Impaired postprandial skeletal muscle vascular responses to a mixed meal challenge in normoglycaemic people with a parent with type 2 diabetes.

AIMS/HYPOTHESIS: Microvascular blood flow (MBF) increases in skeletal muscle postprandially to aid in glucose delivery and uptake in muscle. This vascular action is impaired in individuals who are obese or have type 2 diabetes. Whether MBF is impaired in normoglycaemic people at risk of type 2 diabetes is unknown. We aimed to determine whether apparently healthy people at risk of type 2 diabetes display impaired skeletal muscle microvascular responses to a mixed-nutrient meal. METHODS: In this cross-sectional study, participants with no family history of type 2 diabetes (FH-) for two generations (n = 18), participants with a positive family history of type 2 diabetes (FH+; i.e. a parent with type 2 diabetes; n = 16) and those with type 2 diabetes (n = 12) underwent a mixed meal challenge (MMC). Metabolic responses (blood glucose, plasma insulin and indirect calorimetry) were measured before and during the MMC. Skeletal muscle large artery haemodynamics (2D and Doppler ultrasound, and Mobil-O-graph) and microvascular responses (contrast-enhanced ultrasound) were measured at baseline and 1 h post MMC. RESULTS: Despite normal blood glucose concentrations, FH+ individuals displayed impaired metabolic flexibility (reduced ability to switch from fat to carbohydrate oxidation vs FH-; p < 0.05) during the MMC. The MMC increased forearm muscle microvascular blood volume in both the FH- (1.3-fold, p < 0.01) and FH+ (1.3-fold, p < 0.05) groups but not in participants with type 2 diabetes. However, the MMC increased MBF (1.9-fold, p < 0.01), brachial artery diameter (1.1-fold, p < 0.01) and brachial artery blood flow (1.7-fold, p < 0.001) and reduced vascular resistance (0.7-fold, p < 0.001) only in FH- participants, with these changes being absent in FH+ and type 2 diabetes. Participants with type 2 diabetes displayed significantly higher vascular stiffness (p < 0.001) compared with those in the FH- and FH+ groups; however, vascular stiffness did not change during the MMC in any participant group. CONCLUSIONS/INTERPRETATION: Normoglycaemic FH+ participants display impaired postprandial skeletal muscle macro- and microvascular responses, suggesting that poor vascular responses to a meal may contribute to their increased risk of type 2 diabetes. We conclude that vascular insulin resistance may be an early precursor to type 2 diabetes in humans, which can be revealed using an MMC.

Development and characterisation of a rat model that exhibits both metabolic dysfunction and neurodegeneration seen in type 2 diabetes.

Accurate modelling type 2 diabetes and diabetic complications in rodents has proven a challenge, largely as a result of the long-time course of disease development in humans. In the present study, we aimed to develop and comprehensively characterise a new rodent model of type 2 diabetes. To do this, we fed Sprague-Dawley rats a high fat/high sugar diet (HFD) to induce obesity and dyslipidaemia. After 3 weeks, we s.c. implanted osmotic mini pumps to enable a 14 day, slow infusion of streptozotocin (STZ; lower dose = 100 mg kg-1 ; higher dose = 120 mg kg-1 ) to dose-dependently reduce pancreatic beta cell mass. After removing the mini pumps, we monitored animals for 4 months using a battery of tests to assess both metabolic and neurodegenerative changes across time. Our data demonstrate the combination of the HFD and lower dose STZ leads to induction of early-stage type 2 diabetes defined by moderate hyperglycaemia, hyperinsulinaemia and impaired glucose tolerance, at the same time as the retention of an obese phenotype. By contrast, combining the HFD and higher dose STZ leads to induction of later-stage type 2 diabetes defined by frank hyperglycaemia, hypoinsulinaemia (but not insulin depletion) and severely impaired glucose tolerance, at the same time as retaining an obese phenotype. Regardless of dose of STZ (and level of hyperglycaemia), all diabetic rats exhibited signs of peripheral neurodegeneration in the skin and muscle. Thus, this model recapitulates many of the complex metabolic disturbances seen in type 2 diabetes and provides an excellent platform for investigating the pathophysiological mechanisms that lead to diabetic complications such as peripheral neuropathy. KEY POINTS: Type 2 diabetes is a major health concern and markedly increases risk cardiovascular and neurodegenerative diseases. Accurate modelling of type 2 diabetes is a major challenge and has impeded our ability to understand the mechanisms that contribute to complications of type 2 diabetes. We have developed a method of inducing different stages of type 2 diabetes using a high fat/high sugar diet and 14 day infusion of streptozotocin to dose-dependently destroy pancreatic beta cell mass. Over 4 months, we comprehensively characterised these animals and confirmed that they develop sustained metabolic dysfunction and progressive peripheral neurodegeneration as seen in type 2 diabetes. This new model will improve our ability to investigate the pathophysiological mechanisms that link type 2 diabetes with complications such as neurodegeneration.

Impaired postprandial adipose tissue microvascular blood flow responses to a mixed-nutrient meal in first-degree relatives of adults with type 2 diabetes.

Adipose tissue microvascular blood flow (MBF) is stimulated postprandially to augment delivery of nutrients and hormones to adipocytes. Adipose tissue MBF is impaired in type 2 diabetes (T2D). Whether healthy individuals at-risk of T2D show similar impairments is unknown. We aimed to determine whether adipose tissue MBF is impaired in apparently healthy individuals with a family history of T2D. Overnight-fasted individuals with no family history of T2D for two generations (FH-, n = 13), with at least one parent with T2D (FH+, n = 14) and clinically diagnosed T2D (n = 11) underwent a mixed meal challenge (MMC). Metabolic responses [blood glucose, plasma insulin, plasma nonesterified fatty acids (NEFAs), and fat oxidation] were measured before and during the MMC. MBF in truncal subcutaneous adipose tissue was assessed by contrast ultrasound while fasting and 60 min post-MMC. FH+ had normal blood glucoses, increased adiposity, and impaired post-MMC adipose tissue MBF (Δ0.70 ± 0.22 vs. 2.45 ± 0.60 acoustic intensity/s, P = 0.007) and post-MMC adipose tissue insulin resistance (Adipo-IR index; Δ45.5 ± 13.9 vs. 7.8 ± 5.1 mmol/L × pmol/L, P = 0.007) compared with FH-. FH+ and T2D had an impaired ability to suppress fat oxidation post-MMC. Fat oxidation incremental area under the curve (iAUC) (35-55 min post-MMC, iAUC) was higher in FH+ and T2D than in FH- (P = 0.005 and 0.009, respectively). Postprandial MBF was negatively associated with postprandial fat oxidation iAUC (P = 0.01). We conclude that apparently healthy FH+ individuals display blunted postprandial adipose tissue MBF that occurs in parallel with adipose tissue insulin resistance and impaired suppression of fat oxidation, which may help explain their heightened risk for developing T2D.NEW & NOTEWORTHY Adipose tissue blood flow plays a key role in postprandial nutrient storage. People at-risk of type 2 diabetes have impaired postmeal adipose tissue blood flow. Impaired adipose tissue blood flow is associated with altered fat oxidation. Risk of type 2 diabetes may be elevated by poor adipose tissue blood flow.

Pharmacological PDGFRß inhibitors imatinib and sunitinib cause human brain pericyte death in vitro.

Capillary pericytes have numerous functions important for tissue maintenance. Changes in pericyte function are implicated in diseases such as cancer, where pericyte-mediated angiogenesis contributes to the blood supply that tumors use to survive. Some anti-cancer agents, like imatinib, target platelet-derived growth factor receptor-beta (PDGFRß). Healthy pericytes rely on PDGFRß phosphorylation for their survival. Therefore, we hypothesised that pharmacological agents that block PDGFRß phosphorylation could be used to kill pericytes. We treated human brain vascular pericytes, which express PDGFRß, with three receptor tyrosine kinase inhibitors: imatinib, sunitinib and orantinib. Imatinib and sunitinib, but not orantinib, inhibited PDGFRß phosphorylation in pericytes. Imatinib and sunitinib also reduced viability, prevented proliferation, and induced death, while orantinib only blocked pericyte proliferation. Overall, we found that receptor tyrosine kinase inhibitors that block PDGFRß phosphorylation cause healthy pericytes to die in vitro. While useful in cancer to limit tumor growth, these agents could impair healthy brain pericyte survival and impact brain function.

Insulin Resistance in the Brain: Evidence Supporting a Role for Inflammation, Reactive Microglia, and the Impact of Biological Sex.

Increased intake of highly processed, energy-dense foods combined with a sedentary lifestyle are helping fuel the current overweight and obesity crisis, which is more prevalent in women than in men. Although peripheral organs such as adipose tissue contribute to the physiological development of obesity, emerging work aims to understand the role of the central nervous system to whole-body energy homeostasis and development of weight gain and obesity. The present review discusses the impact of insulin, insulin resistance, free fatty acids, and inflammation on brain function and how these differ between the males and females in the context of obesity. We highlight the potential of microglia, the resident immune cells in the brain, as mediators of neuronal insulin resistance that drive reduced satiety, increased food intake, and thus, obesity.

Novel Short-Chain Quinones to Treat Vision Loss in a Rat Model of Diabetic Retinopathy.

Diabetic retinopathy (DR), one of the leading causes of blindness, is mainly diagnosed based on the vascular pathology of the disease. Current treatment options largely focus on this aspect with mostly insufficient therapeutic long-term efficacy. Mounting evidence implicates mitochondrial dysfunction and oxidative stress in the central etiology of DR. Consequently, drug candidates that aim at normalizing mitochondrial function could be an attractive therapeutic approach. This study compared the mitoprotective compounds, idebenone and elamipretide, side-by-side against two novel short-chain quinones (SCQs) in a rat model of DR. The model effectively mimicked type 2 diabetes over 21 weeks. During this period, visual acuity was monitored by measuring optokinetic response (OKR). Vision loss occurred 5-8 weeks after the onset of hyperglycemia. After 10 weeks of hyperglycemia, visual function was reduced by 65%. From this point, the right eyes of the animals were topically treated once daily with the test compounds. The left, untreated eye served as an internal control. Only three weeks of topical treatment significantly restored vision from 35% to 58-80%, while visual acuity of the non-treated eyes continued to deteriorate. Interestingly, the two novel SCQs restored visual acuity better than idebenone or elamipretide. This was also reflected by protection of retinal pathology against oxidative damage, retinal ganglion cell loss, reactive gliosis, vascular leakage, and retinal thinning. Overall, mitoprotective and, in particular, SCQ-based compounds have the potential to be developed into effective and fast-acting drug candidates against DR.

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.

Postprandial microvascular blood flow in skeletal muscle: Similarities and disparities to the hyperinsulinaemic-euglycaemic clamp.

Skeletal muscle contributes to ~40% of total body mass and has numerous important mechanical and metabolic roles in the body. Skeletal muscle is a major site for glucose disposal following a meal. Consequently, skeletal muscle plays an important role in postprandial blood glucose homeostasis. Over the past number of decades, research has demonstrated that insulin has an important role in vasodilating the vasculature in skeletal muscle in response to an insulin infusion (hyperinsulinaemic-euglycaemic clamp) or following the ingestion of a meal. This vascular action of insulin is pivotal for glucose disposal in skeletal muscle, as insulin-stimulated vasodilation increases the delivery of both glucose and insulin to the myocyte. Notably, in insulin-resistant states such as obesity and type 2 diabetes, this vascular response of insulin in skeletal muscle is significantly impaired. Whereas the majority of work in this field has focussed on the action of insulin alone on skeletal muscle microvascular blood flow and myocyte glucose metabolism, there is less understanding of how the consumption of a meal may affect skeletal muscle blood flow. This is in part due to complex variations in glucose and insulin dynamics that occurs postprandially-with changes in humoral concentrations of glucose, insulin, amino acids, gut and pancreatic peptides-compared to the hyperinsulinaemic-euglycaemic clamp. This review will address the emerging body of evidence to suggest that postprandial blood flow responses in skeletal muscle may be a function of the nutritional composition of a meal.

Are the metabolic benefits of resistance training in type 2 diabetes linked to improvements in adipose tissue microvascular blood flow?

The microcirculation in adipose tissue is markedly impaired in type 2 diabetes (T2D). Resistance training (RT) often increases muscle mass and promotes a favorable metabolic profile in people with T2D, even in the absence of fat loss. Whether the metabolic benefits of RT in T2D are linked to improvements in adipose tissue microvascular blood flow is unknown. Eighteen sedentary people with T2D (7 women/11 men, 52 ± 7 yr) completed 6 wk of RT. Before and after RT, overnight-fasted participants had blood sampled for clinical chemistries (glucose, insulin, lipids, HbA1c, and proinflammatory markers) and underwent an oral glucose challenge (OGC; 50 g glucose × 2 h) and a DEXA scan to assess body composition. Adipose tissue microvascular blood volume and flow were assessed at rest and 1 h post-OGC using contrast-enhanced ultrasound. RT significantly reduced fasting blood glucose ( P = 0.006), HbA1c ( P = 0.007), 2-h glucose area under the time curve post-OGC ( P = 0.014), and homeostatic model assessment of insulin resistance ( P = 0.005). This was accompanied by a small reduction in total body fat ( P = 0.002), trunk fat ( P = 0.023), and fasting triglyceride levels ( P = 0.029). Lean mass ( P = 0.003), circulating TNF-α ( P = 0.006), and soluble VCAM-1 ( P < 0.001) increased post-RT. There were no significant changes in adipose tissue microvascular blood volume or flow following RT; however those who did have a higher baseline microvascular blood flow post-RT also had lower fasting triglyceride levels ( r = -0.476, P = 0.045). The anthropometric, glycemic, and insulin-sensitizing benefits of 6 wk of RT in people with T2D are not associated with an improvement in adipose tissue microvascular responses; however, there may be an adipose tissue microvascular-linked benefit to fasting triglyceride levels.

Regulation of microvascular flow and metabolism: An overview.

Skeletal muscle is an important site for insulin to regulate blood glucose levels. It is estimated that skeletal muscle is responsible for ~80% of insulin-mediated glucose disposal in the post-prandial period. The classical action of insulin to increase muscle glucose uptake involves insulin binding to insulin receptors on myocytes to stimulate glucose transporter 4 (GLUT 4) translocation to the cell surface membrane, enhancing glucose uptake. However, an additional role of insulin that is often under-appreciated is its action to increase muscle perfusion thereby improving insulin and glucose delivery to myocytes. Either of these responses (myocyte and/or vascular) may be impaired in insulin resistance, and both impairments are apparent in type 2 diabetes, resulting in diminished glucose disposal by muscle. The aim of this review is to report on the growing body of literature suggesting that insulin-mediated control of skeletal muscle perfusion is an important regulator of muscle glucose uptake and that impairment of microvascular insulin action has important physiological consequences early in the pathogenesis of insulin resistance. This work was discussed at the 2015 Australian Physiological Society Symposium "Physiological mechanisms controlling microvascular flow and muscle metabolism".

Muscle microvascular blood flow responses in insulin resistance and ageing.

Insulin resistance plays a key role in the development of type 2 diabetes. Skeletal muscle is the major storage site for glucose following a meal and as such has a key role in maintenance of blood glucose concentrations. Insulin resistance is characterised by impaired insulin-mediated glucose disposal in skeletal muscle. Multiple mechanisms can contribute to development of muscle insulin resistance and our research has demonstrated an important role for loss of microvascular function within skeletal muscle. We have shown that insulin can enhance blood flow to the microvasculature in muscle thus improving the access of glucose and insulin to the myocytes to augment glucose disposal. Obesity, insulin resistance and ageing are all associated with impaired microvascular responses to insulin in skeletal muscle. Impairments in insulin-mediated microvascular perfusion in muscle can directly cause insulin resistance, and this event can occur early in the aetiology of this condition. Understanding the mechanisms involved in the loss of microvascular function in muscle has the potential to identify novel treatment strategies to prevent or delay progression of insulin resistance and type 2 diabetes.

No effect of NOS inhibition on skeletal muscle glucose uptake during in situ hindlimb contraction in healthy and diabetic Sprague-Dawley rats.

Nitric oxide (NO) has been shown to be involved in skeletal muscle glucose uptake during contraction/exercise, especially in individuals with Type 2 diabetes (T2D). To examine the potential mechanisms, we examined the effect of local NO synthase (NOS) inhibition on muscle glucose uptake and muscle capillary blood flow during contraction in healthy and T2D rats. T2D was induced in Sprague-Dawley rats using a combined high-fat diet (23% fat wt/wt for 4 wk) and low-dose streptozotocin injections (35 mg/kg). Anesthetized animals had one hindlimb stimulated to contract in situ for 30 min (2 Hz, 0.1 ms, 35 V) with the contralateral hindlimb rested. After 10 min, the NOS inhibitor, N(G)-nitro-l-arginine methyl ester (l-NAME; 5 µM) or saline was continuously infused into the femoral artery of the contracting hindlimb until the end of contraction. Surprisingly, there was no increase in skeletal muscle NOS activity during contraction in either group. Local NOS inhibition had no effect on systemic blood pressure or muscle contraction force, but it did cause a significant attenuation of the increase in femoral artery blood flow in control and T2D rats. However, NOS inhibition did not attenuate the increase in muscle capillary recruitment during contraction in these rats. Muscle glucose uptake during contraction was significantly higher in T2D rats compared with controls but, unlike our previous findings in hooded Wistar rats, NOS inhibition had no effect on glucose uptake during contraction. In conclusion, NOS inhibition did not affect muscle glucose uptake during contraction in control or T2D Sprague-Dawley rats, and this may have been because there was no increase in NOS activity during contraction.

A vascular mechanism for high-sodium-induced insulin resistance in rats.

AIMS/HYPOTHESIS: High sodium (HS) effects on hypertension are well established. Recent evidence implicates a relationship between HS intake and insulin resistance, even in the absence of hypertension. The aim of the current study was to determine whether loss of the vascular actions of insulin may be the driving factor linking HS intake to insulin resistance. METHODS: Sprague Dawley rats were fed a control (0.31% wt/wt NaCl) or HS (8.00% wt/wt NaCl) diet for 4 weeks and subjected to euglycaemic-hyperinsulinaemic clamp (10 mU min(-1) kg(-1)) or constant-flow pump-perfused hindlimb studies following an overnight fast. A separate group of HS rats was given quinapril during the dietary intervention and subjected to euglycaemic-hyperinsulinaemic clamp as above. RESULTS: HS intake had no effect on body weight or fat mass or on fasting glucose, insulin, endothelin-1 or NEFA concentrations. However, HS impaired whole body and skeletal muscle glucose uptake, in addition to a loss of insulin-stimulated microvascular recruitment. These effects were present despite enhanced insulin signalling (Akt) in both liver and skeletal muscle. Constant-flow pump-perfused hindlimb experiments revealed normal insulin-stimulated myocyte glucose uptake in HS-fed rats. Quinapril treatment restored insulin-mediated microvascular recruitment and muscle glucose uptake in vivo. CONCLUSIONS/INTERPRETATION: HS-induced insulin resistance is driven by impaired microvascular responsiveness to insulin, and is not due to metabolic or signalling defects within myocytes or liver. These results imply that reducing sodium intake may be important not only for management of hypertension but also for insulin resistance, and highlight the vasculature as a potential therapeutic target in the prevention of insulin resistance.

Studying the photothermal activation of polydopamine-shelled, phase-change emulsion droplets into microbubbles using small- and ultra-small-angle neutron scattering.

Polydopamine-shelled perfluorocarbon (PDA/PFC) emulsion droplets are promising candidates for medical imaging and drug delivery applications. This study investigates their phase transition into microbubbles under near-infrared (NIR) illumination in situ using small- and ultra-small-angle neutron scattering (SANS and USANS) and contrast variation techniques. Supported by optical microscopy, thermogravimetric analysis, and ultrasound imaging, SANS and USANS results reveal rapid phase transition rates upon NIR illumination, dependent on PFC content and droplet size distribution. Specifically, perfluoropentane droplets rapidly transform into bubbles upon NIR irradiation, whereas perfluorohexane droplets exhibit greater resistance to phase change (bulk boiling points = 30 °C and 60 °C, respectively). Furthermore, smaller emulsion droplets with unimodal distribution resist NIR-triggered phase changes better than their bimodal counterparts. This observation is attributable to the lower boiling points of large emulsion droplets (lower Laplace pressure than smaller droplets) and the faster photothermal heating rates due to their thicker polydopamine shells. The insights gained from these techniques are crucial for designing phase-change emulsions activated by NIR for photothermal therapies and controlled drug delivery.

Induction of Type 2 Diabetes in Mice to Understand Vascular Changes That Drive Diabetic Retinopathy.

Diabetic retinopathy is a common complication of type 2 diabetes. Research into this comorbidity is challenging due to the slow progression of pathological changes and the limited transgenic models available to study disease progression and mechanistic changes. Here, we describe a non-transgenic mouse model of accelerated type 2 diabetes using a high-fat diet in combination with streptozotocin delivered via osmotic mini pump. This model, when subjected to fluorescent gelatin vascular casting, can be used to study vascular changes in type 2 diabetic retinopathy.

An Abductive Inference Approach to Assess the Performance-Enhancing Effects of Drugs Included on the World Anti-Doping Agency Prohibited List.

Some have questioned the evidence for performance-enhancing effects of several substances included on the World Anti-Doping Agency's Prohibited List due to the divergent or inconclusive findings in randomized controlled trials (RCTs). However, inductive statistical inference based on RCTs-only may result in biased conclusions because of the scarcity of studies, inter-study heterogeneity, too few outcome events, or insufficient power. An abductive inference approach, where the body of evidence is evaluated beyond considerations of statistical significance, may serve as a tool to assess the plausibility of performance-enhancing effects of substances by also considering observations and facts not solely obtained from RCTs. Herein, we explored the applicability of an abductive inference approach as a tool to assess the performance-enhancing effects of substances included on the Prohibited List. We applied an abductive inference approach to make inferences on debated issues pertaining to the ergogenic effects of recombinant human erythropoietin (rHuEPO), beta2-agonists and anabolic androgenic steroids (AAS), and extended the approach to more controversial drug classes where RCTs are limited. We report that an abductive inference approach is a useful tool to assess the ergogenic effect of substances included on the Prohibited List-particularly for substances where inductive inference is inconclusive. Specifically, a systematic abductive inference approach can aid researchers in assessing the effects of doping substances, either by leading to suggestions of causal relationships or identifying the need for additional research.

Acute, local infusion of angiotensin II impairs microvascular and metabolic insulin sensitivity in skeletal muscle.

AIMS: Angiotensin II (AngII) is a potent vasoconstrictor implicated in both hypertension and insulin resistance. Insulin dilates the vasculature in skeletal muscle to increase microvascular blood flow and enhance glucose disposal. In the present study, we investigated whether acute AngII infusion interferes with insulin's microvascular and metabolic actions in skeletal muscle. METHODS AND RESULTS: Adult, male Sprague-Dawley rats received a systemic infusion of either saline, AngII, insulin (hyperinsulinaemic euglycaemic clamp), or insulin (hyperinsulinaemic euglycaemic clamp) plus AngII. A final, separate group of rats received an acute local infusion of AngII into a single hindleg during systemic insulin (hyperinsulinaemic euglycaemic clamp) infusion. In all animals' systemic metabolic effects, central haemodynamics, femoral artery blood flow, microvascular blood flow, and skeletal muscle glucose uptake (isotopic glucose) were monitored. Systemic AngII infusion increased blood pressure, decreased heart rate, and markedly increased circulating glucose and insulin concentrations. Systemic infusion of AngII during hyperinsulinaemic euglycaemic clamp inhibited insulin-mediated suppression of hepatic glucose output and insulin-stimulated microvascular blood flow in skeletal muscle but did not alter insulin's effects on the femoral artery or muscle glucose uptake. Local AngII infusion did not alter blood pressure, heart rate, or circulating glucose and insulin. However, local AngII inhibited insulin-stimulated microvascular blood flow, and this was accompanied by reduced skeletal muscle glucose uptake. CONCLUSIONS: Acute infusion of AngII significantly alters basal haemodynamic and metabolic homeostasis in rats. Both local and systemic AngII infusion attenuated insulin's microvascular actions in skeletal muscle, but only local AngII infusion led to reduced insulin-stimulated muscle glucose uptake. While increased local, tissue production of AngII may be a factor that couples microvascular insulin resistance and hypertension, additional studies are needed to determine the molecular mechanisms responsible for these vascular defects.

Metformin improves vascular and metabolic insulin action in insulin-resistant muscle.

Insulin stimulates glucose disposal in skeletal muscle in part by increasing microvascular blood flow, and this effect is blunted during insulin resistance. We aimed to determine whether metformin treatment improves insulin-mediated glucose disposal and vascular insulin responsiveness in skeletal muscle of insulin-resistant rats. Sprague-Dawley rats were fed a normal (ND) or high-fat (HFD) diet for 4 weeks. A separate HFD group was given metformin in drinking water (HFD + MF, 150 mg/kg/day) during the final 2 weeks. After the intervention, overnight-fasted (food and metformin removed) anaesthetised rats underwent a 2-h euglycaemic-hyperinsulinaemic clamp (10 mU/min/kg) or saline infusion. Femoral artery blood flow, hindleg muscle microvascular blood flow, muscle glucose disposal and muscle signalling (Ser473-AKT and Thr172-AMPK phosphorylation) were measured. HFD rats had elevated body weight, epididymal fat pad weight, fasting plasma insulin and free fatty acid levels when compared to ND. HFD-fed animals displayed whole-body and skeletal muscle insulin resistance and blunting of insulin-stimulated femoral artery blood flow, muscle microvascular blood flow and skeletal muscle insulin-stimulated Ser473-AKT phosphorylation. Metformin treatment of HFD rats reduced fasting insulin and free fatty acid concentrations and lowered body weight and adiposity. During euglycaemic-hyperinsulinaemic clamp, metformin-treated animals showed improved vascular responsiveness to insulin, improved insulin-stimulated muscle Ser473-AKT phosphorylation but only partially restored (60%) muscle glucose uptake. This occurred without any detectable levels of metformin in plasma or change in muscle Thr172-AMPK phosphorylation. We conclude that 2-week metformin treatment is effective at improving vascular and metabolic insulin responsiveness in muscle of HFD-induced insulin-resistant rats.