Remodeling of the skeletal muscle microcirculation increases resistance to perfusion in obese Zucker rats

The Role of Methionine-Rich Diet in Unhealthy Cerebrovascular and Brain Aging: Mechanisms and Implications for Cognitive Impairment

As aging societies in the western world face a growing prevalence of vascular cognitive impairment and Alzheimer's disease (AD), understanding their underlying causes and associated risk factors becomes increasingly critical. A salient concern in the western dietary context is the high consumption of methionine-rich foods such as red meat. The present review delves into the impact of this methionine-heavy diet and the resultant hyperhomocysteinemia on accelerated cerebrovascular and brain aging, emphasizing their potential roles in cognitive impairment. Through a comprehensive exploration of existing evidence, a link between high methionine intake and hyperhomocysteinemia and oxidative stress, mitochondrial dysfunction, inflammation, and accelerated epigenetic aging is drawn. Moreover, the microvascular determinants of cognitive deterioration, including endothelial dysfunction, reduced cerebral blood flow, microvascular rarefaction, impaired neurovascular coupling, and blood-brain barrier (BBB) disruption, are explored. The mechanisms by which excessive methionine consumption and hyperhomocysteinemia might drive cerebromicrovascular and brain aging processes are elucidated. By presenting an intricate understanding of the relationships among methionine-rich diets, hyperhomocysteinemia, cerebrovascular and brain aging, and cognitive impairment, avenues for future research and potential therapeutic interventions are suggested.

Application of a novel index for understanding vascular health following pharmacological intervention in a pre-clinical model of metabolic disease

While a thorough understanding of microvascular function in health and how it becomes compromised with progression of disease risk is critical for developing effective therapeutic interventions, our ability to accurately assess the beneficial impact of pharmacological interventions to improve outcomes is vital. Here we introduce a novel Vascular Health Index (VHI) that allows for simultaneous assessment of changes to vascular reactivity/endothelial function, vascular wall mechanics and microvessel density within cerebral and skeletal muscle vascular networks with progression of metabolic disease in obese Zucker rats (OZR); under control conditions and following pharmacological interventions of clinical relevance. Outcomes are compared to "healthy" conditions in lean Zucker rats. We detail the calculation of vascular health index, full assessments of validity, and describe progressive changes to vascular health index over the development of metabolic disease in obese Zucker rats. Further, we detail the improvement to cerebral and skeletal muscle vascular health index following chronic treatment of obese Zucker rats with anti-hypertensive (15%-52% for skeletal muscle vascular health index; 12%-48% for cerebral vascular health index; p < 0.05 for both), anti-dyslipidemic (13%-48% for skeletal muscle vascular health index; p < 0.05), anti-diabetic (12%-32% for cerebral vascular health index; p < 0.05) and anti-oxidant/inflammation (41%-64% for skeletal muscle vascular health index; 29%-42% for cerebral vascular health index; p < 0.05 for both) drugs. The results present the effectiveness of mechanistically diverse interventions to improve cerebral or skeletal muscle vascular health index in obese Zucker rats and provide insight into the superiority of some pharmacological agents despite similar effectiveness in terms of impact on intended targets. In addition, we demonstrate the utility of including a wider, more integrative approach to the study of microvasculopathy under settings of elevated disease risk and following pharmacological intervention. A major benefit of integrating vascular health index is an increased understanding of the development, timing and efficacy of interventions through greater insight into integrated microvascular function in combination with individual, higher resolution metrics.

A novel vascular health index: Using data analytics and population health to facilitate mechanistic modeling of microvascular status

The study of vascular function across conditions has been an intensive area of investigation for many years. While these efforts have revealed many factors contributing to vascular health, challenges remain for integrating results across research groups, animal models, and experimental conditions to understand integrated vascular function. As such, the insights attained in clinical/population research from linking datasets, have not been fully realized in the basic sciences, thus frustrating advanced analytics and complex modeling. To achieve comparable advances, we must address the conceptual challenge of defining/measuring integrated vascular function and the technical challenge of combining data across conditions, models, and groups. Here, we describe an approach to establish and validate a composite metric of vascular function by comparing parameters of vascular function in metabolic disease (the obese Zucker rat) to the same parameters in age-matched, "healthy" conditions, resulting in a common outcome measure which we term the vascular health index (VHI). VHI allows for the integration of datasets, thus expanding sample size and permitting advanced modeling to gain insight into the development of peripheral and cerebral vascular dysfunction. Markers of vascular reactivity, vascular wall mechanics, and microvascular network density are integrated in the VHI. We provide a detailed presentation of the development of the VHI and provide multiple measures to assess face, content, criterion, and discriminant validity of the metric. Our results demonstrate how the VHI captures multiple indices of dysfunction in the skeletal muscle and cerebral vasculature with metabolic disease and provide context for an integrated understanding of vascular health under challenged conditions.

Role of adropin in arterial stiffening associated with obesity and type 2 diabetes

Adropin is a peptide largely secreted by the liver and known to regulate energy homeostasis; however, it also exerts cardiovascular effects. Herein, we tested the hypothesis that low circulating levels of adropin in obesity and type 2 diabetes (T2D) contribute to arterial stiffening. In support of this hypothesis, we report that obesity and T2D are associated with reduced levels of adropin (in liver and plasma) and increased arterial stiffness in mice and humans. Establishing causation, we show that mesenteric arteries from adropin knockout mice are also stiffer, relative to arteries from wild-type counterparts, thus recapitulating the stiffening phenotype observed in T2D db/db mice. Given the above, we performed a set of follow-up experiments, in which we found that 1) exposure of endothelial cells or isolated mesenteric arteries from db/db mice to adropin reduces filamentous actin (F-actin) stress fibers and stiffness, 2) adropin-induced reduction of F-actin and stiffness in endothelial cells and db/db mesenteric arteries is abrogated by inhibition of nitric oxide (NO) synthase, and 3) stimulation of smooth muscle cells or db/db mesenteric arteries with a NO mimetic reduces stiffness. Lastly, we demonstrated that in vivo treatment of db/db mice with adropin for 4 wk reduces stiffness in mesenteric arteries. Collectively, these findings indicate that adropin can regulate arterial stiffness, likely via endothelium-derived NO, and thus support the notion that "hypoadropinemia" should be considered as a putative target for the prevention and treatment of arterial stiffening in obesity and T2D.NEW & NOTEWORTHY Arterial stiffening, a characteristic feature of obesity and type 2 diabetes (T2D), contributes to the development and progression of cardiovascular diseases. Herein we establish that adropin is decreased in obese and T2D models and furthermore provide evidence that reduced adropin may directly contribute to arterial stiffening. Collectively, findings from this work support the notion that "hypoadropinemia" should be considered as a putative target for the prevention and treatment of arterial stiffening in obesity and T2D.

The development of peripheral microvasculopathy with chronic metabolic disease in obese Zucker rats: a retrograde emergence?

The study of peripheral vasculopathy with chronic metabolic disease is challenged by divergent contributions from spatial (the level of resolution or specific tissue being studied) and temporal origins (evolution of the developing impairments in time). Over many years of studying the development of skeletal muscle vasculopathy and its functional implications, we may be at the point of presenting an integrated conceptual model that addresses these challenges within the obese Zucker rat (OZR) model. At the early stages of metabolic disease, where systemic markers of elevated cardiovascular disease risk are present, the only evidence of vascular dysfunction is at postcapillary and collecting venules, where leukocyte adhesion/rolling is elevated with impaired venular endothelial function. As metabolic disease severity and duration increases, reduced microvessel density becomes evident as well as increased variability in microvascular hematocrit. Subsequently, hemodynamic impairments to distal arteriolar networks emerge, manifesting as increasing perfusion heterogeneity and impaired arteriolar reactivity. This retrograde "wave of dysfunction" continues, creating a condition wherein deficiencies to the distal arteriolar, capillary, and venular microcirculation stabilize and impairments to proximal arteriolar reactivity, wall mechanics, and perfusion distribution evolve. This proximal arteriolar dysfunction parallels increasing failure in fatigue resistance, hyperemic responses, and O2 uptake within self-perfused skeletal muscle. Taken together, these results present a conceptual model for the retrograde development of peripheral vasculopathy with chronic metabolic disease and provide insight into the timing and targeting of interventional strategies to improve health outcomes.NEW & NOTEWORTHY Working from an established database spanning multiple scales and times, we studied progression of peripheral microvascular dysfunction in chronic metabolic disease. The data implicate the postcapillary venular endothelium as the initiating site for vasculopathy. Indicators of dysfunction, spanning network structures, hemodynamics, vascular reactivity, and perfusion progress in an insidious retrograde manner to present as functional impairments to muscle blood flow and performance much later. The silent vasculopathy progression may provide insight into clinical treatment challenges.

A randomized clinical trial on the effects of exercise on muscle remodelling following bariatric surgery

BACKGROUND

Muscle atrophy and strength loss are common adverse outcomes following bariatric surgery. This randomized, controlled trial investigated the effects of exercise training on bariatric surgery-induced loss of muscle mass and function. Additionally, we investigated the effects of the intervention on molecular and histological mediators of muscle remodelling.

METHODS

Eighty women with obesity were randomly assigned to a Roux-en-Y gastric bypass (RYGB: n = 40, age = 42 ± 8 years) or RYGB plus exercise training group (RYGB + ET: n = 40, age = 38 ± 7 years). Clinical and laboratory parameters were assessed at baseline, and 3 (POST3) and 9 months (POST9) after surgery. The 6 month, three-times-a-week, exercise intervention (resistance plus aerobic exercise) was initiated 3 months post-surgery (for RYGB + ET). A healthy, lean, age-matched control group was recruited to provide reference values for selected variables.

RESULTS

Surgery resulted in a similar (P = 0.66) reduction in lower-limb muscle strength in RYGB and RYGB+ET (-26% vs. -31%), which was rescued to baseline values in RYGB + ET (P = 0.21 vs. baseline) but not in RYGB (P < 0.01 vs. baseline). Patients in RYGB+ET had greater absolute (214 vs. 120 kg, P < 0.01) and relative (2.4 vs. 1.4 kg/body mass, P < 0.01) muscle strength compared with RYGB alone at POST9. Exercise resulted in better performance in timed-up-and-go (6.3 vs. 7.1 s, P = 0.05) and timed-stand-test (18 vs. 14 repetitions, P < 0.01) compared with RYGB. Fat-free mass was lower (POST9-PRE) after RYBG than RYGB + ET (total: -7.9 vs. -4.9 kg, P < 0.01; lower-limb: -3.8 vs. -2.7 kg, P = 0.02). Surgery reduced Types I (~ - 21%; P = 0.99 between-group comparison) and II fibre cross-sectional areas (~ - 27%; P = 0.88 between-group comparison), which were rescued to baseline values in RYGB+ET (P > 0.05 vs. baseline) but not RYGB (P > 0.01 vs. baseline). RYGB + ET showed greater Type I (5187 vs. 3898 μm² , P < 0.01) and Type II (5165 vs. 3565 μm² , P < 0.01) fCSA than RYGB at POST9. RYGB + ET also resulted in increased capillarization (P < 0.01) and satellite cell content (P < 0.01) than RYGB at POST9. Gene-set normalized enrichment scores for the muscle transcriptome revealed that the ubiquitin-mediated proteolysis pathway was suppressed in RYGB + ET at POST9 vs. PRE (NES: -1.7; P < 0.01), but not in RYGB. Atrogin-1 gene expression was lower in RYGB + ET vs. RYGB at POST9 (0.18 vs. 0.71-fold change, P < 0.01). From both genotypic and phenotypic perspectives, the muscle of exercised patients resembled that of healthy lean individuals.

CONCLUSIONS

This study provides compelling evidence-from gene to function-that strongly supports the incorporation of exercise into the recovery algorithm for bariatric patients so as to counteract the post-surgical loss of muscle mass and function.

Microvessel Density: Integrating Sex-Based Differences and Elevated Cardiovascular Risks in Metabolic Syndrome

Metabolic syndrome (MetS) is a complex pathological state consisting of metabolic risk factors such as hypertension, insulin resistance, and obesity. The interconnectivity of cellular pathways within various biological systems suggests that each individual component of MetS may share common pathological sources. Additionally, MetS is closely associated with vasculopathy, including a reduction in microvessel density (MVD) (rarefaction) and elevated risk for various cardiovascular diseases. Microvascular impairments may contribute to perfusion-demand mismatch, where local metabolic needs are insufficiently met due to the lack of nutrient and oxygen supply, thus creating pathological positive-feedback loops and furthering the progression of disease. Sexual dimorphism is evident in these underlying cellular mechanisms, which places males and females at different levels of risk for cardiovascular disease and acute ischemic events. Estrogen exhibits protective effects on the endothelium of pre-menopausal women, while androgens may be antagonistic to cardiovascular health. This review examines MetS and its influences on MVD, as well as sex differences relating to the components of MetS and cardiovascular risk profiles. Finally, translational relevance and interventions are discussed in the context of these sex-based differences.

Guidelines for the measurement of vascular function and structure in isolated arteries and veins

The measurement of vascular function in isolated vessels has revealed important insights into the structural, functional, and biomechanical features of the normal and diseased cardiovascular system and has provided a molecular understanding of the cells that constitutes arteries and veins and their interaction. Further, this approach has allowed the discovery of vital pharmacological treatments for cardiovascular diseases. However, the expansion of the vascular physiology field has also brought new concerns over scientific rigor and reproducibility. Therefore, it is appropriate to set guidelines for the best practices of evaluating vascular function in isolated vessels. These guidelines are a comprehensive document detailing the best practices and pitfalls for the assessment of function in large and small arteries and veins. Herein, we bring together experts in the field of vascular physiology with the purpose of developing guidelines for evaluating ex vivo vascular function. By using this document, vascular physiologists will have consistency among methodological approaches, producing more reliable and reproducible results.

Obesity-induced cognitive impairment in older adults: a microvascular perspective

Over two-thirds of individuals aged 65 and older are obese or overweight in the United States. Epidemiological data show an association between the degree of adiposity and cognitive dysfunction in the elderly. In this review, the pathophysiological roles of microvascular mechanisms, including impaired endothelial function and neurovascular coupling responses, microvascular rarefaction, and blood-brain barrier disruption in the genesis of cognitive impairment in geriatric obesity are considered. The potential contribution of adipose-derived factors and fundamental cellular and molecular mechanisms of senescence to exacerbated obesity-induced cerebromicrovascular impairment and cognitive decline in aging are discussed.

Vascular Dysfunction and Insulin Resistance in Aging

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Insulin was discovered in 1922 by Banting and Best. Since that time, extensive research on the mechanisms of insulin activity and action has continued. Currently, it is known that the role of insulin is much greater than simply regulating carbohydrate metabolism. Insulin in physiological concentration is also necessary to maintain normal vascular function.

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Insulin resistance is defined as a pathological condition characterized by reduced sensitivity of skeletal muscles, liver, and adipose tissue, to insulin and its downstream metabolic effects under normal serum glucose concentrations. There are also selective forms of insulin resistance with unique features, including vascular insulin resistance. Insulin resistance, both classical and vascular, contributes to vascular impairment resulting in increased risk of cardiovascular disease. Furthermore, in the elderly population, additional factors including redistribution of fat concentrations, low-grade inflammation, and decreased self-repair capacity [or cell senescence] amplify the vascular abnormalities related to insulin resistance.

Skeletal muscle performance in metabolic disease: Microvascular or mitochondrial limitation or both?

One of the clearly established health outcomes associated with chronic metabolic diseases (eg, type II diabetes mellitus) is that the ability of skeletal muscle to maintain contractile performance during periods of elevated metabolic demand is compromised as compared to the fatigue-resistance of muscle under normal, healthy conditions. While there has been extensive effort dedicated to determining the major factors that contribute to the compromised performance of skeletal muscle with chronic metabolic disease, the extent to which this poor outcome reflects a dysfunctional state of the microcirculation, where the delivery and distribution of metabolic substrates can be impaired, versus derangements to normal metabolic processes and mitochondrial function, versus a combination of the two, represents an area of considerable unknown. The purpose of this manuscript is to present some of the current concepts for dysfunction to both the microcirculation of skeletal muscle as well as to mitochondrial metabolism under these conditions, such that these diverse issues can be merged into an integrated framework for future investigation. Based on an interpretation of the current literature, it may be hypothesized that the primary site of dysfunction with earlier stages of metabolic disease may lie at the level of the vasculature, rather than at the level of the mitochondria.

Sympathoexcitation constrains vasodilation in the human skeletal muscle microvasculature during postocclusive reactive hyperemia

We used diffuse correlation spectroscopy to investigate sympathetic vasoconstriction, local vasodilation, and integration of these two responses in the skeletal muscle microvasculature of 20 healthy volunteers. Diffuse correlation spectroscopy probes were placed on the flexor carpi radialis muscle or vastus lateralis muscle, and a blood flow index was derived continuously. We measured hemodynamic responses during sympathoexcitation induced by forehead cooling, after which the effects of the increased sympathetic tone on vasodilatory responses during postocclusive reactive hyperemia (PORH) were examined. PORH was induced by releasing arterial occlusion (3 min) in an arm or leg. To increase sympathetic tone during PORH, forehead cooling was begun 60 s before the occlusion release and ended 60 s after the release. During forehead cooling, mean arterial pressure rose significantly and was sustained at an elevated level. Significant vasoconstriction and decreases in blood flow index followed by gradual blunting of the vasoconstriction also occurred. The time course of these responses is in good agreement with previous observations in animals. The acute sympathoexcitation diminished the peak vasodilation during PORH only in the vastus lateralis muscle, but it hastened the decline in vasodilation after the peak in both the flexor carpi radialis muscle and vastus lateralis muscle. Consequently, the total vasodilatory response assessed as the area of the vascular conductance during the first minute of PORH was significantly diminished in both regions. We conclude that, in humans, the integrated effects of sympathetic vasoconstriction and local vasodilation have an important role in vascular regulation and control of perfusion in the skeletal muscle microcirculation.

NEW & NOTEWORTHY We used diffuse correlation spectroscopy to demonstrate that acute sympathoexcitation constrains local vasodilation in the human skeletal muscle microvasculature during postocclusive reactive hyperemia. This finding indicates that integration of sympathetic vasoconstriction and local vasodilation is importantly involved in vascular regulation and the control of perfusion of the skeletal muscle microcirculation in humans.

Obesity Downregulates MicroRNA-126 Inducing Capillary Rarefaction in Skeletal Muscle: Effects of Aerobic Exercise Training

Background. We investigated the effects of exercise training (ET) on miR-126 levels and skeletal muscle angiogenesis in obese Zucker rats. Results. Zucker rats were randomly assigned to sedentary and swimming-trained groups: lean sedentary (LS) and trained (LTR); obese sedentary (OB) and trained (OBTR). The OB group displayed capillary rarefaction compared with the LS group. In contrast, ET increased the capillary/fiber ratio by 38% in the LTR group and normalized capillary rarefaction in the OBTR group. VEGF, PI3K, and eNOS levels were reduced in the skeletal muscle of the OB group. ET normalized VEGF, PI3K, and eNOS levels in OBTR, contributing to vascular network homeostasis. PI3KR2 inhibits PI3K, a key mediator of the VEGF signaling pathway. Obesity decreased miR-126 and increased PI3KR2 levels compared with the LS group. However, ET normalized miR-126 levels in the OBTR group versus the LS group and decreased expression of PI3KR2. Conclusion. Our findings show that obesity leads to skeletal muscle capillary rarefaction, which is regulated by decreased miR-126 levels and increased PI3KR2. Inversely, ET normalizes miR-126 levels and VEGF signaling and should be considered an important therapeutic strategy for vascular disorders.

Retinal Capillary Rarefaction in Patients with Type 2 Diabetes Mellitus

Purpose

In diabetes mellitus type 2, capillary rarefaction plays a pivotal role in the pathogenesis of end-organ damage. We investigated retinal capillary density in patients with early disease.

Methods

This cross-sectional study compares retinal capillary rarefaction determined by intercapillary distance (ICD) and capillary area (CapA), measured non-invasively and in vivo by scanning laser Doppler flowmetry, in 73 patients with type 2 diabetes, 55 healthy controls and 134 individuals with hypertension stage 1 or 2.

Results

In diabetic patients, ICD was greater (23.2±5.5 vs 20.2±4.2, p = 0.013) and CapA smaller (1592±595 vs 1821±652, p = 0.019) than in healthy controls after adjustment for differences in cardiovascular risk factors between the groups. Compared to hypertensive patients, diabetic individuals showed no difference in ICD (23.1±5.8, p = 0.781) and CapA (1556±649, p = 0.768).

Conclusion

In the early stage of diabetes type 2, patients showed capillary rarefaction compared to healthy individuals.

Impact of increased intramuscular perfusion heterogeneity on skeletal muscle microvascular hematocrit in the metabolic syndrome

OBJECTIVE

To determine HMV and PS in skeletal muscle of OZR and evaluate the impact of increased microvascular perfusion heterogeneity on mass transport/exchange.

METHODS

The in situ gastrocnemius muscle from OZR and LZR was examined under control conditions and following pretreatment with TEMPOL (antioxidant)/SQ-29548 (PGH2 /TxA2 receptor antagonist), phentolamine (adrenergic antagonist), or all agents combined. A spike input of a labeled blood tracer cocktail was injected into the perfusing artery. Tracer washout was analyzed using models for HMV and PS. HT was determined in in situ cremaster muscle of OZR and LZR using videomicroscopy.

RESULTS

HMV was decreased in OZR versus LZR. While TEMPOL/SQ-29548 or phentolamine had minor effects, treatment with all three agents improved HMV in OZR. HT was not different between strains, although variability was increased in OZR, and normalized following treatment with all three agents. PS was reduced in OZR and was not impacted by intervention.

CONCLUSIONS

Increased microvascular perfusion heterogeneity in OZR reduces HMV in muscle vascular networks and increases its variability, potentially contributing to premature muscle fatigue. While targeted interventions can ameliorate this, the reduced microvascular surface area is not acutely reversible.

Microvascular perfusion heterogeneity contributes to peripheral vascular disease in metabolic syndrome

A major challenge facing public health is the increased incidence and prevalence of the metabolic syndrome, a clinical condition characterized by excess adiposity, impaired glycaemic control, dyslipidaemia and moderate hypertension. The greatest concern for this syndrome is the profound increase in risk for development of peripheral vascular disease (PVD) in afflicted persons. However, ongoing studies suggest that reductions in bulk blood flow to skeletal muscle may not be the primary contributor to the premature muscle fatigue that is a hallmark of PVD. Compelling evidence has been provided suggesting that an increasingly spatially heterogeneous and temporally stable distribution of blood flow at successive arteriolar bifurcations in metabolic syndrome creates an environment where a large number of the pre-capillary arterioles have low perfusion, low haematocrit, and are increasingly confined to this state, with limited ability to adapt perfusion in response to a challenged environment. Single pharmacological interventions are unable to significantly restore function owing to a divergence in their spatial effectiveness, although combined therapeutic approaches to correct adrenergic dysfunction, elevated oxidant stress and increased thromboxane A2 improve perfusion-based outcomes. Integrated, multi-target therapeutic interventions designed to restore healthy network function and flexibility may provide for superior outcomes in subjects with metabolic syndrome-associated PVD.

Insulin resistance and skeletal muscle vasculature: significance, assessment and therapeutic modulators

Overnutrition and sedentarism are closely related to the alarming incidence of obesity and type 2 diabetes mellitus (DM2) in the Western world. Resistance to the actions of insulin is a common occurrence in conditions such as obesity, hypertension and DM2. In the skeletal muscle vasculature, insulin promotes vasodilation and its own transport across the vascular wall to reach its target tissue. Furthermore, insulin resistance (IR) in the skeletal muscle vasculature results in impaired skeletal muscle glucose uptake and altered whole-body glucose homeostasis. The development of different invasive and noninvasive techniques has allowed the characterization of the actions of insulin and other vasoactive hormones in the skeletal muscle vasculature in both health and disease. Current treatment strategies for DM2 do not necessarily address the impaired effect of insulin in the vasculature. Understanding the effects of insulin and other metabolically active hormones in the vasculature should facilitate the development of new therapeutic strategies targeted at the modulation of IR and improvement of whole-body glucose tolerance.

Physical exercise restores microvascular function in obese rats with metabolic syndrome

BACKGROUND

Obesity and metabolic syndrome are related to systemic functional microvascular alterations, including a significant reduction in microvessel density. The aim of this study was to investigate the effects of exercise training on functional capillary density in the skeletal muscle and skin of obese rats with metabolic syndrome.

METHODS

We used male Wistar-Kyoto rats that had been fed a standard commercial diet (CON) or high-fat diet (HFD) for 32 weeks. Animals receiving the HFD were randomly divided into sedentary (HFD+SED) and training groups (HFD+TR) at the 20(th) week. After 12 weeks of aerobic treadmill training, the maximal oxygen uptake (VO2max); hemodynamic, biochemical, and anthropometric parameters; and functional capillary density were assessed. In addition, a maximal exercise test was performed.

RESULTS

Exercise training increased the VO2max (69 ± 3 mL/kg per min) and exercise tolerance (30 ± 1 min) compared with the HFD+SED (41 ± 6 mL/kg per min, P < 0.05 and 16 ± 1 min, P < 0.001) and with the CON (52 ± 7 mL/kg per min and 18 ± 1 min, P < 0.05) groups. The HFD+TR group also showed reduced retroperitoneal fat (0.03 ± 0.00 vs. 0.05 ± 0.00 gram/gram, P < 0.001), epididymal fat (0.01 ± 0.00 vs. 0.02 ± 0.00 gram/gram, P < 0.001), and systolic blood pressure (127 ± 2 vs. 150 ± 2 mmHg, P<0.001). The HFD+TR group also demonstrated improved glucose tolerance, as evaluated by an intraperitoneal glucose tolerance test, fasting plasma glucose levels (5.0 ± 0.1 vs. 6.4 ± 0.2 mmol/L, P<0.001) and fasting plasma insulin levels (26.5 ± 2.3 vs. 38.9 ± 3.7 μIU/mL, P < 0.05). Glucose tolerance did not differ between HFD+TR and CON groups. Exercise training also increased the number of spontaneously perfused capillaries in the skeletal muscle (252 ± 9 vs. 207 ± 9 capillaries/mm(2)) of the training group compared with that in the sedentary animals (260 ± 15 capillaries/mm(2)).

CONCLUSIONS

These results demonstrate that exercise training reverses capillary rarefaction in our experimental model of metabolic syndrome and obesity.

Retinal vascular caliber and metabolic syndrome in a Chinese population

BACKGROUND

Microvascular changes have been associated with the metabolic syndrome.

METHODS

We included 869 participants aged ≥ 40 years from the High-risk for Diabetes Changfeng Study, who had gradable fundus photographs. On digital photographs sum retinal arteriolar and venular calibers were measured with a semi-automated system. Metabolic syndrome was defined according to the International Diabetes Federation consensus.

RESULTS

A total of 286 (32.9%) participants was diagnosed with metabolic syndrome. Participants with narrower retinal arteriolar caliber were more often diagnosed with metabolic syndrome (odds ratio 1.78, 95% confidence interval 1.02 – 3.10; lowest vs highest quintile). Additionally adjusting for age, gender, education, smoking and weekly activity, and adding arteriolar and venular caliber simultaneously in the same models did not alter these associations. In the component analyses, participants with narrower retinal arteriolar caliber were more likely to have central obesity, dyslipidaemia or raised blood pressure, and less likely to have raised fasting plasma glucose. The association between wider venular caliber and metabolic syndrome was less pronounced and non-significant (odds ratio 1.34; 95% confidence interval 0.79 – 2.38; highest vs lowest quintile).

CONCLUSION

Retinal arteriolar narrowing and, to a lesser extent, retinal venular dilatation were associated with metabolic syndrome in this Chinese population. These vascular changes, although small in magnitude, may still be important in the pathophysiological mechanisms involved in the metabolic syndrome.

New insights into insulin action and resistance in the vasculature

Two-thirds of adults in the United States are overweight or obese, and another 26 million have type 2 diabetes. Decreased insulin sensitivity in cardiovascular tissue is an underlying abnormality in these individuals. Insulin metabolic signaling increases endothelial cell nitric oxide (NO) production. Impaired vascular insulin sensitivity is an early defect leading to impaired vascular relaxation. In overweight and obese persons, as well as in those with hypertension, systemic and vascular insulin resistance often occur in conjunction with activation of the cardiovascular tissue renin-angiotensin-aldosterone system (RAAS). Activated angiotensin II type 1 receptor and mineralocorticoid receptor signaling promote the development of vascular insulin resistance and impaired endothelial NO-mediated relaxation. Research in this area has implicated excessive serine phosphorylation and proteasomal degradation of the docking protein insulin receptor substrate and enhanced signaling through hybrid insulin/insulin-like growth factor receptor as important mechanisms underlying RAAS impediment of downstream vascular insulin metabolic signaling. This review will present recent evidence supporting the notion that RAAS signaling represents a potential pathway for the development of vascular insulin resistance and impaired endothelial-mediated vasodilation.

The microvasculature: a target for nutritional programming and later risk of cardio-metabolic disease

There is compelling evidence that microvascular deficits affecting multiple tissues and organs play an important role in the aetiopathogenesis of cardio-metabolic disease. Furthermore, both in humans and animal models, deficits in small vessel structure and function can be detected early, often before the onset of macrovascular disease and the development of end-organ damage that is common to hypertension and obesity-associated clinical disorders. This article considers the growing evidence for the negative impact of an adverse maternal diet on the long-term health of her child, and how this can result in a disadvantageous vascular phenotype that extends to the microvascular bed. We describe how structural and functional modifications in the offspring microcirculation during development may represent an important and additional risk determinant to increase susceptibility to the development of cardio-metabolic disease in adult life and consider the cell-signalling pathways associated with endothelial dysfunction that may be 'primed' by the maternal environment. Published studies were identified that reported outcomes related to the microcirculation, endothelium, maternal diet and vascular programming using NCBI PubMed.gov, MEDLINE and ISI Web of Science databases from 1980 until April 2013 using pre-specified search terms. Information extracted from over 230 original reports and review articles was critically evaluated by the authors for inclusion in this review.

Structural and functional microvascular alterations in a rat model of metabolic syndrome induced by a high-fat diet

OBJECTIVE

To investigate microvascular alterations in an experimental model of metabolic syndrome induced by a high-fat diet (HFD) associated with salt supplementation (0.5% NaCl).

DESIGN AND METHODS

Wistar Kyoto rats were fed standard chow (control group, CONT) or HFD for 20 weeks. The functional capillary density (FCD) was assessed using intravital fluorescence videomicroscopy.

RESULTS

The HFD group presented a higher systolic blood pressure, plasma glucose and insulin levels, total and LDL-cholesterol levels, triglycerides, and visceral and epididymal fat when compared with the CONT group. When compared with the CONT group, the HFD group showed a lower FCD in the skeletal muscle (P < 0.05) but not in the skin (P > 0.05). The HFD group also had a lower capillary-to-fiber ratio in the skeletal muscle (P < 0.01). The capillary volume density-to-fiber volume density ratio in the left ventricle of the HFD was also reduced (P < 0.01). Finally, rats fed with HFD showed ventricular hypertrophy and increased cardiomyocyte diameter (P < 0.01).

CONCLUSIONS

The long-term administration of a HFD associated with salt supplementation to rats generates an experimental model of metabolic syndrome characterized by central body fat deposition, insulin resistance, glucose intolerance, hypertriglyceridemia, hypercholesterolemia, arterial hypertension, cardiac remodeling, and rarefaction of the microcirculation in the heart and skeletal muscle.

Rapid onset vasodilatation is blunted in obese humans

AIM

Conduit artery function in obese humans is frequently assessed at rest, but very little is known about resistance artery function in response to muscle contraction. We tested the hypothesis that obese adults will exhibit reduced contraction-induced rapid onset vasodilatation. Single and brief forearm contractions were used to isolate the local effects of muscle contraction on the forearm vasodilatory response, independent of systemic haemodynamic and sympathetic neural influence.

METHODS

We measured forearm blood flow (Doppler ultrasound), blood pressure (finger photoplethysmography) and heart rate (electrocardiogram) on a beat-by-beat basis in 14 obese (body mass index = 36.2 ± 1.7 kg m(-2)) and 14 lean (body mass index = 21.6 ± 0.7 kg m(-2)) young (18-40 years) adults. Percent changes from baseline in forearm vascular conductance (FVC(%) ) were calculated in response to single, brief forearm contractions performed in random order at 15, 20, 25, 30, 40 and 50% of maximal voluntary contraction (MVC).

RESULTS

In both groups, each single contraction evoked a significant (P < 0.05), immediate (within one cardiac cycle) and graded FVC(%) increase from one up to six cardiac cycles post-contraction. Immediate (20-50% MVC), peak (15-50% MVC) and total (area under the curve, 20-50% MVC) vasodilatory responses were reduced with obesity. The degree of impaired vasodilatation increased with increasing workloads.

CONCLUSIONS

These novel findings demonstrate a blunted contraction-induced rapid onset vasodilatation with obesity that is exercise intensity dependent. Impaired rapid onset vasodilatation may negatively impact haemodynamic responses to everyday intermittent activities performed by obese humans.

Heart of the matter: coronary dysfunction in metabolic syndrome

Metabolic syndrome (MetS) is a collection of risk factors including obesity, dyslipidemia, insulin resistance/impaired glucose tolerance, and/or hypertension. The incidence of obesity has reached pandemic levels, as ~20-30% of adults in most developed countries can be classified as having MetS. This increased prevalence of MetS is critical as it is associated with a two-fold elevated risk for cardiovascular disease. Although the pathophysiology underlying this increase in disease has not been clearly defined, recent evidence indicates that alterations in the control of coronary blood flow could play an important role. The purpose of this review is to highlight current understanding of the effects of MetS on regulation of coronary blood flow and to outline the potential mechanisms involved. In particular, the role of neurohumoral modulation via sympathetic α-adrenoceptors and the renin-angiotensin-aldosterone system (RAAS) are explored. Alterations in the contribution of end-effector K(+), Ca(2+), and transient receptor potential (TRP) channels are also addressed. Finally, future perspectives and potential therapeutic targeting of the microcirculation in MetS are discussed. This article is part of a Special Issue entitled "Coronary Blood Flow".

Computational analyses of intravascular tracer washout reveal altered capillary-level flow distributions in obese Zucker rats

Intravascular tracer washout data obtained from gastrocnemius muscle of lean Zucker rats (LZRs) and obese Zucker rats (OZRs) were analysed to investigate flow distributions in the OZR, a model of non-atherosclerotic peripheral vascular disease. A computer model used to simulate the network washout curves was developed based on experimentally observed relative dispersions in large vessels and asymmetrical flow distributions at bifurcations in dichotomous microvascular networks. The model results of simulations were compared to experimental washout data of (125)I-labelled albumin, an intravascular tracer, to uncover flow distributions on the arterial-network and capillary levels. The lean and obese Zucker rats demonstrated distinct capillary-level flow distributions, with higher dispersion and significantly more low-flow capillaries in the OZRs than in the LZRs. Targeted pharmacological treatments against identified sites of vascular dysfunction in OZRs (adrenoreceptor blockade with phentolamine, antioxidant treatment with Tempol and thromboxane receptor antagonism with SQ-29548) were shown to improve the capillary-level flow distributions in treated OZRs toward distributions determined in control LZRs. Combination therapy with multiple pharmacological interventions resulted in a greater degree of recovery. This study demonstrates that the enhanced perfusion heterogeneity at arteriole bifurcations is a potential mechanism underlying perfusion-demand mismatching in OZRs, and suggests that amelioration of this dysfunction must involve a multi-faceted interventional approach.

Postmaximal contraction blood volume responses are blunted in obese and type 2 diabetic subjects in a muscle-specific manner

The purpose of this study was to determine whether there are differences in postisometric contraction blood volume and oxygenation responses among groups of type 2 diabetes mellitus (T2DM), obese, and lean individuals detectable using MRI. Eight T2DM patients were individually matched by age, sex, and race to non-T2DM individuals with similar body mass index (obese) and lean subjects. Functional MRI was performed using a dual-gradient-recalled echo, echo-planar imaging sequence with a repetition time of 1 s and at two echo times (TE = 6 and 46 ms). Data were acquired before, during, and after 10-s isometric dorsiflexion contractions performed at 50 and 100% of maximal voluntary contraction (MVC) force. MRI signal intensity (SI) changes from the tibialis anterior and extensor digitorum longus muscles were plotted as functions of time for each TE. From each time course, the difference between the minimum and the maximum postcontraction SI (ΔSI) were determined for TE = 6 ms (ΔSI(6)) and TE = 46 ms (ΔSI(46)), reflecting variations in blood volume and oxyhemoglobin saturation, respectively. Following 50% MVC contractions, the mean postcontraction ΔSI(6) values were similar in the three groups. Following MVC only, and in the EDL muscle only, T2DM and obese participants had ∼56% lower ΔSI(6) than the lean individuals. Also following MVC only, the ΔSI(46) response in the EDL was lower in T2DM subjects than in lean individuals. These data suggest that skeletal muscle small vessel impairment occurs in T2DM and body mass index-matched subjects, in muscle-specific and contraction intensity-dependent manners.

Differential effects of diet-induced obesity on BKCa {beta}1-subunit expression and function in rat skeletal muscle arterioles and small cerebral arteries

Mechanisms underlying obesity-related vascular dysfunction are unclear. This study examined the effect of diet-induced obesity on expression and function of large conductance Ca(2+)-activated potassium channel (BK(Ca)) in rat pressurized small resistance vessels with myogenic tone. Male Sprague-Dawley rats fed a cafeteria-style high fat diet (HFD; ∼30% energy from fat) for 16-20 wk were ∼30% heavier than controls fed standard chow (∼13% fat). Obesity did not alter BK(Ca) α-subunit function or α-subunit protein or mRNA expression in vessels isolated from the cremaster muscle or middle-cerebral circulations. In contrast, BK(Ca) β(1)-subunit protein expression and function were significantly reduced in cremaster muscle arterioles but increased in middle-cerebral arteries from obese animals. Immunohistochemistry showed α- and β(1)-subunits were present exclusively in the smooth muscle of both vessels. Cremaster muscle arterioles from obese animals showed significantly increased medial thickness, and media-to-lumen ratio and pressurized arterioles showed increased myogenic tone at 30 mmHg, but not at 50-120 mmHg. Myogenic tone was not affected by obesity in middle-cerebral arteries. The BK(Ca) antagonist iberiotoxin constricted both cremaster muscle and middle-cerebral arterioles from control rats; this effect of iberiotoxin was abolished in cremaster muscle arteries only from obese rats. Diet-induced obesity has contrasting effects on BK(Ca) function in different vascular beds, through differential effects on β(1)-subunit expression. However, these alterations in BK(Ca) function had little effect on overall myogenic tone, suggesting that the mechanisms controlling myogenic tone can be altered and compensate for altered BK(Ca) expression and function.

Differential impact of familial hypercholesterolemia and combined hyperlipidemia on vascular wall and network remodeling in mice

Genetic familial hypercholesterolemia (FH) and combined hyperlipidemia (FCH) are characterized by elevated plasma low-density lipoprotein (LDL) (FH) and LDL/triglycerides (FCH), with mouse models represented by LDL receptor (LDLR) and apolipoprotein E (ApoE) gene deletion mice, respectively. Given the impact of FH and FCH on health outcomes, we determined the impact of FH/FCH on vascular structure in LDLR and ApoE mice. LDLR, ApoE and control mice were utilized at 12-13 and 22-23 weeks when gracilis arteries were studied for wall mechanics and gastrocnemius muscles were harvested for microvessel density measurements. Conduit arteries and plasma samples were harvested for biochemical analyses. Arteries from ApoE and LDLR exhibited blunted expansion versus control, reduced distensibility and left-shifted stress versus strain relation (LDLR > ApoE). Microvessel density was reduced in ApoE and LDLR (ApoE > LDLR). Secondary analyses suggested that wall remodeling in LDLR was associated with cholesterol and MCP-1, while rarefaction in ApoE was associated with tumor necrosis factors-alpha, triglycerides and vascular production of TxA(2). Remodeling in ApoE and LDLR appears distinct; as that in LDLR is preferential for vascular walls, while that for ApoE is stronger for rarefaction. Remodeling in LDLR may be associated with cellular adhesion, while that in ApoE may be associated with pro-apoptotsis and constrictor prostanoid generation.

Impact of chronic anticholesterol therapy on development of microvascular rarefaction in the metabolic syndrome

OBJECTIVE

The obese Zucker rat (OZR) model of the metabolic syndrome is partly characterized by moderate hypercholesterolemia, in addition to other contributing comorbidities. Previous results suggest that vascular dysfunction in OZR is associated with chronic reduction in vascular nitric-oxide (NO) bioavailability and chronic inflammation, both frequently associated with hypercholesterolemia. As such, we evaluated the impact of chronic cholesterol-reducing therapy on the development of impaired skeletal muscle arteriolar reactivity and microvessel density in OZR and its impact on chronic inflammation and NO bioavailability.

MATERIALS AND METHODS

Beginning at seven weeks of age, male OZR were treated with gemfibrozil, probucol, atorvastatin, or simvastatin (in chow) for 10 weeks. Subsequently, plasma and vascular samples were collected for biochemical/molecular analyses, while arteriolar reactivity and microvessel network structure were assessed by using established methodologies after 3, 6, and 10 weeks of drug therapy.

RESULTS

All interventions were equally effective at reducing total cholesterol, although only the statins also blunted the progressive reductions to vascular NO bioavailability, evidenced by greater maintenance of acetylcholine-induced dilator responses, an attenuation of adrenergic constrictor reactivity, and an improvement in agonist-induced NO production. Comparably, while minimal improvements to arteriolar wall mechanics were identified with any of the interventions, chronic statin treatment reduced the rate of microvessel rarefaction in OZR. Associated with these improvements was a striking statin-induced reduction in inflammation in OZR, such that numerous markers of inflammation were correlated with improved microvascular reactivity and density. However, using multivariate discriminant analyses, plasma RANTES (regulated on activation, normal T-cell expressed and secreted), interleukin-10, monocyte chemoattractant protein-1, and tumor necrosis factor alpha were determined to be the strongest contributors to differences between groups, although their relative importance varied with time.

CONCLUSIONS

While the positive impact of chronic statin treatment on vascular outcomes in the metabolic syndrome are independent of changes to total cholesterol, and are more strongly associated with improvements to vascular NO bioavailability and attenuated inflammation, these results provide both a spatial and temporal framework for targeted investigation into mechanistic determinants of vasculopathy in the metabolic syndrome.

Non-invasive and quantitative evaluation of peripheral vascular resistances in rats by combined NMR measurements of perfusion and blood pressure using ASL and dynamic angiography

The in vivo determination of peripheral vascular resistances (VR) is crucial for the assessment of arteriolar function. It requires simultaneous determination of organ perfusion (F) and arterial blood pressure (BP). A fully non-invasive method was developed to measure systolic and diastolic BP in the caudal artery of rats based on dynamic NMR angiography. A good agreement was found between the NMR approach and the gold standard techniques (linear regression slope = 0.98, R(2) = 0.96). This method and the ASL-MRI measurement of skeletal muscle perfusion were combined into one single NMR experiment to quantitatively evaluate the local vascular resistances in the calf muscle of anaesthetized rats, in vivo and non-invasively 1) at rest: VR = 7.0 +/- 1.0 mmHg x min 100 g x ml(-1), F = 13 +/- 3 ml min(-1) x 100 g(-1) and mean BP (MBP) = 88 +/- 10 mmHg; 2) under vasodilator challenge (milrinone): VR = 3.7 +/- 1.1 mmHg min x 100 g ml(-1), F = 21 +/- 4 ml min(-1) x 100 g(-1) and MBP = 75 +/- 14 mmHg; 3) under vasopressor challenge (norepinephrine): VR = 9.8 +/- 1.2 mmHg min 100 g ml(-1), F = 14 +/- 3 ml min(-1) x 100 g(-1) and MBP = 137 +/- 2 mmHg.

Role of Angiotensin II in the Remodeling Induced by a Chronic Increase in Flow in Rat Mesenteric Resistance Arteries

Angiotensin II is a potent growth factor involved in arterial wall homeostasis. In resistance arteries, chronic increases in blood flow induce a rise in diameter associated with arterial wall hypertrophy. Nevertheless, the role of angiotensin II in this remodeling is unknown. We investigated the effect of blocking angiotensin II production or receptor activation on flow-induced remodeling of mesenteric resistance arteries. Arteries were ligated in vivo to generate high-flow arteries compared with normal flow (control) vessels located at a distance. Arteries were isolated after 1 week for in vitro analysis. Arterial diameter, media surface, endothelial NO synthase expression, superoxide production, and extracellular signal–regulated kinase 1/2 phosphorylation were higher in high-flow than in control arteries. Angiotensin-converting enzyme inhibition (perindopril) and angiotensin II type 1 receptor blockade (candesartan) prevented arterial wall hypertrophy without affecting diameter enlargement. The nonselective vasodilator hydralazine had no effect on remodeling. Although perindopril and candesartan increased endothelial NO synthase expression in high-flow arteries, hypertrophy remained in rats treated with N G -nitro- l -arginine methyl ester and mice lacking endothelial NO synthase. Perindopril and candesartan reduced oxidative stress in high-flow arteries, but superoxide scavenging did not prevent hypertrophy. Both Tempol and the absence of endothelial NO synthase prevented the rise in diameter in high-flow vessels. Extracellular signal–regulated kinase 1/2 activation in high-flow arteries was prevented by perindopril and candesartan and not by hydralazine. Extracellular signal–regulated kinase 1/2 inhibition in vivo (U0126) prevented hypertrophy in high-flow arteries. Thus, a chronic rise in blood flow in resistance arteries induces a diameter enlargement involving NO and superoxide, whereas hypertrophy was associated with extracellular signal–regulated kinase 1/2 activation by angiotensin II.

Reversal of voltage-dependent erectile responses in the Zucker obese-diabetic rat by rosuvastatin-altered RhoA/Rho-kinase signaling

INTRODUCTION

The combination of independent risk factors for erectile dysfunction, obesity, hypertension, and diabetes are collectively manifested in a condition known as metabolic syndrome X (MSX). However, the regulatory mechanisms responsible for the erectile dysfunction (ED) are not fully understood. Clinical studies suggest that a pleiotropic effect of statin's ability to enhance vascular relaxation might be through an impact on nitric oxide signaling or through a regulation of RhoA activation.

AIM

We hypothesized that regulatory aspects of short-term statin therapy involve the alteration of the RhoA/Rho-kinase signaling cascade and will reverse the ED seen in a rat model of MSX.

MAIN OUTCOME MEASURES

The magnitude and sensitivity of the voltage-dependent maintenance of intracavernosal blood pressure and mean arterial blood pressure. These responses were correlated with tissue protein and mRNA expression levels of RhoA and Rho kinases.

METHODS

Erectile function was evaluated by assessing voltage-dependent stimulation of the cavernosal nerve in 16-20 weeks old lean and obese-diabetic Zucker rats treated with 5 mg/kg/day of rosuvastatin intraperitoneally for 3 days. Cavernosal tissue RhoA and Rho-kinases expression levels were evaluated by real-time reverse transcriptase-polymerase chain reaction, Western blot.

RESULTS

The voltage-dependent erectile responses were suppressed by >30% in the obese-diabetic Zucker rat. The 3-day treatment with rosuvastatin partially restored the erectile response. The Rho-kinase inhibitor, H-1152, dose dependently increased the erectile responses and shifted the voltage sensitivity with statin treatment. Analysis of protein expression levels suggested elevation of RhoA and Rho kinases in obese-diabetics and statin treatment lowering Rho-kinase II. The RhoA and Rho-kinase II mRNA levels were significantly reduced in the rosuvastatin-treated obese-diabetic animals.

CONCLUSIONS

These results support a hypothesis that short-term statin therapy may lower RhoA/Rho-kinase expression levels and improve cavernosal blood pressure response to Rho-kinase inhibition and voltage-stimulation, and reversing an augmented vasoconstricted state associated with diabetes and/or hypertension in MSX.

Preserved coronary arteriolar dilatation in patients with type 2 diabetes mellitus: implications for reactive oxygen species

Type 2 diabetes mellitus is associated with clustering of cardiovascular risk factors that may greatly increase individuals' risk of developing coronary artery disease. Type 2 diabetes is believed to impair coronary function. However, its impact on the vasomotor function of coronary resistance vessels in humans is still debated. Reduced, preserved or even augmented dilations of coronary arterioles have been reported in subjects with type 2 diabetes. Interestingly, recent studies have suggested that reactive oxygen species (ROS), particularly hydrogen peroxide, may compensate for the loss of the vasodilatory function of coronary microvessels during disease development. Recent interventional clinical trials have yielded largely negative results, and there has even been some suggestion of harm caused by attempts to reduce ROS. Thus, it is possible that interference with ROS-related signaling might paradoxically temper the function of coronary microvessels, predisposing patients to myocardial ischemia. In this review, we aim to highlight current findings supporting a potential role for ROS in preserving coronary arteriolar dilation in type 2 diabetes mellitus.

Obesity induced-insulin resistance causes endothelial dysfunction without reducing the vascular response to hindlimb ischemia

Impairment of vascular growth is a hallmark of diabetic complications, but the progression and mechanisms are poorly understood. To determine whether obesity and early diabetes impair endothelium-dependent vasodilatation and vascular response to ischemia, microvascular function as well as angiogenic responses to ischemia were assessed in young (C57) and 6-month-old lean mice (old C57), in obese (db-C57) mice, and in mice suffering an early (db-KsJ) and sustained type 2 diabetes (old db-KsJ). Glycemia gradually increased from the db-C57 to the old db-KsJ. Early and established type II diabetes significantly reduced the level of insulin that was significantly increased in obese mice. Endothelial function was assessed in isolated resistance arteries while the angiogenic response induced by unilateral hindlimb ischemia was analyzed, after 28 days, with a laser Doppler flowmeter and angiography. Aging (-21%), obesity (-45%), as well as early (-58%) and sustained type II diabetes (-69%) induced a progressive impairment of the endothelium-dependent relaxation of the gracilis artery. Laser Doppler measurements demonstrated that only early and sustained type II diabetes impaired skin blood flow recovery. Vascular collateralization was reduced with aging and severely impaired in older db-KsJ mice, the two strains of mice in which ischemia reduced eNOS expression. These results demonstrate that endothelial dysfunction induced by obesity is insufficient to alter the angiogenic response to ischemia. Furthermore, the development of frank type II diabetes or increasing age is required to impair the vascular response to hindlimb ischemia. We conclude that additional risk factors or severe endothelial dysfunction may be requisite to impede the angiogenic response to ischemia.

Differential structural and functional changes in penile and coronary arteries from obese Zucker rats

Erectile dysfunction frequently coexists with coronary artery disease and has been proposed as a potential marker for silent coronary artery disease in type 2 diabetes. In the present study, we comparatively assessed the structural and functional changes of both penile arteries (PAs) and coronary arteries (CAs) from a prediabetic animal model. PAs and CAs from 17- to 18-wk-old obese Zucker rats (OZRs) and from their control counterparts [lean Zucker rats (LZRs)] were mounted in microvascular myographs to evaluate vascular function, and stained arteries were subjected to morphometric analysis. Endothelial nitric oxide (NO) synthase (eNOS) protein expression was also assessed. The internal diameter was reduced and the wall-to-lumen ratio was increased in PAs from OZRs, but structure was preserved in CAs. ACh-elicited relaxations were severely impaired in PAs but not in CAs from OZRs, although eNOS expression was unaltered. Contractions to norepinephrine and 5-HT were significantly enhanced in both PAs and CAs, respectively, from OZRs. Blockade of NOS abolished endothelium-dependent relaxations in PAs and CAs and potentiated norepinephrine and 5-HT contractions in arteries from LZRs but not from OZRs. The vasodilator response to the phosphodiesterase 5 inhibitor sildenafil was reduced in both PAs and CAs from OZRs. Pretreatment with SOD reduced the enhanced vasoconstriction in both PAs and CAs from OZRs but did not restore ACh-induced relaxations in PAs. In conclusion, the present results demonstrate vascular inward remodeling in PAs and a differential impairment of endothelial relaxant responses in PAs and CAs from insulin-resistant OZRs. Enhanced superoxide production and reduced basal NO activity seem to underlie the augmented vasoconstriction in both PAs and CAs. The severity of the structural and functional abnormalities in PAs might anticipate the vascular dysfunction of the more preserved coronary vascular bed.

Integration of skeletal muscle resistance arteriolar reactivity for perfusion responses in the metabolic syndrome

Previous study suggests that with evolution of the metabolic syndrome, patterns of arteriolar reactivity are profoundly altered and may constrain functional hyperemia. This study investigated interactions between parameters of vascular reactivity at two levels of resistance arterioles in obese Zucker rats (OZR), translating these observations into perfusion regulation for in situ skeletal muscle. Dilation of isolated and in situ resistance arterioles from OZR to acetylcholine, arachidonic acid (AA), and hypoxia (isolated arterioles only) were blunted vs. lean Zucker rats (LZR), although dilation to adenosine was intact. Increased adrenergic tone (phenylephrine) or intralumenal pressure (ILP) impaired dilation in both strains (OZR>LZR). Treatment of OZR arterioles with Tempol (superoxide dismutase mimetic) or SQ-29548 (prostaglandin H2/thromboxane A2 receptor antagonist) improved dilator reactivity under control conditions and with increased ILP, but had minimal effect with increased adrenergic tone. Arteriolar dilation to adenosine was well maintained in both strains under all conditions. For in situ cremasteric arterioles, muscle contraction-induced elevations in metabolic demand elicited arteriolar dilations and hyperemic responses that were blunted in OZR vs. LZR, although distal parallel arterioles were characterized by heterogeneous dilator and perfusion responses. alpha-Adrenoreceptor blockade improved outcomes at rest but had minimal effect with elevated metabolic demand. Treatment with Tempol or SQ-29548 had minimal impact at rest, but lessened distal arteriolar perfusion heterogeneity with increased metabolic demand. In blood-perfused gastrocnemius of OZR, perfusion was constrained primarily by adrenergic tone, while myogenic activation and endothelium-dependent dilation did not appear to contribute significantly to ischemia. These results of this novel, integrated approach suggest that adrenergic tone and metabolic dilation are robust determinants of bulk perfusion to skeletal muscle of OZR, while endothelial dysfunction may more strongly regulate perfusion distribution homogeneity via the impact of oxidant stress and AA metabolism.

Differential effects of diet-induced dyslipidemia and hyperglycemia on mesenteric resistance artery structure and function in type 2 diabetes

Type 2 diabetes and dyslipidemia oftentimes present in combination. However, the relative roles of diabetes and diet-induced dyslipidemia in mediating changes in vascular structure, mechanics, and function are poorly understood. Our hypothesis was that addition of a high-fat diet would exacerbate small artery remodeling, compliance, and vascular dysfunction in type 2 diabetes. Vascular remodeling indices [media/lumen (M/L) ratio, collagen abundance and turnover, and matrix metalloproteinase dynamics], mechanical properties (vessel stiffness), and reactivity to pressure and vasoactive factors were measured in third-order mesenteric arteries in control Wistar and type 2 diabetic Goto-Kakizaki (GK) rats fed either a regular or high-fat diet. M/L ratios, total collagen, and myogenic tone were increased in diabetes. Addition of the high-fat diet altered collagen patterns (mature versus new collagen) in favor of matrix accumulation. Addition of a high-fat diet caused increased constriction to endothelin-1 (0.1-100 nM), showed impaired vasorelaxation to both acetylcholine (0.1 nM-1 microM) and sodium nitroprusside (0.1 nM-1 microM), and increased cardiovascular risk factors in diabetes. These results suggest that moderate elevations in blood glucose, as seen in our lean GK model of type 2 diabetes, promote resistance artery remodeling resulting in increased medial thickness, whereas addition of a high-fat diet contributes to diabetic vascular disease predominantly by impairing vascular reactivity in the time frame used for this study. Although differential in their vascular effects, both hyperglycemia and diet-induced dyslipidemia need to be targeted for effective prevention and treatment of diabetic vascular disease.

Attenuated PGI2 synthesis in obese Zucker rats

In obesity, skeletal muscle blood flow during exercise (functional hyperemia) is impaired. We have indirectly demonstrated that an altered arachidonic acid metabolism is responsible for the impaired functional vasodilation in the obese Zucker rat (OZR), a model of obesity. In this study, we tested the hypothesis that there is an impaired release of PGI(2) due to a nitration of PGI(2) synthase (PGIS), which is associated with a decreased prostanoid receptor expression. PGI(2), PGE(2), and thromboxane A(2) (TXA(2)) release were determined in vitro using ELISA under basal conditions and in response to arachidonic acid (AA) administration (50 microM). Immunofluorescence of PGI(2) and TXA(2) receptors (IP and TP, respectively) was determined in dispersed vascular smooth muscle cells (VSMCs). Nitration of tyrosine residues of the PGIS enzyme was determined using immunoprecipitation and Western blot analysis. Following AA administration, PGI(2) and PGE(2) release were attenuated in OZR compared with lean Zucker rats (LZR; controls). Basal and AA-induced TXA(2) release were not significantly different between groups. IP and TP immunofluorescence were not significantly different between OZR and LZR groups. OZR exhibited elevated nitration of tyrosine residues of PGIS compared with LZR. These results suggest that alterations in the PGI(2) pathway (attenuated PGI(2) synthesis), and not the TXA(2) pathway (normal TXA(2) synthesis/no change in TP receptor expression), underlie the attenuated functional hyperemia in the OZR.

K(ATP)-mediated vasodilation is impaired in obese Zucker rats

OBJECTIVE

Skeletal muscle blood flow during exercise is impaired in obesity. We tested the hypothesis that the attenuated vasodilation in skeletal muscle arterioles of obese Zucker rats (OZR) is due to altered K(ATP) channel-mediated vasodilation.

MATERIALS AND METHODS

K(ATP) channel function was determined in isolated skeletal muscle arterioles in response to the K(ATP) opener cromakalim (0.1-10 microM) during normal myogenic tone and alpha-adrenergic-mediated tone (0.1 microM phenylephrine). The spinotrapezius muscle was prepared and the vasodilatory responses to muscle stimulation or iloprost (0.028-2.8 microM) were observed before and after the application of the K(ATP) inhibitor, glibenclamide (10 microM). Channel subunit expression was determined by using western blot analyses.

RESULTS

Cromakalim concentration-response curves were shifted in OZR as compared to lean controls. OZR exhibited impaired functional and iloprost-induced vasodilation as compared to the lean controls. Glibenclamide inhibited the functional and iloprost-induced dilation in the lean rats with no effects in the obese a nimals. Channel subunit expression was similar in femoral arteries.

CONCLUSION

The impaired functional vasodilation in the OZR is associated with altered K(ATP) channel sensitivity.

Skin microcirculatory dysfunction is already present in normoglycemic subjects with metabolic syndrome

The role of microcirculatory dysfunction (MD) in metabolic and cardiovascular diseases is not well established. Considering that metabolic syndrome (MS) is an independent risk factor and diabetic patients have microangiopathy, our aim was to investigate if normoglycemic subjects with MS already have detectable skin MD. Thirty-six subjects with MS (National Cholesterol Education Program-Adult Treatment Panel III criteria) (10 men/26 women, 38.8 +/- 7.9 years, 35.8 +/- 4.9 kg/m(2)) with normal glucose tolerance (American Diabetes Association criteria) and 16 controls (11 men/5 women, 33.6 +/- 8.4 years, 23.9 +/- 3.6 kg/m(2)) were studied using nailfold videocapillaroscopy. Afferent, efferent, and apical capillary diameters; functional capillary density; red blood cell velocity (RBCV) at baseline; and RBCV(max) and time (TRBCV(max)) to reach it during postocclusive reactive hyperemia after 1-minute arterial occlusion were measured. Subjects with MS had smaller afferent, efferent, and apical diameters (4.2 [3.8-4.2] vs 5.6 [4.65-6.25] mum, P < .001; 4.8 [4.2-4.8] vs 6.2 [5.6-7] mum, P < .001; and 5.2 [4.8-5.55] vs 7.4 [6.2-8] mum, P < .001); lower functional capillary density (7.28 [6.37-9.10] vs 10.4 [9.1-11.8] capillaries per square millimeter, P < .001), RBCV (0.62 [0.57-0.65] vs 0.79 [0.76-0.89] mm/s, P < .001], and RBCV(max) (1.14 [1.12-1.210] vs 1.57 [1.45-1.62] mm/s, P < .001); and longer TRBCV(max) (10.0 [10-11] vs 4.5 [4-6] seconds, P < .001) compared with controls. Microcirculatory dysfunction was associated with body mass index. We concluded that subjects with MS already have nutritive skin MD even within the normoglycemic milieu.

Decreased NO signaling leads to enhanced vasoconstrictor responsiveness in skeletal muscle arterioles of the ZDF rat prior to overt diabetes and hypertension

Approximately 40% of patients with type 2 diabetes present with concurrent hypertension at the time of diabetes diagnosis. Increases in peripheral vascular resistance and correspondingly enhanced vasoconstrictor capacity could have profound implications for the development of hypertension and the progression of insulin resistance to overt diabetes. The purpose of this study was to determine whether skeletal muscle arteriolar vasoconstrictor dysfunction precedes or occurs concurrently with the onset of diabetes and hypertension. Male Zucker diabetic fatty (ZDF) rats were studied at 7, 13, and 20 wk of age to represent prediabetic and short-term and long-term diabetic states, respectively. Conscious mean arterial pressure (MAP), fasted plasma insulin and glucose, vasoconstrictor responses, and passive mechanical properties of isolated skeletal muscle arterioles were measured in prediabetic, diabetic, and age-matched control rats. Elevated MAP was manifest in short-term diabetes (control 117 +/- 1, diabetic 135 +/- 3 mmHg) and persisted with long-term diabetes (control 113 +/- 2, diabetic 135 +/- 3 mmHg). This higher MAP was preceded by augmented arteriolar vasoconstrictor responses to norepinephrine and endothelin-1 and followed by diminished beta-adrenergic vasodilation and enhanced myogenic constriction in long-term diabetes. Furthermore, we demonstrate that diminished nitric oxide (NO) signaling underlies the increases in vasoconstrictor responsiveness in arterioles from prediabetic and diabetic rats. Arteriolar stiffness was not different between control and prediabetic or diabetic rats at any time point studied. Collectively, these results indicate that increases in vasoconstrictor responsiveness resulting from diminished NO signaling in skeletal muscle arterioles precede the development of diabetes and hypertension in ZDF rats.

Skeletal muscle capillary responses to insulin are abnormal in late-stage diabetes and are restored by angiotensin-converting enzyme inhibition

Acute physiological hyperinsulinemia increases skeletal muscle capillary blood volume (CBV), presumably to augment glucose and insulin delivery. We hypothesized that insulin-mediated changes in CBV are impaired in type 2 diabetes mellitus (DM) and are improved by angiotensin-converting enzyme inhibition (ACE-I). Zucker obese diabetic rats (ZDF, n = 18) and control rats (n = 9) were studied at 20 wk of age. One-half of the ZDF rats were treated with quinapril (ZDF-Q) for 15 wk prior to study. CBV and capillary flow in hindlimb skeletal muscle were measured by contrast-enhanced ultrasound (CEU) at baseline and at 30 and 120 min after initiation of a euglycemic hyperinsulinemic clamp (3 mU.min(-1).kg(-1)). At baseline, ZDF and ZDF-Q rats were hyperglycemic and hyperinsulinemic vs. controls. Glucose utilization in ZDF rats was 60-70% lower (P < 0.05) than in controls after 30 and 120 min of hyperinsulinemia. In ZDF-Q rats, glucose utilization was impaired at 30 min but similar to controls at 120 min. Basal CBV was lower in ZDF and ZDF-Q rats compared with controls (13 +/- 4, 7 +/- 3, and 9 +/- 2 U, respectively). With hyperinsulinemia, CBV increased by about twofold in control animals at 30 and 120 min, did not change in ZDF animals, and increased in ZDF-Q animals only at 120 min to a level similar to controls. Anatomic capillary density on immunohistology was not different between groups. We conclude that insulin-mediated capillary recruitment in skeletal muscle, which participates in glucose utilization, is impaired in animals with DM and can be partially reversed by chronic ACE-I therapy.

Microcirculation in obesity: an unexplored domain

Obesity is traditionally linked to diabetes and cardiovascular diseases. Very recent experimental, clinical and epidemiological, sometimes provocative, data challenge this automaticity by showing that not the amount but the distribution of fat is the important determinant. Moderate abdominal fat accumulation may thus be more harmful than even consequent overweight. In view of the worldwide burden of obesity, factors leading to it in children and young adults must urgently be identified. Since obesity is a very complex cardiometabolic situation, this will require to focus investigations on uncomplicated obese subjects and adequate animal models. The recent discovery of intergenerational transmissions of obesity risk factors and also the key role played by gestational and perinatal events (epigenetic factors) give rise to completely new concepts and research avenues. Considering the potential close relationship between microcirculation and tissue metabolism, demonstrations of structural and/or functional abnormalities in microvascular physiology very early in life of subjects at risk for obesity might provide a solid basis for further investigations of such links. Microcirculation(arterioles, capillaries and venules) is conceivably a key compartment determining over one or several decades the translation of genetic and epigenetic factors into fat accumulation. Available animal models should serve to answer this cardinal question.

Selective resistance to vasoactive effects of insulin in muscle resistance arteries of obese Zucker (fa/fa) rats

Obesity is related to insulin resistance and hypertension, but the underlying mechanisms are unclear. Insulin exerts both vasodilator and vasoconstrictor effects on muscle resistance arteries, which may be differentially impaired in obesity.

OBJECTIVES

To investigate whether vasodilator and vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats and the roles of Akt and endothelial NO synthase (eNOS).

METHODS/RESULTS

Effects of insulin were studied in resistance arteries isolated from cremaster muscles of lean and obese Zucker rats. In arteries of lean rats, insulin increased activity of both NO and endothelin (ET-1), resulting in a neutral effect under basal conditions. In arteries of obese rats, insulin induced endothelin-mediated vasoconstriction (-15 +/- 5% at 1 nM, P < 0.05 vs. lean). Insulin induced vasodilatation during endothelin receptor blockade in arteries of lean rats (20 +/- 5% at 1 nM) but not in those of obese rats. Inhibition of NO synthesis increased vascular tone (by 12 +/- 2%) and shifted insulin-mediated vasoreactivity to vasoconstriction (-25 +/- 1% at 1 nM) in lean rats but had no effect in arteries of obese rats, indicating reduced NO activity. Protein analysis of resistance arteries revealed that insulin-mediated activation of Akt was preserved in obese rats, whereas expression of eNOS was markedly decreased.

CONCLUSIONS

Vasodilator but not vasoconstrictor effects of insulin are impaired in muscle resistance arteries of obese rats, and this selective impairment is associated with decreased protein levels of eNOS. These findings provide a new mechanism linking obesity to insulin resistance and hypertension.

Altered mechanisms of endothelium-dependent dilation in skeletal muscle arterioles with genetic hypercholesterolemia

With most cardiovascular disease risk factors, endothelium-dependent dilation of skeletal muscle resistance arterioles is compromised, although with hypercholesterolemia, impairments to reactivity are not consistently observed. Using apolipoprotein E (ApoE) and low-density lipoprotein receptor (LDLR) gene deletion male mouse models of hypercholesterolemia at 20 wk of age, we tested the hypothesis that arteriolar dilation would be maintained due to an increased stimulus-induced production of dilator metabolites via cyclooxygenase and cytochrome P-450 epoxygenase pathways. Arterioles from both strains demonstrated mild reductions in dilation to hypoxia and acetylcholine versus responses in C57/Bl/6J (C57) controls. However, although inhibition of nitric oxide synthase (NOS) attenuated dilation in arterioles from C57 controls, this effect was absent in ApoE or LDLR strains. In contrast, cyclooxygenase-dependent portions of dilator reactivity were maintained across the three strains. Notably, although combined NOS and cyclooxygenase inhibition abolished arteriolar responses to hypoxia and acetylcholine in C57 controls, significant reactivity remained in ApoE and LDLR strains. Whereas inhibition of cytochrome P-450 omega-hydroxylase and epoxygenases had no effect on this residual reactivity in ApoE and LDLR strains, inhibition of 12/15-lipoxygenase with nordihydroguaiaretic acid abolished the residual reactivity. With both hypoxic and methacholine challenges, arteries from ApoE and LDLR strains demonstrated an increased production of both 12(S)- and 15(S)-hydroxyeicosatetraenoic acid, end products of arachidonic acid metabolism via 12/15-lipoxygenase, a response that was not present in C57 controls. These results suggest that with development of hypercholesterolemia, mechanisms contributing to dilator reactivity in skeletal muscle arterioles are modified such that net reactivity to endothelium-dependent stimuli is largely intact.