Plasma free fatty acids decrease insulin-stimulated skeletal muscle glucose uptake by suppressing glycolysis in conscious rats

Saturated fatty acids inhibit unsaturated fatty acid induced glucose uptake involving GLUT10 and aerobic glycolysis in bovine granulosa cells

Fatty acids have been shown to modulate glucose metabolism in vitro and in vivo. However, there is still a need for substantial evidence and mechanistic understanding in many cell types whether both saturated and unsaturated fatty acids (SFAs and UFAs) pose a similar effect and, if not, what determines the net effect of fatty acid mixes on glucose metabolism. In the present study, we asked these questions by treating granulosa cells (GCs) with the most abundant non-esterified fatty acid species in bovine follicular fluid. Results revealed that oleic and alpha-linolenic acids (UFAs) significantly increased glucose consumption compared to palmitic and stearic acids (SFAs). A significant increase in lactate production, extracellular acidification rate, and decreased mitochondrial activity indicate glucose channeling through aerobic glycolysis in UFA treated GCs. We show that insulin independent glucose transporter GLUT10 is essential for UFA driven glucose consumption, and the induction of AKT and ERK signaling pathways necessary for GLUT10 expression. To mimic the physiological conditions, we co-treated GCs with mixes of SFAs and UFAs. Interestingly, co-treatments abolished the UFA induced glucose uptake and metabolism by inhibiting AKT and ERK phosphorylation and GLUT10 expression. These data suggest that the net effect of fatty acid induced glucose uptake in GCs is determined by SFAs under physiological conditions.

The Associations of Individual and Subclasses of Nonesterified Fatty Acids With Disability, and Mobility Limitation in Older Adults: The Cardiovascular Health Study

BACKGROUND

We sought to determine the associations between individual nonesterified fatty acids (NEFAs) and disability and mobility limitation.

METHODS

We studied 1 734 participants in the Cardiovascular Health Study (CHS), an ongoing population-based cohort study of community-living older American adults. We measured 35 individual NEFA species in fasting serum samples obtained at the 1996-1997 clinic visit. Using yearly assessments of activities of daily living and self-reported mobility, we identified participants with incident disability or mobility limitation during 15 years of follow-up. Cox proportional hazards regression models were used to determine the associations between per SD increment in the individual NEFAs and incident disability and mobility limitations with adjustment for potential confounding factors.

RESULTS

Higher concentrations of total and a broad range of individual NEFA species were associated with risk of disability and mobility limitation (disability: HR per SD of total NEFA [SD = 174.70] = 1.11, 95% CI = 1.04-1.18, p = .001; mobility limitation: HR per SD of total NEFA = 1.09, 95% CI = 1.02-1.16, p = .01). Among individual saturated NEFAs (SFAs), myristic (14:0) and palmitic (16:0) acids were significantly associated with higher risk of both disability and mobility limitations, but longer-chain FAs were not. Most individual monounsaturated (MUFA), n-6 polyunsaturated fatty acids (PUFAs), and trans FAs were positively significantly associated with higher risks of both disability and mobility limitation. In contrast, most n-3 PUFA species were not associated with disability or mobility limitation.

CONCLUSIONS

Higher risks of disability and mobility limitation were observed for proinflammatory intermediate-chain SFAs, MUFAs, n-6 PUFAs, and trans FAs. Our findings indicated no significant association for anti-inflammatory n-3 PUFAs.

Is insulin resistance tissue-dependent and substrate-specific? The role of white adipose tissue and skeletal muscle

Insulin resistance (IR) refers to a reduction in the ability of insulin to exert its metabolic effects in organs such as adipose tissue (AT) and skeletal muscle (SM), leading to chronic diseases such as type 2 diabetes, hepatic steatosis, and cardiovascular diseases. Obesity is the main cause of IR, however not all subjects with obesity develop clinical insulin resistance, and not all clinically insulin-resistant people have obesity. Recent evidence implies that IR onset is tissue-dependent (AT or SM) and/or substrate-specific (glucometabolic or lipometabolic). Therefore, the aims of the present review are 1) to describe the glucometabolic and lipometabolic activities of insulin in AT and SM in the maintenance of whole-body metabolic homeostasis, 2) to discuss the pathophysiology of substrate-specific IR in AT and SM, and 3) to highlight novel validated tests to assess tissue and substrate-specific IR that are easy to perform in clinical practice. In AT, glucometabolic IR reduces glucose availability for glycerol and fatty acid synthesis, thus decreasing the esterification and synthesis of signaling bioactive lipids. Lipometabolic IR in AT impairs the antilipolytic effect of insulin and lipogenesis, leading to an increase in circulating FFAs and generating lipotoxicity in peripheral tissues. In SM, glucometabolic IR reduces glucose uptake, whereas lipometabolic IR impairs mitochondrial lipid oxidation, increasing oxidative stress and inflammation, all of which lead to metabolic inflexibility. Understanding tissue-dependent and substrate-specific IR is of paramount importance for early detection before clinical manifestations and for the development of more specific treatments or direct interventions to prevent chronic life-threatening diseases.

Palmitic acid hinders extracellular traps of neutrophil from postpartum dairy cow in vitro

Peripartum dairy cows experience negative energy balance, characterized by high concentrations of blood free fatty acids (FFA) and immune dysfunction. Palmitic acid (PA), the most abundant saturated fatty acid in cow blood, is not only an energy precursor, but causes cellular dysfunction when in excess. Neutrophil extracellular traps (NET) are one of the arsenals of weapons neutrophils use to fight invading pathogens. However, given the marked increase in circulating PA during the peripartum period, it remains to be determined what effect (if any) PA has on NET release. Thus, the objective of this study was to evaluate the effect of PA on NET release and the underlying mechanism in vitro. Phorbol-12-myristate-13-acetate (PMA; 100 ng/mL, 3 h) was used to induce the release of NET in vitro. We isolated neutrophils from the peripheral blood of 5 healthy postpartum dairy cows with similar parity (median = 3, range = 2-4), milk yield (median = 27.84 kg/d per cow, range = 25.79-31.43 kg/d per cow), days in milk (median = 7 d, range = 4-10 d), and serum FFA <0.25 mM, β-hydroxybutyric acid <0.6 mM, and glucose >3.5 mM. Inhibition of double-stranded DNA (dsDNA) level, a marker of NET release, in response to PA was used to determine an optimal incubation time and concentration for in vitro experiments. Cells were maintained in RPMI-1640 basic medium without phenol red, treated with 600 μM PA for different times (4, 5, 6, and 7 h) in the presence or absence of PMA. There was a decrease for dsDNA level in the supernatant due to increased duration of PA treatment, with a peak response at 6 h. Thus, 6 h was selected as the challenge time. Then, cells were treated with different concentrations of PA (100, 200, 400, and 600 μM) for 6 h in the presence or absence of PMA. There was a decrease for dsDNA level in the supernatant due to increased dose of PA, with a peak response at 400 μM. Finally, 400 μM PA for 6 h was selected as the treatment for subsequent experiments. Protein abundance of citrullinated histone in the presence or absence of PMA was markedly lower in response to incubation with PA. Morphological observations by laser confocal microscopy and scanning electron microscopy showed that the ratio of NET-releasing cells decreased in response to incubation with PA. Autophagy is a potential key intermediate process in the regulation of NET by PA. To investigate the effect of PA on autophagy, we used chloroquine to block lysosomal degradation. Exogenous PA led to accumulation of sequestosome-1 and microtubule-associated protein 1 light chain 3-II, and no further accumulation in the presence of chloroquine, all of which suggested an impairment of autophagic flux. To verify the role of autophagy in NET, we used rapamycin to promote autophagic flux; 100 nM rapamycin attenuated the suppressive effect of PA on NET release indicated by greater dsDNA levels, accumulation of citrullinated histone, and ratio of NET-releasing neutrophils. Overall, these data demonstrate PA inhibits NET release by suppressing autophagic flux, which provides information for understanding the immune dysfunction in postpartum cows.

The Role of Lipids, Lipid Metabolism and Ectopic Lipid Accumulation in Axon Growth, Regeneration and Repair after CNS Injury and Disease

Axons in the adult mammalian nervous system can extend over formidable distances, up to one meter or more in humans. During development, axonal and dendritic growth requires continuous addition of new membrane. Of the three major kinds of membrane lipids, phospholipids are the most abundant in all cell membranes, including neurons. Not only immature axons, but also severed axons in the adult require large amounts of lipids for axon regeneration to occur. Lipids also serve as energy storage, signaling molecules and they contribute to tissue physiology, as demonstrated by a variety of metabolic disorders in which harmful amounts of lipids accumulate in various tissues through the body. Detrimental changes in lipid metabolism and excess accumulation of lipids contribute to a lack of axon regeneration, poor neurological outcome and complications after a variety of central nervous system (CNS) trauma including brain and spinal cord injury. Recent evidence indicates that rewiring lipid metabolism can be manipulated for therapeutic gain, as it favors conditions for axon regeneration and CNS repair. Here, we review the role of lipids, lipid metabolism and ectopic lipid accumulation in axon growth, regeneration and CNS repair. In addition, we outline molecular and pharmacological strategies to fine-tune lipid composition and energy metabolism in neurons and non-neuronal cells that can be exploited to improve neurological recovery after CNS trauma and disease.

Lipoprotein Lipase Overexpression in Skeletal Muscle Attenuates Weight Regain by Potentiating Energy Expenditure

Moderate weight loss improves numerous risk factors for cardiometabolic disease; however, long-term weight loss maintenance (WLM) is often thwarted by metabolic adaptations that suppress energy expenditure and facilitate weight regain. Skeletal muscle has a prominent role in energy homeostasis; therefore, we investigated the effect of WLM and weight regain on skeletal muscle in rodents. In skeletal muscle of obesity-prone rats, WLM reduced fat oxidative capacity and downregulated genes involved in fat metabolism. Interestingly, even after weight was regained, genes involved in fat metabolism were also reduced. We then subjected mice with skeletal muscle lipoprotein lipase overexpression (mCK-hLPL), which augments fat metabolism, to WLM and weight regain and found that mCK-hLPL attenuates weight regain by potentiating energy expenditure. Irrespective of genotype, weight regain suppressed dietary fat oxidation and downregulated genes involved in fat metabolism in skeletal muscle. However, mCK-hLPL mice oxidized more fat throughout weight regain and had greater expression of genes involved in fat metabolism and lower expression of genes involved in carbohydrate metabolism during WLM and regain. In summary, these results suggest that skeletal muscle fat oxidation is reduced during WLM and regain, and therapies that improve skeletal muscle fat metabolism may attenuate rapid weight regain.

Non-Esterified Fatty Acids and Risks of Frailty, Disability, and Mobility Limitation in Older Adults: The Cardiovascular Health Study

BACKGROUND/OBJECTIVES

Non-esterified fatty acids (NEFAs) play central roles in the relationship between adiposity and glucose metabolism, and they have been implicated in the pathogenesis of cardiovascular disease, but few studies have assessed their effects on complex geriatric syndromes like frailty that cross multiple organ systems. We sought to determine the relationships between NEFAs and incident frailty, disability, and mobility limitation in a population-based cohort of older persons.

METHODS

We analyzed 4,710 Cardiovascular Health Study (CHS) participants who underwent measurement of circulating total fasting NEFAs in 1992-1993 and were assessed for frailty in 1996-1997 and for disability and mobility limitation annually. We used ordinal logistic regression to model incident frailty, linear regression to model components of frailty, and Cox regression to model disability and mobility limitation in relation to baseline NEFAs. To ensure proportional hazards, we truncated follow-up at 9 years for disability and 6.5 years for mobility limitation.

RESULTS

A total of 42 participants became frail and 510 became pre-frail over a 4-year period, and we documented 1,720 cases of disability and 1,225 cases of mobility limitation during follow-up. NEFAs were positively associated in a dose-dependent manner with higher risks of incident frailty, disability, and mobility limitation. The adjusted odds ratios for frailty were 1.37 (95% confidence interval [CI] = 1.01-1.86; P = .04) across extreme tertiles and 1.17 (95% CI = 1.03-1.33; P = .01) per standard deviation increment. The corresponding hazard ratios for incident disability were 1.14 (95% CI = 1.01-1.30; P = .04) and 1.11 (95% CI = 1.06-1.17; P < .0001); those for incident mobility limitation were 1.23 (95% CI = 1.06-1.43; P = .006) and 1.15 (95% CI = 1.08-1.22; P < .0001). Results were largely consistent among both men and women. Among individual components of frailty, NEFAs were significantly associated with self-reported exhaustion (β = .07; standard error = .03; P = .02).

CONCLUSION

Circulating NEFAs are significantly associated with frailty, disability, and mobility limitation among older adults. These results highlight the broad spectrum of adverse health issues associated with NEFA in older adults.

Novel metabolic disorders in skeletal muscle of Lipodystrophic Bscl2/Seipin deficient mice

Bscl2^-/- mice recapitulate many of the major metabolic manifestations in Berardinelli-Seip congenital lipodystrophy type 2 (BSCL2) individuals, including lipodystrophy, hepatosteatosis, muscular hypertrophy, and insulin resistance. Metabolic defects in Bscl2^-/- mice with regard to glucose and lipid metabolism in skeletal muscle have never been investigated. Here, we identified Bscl2^-/- mice displayed reduced intramyocellular triglyceride (IMTG) content but increased glycogen storage predominantly in oxidative type I soleus muscle (SM). These changes were associated with increased incomplete fatty acid oxidation and glycogen synthesis. Interestingly, SM in Bscl2^-/- mice demonstrated a fasting duration induced insulin sensitivity which was further confirmed by hyperinsulinemic-euglycemic clamp in SM of overnight fasted Bscl2^-/- mice but reversed by raising circulating NEFA levels through intralipid infusion. Furthermore, mice with skeletal muscle-specific inactivation of BSCL2 manifested no changes in muscle deposition of lipids and glycogen, suggesting BSCL2 does not play a cell-autonomous role in muscle lipid and glucose homeostasis. Our study uncovers a novel link between muscle metabolic defects and insulin resistance, and underscores an important role of circulating NEFA in regulating oxidative muscle insulin signaling in BSCL2 lipodystrophy.

Role of Fatty Acid Composition in the Development of Metabolic Disorders in Sucrose-Induced Obese Rats

Fatty acids have been shown to be involved in the development of insulin resistance associated with obesity. We used sucrose loading in rats to analyze changes in fatty acid composition in the progression of obesity and the related metabolic disorder. Although rats fed a sucrose diet for 4 weeks had body weights similar to those of control animals, their visceral fat pads were significantly larger, and serum triglyceride levels were higher; however, neither plasma glucose nor insulin levels were significantly higher. After 20 weeks of sucrose loading, body weight and visceral and subcutaneous fat pads had increased significantly compared with those in control rats. Moreover, plasma glucose, Insulin, and triglyceride levels were significantly higher. An analysis of individual fatty acid components in the blood and peripheral tissues demonstrated phase- and tissue-dependent changes. After 20 weeks of sucrose loading, palmitoleic acid (16:1 n-7) and oleic acid (18:1 n-9), the major components of monounsaturated fatty acid, showed a ubiquitous increase in plasma and all tissues analyzed. In contrast, linoleic acid (18:2 n-6) and arachidonic acid (20:4 n-6), the major components of polyunsaturated fatty acid in the n-6 family, decreased in plasma and all tissues analyzed. After 4 weeks of sucrose loading, these changes in fatty acid composition were observed only in the liver and plasma and not in fat and muscle. This led us to conclude that elevation of plasma glucose and insulin develop at the late phase of sucrose-induced obesity, when changes in fatty acid composition appear in fat and muscle. Furthermore, changes in fatty acid composition in liver seen after 4 weeks of sucrose loading, when increases in neither plasma glucose nor insulin were detected, suggest that liver may be the initial site of fatty acid imbalance and that aberrations in hepatic fatty acid composition may lead to fatty acid imbalances in other tissues.

Insulin resistance: cross-talk between adipose tissue and skeletal muscle, through free fatty acids, liver X receptor, and peroxisome proliferator-activated receptor-α signaling

Skeletal muscle and adipose tissue play a major role in the regulation of whole-body glucose homeostasis. Much of the coordinated regulation of whole-body glucose homeostasis results from the regulation of lipid storage and release by adipose tissue and efficient switching between glucose oxidation and fatty acid oxidation in skeletal muscle. A control point for these biochemical actions center around the regulation of the insulin responsive glucose transporter, GLUT4. This review examines the regulation of GLUT4 in adipose tissue and skeletal muscle, in the context of the steroid nuclear hormone receptor signaling.

Abdominal fat depots associated with insulin resistance and metabolic syndrome risk factors in black African young adults

Background

Individuals of black African ethnicity tend to have less visceral adipose tissue (VAT) but more subcutaneous-abdominal adipose tissue (SCAT) than white Caucasians. However, it is unclear whether such distribution of abdominal fat is beneficial for metabolic disease risk in black individuals. Here we compared the associations between these specific abdominal fat depots, insulin sensitivity and metabolic syndrome risk.

Methods

A cross-sectional analysis of 76 black South African young adults (36 men; 40 women) aged 18–19 years participating in the Birth to Twenty Cohort Study had VAT and SCAT measured by MRI. The metabolic syndrome traits (blood pressure, lipid profile, fasting glucose and insulin) were measured and the values were combined into a metabolic syndrome risk score. Fasting glucose and insulin were used to derive the HOMA-index of insulin resistance (HOMA-IR).

Results

Compared to men, women had greater VAT (mean: 16.6 vs. 12.5 cm²) and SCAT (median 164.0 vs. 59.9 cm²). In men, SCAT (r = 0.50) was more strongly correlated to the metabolic syndrome score (MetS) than was VAT (r = 0.23), and was associated with both MetS (P = 0.001) and HOMA-IR (P = 0.001) after adjustment for VAT and total fat mass. In women, both abdominal fat compartments showed comparable positive correlations with MetS (r = 0.26 to 0.31), although these trends were weaker than in men.

Conclusions

In young black South African adults, SCAT appears to be more relevant than VAT to metabolic syndrome traits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12889-015-2147-x) contains supplementary material, which is available to authorized users.

Quantifying insulin sensitivity and entero-insular responsiveness to hyper- and hypoglycemia in ferrets

Ferrets are an important emerging model of cystic fibrosis related diabetes. However, there is little documented experience in the use of advanced techniques to quantify aspects of diabetes pathophysiology in the ferret. Glycemic clamps are the gold standard technique to assess both insulin sensitivity and insulin secretion in humans and animal models of diabetes. We therefore sought to develop techniques for glycemic clamps in ferrets. To assess insulin sensitivity, we performed euglycemic hyperinsulinemic clamps in 5–6 week old ferrets in the anesthetized and conscious states. To assess insulin secretion, we performed hyperglycemic clamps in conscious ferrets. To evaluate responsiveness of ferret islet and entero-insular hormones to low glucose, a portion of the hyperglycemic clamps were followed by a hypoglycemic clamp. The euglycemic hyperinsulinemic clamps demonstrated insulin responsiveness in ferrets similar to that previously observed in humans and rats. The anesthetic isoflurane induced marked insulin resistance, whereas lipid emulsion induced mild insulin resistance. In conscious ferrets, glucose appearance was largely suppressed at 4 mU/kg/min insulin infusion, whereas glucose disposal was progressively increased at 4 and 20 mU/kg/min insulin. Hyperglycemic clamp induced first phase insulin secretion. Hypoglycemia induced a rapid diminishment of insulin, as well as a rise in glucagon and pancreatic polypeptide levels. The incretins GLP-1 and GIP were affected minimally by hyperglycemic and hypoglycemic clamp. These techniques will prove useful in better defining the pathophysiology in ferrets with cystic fibrosis related diabetes.

Suppression of the C/EBP family of transcription factors in adipose tissue causes lipodystrophy

Adipose-specific inactivation of both AP-1 and CCAAT-enhancer-binding protein (C/EBP) families of B-ZIP transcription factors in transgenic mice causes severe lipoatrophy. To evaluate whether inactivation of only C/EBP members was critical for lipoatrophy, A-C/EBP, a dominant-negative protein that specifically inhibits the DNA binding of the C/EBP members, was expressed in adipose tissue. For the first 2 weeks after birth, aP2-A-C/EBP mice had no white adipose tissue (WAT), drastically reduced brown adipose tissue (BAT), and exhibited marked hepatic steatosis, hyperinsulinemia, and hyperlipidemia. However, WAT appeared during the third week, coinciding with significantly improved metabolic functioning. In adults, BAT remained reduced, causing cold intolerance. At 30 weeks, the aP2-A-C/EBP mice had only 35% reduced WAT, with clear morphological signs of lipodystrophy in subcutaneous fat. Circulating leptin and adiponectin levels were less than the wild-type levels, and these mice exhibited impaired triglyceride clearance. Insulin resistance, glucose intolerance, and reduced free fatty acid release in response to β3-adrenergic agonist suggest improper functioning of the residual WAT. Gene expression analysis of inguinal WAT identified reduced mRNA levels of several enzymes involved in fatty acid synthesis and glucose metabolism that are known C/EBPα transcriptional targets. There were increased levels for genes involved in inflammation and muscle differentiation. However, when dermal fibroblasts from aP2-A-C/EBP mice were differentiated into adipocytes in tissue culture, muscle markers were elevated more than the inflammatory markers. These results demonstrate that the C/EBP family is essential for adipose tissue development during the early postnatal period, the regulation of glucose and lipid homeostasis in adults, and the suppression of the muscle lineage.

Resistance to high-fat diet-induced obesity and insulin resistance in mice with very long-chain acyl-CoA dehydrogenase deficiency

Mitochondrial fatty acid oxidation provides an important energy source for cellular metabolism, and decreased mitochondrial fatty acid oxidation has been implicated in the pathogenesis of type 2 diabetes. Paradoxically, mice with an inherited deficiency of the mitochondrial fatty acid oxidation enzyme, very long-chain acyl-CoA dehydrogenase (VLCAD), were protected from high-fat diet-induced obesity and liver and muscle insulin resistance. This was associated with reduced intracellular diacylglycerol content and decreased activity of liver protein kinase Cvarepsilon and muscle protein kinase Ctheta. The increased insulin sensitivity in the VLCAD(-/-) mice were protected from diet-induced obesity and insulin resistance due to chronic activation of AMPK and PPARalpha, resulting in increased fatty acid oxidation and decreased intramyocellular and hepatocellular diacylglycerol content.

Manifestation of renal disease in obesity: pathophysiology of obesity-related dysfunction of the kidney

Albuminuria in individuals whose body mass index exceeds 40 kg/m(2) is associated with the presence of large glomeruli, thickened basement membrane and epithelial cellular (podocyte) distortion. Obstructive sleep apnea magnifies glomerular injury as well, probably through a vasoconstrictive mechanism. Insulin resistance from excess fatty acids is exacerbated by decreased secretion of high molecular weight adiponectin from adipose cells in the obese state. Adiponectin potentiates insulin in its post-receptor signaling resulting in glucose oxidation in mitochondria. Recent studies of podocyte physiology have concentrated on the structural and functional requirements that prevent glomerular albumin leakage. The architecture of the podocyte involves nephrin and podocin, proteins that cooperate to keep slit pores between foot processes competent to retain albumin. Insulin and adiponectin are necessary for high-energy phosphate generation. When fatty acids bind to albumin, the toxicity to proximal renal tubules is magnified. Albumin and fatty acids are elevated in urine of individuals with obesity related nephrotic syndrome. Fatty acid accumulation and resistin inhibit insulin and adiponectin. Study of cytokines produced by adipose tissue (adiponectin and leptin) and macrophages (resistin) has led to a better understanding of the relationship between weight and hypertension. Leptin, is presumably secreted after food intake to inhibit the midbrain/hypothalamic appetite centers. Resistance to leptin results in excess signaling to hypothalamic sympathetics leading to hypertension. Demonstration of the existence of a cerebral receptor mutation provide evidence for a role in hypertension of a central nervous reflex arc in humans. Further understanding of obesity-related renal dysfunction has been accomplished recently using experimental models. Rapid weight loss following bariatric surgery may reverse renal pathology of obesity with restoration of normal blood pressure.

Muscle contraction, but not insulin, increases microvascular blood volume in the presence of free fatty acid-induced insulin resistance

OBJECTIVE

Insulin and contraction each increase muscle microvascular blood volume (MBV) and glucose uptake. Inhibiting nitric oxide synthase blocks insulin's but not contraction's effects. We examined whether contraction could augment the MBV increase seen with physiologic hyperinsulinemia and whether free fatty acid (FFA)-induced insulin resistance differentially affects contraction- versus insulin-mediated increases in MBV.

RESEARCH DESIGN AND METHODS

Rats were fasted overnight. Plasma FFAs were increased by intralipid/heparin infusion (3 h), insulin was increased with a euglycemic clamp (3 mU x min(-1) x kg(-1)), and hindlimb muscle contraction was electrically stimulated. Muscle MBV was measured using contrast-enhanced ultrasound. Insulin transport into muscle was measured using (125)I-insulin. BQ-123 (0.4 mg/h) was used to block the endothelin-1 (ET-1) receptor A.

RESULTS

Superimposing contraction on physiologic hyperinsulinemia increased MBV within 10 min by 37 and 67% for 0.1 or 1 Hz, respectively (P < 0.01). FFA elevation alone did not affect MBV, whereas 0.1 Hz stimulation doubled MBV (P < 0.05) and increased muscle insulin uptake (P < 0.05) despite high FFA. Physiologic hyperinsulinemia during FFA elevation paradoxically decreased MBV (P < 0.05). This MBV decrease was reversed by either 0.1 Hz contraction or ET-1 receptor A antagonism, and the combination raised MBV above basal.

CONCLUSIONS

Contraction recruits microvasculature beyond that seen with physiologic hyperinsulinemia by a distinct mechanism that is not blocked by FFA-induced vascular insulin resistance. The paradoxical MBV decline seen with insulin plus FFA may result from differential inhibition of insulin-stimulated nitric oxide-dependent vasodilation relative to ET-1 vasoconstriction. Our results implicate ET-1 as a potential mediator of FFA-induced vascular insulin resistance.

Impact of short-term high-fat feeding on glucose and insulin metabolism in young healthy men

A high-fat, high-calorie diet is associated with obesity and type 2 diabetes. However, the relative contribution of metabolic defects to the development of hyperglycaemia and type 2 diabetes is controversial. Accumulation of excess fat in muscle and adipose tissue in insulin resistance and type 2 diabetes may be linked with defective mitochondrial oxidative phosphorylation. The aim of the current study was to investigate acute effects of short-term fat overfeeding on glucose and insulin metabolism in young men. We studied the effects of 5 days' high-fat (60% energy) overfeeding (+50%) versus a control diet on hepatic and peripheral insulin action by a hyperinsulinaemic euglycaemic clamp, muscle mitochondrial function by (31)P magnetic resonance spectroscopy, and gene expression by qrt-PCR and microarray in 26 young men. Hepatic glucose production and fasting glucose levels increased significantly in response to overfeeding. However, peripheral insulin action, muscle mitochondrial function, and general and specific oxidative phosphorylation gene expression were unaffected by high-fat feeding. Insulin secretion increased appropriately to compensate for hepatic, and not for peripheral, insulin resistance. High-fat feeding increased fasting levels of plasma adiponectin, leptin and gastric inhibitory peptide (GIP). High-fat overfeeding increases fasting glucose levels due to increased hepatic glucose production. The increased insulin secretion may compensate for hepatic insulin resistance possibly mediated by elevated GIP secretion. Increased insulin secretion precedes the development of peripheral insulin resistance, mitochondrial dysfunction and obesity in response to overfeeding, suggesting a role for insulin per se as well GIP, in the development of peripheral insulin resistance and obesity.

L-carnitine: implications in the treatment of the metabolic syndrome and Type 2 diabetes

The metabolic syndrome (MS) is a conglomeration of inter-related common clinical disorders, including obesity, glucose intolerance, hypertension and dyslipidemia, which predispose to Type 2 diabetes (T2D) and cardiovascular diseases. Hyperinsulinemia, per se, and insulin resistance are the pathogenic factors associated with the metabolic risk factors. Since these risk factors are the most frequent causes for mortality among patients with T2D and the MS, treatments targeting normalization of both lipid and glucose homeostasis are of interest. The crucial role of L-carnitine (CA) as a regulator of lipid and glucose metabolism has raised considerable interest in its use as a potential tool for therapeutic intervention in the MS. Several clinical studies have, therefore, been undertaken to examine the efficacy and other benefits in the treatment of T2D and the MS. Studies from rodent models of MS have also shown the positive effects of CA on several components of the syndrome. CA, being an endogenous water-soluble nutrient, could be a safe adjunct and a relevant future drug for the MS. This review provides an overview on the importance of CA in T2D and the MS and the need for further evaluation of its inclusion in treatment protocols.

Current approaches for assessing insulin sensitivity and resistance in vivo: advantages, limitations, and appropriate usage

Insulin resistance contributes to the pathophysiology of diabetes and is a hallmark of obesity, metabolic syndrome, and many cardiovascular diseases. Therefore, quantifying insulin sensitivity/resistance in humans and animal models is of great importance for epidemiological studies, clinical and basic science investigations, and eventual use in clinical practice. Direct and indirect methods of varying complexity are currently employed for these purposes. Some methods rely on steady-state analysis of glucose and insulin, whereas others rely on dynamic testing. Each of these methods has distinct advantages and limitations. Thus, optimal choice and employment of a specific method depends on the nature of the studies being performed. Established direct methods for measuring insulin sensitivity in vivo are relatively complex. The hyperinsulinemic euglycemic glucose clamp and the insulin suppression test directly assess insulin-mediated glucose utilization under steady-state conditions that are both labor and time intensive. A slightly less complex indirect method relies on minimal model analysis of a frequently sampled intravenous glucose tolerance test. Finally, simple surrogate indexes for insulin sensitivity/resistance are available (e.g., QUICKI, HOMA, 1/insulin, Matusda index) that are derived from blood insulin and glucose concentrations under fasting conditions (steady state) or after an oral glucose load (dynamic). In particular, the quantitative insulin sensitivity check index (QUICKI) has been validated extensively against the reference standard glucose clamp method. QUICKI is a simple, robust, accurate, reproducible method that appropriately predicts changes in insulin sensitivity after therapeutic interventions as well as the onset of diabetes. In this Frontiers article, we highlight merits, limitations, and appropriate use of current in vivo measures of insulin sensitivity/resistance.

Mitochondrial dysfunction due to long-chain Acyl-CoA dehydrogenase deficiency causes hepatic steatosis and hepatic insulin resistance

Alterations in mitochondrial function have been implicated in the pathogenesis of insulin resistance and type 2 diabetes. However, it is unclear whether the reduced mitochondrial function is a primary or acquired defect in this process. To determine whether primary defects in mitochondrial beta-oxidation can cause insulin resistance, we studied mice with a deficiency of long-chain acyl-CoA dehydrogenase (LCAD), a key enzyme in mitochondrial fatty acid oxidation. Here, we show that LCAD knockout mice develop hepatic steatosis, which is associated with hepatic insulin resistance, as reflected by reduced insulin suppression of hepatic glucose production during a hyperinsulinemic-euglycemic clamp. The defects in insulin action were associated with an approximately 40% reduction in insulin-stimulated insulin receptor substrate-2-associated phosphatidylinositol 3-kinase activity and an approximately 50% decrease in Akt2 activation. These changes were associated with increased PKCepsilon activity and an aberrant 4-fold increase in diacylglycerol content after insulin stimulation. The increase in diacylglycerol concentration was found to be caused by de novo synthesis of diacylglycerol from medium-chain acyl-CoA after insulin stimulation. These data demonstrate that primary defects in mitochondrial fatty acid oxidation capacity can lead to diacylglycerol accumulation, PKCepsilon activation, and hepatic insulin resistance.

Sensory nerve inactivation by resiniferatoxin improves insulin sensitivity in male obese Zucker rats

Recent studies have suggested that sensory nerves may influence insulin secretion and action. The present study investigated the effects of resiniferatoxin (RTX) inactivation of sensory nerves (desensitization) on oral glucose tolerance, insulin secretion and whole body insulin sensitivity in the glucose intolerant, hyperinsulinemic, and insulin-resistant obese Zucker rat. After RTX treatment (0.05 mg/kg RTX sc given at ages 8, 10, and 12 wk), fasting plasma insulin was reduced (P < 0.0005), and oral glucose tolerance was improved (P < 0.005). Pancreas perfusion showed that baseline insulin secretion (7 mM glucose) was lower in RTX-treated rats (P = 0.01). Insulin secretory responsiveness to 20 mM glucose was enhanced in the perfused pancreas of RTX-treated rats (P < 0.005) but unaffected in stimulated, isolated pancreatic islets. At the peak of spontaneous insulin resistance in the obese Zucker rat, insulin sensitivity was substantially improved after RTX treatment, as evidenced by higher glucose infusion rates (GIR) required to maintain euglycemia during a hyperinsulinemic euglycemic (5 mU.kg(-1).min(-1)) clamp (GIR(60-120min): 5.97 +/- 0.62 vs. 11.65 +/- 0.83 mg.kg(-1).min(-1) in RTX-treated rats, P = 0.003). In conclusion, RTX treatment and, hence, sensory nerve desensitization of adult male obese Zucker rats improved oral glucose tolerance by enhancing insulin secretion, and, in particular, by improving insulin sensitivity.

Glycine intake decreases plasma free fatty acids, adipose cell size, and blood pressure in sucrose-fed rats

The study investigated the mechanism by which glycine protects against increased circulating nonesterified fatty acids (NEFA), fat cell size, intra-abdominal fat accumulation, and blood pressure (BP) induced in male Wistar rats by sucrose ingestion. The addition of 1% glycine to the drinking water containing 30% sucrose, for 4 wk, markedly reduced high BP in sucrose-fed rats (SFR) (122.3 +/- 5.6 vs. 147.6 +/- 5.4 mmHg in SFR without glycine, P < 0.001). Decreases in plasma triglyceride (TG) levels (0.9 +/- 0.3 vs. 1.4 +/- 0.3 mM, P < 0.001), intra-abdominal fat (6.8 +/- 2.16 vs. 14.8 +/- 4.0 g, P < 0.01), and adipose cell size were observed in SFR treated with glycine compared with SFR without treatment. Total NEFA concentration in the plasma of SFR was significantly decreased by glycine intake (0.64 +/- 0.08 vs. 1.11 +/- 0.09 mM in SFR without glycine, P < 0.001). In control animals, glycine decreased glucose, TGs, and total NEFA but without reaching significance. In SFR treated with glycine, mitochondrial respiration, as an indicator of the rate of fat oxidation, showed an increase in the state IV oxidation rate of the beta-oxidation substrates octanoic acid and palmitoyl carnitine. This suggests an enhancement of hepatic fatty acid metabolism, i.e., in their transport, activation, or beta-oxidation. These findings imply that the protection by glycine against elevated BP might be attributed to its effect in increasing fatty acid oxidation, reducing intra-abdominal fat accumulation and circulating NEFA, which have been proposed as links between obesity and hypertension.

Acute interleukin-6 administration does not impair muscle glucose uptake or whole-body glucose disposal in healthy humans

The cytokine interleukin (IL)-6 has recently been linked with type 2 diabetes mellitus and has been suggested to affect glucose metabolism. To determine whether acute IL-6 administration affects whole-body glucose kinetics or muscle glucose uptake, 18 healthy young men were assigned to one of three groups receiving a high dose of recombinant human IL-6 (HiIL-6; n = 6), a low dose of IL-6 (LoIL-6; n = 6) or saline (Con; n = 6) infused into one femoral artery for 3 h. The stable isotope [6,6-2H2] glucose was infused into a forearm vein throughout the 3 h infusion period and for a further 3 h after the cessation of infusion (recovery) to determine endogenous glucose production and whole-body glucose disposal. Infusion with HiIL-6 and LoIL-6 resulted in a marked (P < 0.05) increase in systemic IL-6 concentration throughout the 3 h of infusion (mean arterial plasma [IL-6]s of 319 and 143 pg ml-1 for HiIL-6 and LoIL-6, respectively), followed by a rapid decline (P < 0.05) during the recovery period. Subjects experienced clinical symptoms such as shivering and discomfort during HiIL-6 administration, but were asymptomatic during LoIL-6 administration. In addition, only HiIL-6 elevated (P < 0.05) plasma adrenaline (epinephrine). IL-6 infusion, irrespective of dose, did not result in any changes to endogenous glucose production, whole-body glucose disposal or leg- glucose uptake. These data demonstrate that acute IL-6 administration does not impair whole-body glucose disposal, net leg-glucose uptake, or increase endogenous glucose production at rest in healthy young humans.

Dual tail catheters for infusion and sampling in rats as an efficient platform for metabolic experiments

The authors present a technique for establishing a virtually stress-free dual-catheter system in rats, allowing researchers to conduct extended, complex experimental procedures involving the simultaneous infusion of multiple compounds and frequent collection of blood samples.

Fatty acid infusion selectively impairs insulin action on Akt1 and protein kinase C lambda /zeta but not on glycogen synthase kinase-3

To determine the mechanism(s) for insulin resistance induced by fatty acids, we measured the ability of insulin to activate phosphoinositide 3-kinase (PI3K) and multiple distal pathways in rats. Following a 5-h infusion of lipid or glycerol (control), rats underwent a euglycemic hyperinsulinemic clamp. Insulin stimulated IRS-1-associated PI3K activity in muscle of glycerol-infused rats 2.4-fold but had no effect in lipid-infused rats. IRS-2- and phosphotyrosine-associated PI3K activity were increased 3.5- and 4.8-fold, respectively, by insulin in glycerol-infused rats but only 1.6- and 2.3-fold in lipid-infused rats. Insulin increased Akt1 activity 3.9-fold in glycerol-infused rats, and this was impaired 41% in lipid-infused rats. Insulin action on Akt2 and p70S6K were not impaired, whereas activation of protein kinase C lambda/zeta activity was reduced 47%. Insulin inhibited glycogen synthase kinase 3alpha (GSK-3alpha) activity by 30% and GSK-3beta activity by approximately 65% and increased protein phosphatase-1 activity by 40-47% in both glycerol- and lipid-infused rats. Insulin stimulated glycogen synthase activity 2.0-fold in glycerol-infused rats but only 1.4-fold in lipid-infused rats. Thus, 1) elevation of fatty acids differentially affects insulin action on pathways distal to PI3K, impairing activation of Akt1 and protein kinase C lambda/zeta and 2) insulin action on glycogen synthase can be regulated independent of effects on GSK-3 and protein phosphatase-1 activity in vivo.

A rational approach to pathogenesis and treatment of type 2 diabetes mellitus, insulin resistance, inflammation, and atherosclerosis

Type 2 diabetes and obesity are major risk factors for the development of coronary artery disease (CAD) and premature atherosclerosis. Both conditions are associated with insulin resistance, oxidative stress, and inflammation. Inflammatory mediators, including plasma interleukin 6, tumor necrosis factor alpha, and tumor necrosis factor R are elevated in these individuals. The elevations of inflammatory mediators may contribute to the pathogenesis of atherosclerosis, because atherosclerosis is an inflammation of the arterial wall. There is evidence that the thiazolidinedione (TZD) class of drugs may alleviate some of the adverse atherosclerotic effects common in patients with type 2 diabetes. Considerable recent data suggest that the TZDs possess anti-inflammatory properties and exert an effect on the atherogenic process, including effects on endothelial function, monocyte/macrophage function, lipid abnormalities, smooth muscle cell migration, and fibrinolysis, all functions that are abnormal in the presence of insulin resistance. These actions of TZDs are consistent with the recently described anti-inflammatory effects of insulin. The use of TZDs as potent anti-inflammatory agents in patients with type 2 diabetes is an approach that would normalize glucose levels, as well as potentially alleviate the long-term risk of atherosclerosis.

Disordered fat storage and mobilization in the pathogenesis of insulin resistance and type 2 diabetes

The primary genetic, environmental, and metabolic factors responsible for causing insulin resistance and pancreatic beta-cell failure and the precise sequence of events leading to the development of type 2 diabetes are not yet fully understood. Abnormalities of triglyceride storage and lipolysis in insulin-sensitive tissues are an early manifestation of conditions characterized by insulin resistance and are detectable before the development of postprandial or fasting hyperglycemia. Increased free fatty acid (FFA) flux from adipose tissue to nonadipose tissue, resulting from abnormalities of fat metabolism, participates in and amplifies many of the fundamental metabolic derangements that are characteristic of the insulin resistance syndrome and type 2 diabetes. It is also likely to play an important role in the progression from normal glucose tolerance to fasting hyperglycemia and conversion to frank type 2 diabetes in insulin resistant individuals. Adverse metabolic consequences of increased FFA flux, to be discussed in this review, are extremely wide ranging and include, but are not limited to: 1) dyslipidemia and hepatic steatosis, 2) impaired glucose metabolism and insulin sensitivity in muscle and liver, 3) diminished insulin clearance, aggravating peripheral tissue hyperinsulinemia, and 4) impaired pancreatic beta-cell function. The precise biochemical mechanisms whereby fatty acids and cytosolic triglycerides exert their effects remain poorly understood. Recent studies, however, suggest that the sequence of events may be the following: in states of positive net energy balance, triglyceride accumulation in "fat-buffering" adipose tissue is limited by the development of adipose tissue insulin resistance. This results in diversion of energy substrates to nonadipose tissue, which in turn leads to a complex array of metabolic abnormalities characteristic of insulin-resistant states and type 2 diabetes. Recent evidence suggests that some of the biochemical mechanisms whereby glucose and fat exert adverse effects in insulin-sensitive and insulin-producing tissues are shared, thus implicating a diabetogenic role for energy excess as a whole. Although there is now evidence that weight loss through reduction of caloric intake and increase in physical activity can prevent the development of diabetes, it remains an open question as to whether specific modulation of fat metabolism will result in improvement in some or all of the above metabolic derangements or will prevent progression from insulin resistance syndrome to type 2 diabetes.

Mechanisms of insulin resistance in cystic fibrosis

Cystic fibrosis (CF) is associated with a high incidence of diabetes. Studies evaluating causes of CF-related diabetes (CFRD) have consistently documented decreased insulin secretion. In patients with CFRD, insulin sensitivity has been documented to be decreased, but controversy exists in patients with normal or impaired glucose tolerance (IGT). We undertook this study 1) to reexplore insulin sensitivity in patients with IGT and 2) to evaluate potential mechanisms of insulin resistance in CF, including GLUT-4 translocation, elevation of serum cytokines, and free fatty acid (FFA) levels. We recruited nine CF subjects with impaired glucose tolerance (IGTCF) and nine age-, gender-, and body mass index-matched control volunteers. Each underwent a hyperinsulinemic euglycemic clamp (200 mU. m(-2). min(-1)) to measure insulin sensitivity. A muscle biopsy was obtained at maximal insulin stimulation for measure of GLUT-4 translocation with sucrose gradients. An oral glucose tolerance test and National Institutes of Health (NIH) clinical status scores were measured in all volunteers. We also measured tumor necrosis factor (TNF)-alpha levels and FFA in all subjects. Additionally, we report the results of TNF-alpha and FFA in 32 CF patients previously studied by our group. Results were that glucose disposal rate (GDR) was significantly lower in the CFIGT subjects than in controls, indicative of impaired insulin action. GLUT-4 translocation was impaired in CF and correlated with GDR. TNF-alpha levels were higher in all CF subjects than in controls and correlated with GDR. There was no difference in FFA between CF and control subjects. Modified NIH clinical status scores were inversely correlated with GDR and TNF-alpha levels. We conclude that IGTCF patients have decreased peripheral insulin sensitivity. Mechanisms include elevation of TNF-alpha and impaired translocation of GLUT-4.

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.

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.

Prolonged fasting significantly changes nutrient oxidation and glucose tolerance after a normal mixed meal

The aim of this study was to establish the experimental paradigm of fasting, followed by refeeding, to investigate individual differences in nutrient partitioning. Eight nonobese men were fed a normal meal (25% of daily energy requirements) on two occasions, after an overnight (13-h) fast and after a prolonged (72-h) fast. During the entire fasting period, subjects were resident in a whole room indirect calorimeter, and blood samples were drawn periodically. Because no other food was consumed over the 12 h after either meal, negative energy balance was observed after the overnight and prolonged fast. Postprandial carbohydrate oxidation was significantly reduced after the 72- vs. 13-h fast (P < 0.0001), whereas fat oxidation was significantly increased (P < 0.0001). Interestingly, carbohydrate balance was positive after the prolonged fast but negative after the overnight fast (24 +/- 17 vs. -57 +/- 16 g/12 h, respectively; P < 0.001), whereas fat balance was negative under both conditions (-78 +/- 7 vs. -47 +/- 8 g/12 h, respectively; P < 0.002). With 72 h of fasting, the glucose and insulin excursions in response to the mixed meal were significantly greater compared with the 13-h fast (P < 0.001). In conclusion, prolonged fasting resulted in a significant decrease in carbohydrate oxidation and an increase in fat oxidation, after a normal mixed meal, in healthy men. This was associated with a significant decrease in glucose tolerance. Because circulating free fatty acids were greatly elevated at all times after the prolonged fast, these may be mediating some of the changes in postprandial metabolism.

Influence of insulin on the metabolism of glucose in uteri isolated from ovariectomized and non ovariectomized underfed rats

The effects of insulin on the metabolism of U14C-glucose in uteri isolated from ovariectomized and non-ovariectomized rats receiving a restricted diet (50% of the normal food intake) for 25 days, were studied. As a result of food restriction, the production of 14CO2 diminishes in intact rats, while results are reversed in ovariectomized ones. Various concentrations of insulin were added to the medium, but only 0.50 IU. ml(-1)was effective in increasing glucose metabolism in intact rats receiving a restricted diet; neither underfed castrated animals nor control ones receiving a normal diet, reacted to this concentration. The increase of 14CO2 produced by insulin is not affected by acetyl salicylic acid. Insulin does not alter the effect of underfeeding over arachidonic acid metabolism. On the contrary, the increase in glucose metabolism was blocked by N(G)methyl-L-arginine or by hemoglobin, increased with the addition of L arginine and is not affected by acetyl salicylic acid. Hemoglobin and L-arginine show no effects without insulin. We can conclude that the stimulating effect of insulin on glucose metabolism in uteri isolated from intact rats subjected to dietary restriction, is nitric oxide dependent.

Limitations to basal and insulin-stimulated skeletal muscle glucose uptake in the high-fat-fed rat

Rats fed a high-fat diet display blunted insulin-stimulated skeletal muscle glucose uptake. It is not clear whether this is due solely to a defect in glucose transport, or if glucose delivery and phosphorylation are also impaired. To determine this, rats were fed standard chow (control rats) or a high-fat diet (HF rats) for 4 wk. Experiments were then performed on conscious rats under basal conditions or during hyperinsulinemic euglycemic clamps. Rats received primed constant infusions of 3-O-methyl-[(3)H]glucose (3-O-MG) and [1-(14)C]mannitol. Total muscle glucose concentration and the steady-state ratio of intracellular to extracellular 3-O-MG concentration [which distributes based on the transsarcolemmal glucose gradient (TSGG)] were used to calculate glucose concentrations at the inner and outer sarcolemmal surfaces ([G](im) and [G](om), respectively) in soleus. Total muscle glucose was also measured in two fast-twitch muscles. Muscle glucose uptake was markedly decreased in HF rats. In control rats, hyperinsulinemia resulted in a decrease in soleus TSGG compared with basal, due to increased [G](im). In HF rats during hyperinsulinemia, [G](im) also exceeded zero. Hyperinsulinemia also decreased muscle glucose in HF rats, implicating impaired glucose delivery. In conclusion, defects in extracellular and intracellular components of muscle glucose uptake are of major functional significance in this model of insulin resistance.

Rapid oscillations in omental lipolysis are independent of changing insulin levels in vivo

Abnormal fat metabolism plays an important role in the pathogenesis of obesity-related type 2 diabetes mellitus. This study examined whether free fatty acid levels (FFAs), like insulin levels, oscillate rapidly in plasma. Peripheral and portal blood samples from dogs were assayed for FFA, glycerol, glucose, and insulin. FFA and glycerol showed correlated oscillatory profiles, with about 8 pulses/hour. Omental lipolysis was also pulsatile, with about 10 pulses/hour, and insulin levels oscillated rapidly in plasma with about 7 pulses/hour. We applied an insulin clamp, beta-adrenergic blockade, or both together, to determine the driving force behind the FFA oscillation, and we analyzed our findings by approximate entropy (ApEn) for which lower values suggest regular pulses and higher values suggest disorder. Under basal conditions, ApEn was 0.3 +/- 0.2. With insulin not oscillating, FFA still cycled at about 9 pulses/hour and the ApEn was 0.2 +/- 0.1. In contrast, beta-blockade, either in the presence or absence of an insulin clamp, removed the FFA oscillation in three of nine dogs. In the other six dogs, the oscillatory profile was unchanged, but ApEn was significantly higher than basal values, suggesting that the regularity of the profile was disrupted. These results suggest that the FFA oscillation is driven by the central nervous system, not by insulin.

Redistribution of substrates to adipose tissue promotes obesity in mice with selective insulin resistance in muscle

Obesity and insulin resistance in skeletal muscle are two major factors in the pathogenesis of type 2 diabetes. Mice with muscle-specific inactivation of the insulin receptor gene (MIRKO) are normoglycemic but have increased fat mass. To identify the potential mechanism for this important association, we examined insulin action in specific tissues of MIRKO and control mice under hyperinsulinemic-euglycemic conditions. We found that insulin-stimulated muscle glucose transport and glycogen synthesis were decreased by about 80% in MIRKO mice, whereas insulin-stimulated fat glucose transport was increased threefold in MIRKO mice. These data demonstrate that selective insulin resistance in muscle promotes redistribution of substrates to adipose tissue thereby contributing to increased adiposity and development of the prediabetic syndrome.

Mechanism of insulin resistance in A-ZIP/F-1 fatless mice

Insulin resistance is a major factor in the pathogenesis of type 2 diabetes and may be related to alterations in fat metabolism. Fatless mice have been created using dominant-negative protein (A-ZIP/F-1) targeted gene expression in the adipocyte and shown to develop diabetes. To understand the mechanism responsible for the insulin resistance in these mice, we conducted hyperinsulinemic-euglycemic clamps in awake fatless and wild type littermates before the development of diabetes and examined insulin action and signaling in muscle and liver. We found the fatless mice to be severely insulin-resistant, which could be attributed to defects in insulin action in muscle and liver. Both of these abnormalities were associated with defects in insulin activation of insulin receptor substrate-1 and -2-associated phosphatidylinositol 3-kinase activity and a 2-fold increase in muscle and liver triglyceride content. We also show that upon transplantation of fat tissue into these mice, triglyceride content in muscle and liver returned to normal as does insulin signaling and action. In conclusion, these results suggest that the development of insulin resistance in type 2 diabetes may be due to alterations in the partitioning of fat between the adipocyte and muscle/liver leading to accumulation of triglyceride in the latter tissues with subsequent impairment of insulin signaling and action.

Comparison of local and systemic effects of insulin on myocardial glucose extraction in ischemic heart disease

Physiological increases in circulating insulin level significantly increase myocardial glucose uptake in vivo. To what extent this represents a direct insulin action on the heart or results indirectly from reduction in circulating concentrations of free fatty acids (FFA) is uncertain. To examine this, we measured myocardial glucose, lactate, and FFA extraction in 10 fasting men (ages 49-76 yr) with stable coronary artery disease during sequential intracoronary (10 mU/min, coronary plasma insulin = 140 +/- 20 microU/ml) and intravenous (100 mU/min, systemic plasma insulin = 168 +/- 26 microU/ml) insulin infusion. Basally, hearts extracted 2 +/- 2% of arterial glucose and extracted 27 +/- 6% of FFA. Coronary insulin infusion increased glucose extraction to 5 +/- 3% (P < 0.01 vs. basal) without changing plasma FFA or heart FFA extraction. Conversion to intravenous infusion lowered plasma FFA by approximately 50% and heart FFA extraction by approximately 75%, increasing heart glucose extraction still further to 8 +/- 3% (P < 0. 01 vs. intracoronary). This suggests the increase in myocardial glucose extraction observed in response to an increment in systemic insulin concentration is mediated equally by a reduction in circulating FFA and by direct insulin action on the heart itself. Coronary insulin infusion increased myocardial lactate extraction as well (from 20 +/- 10% to 29 +/- 9%, P < 0.05), suggesting the local action may include stimulation of a metabolic step distal to glucose transport and glycolysis.

Surgical implantation of adipose tissue reverses diabetes in lipoatrophic mice

In lipoatrophic diabetes, a lack of fat is associated with insulin resistance and hyperglycemia. This is in striking contrast to the usual association of diabetes with obesity. To understand the underlying mechanisms, we transplanted adipose tissue into A-ZIP/F-1 mice, which have a severe form of lipoatrophic diabetes. Transplantation of wild-type fat reversed the hyperglycemia, dramatically lowered insulin levels, and improved muscle insulin sensitivity, demonstrating that the diabetes in A-ZIP/F-1 mice is caused by the lack of adipose tissue. All aspects of the A-ZIP/F-1 phenotype including hyperphagia, hepatic steatosis, and somatomegaly were either partially or completely reversed. However, the improvement in triglyceride and FFA levels was modest. Donor fat taken from parametrial and subcutaneous sites was equally effective in reversing the phenotype. The beneficial effects of transplantation were dose dependent and required near-physiological amounts of transplanted fat. Transplantation of genetically modified fat into A-ZIP/F-1 mice is a new and powerful technique for studying adipose physiology and the metabolic and endocrine communication between adipose tissue and the rest of the body.

Nutrient modulation of cellular insulin action

Abundant evidence supports a crucial role for dietary factors in the induction and maintenance of insulin resistance. At the cellular and tissue level, the availability of substrates for cellular energy production may play an important role in metabolic regulation and, in particular, in determining the response to insulin stimulation. The infusion of amino acids or fatty acids decreases insulin-stimulated glucose disposal in vivo; sustained hyperglycemia also induces insulin resistance. To determine whether nutrients directly affect insulin signaling, we have evaluated the impact of fatty acids, amino acids, and activation of the hexosamine pathway on insulin signaling in both cultured cells and animal models. We demonstrate that fatty acids and amino acids inhibit early post-receptor steps in insulin action, including tyrosine phosphorylation of insulin receptor substrate (IRS) proteins and activation of phosphatidylinositol 3-kinase (PI3-kinase), both in vitro and in several in vivo models. Similarly, activation of the hexosamine pathway by infusion of glucosamine also reduces insulin-stimulated phosphorylation of IRS proteins, activation of PI3-kinase, and activation of glycogen synthase. These data suggest that nutrients directly modulate insulin signaling, perhaps via common pathways, and thus contribute to cellular insulin resistance.

Acute effect of growth hormone to induce peripheral insulin resistance is independent of FFA and insulin levels in rats

To examine whether growth hormone (GH) induces peripheral insulin resistance by altering plasma free fatty acid (FFA) or insulin levels, the effects of GH infusion on insulin-stimulated glucose fluxes were studied in conscious rats under two protocols. In study 1, either saline (n = 7) or human recombinant GH (21 microg. kg(-1). h(-1); n = 8) was infused for 300 min, and insulin-stimulated glucose fluxes were estimated during the final 150-min period of hyperinsulinemic euglycemic clamps. In study 2, hyperinsulinemic euglycemic clamps were first conducted for 150 min (to raise plasma insulin and suppress FFA levels), and saline or GH (n = 7 for each) was subsequently infused for the following 300-min clamp period. In study 1, GH infusion in the basal state did not significantly alter plasma FFA or insulin levels. In contrast, GH infusion decreased insulin-stimulated glucose uptake, glycolysis, and glycogen synthesis by 32, 27, and 40%, respectively (P < 0.05). In study 2, GH infusion during hyperinsulinemic euglycemic clamps did not alter plasma FFA or insulin levels (P > 0.05). GH infusion had no effect on insulin-stimulated glucose uptake during the initial 150 min but eventually decreased insulin-stimulated glucose uptake by 37% (P < 0. 05), similar to the results in study 1. These data indicate that GH induces peripheral insulin resistance independent of plasma FFA and insulin levels. The induction of insulin resistance was preceded by suppression of glycogen synthesis, consistent with the hypothesis that metabolic impairment precedes and causes development of peripheral insulin resistance.

Metabolic control analysis of insulin-stimulated glucose disposal in rat skeletal muscle

Metabolic control analysis was used to calculate the distributed control of insulin-stimulated skeletal muscle glucose disposal in awake rats. Three separate hyperinsulinemic infusion protocols were performed: 1) protocol I was a euglycemic (approximately 6 mM)-hyperinsulinemic (10 mU. kg(-1). min(-1)) clamp, 2) protocol II was a hyperglycemic ( approximately 11 mM)-hyperinsulinemic (10 mU. kg(-1). min(-1)) clamp, and 3) protocol III was a euglycemic (approximately 6 mM)-hyperinsulinemic (10 mU. kg(-1). min(-1))-lipid/heparin (increased plasma free fatty acid) clamp. [1-13C]glucose was administered in all three protocols for a 3-h period, during which time [1-13C]glucose label incorporation into [1-13)]glycogen, [3-13C]lactate, and [3-13C]alanine was detected in the hindlimb of awake rats via 13C-NMR. Combined steady-state and kinetic data were used to calculate rates of glycogen synthesis and glycolysis. Additionally, glucose 6-phosphate (G-6-P) was measured in the hindlimb muscles with the use of in vivo 31P-NMR during the three infusion protocols. The clamped glucose infusion rates were 31.6 +/- 2.9, 49.7 +/- 1.0, and 24.0 +/- 1.5 mg. kg(-1). min(-1) at 120 min in protocols I-III, respectively. Rates of glycolysis were 62.1 +/- 10.3, 71.6 +/- 11.8, and 19.5 +/- 3.6 nmol. g(-1). min(-1) and rates of glycogen synthesis were 125 +/- 15, 224 +/- 23, and 104 +/- 17 nmol. g(-1). min(-1) in protocols I-III, respectively. Insulin-stimulated G-6-P concentrations were 217 +/- 8, 265 +/- 12, and 251 +/- 9 nmol/g in protocols I-III, respectively. A top-down approach to metabolic control analysis was used to calculate the distributed control among glucose transport/phosphorylation [GLUT-4/hexokinase (HK)], glycogen synthesis, and glycolysis from the metabolic flux and G-6-P data. The calculated values for the control coefficients (C) of these three metabolic steps (C(J)(GLUT-4/HK) = 0.55 +/- 0.10, C(J)(glycogen syn) = 0.30 +/- 0.06, and C(J)(glycolysis) = 0.15 +/- 0.02; where J is glucose disposal flux, and glycogen syn is glycogen synthesis) indicate that there is shared control of glucose disposal and that glucose transport/phosphorylation is responsible for the majority of control of insulin-stimulated glucose disposal in skeletal muscle.

Whole body glucose metabolism

This review describes major factors that, singly or together, influence the concentration and distribution of D-glucose in mammals, particularly in humans, with emphasis on rest, physical activity, and alimentation. It identifies areas of uncertainty: distribution and concentrations of glucose in interstitial fluid, kinetics and mechanism of transcapillary glucose transport, kinetics and mechanism of glucose transport via its transporters into cells, detailed mechanisms by which hormones, exercise, and hypoxia affect glucose movement across cell membranes, whether translocation of glucose transporters to the cell membrane accounts completely, or even mainly, for insulin-stimulated glucose uptake, whether exercise stimulates release of a circulating insulinomimetic factor, and the relation between muscle glucose uptake and muscle blood flow. The review points out that there is no compartment of glucose in the body at which all glucose is at the same concentration, and that models of glucose metabolism, including effects of insulin on glucose metabolism based on assumptions of concentration homogeneity, cannot be entirely correct. A fresh approach to modeling is needed.

Clear correlation of genotype with disease phenotype in very-long-chain acyl-CoA dehydrogenase deficiency

Very-long-chain acyl-CoA dehydrogenase (VLCAD) catalyzes the initial rate-limiting step in mitochondrial fatty acid beta-oxidation. VLCAD deficiency is clinically heterogenous, with three major phenotypes: a severe childhood form, with early onset, high mortality, and high incidence of cardiomyopathy; a milder childhood form, with later onset, usually with hypoketotic hypoglycemia as the main presenting feature, low mortality, and rare cardiomyopathy; and an adult form, with isolated skeletal muscle involvement, rhabdomyolysis, and myoglobinuria, usually triggered by exercise or fasting. To examine whether these different phenotypes are due to differences in the VLCAD genotype, we investigated 58 different mutations in 55 unrelated patients representing all known clinical phenotypes and correlated the mutation type with the clinical phenotype. Our results show a clear relationship between the nature of the mutation and the severity of disease. Patients with the severe childhood phenotype have mutations that result in no residual enzyme activity, whereas patients with the milder childhood and adult phenotypes have mutations that may result in residual enzyme activity. This clear genotype-phenotype relationship is in sharp contrast to what has been observed in medium-chain acyl-CoA dehydrogenase deficiency, in which no correlation between genotype and phenotype can be established.

Malonyl-CoA, fuel sensing, and insulin resistance

Malonyl-CoA is an allosteric inhibitor of carnitine palmitoyltransferase (CPT) I, the enzyme that controls the transfer of long-chain fatty acyl (LCFA)-CoAs into the mitochondria where they are oxidized. In rat skeletal muscle, the formation of malonyl-CoA is regulated acutely (in minutes) by changes in the activity of the beta-isoform of acetyl-CoA carboxylase (ACCbeta). This can occur by at least two mechanisms: one involving cytosolic citrate, an allosteric activator of ACCbeta and a precursor of its substrate cytosolic acetyl-CoA, and the other involving changes in ACCbeta phosphorylation. Increases in cytosolic citrate leading to an increase in the concentration of malonyl-CoA occur when muscle is presented with insulin and glucose, or when it is made inactive by denervation, in keeping with a diminished need for fatty acid oxidation in these situations. Conversely, during exercise, when the need of the muscle cell for fatty acid oxidation is increased, decreases in the ATP/AMP and/or creatine phosphate-to-creatine ratios activate an isoform of an AMP-activated protein kinase (AMPK), which phosphorylates ACCbeta and inhibits both its basal activity and activation by citrate. The central role of cytosolic citrate links this malonyl-CoA regulatory mechanism to the glucose-fatty acid cycle concept of Randle et al. (P. J. Randle, P. B. Garland. C. N. Hales, and E. A. Newsholme. Lancet 1: 785-789, 1963) and to a mechanism by which glucose might autoregulate its own use. A similar citrate-mediated malonyl-CoA regulatory mechanism appears to exist in other tissues, including the pancreatic beta-cell, the heart, and probably the central nervous system. It is our hypothesis that by altering the cytosolic concentrations of LCFA-CoA and diacylglycerol, and secondarily the activity of one or more protein kinase C isoforms, changes in malonyl-CoA provide a link between fuel metabolism and signal transduction in these cells. It is also our hypothesis that dysregulation of the malonyl-CoA regulatory mechanism, if it leads to sustained increases in the concentrations of malonyl-CoA and cytosolic LCFA-CoA, could play a key role in the pathogenesis of insulin resistance in muscle. That it may contribute to abnormalities associated with the insulin resistance syndrome in other tissues and the development of obesity has also been suggested. Studies are clearly needed to test these hypotheses and to explore the notion that exercise and some pharmacological agents that increase insulin sensitivity act via effects on malonyl-CoA and/or cytosolic LCFA-CoA.

Exaggerated stress-induced release of nonesterified fatty acids in JCR:LA-corpulent rats

Studies were performed to test the hypothesis that the stress response is exaggerated in obesity and to identify which component of the response is modified. Chronically cannulated lean (+/+) and obese (cp/cp) JCR:LA rats were subjected to mild restraint stress for 15 minutes. Blood pressure and serum glucose, insulin, and corticosterone responses did not differ significantly between genotypes before, during, or after restraint. Lean rats had a significantly greater plasma epinephrine (EPI) response but a similar norepinephrine (NE) response compared with obese rats. Serum nonesterified fatty acid (NEFA) concentrations were unchanged in lean rats, but increased from 0.86 to a mean of 1.48 mmol/L in obese rats within 10 minutes of restraint. All animals recovered to prestress values by 45 minutes postrestraint. In obese rats, handling increased NEFAs to greater than 2 mmol/L before or at 165 minutes after restraint. In lean rats, NEFAs increased when handling occurred at 165 minutes after restraint, but there was no significant NEFA response at the prerestraint point. The sensitivity of adipose tissue to NE-induced lipolysis was not significantly different between genotypes. It is concluded that the exaggerated accumulation of NEFAs in the blood of obese rats results from increased adipose tissue mass. These increases in NEFAs in obese rats resulting from mild stress reached levels normally associated with gross pathology such as ketotic diabetes.

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.