Reduced capacity and affinity of skeletal muscle for insulin-mediated glucose uptake in noninsulin-dependent diabetic subjects. Effects of insulin therapy

Involvement of the extracellular matrix and integrin signalling proteins in skeletal muscle glucose uptake

Whole-body euglycaemia is partly maintained by two cellular processes that encourage glucose uptake in skeletal muscle, the insulin- and contraction-stimulated pathways, with research suggesting convergence between these two processes. The normal structural integrity of the skeletal muscle requires an intact actin cytoskeleton as well as integrin-associated proteins, and thus those structures are likely fundamental for effective glucose uptake in skeletal muscle. In contrast, excessive extracellular matrix (ECM) remodelling and integrin expression in skeletal muscle may contribute to insulin resistance owing to an increased physical barrier causing reduced nutrient and hormonal flux. This review explores the role of the ECM and the actin cytoskeleton in insulin- and contraction-mediated glucose uptake in skeletal muscle. This is a clinically important area of research given that defects in the structural integrity of the ECM and integrin-associated proteins may contribute to loss of muscle function and decreased glucose uptake in type 2 diabetes.

Insulin: The master regulator of glucose metabolism

Insulin is the master regulator of glucose, lipid, and protein metabolism. Following ingestion of an oral glucose load or mixed meal, the plasma glucose concentration rises, insulin secretion by the beta cells is stimulated and the hyperinsulinemia, working in concert with hyperglycemia, causes: (i) suppression of endogenous (primarily reflects hepatic) glucose production, (ii) stimulation of glucose uptake by muscle, liver, and adipocytes, (iii) inhibition of lipolysis leading to a decline in plasma FFA concentration which contributes to the suppression of hepatic glucose production and augmentation of muscle glucose uptake, and (iv) vasodilation in muscle, which contributes to enhanced muscle glucose disposal. Herein, the integrated physiologic impact of insulin to maintain normal glucose homeostasis is reviewed and the molecular basis of insulin's diverse actions in muscle, liver, adipocytes, and vasculature are discussed.

The aetiology and molecular landscape of insulin resistance

Insulin resistance, defined as a defect in insulin-mediated control of glucose metabolism in tissues - prominently in muscle, fat and liver - is one of the earliest manifestations of a constellation of human diseases that includes type 2 diabetes and cardiovascular disease. These diseases are typically associated with intertwined metabolic abnormalities, including obesity, hyperinsulinaemia, hyperglycaemia and hyperlipidaemia. Insulin resistance is caused by a combination of genetic and environmental factors. Recent genetic and biochemical studies suggest a key role for adipose tissue in the development of insulin resistance, potentially by releasing lipids and other circulating factors that promote insulin resistance in other organs. These extracellular factors perturb the intracellular concentration of a range of intermediates, including ceramide and other lipids, leading to defects in responsiveness of cells to insulin. Such intermediates may cause insulin resistance by inhibiting one or more of the proximal components in the signalling cascade downstream of insulin (insulin receptor, insulin receptor substrate (IRS) proteins or AKT). However, there is now evidence to support the view that insulin resistance is a heterogeneous disorder that may variably arise in a range of metabolic tissues and that the mechanism for this effect likely involves a unified insulin resistance pathway that affects a distal step in the insulin action pathway that is more closely linked to the terminal biological response. Identifying these targets is of major importance, as it will reveal potential new targets for treatments of diseases associated with insulin resistance.

Signature of high altitude adaptation in the gluteus proteome of the yak

Yak is the unique Bovidae species in the Qinghai-Tibetan Plateau. A previous proteomic study has compared the yak muscle tissue to one cattle strain using the isobaric tags for relative and absolute quantification approach. In this study, to further investigate the molecular mechanisms underlying yak adaptation, the proteomic profiles of gluteus were compared between yak and one moderate-altitude cattle strain (Tibetan cattle) and two low-altitude cattle strains (Holstein and Sanjiang cattle) using a label-free quantitative method. The comparisons identified 20, 364, 143 upregulated proteins and 4, 6, 37 downregulated proteins in yak, compared with Tibetan, Holstein, and Sanjiang cattle, respectively. Protein-protein interaction analysis indicated that these differentially expressed proteins were mainly related to "oxidative phosphorylation" and "electron transport chain." Further analysis revealed that NADH dehydrogenase [ubiquinone] 1 alpha subcomplex subunit 11, NADH dehydrogenase [ubiquinone] 1 beta subcomplex subunit 4, cytochrome C oxidase subunit 6A2, mitochondrial and cytochrome c oxidase subunit NDUFA4 were all increased in the yak, suggesting that yak might increase mitochondrial capacity to sustain metabolic rates under high altitude conditions, which might be a long-term adaptive mechanism underlying adaptation to high altitude environments. Yak increased the level of thioredoxin reductase 2 to protect themselves from oxidative damages. Moreover, the increased expression levels of phosphatidylinositol 4,5-bisphosphate 3-kinase catalytic subunit alpha isoform and caveolin-1 in yak suggested that yaks promoted glucose uptake for adaptation to high altitude. These results provided more information to better understand the molecular mechanisms underlying yak adaption.

Current trends in small molecule discovery targeting key cellular signaling events towards the combined management of diabetes and obesity

Non-insulin dependent diabetes mellitus, also known as Type 2 diabetes is a polygenic disorder leading to abnormalities in the carbohydrate and lipid metabolism. The major contributors in the pathophysiology of type 2 diabetes (T2D) include resistance to insulin action, β cell dysfunction, an abnormality in glucose metabolism and storage, visceral obesity and to some extent inflammation and oxidative stress. Insulin resistance, along with a defect in insulin secretion by the pancreatic β cells is instrumental towards progression to hyperglycemia. Increased incidence of obesity is also a major contributing factor in the escalating rates of type 2 diabetes. Drug discovery efforts are therefore crucially dependent on identifying individual molecular targets and validating their relevance to human disease. The current review discusses bioactive compounds from medicinal plants offering enhanced therapeutic potential for the combined patho-physiology of diabetes and obesity. We have demonstrated that 3β-taraxerol a pentacyclic triterpenoid (14-taraxeren-3-ol) isolated from the ethyl acetate extract of Mangifera indica, chlorogenic acid isolated from the methanol extract of Cichorium intybus, methyl tetracosanoate from the methanol extract of Costus pictus and vitalboside A derived from methanolic extract of Syzygium cumini exhibited significant effects on insulin stimulated glucose uptake causing insulin sensitizing effects on 3T3L1 adipocytes (an in vitro model mimicking adipocytes). Whereas, (3β)-stigmast-5-en-3-ol isolated from Adathoda vasica and Aloe emodin isolated from Cassia fistula showed significant insulin mimetic effects favoring glucose uptake in L6 myotubes (an in vitro model mimicking skeletal muscle cells). These extracts and molecules showed glucose uptake through activation of PI3K, an important insulin signaling intermediate. Interestingly, cinnamic acid isolated from the hydro-alcohol extract of Cinnamomum cassia was found to activate glucose transport in L6 myotubes through the involvement of GLUT4 via the PI3K-independent pathway. However, the activation of glucose storage was effective in the presence of 3β-taraxerol and aloe emodin though inhibition of GSK3β activity. Therefore, the mechanism of improvement of glucose and lipid metabolism exhibited by the small molecules isolated from our lab is discussed. However, Obesity is a major risk factor for type-2 diabetes leading to destruction of insulin receptors causing insulin resistance. Identification of compounds with dual activity (anti-diabetic and antiadipogenic activity) is of current interest. The protein tyrosine phosphatase 1B (PTP1B) is an important negative regulator of the insulin and leptin-signaling pathway is of significance in target definition and discovery.

Reactive oxygen species enhance mitochondrial function, insulin sensitivity and glucose uptake in skeletal muscle of senescence accelerated prone mice SAMP8

Whereas reactive oxygen species (ROS) can have opposite impacts on insulin signaling, they have mainly been associated with mitochondrial dysfunction in skeletal muscle. We analyzed the relationship between these three features in skeletal muscle of senescence accelerated mice (SAM) prone (P8), which are characterized by enhanced oxidative stress compared to SAM resistant (R1). Oxidative stress, ROS production, antioxidant system, mitochondrial content and functioning, as well as in vitro and in vivo insulin signaling were investigated in gastrocnemius and quadriceps muscles. In SAMP8 compared to SAMR1, muscle content in carbonylated proteins was two-fold (p < 0.01) and ROS production by xanthine oxidase 70% (p < 0.05) higher. Furthermore, insulin-induced Akt phosphorylation measured in vivo and ex vivo as well as muscle glucose uptake measured ex vivo were significantly higher (p < 0.05). Mitochondrial respiration evidenced uncoupling and higher respiration rates with substrates of complexes II and IV, in agreement with higher maximal activity of complexes II and IV (+ 18% and 62%, respectively, p < 0.05). By contrast, maximal activity of complex I was 22% lower (p < 0.05). All strain differences were corrected after 6 months of N-acetylcysteine (NAC) treatment, thus supporting the involvement of high ROS production in these differences. In conclusion in muscle of SAMP8 compared to SAMR1, high ROS production is associated to higher insulin sensitivity and glucose uptake but to lower mitochondrial complex I activity. These conflicting adaptations, with regards to the resulting imbalance between NADH production and use, were associated with intrinsic adjustments in the mitochondrial respiration chain (mitochondrial uncoupling, enhanced complexes II and IV activity). We propose that these bioenergetics adaptations may help at preserving muscle metabolic flexibility of SAMP8.

In vitro Antidiabetic Activity of Polar and Nonpolar Solvent Extracts from Leucas aspera (Willd.) Link Leaves

Background:

Diabetes mellitus is a chronic illness, and the management of diabetes is a global problem. Successful treatment is required to prevent complications and organ damages. Herbal medicines are having minimal adverse effects when compared to the available synthetic drugs to treat such chronic diseases and disorders.

Objective:

The present study was aimed to evaluate the antidiabetic and antioxidant activity of polar and nonpolar solvent extracts of Leucas aspera (Willd.) link leaves under in vitro models.

Materials and Methods:

The in vitro antidiabetic activity of petroleum ether (nonpolar) and ethanol (polar) extracts were evaluated in C2C12 cell lines by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay (cell viability method) and glucose uptake assay. 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging method used for the evaluation of in vitro antioxidant activity.

Results:

Both the polar and nonpolar solvent extracts of L. aspera had shown better antioxidant activity compared to standard (IC₅₀ = 18.96 and 19.90 μg/mL, respectively). Petroleum ether extract exhibited better cytotoxic activity in C2C12 cell line compared to ethanol extract (concentration of test drug needed to inhibit cell growth by 50% 110.75 ± 5.5 vs. 415.25 ± 8.0 μg/mL) whereas ethanol extract showed enhanced glucose uptake activity than petroleum ether extract in C2C12 cell line at same concentrations.

Conclusion:

From our study results, we concluded that L. aspera (Willd.) link leaves had shown better antidiabetic activity and antioxidant activity under in vitro models. Nonpolar solvent extract produced slightly better activity than polar solvent extract. This study warrants further research and experiments on animal models.

SUMMARY

Petroleum ether extract of Leucas aspera (PELA) exhibited slightly higher 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging activity compared to ethanol extract of L. aspera (EELA)

PELA exhibited better cytotoxic activity in C2C12 cell line compared to EELA

EELA had shown enhanced glucose uptake activity than PELA in C2C12 cell line at same concentrations

L. aspera leaf extracts have potential scavenging of DPPH radicals similar to that of ascorbic acid

Overall, PELA (nonpolar) produced slightly better antidiabetic activity and antioxidant activity than EELA (polar).

Abbreviations Used: DM: Diabetes mellitus, EELA: Ethanol extract of Leucas aspera, L. aspera: Leucas aspera, PELA: Petroleum ether extract of Leucas aspera.

Glucose uptake saturation explains glucose kinetics profiles measured by different tests

It is known that for a given insulin level glucose clearance depends on glucose concentration. However, a quantitative representation of the concomitant effects of hyperinsulinemia and hyperglycemia on glucose clearance, necessary to describe heterogeneous tests such as euglycemic and hyperglycemic clamps and oral tests, is lacking. Data from five studies (123 subjects) using a glucose tracer and including all the above tests in normal and diabetic subjects were collected. A mathematical model was developed in which glucose utilization was represented as a Michaelis-Menten function of glucose with constant Km and insulin-controlled Vmax, consistently with the basic notions of glucose transport. Individual values for the model parameters were estimated using a population approach. Tracer data were accurately fitted in all tests. The estimated Km was 3.88 (2.83-5.32) mmol/l [median (interquartile range)]. Median model-derived glucose clearance at 600 pmol/l insulin was reduced from 246 to 158 ml·min(-1)·m(-2) when glucose was raised from 5 to 10 mmol/l. The model reproduced the characteristic lack of increase in glucose clearance when moderate hyperinsulinemia was accompanied by hyperglycemia. In all tests, insulin sensitivity was inversely correlated with BMI, as expected (R(2) = 0.234, P = 0.0001). In conclusion, glucose clearance in euglycemic and hyperglycemic clamps and oral tests can be described with a unifying model, consistent with the notions of glucose transport and able to reproduce the suppression of glucose clearance due to hyperglycemia observed in previous studies. The model may be important for the design of reliable glucose homeostasis simulators.

Muscle microvasculature's structural and functional specializations facilitate muscle metabolism

We review the evolving findings from studies that examine the relationship between the structural and functional properties of skeletal muscle's vasculature and muscle metabolism. Unique aspects of the organization of the muscle microvasculature are highlighted. We discuss the role of vasomotion at the microscopic level and of flowmotion at the tissue level as modulators of perfusion distribution in muscle. We then consider in some detail how insulin and exercise each modulate muscle perfusion at both the microvascular and whole tissue level. The central role of the vascular endothelial cell in modulating both perfusion and transendothelial insulin and nutrient transport is also reviewed. The relationship between muscle metabolic insulin resistance and the vascular action of insulin in muscle continues to indicate an important role for the microvasculature as a target for insulin action and that impairing insulin's microvascular action significantly affects body glucose metabolism.

Determinants of reversibility of β-cell dysfunction in response to short-term intensive insulin therapy in patients with early type 2 diabetes

Short-term intensive insulin therapy (IIT) can improve pancreatic β-cell function when administered early in the course of type 2 diabetes mellitus (T2DM). However, the degree of improvement in response to this therapy varies between patients. Thus, we sought to characterize the determinants of improvement in β-cell function in response to short-term IIT in early T2DM. Sixty-three patients with mean 3.0 ± 2.1 yr duration of T2DM and Hb A1c of 6.8 ± 0.8% underwent 4 wk of IIT consisting of basal insulin detemir and premeal insulin aspart, with oral glucose tolerance test administered at baseline and 1 day post-IIT. β-Cell function before and after IIT was assessed by Insulin Secretion Sensitivity Index-2 (ISSI-2). Reversibility of β-cell dysfunction was defined as percentage change in ISSI-2 of ≥25%. Overall, the study population experienced an increase in ISSI-2 from baseline to post-IIT (P = 0.01), with one-third of participants achieving ≥25% improvement in ISSI-2. Compared with their peers, those with increases in ISSI-2 of ≥25% had greater decrements in fasting glucose (P < 0.0001), Hb A1c (P = 0.001), ALT (P = 0.04), AST (P = 0.02), and HOMA-IR (P < 0.0001). On logistical regression analysis, baseline Hb A1c (OR = 2.83, 95% CI 1.16-6.88, P = 0.02) and change in HOMA-IR (OR = 0.008, 95%CI 0.0004-0.16, P = 0.001) emerged as independent predictors of reversibility of β-cell dysfunction. Indeed, reversibility of β-cell dysfunction was achieved in only those participants in whom IIT yielded an improvement in HOMA-IR. In conclusion, decline in HOMA-IR may be a key determinant of improvement of β-cell function in response to short-term IIT, suggesting a fundamental contribution of insulin resistance to the reversible component of β-cell dysfunction in early T2DM.

Mechanisms for food polyphenols to ameliorate insulin resistance and endothelial dysfunction: therapeutic implications for diabetes and its cardiovascular complications

The rising epidemic of diabetes is a pressing issue in clinical medicine worldwide from both healthcare and economic perspectives. This is fueled by overwhelming increases in the incidence and prevalence of obesity. Obesity and diabetes are characterized by both insulin resistance and endothelial dysfunction that lead to substantial increases in cardiovascular morbidity and mortality. Reciprocal relationships between insulin resistance and endothelial dysfunction tightly link metabolic diseases including obesity and diabetes with their cardiovascular complications. Therefore, therapeutic approaches that target either insulin resistance or endothelial dysfunction alone are likely to simultaneously improve both metabolic and cardiovascular pathophysiology and disease outcomes. Moreover, combination therapies with agents targeting distinct mechanisms are likely to have additive or synergistic benefits. Conventional therapies for diabetes and its cardiovascular complications that are both safe and effective are insufficient to meet rising demand. Large, robust, epidemiologic studies demonstrate beneficial metabolic and cardiovascular health effects for many functional foods containing various polyphenols. However, precise molecular mechanisms of action for food polyphenols are largely unknown. Moreover, translation of these insights into effective clinical therapies has not been fully realized. Nevertheless, some functional foods are likely sources for safe and effective therapies and preventative strategies for metabolic diseases and their cardiovascular complications. In this review, we emphasize recent progress in elucidating molecular, cellular, and physiological actions of polyphenols from green tea (EGCG), cocoa (ECG), and citrus fruits (hesperedin) that are related to improving metabolic and cardiovascular pathophysiology. We also discuss a rigorous comprehensive approach to studying functional foods that is essential for developing novel, effective, and safe medications derived from functional foods that will complement existing conventional drugs.

Lipid-induced insulin resistance is not mediated by impaired transcapillary transport of insulin and glucose in humans

Increased lipid availability reduces insulin-stimulated glucose disposal in skeletal muscle, which is generally explained by fatty acid-mediated inhibition of insulin signaling. It remains unclear whether lipids also impair transcapillary transport of insulin and glucose, which could become rate controlling for glucose disposal. We hypothesized that lipid-induced insulin resistance is induced by inhibiting myocellular glucose uptake and not by interfering with the delivery of insulin or glucose. We measured changes in interstitial glucose and insulin in skeletal muscle of healthy volunteers during intravenous administration of triglycerides plus heparin or glycerol during physiologic and supraphysiologic hyperinsulinemia, by combining microdialysis with oral glucose tolerance tests and euglycemic-hyperinsulinemic clamps. Lipid infusion reduced insulin-stimulated glucose disposal by ~70% (P < 0.05) during clamps and dynamic insulin sensitivity by ~12% (P < 0.05) during oral glucose loading. Dialysate insulin and glucose levels were unchanged or even transiently higher (P < 0.05) during lipid than during glycerol infusion, whereas regional blood flow remained unchanged. These results demonstrate that short-term elevation of free fatty acids (FFAs) induces insulin resistance, which in skeletal muscle occurs primarily at the cellular level, without impairment of local perfusion or transcapillary transport of insulin and glucose. Thus, vascular effects of FFAs are not rate controlling for muscle insulin-stimulated glucose disposal.

Smad3 deficiency in mice protects against insulin resistance and obesity induced by a high-fat diet

OBJECTIVE

Obesity and associated pathologies are major global health problems. Transforming growth factor-β/Smad3 signaling has been implicated in various metabolic processes, including adipogenesis, insulin expression, and pancreatic β-cell function. However, the systemic effects of Smad3 deficiency on adiposity and insulin resistance in vivo remain elusive. This study investigated the effects of Smad3 deficiency on whole-body glucose and lipid homeostasis and its contribution to the development of obesity and type 2 diabetes.

RESEARCH DESIGN AND METHODS

We compared various metabolic profiles of Smad3-knockout and wild-type mice. We also determined the mechanism by which Smad3 deficiency affects the expression of genes involved in adipogenesis and metabolism. Mice were then challenged with a high-fat diet to study the impact of Smad3 deficiency on the development of obesity and insulin resistance.

RESULTS

Smad3-knockout mice exhibited diminished adiposity with improved glucose tolerance and insulin sensitivity. Chromatin immunoprecipitation assay revealed that Smad3 deficiency increased CCAAT/enhancer-binding protein β-C/EBP homologous protein 10 interaction and exerted a differential regulation on proliferator-activated receptor β/δ and proliferator-activated receptor γ expression in adipocytes. Focused gene expression profiling revealed an altered expression of genes involved in adipogenesis, lipid accumulation, and fatty acid β-oxidation, indicative of altered adipose physiology. Despite reduced physical activity with no modification in food intake, these mutant mice were resistant to obesity and insulin resistance induced by a high-fat diet.

CONCLUSIONS

Smad3 is a multifaceted regulator in adipose physiology and the pathogenesis of obesity and type 2 diabetes, suggesting that Smad3 may be a potential target for the treatment of obesity and its associated disorders.

Study of glucose uptake activity of Helicteres isora Linn. fruits in L-6 cell lines

The effect of hot water extract of fruits of Helicteres isora on glucose uptake was studied in rodent skeletal muscle cells (L-6 cells) involved in glucose utilization. H. isora is an antidiabetic medicinal plant being used in Indian traditional medicine. Hot water extracts were analysed for glucose uptake activity and found to be significantly active at 200 mug/ml dose comparable with insulin and metformin. Elevation of glucose uptake by H. isora in association with glucose transport supported the upregulation of glucose uptake. It was concluded that hot water extract of H. isora activate glucose uptake in L-6 cell line of mouse skeletal muscles.

Expression profiling of skeletal muscle following acute and chronic beta2-adrenergic stimulation: implications for hypertrophy, metabolism and circadian rhythm

Background

Systemic administration of β-adrenoceptor (β-AR) agonists has been found to induce skeletal muscle hypertrophy and significant metabolic changes. In the context of energy homeostasis, the importance of β-AR signaling has been highlighted by the inability of β_1-3-AR-deficient mice to regulate energy expenditure and susceptibility to diet induced obesity. However, the molecular pathways and gene expression changes that initiate and maintain these phenotypic modulations are poorly understood. Therefore, the aim of this study was to identify differential changes in gene expression in murine skeletal muscle associated with systemic (acute and chronic) administration of the β₂-AR agonist formoterol.

Results

Skeletal muscle gene expression (from murine tibialis anterior) was profiled at both 1 and 4 hours following systemic administration of the β₂-AR agonist formoterol, using Illumina 46K mouse BeadArrays. Illumina expression profiling revealed significant expression changes in genes associated with skeletal muscle hypertrophy, myoblast differentiation, metabolism, circadian rhythm, transcription, histones, and oxidative stress. Differentially expressed genes relevant to the regulation of muscle mass and metabolism (in the context of the hypertrophic phenotype) were further validated by quantitative RT-PCR to examine gene expression in response to both acute (1-24 h) and chronic administration (1-28 days) of formoterol at multiple timepoints. In terms of skeletal muscle hypertrophy, attenuation of myostatin signaling (including differential expression of myostatin, activin receptor IIB, phospho-Smad3 etc) was observed following acute and chronic administration of formoterol. Acute (but not chronic) administration of formoterol also significantly induced the expression of genes involved in oxidative metabolism, including hexokinase 2, sorbin and SH3 domain containing 1, and uncoupling protein 3. Interestingly, formoterol administration also appeared to influence some genes associated with the peripheral regulation of circadian rhythm (including nuclear factor interleukin 3 regulated, D site albumin promoter binding protein, and cryptochrome 2).

Conclusion

This is the first study to utilize gene expression profiling to examine global gene expression in response to acute β₂-AR agonist treatment of skeletal muscle. In summary, systemic administration of a β₂-AR agonist had a profound effect on global gene expression in skeletal muscle. In terms of hypertrophy, β₂-AR agonist treatment altered the expression of several genes associated with myostatin signaling, a previously unreported effect of β-AR signaling in skeletal muscle. This study also demonstrates a β₂-AR agonist regulation of circadian rhythm genes, indicating crosstalk between β-AR signaling and circadian cycling in skeletal muscle. Gene expression alterations discovered in this study provides insight into many of the underlying changes in gene expression that mediate β-AR induced skeletal muscle hypertrophy and altered metabolism.

Physical inactivity and obesity: links with insulin resistance and type 2 diabetes mellitus

Data from the health survey for England 2006, showed that the prevalence of type 2 diabetes mellitus (T2DM) has more than doubled in men and women since 1991. In the USA certain States have a prevalence of T2DM of greater than 10%. Globally it has been reported that this increase is by no means slowing down and that the number of individuals with the disease is expected to rise from 171 million cases reported in 2000 to 366 million by the year 2030. Physical inactivity and obesity are two major risk factors for the development of T2DM. In this review we will discuss evidence of an association between physical inactivity, obesity and T2DM from prospective cohort studies and clinical trials. We will also discuss some of the potential mechanisms that are thought to link obesity and physical inactivity with the major pathophysiological precursor of T2DM, insulin resistance.

Hyperinsulinemia fails to augment ET-1 action in the skeletal muscle vascular bed in vivo in humans

Endogenous endothelin action is augmented in human obesity and type 2 diabetes and contributes to endothelial dysfunction and impairs insulin-mediated vasodilation in humans. We hypothesized that insulin resistance-associated hyperinsulinemia could preferentially drive endothelin-mediated vasoconstriction. We applied hyperinsulinemic-euglycemic clamps with higher insulin dosing in obese subjects than lean subjects (30 vs. 10 mU.m(-2).min(-1), respectively), with the goal of matching insulin's nitric oxide (NO)-mediated vascular effects. We predicted that, under these circumstances, insulin-stimulated endothelin-1 (ET-1) action (assessed with the type A endothelin receptor antagonist BQ-123) would be augmented in proportion to hyperinsulinemia. NO bioactivity was assessed using the nitric oxide synthase inhibitor N(G)-monomethyl-l-arginine. Insulin-mediated vasodilation and insulin-stimulated NO bioavailability were well matched across groups by this approach. As expected, steady-state insulin levels were approximately threefold higher in obese than lean subjects (109.2 +/- 10.2 pmol/l vs. 518.4 +/- 84.0, P = 0.03). Despite this, the augmentation of insulin-mediated vasodilation by BQ-123 was not different between groups. ET-1 flux across the leg was not augmented by insulin alone but was increased with the addition of BQ-123 to insulin (P = 0.01 BQ-123 effect, P = not significant comparing groups). Endothelin antagonism augmented insulin-stimulated NO bioavailability and NOx flux, but not differently between groups and not proportional to hyperinsulinemia. These findings do not support the hypothesis that insulin resistance-associated hyperinsulinemia preferentially drives endothelin-mediated vasoconstriction.

A potential role for skeletal muscle caveolin-1 as an insulin sensitivity modulator in ageing-dependent non-obese type 2 diabetes: studies in a new mouse model

AIMS/HYPOTHESIS

Type 2 diabetes mellitus is a common age-dependent disease. We discovered that male offspring of non-diabetic C57BL/6 and DBA/2 mice, called JYD mice, develop type 2 diabetes when they grow old. JYD mice show characteristics of insulin resistance, hyperglycaemia and hyperinsulinaemia in old age without obesity. We postulated that the mechanism of age-dependent type 2 diabetes in this model relates to caveolin-1 status in skeletal muscle, which appears to regulate insulin sensitivity in the mice.

METHODS

We compared insulin sensitivity in aged C57BL/6 and JYD mice using glucose and insulin tolerance tests and (18)F-fluorodeoxyglucose positron emission tomography. We also determined insulin signalling molecules and caveolin proteins using western blotting, and altered caveolin-1 levels in skeletal muscle of C57BL/6 and JYD mice using viral vector systems, to examine the effect of this on insulin sensitivity.

RESULTS

In 30-week-old C57BL/6 and JYD mice, the basal levels of IRS-1, Akt and peroxisome proliferator-activated receptor-gamma decreased, as did insulin-stimulated phosphorylation of Akt and insulin receptor beta. However, caveolin-1 was only increased about twofold in 30-week-old JYD mice as compared with 3-week-old mice, whereas an eightfold increase was seen in C57BL/6 mice. Downregulation of caveolin-1 production in C57BL/6 mice caused severe impairment of glucose and insulin tolerance. Upregulation of caveolin-1 in aged diabetic JYD mice significantly improved insulin sensitivity with a concomitant increase of glucose uptake in the skeletal muscle.

CONCLUSIONS/INTERPRETATION

The level of skeletal muscle caveolin-1 is correlated with the progression of age-dependent type 2 diabetes in JYD mice.

Dangnyohwan improves glucose utilization and reduces insulin resistance by increasing the adipocyte-specific GLUT4 expression in Otsuka Long-Evans Tokushima Fatty rats

AIM OF THE STUDY

Dangnyohwan (DNH) has been used for treatment of diabetes mellitus. However, the exact cellular and molecular mechanisms underlying the beneficial effects of DNH are not well understood. Therefore, we investigated how DNH improves hyperglycemia and insulin resistance in obese-type diabetes model.

METHODS AND MATERIALS

We examined the effect of DNH on the expression of glucose transporter 4 (GLUT4), GLUT4 translocation, and glucose transport activity in muscle and adipose tissues from Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Otsuka (LETO) rats.

RESULTS

DNH ameliorated hyperglycemia and impaired glucose tolerance (IGT) observed in 26- and 42-week-old male OLETF rats. The basal and insulin-stimulated [14C]2-Deoxyglucose (2DG) uptake was significantly increased in adipocytes from DNH-treated OLETF rats, as compared with untreated OLETF rats. The expression level of GLUT4 was markedly decreased (by 90-95%) in the adipose tissue of OLETF rats, whereas DNH treatment drastically increased the expression of GLUT4 within 8 weeks. DNH improved GLUT4 recruitment stimulated by insulin in both the 26- and 42-week-old OLETF rat adipocytes.

CONCLUSION

These results suggest that DNH could exert the beneficial effects on hyperglycemia and insulin resistance by increasing the expression and insulin-stimulated translocation of GLUT4 in OLETF rat adipocytes.

Rosiglitazone reduces liver fat and insulin requirements and improves hepatic insulin sensitivity and glycemic control in patients with type 2 diabetes requiring high insulin doses

BACKGROUND

Liver fat is an important determinant of insulin requirements during insulin therapy. Peroxisome proliferator-activated receptor (PPAR)-gamma agonists reduce liver fat. We therefore hypothesized that type 2 diabetic patients using exceptionally high doses of insulin might respond well to addition of a PPARgamma agonist.

METHODS

We determined the effect of the PPARgamma agonist rosiglitazone on liver fat and directly measured hepatic insulin sensitivity in 14 patients with type 2 diabetes (aged 51 +/- 3 yr, body mass index 36.7 +/- 1.1 kg/m2), who were poorly controlled (glycosylated hemoglobin A 1c (HbA 1c) 8.9 +/- 0.4%) despite using high doses of insulin (218 +/- 22 IU/d) in combination with metformin. Liver fat content (1H-magnetic resonance spectroscopy), hepatic insulin sensitivity [6 h hyperinsulinemic euglycemic clamp (insulin 0.3 mU/kg.min) combined with [3-3H]glucose], body composition (magnetic resonance imaging), substrate oxidation rates (indirect calorimetry), clinical parameters, and liver enzymes were measured before and after rosiglitazone treatment (8 mg/d) for 8 months.

RESULTS

During rosiglitazone, HbA(1c) decreased from 8.9 +/- 0.4% to 7.8 +/- 0.3% (P = 0.007) and insulin requirements from 218 +/- 22 to 129 +/- 20 IU/d (P = 0.002). Liver fat content decreased by 46 +/- 9% from 20 +/- 3% to 11 +/- 3% (P = 0.0002). Hepatic insulin sensitivity, measured from the percent suppression of endogenous glucose production by insulin, increased from -40 +/- 7% to -89 +/- 12% (P = 0.001). The percent change in liver fat correlated with the percent decrease in HbA 1c (r = 0.53, P = 0.06), insulin dose (r = 0.66, P = 0.014), and suppression of endogenous glucose production (r = 0.76, P = 0.003).

CONCLUSIONS

Our results suggest that rosiglitazone may be particularly effective in type 2 diabetic patients who are poorly controlled despite using high insulin doses. The mechanism is likely to involve reduced liver fat and enhanced hepatic insulin sensitivity.

Cardiovascular actions of insulin

Insulin has important vascular actions to stimulate production of nitric oxide from endothelium. This leads to capillary recruitment, vasodilation, increased blood flow, and subsequent augmentation of glucose disposal in classical insulin target tissues (e.g., skeletal muscle). Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways regulating endothelial production of nitric oxide share striking parallels with metabolic insulin-signaling pathways. Distinct MAPK-dependent insulin-signaling pathways (largely unrelated to metabolic actions of insulin) regulate secretion of the vasoconstrictor endothelin-1 from endothelium. These and other cardiovascular actions of insulin contribute to coupling metabolic and hemodynamic homeostasis under healthy conditions. Cardiovascular diseases are the leading cause of morbidity and mortality in insulin-resistant individuals. Insulin resistance is typically defined as decreased sensitivity and/or responsiveness to metabolic actions of insulin. This cardinal feature of diabetes, obesity, and dyslipidemia is also a prominent component of hypertension, coronary heart disease, and atherosclerosis that are all characterized by endothelial dysfunction. Conversely, endothelial dysfunction is often present in metabolic diseases. Insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling that in vascular endothelium contributes to a reciprocal relationship between insulin resistance and endothelial dysfunction. The clinical relevance of this coupling is highlighted by the findings that specific therapeutic interventions targeting insulin resistance often also ameliorate endothelial dysfunction (and vice versa). In this review, we discuss molecular mechanisms underlying cardiovascular actions of insulin, the reciprocal relationships between insulin resistance and endothelial dysfunction, and implications for developing beneficial therapeutic strategies that simultaneously target metabolic and cardiovascular diseases.

Effects of insulin therapy on liver fat content and hepatic insulin sensitivity in patients with type 2 diabetes

We determined whether insulin therapy changes liver fat content (LFAT) or hepatic insulin sensitivity in type 2 diabetes. Fourteen patients with type 2 diabetes (age 51+/-2 yr, body mass index 33.1+/-1.4 kg/m2) treated with metformin alone received additional basal insulin for 7 mo. Liver fat (proton magnetic resonance spectroscopy), fat distribution (MRI), fat-free and fat mass, and whole body and hepatic insulin sensitivity (6-h euglycemic hyperinsulinemic clamp combined with infusion of [3-(3)H]glucose) were measured. The insulin dose averaged 75+/-10 IU/day (0.69+/-0.08 IU/kg, range 24-132 IU/day). Glycosylated hemoglobin A1c (Hb A1c) decreased from 8.9+/-0.3 to 7.4+/-0.2% (P<0.001). Whole body insulin sensitivity increased from 2.21+/-0.38 to 3.08+/-0.40 mg/kg fat-free mass (FFM).min (P<0.05). This improvement could be attributed to enhanced suppression of hepatic glucose production (HGP) by insulin (HGP 1.04+/-0.28 vs. 0.21+/-0.19 mg/kg FFM.min, P<0.01). The percent suppression of HGP by insulin increased from 72+/-8 to 105+/-11% (P<0.01). LFAT decreased from 17+/-3 to 14+/-3% (P<0.05). The change in LFAT was significantly correlated with that in hepatic insulin sensitivity (r=0.56, P<0.05). Body weight increased by 3.0+/-1.1 kg (P<0.05). Of this, 83% was due to an increase in fat-free mass (P<0.01). Fat distribution and serum adiponectin concentrations remained unchanged while serum free fatty acids decreased significantly.

CONCLUSIONS

insulin therapy improves hepatic insulin sensitivity and slightly but significantly reduces liver fat content, independent of serum adiponectin.

Calpain inhibition and insulin action in cultured human muscle cells

Variation in the calpain 10 gene has been reported to increase susceptibility to type 2 diabetes. Part of this susceptibility appears to be mediated by a decrease in whole body insulin sensitivity. As skeletal muscle is the primary tissue site of the peripheral insulin resistance in type 2 diabetes, the aim of this study was to use a human skeletal muscle cell culture system to explore the effects of calpain inhibition on insulin action. Calpain 10 mRNA and protein expression was examined in cultured myoblasts, myotubes, and whole skeletal muscle from non-diabetic subjects using RT-PCR and Western blotting. Changes in insulin-stimulated glucose uptake and glycogen synthesis in response to the calpain inhibitors ALLN and ALLM were measured. Calpain 10 expression was confirmed in cultured human myoblasts, myotubes, and native skeletal muscle. Insulin-stimulated glucose uptake was significantly decreased following preincubation with ALLN [404+/-40 vs 505+/-55 (mean+/-SEM)pmol/mg/min; with vs without ALLN: p = 0.04] and ALLM [455+/-38 vs 550+/-50 pmol/mg/min; with vs without ALLM: p = 0.025] in day 7 fused myotubes, but not in myoblasts. Neither ALLN nor ALLM affected insulin-stimulated glycogen synthesis in myoblasts or myotubes. These studies confirm calpain 10 expression in cultured human muscle cells and support a role for calpains in insulin-stimulated glucose uptake in human skeletal muscle cells that may be relevant to the pathogenesis of the peripheral insulin resistance in type 2 diabetes.

Calorie restriction improves whole-body glucose disposal and insulin resistance in association with the increased adipocyte-specific GLUT4 expression in Otsuka Long–Evans Tokushima Fatty rats

Calorie restriction (CR) has been shown to improve peripheral insulin resistance and type 2 diabetes in animal models. However, the exact mechanism of CR on GLUT4 expression and translocation in insulin-sensitive tissues has not been well elucidated. In the present study, we examine the effect of CR on the expression of glucose transporter 4 (GLUT4), GLUT4 translocation, and glucose transport activity in adipose tissue from Otsuka Long-Evans Tokushima Fatty (OLETF) rat and control (LETO) rats. CR (70% of satiated group) ameliorated hyperglycemia and improved impaired glucose tolerance (IGT) in OLETF rats. In skeletal muscle, the expression levels of GLUT4 and GLUT1 were not significantly different between LETO and OLETF rats, and were not affected by CR. By contrast, the expression level of GLUT4 was markedly decreased in the adipose tissue of OLETF rats, but was dramatically increased by CR. The GLUT4 recruitment stimulated by insulin was also improved in OLETF rat adipocytes by CR. The insulin-stimulated 2-deoxyglucose (2DG) uptake was significantly increased in adipocytes from the CR OLETF rats, as compared with the satiated OLETF rats. Taken together, these results suggest that CR improves whole body glucose disposal and insulin resistance in OLETF rats, and that these effects may associate with the increased adipocyte-specific GLUT4 expression.

Changes in blood pressure and plasma catecholamine levels during prolonged hyperinsulinemia

Hyperinsulinemia has been shown to induce activation of the sympathetic nervous system and vasodilatation. Whether these effects result in changes in blood pressure (BP) is discussed controversially. We measured BP and plasma catecholamine levels in 30 healthy men during a 60-minute baseline phase and 360-minute period of insulin infusion. In a double-blind, between-subject comparison, insulin was infused at a low rate (1.5 mU insulin/kg per minute) in one half of the subjects and at a high rate (15 mU/kg per minute) in the other half. Throughout the experiments, blood glucose levels were held constantly within the normal range by a simultaneous infusion of glucose. Serum insulin levels increased to a plateau of 543 +/- 34 pmol/L during low rate and to 24,029 +/- 1,595 pmol/L during high rate of insulin infusion. Compared with baseline, insulin infusion of either rate significantly increased systolic BP, BP amplitude, and heart rate (all P < .05). In comparison with the low rate of insulin infusion, the high rate provoked a more pronounced increase in heart rate (P < .02) and systolic BP (P < .05) but tended to decrease diastolic BP (P < .08) summing up to a distinctly more increased BP amplitude (P < .05). Plasma norepinephrine as well as epinephrine levels did not significantly change during the low-rate insulin infusion but significantly increased during high-rate insulin infusion (both P < .05). By showing a dose-dependent increasing influence of insulin on systolic BP and circulating catecholamine levels, the present study provides experimental evidence for the notion that hyperinsulinemia contributes to the development of hypertension.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Insulin resistance and endothelial dysfunction

Insulin has multiple metabolic actions, including effects on blood vessels. Insulin normally increases blood flow by a mechanism which involves generation of nitric oxide (NO) via the arginine-NO pathway. Although insulin itself is a weak and physiologically unimportant vasodilatator, it appears to markedly potentiate endothelium-dependent vasodilatation. Therefore, anything that impairs insulin action in endothelial cells can be expected to be associated with endothelial dysfunction, i.e. loss of NO bioactivity in the vessel wall. Consistent with the idea that insulin resistance and endothelial dysfunction frequently coexist, all insulin-resistant conditions examined to date have been associated with endothelial dysfunction. However, the latter can also be caused by factors other than insulin resistance-such as a high concentration of low-density lipoprotein (LDL) cholesterol. Therapies which reverse insulin resistance-such as exercise, insulin and inhibitors of the renin-angiotensin-aldosterone (RAA) axis-also reverse endothelial dysfunction, which may thus be an inherent feature of insulin resistance.

Stimulation of glucose uptake in muscle cells by prolonged treatment with scriptide, a histone deacetylase inhibitor

Glucose incorporation is regulated mainly by GLUT4 in skeletal muscles. Here we report that treatment of L6 myotubes with scriptide, a hydroxamic acid-based histone deacetylase (HDAC) inhibitor, stimulated 2-deoxyglucose uptake. The effect appeared only after 24 hr, resulting in 2.4-fold glucose uptake at treatment day 6. Scriptide acted synergistically with insulin, indicating it stimulated a distinct pathway from the insulin signaling pathway. It was not observed in undifferentiated myoblasts or 3T3-L1 adipocytes, suggesting a muscle-specific effect of scriptide. A five-carbon chain and hydroxamic acid, essential for histone deacetylase inhibition, were indispensable for this effect, and trichostatin A stimulated glucose uptake as well. Scriptide increased the cellular content of GLUT4, and induced GLUT4 translocation, but GLUT4 mRNA level did not change, indicating scriptide functions posttranslationally. Our results indicated a novel function for HDAC inhibitors of increasing GLUT4 content and its translocation in muscle cells, resulting in stimulation of glucose uptake.

Fiber type-specific determinants of Vmax for insulin-stimulated muscle glucose uptake in vivo

The aim of this study was to determine barriers limiting muscle glucose uptake (MGU) during increased glucose flux created by raising blood glucose in the presence of fixed insulin. The determinants of the maximal velocity (V(max)) of MGU in muscles of different fiber types were defined. Conscious rats were studied during a 4 mU x kg(-1) x min(-1) insulin clamp with plasma glucose at 2.5, 5.5, and 8.5 mM. [U-(14)C]mannitol and 3-O-methyl-[(3)H]glucose ([(3)H]MG) were infused to steady-state levels (t = -180 to 0 min). These isotope infusions were continued from 0 to 40 min with the addition of a 2-deoxy-[(3)H]glucose ([(3)H]DG) infusion. Muscles were excised at t = 40 min. Glucose metabolic index (R(g)) was calculated from muscle-phosphorylated [(3)H]DG. [U-(14)C]mannitol was used to determine extracellular (EC) H(2)O. Glucose at the outer ([G](om)) and inner ([G](im)) sarcolemmal surfaces was determined by the ratio of [(3)H]MG in intracellular to EC H(2)O and muscle glucose. R(g) was comparable at the two higher glucose concentrations, suggesting that rates of uptake near V(max) were reached. In summary, by defining the relationship of arterial glucose to [G](om) and [G](im) in the presence of fixed hyperinsulinemia, it is concluded that 1) V(max) for MGU is limited by extracellular and intracellular barriers in type I fibers, as the sarcolemma is freely permeable to glucose; 2) V(max) is limited in muscles with predominantly type IIb fibers by extracellular resistance and transport resistance; and 3) limits to R(g) are determined by resistance at multiple steps and are better defined by distributed control rather than by a single rate-limiting step.

Effects of insulin resistance and type 2 diabetes on lipoprotein subclass particle size and concentration determined by nuclear magnetic resonance

The insulin resistance syndrome (IRS) is associated with dyslipidemia and increased cardiovascular disease risk. A novel method for detailed analyses of lipoprotein subclass sizes and particle concentrations that uses nuclear magnetic resonance (NMR) of whole sera has become available. To define the effects of insulin resistance, we measured dyslipidemia using both NMR lipoprotein subclass analysis and conventional lipid panel, and insulin sensitivity as the maximal glucose disposal rate (GDR) during hyperinsulinemic clamps in 56 insulin sensitive (IS; mean +/- SD: GDR 15.8 +/- 2.0 mg. kg(-1). min(-1), fasting blood glucose [FBG] 4.7 +/- 0.3 mmol/l, BMI 26 +/- 5), 46 insulin resistant (IR; GDR 10.2 +/- 1.9, FBG 4.9 +/- 0.5, BMI 29 +/- 5), and 46 untreated subjects with type 2 diabetes (GDR 7.4 +/- 2.8, FBG 10.8 +/- 3.7, BMI 30 +/- 5). In the group as a whole, regression analyses with GDR showed that progressive insulin resistance was associated with an increase in VLDL size (r = -0.40) and an increase in large VLDL particle concentrations (r = -0.42), a decrease in LDL size (r = 0.42) as a result of a marked increase in small LDL particles (r = -0.34) and reduced large LDL (r = 0.34), an overall increase in the number of LDL particles (r = -0.44), and a decrease in HDL size (r = 0.41) as a result of depletion of large HDL particles (r = 0.38) and a modest increase in small HDL (r = -0.21; all P < 0.01). These correlations were also evident when only normoglycemic individuals were included in the analyses (i.e., IS + IR but no diabetes), and persisted in multiple regression analyses adjusting for age, BMI, sex, and race. Discontinuous analyses were also performed. When compared with IS, the IR and diabetes subgroups exhibited a two- to threefold increase in large VLDL particle concentrations (no change in medium or small VLDL), which produced an increase in serum triglycerides; a decrease in LDL size as a result of an increase in small and a reduction in large LDL subclasses, plus an increase in overall LDL particle concentration, which together led to no difference (IS versus IR) or a minimal difference (IS versus diabetes) in LDL cholesterol; and a decrease in large cardioprotective HDL combined with an increase in the small HDL subclass such that there was no net significant difference in HDL cholesterol. We conclude that 1) insulin resistance had profound effects on lipoprotein size and subclass particle concentrations for VLDL, LDL, and HDL when measured by NMR; 2) in type 2 diabetes, the lipoprotein subclass alterations are moderately exacerbated but can be attributed primarily to the underlying insulin resistance; and 3) these insulin resistance-induced changes in the NMR lipoprotein subclass profile predictably increase risk of cardiovascular disease but were not fully apparent in the conventional lipid panel. It will be important to study whether NMR lipoprotein subclass parameters can be used to manage risk more effectively and prevent cardiovascular disease in patients with the IRS.

Are the metabolic effects of rosiglitazone influenced by baseline glycaemic control?

OBJECTIVE

To compare the metabolic effects of rosiglitazone, an antidiabetic agent of the thiazolidinedione class, in patients with type 2 diabetes with fair to moderate glycaemic control (glycosylated haemoglobin (HbA(lc)) < 9%) and poor glycaemic control (HbA(lc) > or = 9%).

RESEARCH DESIGN AND METHODS

Data were pooled from two 26-week, randomised, placebo-controlled, double-blind studies of rosiglitazone (4 and 8 mg/day).

RESULTS

After 26 weeks of treatment, HbA(lc) was significantly reduced (p < 0.05) compared with baseline and placebo in patients taking rosiglitazone 8 mg/day for both HbA(lc) stratifications, with greater reductions in patients with baseline HbA(lc) > or = 9%. After 26 weeks of treatment, reductions in fasting plasma glucose (FPG) were significant (p < 0.05) compared with baseline and placebo in both rosiglitazone treatment groups for both HbA(lc) stratifications, with greater reductions in the group with poor glycaemic control. Rosiglitazone significantly improved insulin sensitivity (p < 0.05) compared with baseline in patients with baseline HbA(lc) < 9%. Rosiglitazone significantly improved beta-cell function (p < 0.05) compared with baseline with more improvement in the group with baseline HbA(lc) > or = 9%. These improvements were statistically significant compared with placebo, regardless of HbA(lc) stratification.

CONCLUSION

Rosiglitazone significantly improved HbA(lc) and FPG levels in patients with type 2 diabetes, with the greatest improvements observed in patients with baseline HbA(lc) levels > or =9%.

Insulin-induced decreases in aortic wave reflection and central systolic pressure are impaired in type 2 diabetes

OBJECTIVE

To determine whether large arteries are resistant to insulin.

RESEARCH DESIGN AND METHODS

Insulin normally acutely decreases central systolic pressure by decreasing wave reflection in vivo. This effect occurs before any changes in peripheral vascular resistance or heart rate under normoglycemic conditions. We determined whether the ability of insulin to decrease central aortic pressure is altered in uncomplicated type 2 diabetes. The study subjects consisted of 16 type 2 diabetic patients (age 54 +/- 2 years, BMI 29 +/- 1 kg/m(2)) and 19 matched nondiabetic individuals (51 +/- 2 years, 29 +/- 1 kg/m(2)) studied under normoglycemic-hyperinsulinemic conditions. Central aortic pressure waveforms were synthesized from those recorded in the periphery using applanation tonometry and a validated reverse transfer function to construct the central aortic pressure waveform every 30 min. This method allowed determination of aortic augmentation (the pressure difference between the first and second central systolic pressure waves) and the augmentation index (augmentation divided by pulse pressure).

RESULTS

Whole-body insulin sensitivity was 31% lower (P < 0.05) in the type 2 diabetic patients than in the normal subjects. Basally, before the insulin infusion, augmentation averaged 8.9 +/- 1.3 and 11.1 +/- 1.2 mmHg (NS) and the augmentation index averaged 23.1 +/- 2.1 and 27.5 +/- 2.1% (NS) in the normal subjects and diabetic patients, respectively. After 30 min of hyperinsulinemia, augmentation decreased significantly to 6.1 +/- 1.1 mmHg (P < 0.001) in the normal subjects but remained unchanged at 9.1 +/- 1.1 mmHg (NS) in type 2 diabetic patients. At 30 min, the augmentation index had decreased significantly (30 +/- 7% decrease) to 17.9 +/- 2.6% in the normal subjects but remained at 24.4 +/- 2.4% in the diabetic patients (13 +/- 4% decrease, P < 0.05 for change vs. normal subjects). Central systolic pressure decreased significantly by 30 min in the normal subjects but only after 120 min in the type 2 diabetic patients. There were no significant changes in heart rate, pulse pressure, or forearm blood flow during the first 120 min of the insulin infusion.

CONCLUSIONS

Insulin resistance in type 2 diabetes involves a delay in the ability of insulin to decrease central aortic pressure. This defect could predispose these patients to develop systolic hypertension.

Troglitazone improves GLUT4 expression in adipose tissue in an animal model of obese type 2 diabetes mellitus

Troglitazone has been shown to improve peripheral insulin resistance in type 2 diabetic patients and animal models. We examined the effect of troglitazone on the expression of glucose transporter 4 (GLUT4) in muscle and adipose tissue from Otsuka Long-Evans Tokushima Fatty (OLETF) rat, an animal model of obese type 2 diabetes mellitus. In addition, the effects of troglitazone on GLUT4 translocation and on glucose transport activity in adipocytes were also evaluated. Muscle and adipose tissues were isolated from 35-week-old male troglitazone-treated and untreated OLETF rats at a dose of 150 mg/kg per day for 14 days. In skeletal muscle, the protein and mRNA levels of GLUT4 were not significantly different between OLETF and control rats and they were not affected by troglitazone. On the other hand, GLUT4 protein and mRNA levels in adipose tissue from OLETF rats were significantly decreased (P<0.01) compared with control rats and they were significantly increased (1.5-fold, P<0.01) by troglitazone. Troglitazone had no major effect on GLUT4 translocation in adipocytes, but it significantly increased (1.4-fold, P<0.05) the basal and insulin-induced amounts of GLUT4 in plasma membrane (PM) in adipocytes from OLETF rats. Consistent with these results, the basal and insulin-induced glucose uptakes in adipocytes from troglitazone-treated OLETF rats were significantly increased (1.5-fold, P<0.05) compared with untreated OLETF rats. Our results suggest that troglitazone may exert beneficial effects on insulin resistance by increasing the expression of GLUT4 in adipose tissue.

No role of calcium- and ATP-dependent potassium channels in insulin-induced vasodilation in humans in vivo

The mechanism of insulin-induced vasodilation has not been completely clarified, but could be important in future treatment strategies of insulin resistance. Recently, a role for calcium-dependent and ATP-dependent potassium (K(Ca) and K(ATP)) channels in insulin-induced vasodilation has been demonstrated in in vitro studies. A role for these channels has never been confirmed in humans in vivo. Therefore, we investigated the role of these channels in insulin-induced vasodilation in humans in vivo. A hyperinsulinemic euglycemic clamp was combined with intra-arterial infusion of placebo, tetraethylammonium (blocker of K(Ca) channels) or glibenclamide (blocker of K(ATP) channels) in three groups of 12 healthy volunteers. Bilateral forearm blood flow was measured with venous occlusion plethysmography. Systemic hyperinsulinemia induced a 20+/-9% vasodilation (p=0.001). Neither tetraethylammonium nor glibenclamide reduced this vasodilation as compared to placebo. According to the results of the present study, insulin-induced vasodilation seems not to be mediated by the opening of K(Ca) and K(ATP) channels in humans in vivo.

Insulin resistance and associated metabolic abnormalities in muscle: effects of exercise

Skeletal muscle is a major site of insulin resistance. In addition to glucose transport, oxidative disposal and storage defects, insulin resistant muscle exhibit many other metabolic abnormalities. After a brief review of insulin resistance determinants, we will focus on muscular abnormalities in obesity and type 2 diabetes. Glucose and lipid metabolism defects will be analysed and their interactions discussed. Exercise can improve many of these muscular abnormalities and the mechanisms underlying exercise-induced benefits have been clarified during the past decades. Therefore, exercise training has proved to be useful in the management of insulin resistant states, i.e. mainly obesity, especially in its truncal distribution, and type 2 diabetes. However, exercise prescription remains poorly codified, and results on glycaemic control are sometimes conflicting. In the last part of this review, we will emphasize the pathophysiological basis for an individualized exercise prescription in insulin resistant subjects.

Evidence for physiological coupling of insulin-mediated glucose metabolism and limb blood flow

We hypothesized that the vasodilation observed during insulin stimulation is closely coupled to the rate of glucose metabolism. Lean (L, n = 13), obese nondiabetic (OB, n = 13), and obese type 2 diabetic subjects (Type 2 DM, n = 16) were studied. Leg blood flow (LBF) was examined under conditions of euglycemic hyperinsulinemia (EH) and hyperglycemic hyperinsulinemia (HH), which produced a steady-state whole body glucose disposal rate (GDR) of approximately 2,000 micromol. m(-2). min(-1). At this GDR, under both conditions, subjects across the range of insulin sensitivity exhibited equivalent LBF (l/min EH: L, 0.42 +/- 0.03; OB, 0.43 +/- 0. 03; Type 2 DM, 0.38 +/- 0.07; P = 0.72 by ANOVA. HH: L, 0.44 +/- 0. 04; OB, 0.39 +/- 0.05; Type 2 DM, 0.41 +/- 0.04; P = 0.71). The continuous relationship between LBF and GDR did not differ across subject groups [slope x 10(-5) l/(micromol. m(-2). min(-1)) by ANOVA. EH: L, 8.6; OB, 9.2; Type 2 DM, 7.9; P = 0.91. HH: L, 4.2; OB, 2.5; Type 2 DM, 4.1; P = 0.77], although this relationship did differ between the EH and HH conditions (P = 0.001). These findings support a physiological coupling of LBF and insulin-mediated glucose metabolism. The mechanism(s) linking substrate delivery and metabolism appears to be intact in insulin-resistant states.

Impairment of insulin-induced vasodilation is associated with muscle insulin resistance in type 2 diabetes

To clarify the association between the actions of insulin on the vascular wall and on the muscles in diabetes, we evaluated insulin-mediated vasodilation and muscle glucose uptake simultaneously using the euglycemic hyperinsulinemic glucose clamp technique and the calculation of total peripheral vascular resistance (TPR) from arterial pulse wave analysis in 19 Japanese patients with type 2 diabetes who had no signs of atherosclerosis. During the clamp study, the plasma norepinephrine (NE) level and plasma renin activity (PRA) increased without showing any significant correlation to the glucose infusion rate (GIR); a marker of muscle insulin sensitivity, and no changes of other plasma vasoactive hormone levels were observed. TPR decreased over time during the clamp study. The decrease of TPR from baseline was 0.88 +/- 0.02 at 1 h (mean +/- S.E.M., P < 0.01) and 0.79 +/- 0.03 at 2 h (P < 0.01), and the relative change in TPR from baseline was negatively correlated with GIR (r = -0.48 at 1 and 2 h; both P < 0.05). Our results suggest that there is also insulin resistance in the vascular wall, and this phenomenon may be associated with muscle insulin resistance in type 2 diabetes.

Insulin, nitric oxide and the sympathetic nervous system: at the crossroads of metabolic and cardiovascular regulation

Epidemiological studies demonstrate an association between insulin resistance, hypertension and cardiovascular morbidity. Over the past decade, evidence has accumulated indicating that short-term insulin administration, in addition to its metabolic effects, also has important cardiovascular actions. The sympathetic nervous system and the L-arginine-nitric oxide pathway have emerged as central players in the mediation of insulin's cardiovascular actions. The underlying mechanisms and the factors that may govern the interaction between insulin and these two major cardiovascular regulatory systems have been studied extensively in healthy people and insulin-resistant subjects. Here we summarize the current understanding and gaps in knowledge on insulin's cardiovascular actions in humans, and discuss possible pathophysiological consequences of their alteration. Based on recent new insight, we propose that a genetic and/or acquired defect of nitric oxide synthesis could represent a central defect triggering many of the metabolic, vascular and sympathetic abnormalities characteristic of insulin-resistant states, all of which may predispose to cardiovascular disease.

The role of adenosine in insulin-induced vasodilation

It was previously shown that systemic hyperinsulinemia induces vasodilation in human skeletal muscle. The mechanism mediating this vasodilation is not yet completely clarified. Based on data from animal experiments, we hypothesized that stimulation of the adenosine receptor is involved in insulin-induced vasodilation. To test this hypothesis, a 105-min hyperinsulinemic euglycemic clamp was performed in three groups of eight healthy volunteers. In group 1, placebo was infused into the left brachial artery (experimental forearm). In the second and third group, respectively, draflazine (an adenosine-uptake blocker) and theophylline (an adenosine-receptor antagonist) were administered by intrabrachial infusion. Forearm blood flow (FBF) was measured by venous-occlusion plethysmography, both at the experimental and the control forearms. The percentage decrease in flow ratio (FBF experimental arm/control arm) in the draflazine group was significantly less pronounced than that in the placebo group, whereas the percentage decrease in flow ratio was larger in the theophylline group. These results demonstrate that the insulin-induced increase in blood flow in the experimental arm was more pronounced at the site of adenosine-uptake blockade by draflazine, whereas this was reduced during adenosine-receptor antagonism by theophylline. Our observations are compatible with the hypothesis that insulin-induced vasodilation is mediated by the release of adenosine.

Insulin resistance, impaired glucose tolerance and non-insulin-dependent diabetes, pathologic mechanisms and treatment: current status and therapeutic possibilities

Impaired glucose tolerance and non-insulin-dependent diabetes (NIDDM) are the pathologic consequence of two co-incident and interacting conditions, namely insulin resistance and relative insulin deficiency. Recognised by the World Health Authority as a global health problem there are at 1995 estimates at least 110 million diagnosed diabetics world wide with at least the same number undiagnosed. Diabetes is the 4th leading cause of death in developed countries and its management exerts a vast economic and social burden. Insulin resistance is established as the characteristic pathologic feature of patients with glucose intolerance and NIDDM describing a state in which insulin stimulated glucose uptake and utilisation in liver, skeletal muscle and adipose tissue is impaired and coupled to impaired suppression of hepatic glucose output. Although the biochemical mechanisms underpinning both defects are becoming better understood, the genetic and molecular causes remain elusive; and whether insulin resistance or relative insulin deficiency represents the primary defect in patients with NIDDM is the matter of some debate. In this article we review the biochemical and molecular nature of the defects in insulin sensitivity and glucose uptake, and discuss some of the potential causative mechanisms. The genetic and environmental basis of insulin resistance is reviewed and presented, and potential therapeutic targets including thiazolidinediones are discussed.

Actions of novel antidiabetic thiazolidinedione, T-174, in animal models of non-insulin-dependent diabetes mellitus (NIDDM) and in cultured muscle cells

1. The antihyperglycaemic effect and the possible mechanism of action of T-174, a novel thiazolidinedione derivative, was determined in vivo and in vitro. 2. Oral administration of T-174 markedly improved hyperglycaemia, hyperinsulinaemia, hyperlipidaemia, and glucose intolerance in genetically obese and diabetic yellow KK (KK-Ay) mice (0.2-15.5 mg kg(-1) day(-1), for 7 days) and Zucker fatty rats (1.4-11.4 mg kg(-1) day(-1), for 6 days). The ED50 values for the glucose lowering action of T-174 and pioglitazone, another thiazolidinedione antidiabetic, were 1.8 and 29 mg kg(-1) day(-1), respectively in KK-Ay mice; T-174 was about 16 times more potent than pioglitazone. 3. The hypoglycaemic effect of exogenous insulin in KK-Ay mice was enhanced after the administration of T-174. A hyperinsulinaemic euglycaemic clamp study in Zucker fatty rats showed an amelioration of whole-body insulin resistance by the T-174 treatment. 4. Insulin-stimulated glucose metabolism was enhanced in adipocytes from KK-Ay mice treated with T-174. The insulin receptor number of the adipocytes was increased without a change in the affinity of the receptor. 5. The hypomagnesaemia in KK-Ay mice was completely restored by T-174. 6. In cultured L6 myotubes, glucose consumption and [3H]-2-deoxy-glucose transport were enhanced by T-174 (EC50; 6 and 4 microM, respectively). Combination of insulin with T-174 was additive to stimulate glucose disposal. 7. These results suggest that the antihyperglycaemic effect of T-174 was mediated by enhanced insulin action. This was associated with amelioration of the hypomagnesaemia and T-174 directly increased basal and insulin-stimulated glucose utilization by cultured muscle cells.

Relationship between blood pressure, metabolic variables and blood flow in obese subjects with or without non-insulin-dependent diabetes mellitus

BACKGROUND

To assess the relationship between systemic blood pressure, metabolic variables and adipose tissue blood flow, we studied 55 subjects before and 36 subjects after an oral glucose load (100 g).

METHODS

The subjects were divided into four different groups: (a) young lean control subjects [age 31 +/- 1 years, mean +/- SE, BMI (body mass index) 22.7 +/- 0.4 kg m-2]; (b) young obese subjects (age 29 +/- 2 years, BMI 37.8 +/- 1.8 kg m-2); (c) middle-aged obese subjects (age 50 +/- 2 years, BMI 30.2 +/- 0.9 kg m-2); and (d) middle-aged obese non-insulin-dependent diabetic (NIDDM) subjects (age 54 +/- 2 years, BMI 30.0 +/- 0.7 kg m-2).

RESULTS

Groups 2-4 demonstrated a low fasting adipose tissue blood flow (ATBF) and the increase in ATBF after oral glucose was impaired. A further impairment was present in NIDDM subjects. Systolic and diastolic blood pressure were also increased in groups 2-4 and further so in group 4. Fasting glucose, lactate and free fatty acid (FFA) levels correlated positively with the systolic blood pressure, whereas ATBF correlated negatively with the diastolic blood pressure. Furthermore, in the NIDDM subjects fasting lactate correlated closely with both the systolic (r = 0.649, P = 0.01) and diastolic (r = 0.626, P = 0.013) blood pressure.

CONCLUSION

These data suggest a close relationship between insulin resistance and regulation of adipose tissue blood flow as well as blood pressure.