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Controversy exists about rates of bone mineral loss from the lumbar spine at the menopause. Cross-sectional studies of lumbar bone mineral against age have been interpreted as evidence for or against a menopause-related change in lumbar bone loss, depending on their goodness of fit to either linear or nonlinear equations. To investigate the ability of cross-sectional studies to detect accelerated lumbar bone loss at the menopause, we constructed models of different patterns of loss and evaluated the findings in cross-sectional analyses of computer-simulated populations of normal women. A sample size of >300 was needed to distinguish between linear and nonlinear patterns of bone loss with a power of 90% when the data was analyzed throughout life. Examining for differences between the slopes of the regressions of pre- and postmenopausal women was more useful for detecting nonlinear bone loss under some circumstances. A difference between slopes was apparent in studies containing 100 subjects if lumbar bone density (BMD) was assumed to be unchanged prior to the menopause, but a larger study size was needed (> 1000) if BMD was assumed to fall before the menopause. We also measured lumbar BMD by dual photon absorptiometry in 141 normal females. When the data was analyzed against age throughout life, a nonlinear pattern of bone loss was found, and comparison of preand postmenopausal subjects showed a significant difference in the slopes of the linear regressions. Our patient data support the concept of a relatively stable lumbar BMD prior to the menopause with a rapid but transient decline postmenopausally. The modeling studies emphasize the importance of an adequate study size in cross-sectional studies to identify accelerated lumbar bone loss at the menopause.

Accumulation of intracellular lipid in obesity is associated with metabolic disease in many tissues including liver. Storage of fatty acid as triglyceride (TG) requires the activation of fatty acids to long-chain acyl-CoAs (LC-CoA) by the enzyme acyl-CoA synthetase (ACSL). There are five known isoforms of ACSL (ACSL1, -3, -4, -5, -6), which vary in their tissue specificity and affinity for fatty acid substrates. To investigate the role of ACSL1 in the regulation of lipid metabolism, we used adenoviral-mediated gene transfer to overexpress ACSL1 in the human hepatoma cell-line HepG2 and in liver of rodents. Infection of HepG2 cells with the adenoviral construct AdACSL1 increased ACSL activity >10-fold compared with controls after 24 h. HepG2 cells overexpressing ACSL1 had a 40% higher triglyceride (TG) content (93 ± 3 vs. 67 ± 2 nmol/mg protein in controls, P < 0.05) after 24-h exposure to 1 mM oleate. Furthermore, ACSL1 overexpression produced a 60% increase in cellular LCA-CoA content (160 ± 6 vs. 100 ± 6 nmol/g protein in controls, P < 0.05) and increased [14C]oleate incorporation into TG without significantly altering fatty acid oxidation. In mice, AdACSL1 administration increased ACSL1 mRNA and protein more than fivefold over controls at 4 days postinfection. ACSL1 overexpression caused a twofold increase in TG content in mouse liver (39 ± 4 vs. 20 ± 2 μmol/g wet wt in controls, P < 0.05), and overexpression in rat liver increased [1-14C]palmitate clearance into liver TG. These in vitro and in vivo results suggest a pivotal role for ACSL1 in regulating TG synthesis in liver.

Lipid accumulation in non-adipose tissues is strongly associated with the metabolic syndrome, possibly due to aberrant partitioning of intracellular fatty acids between storage and oxidation. In the present study, we administered the non-metabolizable fatty acid analog [9,10-3H]-(R)-2-bromopalmitate, and authentic 14C-palmitate to conscious rats, in order to directly examine the initial intracellular fate of fatty acids in a range of insulin-sensitive tissues, including white and red muscles, liver, white adipose tissue, and heart. Rats were studied after administration of an oral glucose load to examine the effect of physiological elevation of glucose and insulin. The tracer results showed that glucose administration partitioned fatty acid toward storage in white muscle (storage:uptake ratios, vehicle vs glucose; 0.64 ± 0.02 vs 0.92 ± 0.09, P < 0.05), and in liver (0.66 ± 0.07 vs 0.98 ± 0.04, P < 0.05), but not in red muscle (1.18 ± 0.07 vs 1.36 ± 0.11, P = not significant). These results demonstrate the physiological relevance of the so-called ‘reverse’ Randle cycle, but surprisingly show that it may be more important in white rather than oxidative red muscle.

Studies of cardiac fuel metabolism in mice have been almost exclusively conducted ex vivo. The major aim of this study was to assess in vivo plasma FFA and glucose utilization by the hearts of healthy control ( db/+) and diabetic ( db/db) mice, based on cardiac uptake of (R)-2-[9,10-3H]bromopalmitate ([3H]R-BrP) and 2-deoxy-d-[U-14C]glucose tracers. To obtain quantitative information about the evaluation of cardiac FFA utilization with [3H]R-BrP, simultaneous comparisons of [3H]R-BrP and [14C]palmitate ([14C]P) uptake were first made in isolated perfused working hearts from db/+ mice. It was found that [3H]R-BrP uptake was closely correlated with [14C]P oxidation ( r2= 0.94, P < 0.001). Then, methods for in vivo application of [3H]R-BrP and [14C]2-DG previously developed for application in the rat were specially adapted for use in the mouse. The method yields indexes of cardiac FFA utilization ( Rf*) and clearance ( Kf*), as well as glucose utilization ( Rg′). Finally, in the main part of the study, the ability of the heart to switch between FFA and glucose fuels (metabolic flexibility) was investigated by studying anesthetized, 8-h-fasted control and db/db mice in either the basal state or during glucose infusion. In control mice, glucose infusion raised plasma levels of glucose and insulin, raised Rg′ (+58%), and lowered plasma FFA level (−48%), Kf* (−45%), and Rf* (−70%). This apparent reciprocal regulation of glucose and FFA utilization by control hearts illustrates metabolic flexibility for substrate use. By contrast, in the db/db mice, glucose infusion raised glucose levels with no apparent influence on cardiac FFA or glucose utilization. In conclusion, tracer methodology for assessing in vivo tissue-specific plasma FFA and glucose utilization has been adapted for use in mice and reveals a profound loss of metabolic flexibility in the diabetic db/db heart, suggesting a fixed level of FFA oxidation in fasted and glucose-infused states.

In order to enable detailed studies of free fatty acid (FFA) metabolism, we recently introduced a method for the evaluation of tissue-specific FFA metabolism in vivo. The method is based on the simultaneous use of 14C-palmitate (14C-P) and the non-beta-oxidizable FFA analogue, [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP). Indices of total FFA utilization and incorporation into storage products are obtained from tissue concentrations of 3H and 14C, respectively, following intravenous administration of 3H-R-BrP and 14C-P and their disappearance from plasma into tissues. This review covers the basis for, and developments in, the methodology, as well as some of the applications to date. In the rat, the method has been used to characterize tissue-specific alterations in FFA metabolism in various situations, including skeletal muscle contraction, fasting, hyperinsulinemia, and various pharmacological manipulations. The results of all these studies clearly demonstrate tissue-level control of FFA utilization and metabolic fate, refuting the traditional view that FFA utilization is simply supply-driven. Recent developments enable the simultaneous evaluation of both tissue-specific FFA and glucose metabolism by integrating the use of 2-deoxyglucose and stable isotope-labeled glucose tracers. In conclusion, the 3H-R-BrP methodology, especially in combination with other tracers, represents a powerful tool for elucidation of tissue-specific fatty acid metabolism in vivo.

OBJECTIVE—Inflammatory markers such as C-reactive protein (CRP) are associated with insulin resistance, adiposity, and type 2 diabetes. Whether inflammation causes insulin resistance or is an epiphenomenon of obesity remains unresolved. We aimed to determine whether first-degree relatives of type 2 diabetic subjects differ in insulin sensitivity from control subjects without a family history of diabetes, whether first-degree relatives of type 2 diabetic subjects and control subjects differ in CRP, adiponectin, and complement levels, and whether CRP is related to insulin sensitivity independently of adiposity. RESEARCH DESIGN AND METHODS—We studied 19 young normoglycemic nonobese first-degree relatives of type 2 diabetic subjects and 22 control subjects who were similar for age, sex, and BMI. Insulin sensitivity (glucose infusion rate [GIR]) was measured by the euglycemic-hyperinsulinemic clamp. Dual-energy X-ray absorptiometry determined total and abdominal adiposity. Magnetic resonance imaging measured abdominal adipose tissue volumes. RESULTS—First-degree relatives of type 2 diabetic subjects had a 20% lower GIR than the control group (51.8 ± 3.9 vs. 64.9 ± 4.6 μmol · min−1 · kg fat-free mass−1, P = 0.04). However, first-degree relatives of subjects with type 2 diabetes and those without a family history of diabetes had normal and comparable levels of CRP, adiponectin, and complement proteins. When the cohort was examined as a whole, CRP was inversely related to GIR (r = −0.33, P = 0.04) and adiponectin (r = −0.34, P = 0.03) and positively related to adiposity (P < 0.04). However, CRP was not related to GIR independently of fat mass. In contrast to C3 (r = 0.41, P = 0.009) and factor B (r = 0.43, P = 0.005), CRP was unrelated to factor D. CONCLUSIONS—The insulin-resistant state is not associated with changes in inflammatory markers or complement proteins in subjects at high risk of type 2 diabetes. Our study confirms a strong relationship between CRP and fat mass. Increasing adiposity and insulin resistance may interact to raise CRP levels.

Agonists of peroxisome proliferator-activated receptors (PPARs) have emerged as important pharmacological agents for improving insulin action. A major mechanism of action of PPAR agonists is thought to involve the alteration of the tissue distribution of nonesterified fatty acid (NEFA) uptake and utilization. To test this hypothesis directly, we examined the effect of the novel PPARα/γ agonist tesaglitazar on whole-body insulin sensitivity and NEFA clearance into epididymal white adipose tissue (WAT), red gastrocnemius muscle, and liver in rats with dietary-induced insulin resistance. Wistar rats were fed a high-fat diet (59% of calories as fat) for 3 wk with or without treatment with tesaglitazar (1 μmol·kg−1·d−1, 7 d). NEFA clearance was measured using the partially metabolizable NEFA tracer, 3H-R-bromopalmitate, administered under conditions of basal or elevated NEFA availability. Tesaglitazar improved the insulin sensitivity of high-fat-fed rats, indicated by an increase in the glucose infusion rate during hyperinsulinemic-euglycemic clamp (P < 0.01). This improvement in insulin action was associated with decreased diglyceride (P < 0.05) and long chain acyl coenzyme A (P < 0.05) in skeletal muscle. NEFA clearance into WAT of high-fat-fed rats was increased 52% by tesaglitazar under basal conditions (P < 0.001). In addition the PPARα/γ agonist moderately increased hepatic and muscle NEFA utilization and reduced hepatic triglyceride accumulation (P < 0.05). This study shows that tesaglitazar is an effective insulin-sensitizing agent in a mild dietary model of insulin resistance. Furthermore, we provide the first direct in vivo evidence that an agonist of both PPARα and PPARγ increases the ability of WAT, liver, and skeletal muscle to use fatty acids in association with its beneficial effects on insulin action in this model.

Insulin-stimulated glucose uptake is increased in white but not red muscle of insulin-resistant high-fat-fed (HF) rats after administration of the AMP-activated protein kinase (AMPK) activator 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR). To investigate whether a lesser AICAR effect on glucose uptake in red muscle was offset by a greater effect on fatty acid (FA) uptake, we examined acute effects of AICAR on muscle glucose and FA fluxes in HF rats. HF rats received AICAR (250 mg/kg) subcutaneously. At 30 min, a mixture of either (3)H-(R)-2-bromopalmitate/(14)C-palmitate or (3)H-2-deoxyglucose/(14)C-glucose was administered intravenously to assess muscle FA and glucose uptake. AICAR decreased plasma levels of glucose (approximately 25%), insulin (approximately 60%), and FAs (approximately 30%) at various times over the next 46 min (P0.05 vs. controls). In white muscle, AICAR increased both FA (2.4-fold) and glucose uptake (4.9-fold), associated with increased glycogen synthesis (6-fold). These effects were not observed in red muscle. We conclude that both glucose and FA fluxes are enhanced by AICAR more in white versus red muscle, consistent with the relative degree of activation of AMPK. Therefore, a lesser effect of AICAR to alleviate muscle insulin resistance in red versus white muscle is not explained by a relatively greater effect on FA uptake in the red muscle.

Objective: Abdominal fat and myocyte triglyceride levels relate negatively to insulin sensitivity, but their interrelationships are inadequately characterized in the overweight. Using recent methods for measuring intramyocyte triglyceride, these relationships were studied in men with a broad range of adiposity. Research Methods and Procedures: Myocyte triglyceride content (1H-magnetic resonance spectroscopy of soleus and tibialis anterior muscles and biochemical assessment of vastus lateralis biopsies), regional fat distribution (DXA and abdominal magnetic resonance imaging), serum lipids, insulin action (euglycemic hyperinsulinemic clamp), and substrate oxidation rates (indirect calorimetry) were measured in 39 nondiabetic men (35.1 ± 7.8 years) with a broad range of adiposity (BMI 28.6 ± 4.1 kg/m2, range 20.1 to 37.6 kg/m2). Results: Relationships between insulin-stimulated glucose disposal and regional body fat depots appeared more appropriately described by nonlinear than linear models. When the group was subdivided using median total body fat as the cut-point, insulin-stimulated glucose disposal correlated negatively to all regional body fat measures (all p ≤ 0.004), serum triglycerides and free fatty acids (p < 0.02), and both soleus intramyocellular lipid (p = 0.003) and vastus lateralis triglyceride (p = 0.04) in the normal/less overweight group. In contrast, only visceral abdominal fat showed significant negative correlation with insulin-stimulated glucose disposal in more overweight men (r = −0.576, p = 0.01), some of whom surprisingly had lower than expected myocyte lipid levels. These findings persisted when the group was subdivided using different cut-points or measures of adiposity. Discussion: Interrelationships among body fat depots, myocyte triglyceride, serum lipids, and insulin action are generally absent with increased adiposity. However, visceral abdominal fat, which corresponds less closely to total adiposity, remains an important predictor of insulin resistance in men with both normal and increased adiposity.

OBJECTIVE: To investigate whether normal glucose-tolerant and type II diabetic overweight adults differ in response to weight regain with regard to substrate oxidation and metabolic parameters. METHODS: A total of 15 overweight-obese subjects: seven normal glucose tolerant (NGT) and eight with type II diabetes (DM) were restudied 5 y after significant weight loss. Prediet, after 28 days calorie restriction and at 5 y, subjects were characterised for weight, height, waist-to-hip ratio (WHR) and body composition by dual-energy X-ray absorptiometry. Fasting glucose, insulin, leptin and lipid levels were measured and subjects underwent euglycaemic–hyperinsulinaemic clamp (insulin 0.25 U/kg/h for 150 min). Indirect calorimetry was performed resting and in the final 30 min of the clamp. Dietary assessment was by 4-day diet-diary. RESULTS: Both NGT and DM groups regained weight at 5 y and were not different to prediet. Total body fat (%) and WHR were higher at 5 y compared to prediet in both groups. Fasting glucose was increased in NGT subjects at 5 y, and fasting insulin was higher in both groups at 5 y compared to prediet. Insulin sensitivity (GIR) was similar at 5 y compared to prediet, but at 5 y DM subjects were more insulin resistant than NGT subjects. At 5 y, both DM and NGT groups had significantly reduced basal fat oxidation and no significant suppression of fat oxidation with insulin. Clamp respiratory quotient levels at 5 y were significantly higher in NGT compared to DM subjects. CONCLUSION: Reduced basal fat oxidation, and reduced variation in substrate oxidation in response to insulin develop with fat regain and fasting hyperinsulinaemia in both NGT and DM obese adults.

There is interest in how altered lipid metabolism could contribute to muscle insulin resistance. Many animal and human states of insulin resistance have increased muscle triglyceride content, and there are now plausible mechanistic links between muscle lipid accumulation and insulin resistance, which go beyond the classic glucose-fatty acid cycle. We postulate that muscle cytosolic accumulation of the metabolically active long-chain fatty acyl CoAs (LCACoA) is involved, leading to insulin resistance and impaired insulin signalling or impaired enzyme activity (e.g. glycogen synthase or hexokinase) either directly or via chronic translocation/activation of mediators such as a protein kinase C (particularly PKC theta and epsilon ). Ceramides and diacylglycerols (DAGs) have also been implicated in forms of lipid-induced muscle insulin resistance. Dietary lipid-induced muscle insulin resistance in rodents is relatively easily reversed by manipulations that lessen cytosolic lipid accumulation (e.g. diet change, exercise or fasting). PPAR agonists (both gamma and alpha) also lower muscle LCACoA and enhance insulin sensitivity. Activation of AMP-activated protein kinase (AMPK) by AICAR leads to muscle enhancement (especially glycolytic muscle) of insulin sensitivity, but involvement of altered lipid metabolism is less clear cut. In rodents there are similarities in the pattern of muscle lipid accumulation/PKC translocation/altered insulin signalling/insulin resistance inducible by 3-5-h acute free fatty acid elevation, 1-4 days intravenous glucose infusion or several weeks of high-fat feeding. Recent studies extend findings and show relevance to humans. Muscle cytosolic lipids may accumulate either by increased fatty acid flux into muscle, or by reduced fatty acid oxidation. In some circumstances muscle insulin resistance may be an adaptation to optimize use of fatty acids when they are the predominant available energy fuel. The interactions described here are fundamental to optimizing therapy of insulin resistance based on alterations in muscle lipid metabolism.

Aims To examine the relationships between body composition and changes in fasting glycaemia, and in indices of insulin secretion and insulin action over 6 years in females with a family history of Type 2 diabetes with or without prior gestational diabetes (‘at risk’ group, AR) and control females (control group, C). Methods At baseline and at follow-up, an oral glucose tolerance test and dual energy X-ray absorptiometry assessment of body composition were performed. Indices of insulin resistance (HOMA R′) and insulin secretion (HOMA β′) were obtained from fasting insulin and glucose concentrations. Results At baseline, the groups were similar for age, body mass index, fasting levels of plasma glucose and insulin, HOMA R′ and HOMA β′. Despite similar total body fatness, AR had significantly greater waist circumference and central fat (both P

An increase in muscle lipid content has been postulated to relate closely to the evolution of insulin resistance. We aimed to test whether the multiple indexes of lipid supply within man [namely, circulating triglycerides, skeletal muscle triglycerides (SMT), total and central fat mass, and circulating leptin] were independent predictors of insulin resistance, or whether triglycerides from different sources are additive in their influence on whole body insulin sensitivity. Whole body insulin sensitivity, body composition, and SMT content were determined in 49 sedentary, nondiabetic males (age, 20–74 yr; body mass index, 20–38 kg/m2). Insulin sensitivity was inversely associated with central abdominal fat (r2 = 0.38; P < 0.0001), total body fat (r2 = 0.21; P = 0.0003), SMT content (r2 = 0.16; P = 0.005), and fasting triglycerides (r2 = 0.24; P = 0.0003), nonesterified free fatty acid (r2 = 0.19; P = 0.002), and leptin (r2 = 0.35; P < 0.0001) levels. However, only central abdominal fat was significantly related to SMT content (r2 = 0.10; P = 0.03). SMT content, circulating triglycerides, and measurements of total or central adiposity were independent predictors of whole body insulin sensitivity.

Thiazolidinediones lower lipids, but it is unclear whether this is essential for their insulin-sensitizing action. We investigated relationships between lipid-lowering and insulin-sensitizing actions of a thiazolidinedione. Normal rats were pretreated with or without Pioglitazone (Pio, 3 mg/kg.d) for 2 wk. Insulin sensitivity was assessed by hyperinsulinemic-euglycemic clamp with elevation of free fatty acids (FFA) by Intralipid/heparin infusion over 6 h. In untreated rats insulin sensitivity decreased by 46% over 3-6 h of elevated FFA, whereas it remained normal but with a 50% increase in FFA clearance in Pio-treated rats. After matching plasma FFA, insulin sensitivity was still partially (30%) protected in Pio-treated rats, substantially by maintaining insulin suppressibility of hepatic glucose output. This was associated with lower hepatic long-chain acyl-coenzyme A. Plasma adiponectin was increased 2-fold in Pio-treated rats and was negatively correlated with hepatic glucose output (r2 = 0.70, P < 0.001) and liver long-chain acyl-coenzyme A (r2 = 0.39, P < 0.005). Pio-induced muscle insulin sensitization was largely diminished after matching plasma FFA elevation, but insulin-stimulated protein kinase B phosphorylation was protected. We conclude that thiazolidinediones can protect against lipid-induced insulin resistance with a significant component (mainly liver) of the protective effect not requiring lipid lowering. This may be related to chronic elevation of adiponectin by thiazolidinediones.

Objective: Insulin resistance is closely associated with two disparate aspects of lipid storage: the intracellular lipid content of skeletal muscle and the magnitude of central adipose beds. Our aim was to determine their relative contribution to impaired insulin action. Research Methods and Procedures: Eighteen older (56 to 75 years of age) men were studied before elective knee surgery. Insulin sensitivity (M/ΔI) was determined by hyperinsulinemic–euglycemic clamp. Central abdominal fat (CF) was assessed by DXA. Skeletal muscle was excised at surgery and assayed for content of metabolically active long-chain acyl-CoA esters (LCAC). Results: Significant inverse relationships were observed between LCAC and M/ΔI (R2 = 0.34, p = 0.01) and between CF and M/ΔI (R2 = 0.38, p = 0.006), but not between CF and LCAC (R2 = 0.0005, p = 0.93). In a multiple regression model (R2 = 0.71, p < 0.0001), both CF (p = 0.0006) and LCAC (p = 0.0009) were independent statistical predictors of M/ΔI. Leptin levels correlated inversely with M/ΔI (R2 = 0.60, p = 0.0002) and positively with central (R2 = 0.41, p = 0.006) and total body fat (R2 = 0.63, p = 0.0001). Discussion: The mechanisms by which altered lipid metabolism in skeletal muscle influences insulin action may not be related directly to those linking central fat and insulin sensitivity. In particular, it is unlikely that muscle accumulation of lipids directly derived from labile central fat depots is a principal contributor to peripheral insulin resistance. Instead, our results imply that circulating factors, other than nonesterified fatty acids or triglyceride, mediate between central fat depots and skeletal muscle tissue. Leptin was not exclusively associated with central fat, but other factors, secreted specifically from central fat cells, could modulate muscle insulin sensitivity.

Peroxisome proliferator-activated receptor gamma (PPAR-gamma) activation in adipose tissue is known to regulate genes involved in adipocyte differentiation and lipid metabolism. However, the role of PPAR-gamma in muscle remains unclear. To examine the potential regulation of genes by PPAR-gamma in human skeletal muscle, we used semiquantitative RT-PCR to determine the expression of PPAR-gamma, lipoprotein lipase (LPL), muscle carnitine palmitoyl transferase-1 (mCPT1), fatty acid-binding protein (FABP), carnitine acylcarnitine transferase (CACT), and glucose transporter-4 (GLUT4) in freeze-dried muscle samples from 14 male subjects. These samples were dissected free of adipose and other tissue contamination, as confirmed by minimal or absent adipsin expression. Between individuals, the messenger ribonucleic acid concentration of PPAR-gamma varied up to 3-fold, whereas LPL varied up to 6.5-fold, mCPT1 13-fold, FABP 4-fold, CACT 4-fold, and GLUT4 up to 3-fold. The expression of LPL (r2 = 0.54; P = 0.003), mCPT1 (r2 = 0.42; P = 0.012), and FABP (r2 = 0.324; P = 0.034) all correlated significantly with PPAR-gamma expression in the same samples. No significant correlation was observed between the expression of CACT and PPAR-gamma or between GLUT4 and PPAR-gamma. These findings demonstrate a relationship between PPAR-gamma expression and the expression of other genes of lipid metabolism in muscle and support the hypothesis that PPAR-gamma activators such as the antidiabetic thiazolidinediones may regulate fatty acid metabolism in skeletal muscle as well as in adipose tissue.

Long-chain acyl-CoAs (LCACoA) are an activated lipid species that are key metabolites in lipid metabolism; they also have a role in the regulation of other cellular processes. However, few studies have linked LCACoA content in rat and human muscle to changes in nutritional status and insulin action. Fasting rats for 18 h significantly elevated the three major LCACoA species in muscle ( P < 0.001), whereas high-fat feeding of rats with a safflower oil (18:2) diet produced insulin resistance and increased total LCACoA content ( P < 0.0001) by specifically increasing 18:2-CoA. The LCACoA content of red muscle from rats (4–8 nmol/g) was 4- to 10-fold higher than adipose tissue (0.4–0.9 nmol/g, P < 0.001), suggesting that any contamination of muscle samples with adipocytes would contribute little to the LCACoA content of muscle. In humans, the LCACoA content of muscle correlated significantly with a measure of whole body insulin action in 17 male subjects ( r2= 0.34, P = 0.01), supporting a link between muscle lipid metabolism and insulin action. These results demonstrate that the LCACoA pool reflects lipid metabolism and nutritional state in muscle. We conclude that the LCACoA content of muscle provides a direct index of intracellular lipid metabolism and its links to insulin action, which, unlike triglyceride content, is not subject to contamination by closely associated adipose tissue.

We describe a method for assessing tissue-specific plasma free fatty acid (FFA) utilization in vivo using a non-β-oxidizable FFA analog, [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP). Ideally 3H-R-BrP would be transported in plasma, taken up by tissues and activated by the enzyme acyl-CoA synthetase (ACS) like native FFA, but then 3H-labeled metabolites would be trapped. In vitro we found that 2-bromopalmitate and palmitate compete equivalently for the same ligand binding sites on albumin and intestinal fatty acid binding protein, and activation by ACS was stereoselective for the R-isomer. In vivo, oxidative and non-oxidative FFA metabolism was assessed in anesthetized Wistar rats by infusing, over 4 min, a mixture of 3H-R-BrP and [U-14C] palmitate (14C-palmitate). Indices of total FFA utilization (R*f) and incorporation into storage products (Rfs′) were defined, based on tissue concentrations of 3H and 14C, respectively, 16 min after the start of tracer infusion. R*f, but not Rfs′, was substantially increased in contracting (sciatic nerve stimulated) hindlimb muscles compared with contralateral non-contracting muscles. The contraction-induced increases in R*f were completely prevented by blockade of β-oxidation with etomoxir. These results verify that 3H-R-BrP traces local total FFA utilization, including oxidative and non-oxidative metabolism. Separate estimates of the rates of loss of 3H activity indicated effective 3H metabolite retention in most tissues over a 16-min period, but appeared less effective in liver and heart. In conclusion, simultaneous use of 3H-R-BrP and [14C]palmitate tracers provides a new useful tool for in vivo studies of tissue-specific FFA transport, utilization and metabolic fate, especially in skeletal muscle and adipose tissue.—Oakes, N. D., A. Kjellstedt, G-B. Forsberg, T. Clementz, G. Camejo, S. M. Furler, E. W. Kraegen, M. Ölwegård-Halvarsson, A. B. Jenkins, and B. Ljung. Development and initial evaluation of a novel method for assessing tissue-specific plasma free fatty acid utilization in vivo using (R)-2-bromopalmitate tracer. J. Lipid Res. 1999. 40: 1155–1169.

Elevated non-esterified fatty acid (NEFA) levels may influence insulin secretion and contribute to the development of Type 2 DM. We investigated the effects of acute NEFA elevation in controls (n = 6) and subjects predisposed to Type 2 DM (n = 6) on basal insulin levels, and following glucose and arginine stimulation. Each subject had one study with a triglyceride (TG) plus heparin infusion (elevated NEFA levels) and another with normal saline. Twenty minutes after the TG or saline infusion began a glucose bolus was given and 10 min later a 90-min hyperglycaemic clamp (approximately 9 mmol l(-1)) was started. Intravenous arginine was given at 110 min. Elevated NEFA levels (approximately 4000 micromol l(-1)) did not enhance basal or first phase glucose stimulated insulin levels. During hyperglycaemia, NEFA elevation further increased insulin levels in both groups by 20-44% (p < 0.05) and C-peptide levels by 17-25% (p < 0.05). The post-arginine insulin levels during hyperglycaemia were increased by 45% in the Type 2 DM-risk group (p < 0.02). The glucose infusion rate maintaining matched hyperglycaemia was similar during NEFA elevation and for saline control for both groups. We conclude that acute elevation of NEFA levels enhances glucose and non-glucose-induced insulin secretion.

Because of a failure to detect significant quantities of intracellular glucose, it has been generally accepted that transport rather than phosphorylation is the rate-limiting process of muscle glucose metabolism under most (but not all) physiological conditions. Here, we have measured tissue free levels of the glucose analog 2-deoxy- d -glucose (2DG) in red quadriceps muscle of rats fed a high-fat diet (59% of energy from fat) for 3 weeks, to identify the barrier to insulin-stimulated glucose uptake previously seen in such animals. Measurements were performed on pentobarbital-anesthetized rats following exogenous infusion of radiolabeled 2DG. A glucose clamp was used to maintain plasma insulin at high physiological levels (≈ 120 mU/L). Three other treatment groups representing normal insulin action (chow-fed), extreme glucose uptake (maximal insulin stimulation + hyperglycemia), and insulin resistance with elevated free intracellular glucose (epinephrine infusion) were also studied for comparison. In chow-fed animals, no muscle free 2DG was detected, confirming transport as the rate-limiting process. In fat-fed animals, a significant elevation in muscle free 2DG was observed ( P v ) chow-fed controls). The elevation was similar in magnitude to that in epinephrine-infused rats, and implied a limitation of insulin action at a posttransport step. This result was confirmed with a more complex modeling analysis. We conclude that posttransport steps influence insulin-stimulated in vivo muscle glucose metabolism in long-term high-fat—fed rats.

OBJECTIVE Subjects with IDDM should take carbohydrate before exercise to avoid hypoglycemia. However, there is little information on the glycemic effect of recommended supplementation. This study is aimed to determine the glycemic effects of oral glucose or bread (30 g carbohydrate) before 45 min of moderate exercise. RESEARCH DESIGN AND METHODS Nine subjects with uncomplicated IDDM did 45 min of bicycle ergometer exercise at 60% VO2max in the morning before insulin injection on three occasions: 1) with no carbohydrate supplement, 2) with 30 g glucose in water at −5 min, and 3) with 30 g carbohydrate as white bread with water at −20 min. The glycemic responses were determined. The glycemic responses to glucose and bread were also determined without exercise in six subjects. RESULTS Without carbohydrate, exercise caused a small fall (−1.2 ± 0.6 mmol/l, mean ± SE) in plasma glucose (PG). With either glucose or bread, PG rose (the change in plasma glucose relative to basal [Δ PG] = 5.1 ± 0.8 and 2.6 ± 0.8, respectively). The rise was greater (P < 0.01) without exercise (Δ PG = 6.9 ± 0.7 and 4.5 ± 0.7, respectively). During exercise, glucose increased PG levels more than bread increased glucose levels P < 0.05). CONCLUSIONS Before morning insulin injection, the fall in PG during moderate exercise in IDDM subjects is generally small or absent. The glycemic effects of complex carbohydrate are slightly < glucose before exercise. Under these circumstances, the usually recommended amount of carbohydrate tends to cause an unwanted elevation of PG; thus, IDDM subjects should anticipate reducing or even omitting carbohydrate supplementation after monitoring their individual glycemic response.

SUMMARY objective We aimed to investigate the impact of a longacting somatostatin analogue, octreotide, on glucose tolerance and on insulin sensitivity in acromegaly design We performed a non-randomized controlled trial patients Seven patients with active acromegaly were assessed before and during octreotide therapy given in a dose of 500 μ g three times daily subcutaneously. measurements The effects of octreotide on carbohydrate metabolism were assessed by performing a glucose tolerance test and a euglycaemlc hyperinsulinaemic clamp. These latter tests were undertaken 8 hours after the last dose, allowing GH and glucagon to return to pretreatment levels during the study results Octreotide significantly reduced (P > 0·05) mean ± SEM 12-h GH (from 42·13 to 10·3 mIU/l) and IGF-I (from 4·2±0·5 to 2·1±0·5 U/ml) concentrations. Glucose tolerance was normalized in four of five patients with impaired glucose tolerance without a significant change In mean insulin concentrations. The improvement In fasting and mean blood glucose during glucose tolerance testing was dependent on the pretherapy blood glucose concentrations (r=–0·95, P= 0·002). The glucose infusion rate during the hyperinsulinaemic (5 U/h) clamp was significantly increased (P > 0·05, 15·3±1·8 vs 24·2±5·4 μ mol/kg min) following octreotide treatment. Insulin infusion during the glucose clamp completely suppressed hepatic glucose production during but not before octreotide treatment (79·2±4 vs 0·7±2·2 μ mol/kg min, P= 0·02). Insulin-mediated stimulation of peripheral glucose uptake was unaffected by treatment. Mean GH and glucagon levels during both clamp studies were no: significantly different. conclusions Octreotide improves whole body insulln sensitivity by an increased ability of Insulln to suppress hepatic glucose production without affecting the substantial Impairment of perpheral insutin action. Octrectide has beneficial attects on carbohydrate metabollsm in acromegallc patients with glucose intoferance

SummaryActivation of AMP-activated protein kinase (AMPK) is thought to convey many of the beneficial effects of exercise via its inhibitory effect on acetyl-CoA carboxylase 2 (ACC2) and promotion of fatty acid oxidation. Hence, AMPK and ACC have become major drug targets for weight loss and improved insulin action. However, it remains unclear whether or how activation of the fatty acid oxidation pathway without a concomitant increase in energy expenditure could be beneficial. Here, we have used either pharmacological (administration of the AMPK agonist 5′ aminoimidazole-4-carboxamide-riboside) or genetic means (mutation of the ACC2 gene in mice) to manipulate fatty acid oxidation to determine whether this is sufficient to promote leanness. Both of these strategies increased whole-body fatty acid oxidation without altering energy expenditure or adiposity. We conclude that negative energy balance is a prerequisite for weight reduction, and increased fatty acid oxidation per se has little, if any, effect to reduce adiposity.

Skeletal muscle insulin resistance, defined as a reduced ability of insulin to stimulate tissue utilization and storage of glucose, is an early and major perturbation in the metabolic syndrome. Clinical conditions under the umbrella of the metabolic syndrome include obesity, type 2 diabetes, dyslipidemia, and hypertension, and are known to be influenced by lifestyle factors such as diet and activity. The metabolic syndrome is reaching alarming proportions in many societies. This chapter considers the increasingly compelling evidence that links insulin resistance with the situation when fatty acid supply exceeds energy demand in muscle, resulting in an accumulation of lipid in this tissue. This chapter is an update of a similar-titled chapter written some five years ago (1), and we highlight how knowledge has progressed since 2000. Considered are evidence for the association between fatty acid metabolism and muscle insulin resistance, possible causal links whereby increased muscle lipid accumulation could result in impaired insulin signalling and insulin resistance, and therapeutic options for ameliorating insulin resistance based on a “lipid-lowering” approach. We also consider how a scenario of excess fatty acid supply to muscle stands in relation to other emerging theories of factors contributing to muscle insulin resistance in obesity and similar states.

Objective: To test in humans the hypothesis that part of the association of adiponectin with insulin sensitivity is independent of lipid availability. Research Methods and Procedures: We studied relationships among plasma adiponectin, insulin sensitivity (by hyperinsulinemic-euglycemic clamp), total adiposity (by DXA), visceral adiposity (VAT; by magnetic resonance imaging), and indices of lipid available to muscle, including circulating and intramyocellular lipid (IMCL; by 1H-magnetic resonance spectroscopy). Our cohort included normal weight to obese men (n = 36). Results: Plasma adiponectin was directly associated with insulin sensitivity and high-density lipoprotein-cholesterol and inversely with plasma triglycerides but not IMCL. These findings are consistent with adiponectin promoting lipid uptake and subsequent oxidation in muscle and inhibiting TG synthesis in the liver. In multiple regression models that also included visceral and total fat, free fatty acids, TGs, and IMCL, either alone or in combination, adiponectin independently predicted insulin sensitivity, consistent with some of its insulin-sensitizing effects being mediated through mechanisms other than modulation of lipid metabolism. Because VAT directly correlated with total fat and all three indices of local lipid availability, free fatty acids, and IMCL, an efficient regression model of insulin sensitivity (R2 = 0.69, p < 0.0001) contained only VAT (part R2 = 0.12, p < 0.002) and adiponectin (part R2 = 0.41, p < 0.0001) as independent variables. Discussion: Given the broad range of total adiposity and body fat distribution in our cohort, we suggest that insulin sensitivity is robustly associated with adiponectin and VAT.

Thiazolidinediones can enhance clearance of whole-body non-esterified fatty acids and protect against the insulin resistance that develops during an acute lipid load. The present study used [(3)H]-R-bromopalmitate to compare the effects of the thiazolidinedione, rosiglitazone, and the biguanide, metformin, on insulin action and the tissue-specific fate of non-esterified fatty acids in rats during lipid infusion.Normal rats were treated with rosiglitazone or metformin for 7 days. Triglyceride/heparin (to elevate non-esterified fatty acids) or glycerol (control) were then infused for 5 h, with a hyperinsulinaemic clamp being performed between the 3rd and 5th hours.Rosiglitazone and metformin prevented fatty-acid-induced insulin resistance (reduced clamp glucose infusion rate). Both drugs improved insulin-mediated suppression of hepatic glucose output but only rosiglitazone enhanced systemic non-esterified fatty acid clearance (plateau plasma non-esterified fatty acids reduced by 40%). Despite this decrease in plateau plasma non-esterified fatty acids, rosiglitazone increased fatty acid uptake (two-fold) into adipose tissue and reduced fatty acid uptake into liver (by 40%) and muscle (by 30%), as well as reducing liver long-chain fatty acyl CoA accumulation (by 30%). Both rosiglitazone and metformin increased liver AMP-activated protein kinase activity, a possible mediator of the protective effects on insulin action, but in contrast to rosiglitazone, metformin had no significant effect on non-esterified fatty acid kinetics or relative tissue fatty acid uptake.These results directly demonstrate the "lipid steal" mechanism, by which thiazolidinediones help prevent fatty-acid-induced insulin resistance. The contrasting mechanisms of action of rosiglitazone and metformin could be beneficial when both drugs are used in combination to treat insulin resistance.

A common observation in animal models and in humans is that accumulation of muscle triglyceride is associated with the development of insulin resistance. In animals, this is true of genetic models of obesity and nutritional models of insulin resistance generated by high-fat feeding, infusion of lipid, or infusion of glucose. Although there is a strong link between the accumulation of triglycerides (TG) in muscle and insulin resistance, it is unlikely that TG are directly involved in the generation of muscle insulin resistance. There are now other plausible mechanistic links between muscle lipid metabolites and insulin resistance, in addition to the classic substrate competition proposed by Randle's glucose-fatty acid cycle. The first step in fatty acid metabolism (oxidation or storage) is activation to the long-chain fatty acyl CoA (LCACoA). This review covers the evidence suggesting that cytosolic accumulation of this active form of lipid in muscle can lead to impaired insulin signaling, impaired enzyme activity, and insulin resistance, either directly or by conversion to other lipid intermediates that alter the activity of key kinases and phosphatases. Actions of fatty acids to bind specific nuclear transcription factors provide another mechanism whereby different lipids could influence metabolism.

This review concentrates on the physiological and metabolic manifestations of PPARγ agonist action, particularly as they relate to muscle lipid metabolism and their links with insulin sensitivity. Recently we have also been concerned with PPARα action [1], and believe that insights can be gained into both PPARα and PPARγ action by comparisons of the result of their respective activation on muscle metabolism. Our studies have mainly involved dietary animal models of insulin resistance, and we have made use of the high fat fed rat model [2], discussed in this review. The majority of our studies point to a reduction in muscle lipid accumulation as an important mediator of PPARγ agonist enhancement of muscle insulin sensitivity. Consistently the PPARα agonist WY14643 also lowers muscle lipid accumulation and enhances muscle insulin sensitivity in our experimental model, although the whole body lipid lowering actions of the two agonists are quite different. Recent evidence we have obtained suggests that there is also a component of PPARγ agonist action in muscle which does not depend on a lowering of muscle lipid supply.

This review considers evidence for, and putative mechanisms of, lipid-induced muscle insulin resistance. Acute free fatty acid elevation causes muscle insulin resistance in a few hours, with similar muscle lipid accumulation as accompanies more prolonged high fat diet-induced insulin resistance in rodents. Although causal relations are not as clearcut in chronic human insulin resistant states such as obesity and type 2 diabetes, it is now recognised that muscle lipids also accumulate in these states. The classic Randle glucose-fatty acid cycle is only one of a number of mechanisms by which fatty acids might influence muscle glucose metabolism and insulin action. A key factor is seen to be accumulation of muscle long chain acyl CoAs, which could alter insulin action via several mechanisms including chronic activation of protein kinase C isoforms or ceramide accumulation. These interactions are fundamental to understanding metabolic effects of new insulin "sensitizers", e.g. thiazolidinediones, which alter lipid metabolism and improve muscle insulin sensitivity in insulin resistant states. Recent work has also pointed to a possible role of lipids in beta cell deterioration ("lipotoxicity") associated with type 2 diabetes.

Insulin-resistant states are associated with accumulation of muscle lipid, suggesting an imbalance between lipid uptake and oxidation. We have employed a new fatty-acid tracer [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP) to study individual-tissue nonesterified fatty acid (NEFA) uptake in states with diminished or enhanced lipid oxidation. 3H-R-BrP was administered to conscious male Wistar rats (approximately 300 g) during fasting (5, 18, or 36 h), acute blockade of beta-oxidation (etomoxir, 15 micromol/kg), and insulin infusion (0.25 U x kg(-1) x h(-1)). Estimates of NEFA clearance rates (K(f)*) and absolute rates of uptake (R(f)*) were calculated from tissue accumulation of 3H-R-BrP products. In the basal state, NEFA uptake was dependent on the oxidative capacity of tissues: R(f)* in brown adipose tissue (BAT) > heart (HRT) > diaphragm (DPHM) > red quadriceps (RQ) > white quadriceps (WQ) > white adipose tissue (WAT). Fasting increased (P < 0.001) K(f)* in WAT but did not change NEFA clearance in other tissues. However, plasma NEFA levels were raised (P < 0.01), tending to elevate R(f)* in most tissues (P < 0.05: WAT, BAT, WQ, DPHM). Etomoxir reduced (P < 0.01) K(f)* only in oxidative tissues (BAT, RQ, DPHM, HRT). Insulin lowered plasma NEFA levels (P < 0.001) and significantly decreased R(f)* in most tissues (P < 0.05: WAT, RQ, DPHM, HRT). An increased (P < 0.05) clearance was observed in WAT, BAT, and WQ; a decrease (P < 0.01) in K(f)* was observed in HRT. This study is the first to measure tissue-specific NEFA uptake in conscious rats in the postabsorptive, fasted, and insulin-stimulated states. We have demonstrated that tissue NEFA utilization is not exclusively determined by systemic availability, but that the early steps of NEFA uptake or metabolic sequestration can also be rapidly modulated by local processes such as NEFA oxidation.

OBJECTIVE To examine the mechanisms by which weight loss improves glycemic control in overweight subjects with NIDDM, particularly the relationships between energy restriction, improvement in insulin sensitivity, andregional and overall adipose tissue loss. RESEARCH DESIGN AND METHODS Hyperinsulinemic glucose clamps wereperformed in 20 subjects (BMI = 32.0 ± 0.5 [SEM] kg/m2, age = 48.4 ± 2.7 years) with normal glucose tolerance (NGT) (n = 10) or mild NIDDM (n = 10) before and on the 4th (d4) and 28th (d28) days of a reduced-energy (1,100 ± 250 [SD] kcal/day) formula diet. Body composition changes were assessed by dual energy x-ray absorptiometry and insulin secretory changes were measured by insulin response to intravenous glucose before and after weight loss. RESULTS In both groups, energy restriction (d4) reduced fasting plasma glucose (FPG) (ΔFPG: NGT = −0.4 ± 0.2 mmol/1 and NIDDM = −1.1 ± 0.03 mmol/1, P = 0.002), which was independently related to reduced carbohydrate intake (partial r = 0.64, P = 0.003). There was a marked d4 increase in percent of insulin suppression of hepatic glucose output(HGO) in both groups (ΔHGO suppression: NGT = 28 ± 15% and NIDDM = 32 ± 8%, P = 0.002). By d28, with 6.3 ± 0.4 kg weight loss, FPG was further reduced (d4 vs. d28) in NIDDM only (P = 0.05), and insulin sensitivity increased in both groups (P = 0.02). Only loss of abdominal fat related to improvements in FPG (r = 0.51, P = 0.03) and insulin sensitivity afterweight loss (r = 0.48, P = 0.05). In contrast to insulin action, there were only small changes in insulin secretion. CONCLUSIONS Both energy restriction and weight loss have beneficial effects on insulin action and glycemic control in obesity and mild NIDDM. The effect of energy restriction is related to changes in individual macronutrients, whereas weight loss effects relate to changes in abdominal fat.

Glycogen depletion is thought to be a potent stimulus for the substantially increased glucose fluxes observed in skeletal muscle following exercise. The aim of this study was to establish the relationships between the glycogen mass and the rates of glucose uptake (Rg') and glucose incorporation into glycogen (Rgly) in individual muscles of conscious adult Wistar rats following moderate nonexhausting treadmill exercise (15 m/min at a 10 degree slope for 45 minutes, approximately 65% VO2max). Muscle glycogen content was determined at 0, 20, 45, 90, or 135 minutes following exercise and compared with Rg' and Rgly measurements at matched times. Muscle types varied in the rate of glycogen resynthesis. Glycogen depots of glycolytic muscle (white gastrocnemius) were still significantly (P.01) lower than preexercise levels after 135 minutes; red oxidative muscles (soleus and red gastrocnemius) were essentially repleted by 90 minutes. Immediately following exercise, Rg' and Rgly in red gastrocnemius and soleus were 42 +/- 4 and 42 +/- 5 and 36 +/- 2 and 33 +/- 7 micromol/(min . 100 g), greater than the rates induced by maximal insulin stimulation in previous studies. In red muscles, there was a strong inverse relationship between Rgly and tissue glycogen content, consistent with a dominant role for the glycogen mass in the regulation of glycogen resynthesis.

LAPSYS, NM, SM FURLER, KR MOORE, TV NGUYEN, H HERZOG, G HOWARD, K SAMARAS, DG CAREY, NA MORRISON, JA EISMAN, DJ CHISHOLM. Relationship of a novel polymorphic marker near the human obese (OB) gene to fat mass in healthy women. The cloning of the murine obese (ob)gene and its human homologue has recently been reported. Mutations in the mouse obgene result in hereditary obesity; however, the role of variations of OBin the regulation of bodyweight in humans has yet to be determined. The contribution of putative genetic variations in the human OBgene to total and regional fat mass in a normal twin population has been analyzed through linkage and association with a novel polymorphic marker, located in proximity to this gene. The polymorphic dinucleotide repeat, isolated from a PI clone containing the human OB gene, was physically localized by long-range restriction mapping to within 30 kilobases of the OB locus. The marker was genotyped in a population of 47 healthy female/female dizygotic (DZ) twin pairs for which direct measures of central abdominal and whole body fat had been obtained by dual X-ray absorbtiometry. Possible linkage between the microsatellite marker and whole-body (p=0.008), but not central abdominal (p=0.09), fat deposits was indicated. No association between fat depot phenotype and marker genotype was detected. These results suggest that genetic variation in or close to the human OB gene may play a role in the size of body fat stores in healthy women.

Chronic high-fat feeding in rats induces profound whole-body insulin resistance, mainly due to effects in oxidative skeletal muscle. The mechanisms of this reaction remain unclear, but local lipid availability has been implicated. The aim of this study was to examine the influence of three short-term physiological manipulations intended to lower muscle lipid availability on insulin sensitivity in high-fat–fed rats. Adult male Wistar rats fed a high-fat diet for 3 weeks were divided into four groups the day before the study: one group was fed the normal daily high-fat meal (FM); another group was fed an isocaloric low-fat high-glucose meal (GM); a third group was fasted overnight (NM); and a fourth group underwent a single bout of exercise (2-h swim), then were fed the normal high-fat meal (EX). In vivo insulin action was assessed using the hyperinsulinemic glucose clamp (plasma insulin 745 pmol/l, glucose 7.2 mmol/l). Prior exercise, a single low-fat meal, or fasting all significantly increased insulin-stimulated glucose utilization, estimated at either the whole-body level (P < 0.01 vs. FM) or in red quadriceps muscle (EX 18.2, GM 28.1, and NM 19.3 vs. FM 12.6 ± 1.1 μmol · 100 g−1 · min−1; P < 0.05), as well as increased insulin suppressibility of muscle total long-chain fatty acyl-CoA (LCCoA), the metabolically available form of fatty acid (EX 24.0, GM 15.5, and NM 30.6 vs. FM 45.4 nmol/g; P < 0.05). There was a strong inverse correlation between glucose uptake and LC-CoA in red quadriceps during the clamp (r = −0.7, P = 0.001). Muscle triglyceride was significantly reduced by short-term dietary lipid withdrawal (GM −22 and NM −24% vs. FM; P < 0.01), but not prior exercise. We concluded that muscle insulin resistance induced by high-fat feeding is readily ameliorated by three independent, short-term physiological manipulations. The data suggest that insulin resistance is an important factor in the elevated muscle lipid availability induced by chronic high-fat feeding.

Thiazolidinediones are ora insulin-sensitizing agents that may be useful for the treatment of non—insulin-dependent diabetes mellitus (NIDDM). BRL 49653 ameliorates insulin resistance and improves glucoregulation in high-fat—fed (HF) rats. It is known that thiazolidinediones bind to the peroxisome proliferator—activated receptor (PPARγ) in fat cells, but the extent to which the improved glucoregulation and hypolipidemic effects relate to adipose tissue requires clarification. We therefore examined BRL 49653 effects on lipid metabolism in HF and control (high-starch—fed [HS]) rats. The diet period was 3 weeks, with BRL 49653 (10 μmol/kg/d) or vehicle garage administered over the last 4 days. Studies were performed on animals in the conscious fasted state. In HF rats, rate constants governing 3H-palmitate clearance were unaffected by BRL 49653. This finding, taken with a concurrent decrease of fasting plasma nonesterified fatty acids (NEFA) (P < .01, ANOVA), demonstrated that systemic NEFA supply and hence absolute utilization are reduced by BRL 49653. Hepatic triglyceride (TG) production (HTGP) assessed using Triton WFI1339 was unaffected by diet or BRL 49653. In liver, BRL 49653 increased insulin-stimulated conversion of glucose into fatty acid in both HF (by 270%) and HS (by 30%) groups (P < .05), Relative to HS rats, HF animals had substantially elevated levels of muscle diglyceride (diacylglycerol [DG] by 240%, P < .001). BRL 49653 significantly reduced muscle DG in HF (by 30%, P < .05) but not in HS rats. The agent did not reduce the intake of dietary lipid. In conclusion, these results are consistent with a primary action of BRL 49653 in adipose tissue to conserve lipid by reducing systemic lipid supply and subsequent utilization. The parallel effects of diet and BRL 49653 treatment on insulin resistance and muscle acylglyceride levels support the involvement of local lipid oversupply in the generation of muscle insulin resistance.

Adiponectin is an abundant circulating adipocytokine with anti-inflammatory properties (1) linked to cardiovascular disease, type 2 diabetes, and obesity (2–5). Numerous reports (3–5), including the present one, confirm plasma adiponectin levels to be inversely related to insulin resistance. Longer term, a rise in adiponectin has been shown to occur in response to weight loss and glitazone therapy, but not after chronic exercise training. However, understanding of the shorter-term regulation of adiponectin in particular remains unclear. As an extension to a previously reported exercise intervention in sedentary males by our group (6), we have now examined the effects of this training intervention on adiponectin levels in overweight males. We demonstrate that the short-term exercise training increased circulating adiponectin levels with accompanied improved insulin sensitivity. Twenty-six overweight males participated in an exercise program, as previously described (6). Full data were available on 19 subjects who completed the entire program. At …

To examine the effect of different profiles of insulin administration on glycaemia and thermogenesis, we studied 10 subjects with mild non-insulin-dependent diabetes mellitus on four occasions after a standard mixed meal: (1) with no supplementary insulin (control), (2) with intravenous insulin (1.8U over 15 min = short), (3) as for short but extended over 30 min to simulate the normal initial rise in portal vein insulin levels (medium), (4) as for medium with additional insulin to normalize the profile from 30-60 min (3.6U over 60 min, long). All studies in which supplemental insulin was administered lowered the integrated glucose response above baseline versus the control study (short 76%, medium 71%, and long 56% of control, p = 0.003). The insulin infusions also increased the non-protein respiratory quotient in the first hour following the meal (0.82 +/- 0.01 (control) vs 0.87 +/- 0.01 (short), 0.86 +/- 0.01 (medium) and 0.87 +/- 0.01 (long), p = 0.003) and augmented thermogenesis (7.6 +/- 1.5 (control) vs 10.5 +/- 2.9 (short), 13.0 +/- 1.9 (medium) and 13.2 +/- 2.8% (long), p = 0.02). Total integrated insulin area above baseline was significantly greater in the long study (short 121, medium 111 vs long 179% of control, p = 0.02). Thus the greatest glycaemic benefit in relation to insulinaemia was obtained with the two shorter insulin infusions (short and medium). In conclusion, this study confirms the role of early prandial insulin secretion (or delivery) in limiting prandial glycaemia in NIDDM and increasing thermogenesis and highlights the pivotal role of the timing of elevation of insulin levels in modulating hyperglycaemia and hyperinsulinaemia.

We have tested the hypothesis that hepatic glucose output (Ra) during exercise in humans is subject to feedback control by circulating glucose within a control range that is determined by the circulating insulin concentration. Three exercise protocols based on 60-min cycle ergometer exercise at 55% maximal O2 consumption were used: 1) control, 2) insulin infusion with a euglycemic clamp, and 3) insulin infusion with a fixed-rate glucose infusion. Ra was measured using a constant infusion of [3H]glucose. During the glucose clamp there was no Ra response to exercise. There were significant inverse relationships between Ra and plasma glucose during control exercise (r = -0.73, P less than 0.001) and exercise with fixed-rate glucose and insulin infusion (r = -0.96, P less than 0.001). During the fixed-rate glucose and insulin infusion, plasma glucose fell from the commencement of exercise but stabilized at a lower level. These results are interpreted in terms of a simple difference controller where Ra is proportional to the deviation of plasma glucose from a defined set point. Insulin affects Ra and regulates the steady-state glucose level by altering the sensitivity of this control system.

The presence and physiologic importance of cephalic phase insulin release in humans remains controversial. The aim of these studies was to determine whether cephalic phase insulin release could be demonstrated in normal weight subjects and whether it would be associated with changes in blood glucose, free fatty acid, and pancreatic polypeptide levels. The studies were followed by a hyperglycemic clamp to determine whether cephalic responses would alter overall glucose disposal or glucose-stimulated insulin secretion. In all, 17 subjects were studied on two occasions with and without (control study) presentation of food stimuli. Tease-feeding alone (n = 6), or the administration of a sweet taste alone (aspartame, n = 5) failed to stimulate cephalic responses. However, the presentation of the combined stimuli (tease meals plus sweet taste, n = 7) resulted in a significant fall (P less than .005) in blood glucose levels and a variable rise in serum insulin (% insulin rise 38 +/- 15%, P less than .05) and C-peptide levels (7 +/- 6%, NS) within five minutes of the food presentation when compared with control studies, with no change seen in free fatty acid or pancreatic polypeptide levels. The blood glucose fall correlated strongly (r = .90, P less than .01) with a score of the subjective response to the food and taste.(ABSTRACT TRUNCATED AT 250 WORDS)

In normal subjects during moderate exercise there is a strong negative correlation between plasma glucose and hepatic glucose output (HGO) suggesting a negative feedback regulation of HGO by plasma glucose. Little information is available about HGO responses to exercise in non-insulin-dependent diabetes mellitus (NIDDM). To determine whether the same feedback relationship is operative, we have compared the glucose turnover responses to moderate exercise (50% Vo2max for 60 minutes) of nonobese non-insulin-dependent diabetic subjects (NIDDM, n = 7) with a group of age-matched controls (n = 5). Glucose turnover responses to exercise in NIDDM were heterogeneous. Plasma glucose showed sustained falls, no change, or sustained rises in different individuals. Similarly, HGO responses ranged from undetectable to responses comparable to those of normal subjects. The mean integrated HGO response in NIDDM was significantly reduced compared with controls (11 +/- 6 [SEM] v 33 +/- 7 mmol/h/70 kg, P less than .05); mean glucose utilization response was also reduced but not significantly different from controls (NIDDM 18 +/- 5 v control 35 +/- 6). In NIDDM there was no significant feedback-control relationship between plasma glucose and HGO (r = -0.20, P = NS) in contrast to controls (r = -0.87, P less than .01). We conclude that feedback control of HGO by plasma glucose during moderate exercise is impaired in NIDDM. This impairment may be due to defective nonpancreatic glucoregulatory mechanisms.

Despite its increasing use in physiological animal investigations, there has been no systematic study of the whole body kinetics of 2-deoxy-D-glucose (2DG) or its modification by insulin. A previously proposed model that included processes representing transport across cell walls and intracellular phosphorylation of 2DG was investigated. The model predictions were compared with the plasma disappearance of 2DG observed in the rat following intravenous bolus injection. Experiments were performed during euglycemia at varying levels of hyperinsulinemia. The model was adequate to describe empirically the experimental data after a correction was made for urine loss. However, the variation in model parameters with plasma insulin concentration was not consistent with the expected action of insulin on cellular efflux. A possible explanation could be a shift in the rate-limiting step from glucose transport to another prephosphorylation process under conditions of high tissue uptake. This suggests that either intracellular or extracellular diffusion may constitute a significant barrier to 2DG (and glucose) uptake under some conditions.

Fish oils, containing omega-3 fatty acids (ω3FAs), favorably influence plasma lipoproteins in nondiabetic humans and prevent the development of insulin resistance induced by fat feeding in rats. We studied the effects of fish oils in 10 subjects (aged 42–65 yr) with mild non-insulin-dependent diabetes mellitus (NIDDM). Subjects were fed a standard diabetic diet plus 1) no supplementation (baseline), 2) 10 g fish oil concentrate (30% ω3FAs) daily, and 3) 10 g safflower oil daily over separate 3-wk periods, the latter two supplements being given in radom order by use of a double-blind crossover design. At the end of each diet period, fasting blood glucose (FBG), insulin, and lipids were measured, and insulin sensitivity was assessed with a hyperinsulinemic-euglycemic clamp performed with [3-3H]glucose. FBG increased 14% during fish oil and 11% during safflower oil supplementation compared with baseline (P < .05), whereas body weight, fasting serum insulin levels, and insulin sensitivity were unchanged. The absolute increase in FBG during each supplementation period correlated with the baseline FBG (fish oil, r = .83, P < .005; safflower oil, r = .75, P = .012). Fasting plasma triglyceride levels decreased during fish oil supplementation in the 4 subjects with baseline hypertriglyceridemia (>2 mM) but were not significantly reduced overall. There was no significant change in fasting plasma total, high-density lipoprotein, and low-density lipoprotein cholesterol levels. In summary, dietary fish oil supplementation adversely affected glycemic control in NIDDM subjects without producing significant beneficial effects on plasma lipids. The effect of safflower oil supplementation was not significantly different from fish oil, suggesting that the negative effects on glucose metabolism may be related to the extra energy or fat intake. These data indicate that fish oil supplementation should be used with caution in subjects with NIDDM.

A semiclosed loop, bedside insulin infusion system using a simple basal infusion algorithm consisting of a linear transition between two insulin delivery rates as blood glucose (BG) increases has been developed. A theoretical study using computer simulation has now been undertaken to examine the effect of BG sampling frequency and algorithm parameters on BG control. A model for BG control by exogenous insulin in the individual with diabetes was developed from a model for healthy subjects and from clinical data in the literature. Results of computer simulation using this model showed a decrease in BG stability as the sampling interval increased from 1 to 4 h. Simulations also showed a decrease in BG stability as the sensitivity of the control algorithm increased. Choice of an appropriate basal control algorithm involved a compromise between stability, sampling interval, and metabolic control. We conclude that satisfactory metabolic control can be obtained using intermittent BG sampling in the basal state; sampling at intervals of 3 h combined with a basal control algorithm whereby insulin delivery rate increases linearly from 0.5 to 2.5 U/h over the BG range 2–12 mmol/L appears suitable for most diabetic persons. Three-hour sampling offers a good compromise between degree of metabolic control and clinical effort involved.

As information on the absorption kinetics and local degradation of infused insulin is relevant to programming strategies for continuous subcutaneous insulin infusion, we examined the time relationship of systemic insulin appearance and quantitated subcutaneous degradation during a near-basal rate of continuous subcutaneous insulin infusion in five insulin-dependent diabetic patients. Plasma free insulin was monitored for 8 h during and 3 h after a subcutaneous (abdominal wall) infusion of neutral insulin at 2.4 U/h. An identical intravenous infusion (2-4 h) was given on a separate occasion. Plateau levels of free insulin were not significantly different during the subcutaneous (37 +/- 8 mU/l) and intravenous (40 +/- 7 mU/l) infusions. Fitting of the free insulin data to our two-pool model of the subcutaneous space gave a mean estimate of 9.2 units insulin (= 3.8 h infusion) for the subcutaneous depot after 8 h. Model estimates of systemic insulin appearance, as a percentage of subcutaneous infusion rate, were 59% and 93% after 4 and 8 h respectively, and 76% 2 h after cessation of infusion. In insulin-dependent diabetic patients subcutaneous degradation of infused insulin is negligible but local accumulation in the subcutaneous space is considerable. The delay in absorption has important clinical implications for interruption and resumption of continuous subcutaneous insulin infusion and also for programming of variable basal rates.

The nature of and rates of loss of products of systemic radiolabelled 2-deoxy-D-glucose in rat tissues in vivo were investigated to validate the use of this tracer to measure rates of metabolism of circulating glucose by tissues in vivo. Apparent first order rate constants for loss of products ranged from 8.0 +/- 0.10 (SD) X 10(-3) min-1 (liver) to 2.2 +/- 0.8 X 10(-3) min-1 (skeletal muscle). 2-deoxyglucose 6-phosphate was the major product found in all tissues examined except liver; all tissues contained other minor products. Products were effectively trapped by rat tissues in vivo allowing the use of this tracer for the measurement of rates of circulating glucose utilisation by tissues in vivo.