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OBJECTIVE This crossover study explored the impact of a single bout of exercise on insulin-stimulated responses in conduit arteries and capillaries. METHODS Twelve sedentary adults (49.5 [7.8] years; maximal oxygen consumption [VO2 max]: 23.7 [5.4] mL/kg/min) with obesity (BMI 34.5 [4.3] kg/m2 ) completed a control and exercise bout (70% VO2 max to expend 400 kcal). Sixteen hours later, participants underwent a 2-hour euglycemic-hyperinsulinemic clamp (90 mg/dL; 40 mU/m2 /min) to determine vascular and metabolic insulin sensitivity. Endothelial and capillary functions were assessed by brachial artery flow-mediated dilation and contrast-enhanced ultrasound, respectively. Metabolized glucose infusion rate, substrate oxidation (indirect calorimetry), nonoxidative glucose disposal (NOGD), and inflammation were also determined. RESULTS Exercise increased insulin-stimulated preocclusion diameter (p = 0.01) and microvascular blood flow (condition effect: p = 0.04) compared with control. Furthermore, exercise improved metabolic insulin sensitivity by 21%, which paralleled rises in NOGD (p = 0.05) and decreases in soluble receptors for advanced glycation end products (condition effect: p = 0.01). Interestingly, changes in NOGD were related to increased insulin-stimulated microvascular blood flow (r = 0.57, p = 0.05). CONCLUSIONS A single bout of exercise increases vascular insulin sensitivity in adults with obesity. Additional work is needed to determine vascular responses following different doses of exercise in order to design lifestyle prescriptions for reducing chronic disease risk.

Context Obesity and type 2 diabetes are associated with chronic hyperinsulinemia, elevated plasma levels of dipeptidyl peptidase-4 (DPP4), and a pro-atherosclerotic milieu. Evidence Acquisition PubMed search of the term “insulin and atherosclerosis,” “hyperinsulinemia,” “atherosclerosis,” or “cardiovascular outcomes” cross-referenced with “DPP4.” Relevant research and review articles were reviewed. Evidence Synthesis Hyperinsulinemia in the setting of insulin resistance promotes vascular inflammation, vascular smooth muscle cell growth, pathological cholesterol profile, hypertension, and recruitment of immune cells to the endothelium, all contributing to atherosclerosis. DPP4 has pleiotropic functions and its activity is elevated in obese humans. DPP4 mirrors hyperinsulinemia’s atherogenic actions in the insulin resistant state, and genetic deletion of DPP4 protects rodents from developing insulin resistance and improves cardiovascular outcomes. DPP4 inhibition in pro-atherosclerotic preclinical models results in reduced inflammation and oxidative stress, improved endothelial function, and decreased atherosclerosis. Increased incretin levels may have contributed to but do not completely account for these benefits. Small clinical studies with DPP4 inhibitors demonstrate reduced carotid intimal thickening, improved endothelial function, and reduced arterial stiffness. To date, this has not been translated to cardiovascular risk reduction for individuals with type 2 diabetes with prior or exaggerated risk of cardiovascular disease. Conclusion DPP4 may represent a key link between central obesity, insulin resistance, and atherosclerosis. The gaps in knowledge in DPP4 function and discrepancy in cardiovascular outcomes observed in preclinical and large-scale randomized controlled studies with DPP4 inhibitors warrant additional research.

Muscle microvasculature critically regulates skeletal and cardiac muscle health and function. It provides endothelial surface area for substrate exchange between the plasma compartment and the muscle interstitium. Insulin fine-tunes muscle microvascular perfusion to regulate its own action in muscle and oxygen and nutrient supplies to muscle. Specifically, insulin increases muscle microvascular perfusion, which results in increased delivery of insulin to the capillaries that bathe the muscle cells and then facilitate its own transendothelial transport to reach the muscle interstitium. In type 2 diabetes, muscle microvascular responses to insulin are blunted and there is capillary rarefaction. Both loss of capillary density and decreased insulin-mediated capillary recruitment contribute to a decreased endothelial surface area available for substrate exchange. Vasculature expresses abundant glucagon-like peptide 1 (GLP-1) receptors. GLP-1, in addition to its well-characterized glycemic actions, improves endothelial function, increases muscle microvascular perfusion, and stimulates angiogenesis. Importantly, these actions are preserved in the insulin resistant states. Thus, treatment of insulin resistant patients with GLP-1 receptor agonists may improve skeletal and cardiac muscle microvascular perfusion and increase muscle capillarization, leading to improved delivery of oxygen, nutrients, and hormones such as insulin to the myocytes. These actions of GLP-1 impact skeletal and cardiac muscle function and systems biology such as functional exercise capacity. Preclinical studies and clinical trials involving the use of GLP-1 receptor agonists have shown salutary cardiovascular effects and improved cardiovascular outcomes in type 2 diabetes mellitus. Future studies should further examine the different roles of GLP-1 in cardiac as well as skeletal muscle function.肌肉微血管系统对骨骼肌和心肌的健康和功能起着至关重要的调节作用。它为血浆间隙和肌肉间质之间的底物交换提供内皮表面积。胰岛素微调肌肉微血管灌注, 以调节自身在肌肉中的活动, 并为肌肉提供氧气和营养。具体地说, 胰岛素增加了肌肉微血管灌注, 使得更多的胰岛素转运到毛细血管周边的肌肉细胞, 然后通过其自身的透皮转运到达肌肉间质。在2型糖尿病中, 肌肉微血管对胰岛素的反应迟钝, 毛细血管稀疏。毛细血管密度的减少和胰岛素介导的毛细血管募集的降低, 都会导致可用于底物交换的内皮表面积减少。血管系统表达丰富的胰高血糖素样肽-1(GLP-1)受体。GLP-1可以改善内皮功能, 增加肌肉微血管灌注, 并刺激血管生成。重要的是, 这些作用在胰岛素抵抗状态下得以保留。因此, 使用GLP-1受体激动剂治疗胰岛素抵抗患者, 可能会改善骨骼肌和心肌微血管灌注, 增加肌肉的毛细血管, 从而改善氧、营养物质和胰岛素等激素向心肌细胞的输送。GLP-1的这些作用影响骨骼肌和心肌功能以及生物系统, 如功能性运动能力。涉及使用GLP-1受体激动剂的临床前研究和临床试验已经显示了有益的心血管效应, 并改善了2型糖尿病的心血管结局。未来的研究应该进一步研究GLP-1在心肌和骨骼肌功能中的不同作用.

Insulin increases muscle microvascular perfusion and enhances tissue insulin and nutrient delivery. Our aim was to determine phenotypic traits that foretell human muscle microvascular insulin responses. Hyperinsulinemic euglycemic clamps were performed in 97 adult humans who were lean and healthy, had class 1 obesity without comorbidities, or controlled type 1 diabetes without complications. Insulin-mediated whole-body glucose disposal rates (M-value) and insulin-induced changes in muscle microvascular blood volume (ΔMBV) were determined. Univariate and multivariate analyses were conducted to examine bivariate and multivariate relationships between outcomes, ΔMBV and M-value, and predictor variables, body mass index (BMI), total body weight (WT), percent body fat (BF), lean body mass, blood pressure, maximum consumption of oxygen (VO2max), plasma LDL (LDL-C) and HDL cholesterol, triglycerides (TG), and fasting insulin (INS) levels. Among all factors, only M-value (r = 0.23, p = 0.02) and VO2max (r = 0.20, p = 0.047) correlated with ΔMBV. Conversely, INS (r = − 0.48, p ≤ 0.0001), BF (r = − 0.54, p ≤ 0.001), VO2max (r = 0.5, p ≤ 0.001), BMI (r = − 0.40, p 2max correlated with ΔMBV but only M-value provided unique predictive information about ΔMBV. This suggests that metabolic and microvascular insulin responses are important predictors of one another, but most metabolic insulin resistance predictors do not predict microvascular insulin responses.

Endothelium, acting as a barrier, protects tissues against factors that provoke insulin resistance and type 2 diabetes and itself responds to the insult of insulin resistance inducers with altered function. Endothelial insulin resistance and vascular dysfunction occur early in the evolution of insulin resistance-related disease, can co-exist with and even contribute to the development of metabolic insulin resistance, and promote vascular complications in those affected. The impact of endothelial insulin resistance and vascular dysfunction varies depending on the blood vessel size and location, resulting in decreased arterial plasticity, increased atherosclerosis and vascular resistance, and decreased tissue perfusion. Women with insulin resistance and diabetes are disproportionately impacted by cardiovascular disease, likely related to differential sex-hormone endothelium effects. Thus, reducing endothelial insulin resistance and improving endothelial function in the conduit arteries may reduce atherosclerotic complications, in the resistance arteries lead to better blood pressure control, and in the microvasculature lead to less microvascular complications and more effective tissue perfusion. Multiple diabetes therapeutic modalities, including medications and exercise training, improve endothelial insulin action and vascular function. This action may delay the onset of type 2 diabetes and/or its complications, making the vascular endothelium an attractive therapeutic target for type 2 diabetes and potentially type 1 diabetes.

Insulin recruits skeletal and cardiac muscle microvasculature to increase insulin and nutrient delivery, but this action is lost in insulin resistant states. Glucagon-like peptide 1 (GLP-1) improves microvascular perfusion in humans with obesity and the GLP-1 receptor agonist liraglutide attenuates muscle microvascular insulin resistance (IR) in rodents. To examine whether GLP-1 receptor agonism improves insulin’s microvascular actions in humans with acute IR, 18 healthy human subjects randomly received a systemic infusion of either saline or lipid emulsion (20%, 45 mL x 1 h and then 30 mL x 4 h) for 5 h or lipid emulsion for 5 h + liraglutide pre-treatment (0.6, 1.2, then 1.8 mg daily SC each x 2 days) with euglycemic insulin clamp (1 mU/kg/min) superimposed for the last 120 min. Skeletal and cardiac muscle microvascular blood volume (MBV), flow-mediated dilation (FMD), pulse wave velocity (PWV), and augmentation index (AI) were determined before and after insulin clamp. Compared with saline admission, insulin failed to increase MBV following lipid infusion in both skeletal (28.4 ± 14.1% vs. -1.3 ± 9.6% respectively, p=0.011) and cardiac (28.4 ± 14.1% vs. -1.3 ± 8.9%, p=0.027) muscle, consistent with acute IR induced by lipid infusion in both microvascular beds. Liraglutide pre-treatment improved insulin-mediated increases in MBV in the presence of lipid infusion in skeletal (8.9 ± 13.5%, p=0.117 vs. saline) and cardiac (12.6 ± 12.1%, p=0.101 vs. saline) muscle. There was no significant change in PWV, AI, or FMD amongst admissions. Glucose infusion rates were not significantly different amongst admissions (saline 6.02 ± 0.52, lipid 5.58 ± 0.62, vs. liraglutide + lipid 5.46 ± 0.46 mg/kg/min). In conclusion, liraglutide pretreatment effectively ameliorates lipid-induced skeletal and cardiac muscle microvascular IR in healthy humans. These data provide a mechanistic insight into the metabolic and cardiovascular benefits of GLP-1 receptor agonism seen in humans with obesity and type 2 diabetes. Disclosure K. Love: None. L. Jahn: None. L. Hartline: None. Z. Liu: None. Funding American Diabetes Association (1-17-ICST-059 to Z.L.); National Institutes of Health (1R01DK102359-01A1)

Insulin recruits muscle microvasculature which contributes to insulin’s metabolic action but is blunted in humans with obesity. Both exercise and glucagon-like peptide 1 (GLP-1) are potent enhancers of muscle microvascular perfusion and their actions are preserved in the insulin resistant states. To examine whether light exercise and GLP-1 receptor agonism modulate insulin’s microvascular and metabolic actions during the development of obesity, adult male rats were fed a high-fat diet (HFD) for 2 weeks with either free access to running wheel (light exercise), liraglutide (200 mcg/kg subcutaneously twice daily), or light exercise + liraglutide. Rats were then fasted overnight and received a euglycemic insulin clamp (3 mU/kg/min) for 120 min. Insulin-mediated muscle microvascular recruitment was measured using contrast-enhanced ultrasound. Rats on HFD alone (control) displayed marked microvascular insulin resistance. Light exercise increased basal plasma nitric oxide levels and fully rescued insulin’s microvascular action. Liraglutide similarly increased basal plasma NO contents but did not reverse microvascular insulin resistance. Both light exercise and liraglutide improved insulin-stimulated whole body glucose disposal in the first 30 min but did not significantly increase the steady-state glucose infusion rates. Combined light exercise and liraglutide treatment not only fully restored microvascular insulin responses but also significantly enhanced insulin-stimulated whole body glucose disposal (>50%, p Disclosure J. Liu: None. K.W. Aylor: None. Z. Liu: None. Funding American Diabetes Association (1-17-ICTS-059 to Z.L.); National Institutes of Health (R01HL094722, R01DK102359)

Both type 1 and type 2 diabetes adversely affect the microvasculature in multiple organs. Our understanding of the genesis of this injury and of potential interventions to prevent, limit, or reverse injury/dysfunction is continuously evolving. This statement reviews biochemical/cellular pathways involved in facilitating and abrogating microvascular injury. The statement summarizes the types of injury/dysfunction that occur in the three classical diabetes microvascular target tissues, the eye, the kidney, and the peripheral nervous system; the statement also reviews information on the effects of diabetes and insulin resistance on the microvasculature of skin, brain, adipose tissue, and cardiac and skeletal muscle. Despite extensive and intensive research, it is disappointing that microvascular complications of diabetes continue to compromise the quantity and quality of life for patients with diabetes. Hopefully, by understanding and building on current research findings, we will discover new approaches for prevention and treatment that will be effective for future generations.

A high-fat diet (HFD) can rapidly recruit neutrophils to insulin target tissues and within days induce microvascular insulin resistance (IR). Myeloperoxidase (MPO) is highly enriched in neutrophils, can inhibit nitric oxide-mediated vasorelaxation in vitro and is associated with increased cardiovascular disease risk. AZD5904 irreversibly inhibits MPO and in human clinical trials. MPO knockout, or chemical inhibition, blunts HFD-induced metabolic IR in mice. Whether MPO affects microvascular IR or muscle metabolic insulin sensitivity in vivo is unknown. We used contrast-enhanced ultrasound and the euglycemic insulin clamp to test whether inhibiting MPO could prevent the development or reverse established HFD-induced metabolic and/or microvascular IR in Sprague-Dawley rats. Two weeks of HFD feeding blocked insulin-mediated skeletal muscle capillary recruitment, inhibited glucose utilization, and insulin signaling to muscle. Continuous subcutaneous AZD5904 infusion during the 2 wk selectively blocked HFD’s microvascular effect. Furthermore, AZD5904 infusion during the last 2 of 4 wk of HFD feeding restored microvascular insulin sensitivity but not metabolic IR. We conclude that inhibiting MPO selectively improves vascular IR. This selective microvascular effect may connote a therapeutic potential for MPO inhibition in the prevention of vascular disease/dysfunction seen in IR humans.

Latent autoimmune diabetes in adults (LADA) is the most common form of autoimmune diabetes diagnosed in adults. Similar to type 1 diabetes, the prevalence of LADA is impacted by ethnicity and geography. LADA is characterized by β cell loss due to autoimmunity and insulin resistance and has highly heterogeneous clinical features, autoimmunity, and genetics in a glutamic acid decarboxylase antibody (GADA) titre-dependent manner, suggesting LADA is part of a continuum spectrum between type 1 and type 2 diabetes. Although LADA is the most frequent form of autoimmune diabetes diagnosed in adults, clinical trials involving LADA are scarce. Here we review the recent advancements in LADA epidemiology, clinical features, pathogenesis, and interventions. We also highlight the environmental factors that are thought to play an important role in addition to genetics in the pathogenesis of LADA. In the future, high-throughput molecular profiles might shed light on the nature of LADA among the wide spectrum of diabetes and offer new opportunities to identify novel LADA-specific biomarkers.

OBJECTIVE Obesity is associated with microvascular insulin resistance, which is characterized by impaired insulin-mediated microvascular recruitment. Glucagon-like peptide 1 (GLP-1) recruits skeletal and cardiac muscle microvasculature, and this action is preserved in insulin-resistant rodents. We aimed to examine whether GLP-1 recruits microvasculature and improves the action of insulin in obese humans. RESEARCH DESIGN AND METHODS Fifteen obese adults received intravenous infusion of either saline or GLP-1 (1.2 pmol/kg/min) for 150 min with or without a euglycemic insulin clamp (1 mU/kg/min) superimposed over the last 120 min. Skeletal and cardiac muscle microvascular blood volume (MBV), flow velocity and blood flow, brachial artery diameter and blood flow, and pulse wave velocity (PWV) were determined. RESULTS Insulin failed to change MBV or flow in either skeletal or cardiac muscle, confirming the presence of microvascular insulin resistance. GLP-1 infusion alone increased MBV by ∼30% and ∼40% in skeletal and cardiac muscle, respectively, with no change in flow velocity, leading to a significant increase in microvascular blood flow in both skeletal and cardiac muscle. Superimposition of insulin to GLP-1 infusion did not further increase MBV or flow in either skeletal or cardiac muscle but raised the steady-state glucose infusion rate by ∼20%. Insulin, GLP-1, and GLP-1 + insulin infusion did not alter brachial artery diameter and blood flow or PWV. The vasodilatory actions of GLP-1 are preserved in both skeletal and cardiac muscle microvasculature, which may contribute to improving metabolic insulin responses and cardiovascular outcomes. CONCLUSIONS In obese humans with microvascular insulin resistance, GLP-1’s vasodilatory actions are preserved in both skeletal and cardiac muscle microvasculature, which may contribute to improving metabolic insulin responses and cardiovascular outcomes.

Endothelial dysfunction and vascular insulin resistance coexist and both contribute to the development of type 2 diabetes mellitus (T2DM) and increased cardiovascular risks. Insulin relaxes precapillary arterioles to increase insulin delivery to and action in muscle via recruiting muscle microvasculature and this action is impaired in the insulin resistant states. Metformin is the first line therapy for T2DM and has been shown to improve endothelial function beyond its hypoglycemic effects. Here we aim to examine whether metformin could restore vascular insulin responses in the setting of insulin resistance induced by high fat diet (HFD) feeding via the modulation of endothelial function. Adult, male Sprague-Dawley rats were fed on HFD with or without metformin treatment for 2 weeks. A chow fed group served as control. After dietary intervention, rats were fasted overnight and received a euglycemic-hyperinsulinemic clamp for 120 min. Insulin-mediated muscle microvascular recruitment was assessed before and during insulin infusion using contrast-enhanced ultrasound. Distal saphenous artery was isolated and used for determination of endothelial function and vascular insulin responses. Compared with controls, HFD feeding abolished insulin-induced muscle microvascular recruitment in muscle, decreased insulin-mediated whole body glucose disposal and blunted the vasodilatory response to insulin in small resistance vessel. Metformin treatment significantly enhanced insulin’s microvascular action, effectively rescued small arteriole endothelial function, but did not improve insulin-stimulated whole body glucose disposal in HFD rats. We conclude that metformin could reverse microvascular insulin resistance and attenuate endothelial dysfunction induced by HFD feeding. There findings may provide a novel mechanistic insight into metformin’s cardiovascular protective effect in patients with diabetes. Disclosure J. Liu: None. W. Chai: None. L. Gong: None. K.W. Aylor: None. Z. Liu: None. Funding American Diabetes Association (1-17-ICTS-059 to Z.L.); National Institutes of Health (R01HL094722, R01DK102359)

Insulin is a vascular hormone and regulates vascular tone and reactivity. Muscle is a major insulin target that is responsible for the majority of insulin-stimulated glucose use. Evidence confirms that muscle microvasculature is an important insulin action site and critically regulates insulin delivery to muscle and action on myocytes, thereby affecting insulin-mediated glucose disposal. Insulin via activation of its signaling cascade in the endothelial cells increases muscle microvascular perfusion, which leads to an expansion of the endothelial exchange surface area. Insulin’s microvascular actions closely couple with its metabolic actions in muscle and blockade of insulin-mediated microvascular perfusion reduces insulin-stimulated muscle glucose disposal. Type 2 diabetes is associated with chronic low-grade inflammation, which engenders both metabolic and microvascular insulin resistance through endocrine, autocrine and paracrine actions of multiple pro-inflammatory factors. Here, we review the crucial role of muscle microvasculature in the regulation of insulin action in muscle and how inflammation in the muscle microvasculature affects insulin’s microvascular actions as well as metabolic actions. We propose that microvascular insulin resistance induced by inflammation is an early event in the development of metabolic insulin resistance and eventually type 2 diabetes and its related cardiovascular complications, and thus is a potential therapeutic target for the prevention or treatment of obesity and diabetes.

Aims To investigate the prevalence of adult-onset autoimmune diabetes (ADM) and predisposition to autoimmune diseases by quantifying serum organ-specific autoantibodies in people with phenotype of type 2 diabetes (T2D). Materials and methods We included a nationally representative sample of 46 239 adults aged ≥20 years from 14 provinces, of whom 4671 had diabetes, plus 1000 control subjects with normal glucose tolerance (NGT). Participants were screened centrally for autoantibodies to glutamic acid decarboxylase (GAD), islet antigen 2 (IA2) and zinc transporter isoform-8 (Znt8) and were defined as having ADM where positive for these antibodies. We then assayed thyroid peroxidase (TPO), tissue transglutaminase (tTG) and 21-hydroxylase (21-OH) autoantibodies in randomly selected participants with ADM and in age-matched, sex-matched and non-ADM controls with T2D plus controls with NGT. Results Post-normalization, the standardized prevalence rate of ADM was 6.0% (95% confidence interval [CI] 5.3-6.8) in initially non-insulin-requiring participants with ADM, corresponding to six million adults in China, in whom adjusted antibody positivity was: TPO autoantibodies 16.3% (95% CI 10.8-21.8), tTG autoantibodies 2.1% (95% CI 0.0-4.2), and 21-OH autoantibodies 1.8% (95% CI -0.2 to 3.8). Those participants with ADM who were GAD autoantibody-positive had high risk of TPO autoantibody positivity (odds ratio [OR] 2.39, P = 0.0031) and tTG autoantibody positivity (OR 6.98, P = 0.027), while those positive for IA2 autoantibodies had a high risk of tTG autoantibody positivity (OR 19.05, P = 0.001). Conclusions A proportion of people with phenotype of T2D in China have ADM, with diabetes-associated autoantibodies, and may be at risk of developing other organ-specific autoimmune diseases; therefore, it may be clinically relevant to consider screening such Chinese populations.

Obesity is associated with metabolic and microvascular insulin resistance. The latter is characterized by impaired insulin-mediated microvascular recruitment. GLP-1 engenders vasodilation in addition to its glycemic actions. At physiological concentrations, GLP-1 acutely recruits skeletal and cardiac muscle microvasculature in healthy humans and this action is preserved in the insulin resistant rodents. To examine whether GLP-1 recruits microvasculature and improves insulin’s action in obese humans, 15 obese adults (age 27 ± 2.2, BMI 33.7 ± 1.2) were studied thrice at random order after an overnight fast. Subjects received an intravenous infusion of GLP-1 (1.2 pmol/kg/min) for 150 mins or an intravenous infusion of either normal saline or GLP-1 (1.2 pmol/kg/min) for 150 mins with a euglycemic insulin clamp (1 mU/kg/min) superimposed over the last 120 mins. Skeletal and cardiac muscle microvascular blood volume (MBV), flow velocity (MFV) and blood flow (MBF) were determined at 0, 30 and 150 min. Steady-state glucose infusion rate (GIR) was calculated. Insulin infusion alone did not change MBV or MBF in either skeletal or cardiac muscle, confirming the presence of microvascular insulin resistance. GLP-1 infusion alone increased MBV by ∼30% and ∼40% in skeletal and cardiac muscle respectively (p We conclude that in obese humans with microvascular insulin resistance GLP-1's vasodilatory actions are preserved in both skeletal and cardiac muscle microvasculature. This may contribute to improving metabolic responses to insulin and decreasing cardiovascular morbidity and mortality associated with obesity and diabetes. Disclosure N. Wang: None. A. Tan: None. L. Jahn: None. L. Hartline: None. K.W. Aylor: None. E. Barrett: None. Z. Liu: None.

Diabetes mellitus (DM) has been proven to be a key risk factor for cognitive impairment. Previous studies have implicated hippocampal neuronal apoptosis in diabetes-related cognitive impairment. However, the underlying mechanism remains unknown. Sirtuin 1 (SIRT1) is a protein deacetylase depended on nicotinamide adenine dinucleotide. Furthermore, it is indispensable in normal learning and memory. Whether SIRT1 is taken part in diabetes-induced neuronal apoptosis and thus involve in the development of diabetic cognitive impairment is still not clear. To address this issue, we examined the possible role of SIRT1 in hippocampal neuronal apoptosis in streptozotocin-induced diabetic mice. Furthermore, the possible mechanism was investigated in high glucose-induced SH-SY5Y cells. We found that downregulation of the activity and expression of SIRT1 was associated with increased hippocampal neuronal apoptosis in mice. In vitro, cell apoptosis induced by high glucose which was accompanied by a downregulation of SIRT1 and an increased acetylation of p53. On the contrary, activation of SIRT1 using its agonist resveratrol ameliorated cell apoptosis via deacetylating p53. Our data suggest that high concentration of glucose can induce neuronal apoptosis through downregulation of SIRT1 and increased acetylation of p53, which likely contribute to the development of cognitive impairment in diabetes.

Obesity and diabetes are associated with inflammation, endothelial dysfunction and insulin resistance in the muscle microvasculature. Inflammation-induced microvascular insulin resistance is an early event and plays a causative role in the development of metabolic insulin resistance in diet-induced obesity., Endothelial dysfunction and vascular insulin resistance usually coexist and chronic inflammation engenders both. In the present study, we investigate the temporal relationship between vascular insulin resistance and metabolic insulin resistance. We assessed insulin responses in all arterial segments, including aorta, distal saphenous artery and the microvasculature, as well as the metabolic insulin responses in muscle in rats fed on a high-fat diet (HFD) for various durations ranging from 3 days to 4 weeks with or without sodium salicylate treatment. Compared with controls, HFD feeding significantly blunted insulin-mediated Akt (protein kinase B) and eNOS [endothelial nitric oxide (NO) synthase] phosphorylation in aorta in 1 week, blunted vasodilatory response in small resistance vessel in 4 weeks and microvascular recruitment in as early as 3 days. Insulin-stimulated whole body glucose disposal did not begin to progressively decrease until after 1 week. Salicylate treatment fully inhibited vascular inflammation, prevented microvascular insulin resistance and significantly improved muscle metabolic responses to insulin. We conclude that microvascular insulin resistance is an early event in diet-induced obesity and insulin resistance and inflammation plays an essential role in this process. Our data suggest microvascular insulin resistance contributes to the development of metabolic insulin resistance in muscle and muscle microvasculature is a potential therapeutic target in the prevention and treatment of diabetes and its related complications.

Insulin enhances the compliance of conduit arteries, relaxes resistance arterioles to increase tissue blood flow, and dilates precapillary arterioles to expand muscle microvascular blood volume. These actions are impaired in the insulin resistant states. Exercise ameliorates endothelial dysfunction and improves insulin responses in insulin resistant patients, but the precise underlying mechanisms remain unclear. The microvasculature critically regulates insulin action in muscle by modulating insulin delivery to the capillaries nurturing the myocytes and trans-endothelial insulin transport. Recent data suggest that exercise may exert its insulin-sensitizing effect via recruiting muscle microvasculature to increase insulin delivery to and action in muscle. The current review focuses on how the interplay among exercise, insulin action, and the vasculature contributes to exercise-mediated insulin sensitization in muscle.

BACKGROUND Latent autoimmune diabetes in adults (LADA) exhibits significant clinical heterogeneity, but the underlying causes remain unclear. The aim of this study was to investigate whether age of onset of LADA contributes to the observed clinical heterogeneity by comparing the clinical, metabolic, and immunogenetic characteristics between elderly and young LADA patients. METHODS The cross-sectional study included a total of 579 patients with LADA which was further divided into elderly LADA (E-LADA) group (n = 135, age of onset ≥60 years) and young LADA (Y-LADA) group (n = 444, age of onset

Angiotensin II receptors regulate muscle microvascular recruitment and the delivery of nutrients, oxygen, and insulin to muscle. Although angiotensin type 1 receptor antagonism increases muscle microvascular perfusion and insulin action, angiotensin type 2 receptor blockade markedly restricts muscle microvascular blood volume and decreases muscle delivery of insulin. To examine the effects of direct type 2 receptor stimulation using Compound 21 (C21) on microvascular perfusion, insulin delivery and action, and tissue oxygenation in muscle, overnight-fasted adult male rats were infused with C21 systemically. C21 potently increased microvascular blood volume without altering microvascular flow velocity or blood pressure, resulting in a net increase in microvascular blood flow in muscle. This was associated with a substantial increase in muscle interstitial oxygen saturation and insulin delivery into the skeletal and cardiac muscle. These effects were neutralized by coinfusion of the type 2 receptor antagonist or nitric oxide synthase inhibitor. Superimposing C21 infusion on insulin infusion increased insulin-mediated whole body glucose disposal by 50%. C21 significantly relaxed the preconstricted distal saphenous artery ex vivo. We have concluded that direct type 2 receptor stimulation markedly increases muscle microvascular perfusion through nitric oxide biosynthesis and enhances insulin delivery and action in muscle. These findings provide a physiologic mechanistic insight into type 2 receptor modulation of insulin action and suggest that type 2 receptor agonists might have therapeutic potential in the management of diabetes and its associated complications.

We have previously shown that insulin plays an important role in the nutrient-induced insulin resistance. In this study, we tested the hypothesis that chronic exposure to excess long-acting insulin (glargine) can cause typical type 2 diabetes mellitus (T2DM) in normal mice fed on a chow diet. C57BL/6 mice were treated with glargine once a day for 8 weeks, followed by evaluations of food intake, body weight, blood levels of glucose, insulin, lipids, and cytokines, insulin signaling, histology of pancreas, ectopic fat accumulation, oxidative stress level, and cholesterol content in mitochondria in tissues. Cholesterol content in mitochondria and its association with oxidative stress in cultured hepatocytes and β-cells were also examined. Results show that chronic exposure to glargine caused insulin resistance, hyperinsulinemia, and relative insulin deficiency (T2DM). Treatment with excess glargine led to loss of pancreatic islets, ectopic fat accumulation in liver, oxidative stress in liver and pancreas, and increased cholesterol content in mitochondria of liver and pancreas. Prolonged exposure of cultured primary hepatocytes and HIT-TI5 β-cells to insulin induced oxidative stress in a cholesterol synthesis-dependent manner. Together, our results show that chronic exposure to excess insulin can induce typical T2DM in normal mice fed on a chow diet.

Key points • Ranolazine, an anti-anginal compound, improves glycaemic control in clinical trials and increases myocardial perfusion via a direct vasodilatatory effect on coronary arteries. • Skeletal muscle microvasculature controls the delivery of nutrient and hormones into muscle and their exchanges between plasma and muscle interstitium by providing microvascular exchange surface area. • In this study we examined whether ranolazine improves glycaemic control via exerting vasodilatatory action on the pre-capillary arterioles to recruit muscle microvasculature. • We demonstrate that ranolazine potently recruits muscle microvasculature, which expands microvascular endothelial surface area in muscle and results in increased muscle delivery and action of insulin. • The results help us better understand the physiological mechanism by which ranolazine improves glycaemic control and its potential as an insulin-sensitizing agent. Abstract Ranolazine, an anti-anginal compound, has been shown to significantly improve glycaemic control in large-scale clinical trials, and short-term ranolazine treatment is associated with an improvement in myocardial blood flow. As microvascular perfusion plays critical roles in insulin delivery and action, we aimed to determine if ranolazine could improve muscle microvascular blood flow, thereby increasing muscle insulin delivery and glucose use. Overnight-fasted, anaesthetized Sprague-Dawley rats were used to determine the effects of ranolazine on microvascular recruitment using contrast-enhanced ultrasound, insulin action with euglycaemic hyperinsulinaemic clamp, and muscle insulin uptake using 125I-insulin. Ranolazine's effects on endothelial nitric oxide synthase (eNOS) phosphorylation, cAMP generation and endothelial insulin uptake were determined in cultured endothelial cells. Ranolazine-induced myographical changes in tension were determined in isolated distal saphenous artery. Ranolazine at therapeutically effective dose significantly recruited muscle microvasculature by increasing muscle microvascular blood volume (∼2-fold, P < 0.05) and increased insulin-mediated whole body glucose disposal (∼30%, P= 0.02). These were associated with an increased insulin delivery into the muscle (P < 0.04). In cultured endothelial cells, ranolazine increased eNOS phosphorylation and cAMP production without affecting endothelial insulin uptake. In ex vivo studies, ranolazine exerted a potent vasodilatatory effect on phenylephrine pre-constricted arterial rings, which was partially abolished by endothelium denudement. In conclusion, ranolazine treatment vasodilatates pre-capillary arterioles and increases microvascular perfusion, which are partially mediated by endothelium, leading to expanded microvascular endothelial surface area available for nutrient and hormone exchanges and resulting in increased muscle delivery and action of insulin. Whether these actions contribute to improved glycaemic control in patients with insulin resistance warrants further investigation.

Insulin delivery and transendothelial insulin transport are two discrete steps that limit muscle insulin action. Angiotensin II type 1 receptor (AT1R) blockade recruits microvasculature and increases glucose use in muscle. Increased muscle microvascular perfusion is associated with increased muscle delivery and action of insulin. To examine the effect of acute AT1R blockade on muscle insulin uptake and action, rats were studied after an overnight fast to examine the effects of losartan on muscle insulin uptake ( protocol 1), microvascular perfusion ( protocol 2), and insulin's microvascular and metabolic actions in the state of insulin resistance ( protocol 3). Endothelial cell insulin uptake was assessed, using125I-insulin as tracer. Systemic lipid infusion was used to induce insulin resistance. Losartan significantly increased muscle insulin uptake (∼60%, P < 0.03), which was associated with a two- to threefold increase in muscle microvascular blood volume (MBV; P = 0.002) and flow (MBF; P = 0.002). Losartan ± angiotensin II had no effect on insulin internalization in cultured endothelial cells. Lipid infusion abolished insulin-mediated increases in muscle MBV and MBF and lowered insulin-stimulated whole body glucose disposal ( P = 0.0001), which were reversed by losartan administration. Inhibition of nitric oxide synthase abolished losartan-induced muscle insulin uptake and reversal of lipid-induced metabolic insulin resistance. We conclude that AT1R blockade increases muscle insulin uptake mainly via microvascular recruitment and rescues insulin's metabolic action in the insulin-resistant state. This may contribute to the clinical findings of decreased cardiovascular events and new onset of diabetes in patients receiving AT1R blockers.

Vascular endothelium is an important insulin target and plays a pivotal role in the development of metabolic insulin resistance provoked by the Western lifestyle. It acts as a “first-responder” to environmental stimuli such as nutrients, cytokines, chemokines and physical activity and regulates insulin delivery to muscle and adipose tissue and thereby affecting insulin-mediated glucose disposal by these tissues. In addition, it also regulates the delivery of insulin and other appetite regulating signals from peripheral tissues to the central nervous system thus influencing the activity of nuclei that regulate hepatic glucose production, adipose tissue lipolysis and lipogenesis, as well as food consumption. Resistance to insulin’s vascular actions therefore broadly impacts tissue function and contribute to metabolic dysregulation. Moreover, vascular insulin resistance negatively impacts vascular health by affecting blood pressure regulation, vessel wall inflammation and atherogenesis thereby contributing to the burden of vascular disease seen with diabetes and metabolic syndrome. In the current review, we examined the evidence that supports the general concept of vascular endothelium as a target of insulin action and discussed the biochemical and physiological consequences of vascular insulin resistance.

Insulin action on hippocampus improves cognitive function, and obesity and type 2 diabetes are associated with decreased cognitive function. Cerebral microvasculature plays a critical role in maintaining cerebral vitality and function by supplying nutrients, oxygen, and hormones such as insulin to cerebral parenchyma, including hippocampus. In skeletal muscle, insulin actively regulates microvascular opening and closure, and this action is impaired in the insulin-resistant states. To examine insulin’s action on hippocampal microvasculature and parenchyma and the impact of diet-induced obesity, we determined cognitive function and microvascular insulin responses, parenchyma insulin responses, and capillary density in the hippocampus in 2- and 8-mo-old rats on chow diet and 8-mo-old rats on a long-term high-fat diet (6 mo). Insulin infusion increased hippocampal microvascular perfusion in rats on chow diet by ~80–90%. High-fat diet feeding completely abolished insulin-mediated microvascular responses and protein kinase B phosphorylation but did not alter the capillary density in the hippocampus. This was associated with a significantly decreased cognitive function assessed using both the two-trial spontaneous alternation behavior test and the novel object recognition test. As the microvasculature provides the needed endothelial surface area for delivery of nutrients, oxygen, and insulin to hippocampal parenchyma, we conclude that hippocampal microvascular insulin resistance may play a critical role in the development of cognitive impairment seen in obesity and diabetes. Our results suggest that improvement in hippocampal microvascular insulin sensitivity might help improve or reverse cognitive function in the insulin-resistant states.

Muscle microvasculature critically regulates endothelial exchange surface area to facilitate transendothelial delivery of insulin, nutrients, and oxygen to myocytes. Insulin resistance blunts insulin-mediated microvascular recruitment and decreases muscle capillary density; both contribute to lower microvascular blood volume. Glucagon-like peptide 1 (GLP-1) and its analogs are able to dilate blood vessels and stimulate endothelial cell proliferation. In this study, we aim to determine the effects of sustained stimulation of the GLP-1 receptors on insulin-mediated capillary recruitment and metabolic insulin responses, small arterial endothelial function, and muscle capillary density. Rats were fed a high-fat diet (HFD) for 4 wk with or without simultaneous administration of liraglutide and subjected to a euglycemic hyperinsulinemic clamp for 120 min after an overnight fast. Insulin-mediated muscle microvascular recruitment and muscle oxygenation were determined before and during insulin infusion. Muscle capillary density was determined and distal saphenous artery used for determination of endothelial function and insulin-mediated vasodilation. HFD induced muscle microvascular insulin resistance and small arterial vessel endothelial dysfunction and decreased muscle capillary density. Simultaneous treatment of HFD-fed rats with liraglutide prevented all of these changes and improved insulin-stimulated glucose disposal. These were associated with a significantly increased AMPK phosphorylation and the expressions of VEGF and its receptors. We conclude that GLP-1 receptor agonists may exert their salutary glycemic effect via improving microvascular insulin sensitivity and muscle capillary density during the development of insulin resistance, and early use of GLP-1 receptor agonists may attenuate metabolic insulin resistance as well as prevent cardiovascular complications of diabetes.

Prediabetes is a heterogeneous term that encompasses different origins of insulin resistance and insulin secretion that contribute to distinct patterns of hyperglycemia. In fact, prediabetes is an umbrella term that characterizes individuals at high risk for developing type 2 diabetes (T2D) and/or cardiovascular disease (CVD). Based on current definitions there are at least 3 distinct phenotypes of prediabetes: impaired fasting glucose (IFG), impaired glucose tolerant (IGT), or the combination of both (IFG + IGT). Each phenotype is clinically relevant as they are uniquely recognized as having different levels of risk for progressing to T2D and CVD. Herein, we discuss the underlying pathophysiology that characterizes IFG, IGT and the combination, as well as examine how some of these phenotypes appear resistant to traditional exercise interventions. We propose that substrate metabolism differences between the prediabetes phenotypes may be a unifying mechanism that explains the inter-subject variation in response to exercise seen across obese, metabolic syndrome, pre-diabetic and T2D patients in the current literature. Ultimately, a better understanding of the pathophysiologic mechanisms that govern disturbances responsible for fasting vs. postprandial hyperglycemia and the combination of both is important for designing optimal and personalized exercise treatment strategies that treat and prevent hyperglycemia and CVD risk.

Angiotensin II type 1 receptor (AT(1)R) tone restricts muscle microvascular blood volume (MBV) and decreases muscle insulin delivery and glucose use.The objective of the study was to examine whether acute AT(1)R blockade alters microvascular perfusion in skeletal and cardiac muscle in humans.The study was conducted at the General Clinical Research Center at the University of Virginia.Eight overnight-fasted healthy young adults were studied thrice in random order. In study 1, each subject received candesartan (32 mg) orally at time 0. In study 2, each subject received placebo at time 0 and a 1 mU/min · kg euglycemic insulin clamp from time 240 to 360 min. In study 3, each subject received candesartan (32 mg) orally at time 0 and insulin infusion from 240 to 360 min. Forearm skeletal and cardiac muscle MBV, microvascular flow velocity, and microvascular blood flow (MBF) were determined at baseline and at 240 and 360 min.Candesartan treatment acutely recruited microvasculature in both skeletal and cardiac muscle by significantly increasing MBV (P0.03 and P = 0.02, respectively) and MBF (P0.03 for both) without altering microvascular flow velocity. Insulin infusion significantly increased cardiac MBV (P = 0.02) and MBF (P0.02). Superimposing insulin infusion 4 h after candesartan ingestion did not further recruit microvasculature. Insulin-mediated whole-body glucose disposal did not differ with or without candesartan pretreatment.Acute AT(1)R blockade with candesartan recruits skeletal as well as cardiac muscle microvasculature in healthy humans without altering insulin-mediated whole-body glucose disposal. This may contribute to the observed improvement in the cardiovascular outcomes in patients receiving prolonged treatment with AT(1)R blockers.

Insulin increases microvascular perfusion and substrate exchange surface area in muscle, which is pivotal for hormone action and substrate exchange, by activating insulin signaling cascade in the endothelial cells to produce nitric oxide. This action of insulin is closely coupled with its metabolic action and type 2 diabetes is associated with both metabolic and microvascular insulin resistance. Muscle microvascular perfusion/volume can be assessed by 1-methylxanthine metabolism, contrast-enhanced ultrasound and positron emission tomography. In addition to insulin, several factors have been shown to recruit muscle microvasculature, including exercise or muscle contraction, mixed meals, glucagon-like peptide 1 and angiotensin II type 1 receptor (AT(1)R) blocker. On the other hand, factors that cause metabolic insulin resistance, such as inflammatory cytokines, free fatty acids, and selective activation of the AT(1)R, are capable of causing microvascular insulin resistance. Therapies targeting microvascular insulin resistance may help prevent or control diabetes and decrease the associated cardiovascular morbidity and mortality.

OBJECTIVE Angiotensin (ANG) II interacts with insulin-signaling pathways to regulate insulin sensitivity. The type 1 (AT1R) and type 2 (AT2R) receptors reciprocally regulate basal perfusion of muscle microvasculature. Unopposed AT2R activity increases muscle microvascular blood volume (MBV) and glucose extraction, whereas unopposed AT1R activity decreases both. The current study examined whether ANG II receptors modulate muscle insulin delivery and sensitivity. RESEARCH DESIGN AND METHODS Overnight-fasted rats were studied. In protocol 1, rats received a 2-h infusion of saline, insulin (3 mU/kg/min), insulin plus PD123319 (AT2R blocker), or insulin plus losartan (AT1R blocker, intravenously). Muscle MBV, microvascular flow velocity, and microvascular blood flow (MBF) were determined. In protocol 2, rats received 125I-insulin with or without PD123319, and muscle insulin uptake was determined. RESULTS Insulin significantly increased muscle MBV and MBF. AT2R blockade abolished insulin-mediated increases in muscle MBV and MBF and decreased insulin-stimulated glucose disposal by ~30%. In contrast, losartan plus insulin increased muscle MBV by two- to threefold without further increasing insulin-stimulated glucose disposal. Plasma nitric oxide increased by >50% with insulin and insulin plus losartan but not with insulin plus PD123319. PD123319 markedly decreased muscle insulin uptake and insulin-stimulated Akt phosphorylation. CONCLUSIONS We conclude that both AT1Rs and AT2Rs regulate insulin’s microvascular and metabolic action in muscle. Although AT1R activity restrains muscle metabolic responses to insulin via decreased microvascular recruitment and insulin delivery, AT2R activity is required for normal microvascular responses to insulin. Thus, pharmacologic manipulation aimed at increasing the AT2R-to-AT1R activity ratio may afford the potential to improve muscle insulin sensitivity and glucose metabolism.

The main goal of this review is to provide more specific and effective targets for prevention and treatment of insulin resistance and associated atherosclerosis. Modern technologies and medicine have vastly improved human health and prolonged the average life span of humans primarily by eliminating various premature deaths and infectious diseases. The modern technologies have also provided us abundant food and convenient transportation tools such as cars. As a result, more people are becoming overfed and sedentary. People are generally ingesting more calories than their bodies' need, leading to the so-called "positive energy imbalance", which is inseparable from the development of insulin resistance and its associated atherosclerosis. A direct consequence of insulin resistance is hyperinsulinemia. The current general view is that insulin is not functional properly in the presence of insulin resistance. Thus, the role of insulin itself in the development of insulin resistance and associated atherosclerosis has not been recognized. We have recently observed that the basal level of insulin signaling is increased in the presence of insulin resistance and hyperinsulinemia. In this review, we will explain how the increased basal insulin signaling contributes to the development of insulin resistance and associated atherosclerosis. We will first explain how insulin causes insulin resistance through two arbitrary stages (before and after the presence of obvious insulin resistance), and, then, explain how the excess exposure to insulin and the relative insulin insufficiency contributes to the atherosclerotic diseases. We propose that blockade of the excess insulin signaling is a viable approach to prevent and/or reverse insulin resistance and its associated atherosclerosis.

Resveratrol, a polyphenol found in many plants, has antioxidant and anti-inflammatory actions. It also improves endothelial function and may be cardioprotective. Tumor necrosis factor-α (TNFα) causes oxidative stress and microvascular endothelial dysfunction. Whether resveratrol affects microvascular function in vivo and, if so, whether inflammatory cytokines antagonize its microvascular action are not clear. In cultured bovine aortic endothelial cells (BAECs), reserveratrol (100 nM) increased the phosphorylation of protein kinase B (Akt), endothelial nitric oxide (NO) synthase (eNOS), and ERK1/2 within 15 min by more than twofold, and this effect lasted for at least 2 h. Treatment of BAECs with TNFα (10 ng/ml) significantly increased the NADPH oxidase activity and the production of hydrogen peroxide and superoxide. Pretreatment of cells with resveratrol (100 nM) prevented each of these. Injection (ip) of resveratrol in rats potently increased muscle microvascular blood volume (MBV; P = 0.007) and flow (MBF; P < 0.02) within 30 min, and this was sustained for at least 2 h. The phosphorylation of Akt in liver or muscle was unchanged. Superimposed systemic infusion of l-NAME (NOS inhibitor) completely abolished resveratrol-induced increases in MBV and MBF. Similarly, systemic infusion of TNFα prevented resveratrol-induced muscle microvascular recruitment. In conclusion, resveratrol activates eNOS and increases muscle microvascular recruitment via an NO-dependent mechanism. Despite the potent antioxidant effect of resveratrol, TNFα at concentrations that block insulin-mediated muscle microvascular recruitment completely neutralized resveratrol's microvascular action. Thus, chronic inflammation, as seen in type 2 diabetes, may limit resveratrol's vasodilatory actions on muscle microvasculature.

Type 2 diabetes is in essence a vascular disease and is frequently associated with hypertension, macrovascular events, and microvascular complications. Microvascular dysfunction, including impaired recruitment and capillary rarefaction, has been implicated in the pathogenesis of diabetic complications. Microvascular insulin resistance and renin-angiotensin system upregulation are present in diabetes, and each contributes to the development of hypertension and microvascular dysfunction. In the insulin-sensitive state, insulin increases microvascular perfusion by increasing endothelial nitric oxide production, but this effect is abolished by insulin resistance. Angiotensin II, acting via the type 1 receptors, induces inflammation and oxidative stress, leading to impaired insulin signaling, reduced nitric oxide availability, and vasoconstriction. Conversely, it acts on the type 2 receptors to cause vasodilatation. Because substrate and hormonal exchanges occur in the microvasculature, antihypertensive agents targeted to improve microvascular insulin sensitivity and function may have beneficial effects beyond their capacity to lower blood pressure in patients with diabetes.

Angiotensin II causes vasoconstriction via the type 1 receptor (AT 1 R) and vasodilatation through the type 2 receptor (AT 2 R). Both are expressed in muscle microvasculature, where substrate exchanges occur. Whether they modulate basal muscle microvascular perfusion and substrate metabolism is not known. We measured microvascular blood volume (MBV), a measure of microvascular surface area and perfusion, in rats during systemic infusion of angiotensin II at either 1 or 100 ng/kg per minute. Each caused a significant increase in muscle MBV. Likewise, administration of the AT 1 R blocker losartan increased muscle MBV by >3-fold ( P 2 R antagonist PD123319 significantly decreased muscle MBV by ≥80% ( P 2 R antagonism and inhibition of NO synthase each blocked the losartan-induced increase in muscle MBV and glucose uptake. In conclusion, angiotensin II acts on both AT 1 R and AT 2 R to regulate basal muscle microvascular perfusion. Basal AT 1 R tone restricts muscle MBV and glucose extraction, whereas basal AT 2 R activity increases muscle MBV and glucose uptake. Pharmacological manipulation of the balance of AT 1 R and AT 2 R activity affords the potential to improve glucose metabolism.

Autophagy is essential for maintaining both survival and health of cells. Autophagy is normally suppressed by amino acids and insulin. It is unclear what happens to the autophagy activity in the presence of insulin resistance and hyperinsulinemia. In this study, we examined the autophagy activity in the presence of insulin resistance and hyperinsulinemia and the associated mechanism. Insulin resistance and hyperinsulinemia were induced in mice by a high fat diet, followed by measurements of autophagy markers. Our results show that autophagy was suppressed in the livers of mice with insulin resistance and hyperinsulinemia. Transcript levels of some key autophagy genes were also suppressed in the presence of insulin resistance and hyperinsulinemia. Conversely, autophagy activity was increased in the livers of mice with streptozotocin-induced insulin deficiency. Levels of vps34, atg12, and gabarapl1 transcripts were elevated in the livers of mice with insulin deficiency. To study the mechanism, autophagy was induced by nutrient deprivation or glucagon in cultured hepatocytes in the presence or absence of insulin. Autophagy activity and transcript levels of vps34, atg12, and gabarapl1 genes were reduced by insulin. The effect of insulin was largely prevented by overexpression of the constitutive nuclear form of FoxO1. Importantly, autophagy of mitochondria (mitophagy) in cultured cells was suppressed by insulin in the presence of insulin resistance. Together, our results show that autophagy activity and expression of some key autophagy genes were suppressed in the presence of insulin resistance and hyperinsulinemia. Insulin suppression of autophagy involves FoxO1-mediated transcription of key autophagy genes.

Subjects with type 1 diabetes mellitus (T1DM) eventually develop insulin resistance and other features of T2DM such as cardiovascular disorders. The exact mechanism has been not been completely understood. In this study, we tested the hypothesis that excessive or inappropriate exposure to insulin is a primary mediator of insulin resistance in T1DM. We found that continuous exposure of mice with non-obese diabetes to insulin detemir, which is similar to some current conventional treatment of human T1DM, induced severe insulin resistance, whereas untreated hyperglycemia for the same amount of time (2 weeks) did not cause obvious insulin resistance. Insulin resistance was accompanied by decreased mitochondrial production as evaluated by mitochondrial DNA and levels of transcripts and proteins of mitochondrion-associated genes, increased ectopic fat accumulation in liver and skeletal muscle (gastrocnemius) evaluated by measurements of triglyceride content, and elevated oxidative stress detected by the GSH/GSSG ratio. Prolonged exposure of cultured hepatocytes to insulin induced significant insulin resistance, whereas the same length of exposure to a high level of glucose (33 mm) did not cause obvious insulin resistance. Furthermore, our results showed that prolonged exposure to insulin caused oxidative stress, and blockade of mitochondrion-derived oxidative stress by overexpression of manganese-superoxide dismutase prevented insulin resistance induced by the prolonged exposure to insulin. Together, our results show that excessive exposure to insulin is a primary inducer of insulin resistance in T1DM in mice.

Insulin recruits muscle microvasculature, which increases the endothelial exchange surface area to facilitate substrate delivery. Elevated plasma concentrations of free fatty acids (FFAs) cause insulin resistance.The aim of the study was to examine whether FFAs cause insulin resistance in human muscle microvasculature.The study was conducted at the General Clinical Research Center at the University of Virginia.Twenty-two healthy subjects were studied under two protocols designed to raise plasma insulin concentrations to postprandial levels using either an insulin infusion or a mixed meal challenge. Within each protocol, subjects were studied twice. In random order, they received a 5-h systemic infusion of either saline or Intralipid/heparin. Three hours into the infusion, baseline muscle microvascular blood volume (MBV), microvascular flow velocity, and microvascular blood flow (MBF) were measured. Each subject was then given either the mixed meal or a 1 mU/kg x min insulin clamp for 2 h. Microvascular parameters were again obtained 2 h after the meal or at the end of insulin infusion.Meal feeding and insulin infusion raised plasma insulin concentrations to approximately 200 pm, and each significantly increased muscle MBV (P = 0.03 and P0.01, respectively). MBF trended up after meal feeding (P = 0.08) and increased significantly after insulin infusion (P = 0.02). In the presence of Intralipid, neither the meal nor the insulin infusion increased muscle MBV and MBF.Compared to saline, lipid infusion raises plasma FFA concentrations and blocks the ability of insulin or meal to recruit muscle microvasculature. High plasma FFA concentrations may contribute to muscle insulin resistance and the microvascular complications of diabetes.

Insulin is the central regulator of metabolism and is necessary for storing energy as fat efficiently. Mitochondria are primary sites of energy consumption of most cells. Increased plasma insulin level and mitochondrial dysfunction are features of insulin resistance. The exact role of insulin in regulation of mitochondrial production and function remains unestablished. In this study, we observed that mitochondrial production in liver and skeletal muscle gastrocnemius was increased in mice with insulin deficiency (streptozotocin-induced type 1 diabetes). In contrast, prolonged exposure (24 h) of isolated hepatocytes to insulin decreased mitochondrial mass, mitochondrial DNA (mtDNA), intracellular ATP content, and cellular O2 consumption. Transcript levels of genes associated with mitochondrial production and β oxidation were decreased, whereas those of lipogenic genes were increased by the prolonged exposure to insulin. Insulin-induced changes in mtDNA, mitochondrial mass, intracellular ATP content, and transcripts of mitochondrion-associated genes were prevented by blockade of Akt activation with the phosphatidylinositol 3-kinase inhibitor LY294002. Conversely, levels of mtDNA, intracellular ATP content, and expression of mitochondrion-associated genes were decreased by overexpression of the constitutively active Akt. Finally, insulin suppression of mtDNA, ATP production, and expression of mitochondrion-related genes was largely prevented by inhibition of cyclic nucleotide phosphodiesterase with isobutylmethylxanthine. Together, our results show prolonged exposure of isolated hepatocytes to insulin suppresses mitochondrial production and function through the classical Akt-dependent insulin signaling pathway.

Evidence suggests that insulin delivery to skeletal muscle interstitium is the rate-limiting step in insulin-stimulated muscle glucose uptake and that this process is impaired by insulin resistance. In this review we examine the basis for the hypothesis that insulin acts on the vasculature at three discrete steps to enhance its own delivery to muscle: (1) relaxation of resistance vessels to increase total blood flow; (2) relaxation of pre-capillary arterioles to increase the microvascular exchange surface perfused within skeletal muscle (microvascular recruitment); and (3) the trans-endothelial transport (TET) of insulin. Insulin can relax resistance vessels and increase blood flow to skeletal muscle. However, there is controversy as to whether this occurs at physiological concentrations of, and exposure times to, insulin. The microvasculature is recruited more quickly and at lower insulin concentrations than are needed to increase total blood flow, a finding consistent with a physiological role for insulin in muscle insulin delivery. Microvascular recruitment is impaired by obesity, diabetes and nitric oxide synthase inhibitors. Insulin TET is a third potential site for regulating insulin delivery. This is underscored by the consistent finding that steady-state insulin concentrations in plasma are approximately twice those in muscle interstitium. Recent in vivo and in vitro findings suggest that insulin traverses the vascular endothelium via a trans-cellular, receptor-mediated pathway, and emerging data indicate that insulin acts on the endothelium to facilitate its own TET. Thus, muscle insulin delivery, which is rate-limiting for its metabolic action, is itself regulated by insulin at multiple steps. These findings highlight the need to further understand the role of the vascular actions of insulin in metabolic regulation.

Type 2 diabetes is associated with insulin resistance, endothelial dysfunction and accelerated atherosclerotic diseases. Though underlying mechanisms remain to be unraveled, p38 mitogen-activated protein kinase (MAPK) appears to play important roles in their pathogenesis. As a member of the MAPK family, it regulates the activities of many transcription factors and proteins/enzymes and thus has a wide-spectrum of biological effects. Patients with insulin resistance and/or type 2 diabetes have high levels of plasma free fatty acids, inflammatory cytokines, and/or glucose, and over-activation of the cardiovascular renin-angiotensin system, all are capable of activating p38 MAPK. p38 MAPK plays a central role in hepatic glucose and lipid metabolism, leading to increased hepatic glucose production and decreased hepatic lipogenesis. The roles of p38 MAPK in insulin-mediated glucose uptake in skeletal muscle and adipose tissue remain controversial. p38 MAPK also mediates inflammatory processes and cell apoptosis. Recent evidence suggests that p38 MAPK may be the key node linking cardiovascular insulin resistance, endothelial dysfunction and the pathogenesis of atherosclerotic diseases through its influences on monocytes/macrophages, vascular endothelial cells, and vascular smooth muscle cells in type 2 diabetes. In addition, p38 MAPK also contributes significantly to cardiac injury during ischemia-reperfusion.

Endothelial dysfunction is common in patients with GH deficiency who are at increased risk for premature cardiovascular death. GH regulates vascular tone and reactivity in humans.Our objective was to explore the mechanisms underlying the GH's acute vascular effects. DESIGN AND STUDY SETTING: There were 10 healthy, lean and young, volunteers studied after an overnight fast. GH was infused systemically for 6 h at 0.06 microg/kg.min. Biopsy of the vastus lateralis muscle was done in seven subjects before and after GH infusion. Human aortic endothelial cells (HAECs) were incubated with GH in vitro.GH infusion increased plasma GH to 32.9 +/- 1.5 ng/ml and forearm blood flow by 66% (P0.001). GH infusion did not significantly change plasma IGF-I concentrations, muscle IGF-I mRNA expression, and muscle Akt phosphorylation, suggesting a lack of IGF-I action in muscle. Because it was reported that GH exerts an acute vascular effect via a nitric oxide (NO)-dependent mechanism, we performed additional in vitro experiments using HAECs. HAECs express abundant GH receptors. Incubating HAECs with GH at 30 ng/ml for 3 or 6 h did not alter endothelial NO synthase (eNOS) protein content but time dependently increased the phosphorylation and activity of eNOS, thus demonstrating a direct effect of GH on endothelial cells.GH exerts an acute vascular effect independent of both systemic and local IGF-I production, and this effect is likely via direct action on GH receptors and eNOS in the vascular endothelium.

Myocardial ischemia-reperfusion injury contributes significantly to morbidity and mortality in patients with diabetes. Insulin decreases myocardial infarct size in animals and the rate of apoptosis in cultured cells. Ischemia-reperfusion activates p38 mitogen-activated protein kinase (MAPK), which regulates cellular apoptosis. To examine whether p38 MAPK affects insulin's cardioprotection against ischemia-reperfusion injury, we studied overnight-fasted adult male rats by use of an in vivo rat model of myocardial ischemia-reperfusion. A euglycemic clamp (3 mU·min−1·kg−1) was begun either 10 min before ischemia (InsulinBI), 5 min before reperfusion (InsulinBR), or 30 min after the onset of reperfusion (InsulinAR), and continued until the end of the study. Compared with saline control, insulin decreased the infarct size in both InsulinBI ( P < 0.001) and InsulinBR ( P < 0.02) rats but not in InsulinAR rats. The ischemic area showed markedly increased phosphorylation of p38 MAPK compared with the nonischemic area in saline animals. Acute activation of p38 MAPK with anisomycin (2 mg/kg iv 10 min before ischemia) had no effect on infarct size in saline rats. However, it completely abolished insulin's protective effect in InsulinBI and InsulinBR rats. Activation of p38 MAPK by anisomycin was associated with marked and persistent elevation in IRS-1 serine phosphorylation. Treatment of animals with SB-239063, a potent and specific inhibitor of p38 MAPK, 10 min before reperfusion enabled insulin-mediated myocardial protection in InsulinAR rats. We conclude that insulin protects myocardium against ischemia-reperfusion injury when given prior to ischemia or reperfusion, and activation of p38 MAPK abolishes insulin's cardioprotective effect.

Plasma free fatty acids are elevated in patients with type 2 diabetes and contribute to the pathogenesis of insulin resistance and endothelial dysfunction. The p38 MAPK mediates stress, inflammation, and apoptosis. Whether free fatty acids induce apoptosis and/or activate nuclear factor-kappaB inflammatory pathway in human coronary artery endothelial cells (hCAECs) and, if so, whether this involves the p38 MAPK pathway is unknown. hCAECs (passages 4-6) were grown to 70% confluence and then incubated with palmitate at concentrations of 0-300 microm for 6-48 h. Palmitate at 100, 200, or 300 microm markedly increased apoptosis after 12 h of incubation. This apoptotic effect was time (P=0.008) and dose (P=0.006) dependent. Palmitate (100 microm for 24 h) induced a greater than 2-fold increase in apoptosis, which was accompanied with a 4-fold increase in p38 MAPK activity (P

We have previously shown that skeletal muscle capillaries are rapidly recruited by physiological doses of insulin in both humans and animals. This facilitates glucose and insulin delivery to muscle, thus augmenting glucose uptake. In obese rats, both insulin-mediated microvascular recruitment and glucose uptake are diminished; however, this action of insulin has not been studied in obese humans. Here we used contrast ultrasound to measure microvascular blood volume (MBV) (an index of microvascular recruitment) in the forearm flexor muscles of lean and obese adults before and after a 120-min euglycemic-hyperinsulinemic (1 mU . min(-1) . kg(-1)) clamp. We also measured brachial artery flow, fasting lipid profile, and anthropomorphic variables. Fasting plasma glucose (5.4 +/- 0.1 vs. 5.1 +/- 0.1 mmol/l, P = 0.05), insulin (79 +/- 11 vs. 38 +/- 6 pmol/l, P = 0.003), and percent body fat (44 +/- 2 vs. 25 +/- 2%, P = 0.001) were higher in the obese than the lean adults. After 2 h of insulin infusion, whole-body glucose infusion rate was significantly lower in the obese versus lean group (19.3 +/- 3.2 and 37.4 +/- 2.6 mumol . min(-1) . kg(-1) respectively, P0.001). Compared with baseline, insulin increased MBV in the lean (18.7 +/- 3.3 to 25.0 +/- 4.1, P = 0.019) but not in the obese group (20.4 +/- 3.6 to 18.8 +/- 3.8, NS). Insulin increased brachial artery diameter and flow in the lean but not in the obese group. We observed a significant, negative correlation between DeltaMBV and BMI (R = -0.482, P = 0.027) in response to insulin. In conclusion, obesity eliminated the insulin-stimulated muscle microvascular recruitment and increased brachial artery blood flow seen in lean individuals.

For many decades, it has been recognized that insulin, growth hormone, glucocorticoids, insulin-like growth factor 1, thyroid hormones, and other hormones regulate body protein metabolism. It has been more recently recognized, but not understood, that humor factors present in states of acute and chronic inflammation could have a strong impact on protein turnover. Most recently, the role of amino acids, acting as signaling molecules, has become increasingly clarified. In aggregate, these factors (together with neuromuscular activity) determine the balance of body protein mass. We will review some of these data, particularly focusing on amino acids, insulin, and the growth hormone axis and their actions in muscle and how these relate to whole-body protein metabolism.

Amino acids are unique anabolic agents in that they nutritively signal to mRNA translation initiation and serve as substrates for protein synthesis in skeletal muscle. Glucocorticoid excess antagonizes the anabolic action of amino acids on protein synthesis in laboratory animals. To examine whether excessive glucocorticoids modulate mixed amino acid-signaled translation initiation in human skeletal muscle, we infused an amino acid mixture (10% Travasol) systemically to 16 young healthy male volunteers for 6 h in the absence ( n = 8) or presence ( n = 8) of glucocorticoid excess (dexamethasone 2 mg orally every 6 h for 3 days). Vastus lateralis muscles were biopsied before and after amino acid infusion, and the phosphorylation of eukaryotic initiation factor (eIF) 4E-binding protein 1 (4E-BP1), ribosomal protein S6 kinase (p70S6K), and eIF2α and the guanine nucleotide exchange activity of eIF2B were measured. Systemic infusion of mixed amino acids significantly stimulated the phosphorylation of 4E-BP1 ( P < 0.04) and p70S6K( P < 0.001) and the dephosphorylation of eIF2α ( P < 0.003) in the control group. Dexamethasone treatment did not alter the basal phosphorylation state of 4E-BP1, p70S6K, or eIF2α; however, it abrogated the stimulatory effect of amino acid infusion on the phosphorylation of 4E-BP1 ( P = 0.31) without affecting amino acid-induced phosphorylation of p70S6K( P = 0.002) or dephosphorylation of eIF2α ( P = 0.003). Neither amino acid nor dexamethasone treatment altered the guanine nucleotide exchange activity of eIF2B. We conclude that changes of amino acid concentrations within the physiological range stimulate mRNA translation by enhancing the binding of mRNA to the 43S preinitiation complex, and the activity of p70S6Kand glucocorticoid excess blocks the former action in vivo in human skeletal muscle.