Effect of increased free fatty acid supply on glucose metabolism and skeletal muscle glycogen synthase activity in normal man

Epigenetic Modifications Associated with the Pathogenesis of Type 2 Diabetes Mellitus

BACKGROUND AND OBJECTIVE

Type 2 diabetes mellitus (T2DM) is a multifactorial metabolic disorder. Pancreatic β-cell dysfunction and insulin resistance are the most common and crucial events of T2DM. Increasing evidence suggests the association of epigenetic modifications with the pathogenesis of T2DM through the changes in important biological processes including pancreatic β- cell differentiation, development and maintenance of normal β-cell function. Insulin sensitivity by the peripheral glucose uptake tissues is also changed by the altered epigenetic mechanisms. In this review, we discussed the major epigenetic alterations and their effects on β-cell function, insulin secretion and insulin resistance in context of T2DM.

METHODS

We investigated the presently available epigenetic modifications including DNA methylation, posttranslational histone modifications, ATP-dependent chromatin remodeling and non-coding RNAs related to the pathogenesis of T2DM. Published literatures on this topic were searched both on Google Scholar and Pubmed with related keywords and investigated for relevant information.

RESULTS

The epigenetic modifications introduce changes in gene expression which are essential for appropriate β-cell development and functions, insulin secretion and sensitivity resulting in the pathogenesis of T2DM. Interestingly, T2DM could also be a prominent reason for the mentioned epigenetic alterations.

CONCLUSION

This review article emphasized on the epigenetic modifications associated with T2DM and discussed the consequences in deterioration of the disease condition.

Consuming snacks mid-afternoon compared with just after lunch improves mean amplitude of glycaemic excursions in patients with type 2 diabetes: A randomized crossover clinical trial

AIMS

Our aim was to explore the acute effects of consuming snacks at different times on glucose excursions in patients with type 2 diabetes (T2D).

METHODS

Seventeen patients with T2D [means±SD: age 67.4±9.4-years; BMI 23.5±3.1kg/m²; HbA_1c 55±6mmol/mol (7.2±1.0%)] were randomly assigned in this crossover study. Each participant wore a continuous glucose monitoring device for 4 days and consumed identical test meals on the second and third days, comprising breakfast at 0700h, lunch at 1200h and dinner at 1900h. Half the participants consumed 75kcal biscuits at 1230h (just after lunch) on the second day and at 1530h (mid-afternoon) on the third day, while the other half consumed snacks at the same times, but vice versa. Each patient's glucose parameters were compared against baseline for the 2days of snacking at different times of day.

RESULTS

Consuming snacks in the mid-afternoon led to significantly lower mean amplitudes of glycaemic excursions (mean±SEM: 5.19±0.48 vs. 6.90±0.69mmol/L, P<0.01; standard deviation: 1.75±0.17 vs. 2.16±0.21mmol/L, P<0.01) and incremental areas under the curve for glucose after dinner (479±76 vs. 663±104mmol/L per min, P<0.01) compared with snacking just after lunch, whereas mean glucose levels did not differ over the 2days.

CONCLUSION

These results suggest that consuming snacks well separated from lunch may be an effective way to suppress postprandial glucose levels and glycaemic excursions.

Royal jelly supplementation reduces skeletal muscle lipotoxicity and insulin resistance in aged obese rats

BACKGROUND

Consumption of a high-fat diet (HFD) in aged rats is associated with several metabolic disorders. The mechanism of skeletal muscle lipotoxicity and insulin resistance (IR) is multi-factorial, but the exact mechanism of how aging affects these processes unknown. Royal jelly (RJ) is a dietary supplement with many physiological and pharmacological properties. No previous studies have demonstrated the protective effects and mechanism of RJ in aged obese rats.

OBJECTIVES

The study was carried to investigate the effects of aging and HFD on skeletal muscles, and adipose tissue metabolism and inflammation, in aged rats, and whether RJ could combat such adverse effects.

METHODOLOGY

A total of 40 male rats were divided into5 groups; young rats fed a standard diet, aged rats fed a standard diet, aged rats fed RJ, aged rats fed a HFD, and aged rats fed both a HFD and RJ for 8 weeks. We investigated changes in body weights (BW), abdominal fat weights, total cholesterol, triglycerides (TG), low density lipoprotein-cholesterol (LDL-c), high density lipoprotein-cholesterol (HDL-c), muscle TG, and IR levels. Also, concentrations of TNF-α receptor 1(TNFR1) were estimated in the serum and adipose tissues.

RESULTS

Aged, obese rats showed increased BW, adipose weights, IR, and disturbed serum and muscle lipids. Also, TNFR1 was increased. Rats fed RJ showed decreased adiposity, improved lipids' profiles, improved IR, and decreased TNFR1.

CONCLUSION

Aging and HFD were associated with disturbed metabolism, and muscle lipotoxicity and inflammation, while RJ could counteract muscle lipotoxicity in rats and reduce IR, most likely due to an anti-inflammatory effect.

DNA methylation in the pathogenesis of type 2 diabetes in humans

Background

Type 2 diabetes (T2D) is a multifactorial, polygenic disease caused by impaired insulin secretion and insulin resistance. Genome-wide association studies (GWAS) were expected to resolve a large part of the genetic component of diabetes; yet, the single nucleotide polymorphisms identified by GWAS explain less than 20% of the estimated heritability for T2D. There was subsequently a need to look elsewhere to find disease-causing factors. Mechanisms mediating the interaction between environmental factors and the genome, such as epigenetics, may be of particular importance in the pathogenesis of T2D.

Scope of Review

This review summarizes knowledge of the impact of epigenetics on the pathogenesis of T2D in humans. In particular, the review will focus on alterations in DNA methylation in four human tissues of importance for the disease; pancreatic islets, skeletal muscle, adipose tissue, and the liver. Case–control studies and studies examining the impact of non-genetic and genetic risk factors on DNA methylation in humans will be considered. These studies identified epigenetic changes in tissues from subjects with T2D versus non-diabetic controls. They also demonstrate that non-genetic factors associated with T2D such as age, obesity, energy rich diets, physical activity and the intrauterine environment impact the epigenome in humans. Additionally, interactions between genetics and epigenetics seem to influence the pathogenesis of T2D.

Conclusions

Overall, previous studies by our group and others support a key role for epigenetics in the growing incidence of T2D.

Divided consumption of late-night-dinner improves glucose excursions in young healthy women: A randomized cross-over clinical trial

AIMS

Our aim was to explore the acute effect of the late-night-dinner and the divided-dinner on postprandial glucose levels in young healthy women.

METHODS

Fourteen women (22.6 ± 2.6 years, BMI 20.2 ± 1.5 kg/m²: mean ± SD) were randomly assigned to this crossover study. Each participant wore a continuous glucose monitor for 5 days and consumed identical test meals from the second to the fourth day at home. Each participant consumed the test meals of breakfast at 0800 h, lunch at 1300 h, and the half of the participants consumed dinner at 2100 h (D21) on the second day, 1800 h (D18) on the third day, and divided dinner (DD: vegetable and rice at 1800 h, and vegetable and the main dish at 2100 h) on the fourth day. The rest of the participants consumed DD on the second day, and D21 on the fourth day.

RESULTS

D21 demonstrated higher incremental glucose peak (IGP 2.74 ± 0.38 vs. 1.57 ± 0.23 mmol/L, p < .05, mean ± SEM) and incremental area under the curve for glucose (IAUC) 2300-0800 h (271 ± 63 vs. 111 ± 37 mmol/L × min, p < .05) than D18. On the other hand, DD ameliorated IGP (1.96 ± 0.29 mmol/L, p < .05), IAUC 2300-0800 h (80 ± 29 mmol/L × min, p < .001), and the mean amplitude of glycemic excursion (DD 2.34 ± 0.25 vs. D21 2.91 ± 0.28 mmol/L, p < .05) than D21.

CONCLUSIONS

Consuming late-night-dinner increased postprandial glucose levels, compared to DD, suggesting DD could be a practical strategy for reduction of postprandial glucose levels in young healthy women.

Foundations of Immunometabolism and Implications for Metabolic Health and Disease

Highly ordered interactions between immune and metabolic responses are evolutionarily conserved and paramount for tissue and organismal health. Disruption of these interactions underlies the emergence of many pathologies, particularly chronic non-communicable diseases such as obesity and diabetes. Here, we examine decades of research identifying the complex immunometabolic signaling networks and the cellular and molecular events that occur in the setting of altered nutrient and energy exposures and offer a historical perspective. Furthermore, we describe recent advances such as the discovery that a broad complement of immune cells play a role in immunometabolism and the emerging evidence that nutrients and metabolites modulate inflammatory pathways. Lastly, we discuss how this work may eventually lead to tangible therapeutic advancements to promote health.

Divided consumption of late-night-dinner improves glycemic excursions in patients with type 2 diabetes: A randomized cross-over clinical trial

AIMS

To explore the acute effect of late-night-dinner and divided dinner on postprandial glucose levels in patients with type 2 diabetes.

METHODS

Sixteen patients were randomly assigned to this cross-over study. Each patient wore a continuous glucose monitor for 5days and consumed identical test meals for 3days. Patients consumed the test meals of dinner at 2100h (D21) or divided dinner (vegetable and rice at 1800h and the vegetable and the main dish at 2100h) on the second or fourth day, and dinner at 1800h (D18) on the third day. The daily glucose parameters were compared within-patient for 3days.

RESULTS

D21 demonstrated significantly higher values of incremental area under the curve (IAUC) for glucose 2300 to 0800h (644±156vs. 147±63mmol/L×min, p<0.01, mean±standard error of the mean) and incremental glucose peak (IGP) after dinner (6.78±0.79 vs. 3.09±0.62mmol/L, p<0.01) compared to those of D18. Moreover, the mean amplitude of glycemic excursion (MAGE) of D21 tended to be higher than that of D18 (6.99±0.60 vs. 5.35±0.51mmol/L, p=0.077). However, the divided dinner significantly reduced IAUC 2300 to 0800h (142±60mmol/L×min, p<0.01), IGP after dinner (3.75±0.58mmol/L, p<0.01), and MAGE (5.33±0.41mmol/L, p<0.01) compared to those of D21.

CONCLUSION

Our findings demonstrated that consuming late-night-dinner led to postprandial hyperglycemia, and this postprandial hyperglycemia can be ameliorated by consuming a divided dinner.

Glycogen metabolism in humans

In the human body, glycogen is a branched polymer of glucose stored mainly in the liver and the skeletal muscle that supplies glucose to the blood stream during fasting periods and to the muscle cells during muscle contraction. Glycogen has been identified in other tissues such as brain, heart, kidney, adipose tissue, and erythrocytes, but glycogen function in these tissues is mostly unknown. Glycogen synthesis requires a series of reactions that include glucose entrance into the cell through transporters, phosphorylation of glucose to glucose 6-phosphate, isomerization to glucose 1-phosphate, and formation of uridine 5'-diphosphate-glucose, which is the direct glucose donor for glycogen synthesis. Glycogenin catalyzes the formation of a short glucose polymer that is extended by the action of glycogen synthase. Glycogen branching enzyme introduces branch points in the glycogen particle at even intervals. Laforin and malin are proteins involved in glycogen assembly but their specific function remains elusive in humans. Glycogen is accumulated in the liver primarily during the postprandial period and in the skeletal muscle predominantly after exercise. In the cytosol, glycogen breakdown or glycogenolysis is carried out by two enzymes, glycogen phosphorylase which releases glucose 1-phosphate from the linear chains of glycogen, and glycogen debranching enzyme which untangles the branch points. In the lysosomes, glycogen degradation is catalyzed by α-glucosidase. The glucose 6-phosphatase system catalyzes the dephosphorylation of glucose 6-phosphate to glucose, a necessary step for free glucose to leave the cell. Mutations in the genes encoding the enzymes involved in glycogen metabolism cause glycogen storage diseases.

Partial Adenosine A1 Agonist in Heart Failure

Adenosine exerts a variety of physiological effects by binding to cell surface G-protein-coupled receptor subtypes, namely, A1, A2a, A2b, and A3. The central physiological role of adenosine is to preclude tissue injury and promote repair in response to stress. In the heart, adenosine acts as a cytoprotective modulator, linking cardiac function to metabolic demand predominantly via activation of adenosine A1 receptors (A1Rs), which leads to inhibition of adenylate cyclase activity, modulation of protein kinase C, and opening of ATP-sensitive potassium channels. Activation of myocardial adenosine A1Rs has been shown to modulate a variety of pathologies associated with ischemic cardiac injury, including arrhythmogenesis, coronary and ventricular dysfunction, apoptosis, mitochondrial dysfunction, and ventricular remodeling. Partial A1R agonists are agents that are likely to elicit favorable pharmacological responses in heart failure (HF) without giving rise to the undesirable cardiac and extra-cardiac effects observed with full A1R agonism. Preclinical data have shown that partial adenosine A1R agonists protect and improve cardiac function at doses that do not result in undesirable effects on heart rate, atrioventricular conduction, and blood pressure, suggesting that these compounds may constitute a valuable new therapy for chronic HF. Neladenoson bialanate (BAY1067197) is the first oral partial and highly selective A1R agonist that has entered clinical development for the treatment of HF. This review provides an overview of adenosine A1R-mediated signaling in the heart, summarizes the results from preclinical and clinical studies of partial A1R agonists in HF, and discusses the potential benefits of these drugs in the clinical setting.

In vivo high-resolution magic angle spinning magnetic and electron paramagnetic resonance spectroscopic analysis of mitochondria-targeted peptide in Drosophila melanogaster with trauma-induced thoracic injury

Trauma is the most common cause of mortality among individuals aged between 1 and 44 years and the third leading cause of mortality overall in the US. In this study, we examined the effects of trauma on the expression of genes in Drosophila melanogaster, a useful model for investigating genetics and physiology. After trauma was induced by a non-lethal needle puncture of the thorax, we observed the differential expression of genes encoding for mitochondrial uncoupling proteins, as well as those encoding for apoptosis-related and insulin signaling-related proteins, thus indicating muscle functional dysregulation. These results prompted us to examine the link between insulin signaling and mitochondrial dysfunction using in vivo nuclear magnetic resonance (NMR) with complementary electron paramagnetic resonance (EPR) spectroscopy. Trauma significantly increased insulin resistance biomarkers, and the NMR spectral profile of the aged flies with trauma-induced thoracic injury resembled that of insulin-resistant chico mutant flies. In addition, the mitochondrial redox status, as measured by EPR, was significantly altered following trauma, indicating mitochondrial uncoupling. A mitochondria-targeted compound, Szeto-Schiller (SS)-31 that promotes adenosine triphosphate (ATP) synthesis normalized the NMR spectral profile, as well as the mitochondrial redox status of the flies with trauma-induced thoracic injury, as assessed by EPR. Based on these findings, we propose a molecular mechanism responsible for trauma-related mortality and also propose that trauma sequelae in aging are linked to insulin signaling and mitochondrial dysfunction. Our findings further suggest that SS-31 attenuates trauma-associated pathological changes.

Fasting until noon triggers increased postprandial hyperglycemia and impaired insulin response after lunch and dinner in individuals with type 2 diabetes: a randomized clinical trial

OBJECTIVE

Skipping breakfast has been consistently associated with high HbA1c and postprandial hyperglycemia (PPHG) in patients with type 2 diabetes. Our aim was to explore the effect of skipping breakfast on glycemia after a subsequent isocaloric (700 kcal) lunch and dinner.

RESEARCH DESIGN AND METHODS

In a crossover design, 22 patients with diabetes with a mean diabetes duration of 8.4 ± 0.7 years, age 56.9 ± 1.0 years, BMI 28.2 ± 0.6 kg/m(2), and HbA1c 7.7 ± 0.1% (61 ± 0.8 mmol/mol) were randomly assigned to two test days: one day with breakfast, lunch, and dinner (YesB) and another with lunch and dinner but no breakfast (NoB). Postprandial plasma glucose, insulin, C-peptide, free fatty acids (FFA), glucagon, and intact glucagon-like peptide-1 (iGLP-1) were assessed.

RESULTS

Compared with YesB, lunch area under the curves for 0-180 min (AUC0-180) for plasma glucose, FFA, and glucagon were 36.8, 41.1, and 14.8% higher, respectively, whereas the AUC0-180 for insulin and iGLP-1 were 17% and 19% lower, respectively, on the NoB day (P < 0.0001). Similarly, dinner AUC0-180 for glucose, FFA, and glucagon were 26.6, 29.6, and 11.5% higher, respectively, and AUC0-180 for insulin and iGLP-1 were 7.9% and 16.5% lower on the NoB day compared with the YesB day (P < 0.0001). Furthermore, insulin peak was delayed 30 min after lunch and dinner on the NoB day compared with the YesB day.

CONCLUSIONS

Skipping breakfast increases PPHG after lunch and dinner in association with lower iGLP-1 and impaired insulin response. This study shows a long-term influence of breakfast on glucose regulation that persists throughout the day. Breakfast consumption could be a successful strategy for reduction of PPHG in type 2 diabetes.

Endogenous circadian system and circadian misalignment impact glucose tolerance via separate mechanisms in humans

Glucose tolerance is lower in the evening and at night than in the morning. However, the relative contribution of the circadian system vs. the behavioral cycle (including the sleep/wake and fasting/feeding cycles) is unclear. Furthermore, although shift work is a diabetes risk factor, the separate impact on glucose tolerance of the behavioral cycle, circadian phase, and circadian disruption (i.e., misalignment between the central circadian pacemaker and the behavioral cycle) has not been systematically studied. Here we show--by using two 8-d laboratory protocols--in healthy adults that the circadian system and circadian misalignment have distinct influences on glucose tolerance, both separate from the behavioral cycle. First, postprandial glucose was 17% higher (i.e., lower glucose tolerance) in the biological evening (8:00 PM) than morning (8:00 AM; i.e., a circadian phase effect), independent of the behavioral cycle effect. Second, circadian misalignment itself (12-h behavioral cycle inversion) increased postprandial glucose by 6%. Third, these variations in glucose tolerance appeared to be explained, at least in part, by different mechanisms: during the biological evening by decreased pancreatic β-cell function (27% lower early-phase insulin) and during circadian misalignment presumably by decreased insulin sensitivity (elevated postprandial glucose despite 14% higher late-phase insulin) without change in early-phase insulin. We explored possible contributing factors, including changes in polysomnographic sleep and 24-h hormonal profiles. We demonstrate that the circadian system importantly contributes to the reduced glucose tolerance observed in the evening compared with the morning. Separately, circadian misalignment reduces glucose tolerance, providing a mechanism to help explain the increased diabetes risk in shift workers.

Banting Memorial lecture 2012: reversing the twin cycles of type 2 diabetes

It has become widely accepted that Type 2 diabetes is inevitably life-long, with irreversible and progressive beta cell damage. However, the restoration of normal glucose metabolism within days after bariatric surgery in the majority of people with Type 2 diabetes disproves this concept. There is now no doubt that this reversal of diabetes depends upon the sudden and profound decrease in food intake, and does not relate to any direct surgical effect. The Counterpoint study demonstrated that normal glucose levels and normal beta cell function could be restored by a very low calorie diet alone. Novel magnetic resonance methods were applied to measure intra-organ fat. The results showed two different time courses: a) resolution of hepatic insulin sensitivity within days along with a rapid fall in liver fat and normalisation of fasting glucose levels; and b) return of normal beta cell insulin secretion over weeks in step with a fall in pancreas fat. Now that it has been possible to observe the pathophysiological events during reversal of Type 2 diabetes, the reverse time course of events which determine the onset of the condition can be identified. The twin cycle hypothesis postulates that chronic calorie excess leads to accumulation of liver fat with eventual spill over into the pancreas. These self-reinforcing cycles between liver and pancreas eventually cause metabolic inhibition of insulin secretion after meals and onset of hyperglycaemia. It is now clear that Type 2 diabetes is a reversible condition of intra-organ fat excess to which some people are more susceptible than others.

Mechanisms underlying skeletal muscle insulin resistance induced by fatty acids: importance of the mitochondrial function

Insulin resistance condition is associated to the development of several syndromes, such as obesity, type 2 diabetes mellitus and metabolic syndrome. Although the factors linking insulin resistance to these syndromes are not precisely defined yet, evidence suggests that the elevated plasma free fatty acid (FFA) level plays an important role in the development of skeletal muscle insulin resistance. Accordantly, in vivo and in vitro exposure of skeletal muscle and myocytes to physiological concentrations of saturated fatty acids is associated with insulin resistance condition. Several mechanisms have been postulated to account for fatty acids-induced muscle insulin resistance, including Randle cycle, oxidative stress, inflammation and mitochondrial dysfunction. Here we reviewed experimental evidence supporting the involvement of each of these propositions in the development of skeletal muscle insulin resistance induced by saturated fatty acids and propose an integrative model placing mitochondrial dysfunction as an important and common factor to the other mechanisms.

In vivo high-resolution magic angle spinning magnetic resonance spectroscopy of Drosophila melanogaster at 14.1 T shows trauma in aging and in innate immune-deficiency is linked to reduced insulin signaling

In vivo magnetic resonance spectroscopy (MRS), a non-destructive biochemical tool for investigating live organisms, has yet to be used in the fruit fly Drosophila melanogaster, a useful model organism for investigating genetics and physiology. We developed and implemented a high-resolution magic-angle-spinning (HRMAS) MRS method to investigate live Drosophila at 14.1 T. We demonstrated, for the first time, the feasibility of using HRMAS MRS for molecular characterization of Drosophila with a conventional MR spectrometer equipped with an HRMAS probe. We showed that the metabolic HRMAS MRS profiles of injured, aged wild-type (wt) flies and of immune deficient (imd) flies were more similar to chico flies mutated at the chico gene in the insulin signaling pathway, which is analogous to insulin receptor substrate1-4 (IRS1-4) in mammals and less to those of adipokinetic hormone receptor (akhr) mutant flies, which have an obese phenotype. We thus provide evidence for the hypothesis that trauma in aging and in innate immune-deficiency is linked to insulin signaling. This link may explain the mitochondrial dysfunction that accompanies insulin resistance and muscle wasting that occurs in trauma, aging and immune system deficiencies, leading to higher susceptibility to infection. Our approach advances the development of novel in vivo non-destructive research approaches in Drosophila, suggests biomarkers for investigation of biomedical paradigms, and thus may contribute to novel therapeutic development.

The second-meal phenomenon is associated with enhanced muscle glycogen storage in humans

The rise in blood glucose after lunch is less if breakfast has been eaten. The metabolic basis of this second-meal phenomenon remains uncertain. We hypothesized that storage of ingested glucose as glycogen could be responsible during the post-meal suppression of plasma NEFAs (non-esterified fatty acids; 'free' fatty acids). In the present study we determined the metabolic basis of the second-meal phenomenon. Healthy subjects were studied on two separate days, with breakfast and without breakfast in a random order. We studied metabolic changes after a standardized test lunch labelled with 3 g of 13C-labelled (99%) glucose. Changes in post-prandial muscle glycogen storage were measured using 13C magnetic resonance spectroscopy. The rise in plasma glucose after lunch was significantly less if breakfast had been taken (0.9+/-0.3 compared with 3.2+/-0.3 mmol/l, with and without breakfast respectively; P<0.001), despite comparable insulin responses. Pre-lunch NEFAs were suppressed after breakfast (0.13+/-0.03 compared with 0.51+/-0.04 mmol/l) and levels correlated positively with the maximum glucose rise after lunch (r=0.62, P=0.001). The increase in muscle glycogen signal was greater 5 h after lunch on the breakfast day (103+/-21 compared with 48+/-12 units; P<0.007) and correlated negatively with plasma NEFA concentrations before lunch (r=-0.48, P<0.05). The second-meal effect is associated with priming of muscle glycogen synthesis consequent upon sustained suppression of plasma NEFA concentrations.

The second-meal phenomenon in type 2 diabetes

OBJECTIVE

In health, the rise in glucose after lunch is less if breakfast is eaten. We evaluated the second-meal effect in type 2 diabetes.

RESEARCH DESIGN AND METHODS

Metabolic changes after lunch in eight obese type 2 diabetic subjects were compared on 3 days: breakfast eaten, no breakfast, and no breakfast but intravenous arginine 1 h before lunch.

RESULTS

Despite comparable insulin levels, the rise in plasma glucose after lunch was considerably less if breakfast had been eaten (0.68 +/- 1.49 vs. 12.32 +/- 1.73 vs. 7.88 +/- 1.03 mmol x h(-1) x l(-1); P < 0.0001). Arginine administration almost halved the lunch rise in plasma glucose (12.32 +/- 1.73 vs. 7.88 +/- 1.03 mmol x h(-1) x l(-1)). The plasma free fatty acid concentration at lunchtime directly related to plasma glucose rise after lunch (r = 0.67, P = 0.0005).

CONCLUSIONS

The second-meal effect is preserved in type 2 diabetes. Premeal administration of a nonglucose insulin secretagogue results in halving the postprandial glucose rise and has therapeutic potential.

Updating the effects of fatty acids on skeletal muscle

In this review we updated the fatty acid (FA) effects on skeletal muscle metabolism. Abnormal FA availability induces insulin resistance and accounts for several of its symptoms and complications. Efforts to understand the pathogenesis of insulin resistance are focused on disordered lipid metabolism and consequently its effect on insulin signaling pathway. We reviewed herein the FA effects on metabolism, signaling, regulation of gene expression and oxidative stress in insulin resistance. The elevated IMTG content has been associated with increased intracellular content of diacylglycerol (DAG), ceramides and long-chain acyl-coenzyme A (LCA-CoA). This condition has been shown to promote insulin resistance by interfering with phosphorylation of proteins of the insulin pathway including insulin receptor substrate-1/2 (IRS), phosphatidylinositol-3-kinase, (PI3-kinase) and protein kinase C. Although the molecular mechanism is not completely understood, elevated reactive oxygen (ROS) and nitrogen species (RNS) are involved in this process. Elevated ROS/RNS activates nuclear factor-kappaB (NFkB), which promotes the transcription of proinflammatory tumoral necrosis factor alpha (TNFalpha), decreasing the insulin response. Therefore, oxidative stress induced by elevated FA availability may constitute one of the major causes of insulin resistance in skeletal muscle.

The role of skeletal muscle insulin resistance in the pathogenesis of the metabolic syndrome

We examined the hypothesis that insulin resistance in skeletal muscle promotes the development of atherogenic dyslipidemia, associated with the metabolic syndrome, by altering the distribution pattern of postprandial energy storage. Following ingestion of two high carbohydrate mixed meals, net muscle glycogen synthesis was reduced by approximately 60% in young, lean, insulin-resistant subjects compared with a similar cohort of age-weight-body mass index-activity-matched, insulin-sensitive, control subjects. In contrast, hepatic de novo lipogenesis and hepatic triglyceride synthesis were both increased by >2-fold in the insulin-resistant subjects. These changes were associated with a 60% increase in plasma triglyceride concentrations and an approximately 20% reduction in plasma high-density lipoprotein concentrations but no differences in plasma concentrations of TNF-alpha, IL-6, adiponectin, resistin, retinol binding protein-4, or intraabdominal fat volume. These data demonstrate that insulin resistance in skeletal muscle, due to decreased muscle glycogen synthesis, can promote atherogenic dyslipidemia by changing the pattern of ingested carbohydrate away from skeletal muscle glycogen synthesis into hepatic de novo lipogenesis, resulting in an increase in plasma triglyceride concentrations and a reduction in plasma high-density lipoprotein concentrations. Furthermore, insulin resistance in these subjects was independent of changes in the plasma concentrations of TNF-alpha, IL-6, high-molecular-weight adiponectin, resistin, retinol binding protein-4, or intraabdominal obesity, suggesting that these factors do not play a primary role in causing insulin resistance in the early stages of the metabolic syndrome.

Proton NMR spectroscopy shows lipids accumulate in skeletal muscle in response to burn trauma‐induced apoptosis

Burn trauma triggers hypermetabolism and muscle wasting via increased cellular protein degradation and apoptosis. Proton nuclear magnetic resonance (1H NMR) spectroscopy can detect mobile lipids in vivo. To examine the local effects of burn in skeletal muscle, we performed in vivo 1H NMR on mice 3 days after burn trauma; and ex vivo, high-resolution, magic angle spinning (1)H NMR on intact excised mouse muscle samples before and 1 and 3 days after burn. These samples were then analyzed for apoptotic nuclei using a terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assay. To confirm our NMR and cell biology results, we used transcriptome analysis to demonstrate that burn trauma alters the expression of genes involved in lipid metabolism and apoptosis. Our results demonstrate that burn injury results in a localized intramyocellular lipid accumulation, which in turn is accompanied by burn-induced apoptosis and mitochondrial dysfunction, as seen by the up-regulation of apoptotic genes and down-regulation of genes that encode lipid oxidation and the peroxisomal proliferator activator receptor gamma coactivator PGC-1beta. Moreover, the increased levels of bisallylic methylene fatty acyl protons (2.8 ppm) and vinyl protons (5.4 ppm), in conjunction with the TUNEL assay results, further suggest that burn trauma results in apoptosis. Together, our results provide new insight into the local physiological changes that occur in skeletal muscle after severe burn trauma.

Dose-response effect of elevated plasma free fatty acid on insulin signaling

The dose-response relationship between elevated plasma free fatty acid (FFA) levels and impaired insulin-mediated glucose disposal and insulin signaling was examined in 21 lean, healthy, normal glucose-tolerant subjects. Following a 4-h saline or Liposyn infusion at 30 (n = 9), 60 (n = 6), and 90 (n = 6) ml/h, subjects received a 2-h euglycemic insulin (40 mU . m(-2) . min(-1)) clamp. Basal plasma FFA concentration ( approximately 440 micromol/l) was increased to 695, 1,251, and 1,688 micromol/l after 4 h of Liposyn infusion and resulted in a dose-dependent reduction in insulin-stimulated glucose disposal (R(d)) by 22, 30, and 34%, respectively (all P < 0.05 vs. saline control). At the lowest lipid infusion rate (30 ml/h), insulin receptor and insulin receptor substrate (IRS)-1 tyrosine phosphorylation, phosphatidylinositol (PI) 3-kinase activity associated with IRS-1, and Akt serine phosphorylation were all significantly impaired (P < 0.05-0.01). The highest lipid infusion rate (90 ml/h) caused a further significant reduction in all insulin signaling events compared with the low-dose lipid infusion (P < 0.05-0.01) whereas the 60-ml/h lipid infusion caused an intermediate reduction in insulin signaling. However, about two-thirds of the maximal inhibition of insulin-stimulated glucose disposal already occurred at the rather modest increase in plasma FFA induced by the low-dose (30-ml/h) lipid infusion. Insulin-stimulated glucose disposal was inversely correlated with both the plasma FFA concentration after 4 h of lipid infusion (r = -0.50, P = 0.001) and the plasma FFA level during the last 30 min of the insulin clamp (r = -0.54, P < 0.001). PI 3-kinase activity associated with IRS-1 correlated with insulin-stimulated glucose disposal (r = 0.45, P < 0.01) and inversely with both the plasma FFA concentration after 4 h of lipid infusion (r = -0.39, P = 0.01) and during the last 30 min of the insulin clamp (r = -0.43, P < 0.01). In summary, in skeletal muscle of lean, healthy subjects, a progressive increase in plasma FFA causes a dose-dependent inhibition of insulin-stimulated glucose disposal and insulin signaling. The inhibitory effect of plasma FFA was already significant following a rather modest increase in plasma FFA and develops at concentrations that are well within the physiological range (i.e., at plasma FFA levels observed in obesity and type 2 diabetes).

Discordant effects of a chronic physiological increase in plasma FFA on insulin signaling in healthy subjects with or without a family history of type 2 diabetes

Muscle insulin resistance develops when plasma free fatty acids (FFAs) are acutely increased to supraphysiological levels (approximately 1,500-4,000 micromol/l). However, plasma FFA levels >1,000 micromol/l are rarely observed in humans under usual living conditions, and it is unknown whether insulin action may be impaired during a sustained but physiological FFA increase to levels seen in obesity and type 2 diabetes mellitus (T2DM) (approximately 600-800 micromol/l). It is also unclear whether normal glucose-tolerant subjects with a strong family history of T2DM (FH+) would respond to a low-dose lipid infusion as individuals without any family history of T2DM (CON). To examine these questions, we studied 7 FH+ and 10 CON subjects in whom we infused saline (SAL) or low-dose Liposyn (LIP) for 4 days. On day 4, a euglycemic insulin clamp with [3-3H]glucose and indirect calorimetry was performed to assess glucose turnover, combined with vastus lateralis muscle biopsies to examine insulin signaling. LIP increased plasma FFA approximately 1.5-fold, to levels seen in T2DM. Compared with CON, FH+ were markedly insulin resistant and had severely impaired insulin signaling in response to insulin stimulation. LIP in CON reduced insulin-stimulated glucose disposal (Rd) by 25%, insulin-stimulated insulin receptor tyrosine phosphorylation by 17%, phosphatidylinositol 3-kinase activity associated with insulin receptor substrate-1 by 20%, and insulin-stimulated glycogen synthase fractional velocity over baseline (44 vs. 15%; all P < 0.05). In contrast to CON, a physiological elevation in plasma FFA in FH+ led to no further deterioration in Rd or to any additional impairment of insulin signaling. In conclusion, a 4-day physiological increase in plasma FFA to levels seen in obesity and T2DM impairs insulin action/insulin signaling in CON but does not worsen insulin resistance in FH+. Whether this lack of additional deterioration in insulin signaling in FH+ is due to already well-established lipotoxicity, or to other molecular mechanisms related to insulin resistance that are nearly maximally expressed early in life, remains to be determined.

How free fatty acids inhibit glucose utilization in human skeletal muscle

Rat muscle studies suggest competition between free fatty acids (FFA) and glucose for oxidation, resulting in glucose-6-phosphate accumulation. However, FFA decrease glucose-6-phosphate in human skeletal muscle, indicating direct inhibition of glucose transport/phosphorylation. This mechanism could redirect glucose from muscle to brain during fasting and explain the insulin resistance associated with high-lipid diets and obesity.

Mitochondrial dysfunction in the elderly: possible role in insulin resistance

Insulin resistance is a major factor in the pathogenesis of type 2 diabetes in the elderly. To investigate how insulin resistance arises, we studied healthy, lean, elderly and young participants matched for lean body mass and fat mass. Elderly study participants were markedly insulin-resistant as compared with young controls, and this resistance was attributable to reduced insulin-stimulated muscle glucose metabolism. These changes were associated with increased fat accumulation in muscle and liver tissue assessed by 1H nuclear magnetic resonance (NMR) spectroscopy, and with a approximately 40% reduction in mitochondrial oxidative and phosphorylation activity, as assessed by in vivo 13C/31P NMR spectroscopy. These data support the hypothesis that an age-associated decline in mitochondrial function contributes to insulin resistance in the elderly.

The role of glycogen synthase kinase-3 in insulin resistance and type 2 diabetes

Glycogen synthase kinase-3 (GSK-3) is a ubiquitous cytosolic serine/threonine protein kinase that has been implicated in multiple receptor-mediated intracellular processes. Its unique feature, which distinguishes it from other protein kinases, is that it is constitutively active in resting conditions and acts as a suppressor of signalling pathways. The fact that the function of two key targets of insulin action, glycogen synthase and insulin receptor substrate-1, are suppressed by GSK-3, as well as the fact that GSK-3 activity is higher in diabetic tissues, makes it a promising drug discovery target for insulin resistance and Type 2 diabetes. Thus, the development of GSK-3 inhibitors has received attention as an attempt to control both the spread of the disease and its severity.

Effects of individual fatty acids on glucose uptake and glycogen synthesis in soleus muscle in vitro

Soleus muscle strips from Wistar rats were preincubated with palmitate in vitro before the determination of insulin-mediated glucose metabolism in fatty acid-free medium. Palmitate decreased insulin-stimulated glycogen synthesis to 51% of control in a time- (0-6 h) and concentration-dependent (0-2 mM) manner. Basal and insulin-stimulated glucose transport/phosphorylation also decreased with time, but the decrease occurred after the effect on glycogen synthesis. Preincubation with 1 mM palmitate, oleate, linoleate, or linolenate for 4 h impaired glycogen synthesis stimulated with a submaximal physiological insulin concentration (300 microU/ml) to 50-60% of the control response, and this reduction was associated with impaired insulin-stimulated phosphorylation of protein kinase B (PKB). Preincubation with different fatty acids (all 1 mM for 4 h) had varying effects on insulin-stimulated glucose transport/phosphorylation, which was decreased by oleate and linoleate, whereas palmitate and linolenate had little effect. Across groups, the rates of glucose transport/phosphorylation correlated with the intramuscular long-chain acyl-CoA content. The similar effects of individual fatty acids on glycogen synthesis but different effects on insulin-stimulated glucose transport/phosphorylation provide evidence that lipids may interact with these two pathways via different mechanisms.

Elevation of plasma fatty acids by ten-hour intralipid infusion has no effect on basal or glucose-stimulated insulin secretion in normal man

There is controversy over the effect of free fatty acids (FFAs) on insulin secretion. Previous studies have shown opposite effects of short- and long-term exposure to elevated concentrations of FFAs. We studied 8 normal subjects (mean age, 30 years; mean body mass index, 23.4 kg/m2) on 2 occasions. Each had a 10-hour overnight infusion of Intralipid 20% (Pharmacia, Milton Keynes, UK) with simultaneous infusion of heparin (0.4 U/kg body weight/min) or a control infusion of saline (150 mmol/L). Insulin secretion was assessed immediately after completion of the 10-hour infusion by an intravenous glucose tolerance test. Results were analyzed using paired ttests. Intralipid infusion caused an increase in plasma FFAs of more than 9-fold (P < .01), with a simultaneous increase in glycerol (P < .01) and hydroxybutyrate (P < .01). There was no difference in blood glucose concentrations during the infusion or intravenous glucose tolerance test. Similarly, insulin secretion was not significantly different during Intralipid infusion or in the intravenous glucose tolerance test (peak insulin achieved in glucose tolerance test, P = .51; total insulin secretion during intravenous glucose tolerance test, P = .27). In conclusion, after increasing plasma FFA concentrations over 9-fold during a 10-hour infusion of Intralipid and heparin, we found no difference in basal or glucose-stimulated insulin secretion.

Effect of fasting on the intracellular metabolic partition of intravenously infused glucose in humans

The effects of fasting on the pathways of insulin-stimulated glucose disposal were explored in three groups of seven normal subjects. Group 1 was submitted to a euglycemic hyperinsulinemic clamp ( approximately 100 microU/ml) after both a 12-h and a 4-day fast. The combined use of [3-(3)H]- and [U-(14)C]glucose allowed us to demonstrate that fasting inhibits, by approximately 50%, glucose disposal, glycolysis, glucose oxidation, and glycogen synthesis via the direct pathway. In group 2, in which the clamp glucose disposal during fasting was restored by hyperglycemia (155 +/- 15 mg/dl), fasting stimulated glycogen synthesis (+29 +/- 2%) and inhibited glycolysis (-32 +/- 3%) but only in its oxidative component (-40 +/- 3%). Results were similar in group 3 in which the clamp glucose disposal was restored by a pharmacological elevation of insulin ( approximately 2,800 microU/ml), but in this case, both glycogen synthesis and nonoxidative glycolysis participated in the rise in nonoxidative glucose disposal. In all groups, the reduction in total carbohydrate oxidation (indirect calorimetry) induced by fasting markedly exceeded the reduction in circulating glucose oxidation, suggesting that fasting also inhibits intracellular glycogen oxidation. Thus prior fasting favors glycogen retention by three mechanisms: 1) stimulation of glycogen synthesis via the direct pathway; 2) preferential inhibition of oxidative rather than nonoxidative glycolysis, thus allowing carbon conservation for glycogen synthesis via the indirect pathway; and 3) suppression of intracellular glycogen oxidation.

Effects of FFA on insulin-stimulated glucose fluxes and muscle glycogen synthase activity in rats

To examine effects of free fatty acids (FFA) on insulin-stimulated glucose fluxes, euglycemic hyperinsulinemic (86 pmol . kg-1 . min-1) clamps were performed for 5 h in conscious rats with (n = 8) or without (n = 8) lipid-heparin infusion. Glucose infusion rate required to maintain euglycemia was not different between the two groups during the first 2 h of clamps but became significantly lower with lipid-heparin infusion in the 3rd h and thereafter. To investigate changes in intracellular glucose metabolism during lipid-heparin infusion, additional clamps (n = 8 each) were performed for 1, 2, 3, or 5 h with an infusion of [3-3H]glucose. Insulin-stimulated whole body glucose utilization (Rd), glycolysis, and glycogen synthesis were estimated on the basis of tracer concentrations in plasma during the final 40 min of each clamp. Similar to changes in glucose infusion rate, Rd was not different between the two groups in the 1st and 2nd h but was significantly lower with lipid-heparin infusion in the 3rd h and thereafter. Whole body glycolysis was significantly lower with lipid-heparin infusion in all time periods, i.e., 1st, 2nd, 3rd, and 5th h of clamps. In contrast, whole body glycogen synthesis was higher with lipid-heparin infusion in the 1st and 2nd h but lower in the 5th h. Similarly, accumulation of [3H]glycogen radioactivity in muscle glycogen was significantly higher with lipid-heparin during the 1st and 2nd h but lower during the 3rd and 5th h. Glucose 6-phosphate (G-6-P) concentrations in gastrocnemius muscles were significantly higher with lipid-heparin infusion throughout the clamps. Muscle glycogen synthase (GS) activity was not altered with lipid-heparin infusion at 1, 2, and 3 h but was significantly lower at 5 h. Thus increased availability of FFA significantly reduced whole body glycolysis, but compensatory increase in skeletal muscle glycogen synthesis in association with accumulation of G-6-P masked this effect, and Rd was not affected in the early phase (within 2 h) of lipid-heparin infusion. Rd was reduced in the later phase (>2 h) of lipid-heparin infusion, when glycogen synthesis was reduced in association with reduced skeletal muscle GS activity.

Influence of muscle glycogen content on metabolic regulation

Euglycemia was maintained in 13 subjects with low muscle glycogen [low glycogen, euglycemic (LGE), n = 8; low glycogen, euglycemic, hyperinsulinemic (LGEI), n = 5] and 6 subjects with normal muscle glycogen (NGE), whereas hyperglycemia was maintained in 8 low muscle glycogen subjects (LGH). All subjects cycled for 145 min at 70% of maximal oxygen uptake during the infusions. Insulin was infused in LGEI at 0.2 mU.kg-1.min-1. During exercise, respiratory exchange ratio (RER) was lower and norepinephrine higher in LGE than in NGE. In LGEI and LGH, RER at the start of exercise was the same as in LGE but did not decrease as in LGE. Free fatty acids (FFA) were higher and plasma insulin concentrations lower in LGE than NGE, LGEI, or LGH over the first 45 min of exercise. Rate of glucose infusion (Ri) and rate of glucose oxidation (Rox) were higher in LGH and LGEI than in NGE or LGE, and Ri matched Rox in all groups except LGH, in which Ri was greater than Rox. Muscle glycogen disappearance was greater in NGE than LGE, LGEI, or LGH, but the latter three groups did not differ. In conclusion, this study showed that low muscle glycogen content results in a decrease in RER, an increase in FFA, fat oxidation, and norepinephrine both at rest and during exercise, and does not affect Rox when euglycemia is maintained by infusion of glucose alone. Rox was increased only during insulin and hyperglycemia.

Mechanism of free fatty acid-induced insulin resistance in humans

To examine the mechanism by which lipids cause insulin resistance in humans, skeletal muscle glycogen and glucose-6-phosphate concentrations were measured every 15 min by simultaneous 13C and 31P nuclear magnetic resonance spectroscopy in nine healthy subjects in the presence of low (0.18 +/- 0.02 mM [mean +/- SEM]; control) or high (1.93 +/- 0.04 mM; lipid infusion) plasma free fatty acid levels under euglycemic (approximately 5.2 mM) hyperinsulinemic (approximately 400 pM) clamp conditions for 6 h. During the initial 3.5 h of the clamp the rate of whole-body glucose uptake was not affected by lipid infusion, but it then decreased continuously to be approximately 46% of control values after 6 h (P < 0.00001). Augmented lipid oxidation was accompanied by a approximately 40% reduction of oxidative glucose metabolism starting during the third hour of lipid infusion (P < 0.05). Rates of muscle glycogen synthesis were similar during the first 3 h of lipid and control infusion, but thereafter decreased to approximately 50% of control values (4.0 +/- 1.0 vs. 9.3 +/- 1.6 mumol/[kg.min], P < 0.05). Reduction of muscle glycogen synthesis by elevated plasma free fatty acids was preceded by a fall of muscle glucose-6-phosphate concentrations starting at approximately 1.5 h (195 +/- 25 vs. control: 237 +/- 26 mM; P < 0.01). Therefore in contrast to the originally postulated mechanism in which free fatty acids were thought to inhibit insulin-stimulated glucose uptake in muscle through initial inhibition of pyruvate dehydrogenase these results demonstrate that free fatty acids induce insulin resistance in humans by initial inhibition of glucose transport/phosphorylation which is then followed by an approximately 50% reduction in both the rate of muscle glycogen synthesis and glucose oxidation.

In vitro effect of adenosine agonist GR79236 on the insulin sensitivity of glucose utilisation in rat soleus and human rectus abdominus muscle

The dose-response effects of a new adenosine agonist, GR79236, were examined in isolated rat soleus muscle strips and human rectus abdominus muscle strips. Effects on the insulin sensitivity of carbohydrate metabolism were examined, in particular upon insulin stimulated glycogen synthesis and glycolytic flux. In the presence of adenosine deaminase (ADA), GR79236 increased insulin sensitivity of pyruvate release from rat soleus muscle strips by 24% from 82.5 +/- 10.0 to 102.5 +/- 10.0 (P < 0.01), by 27% to 105.0 +/- 12.5 (P < 0.01) and by 24% to 102.5 +/- 10.0 (P < 0.01) nmol/25 mg per h at 0.1 and 10 microM GR79236, respectively. Rates of lactate release followed a similar but non-significant trend. Addition of GR79236 in the presence of ADA had no effect on rates of glycogen synthesis. Insulin stimulated rates of pyruvate or lactate release or of glycogen synthesis were unaffected by the addition of adenosine deaminase or GR79236 in human rectus abdominus muscle strips. Adenosine agonists may act indirectly to modulate insulin sensitivity of carbohydrate metabolism.

Infusion of long-chain or medium-chain triglycerides inhibits peripheral glucose metabolism in men

To investigate whether increased availability of lipids affects glucose metabolism in healthy postabsorptive men when lipid and glucose are infused in amounts used in parenteral nutrition, we infused glucose (4 mg/kg.min-1) for 6 hours and clamped plasma glucose at basal level during the first 3 hours. After 3 hours, either long-chain triglycerides (LCTs) (0.07 g/kg.h-1) (n = 7) or a mixture of LCTs and medium-chain triglycerides (MCTs) (MCTs/LCTs, 50/50%, 0.07 g/kg.h-1) (n = 7) was administered, and the infusion rates of glucose and insulin were unchanged compared with the first 3 hours. In a control study, glucose was infused for a period of 6 hours without the lipid infusion (n = 5). After 6 hours, the plasma glucose concentration and glucose tissue uptake were not affected by LCT or MCT/LCT infusion. Nonetheless, glucose oxidation decreased in the LCT group (from 6.42 +/- 1.04 to 2.31 +/- 0.85 mumol/kg.min-1, p < .001) and in the MCT/LCT group (from 7.62 +/- 1.50 to 5.50 +/- 0.76 mumol/kg.min-1, p < .01) but not in the control group. Concentrations of the glucoregulatory hormones were not different among the three groups. In conclusion, MCTs/LCTs administered concomitantly with glucose infusion, in amounts similar to those used in total parenteral nutrition, inhibit glucose oxidation without affecting glucose tissue uptake, just as LCTs do.

Mechanisms modifying glucose oxidation in diabetes mellitus

The Glucose Fatty Acid Cycle as formulated 30 years ago and reviewed in the Minkowski lecture in 1966 described short term effects of fatty acids (minutes) to decrease uptake, glycolysis and oxidation of glucose in heart and skeletal muscles. Such short term effects have since been extended to include inhibition of glucose uptake and glycolysis and stimulation of gluconeogenesis in liver and these effects have also been convincingly demonstrated in man in vivo. More recently a longer term effect of fatty acid metabolism to decrease glucose oxidation (hours) has been shown in heart and skeletal muscle and liver. This effect increases the specific activity of pyruvate dehydrogenase kinase, which in turn results in enhanced phosphorylation and inactivation of the pyruvate dehydrogenase complex. Activity of the pyruvate dehydrogenase complex is the major determinant of glucose oxidation rate. It seems likely that longer term effects of fatty acids on this and other aspects of glucose metabolism could be important in the development of insulin resistance in diabetes mellitus in man.

Glucose fatty acid interactions and the regulation of glucose disposal

Glucose is essential for the energy metabolism of some cells and conservation of glucose is obligatory for survival during starvation. The principal site of this glucose conservation is the mitochondrial pyruvate dehydrogenase (PDH) complex, which is regulated by reversible phosphorylation (phosphorylation is inactivating). In cells in which glucose oxidation is switched off during starvation, fatty acids are used as fuel, and acetyl CoA and NADH formed by beta-oxidation promote phosphorylation of PDH complex by activation of PDH kinase. A longer-term mechanism further increases PDH kinase activity in response to cAMP and products of beta-oxidation of fatty acids. Coordinated inhibition of glycolytic flux mediated by effects of citrate on PFK1 and PFK2 in muscles and liver results in an associated inhibition of glucose uptake. Similar mechanisms lead to impaired glucose oxidation in diabetes.

Obese men with type IIB hyperlipidemia are insulin resistant

By using the euglycemic clamp technique and indirect calorimetry, we determined the degree of insulin resistance in 12 obese (body mass index > 27.0 kg/m2), normotensive patients with type IIB hyperlipidemia (HLIIB) (total cholesterol > or = 6.5 mmol/L and total triglycerides > or = 2.0 mmol/L) and 17 control subjects (total cholesterol < or = 6.1 mmol/L and total triglycerides < 1.8 mmol/L) who were carefully matched for sex, age, and obesity. Fasting plasma insulin was higher in HLIIB patients than in control subjects (18.4 +/- 4.6 versus 8.9 +/- 1.2 mU/L, respectively; P = .010). The rates of whole-body glucose uptake were significantly lower in HLIIB patients than in control subjects during the last hour of the clamp (42.2 +/- 3.9 versus 54.6 +/- 2.8 mumol/kg per minute, respectively; P = .013). Glucose oxidation during the last 30 minutes of the euglycemic clamp was lower in HLIIB patients than in control subjects (14.6 +/- 0.9 versus 19.0 +/- 1.3 mumol/kg per minute, respectively; P = .017). Nonoxidative glucose disposal during the last 30 minutes of the euglycemic clamp was also lower in HLIIB patients than in control subjects, but the difference was not statistically significant (27.6 +/- 3.3 versus 35.8 +/- 2.8 mumol/kg per minute, respectively; P = .069). Lipid oxidation during the clamp was completely suppressed in control subjects (-0.24 +/- 0.44 mumol/kg per minute) but was significantly less suppressed in the HLIIB patients (0.94 +/- 0.29 mumol/kg per minute, P = .024).(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between glucose and free fatty acid metabolism in human skeletal muscle

The mechanism by which FFA metabolism inhibits intracellular insulin-mediated muscle glucose metabolism in normal humans is unknown. We used the leg balance technique with muscle biopsies to determine how experimental maintenance of FFA during hyperinsulinemia alters muscle glucose uptake, oxidation, glycolysis, storage, pyruvate dehydrogenase (PDH), or glycogen synthase (GS). 10 healthy volunteers had two euglycemic insulin clamp experiments. On one occasion, FFA were maintained by lipid emulsion infusion; on the other, FFA were allowed to fall. Leg FFA uptake was monitored with [9,10-3H]-palmitate. Maintenance of FFA during hyperinsulinemia decreased muscle glucose uptake (1.57 +/- 0.31 vs 2.44 +/- 0.39 mumol/min per 100 ml tissue, P < 0.01), leg respiratory quotient (0.86 +/- 0.02 vs 0.93 +/- 0.02, P < 0.05), contribution of glucose to leg oxygen consumption (53 +/- 6 vs 76 +/- 8%, P < 0.05), and PDH activity (0.328 +/- 0.053 vs 0.662 +/- 0.176 nmol/min per mg, P < 0.05). Leg lactate balance was increased. The greatest effect of FFA replacement was reduced muscle glucose storage (0.36 +/- 0.20 vs 1.24 +/- 0.25 mumol/min per 100 ml, P < 0.01), accompanied by decreased GS fractional velocity (0.129 +/- 0.26 vs 0.169 +/- 0.033, P < 0.01). These results confirm in human skeletal muscle the existence of competition between glucose and FFA as oxidative fuels, mediated by suppression of PDH. Maintenance of FFA levels during hyperinsulinemia most strikingly inhibited leg muscle glucose storage, accompanied by decreased GS activity.