATP production. In the present study, we assessed UCP2's role in regulating insulin secretion. UCP2-deficient mice had higher islet ATP levels and increased glucose-stimulated insulin secretion, establishing that UCP2 negatively regulates insulin secretion. Of pathophysiologic significance, UCP2 was markedly upregulated in islets of ob/ob mice, a model of obesity-induced diabetes. Importantly, ob/ob mice lacking UCP2 had restored first-phase insulin secretion, increased serum insulin levels, and greatly decreased levels of glycemia. These results establish UCP2 as a key component of beta cell glucose sensing, and as a critical link between obesity, beta cell dysfunction, and type 2 diabetes.

Glucose toxicity and the development of diabetes in mice with muscle-specific inactivation of GLUT4.

Using cre/loxP gene targeting, transgenic mice with muscle-specific inactivation of the GLUT4 gene (muscle GLUT4 KO) were generated and shown to develop a diabetes phenotype. To determine the mechanism, we examined insulin-stimulated glucose uptake and metabolism during hyperinsulinemic-euglycemic clamp in control and muscle GLUT4 KO mice before and after development of diabetes. Insulin-stimulated whole body glucose uptake was decreased by 55% in muscle GLUT4 KO mice, an effect that could be attributed to a 92% decrease in insulin-stimulated muscle glucose uptake. Surprisingly, insulin's ability to stimulate adipose tissue glucose uptake and suppress hepatic glucose production was significantly impaired in muscle GLUT4 KO mice. To address whether these latter changes were caused by glucose toxicity, we treated muscle GLUT4 KO mice with phloridzin to prevent hyperglycemia and found that insulin-stimulated whole body and skeletal muscle glucose uptake were decreased substantially, whereas insulin-stimulated glucose uptake in adipose tissue and suppression of hepatic glucose production were normal after phloridzin treatment. In conclusion, these findings demonstrate that a primary defect in muscle glucose transport can lead to secondary defects in insulin action in adipose tissue and liver due to glucose toxicity. These secondary defects contribute to insulin resistance and to the development of diabetes.

Insulin resistance and a diabetes mellitus-like syndrome in mice lacking the protein kinase Akt2 (PKB beta).

Glucose homeostasis depends on insulin responsiveness in target tissues, most importantly, muscle and liver. The critical initial steps in insulin action include phosphorylation of scaffolding proteins and activation of phosphatidylinositol 3-kinase. These early events lead to activation of the serine-threonine protein kinase Akt, also known as protein kinase B. We show that mice deficient in Akt2 are impaired in the ability of insulin to lower blood glucose because of defects in the action of the hormone on liver and skeletal muscle. These data establish Akt2 as an essential gene in the maintenance of normal glucose homeostasis.

Tissue-specific overexpression of lipoprotein lipase causes tissue-specific insulin resistance.

Insulin resistance in skeletal muscle and liver may play a primary role in the development of type 2 diabetes mellitus, and the mechanism by which insulin resistance occurs may be related to alterations in fat metabolism. Transgenic mice with muscle- and liver-specific overexpression of lipoprotein lipase were studied during a 2-h hyperinsulinemic-euglycemic clamp to determine the effect of tissue-specific increase in fat on insulin action and signaling. Muscle-lipoprotein lipase mice had a 3-fold increase in muscle triglyceride content and were insulin resistant because of decreases in insulin-stimulated glucose uptake in skeletal muscle and insulin activation of insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity. In contrast, liver-lipoprotein lipase mice had a 2-fold increase in liver triglyceride content and were insulin resistant because of impaired ability of insulin to suppress endogenous glucose production associated with defects in insulin activation of insulin receptor substrate-2-associated phosphatidylinositol 3-kinase activity. These defects in insulin action and signaling were associated with increases in intracellular fatty acid-derived metabolites (i.e., diacylglycerol, fatty acyl CoA, ceramides). Our findings suggest a direct and causative relationship between the accumulation of intracellular fatty acid-derived metabolites and insulin resistance mediated via alterations in the insulin signaling pathway, independent of circulating adipocyte-derived hormones.

Stimulating effects of low-dose fructose on insulin-stimulated hepatic glycogen synthesis in humans.

Fructose has been shown to have a catalytic effect on glucokinase activity in vitro; however, its effects on hepatic glycogen metabolism in humans is unknown. To address this question, we used (13)C nuclear magnetic resonance (NMR) spectroscopy to noninvasively assess rates of hepatic glycogen synthesis and glycogenolysis under euglycemic (approximately 5 mmol/l) hyperinsulinemic conditions (approximately 400 pmol/l) with and without a low-dose infusion of fructose (approximately 3.5 micromol. kg(-1). min(-1)). Six healthy overnight-fasted subjects were infused for 4 h with somatostatin (0.1 micromol. kg(-1). min(-1)) and insulin (240 pmol. m(-2). min(-1)). During the initial 120 min, [1-(13)C]glucose was infused to assess glycogen synthase flux followed by an approximately 120-min infusion of unlabeled glucose to assess rates of glycogen phosphorylase flux. Acetaminophen was given to assess the percent contribution of the direct and indirect (gluconeogenic) pathways of glycogen synthesis by the (13)C enrichment of plasma UDP-glucuronide and C-1 of glucose. In the control studies, the flux through glycogen synthase and glycogen phosphorylase was 0.31 +/- 0.06 and 0.17 +/- 0.04 mmol/l per min, respectively, and the rate of net hepatic glycogen synthesis was 0.14 +/- 0.05 mmol/l per min. In the fructose studies, the glycogen synthase flux increased 2.5-fold to 0.79 +/- 0.16 mmol/l per min (P = 0.018 vs. control), whereas glycogen phosphorylase flux remained unchanged (0.24 +/- 0.06; P = 0.16 vs. control). The infusion of fructose resulted in a threefold increase in rates of net hepatic glycogen synthesis (0.54 +/- 0.12 mmol/l per min; P = 0.008 vs. control) without affecting the pathways of hepatic glycogen synthesis (direct pathway approximately 60% in both groups). We conclude that during euglycemic hyperinsulinemia, a low-dose fructose infusion causes a threefold increase in net hepatic glycogen synthesis exclusively through stimulation of glycogen synthase flux. Because net hepatic glycogen synthesis has been shown to be diminished in patients with poorly controlled type 1 and type 2 diabetes, stimulation of hepatic glycogen synthesis by this mechanism may be of potential therapeutic value.

Syntaxin 4 heterozygous knockout mice develop muscle insulin resistance.

To investigate the physiological function of syntaxin 4 in the regulation of GLUT4 vesicle trafficking, we used homologous recombination to generate syntaxin 4-knockout mice. Homozygotic disruption of the syntaxin 4 gene results in early embryonic lethality, whereas heterozygous knockout mice, Syn4(+/-), had normal viability with no significant impairment in growth, development, or reproduction. However, the Syn4(+/-) mice manifested impaired glucose tolerance with a 50% reduction in whole-body glucose uptake. This defect was attributed to a 50% reduction in skeletal muscle glucose transport determined by 2-deoxyglucose uptake during hyperinsulinemic-euglycemic clamp procedures. In parallel, insulin-stimulated GLUT4 translocation in skeletal muscle was also significantly reduced in these mice. In contrast, Syn4(+/-) mice displayed normal insulin-stimulated glucose uptake and metabolism in adipose tissue and liver. Together, these data demonstrate that syntaxin 4 plays a critical physiological role in insulin-stimulated glucose uptake in skeletal muscle. Furthermore, reduction in syntaxin 4 protein levels in this tissue can account for the impairment in whole-body insulin-stimulated glucose metabolism in this animal model.

Effect of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside infusion on in vivo glucose and lipid metabolism in lean and obese Zucker rats.

Activation of AMP-activated protein kinase (AMPK) with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofurano-side (AICAR) increases glucose transport in skeletal muscle via an insulin-independent pathway. To examine the effects of AMPK activation on skeletal muscle glucose transport activity and whole-body carbohydrate and lipid metabolism in an insulin-resistant rat model, awake obese Zuckerfa/fa rats (n = 26) and their lean (n = 23) littermates were infused for 90 min with AICAR, insulin, or saline. The insulin infusion rate (4 mU.kg(-1).min(-1)) was selected to match the glucose requirements during AICAR (bolus, 100 mg/kg; constant, 10 mg.kg(-1).min(-1)) isoglycemic clamps in the lean rats. The effects of these identical AICAR and insulin infusion rates were then examined in the obese Zucker rats. AICAR infusion increased muscle AMPK activity more than fivefold (P < 0.01 vs. control and insulin) in both lean and obese rats. Plasma triglycerides, fatty acid concentrations, and glycerol turnover, as assessed by [2-13C]glycerol, were all decreased in both lean and obese rats infused with AICAR (P < 0.05 vs. basal), whereas insulin had no effect on these parameters in the obese rats. Endogenous glucose production rates, measured by [U-13C]glucose, were suppressed by >50% during AICAR and insulin infusions in both lean and obese rats (P < 0.05 vs. basal). In lean rats, rates of whole-body glucose disposal increased by more than two-fold (P < 0.05 vs. basal) during both AICAR and insulin infusion; [3H]2-deoxy-D-glucose transport activity increased to a similar extent, by >2.2-fold (both P < 0.05 vs. control), in both soleus and red gastrocnemius muscles of lean rats infused with either AICAR or insulin. In the obese Zucker rats, neither AICAR nor insulin stimulated whole-body glucose disposal or soleus muscle glucose transport activity. However, AICAR increased glucose transport activity by approximately 2.4-fold (P < 0.05 vs. control) in the red gastrocnemius from obese rats, whereas insulin had no effect. In summary, acute infusion of AICAR in an insulin-resistant rat model activates skeletal muscle AMPK and increases glucose transport activity in red gastrocnemius muscle while suppressing endogenous glucose production and lipolysis. Because type 2 diabetes is characterized by diminished rates of insulin-stimulated glucose uptake as well as increased basal rates of endogenous glucose production and lipolysis, these results suggest that AICAR-related compounds may represent a new class of antidiabetic agents.

Overexpression of the LAR (leukocyte antigen-related) protein-tyrosine phosphatase in muscle causes insulin resistance.

Previous reports indicate that the expression and/or activity of the protein-tyrosine phosphatase (PTP) LAR are increased in insulin-responsive tissues of obese, insulin-resistant humans and rodents, but it is not known whether these alterations contribute to the pathogenesis of insulin resistance. To address this question, we generated transgenic mice that overexpress human LAR, specifically in muscle, to levels comparable to those reported in insulin-resistant humans. In LAR-transgenic mice, fasting plasma insulin was increased 2.5-fold compared with wild-type controls, whereas fasting glucose was normal. Whole-body glucose disposal and glucose uptake into muscle in vivo were reduced by 39-50%. Insulin injection resulted in normal tyrosyl phosphorylation of the insulin receptor and insulin receptor substrate 1 (IRS-1) in muscle of transgenic mice. However, phosphorylation of IRS-2 was reduced by 62%, PI3' kinase activity associated with phosphotyrosine, IRS-1, or IRS-2 was reduced by 34-57%, and association of p85alpha with both IRS proteins was reduced by 39-52%. Thus, overexpression of LAR in muscle causes whole-body insulin resistance, most likely due to dephosphorylation of specific regulatory phosphotyrosines on IRS proteins. Our data suggest that increased expression and/or activity of LAR or related PTPs in insulin target tissues of obese humans may contribute to the pathogenesis of insulin resistance.

Contribution of net hepatic glycogen synthesis to disposal of an oral glucose load in humans.

The contribution of hepatic glycogen synthesis to whole body glucose disposal after an oral glucose load was examined using (13)C nuclear magnetic resonance (NMR) spectroscopy to measure liver glycogen content in healthy, volunteers after an overnight fast. In group 1 (n = 14), hepatic glycogen synthesis was measured using (13)C-NMR spectroscopy for 240 minutes after ingestion of 98 +/- 1 g glucose. Liver volumes were measured using magnetic resonance imaging (MRI). To assess the direct (glucose --> glucose-6-P --> glucose-1-P --> uridine diphosphate (UDP)-glucose --> glycogen) and indirect (3-carbon units --> --> glycogen) pathways of liver glycogen synthesis, group 2 (n = 6) was studied with an identical glucose load enriched with [1-(13)C]glucose along with acetaminophen to noninvasively assess the (13)C enrichment in hepatic UDP-glucose. The fasting hepatic glycogen content was 305 +/- 17 mmol/L liver, and the liver volume was 1.46 +/- 0.07 L. For the initial 180 minutes after ingestion of glucose, hepatic glycogen concentrations increased linearly (r =.94, P =.0006) achieving a maximum concentration of 390 +/- 7 mmol/L liver and then remained constant until the end of the study. The mean maximum rate of net hepatic glycogen synthesis was 0.48 +/- 0.07 mmol/L liver-minute. Total liver glycogen synthesis could account for 16.7 +/- 3.8 g (17% +/- 4%) of the glucose ingested, and of this, 10.5 +/- 2.4 g (63% +/- 7%) was synthesized by the direct pathway. In conclusion, after ingestion of 98 g of glucose: (1) 16.7 +/- 3.8 g (17% +/- 4%) glucose was stored in the liver as glycogen, and (2) 63% +/- 7% (10.5 +/- 2.4 g) of this glycogen was formed via the direct pathway.

Nuclear magnetic resonance studies of hepatic glucose metabolism in humans.

Nuclear magnetic resonance (NMR) spectroscopy has made noninvasive and repetitive measurements of human hepatic glycogen concentrations possible. Monitoring of liver glycogen in real-time mode has demonstrated that glycogen concentrations decrease linearly and that net hepatic glycogenolysis contributes only about 50 percent to glucose production during the early period of a fast. Following a mixed meal, hepatic glycogen represents approximately 20 percent of the ingested carbohydrates, while only about 10 percent of an intravenous glucose load is retained by the liver as glycogen. During mixed-meal ingestion, poorly controlled type 1 diabetic patients synthesize only about 30 percent of the glycogen stored in livers of nondiabetic humans studied under similar conditions. Reduced net glycogen synthesis can be improved but not normalized by short-term, intensified insulin treatment. A decreased increment in liver glycogen content following meals was also found in patients with maturity-onset diabetes of the young due to glucokinase mutations (MODY-2). In patients with poorly controlled type 2 diabetes, fasting hyperglycemia can be attributed mainly to increased rates of endogenous glucose production, which was found by 13C NMR to be due to increased rates of gluconeogenesis. Metformin treatment improved fasting hyperglycemia in these patients through a reduction in hepatic glucose production, which could be attributed to a decrease in gluconeogenesis. In conclusion, NMR spectroscopy has provided new insights into the pathogenesis of hyperglycemia in type 1, type 2, and MODY diabetes and offers the potential of providing new insights into the mechanism of action of novel antidabetic therapies.

In vivo effects of uncoupling protein-3 gene disruption on mitochondrial energy metabolism.

To clarify the role of uncoupling protein-3 (UCP3) in skeletal muscle, we used NMR and isotopic labeling experiments to evaluate the effect of UCP3 knockout (UCP3KO) in mice on the regulation of energy metabolism in vivo. Whole body energy expenditure was determined from the turnover of doubly labeled body water. Coupling of mitochondrial oxidative phosphorylation in skeletal muscle was evaluated from measurements of rates of ATP synthesis (using (31)P NMR magnetization transfer experiments) and tricarboxylic acid (TCA) cycle flux (calculated from the time course of (13)C enrichment in C-4 and C-2 of glutamate during an infusion of [2-(13)C]acetate). At the whole body level, we observed no change in energy expenditure. However, at the cellular level, skeletal muscle UCP3KO increased the rate of ATP synthesis from P(i) more than 4-fold under fasting conditions (wild type, 2.2 +/- 0.6 versus knockout, 9.1 +/- 1.4 micromol/g of muscle/min, p < 0.001) with no change in TCA cycle flux rate (wild type, 0.74 +/- 0.04 versus knockout, 0.71 +/- 0.03 micromol/g of muscle/min). The increased efficiency of ATP production may account for the significant (p < 0.05) increase in the ratio of ATP to ADP in the muscle of UCP3KO mice (5.9 +/- 0.3) compared with controls (4.5 +/- 0.4). The data presented here provide the first evidence of uncoupling activity by UCP3 in skeletal muscle in vivo.