Insulin-mediated Muscle Glucose Uptake Research Articles

Skeletal Muscle Health in Polycystic Ovary Syndrome: Protective Effect of Hyperandrogenism or Detrimental Effect of Insulin Resistance? A Systematic Review and Meta-Analysis

Women with polycystic ovary syndrome (PCOS) exhibit reduced skeletal muscle insulin-mediated glucose uptake. Altered muscle mass may affect insulin resistance (IR) and inflammation, thereby potentially aggravating reproductive status including ovulatory cyclicity and fertility potential. However, the relationship between PCOS and skeletal muscle mass is elusive given conflicting reports on protective or detrimental influence of PCOS endocrine derangements (hyperandrogenism, IR) on muscle. We evaluated whether muscle mass and function are affected by PCOS in response to a call to elucidate musculoskeletal alterations in the International Evidence-based Guideline for the Assessment and Management of PCOS. Databases of MEDLINE, Web of Science, and Scopus were searched (January 1990 to September 2020) to identify observational studies on skeletal muscle mass (lean tissue mass) and function (strength) in PCOS and control groups. The primary outcome was total lean body mass (LBM) or fat-free mass (FFM). Data were pooled by random-effects models and expressed as weighted mean differences and 95% confidence intervals. Forty-five studies (n = 3,676 [1,854, PCOS; 1,822, controls]) were eligible. Forty-one evaluated lean tissue mass and five strength. PCOS groups had increased total (0.83 [0.08, 1.58] kg; P=0.03; I2 = 72.0%) yet comparable trunk (0.84 [-0.37, 2.05] kg; P = 0.15; I2 = 73.0%) LBM/FFM. There were no associations between mean differences of groups in total testosterone (TT) or homeostatic model assessment of IR (HOMA-IR) and total/trunk LBM/FFM (All: P ≥ 0.75) by meta-regressions. However, mean differences of groups in body mass index (BMI) were associated with total (0.65 [0.23, 1.06] kg; P < 0.01; I2 = 56.9%) and trunk (0.56 [0.11, 1.01] kg; P = 0.02; I2 = 42.8%) LBM/FFM. Accordingly, PCOS sub-group with overweight/obesity (BMI ≥ 25 kg/m2) exhibited greater total LBM/FFM than controls (1.58 [0.82, 2.34] kg; P < 0.01; I2 = 64.0%) unlike a lean (BMI < 25 kg/m2) sub-group (-0.45 [-1.94, 1.05] kg; P = 0.53; I2 = 69.5%). Some study results were contradictory (i.e., increased appendicular mass or strength in PCOS group or comparable findings between groups) and study methodology varied; thus, inclusion in meta-analyses was not possible. PCOS cohorts have a tendency for increased total and trunk lean tissue mass likely attributed to obesity. However, most critically, whether PCOS influences other lean tissue areas (appendicular), morphology, and function is unclear. Our observations do not support any protective/detrimental influence of hyperandrogenism (TT) or IR (HOMA-IR) on lean mass. Heterogeneity among studies warrants research to address any contributions of lifestyle, healthcare, and biological factors to observed differences for future guideline recommendations to improve PCOS musculoskeletal and reproductive health (www.crd.york.ac.uk/PROSPERO ID, CRD42020203490).

Obesity, but not hyperandrogenism or insulin resistance, predicts skeletal muscle mass in reproductive-aged women with polycystic ovary syndrome: A systematic review and meta-analysis of 45 observational studies.

Women with polycystic ovary syndrome (PCOS) exhibit reduced muscle insulin-mediated glucose uptake, potentially attributed to altered muscle mass; however, this is inconclusive. Altered muscle mass may aggravate PCOS complications. Our systematic review and meta-analysis evaluated whether PCOS alters muscle mass and function. Databases (MEDLINE, Web of Science, Scopus) were searched through September 2, 2020, for studies documenting skeletal muscle mass (lean tissue mass) and function (strength) in PCOS and control groups. The primary outcome was total lean body mass (LBM) or fat-free mass (FFM). Data were pooled by random-effects models and expressed as mean differences and 95% confidence intervals. Forty-five studies (n= 3676 participants) were eligible. Women with PCOS had increased total (0.83 [0.08,1.58]kg; p= 0.03; I2 = 72.0%) yet comparable trunk (0.84 [-0.37,2.05]kg; p= 0.15; I2 = 73.0%) LBM or FFM versus controls. Results of meta-regression analyses showed no associations between mean differences between groups in total testosterone or homeostatic model assessment of insulin resistance and total or trunk LBM or FFM (All: p≥ 0.75). Mean differences in body mass index (BMI) were associated with total (0.65 [0.23,1.06]kg; p< 0.01; I2 = 56.9%) and trunk (0.56 [0.11,1.01]kg; p= 0.02; I2 = 42.8%) LBM or FFM. The PCOS subgroup with BMI ≥ 25 kg/m2 had greater total LBM or FFM versus controls (1.58 [0.82,2.34]kg; p< 0.01; I2 = 64.0%) unlike the PCOS subgroup with BMI < 25 kg/m2 (-0.45 [-1.94,1.05]kg; p= 0.53; I2 = 69.5%). Appendicular lean mass and muscle strength data were contradictory and described narratively, as meta-analyses were impossible. Women with PCOS have higher total and trunk lean tissue mass attributed to overweight/obesity, unlike hyperandrogenism or insulin resistance.

Interrupting prolonged sitting with repeated chair stands or short walks reduces postprandial insulinemia in healthy adults

We determined if interrupting prolonged sitting with practical "activity snacks" could reduce postprandial glycemia and insulinemia in healthy adults. Fourteen participants (7 males, 7 females; 24 ± 5 yr; 25 ± 5 kg/m2; 40 ± 8 mL/kg/min; 7,033 ± 2,288 steps/day) completed three 7.5-h trials in a randomized order consisting of uninterrupted sitting (SIT), sitting with intermittent (every 30 min) walking (WALK; 2 min at 3.1 mph), or sitting with intermittent squats (SQUAT; 15 chair stands with calf raise). Mixed-macronutrient liquid meals provided 20% ("breakfast") and 30% ("lunch") of daily energy needs to mimic Western meal patterns. Blood samples were obtained for analysis of postprandial plasma glucose and insulin concentrations, and skeletal muscle biopsy samples were collected to measure markers of contraction- and insulin-mediated glucose uptake signaling. Postprandial glucose and insulin did not differ across conditions following breakfast. After lunch, peak insulin concentration was lower in SQUAT (52 ± 27, P < 0.01) and WALK (62 ± 35, P < 0.05) compared with SIT (79 ± 43 μIU/mL). The insulin incremental area under the curve (iAUC) 1 h following lunch was 37 and 29% lower in SQUAT (P < 0.01) and WALK (P < 0.05) compared with SIT, respectively; however, 3-h insulin iAUC was reduced in SQUAT only (24% vs. SIT, P < 0.05). The 3-h insulin:glucose iAUC was reduced following lunch in both SQUAT (30%) and WALK (23%) compared with SIT (P < 0.05). Phosphorylation of AKTThr308, AKTSer473, and AS160Ser318 was not different between conditions (P > 0.05). Interrupting prolonged sitting with short walks or repeated chair stands reduces postprandial insulinemia in healthy adults. Our results may have implications for mitigating cardiometabolic disease risk in adults who engage in periods of prolonged sitting.NEW & NOTEWORTHY Breaking up prolonged sitting with intermittent walking breaks can improve glycemic control. Here, we demonstrated that interrupting prolonged sitting every 30 min with 1 min of repeated chair stands was as effective as 2-min treadmill walks for lowering postprandial insulinemia in healthy adults. Markers of contraction- and insulin-mediated muscle glucose uptake were unchanged. Repeated chair stands as a form of body-weight resistance activity may represent a cost- and space-efficient activity break for mitigating cardiometabolic-disease risk.

Green tea polyphenols ameliorate metabolic abnormalities and insulin resistance by enhancing insulin signalling in skeletal muscle of Zucker fatty rats.

In the present study, we evaluated the metabolic effects of green tea polyphenols (GTPs) in high-fat diet (HFD) fed Zucker fatty (ZF) rats, in particular the effects of GTP on skeletal muscle insulin sensitivity. Body weight, visceral fat, glucose tolerance, lipid profiles and whole-body insulin sensitivity were measured in HFD-fed ZF rats after 8-week-treatment with GTP (200 mg/kg of body weight) or saline (5 ml/kg of body weight). Zucker lean rats were studied as controls. Ex vivo insulin-mediated muscle glucose uptake was assessed. Immunoblotting was used to evaluate the expression of key insulin signalling proteins in skeletal muscle. GTP treatment attenuated weight gain (P<0.05) and visceral fat accumulation (27.6%, P<0.05), and significantly reduced fasting serum glucose (P<0.05) and insulin (P<0.01) levels. Homoeostasis model assessment of insulin resistance (HOMA-IR), a measure of insulin resistance, was lower (P<0.01) in GTP-treated animals compared with ZF controls. Moreover, insulin-stimulated glucose uptake by isolated soleus muscle was increased (P<0.05) in GTP-ZF rats compared with ZF-controls. GTP treatment attenuated the accumulation of ectopic lipids (triacyl- and diacyl-glycerols), enhanced the expression and translocation of glucose transporter-4, and decreased pSer612IRS-1 and increased pSer473Akt2 expression in skeletal muscle. These molecular changes were also associated with significantly decreased activation of the inhibitory (muscle-specific) protein kinase (PKC) isoform, PKC-θ. Taken together, the present study has shown that regular ingestion of GTP exerts a number of favourable metabolic and molecular effects in an established animal model of obesity and insulin resistance. The benefits of GTP are mediated in part by inhibiting PKC-θ and improving muscle insulin sensitivity.

Metabolic-vascular coupling in skeletal muscle: A potential role for capillary pericytes?

The matching of capillary blood flow to metabolic rate of the cells within organs and tissues is a critical microvascular function which ensures appropriate delivery of hormones and nutrients, and the removal of waste products. This relationship is particularly important in tissues where local metabolism, and hence capillary blood flow, must be regulated to avoid a mismatch between nutrient demand and supply that would compromise normal function. The consequences of a mismatch in microvascular blood flow and metabolism are acutely apparent in the brain and heart, where a sudden cessation of blood flow, for example following an embolism, acutely manifests as stroke or myocardial infarction. Even in more resilient tissues such as skeletal muscle, a short-term mismatch reduces muscle performance and exercise tolerance, and can cause intermittent claudication. In the longer-term, a microvascular-metabolic mismatch in skeletal muscle reduces insulin-mediated muscle glucose uptake, leading to disturbances in whole-body metabolic homeostasis. While the notion that capillary blood flow is fine-tuned to meet cellular metabolism is well accepted, the mechanisms that control this function and where and how different parts of the vascular tree contribute to capillary blood flow regulation remain poorly understood. Here, we discuss the emerging evidence implicating pericytes, mural cells that surround capillaries, as key mediators that match tissue metabolic demand with adequate capillary blood flow in a number of organs, including skeletal muscle.

Adipose tissue and skeletal muscle insulin-mediated glucose uptake in insulin resistance: role of blood flow and diabetes

Adipose tissue glucose uptake is impaired in insulin-resistant states, but ex vivo studies of human adipose tissue have yielded heterogeneous results. This discrepancy may be due to different regulation of blood supply. The aim of this study was to test the flow dependency of in vivo insulin-mediated glucose uptake in fat tissues, and to contrast it with that of skeletal muscle. We reanalyzed data from 159 individuals in which adipose tissue depots-subcutaneous abdominal and femoral, and intraperitoneal-and femoral skeletal muscle were identified by MRI, and insulin-stimulated glucose uptake ([18F]-fluoro-2-deoxyglucose) and blood flow ([15O]-H2O) were measured simultaneously by positron emission tomography scanning. Individuals in the bottom tertile of whole-body glucose uptake [median (IQR) 36 (17) µmol. kg fat-free mass (kgFFM)-1 . min-1 .nM-1] displayed all features of insulin resistance compared with the rest of the group [median (IQR) 97 (71) µmol . kgFFM-1 .min-1 . nM-1]. Rates of glucose uptake were directly related to the degree of insulin resistance in all fat depots as well as in skeletal muscle. However, blood flow was inversely related to insulin sensitivity in each fat depot (all P≤0.03), whereas femoral muscle blood flow was not significantly different between insulin-resistant and insulin-sensitive subjects, and was not related to insulin sensitivity. Furthermore, in subjects performing one-leg exercise, blood flow increased 5- to 6-fold in femoral muscle but not in the overlying adipose tissue. The presence of diabetes was associated with a modest increase in fat and muscle glucose uptake independent of insulin resistance. Reduced blood supply is an important factor for the impairment of in vivo insulin-mediated glucose uptake in both subcutaneous and visceral fat. In contrast, the insulin resistance of glucose uptake in resting skeletal muscle is predominantly a cellular defect. Diabetes provides a modest compensatory increase in fat and muscle glucose uptake that is independent of insulin resistance.

Maltitol inhibits small intestinal glucose absorption and increases insulin mediated muscle glucose uptake ex vivo but not in normal and type 2 diabetic rats

This study investigated the effects of maltitol on intestinal glucose absorption and muscle glucose uptake using ex vivo and in vivo experimental models. The ex vivo experiment was conducted in isolated jejunum and psoas muscle from normal rats. The in vivo study investigated the effects of a single bolus dose of maltitol on gastric emptying, intestinal glucose absorption and digesta transit in normal and type 2 diabetic rats. Maltitol inhibited glucose absorption in isolated rat jejunum and increased glucose uptake in isolated rat psoas muscle in the presence of insulin but not in the absence of insulin. In contrast, maltitol did not significantly (p > 0.05) alter small intestinal glucose absorption or blood glucose levels as well as gastric emptying and digesta transit in normal or type 2 diabetic rats. The results suggest that maltitol may not be a suitable dietary supplement for anti-diabetic food and food products to improve glycemic control.

Differential impact of selective GH deficiency and endogenous GH excess on insulin-mediated actions in muscle and liver of male mice.

A reciprocal relationship between insulin sensitivity and glucose tolerance has been reported in some mouse models and humans with isolated changes in growth hormone (GH) production and signaling. To determine if this could be explained in part by tissue-specific changes in insulin sensitivity, hyperinsulinemic-euglycemic clamps were performed in mice with adult-onset, isolated GH deficiency and in mice with elevated endogenous GH levels due to somatotrope-specific loss of IGF-I and insulin receptors. Our results demonstrate that circulating GH levels are negatively correlated with insulin-mediated glucose uptake in muscle but positively correlated with insulin-mediated suppression of hepatic glucose production. A positive relationship was also observed between GH levels and endpoints of hepatic lipid metabolism known to be regulated by insulin. These results suggest hepatic insulin resistance could represent an early metabolic defect in GH deficiency.

Type 2 Diabetes Mellitus in Children and Adolescents

Type 2 Diabetes Mellitus in Children and Adolescents

Type 2 Diabetes Mellitus in Children and Adolescents

On the basis of strong research evidence and consensus, type 1 diabetes mellitus (DM) remains the most common form of DM in children and adolescents. The incidence of type 2 DM in the pediatric population is rapidly increasing because of the obesity epidemic, and minority groups are disproportionately affected. (2) (10) (19) On the basis of some research evidence and consensus, it can be challenging to initially differentiate between type 2 DM and type 1 DM clinically because of the increased prevalence of obesity, the complex interplay of autoimmunity and obesity, and common symptoms at presentation. (1) (10) (19) Significant evidence and consensus support a genetic basis for the development of type 2 DM in children. Physicians should routinely screen at risk children older than age 10 years for DM. Screening criteria include obesity, a family history of type 2 DM, a minority racial or ethnic background, acanthosis nigricans, or other diseases associated with insulin resistance, including polycystic ovary syndrome, hypertension, or dyslipidemia. (1) (10) (18) (19) On the basis of consensus, diagnosis of type 2 DM can be confirmed by an elevated fasting blood glucose level greater than 126 mg/dl (7.0 mmol/L), an elevated 2-hour plasma glucose greater than 200 mg/dL (11.1 mmol/L) on an oral glucose tolerance test, an elevated random blood glucose greater than 200 mg/dL (11.1 mmol/L), or a hemoglobin A1c level greater than 6.5% with suggestive symptoms. (10) According to strong research evidence and consensus, once the diagnosis has been made, treatment should be based on the acuity of presentation and should focus on lifestyle modification and on normalizing hyperglycemia to minimize complications. Metformin is currently first-line treatment for type 2 DM in children and adolescents older than age 10 years who present nonacutely. (18) (19) Strong research evidence and consensus demonstrate that because type 2 DM has an insidious onset, microvascular and macrovascular complications can be present at the time of diagnosis. Patients should be screened for the presence of complications when the diagnosis of type 2 DM is made and in follow-up. (6) (10).

Local NOS inhibition impairs vascular and metabolic actions of insulin in rat hindleg muscle in vivo

Insulin stimulates microvascular recruitment in skeletal muscle, and this vascular action augments muscle glucose disposal by ∼40%. The aim of the current study was to determine the contribution of local nitric oxide synthase (NOS) to the vascular actions of insulin in muscle. Hooded Wistar rats were infused with the NOS inhibitor N(ω)-nitro-L-arginine methylester (L-NAME, 10 μM) retrogradely via the epigastric artery in one leg during a systemic hyperinsulinemic-euglycemic clamp (3 mU·min(-1)·kg(-1) × 60 min) or saline infusion. Femoral artery blood flow, microvascular blood flow (assessed from 1-methylxanthine metabolism), and muscle glucose uptake (2-deoxyglucose uptake) were measured in both legs. Local L-NAME infusion did not have any systemic actions on blood pressure or heart rate. Local L-NAME blocked insulin-stimulated changes in femoral artery blood flow (84%, P < 0.05) and microvascular recruitment (98%, P < 0.05), and partially blocked insulin-mediated glucose uptake in muscle (reduced by 34%, P < 0.05). L-NAME alone did not have any metabolic effects in the hindleg. We conclude that insulin-mediated microvascular recruitment is dependent on local activation of NOS in muscle and that this action is important for insulin's metabolic actions.

Perivascular Adipose Tissue and Cardiometabolic Disease

BACKGROUND: Obesity is associated with insulin resistance, hypertension, and cardiovascular disease, but the mechanisms underlying these associations are incompletely understood. Microvascular dysfunction may play an important role in the pathogenesis of both insulin resistance and hypertension in obesity.CONTENT: Perivascular adipose tissue (PVAT) is a local deposit of adipose tissue surrounding the vasculature. PVAT is present throughout the body and has been shown to have a local effect on blood vessels. The influence of PVAT on the vasculature changes with increasing adiposity. PVAT similarly to other fat depots, is metabolically active, secreting a wide array of bioactive substances, termed ‘adipokines’. Adipokines include cytokines, chemokines and hormones that can act in a paracrine, autocrine or endocrine fashion. Many of the proinflammatory adipokines upregulated in obesity are known to influence vascular function, including endothelial function, oxidative stress, vascular stiffness and smooth muscle migration. Adipokines also stimulate immune cell migration into the vascular wall, potentially contributing to the inflammation found in atherosclerosis. Finally, adipokines modulate the effect of insulin on the vasculature, thereby decreasing insulin-mediated muscle glucose uptake. This leads to alterations in nitric oxide signaling, insulin resistance and potentially atherogenesis.SUMMARY: PVAT surrounds blood vessels. PVAT and the adventitial layer of blood vessels are in direct contact with each other. Healthy PVAT secretes adipokines and regulates vascular function. Obesity is associated with changes in adipokine secretion and the resultant inflammation of PVAT. The dysregulation of adipokines changes the effect of PVAT on the vasculature. Changes in perivascular adipokines secretion in obesity appear to contribute to the development of obesity-mediated vascular disease.KEYWORDS: obesity, perivascular adipose tissue, PVAT, cardiometabolic disease, adipokine

Muscle insulin resistance resulting from impaired microvascular insulin sensitivity in Sprague Dawley rats

Enhanced microvascular perfusion of skeletal muscle is important for nutrient exchange and contributes ∼40% insulin-mediated muscle glucose disposal. High fat-fed (36% fat wt./wt.) rats are a commonly used model of insulin-resistance that exhibit impairment of insulin-mediated microvascular recruitment and muscle glucose uptake, which is accompanied by myocyte insulin-resistance. Distinguishing the contribution of impaired microvascular recruitment and impaired insulin action in the myocyte to decreased muscle glucose uptake in these high-fat models is difficult. It is unclear whether microvascular and myocyte insulin-resistance develop simultaneously. To assess this, we used a rat diet model with a moderate increase (two-fold) in dietary fat. Sprague Dawley rats fed normal (4.8% fat wt./wt., 5FD) or high (9.0% fat wt./wt., 9FD) fat diets for 4 weeks were subject to euglycaemic hyperinsulinemic clamp (10 mU/min/kg insulin or saline) or isolated hindlimb perfusion (1.5 or 15 nM insulin or saline). Body weight, epididymal fat mass, and fasting plasma glucose were unaffected by diet. Fasting plasma insulin and non-esterified fatty acid concentrations were significantly elevated in 9FD. Glucose infusion rate and muscle glucose uptake were significantly impaired during insulin clamps in 9FD. Insulin-stimulated microvascular recruitment was significantly blunted in 9FD. Insulin-mediated muscle glucose uptake between 5FD and 9FD were not different during hindlimb perfusion. Impaired insulin-mediated muscle glucose uptake in vivo can be the direct result of reduced microvascular blood flow responses to insulin, and can result from small (two-fold) increases in dietary fat. Thus, microvascular insulin-resistance can occur independently to the development of myocyte insulin-resistance.

Early Microvascular Recruitment Modulates Subsequent Insulin-Mediated Skeletal Muscle Glucose Metabolism During Lipid Infusion

OBJECTIVETo test whether early, insulin-mediated microvascular recruitment in skeletal muscle predicts steady-state glucose metabolism in the setting of physiological elevation of free fatty acid concentrations.RESEARCH DESIGN AND METHODSWe measured insulin’s microvascular and metabolic effects in 14 healthy young adults during a 2-h euglycemic insulin clamp. Plasma free fatty acid concentrations were raised (Intralipid and heparin infusion) for 3 h before the clamp and maintained at postprandial concentrations during the clamp. Microvascular blood volume (MBV) was measured by contrast-enhanced ultrasound (CEU) continuously from baseline through the first 30 min of the insulin clamp. Muscle glucose and insulin uptake were measured by the forearm balance method.RESULTSThe glucose infusion rate (GIR) necessary to maintain euglycemia during the clamp varied by fivefold across subjects (2.5–12.5 mg/min/kg). The early MBV responses to insulin, as indicated by CEU video intensity, ranged widely, from a 39% decline to a 69% increase. During the clamp, steady state forearm muscle glucose uptake and GIR each correlated significantly with the change in forearm MBV (P < 0.01). To explore the basis for the wide range of vascular and metabolic insulin sensitivity observed, we also measured Vo2max in a subset of eight subjects. Fitness (Vo2max) correlated significantly with the GIR, the forearm glucose uptake, and the percentage change in MBV during the insulin clamp (P < 0.05 for each).CONCLUSIONSEarly microvascular responses to insulin strongly associate with steady state skeletal muscle insulin-mediated glucose uptake. Physical fitness predicts both metabolic and vascular insulin responsiveness.

Meal-Related Increases in Microvascular Vasomotion Are Impaired in Obese Individuals

Obesity is associated with insulin resistance, hypertension, cardiovascular disease, and type 2 diabetes (1), but the mechanisms underlying these associations are incompletely understood. This article reviews and adds original data (i.e., in the postprandial state) to the evidence for microvascular dysfunction, including impairment of insulin-stimulated microvascular perfusion as a key element in the pathogenesis of obesity-related hypertension and insulin resistance (2,3). The microcirculation is widely taken to encompass vessels <150 μm (i.e., arterioles, capillaries, and venules) (4) and has two important functions. First, arterioles regulate hydrostatic pressure and peripheral vascular resistance (4). Importantly, dysfunction of the microcirculation, with concomitant increases in vasoconstrictor tone, will increase total peripheral resistance and, other things being equal, blood pressure, as reviewed elsewhere (2,3). Second, it regulates tissue perfusion to optimize the delivery of nutrients and removal of waste products within tissues in response to variations in demand. In this respect, insulin has been shown to play an important role (5). Insulin redirects blood flow within the muscle microvascular bed to increase available capillary surface area, an effect referred to as “capillary recruitment” (6,7). In addition, insulin induces vasodilation of resistance vessels, resulting in an increase in total muscle blood flow (8). Whether this increase in total muscle blood flow, which occurs later in time compared with the redirection of flow to nutritive capillary beds, serves to enhance insulin-mediated glucose uptake remains controversial (9). However, it has been generally accepted that capillary recruitment is crucial for the delivery of insulin and glucose to tissue. Indeed, several studies have shown that insulin-mediated increases in capillary recruitment account for approximately half of insulin-mediated muscle glucose uptake in vivo (10–14). Insulin’s effect on microvascular blood flow is, therefore, an important regulator of insulin-mediated muscle glucose uptake, the main …

Exercise Reduces Intrahepatic Triglyceride And Improves Insulin Action In Adults With Nonalcoholic Fatty Liver Disease

The effects of aerobic exercise (AE) alone without weight loss on reducing intrahepatic triglyceride content (IHTG) and improving insulin action in patients with nonalcoholic fatty liver disease (NAFLD, IHTG >5%) is unknown. PURPOSE: We determined the effects of 4 months of a minimum of 150min/wk of verified brisk walking (∼50% of VO2max) in previously sedentary, overweight (mean ± SE, BMI =27.7 ± 2.9) adults (age 21.0±2.2yrs) on IHTG and hepatic and skeletal muscle insulin sensitivity. METHODS: Participants were randomized to AE (N=12, 6 Male (M), 6 Female (F)) or control (C, N=6, 3M, 3 F) groups. At baseline and after 4 months of verified AE the following procedures were performed: 1) a euglycemic-hyperinsulinemic clamp procedure, in conjunction with isotope tracer([2H2]glucose) infusion, was conducted to assess hepatic and muscle insulin sensitivity and 2) proton magnetic resonance spectroscopy was performed to determine IHTG. Both groups maintained their normal ad-libitum diet and activities of daily living. RESULTS: Subjects in the AE completed 96% ± 2% of the recommended exercise sessions and averaged 225 ± 22min/wk of verified exercise. Subjects in the AE group were weight stable from baseline (105.9 ± 5.1kg) to 4 months (105.5±6.2kg) as well as the C group (113.8 ± 6.3kg vs. 114.2 ± 5.8kg). IHTG significantly (P<0.05) decreased (-15% ± 5%) in the AE group compared to a slight non-significant increase (5.6%±5%) in the C group resulting in a significant between group difference, P<0.05. Hepatic insulin sensitivity index, a function of hepatic glucose production rate and plasma insulin concentration, increased by 22 ±16% (p<0.001) after 4 months of AE compared to no change in the C group for a significant between group difference, P<0.05. Insulin-mediated muscle glucose uptake significantly increased in the AE (15 ± 14.2%) (p≤0.01), but not in the C group (2.0 ± 10.2%) resulting in a significant between group difference, P<0.05. CONCLUSIONS: In adults with NAFLD, aerobic exercise, independent of weight loss, is an effective treatment for reducing IHTG and improving insulin sensitivity in both skeletal muscle and liver.

Current views on type 2 diabetes

Type 2 diabetes mellitus (T2DM) affects a large population worldwide. T2DM is a complex heterogeneous group of metabolic disorders including hyperglycemia and impaired insulin action and/or insulin secretion. T2DM causes dysfunctions in multiple organs or tissues. Current theories of T2DM include a defect in insulin-mediated glucose uptake in muscle, a dysfunction of the pancreatic beta-cells, a disruption of secretory function of adipocytes, and an impaired insulin action in liver. The etiology of human T2DM is multifactorial, with genetic background and physical inactivity as two critical components. The pathogenesis of T2DM is not fully understood. Animal models of T2DM have been proved to be useful to study the pathogenesis of, and to find a new therapy for, the disease. Although different animal models share similar characteristics, each mimics a specific aspect of genetic, endocrine, metabolic, and morphologic changes that occur in human T2DM. The purpose of this review is to provide the recent progress and current theories in T2DM and to summarize animal models for studying the pathogenesis of the disease.

A role of lipin in human obesity and insulin resistance: relation to adipocyte glucose transport and GLUT4 expression

The mouse lipin gene, Lpin1, is important for adipose tissue development and is a candidate gene for insulin resistance. Here, we investigate the adipose tissue expression levels of the human LPIN1 gene in relation to various clinical variables as well as adipocyte function. LPIN1 gene expression was induced at an early step in human preadipocyte differentiation in parallel with peroxisome proliferator-activated receptor gamma. Lipin mRNA levels were higher in fat cells than in adipose tissue segments but showed no difference between subcutaneous and omental depots. Moreover, LPIN1 expression levels were reduced in obesity, improved following weight reduction in obese subjects, and were downregulated in women with the metabolic syndrome. With respect to adipocyte function, adipose LPIN1 gene expression was strongly associated with both basal and insulin-mediated subcutaneous adipocyte glucose transport as well as mRNA levels of glucose transporter 4 (GLUT4). We show that body fat accumulation is a major regulator of human adipose LPIN1 expression and suggest a role of LPIN1 in human preadipocyte as well as mature adipocyte function.

The sibutramine metabolite M2 improves muscle glucose uptake and reduces hepatic glucose output: preliminary data

The satiety agent sibutramine acts in part through a primary amine metabolite, M2. To investigate whether M2 could affect glycaemia independently of satiety and weight loss, groups of normal mice received a single dose of M2 (1 or 10 mg/kg) and food was withheld. Compared with controls (who received vehicle only), M2 (10 mg/kg) decreased basal plasma glucose concentrations, with a maximal decrease of about 25% at 48 hours (p < 0.05). Soleus muscles were isolated from the mice at intervals: insulin-mediated glucose uptake by the muscles from controls progressively decreased over 24 hours whereas uptake was maintained by muscles from M2-treated mice. Hepatic gluconeogenesis was reduced about 40% by liver snips isolated from M2-treated mice after 24 hours (p < 0.05). These preliminary results suggest that the M2 metabolite of sibutramine can reduce glycaemia, maintain insulin-mediated muscle glucose uptake and reduce hepatic gluconeogenesis independently of satiety and weight loss.

Microvascular dysfunction: causative role in the association between hypertension, insulin resistance and the metabolic syndrome?

The metabolic syndrome defines a clustering of metabolic risk factors that confers an increased risk for type 2 diabetes and cardiovascular disease. The metabolic syndrome seems to have multiple etiological factors and microvascular dysfunction may be one potential factor explaining the clustering of multiple metabolic risk factors including hypertension, obesity, insulin resistance and glucose intolerance. Microvascular dysfunction may increase not only peripheral vascular resistance and blood pressure, but may also decrease insulin-mediated glucose uptake in muscle. The present article summarizes some of the data concerning the role of microvascular dysfunction in the metabolic syndrome.