Weight loss-induced plasticity of glucose transport and phosphorylation in the insulin resistance of obesity and type 2 diabetes

We tested the hypothesis that weight loss alleviates insulin resistance in skeletal muscle within the proximal steps of glucose metabolism, namely substrate delivery, glucose transport, and glucose phosphorylation. In obese subjects with and without type 2 diabetes, in vivo skeletal muscle assessments were obtained with dynamic positron emission tomography (PET) imaging performed during euglycemic clamps at moderate hyperinsulinemia (40 mU x min(-1) x m(-2)), using [(15)O]H(2)O and [(18)F]fluoro-deoxyglucose ([(18)F]FDG) to quantify tissue perfusion and glucose metabolism. Dynamic [(18)F]FDG PET data were analyzed using both a novel muscle-specific compartmental model and a compartmental model originally developed for the brain and often used for [(18)F]FDG muscle image quantification. Weight loss in obese subjects with (n = 9) and without (n = 9) type 2 diabetes over a 4-month intervention was substantial (14 +/- 2 kg, P < 0.05). Muscle insulin resistance, assessed by insulin-stimulated [(18)F]FDG uptake, decreased threefold in diabetic subjects and twofold in nondiabetic subjects (P < 0.001). Kinetic parameters for [(18)F]FDG transport and phosphorylation improved substantially in both groups, whereas tissue blood flow did not change. In particular, clinically significant weight loss fully corrected insulin resistance in type 2 diabetes at the step of glucose phosphorylation and largely, but incompletely, corrected insulin resistance at the glucose transport step.     

Interactions Among Glucose Delivery,Transport, and Phosphorylation That Underlie Skeletal Muscle Insulin Resistance in Obesity and Type 2 Diabetes: Studies With Dynamic PET Imaging

Dynamic positron emission tomography (PET) imaging was performed using sequential tracer injections ([¹⁵O]H₂O, [¹¹C]3-O-methylglucose [3-OMG], and [¹⁸F]fluorodeoxyglucose [FDG]) to quantify, respectively, skeletal muscle tissue perfusion (glucose delivery), kinetics of bidirectional glucose transport, and glucose phosphorylation to interrogate the individual contribution and interaction among these steps in muscle insulin resistance (IR) in type 2 diabetes (T2D). PET imaging was performed in normal weight nondiabetic subjects (NW) (n = 5), obese nondiabetic subjects (OB) (n = 6), and obese subjects with T2D (n = 7) during fasting conditions and separately during a 6-h euglycemic insulin infusion at 40 mU·m⁻²·min⁻¹. Tissue tracer activities were derived specifically within the soleus muscle with PET images and magnetic resonance imaging. During fasting, NW, OB, and T2D subjects had similar [¹¹C]3-OMG and [¹⁸F]FDG uptake despite group differences for tissue perfusion. During insulin-stimulated conditions, IR was clearly evident in T2D (P < 0.01), and [¹⁸F]FDG uptake by muscle was inversely correlated with systemic IR (P < 0.001). The increase in insulin-stimulated glucose transport was less (P < 0.01) in T2D (twofold) than in NW (sevenfold) or OB (sixfold) subjects. The fractional phosphorylation of [¹⁸F]FDG during insulin infusion was also significantly lower in T2D (P < 0.01). Dynamic triple-tracer PET imaging indicates that skeletal muscle IR in T2D involves a severe impairment of glucose transport and additional impairment in the efficiency of glucose phosphorylation.     

Interactions between delivery, transport, and phosphorylation of glucose in governing uptake into human skeletal muscle

Skeletal muscle accounts for a large proportion of insulin-stimulated glucose utilization. It is generally regarded that much of the control over rates of uptake is posited within the proximal steps of delivery, transport, and phosphorylation of glucose, with glucose transport as the main locus of control. Whether insulin modulates the distribution of control across these steps and in what manner remains uncertain. The current study addressed this in vivo using dynamic positron emission tomography (PET) imaging of human muscle with sequential injections of three tracers ([(15)O]H(2)O, [(11)C]3-O-methyl glucose [3-OMG], and [(18)F]fluoro-deoxy glucose [FDG]) that enabled quantitative determinations of glucose delivery, transport, and its phosphorylation, respectively. Lean, healthy, research volunteers were studied during fasting conditions (n = 8) or during a euglycemic insulin infusion at 30 mU/min per m(2) (n = 8). PET images were coregistered with magnetic resonance imaging to contrast glucose kinetics in soleus, a highly oxidative muscle, with tibialis anterior, a less oxidative muscle. During fasting conditions, uptake of [(11)C]3-OMG was similar in soleus and tibialis anterior muscles, despite higher delivery to soleus (by 35%; P < 0.01). Uptake of [(18)F]FDG was also similar between muscle during fasting, and glucose transport was found to be the dominant locus of control (90%) for glucose uptake under this condition. Insulin increased uptake of [(11)C]3-OMG substantially and strongly stimulated the kinetics of bidirectional glucose transport. Uptake of [(11)C]3-OMG was higher in soleus than tibialis anterior muscle (by 22%; P < 0.01), a difference partially due to higher delivery, which was again found to be 35% higher to soleus (P < 0.01). The uptake of [(18)F]FDG was 65% greater in soleus compared with tibialis anterior muscle, a larger difference than for [(11)C]3-OMG (P < 0.01), indicating an added importance of glucose phosphorylation in defining insulin sensitivity. Analysis of the distribution of control during insulin-stimulated conditions revealed that most of the control was posited at delivery and transport and was equally divided between these steps. Thus, insulin evokes a broader distribution of control than during fasting conditions in governing glucose uptake into skeletal muscle. This redistribution of control is triggered by the robust stimulation of glucose transport, which in turn unmasks a greater dependence upon delivery and glucose phosphorylation.     

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

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

Endothelin limits insulin action in obese/insulin-resistant humans

The normal action of insulin to vasodilate and redistribute blood flow in support of skeletal muscle metabolism is impaired in insulin-resistant states. Increased endogenous endothelin contributes to endothelial dysfunction in obesity and diabetes. Here, we test the hypothesis that increased endogenous endothelin action also contributes to skeletal muscle insulin resistance via impairments in insulin-stimulated vasodilation. We studied nine lean and seven obese humans, measuring the metabolic and hemodynamic effects of insulin (300 mU . m(-2) . min(-1)) alone and during femoral artery infusion of BQ123 (an antagonist of type A endothelin receptors, 1 micromol/min). Endothelin antagonism augmented skeletal muscle responses to insulin in obese subjects through changes in both leg blood flow (LBF) and glucose extraction. Insulin-stimulated LBF was significantly increased in obese subjects only. These changes, combined with differential effects on glucose extraction, resulted in augmented insulin-stimulated leg glucose uptake in obese subjects (54.7 +/- 5.7 vs. 107.4 +/- 18.9 mg/min with BQ123), with no change in lean subjects (103.7 +/- 11.4 vs. 88.9 +/- 16.3, P = 0.04 comparing BQ123 across groups). BQ123 allowed augmented leg glucose extraction in obese subjects even in the face of NOS antagonism. These findings suggest that increased endogenous endothelin action contributes to insulin resistance in skeletal muscle of obese humans, likely through both vascular and tissue effects.     

Single nucleotide polymorphisms in the peroxisome proliferator-activated receptor delta gene are associated with skeletal muscle glucose uptake

The peroxisome proliferator-activated receptors (PPARs) belong to a superfamily of nuclear receptors. It includes PPAR-delta, a key regulator of fatty acid oxidation and energy uncoupling, universally expressed in different tissues. The PPAR-delta gene (PPARD) maps to 6p21.2-p21.1 and has 11 exons and spans 35 kbp. We investigated the effects of single nucleotide polymorphisms (SNPs) of PPARD on whole-body, skeletal muscle, and subcutaneous adipose tissue glucose uptake in 129 healthy individuals using the hyperinsulinemic-euglycemic clamp technique combined with fluorine-18-labeled fluorodeoxyglucose ([18F]FDG) and positron emission tomography (PET). Three of six SNPs of PPARD and their haplogenotypes were significantly associated with whole-body insulin sensitivity. [18F]FDG-PET scanning indicated that SNPs of PPARD primarily affected insulin sensitivity by modifying glucose uptake in skeletal muscle but not in adipose tissue. Our results give evidence that SNPs of PPARD regulate insulin sensitivity particularly in skeletal muscle.     

Isoform-specific regulation of 5' AMP-activated protein kinase in skeletal muscle from obese Zucker (fa/fa) rats in response to contraction

Glucose transport can be activated in skeletal muscle in response to insulin via activation of phosphoinositide (PI) 3-kinase and in response to contractions or hypoxia, presumably via activation of 5' AMP-activated protein kinase (AMPK). We determined the effects of insulin and muscle contraction/hypoxia on PI 3-kinase, AMPK, and glucose transport activity in epitrochlearis skeletal muscle from insulin-resistant Zucker (fa/ fa) rats. Insulin-stimulated glucose transport in isolated skeletal muscle was reduced 47% in obese versus lean rats, with a parallel 42% reduction in tyrosine-associated PI 3-kinase activity. Contraction and hypoxia elicited normal responses for glucose transport in skeletal muscle from insulin-resistant obese rats. Isoform-specific AMPK activity was measured in skeletal muscle in response to insulin, contraction, or hypoxia. Contraction increased AMPKalpha1 activity 2.3-fold in lean rats, whereas no effect was noted in obese rats. Hypoxia increased AMPKalpha1 activity to a similar extent (more than sixfold) in lean and obese rats. Regardless of genotype, contraction, and hypoxia, each increased AMPKalpha2 activity more than fivefold, whereas insulin did not alter either AMPKalpha1 or -alpha2 activity in skeletal muscle. In conclusion, obesity-related insulin resistance is associated with an isoform-specific impairment in AMPKalpha1 in response to contraction. However, this impairment does not appear to affect contraction-stimulated glucose transport. Activation of AMPKalpha2 in response to muscle contraction/ exercise is associated with a parallel and normal increase in glucose transport in insulin-resistant skeletal muscle.     

Impaired insulin-mediated skeletal muscle blood flow in patients with NIDDM.

Patients with non-insulin-dependent diabetes mellitus (NIDDM) exhibit decreased rates of skeletal muscle insulin-mediated glucose uptake (IMGU). Because IMGU is equal to the product of the arteriovenous glucose difference (AVG delta) across and blood flow (F) into muscle (IMGU = AVG delta x F), reduced tissue permeability (AVG delta) and/or glucose and insulin delivery (F) can potentially lead to decreased IMGU. The components of skeletal muscle IMGU were studied in six obese NIDDM subjects (103 +/- 9 kg) and compared with those previously determined in six lean (weight 68 +/- 3 kg), and six obese (94 +/- 3 kg) with normal glucose tolerance. The insulin dose-response curves for whole body and leg muscle IMGU were constructed using the combined euglycemic clamp and leg balance techniques during sequential insulin infusions (range of serum insulin 130-80,000 pmol/L). In lean, obese, and NIDDM subjects, whole body IMGU, femoral AVG delta, and leg IMGU increased in a dose-dependent fashion over the range of insulin with an ED50 of 400-500 pmol/L in lean, 1000-1200 pmol/L in obese, and 4000-7000 pmol/L in NIDDM subjects (P less than 0.01 lean vs. obese and NIDDM). In lean and obese subjects, maximally effective insulin concentrations increased leg blood flow approximately 2-fold from basal with an ED50 of 266 pmol/L and 957 pmol/L, respectively (P less than 0.01 lean vs. obese). In contrast, leg F did not increase from the basal value in NIDDM subjects (2.7 +/- 0.1 vs. 3.5 +/- 0.5 dl/min, NS).(ABSTRACT TRUNCATED AT 250 WORDS)     

Development of obesity in Zucker rats. Early insulin resistance in muscles but normal sensitivity in white adipose tissue

Euglycemic-hyperinsulinemic clamps were performed on 4- and 12-wk-old anesthetized lean and obese Zucker rats. During the clamp studies, total glucose production and utilization were assessed with a 3-[3H]glucose perfusion, whereas local glucose utilization was determined by measuring 2-deoxy-1-[3H]glucose 6-phosphate accumulation in various tissues. In the basal state, 4 wk-old obese rats were hyperinsulinemic (159 +/- 8 vs. 82 +/- 9 microU/ml), whereas glucose turnover rate was similar to that observed in lean rats (14.9 +/- 1.9 vs. 12.5 +/- 1.9 mg X min-1 X kg-1). Glucose utilization was identical in skeletal muscles, whereas it was increased in white adipose tissue of obese rats (22 +/- 4 vs. 8 +/- 2 ng X min-1 X mg-1). At plasma insulin level of 500 microU/ml, glucose production was totally suppressed in both groups, whereas overall glucose utilization was slightly less in 4-wk-old obese than in lean rats. This was due to a reduced stimulation of glucose utilization in skeletal muscles and brown adipose tissue. In contrast, glucose utilization in periovarian white adipose tissue was similarly increased in lean and obese rats. For a maximal insulin concentration (1500 microU/ml), all the differences were abolished between lean and obese young Zucker rats. In older (12-wk-old) obese rats, glucose utilization in various tissues was markedly reduced at maximal insulin level compared with that observed in age-matched lean animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Delayed insulin transport across endothelium in insulin-resistant JCR:LA-cp rats

Capillary endothelial cells are thought to limit the transport of insulin across the endothelium, resulting in attenuated insulin action at target sites. Whether endothelial insulin transport is altered in dysglycemic insulin-resistant states is not clear and was therefore investigated in the JCR:LA-cp corpulent male rat, which exhibits the metabolic syndrome of obesity, insulin resistance, hyperlipidemia, and hyperinsulinemia. Lean littermates that did not develop these alterations served as controls. Animals of both groups were normotensive (mean arterial pressure 136+/-2 mmHg). Hearts from obese and lean rats aged 7 (n = 6) or 18 (n = 8) weeks were perfused in vitro at 10 ml/min per gram wet wt over 51 min with Krebs-Henseleit buffer containing 0.1 or 0.5 U human insulin/l (equivalent to 0.6 and 3 nmol/l). Interstitial fluid was collected using a validated method, and interstitial insulin was determined with a radioimmunoassay. At 0.1 U/l, insulin transfer velocity was similar in both experimental groups (half-times of transfer: 11+/-0.2 min in obese and 18+/-4 min in lean rats; NS), but at 0.5 U/l, the respective half-times were 7+/-1 min in lean and 13+/-2 min in obese rats (P < 0.05). The steady-state level of insulin in the interstitium was 34+/-1% of the vascular level at 0.1 U/l and reached the vascular level (102+/-2%) at 0.5 U/l in both lean and obese rats. In rats aged 18 weeks, the half-times of insulin transfer were 31+/-2 and 14+/-l min in obese rats and 10+/-0.3 and 7+/-0.3 min in lean rats (P < 0.05). Again, interstitial steady-state levels were similar in both groups. Finally, postprandial insulin dynamics were simulated over a period of 120 min with a peak concentration of 0.8 U/l in rats aged 27 weeks (n = 4). The maximal interstitial level was 0.38+/-0.02 U/l in lean rats and 0.24+/-0.02 U/l in obese rats (P < 0.05), and a similar difference was noted throughout insulin infusion (areas under the transudate concentration-time curves: 17 and 11 U/min per 1, respectively). These data show, for the first time in a genetic animal model of insulin resistance, that transfer of insulin across the endothelium is substantially delayed in obese insulin-resistant rats and that it likely contributes to the postprandial alterations of glucose metabolism observed in the metabolic syndrome.     

Deficiency of subsarcolemmal mitochondria in obesity and type 2 diabetes

The current study addresses a novel hypothesis of subcellular distribution of mitochondrial dysfunction in skeletal muscle in type 2 diabetes. Vastus lateralis muscle was obtained by percutaneous biopsy from 11 volunteers with type 2 diabetes; 12 age-, sex-, and weight-matched obese sedentary nondiabetic volunteers; and 8 lean volunteers. Subsarcolemmal and intermyofibrillar mitochondrial fractions were isolated by differential centrifugation and digestion techniques. Overall electron transport chain activity was similar in type 2 diabetic and obese subjects, but subsarcolemmal mitochondria electron transport chain activity was reduced in type 2 diabetic subjects (0.017 +/- 0.003 vs. 0.034 +/- 0.007 units/mU creatine kinase [CK], P = 0.01) and sevenfold reduced compared with lean subjects (P < 0.01). Electron transport chain activity in intermyofibrillar mitochondria was similar in type 2 diabetic and obese subjects, though reduced compared with lean subjects. A reduction in subsarcolemmal mitochondria was confirmed by transmission electron microscopy. Although mtDNA was lower in type 2 diabetic and obese subjects, the decrement in electron transport chain activity was proportionately greater, indicating functional impairment. Because of the potential importance of subsarcolemmal mitochondria for signal transduction and substrate transport, this deficit may contribute to the pathogenesis of muscle insulin resistance in type 2 diabetes.     

Body composition, adipocyte size, free fatty acid concentration, and glucose tolerance in children of diabetic pregnancies

Previous studies show that children of women who are diabetic during pregnancy are more obese and have a higher prevalence of non-insulin-dependent diabetes mellitus (NIDDM) than children of women who first developed NIDDM greater than 1 yr after the pregnancy (prediabetic mothers) and children of women who have never developed diabetes (nondiabetic mothers). To determine whether lean and obese children of glucose-intolerant pregnancies can be distinguished from similar children of glucose-tolerant pregnancies, we measured body composition, abdominal and gluteal adipocyte size, fasting free fatty acid (FFA), and fasting and stimulated glucose and insulin concentrations during an oral glucose tolerance test in prepubertal children of glucose-intolerant and prediabetic mothers. Each group ranged in adipocity from 6 to 40% body fat. Age, weight, height, and percentage of body fat were similar in the two groups. There were no significant differences in adipocyte size or in glucose, FFA, C-peptide, and insulin concentrations between the groups. The correlation between abdominal adipocyte size and fasting insulin concentration (r = .91 and .18, t = 2.8, P = .01) was stronger in children from glucose-intolerant than from glucose-tolerant pregnancies, respectively. In terms of the parameters we measured, there are no major differences between children of glucose-intolerant and glucose-tolerant pregnancies.     

Oral glucosamine for 6 weeks at standard doses does not cause or worsen insulin resistance or endothelial dysfunction in lean or obese subjects

Glucosamine is a popular nutritional supplement used to treat osteoarthritis. Intravenous administration of glucosamine causes insulin resistance and endothelial dysfunction. However, rigorous clinical studies evaluating the safety of oral glucosamine with respect to metabolic and cardiovascular pathophysiology are lacking. Therefore, we conducted a randomized, placebo-controlled, double-blind, crossover trial of oral glucosamine at standard doses (500 mg p.o. t.i.d.) in lean (n = 20) and obese (n = 20) subjects. Glucosamine or placebo treatment for 6 weeks was followed by a 1-week washout and crossover to the other arm. At baseline, and after each treatment period, insulin sensitivity was assessed by hyperinsulinemic-isoglycemic glucose clamp (SI(Clamp)) and endothelial function evaluated by brachial artery blood flow (BAF; Doppler ultrasound) and forearm skeletal muscle microvascular recruitment (ultrasound with microbubble contrast) before and during steady-state hyperinsulinemia. Plasma glucosamine pharmacokinetics after oral dosing were determined in each subject using a high-performance liquid chromatography method. As expected, at baseline, obese subjects had insulin resistance and endothelial dysfunction when compared with lean subjects (SI(Clamp) [median {25th-75th percentile}] = 4.3 [2.9-5.3] vs. 7.3 [5.7-11.3], P < 0.0001; insulin-stimulated changes in BAF [% over basal] = 12 [-6 to 84] vs. 39 [2-108], P < 0.04). When compared with placebo, glucosamine did not cause insulin resistance or endothelial dysfunction in lean subjects or significantly worsen these findings in obese subjects. The half-life of plasma glucosamine after oral dosing was approximately 150 min, with no significant changes in steady-state glucosamine levels detectable after 6 weeks of therapy. We conclude that oral glucosamine at standard doses for 6 weeks does not cause or significantly worsen insulin resistance or endothelial dysfunction in lean or obese subjects.     

The stimulatory effect of globular adiponectin on insulin-stimulated glucose uptake and fatty acid oxidation is impaired in skeletal muscle from obese subjects

Adiponectin is an adipose-derived hormone that plays an important role in regulating insulin sensitivity in rodents. However, little is known regarding the effect of adiponectin on metabolism in human skeletal muscle. Therefore, we examined whether the globular head of adiponectin, gAcrp30, acutely activates fatty acid oxidation and glucose uptake in isolated human skeletal muscle. Furthermore, we aimed to determine whether these effects would differ in muscle from lean versus obese individuals. Treatment with gAcrp30 (2.5 microg/ml) increased fatty acid oxidation in lean muscle (70%, P < 0.0001) and to a lesser extent in obese muscle (30%, P < 0.01). In the absence of insulin, gAcrp30 increased glucose uptake 37% in lean (P < 0.05) and 33% in obese muscle (P < 0.05). Combined exposure of insulin and gAcrp30 demonstrated an additive effect on glucose uptake in lean and obese individuals, but this effect was reduced by 50% in obese muscle (P < 0.05). These metabolic effects were attributable to an increase in AMP-activated protein kinase-alpha1 (AMPKalpha1) and AMPKalpha2 activity. However, in obese muscle the activation of AMPKalpha2 by gAcrp30 was blunted. This study provides evidence that gAcrp30 plays a role in regulating fatty acid and glucose metabolism in human skeletal muscle. However, the effects are blunted in obesity, indicating the possible development of adiponectin resistance.     

Obesity and type 2 diabetes impair insulin-induced suppression of glycogenolysis as well as gluconeogenesis

To determine whether the hepatic insulin resistance of obesity and type 2 diabetes is due to impaired insulin-induced suppression of glycogenolysis as well as gluconeogenesis, 10 lean nondiabetic, 10 obese nondiabetic, and 11 obese type 2 diabetic subjects were studied after an overnight fast and during a hyperinsulinemic-euglycemic clamp. Gluconeogenesis and glycogenolysis were measured using the deuterated water method. Before the clamp, when glucose and insulin concentrations differed among the three groups, gluconeogenesis was higher in the diabetic than in the obese nondiabetic subjects (P < 0.05) and glycogenolysis was higher in the diabetic than in the lean nondiabetic subjects (P < 0.05). During the clamp, when glucose and insulin concentrations were matched and glucagon concentrations were suppressed, both glycogenolysis and gluconeogenesis were higher (P < 0.01) in the diabetic versus the obese and lean nondiabetic subjects. Furthermore, glycogenolysis and gluconeogenesis were higher (P < 0.01) in the obese than in the lean nondiabetic subjects. Plasma free fatty acid concentrations correlated (P < 0.001) with glucose production and gluconeogenesis both before and during the clamp and with glycogenolysis during the clamp (P < 0.01). We concluded that defects in the regulation of glycogenolysis as well as gluconeogenesis cause hepatic insulin resistance in obese nondiabetic and type 2 diabetic humans.     

A novel method to measure glucose uptake and myosin heavy chain isoform expression of single fibers from rat skeletal muscle

Skeletal muscle includes many individual fibers with diverse phenotypes. A barrier to understanding muscle glucose uptake at the cellular level has been the absence of a method to measure glucose uptake by single fibers from mammalian skeletal muscle. This study's primary objective was to develop a procedure to measure glucose uptake by single fibers from rat skeletal muscle. Rat epitrochlearis muscles were incubated ex vivo with [(3)H]-2-deoxy-d-glucose, with or without insulin or AICAR, before isolation of ~10-30 single fibers from each muscle. Fiber type (myosin heavy chain [MHC] isoform) and glucose uptake were determined for each single fiber. Insulin-stimulated glucose uptake (which was cytochalasin B inhibitable) varied according to MHC isoform expression, with ~2-fold greater values for IIA versus IIB or IIX fibers and ~1.3-fold greater for hybrid (IIB/X) versus IIB fibers. In contrast, AICAR-stimulated glucose uptake was ~1.5-fold greater for IIB versus IIA fibers. A secondary objective was to assess insulin resistance of single fibers from obese versus lean Zucker rats. Genotype differences were observed for insulin-stimulated glucose uptake and inhibitor κB (IκB)-β abundance in single fibers (obese less than lean), with decrements for glucose uptake (44-58%) and IκB-β (25-32%) in each fiber type. This novel method creates a unique opportunity for future research focused on understanding muscle glucose uptake at the cellular level.     

No effect of insulin on glucose blood-brain barrier transport and cerebral metabolism in humans.

The effect of hyperinsulinemia on glucose blood-brain barrier (BBB) transport and cerebral metabolism (CMRglc) was studied using the intravenous double-indicator method and positron emission tomography using [18F]fluorodeoxyglucose as tracer (PET-FDG). Sixteen normal healthy control subjects (25 +/- 4 years old) were studied twice during a euglycemic and a euglycemic-hyperinsulinemic condition. Our hypothesis was that high physiologic levels of insulin did not affect the BBB transport or net metabolism of glucose. During insulin infusion, arterial plasma insulin levels increased from 48.5 to 499.4 pmol/l. The permeability-surface area products for glucose and FDG BBB transport obtained with the double-indicator method remained constant during hyperinsulinemia. Similarly using PET-FDG, no changes were observed in the unidirectional clearance of FDG from blood to brain. k2* (FDG transport from brain to blood) increased significantly by 15 and 18% (gray and white matter, respectively), and k4* (dephosphorylation of FDG) increased by 18%. The increase in k2* may be caused by insulin inducing a decrease in the available FDG brain pool. The increase in k4* may be related to an increased loss of labeled products during insulin fusion. Irrespective of these changes, CMRglc remained unchanged in all brain regions. We conclude that hyperinsulinemia within the normal physiologic range does not affect BBB glucose transport or net cerebral glucose metabolism.     

Increased fat mass compensates for insulin resistance in abdominal obesity and type 2 diabetes: a positron-emitting tomography study

To evaluate the relative impact of abdominal obesity and newly diagnosed type 2 diabetes on insulin action in skeletal muscle and fat tissue, we studied 61 men with (n = 31) or without (n = 30) diabetes, subgrouped into abdominally obese or nonobese according to the waist circumference. Adipose tissue depots were quantified by magnetic resonance imaging, and regional glucose uptake was measured using 2-[(18)F]fluoro-2-deoxyglucose/positron emission tomography during euglycemic hyperinsulinemia. Across groups, glucose uptake per unit tissue weight was higher in visceral (20.5 +/- 1.4 micromol . min(-1) . kg(-1)) than in abdominal (9.8 +/- 0.9 micromol min(-1) . kg(-1), P < 0.001) or femoral (12.3 +/- 0.6 micromol . min(-1) . kg(-1), P < 0.001) subcutaneous tissue and approximately 40% lower than in skeletal muscle (33.1 +/- 2.5 micromol . min(-1) . kg(-1), P < 0.0001). Abdominal obesity was associated with a marked reduction in glucose uptake per unit tissue weight in all fat depots and in skeletal muscle (P < 0.001 for all regions). Recent type 2 diabetes per se had little additional effect. In both intra-abdominal adipose (r = -0.73, P < 0.0001) and skeletal muscle (r = -0.53, P < 0.0001) tissue, glucose uptake was reciprocally related to intra-abdominal fat mass in a curvilinear fashion. When regional glucose uptake was multiplied by tissue mass, total glucose uptake per fat depot was similar irrespective of abdominal obesity or type 2 diabetes, and its contribution to whole-body glucose uptake increased by approximately 40% in obese nondiabetic and nonobese diabetic men and was doubled in obese diabetic subjects. We conclude that 1) in abdominal obesity, insulin-stimulated glucose uptake rate is markedly reduced in skeletal muscle and in all fat depots; 2) in target tissues, this reduction is reciprocally (and nonlinearly) related to the amount of intra-abdominal fat; 3) mild, recent diabetes adds little insulin resistance to that caused by abdominal obesity; and 4) despite fat insulin resistance, an expanded fat mass (especially subcutaneous) provides a sink for glucose, resulting in a compensatory attenuation of insulin resistance at the whole-body level in men.     

Increased fatty acid uptake and altered fatty acid metabolism in insulin-resistant muscle of obese Zucker rats

Altered muscle fatty acid (FA) metabolism may contribute to the presence of muscle insulin resistance in the genetically obese Zucker rat. To determine whether FA uptake and disposal are altered in insulin-resistant muscle, we measured palmitate uptake, oxidation, and incorporation into di- and triglycerides in isolated rat hindquarters, as well as muscle plasma membrane fatty acid-binding protein (FABP(PM)) content of lean (n = 16, fa/+) and obese (n = 15, fa/fa) Zucker rats (12 weeks of age). Hindquarters were perfused with 7 mmol/l glucose, 1,000 micromol/l albumin-bound palmitate, and albumin-bound [1-(14)C]palmitate at rest (no insulin). Glucose uptake was 42% lower in the obese than in the lean rats and indicated the presence of muscle insulin resistance. Fractional and total rates of palmitate uptake were 42 and 74% higher in the obese than in the lean rats and were associated with higher muscle FABP(PM) content (r(2) = 0.69, P < 0.05). The percentage of palmitate oxidized was not significantly different between groups. FA disposal to storage was altered according to fiber type. When compared with lean rats, the rate of triglyceride synthesis in red muscle was 158% higher in obese rats, and the rate of palmitate incorporation into diglycerides in white muscle was 93% higher in obese rats. Pre- and postperfusion muscle triglyceride levels were higher in both red and white muscles of the obese rats. These results show that increased FA uptake and altered FA disposal to storage may contribute to the development of muscle insulin resistance in obese Zucker rats.