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

Cerebral hemodynamics in obesity: relationship with sex, age, and adipokines in a cohort-based study

Overweight and obesity are well-known independent risk factors for stroke in the general population although uncertain in the case of the elderly, according to the obesity paradox. Little is known about underlying mechanisms. Our study aims to assess whether there is a relationship between excess body weight (measured as waist circumference) and poor cerebral hemodynamics (measured by transcranial Doppler parameters: basal, mean flow velocity (MFV), and dynamic, cerebrovascular reserve (CvR) in the right middle cerebral artery (RMCA)). A possible underlying molecular mechanism was analyzed via plasma leptin, adiponectin, TNF-α, IL-6, VCAM, and CRP levels. One hundred sixty-five subjects were included. Bivariate and multivariate regression showed a linear correlation between waist circumference and hemodynamics in RMCA, with clear gender effects: MFV (global NS, men β - 0.26 p < 0.01; women NS), CvR (global: β - 0.15 p < 0.01; men: β - 0.29 p < 0.01, women: β - 0.19 p < 0.09). For subjects above 65 years, there is no significant relationship between AbP and cerebral hemodynamics. In multivariate regression models, only leptin correlated independently with MFV in RMCA (β 7.24, p < 0.01) and CvR (β - 0.30, p < 0.01). In both cases, waist circumference remains significantly related to both parameters. There is an inverse linear correlation between excess body weight and cerebral hemodynamics, independent of other vascular risk factors and clearly influenced by gender. This relation disappears in the elderly population. Leptin might play a role in this relationship. Nevertheless, there must be another associated mechanism, not identified in this study.

High-Fat Overfeeding Impairs Peripheral Glucose Metabolism and Muscle Microvascular eNOS Ser1177 Phosphorylation

CONTEXT

The mechanisms responsible for dietary fat-induced insulin resistance of skeletal muscle and its microvasculature are only partially understood.

OBJECTIVE

To determine the impact of high-fat overfeeding on postprandial glucose fluxes, muscle insulin signaling, and muscle microvascular endothelial nitric oxide synthase (eNOS) content and activation.

DESIGN

Fifteen non-obese volunteers consumed a high-fat (64%) high-energy (+47%) diet for 7 days. Experiments were performed before and after the diet. Stable isotope tracers were used to determine glucose fluxes in response to carbohydrate plus protein ingestion. Muscle insulin signaling was determined as well as the content and activation state of muscle microvascular eNOS.

RESULTS

High-fat overfeeding impaired postprandial glycemic control as demonstrated by higher concentrations of glucose (+11%; P = 0.004) and insulin (+19%; P = 0.035). Carbohydrate plus protein ingestion suppressed endogenous glucose production to a similar extent before and after the diet. Conversely, high-fat overfeeding reduced whole-body glucose clearance (-16%; P = 0.021) and peripheral insulin sensitivity (-26%; P = 0.006). This occurred despite only minor alterations in skeletal muscle insulin signaling. High-fat overfeeding reduced eNOS content in terminal arterioles (P = 0.017) and abolished the increase in eNOS Ser1177 phosphorylation that was seen after carbohydrate plus protein ingestion.

CONCLUSION

High-fat overfeeding impaired whole-body glycemic control due to reduced glucose clearance, not elevated endogenous glucose production. The finding that high-fat overfeeding abolished insulin-mediated eNOS Ser1177 phosphorylation in the terminal arterioles suggests that impairments in the vasodilatory capacity of the skeletal muscle microvasculature may contribute to early dietary fat-induced impairments in glycemic control.

Impaired Tissue Oxygenation in Metabolic Syndrome Requires Increased Microvascular Perfusion Heterogeneity

Metabolic syndrome (MS) in obese Zucker rats (OZR) is associated with impaired skeletal muscle performance and blunted hyperemia. Studies suggest that reduced O₂ diffusion capacity is required to explain compromised muscle performance and that heterogeneous microvascular perfusion distribution is critical. We modeled tissue oxygenation during muscle contraction in control and OZR skeletal muscle using physiologically realistic relationships. Using a network model of Krogh cylinders with increasing perfusion asymmetry and increased plasma skimming, we predict increased perfusion heterogeneity and decreased muscle oxygenation in OZR, with partial recovery following therapy. Notably, increasing O₂ delivery had less impact on VO₂ than equivalent decreases in O₂ delivery, providing a mechanism for previous empirical work associating perfusion heterogeneity and impaired O₂ extraction. We demonstrate that increased skeletal muscle perfusion asymmetry is a defining characteristic of MS and must be considered to effectively model and understand blood-tissue O₂ exchange in this model of human disease.

Increased peripheral vascular disease risk progressively constrains perfusion adaptability in the skeletal muscle microcirculation

To determine the impact of progressive elevations in peripheral vascular disease (PVD) risk on microvascular function, we utilized eight rat models spanning "healthy" to "high PVD risk" and used a multiscale approach to interrogate microvascular function and outcomes: healthy: Sprague-Dawley rats (SDR) and lean Zucker rats (LZR); mild risk: SDR on high-salt diet (HSD) and SDR on high-fructose diet (HFD); moderate risk: reduced renal mass-hypertensive rats (RRM) and spontaneously hypertensive rats (SHR); high risk: obese Zucker rats (OZR) and Dahl salt-sensitive rats (DSS). Vascular reactivity and biochemical analyses demonstrated that even mild elevations in PVD risk severely attenuated nitric oxide (NO) bioavailability and caused progressive shifts in arachidonic acid metabolism, increasing thromboxane A2 levels. With the introduction of hypertension, arteriolar myogenic activation and adrenergic constriction were increased. However, while functional hyperemia and fatigue resistance of in situ skeletal muscle were not impacted with mild or moderate PVD risk, blood oxygen handling suggested an increasingly heterogeneous perfusion within resting and contracting skeletal muscle. Analysis of in situ networks demonstrated an increasingly stable and heterogeneous distribution of perfusion at arteriolar bifurcations with elevated PVD risk, a phenomenon that was manifested first in the distal microcirculation and evolved proximally with increasing risk. The increased perfusion distribution heterogeneity and loss of flexibility throughout the microvascular network, the result of the combined effects on NO bioavailability, arachidonic acid metabolism, myogenic activation, and adrenergic constriction, may represent the most accurate predictor of the skeletal muscle microvasculopathy and poor health outcomes associated with chronic elevations in PVD risk.

Modulation of endothelial cell phenotype by physical activity: impact on obesity-related endothelial dysfunction

Increased levels of physical activity are associated with reduced cardiovascular disease (CVD) risk and mortality in obesity and diabetes. Available evidence suggests that local factors, including local hemodynamics, account for a significant portion of this CVD protection, and numerous studies have interrogated the therapeutic benefit of physical activity/exercise training in CVD. Less well established is whether basal differences in endothelial cell phenotype between/among vasculatures related to muscle recruitment patterns during activity may account for reports of nonuniform development of endothelial dysfunction in obesity. This is the focus of this review. We highlight recent work exploring the vulnerability of two distinct vasculatures with established differences in endothelial cell phenotype. Specifically, based largely on dramatic differences in underlying hemodynamics, arteries perfusing soleus muscle (slow-twitch muscle fibers) and those perfusing gastrocnemius muscle (fast-twitch muscle fibers) in the rat exhibit an exercise training-like versus an untrained endothelial cell phenotype, respectively. In the context of obesity, therefore, arteries to soleus muscle exhibit protection from endothelial dysfunction compared with vulnerable arteries to gastrocnemius muscle. This disparate vulnerability is consistent with numerous animal and human studies, demonstrating increased skeletal muscle blood flow heterogeneity in obesity coincident with reduced muscle function and exercise intolerance. Mechanistically, we highlight emerging areas of inquiry exploring novel aspects of hemodynamic-sensitive signaling in endothelial cells and the time course of physical activity-associated endothelial adaptations. Lastly, further exploration needs to consider the impact of endothelial heterogeneity on the development of endothelial dysfunction because endothelial dysfunction independently predicts CVD events.

Sprint interval and moderate-intensity continuous training have equal benefits on aerobic capacity, insulin sensitivity, muscle capillarisation and endothelial eNOS/NAD(P)Hoxidase protein ratio in obese men

KEY POINTS

Skeletal muscle capillary density and vasoreactivity are reduced in obesity, due to reduced nitric oxide bioavailability. Sprint interval training (SIT) has been proposed as a time efficient alternative to moderate-intensity continuous training (MICT), but its effect on the skeletal muscle microvasculature has not been studied in obese individuals. We observed that SIT and MICT led to equal increases in capillarisation and endothelial eNOS content, while reducing endothelial NOX2 content in microvessels of young obese men. We conclude that SIT is equally effective at improving skeletal muscle capillarisation and endothelial enzyme balance, while being a time efficient alternative to traditional MICT.

ABSTRACT

Sprint interval training (SIT) has been proposed as a time efficient alternative to moderate-intensity continuous training (MICT), leading to similar improvements in skeletal muscle capillary density and microvascular function in young healthy humans. In this study we made the first comparisons of the muscle microvascular response to SIT and MICT in an obese population. Sixteen young obese men (age 25 ± 1 years, BMI 34.8 ± 0.9 kg m(-2) ) were randomly assigned to 4 weeks of MICT (40-60 min cycling at ∼65% V̇O2 peak , 5 times per week) or constant load SIT (4-7 constant workload intervals of 200% Wmax 3 times per week). Muscle biopsies were taken before and after training from the m. vastus lateralis to measure muscle microvascular endothelial eNOS content, eNOS serine(1177) phosphorylation, NOX2 content and capillarisation using quantitative immunofluorescence microscopy. Maximal aerobic capacity (V̇O2 peak ), whole body insulin sensitivity and arterial stiffness were also assessed. SIT and MICT increased skeletal muscle microvascular eNOS content and eNOS ser(1177) phosphorylation in terminal arterioles and capillaries (P < 0.05), but the latter effect was eliminated when normalised to eNOS content (P = 0.217). SIT and MICT also reduced microvascular endothelial NOX2 content (P < 0.05) and both increased capillary density and capillary-fibre perimeter exchange index (P < 0.05). In parallel, SIT and MICT increased V̇O2 peak (P < 0.05) and whole body insulin sensitivity (P < 0.05), and reduced central artery stiffness (P < 0.05). As no significant differences were observed between SIT and MICT it is concluded that SIT is a time efficient alternative to MICT to improve aerobic capacity, insulin sensitivity and muscle capillarisation and endothelial eNOS/NAD(P)Hoxidase protein ratio in young obese men.

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.

Neural control of blood flow during exercise in human metabolic syndrome

α-Adrenergic-mediated vasoconstriction is greater during simulated exercise in animal models of metabolic syndrome (MetSyn) when compared with control animals. In an attempt to translate such findings to humans, we hypothesized that adults with MetSyn (n = 14, 35 ± 3 years old) would exhibit greater α-adrenergic responsiveness during exercise when compared with age-matched healthy control subjects (n = 16, 31 ± 3 years old). We measured muscle sympathetic nerve activity (MSNA; microneurography) and forearm blood flow (Doppler ultrasound) during dynamic forearm exercise (15% of maximal voluntary contraction). α-Adrenergic agonists (phenylephrine and clonidine) and an antagonist (phentolamine) were infused intra-arterially to assess α-adrenergic receptor responsiveness and restraint, respectively. Resting MSNA was ∼35% higher in adults with MetSyn (P < 0.05), but did not change in either group with dynamic exercise. Clonidine-mediated vasoconstriction was greater in adults with MetSyn (P < 0.01). Group differences in vascular responses to phenylephrine and phentolamine were not detected (P > 0.05). Interestingly, exercise-mediated vasodilatation was greater in MetSyn (P < 0.05). Adults with MetSyn exhibit greater resting MSNA and clonidine-mediated vasoconstriction, yet preserved functional sympatholysis and higher exercise blood flow during low-intensity hand-grip exercise when compared with age-matched healthy control subjects. These results suggest that adults with MetSyn exhibit compensatory vascular control mechanisms capable of preserving blood flow responses to exercise in the face of augmented sympathetic adrenergic activity.

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.

Blunted temporal activity of microvascular perfusion heterogeneity in metabolic syndrome: a new attractor for peripheral vascular disease?

A key clinical outcome for peripheral vascular disease (PVD) in patients is a progressive decay in skeletal muscle performance and its ability to resist fatigue with elevated metabolic demand. We have demonstrated that PVD in obese Zucker rats (OZR) is partially due to increased perfusion distribution heterogeneity at successive microvascular bifurcations within skeletal muscle. As this increased heterogeneity (γ) is longitudinally present in the network, its cumulative impact is a more heterogeneous distribution of perfusion between terminal arterioles than normal, causing greater regional tissue ischemia. To minimize this negative outcome, a likely compensatory mechanism against an increased γ should be an increased temporal switching at arteriolar bifurcations to minimize downstream perfusion deficits. Using in situ cremaster muscle, we determined that temporal activity (the cumulative sum of absolute differences between successive values of γ, taken every 20 s) was lower in OZR than in control animals, and this difference was present in both proximal (1A-2A) and distal (3A-4A) arteriolar bifurcations. Although adrenoreceptor blockade (phentolamine) improved temporal activity in 1A-2A arteriolar bifurcations in OZR, this was without impact in the distal microcirculation, where only interventions against oxidant stress (Tempol) and thromboxane A(2) activity (SQ-29548) were effective. Analysis of the attractor for γ indicated that it was not only elevated in OZR but also exhibited severe reductions in range, suggesting that the ability of the microcirculation to respond to any challenge is highly restricted and may represent the major contributor to the manifestation of poor muscle performance at this age in OZR.

Spatial heterogeneity in skeletal muscle microvascular blood flow distribution is increased in the metabolic syndrome

Previous studies have demonstrated that the metabolic syndrome is associated with impaired skeletal muscle arteriolar function, although integrating observations into a conceptual framework for impaired perfusion in peripheral vascular disease (PVD) has been limited. This study builds on previous work to evaluate in situ arteriolar hemodynamics in cremaster muscle of obese Zucker rats (OZR) to integrate existing knowledge into a greater understanding of impaired skeletal muscle perfusion. In OZR cremaster muscle, perfusion distribution at microvascular bifurcations (γ) was consistently more heterogeneous than in controls. However, while consistent, the underlying mechanistic contributors were spatially divergent as altered adrenergic constriction was the major contributor to altered γ at proximal microvascular bifurcations, with a steady decay with distance, while endothelial dysfunction was a stronger contributor in distal bifurcations with no discernible role proximally. Using measured values of γ, we found that simulations predict that successive alterations to γ in OZR caused more heterogeneous perfusion distribution in distal arterioles than in controls, an effect that could only be rectified by combined adrenoreceptor blockade and improvements to endothelial dysfunction. Intravascular (125)I-labeled albumin tracer washout from in situ gastrocnemius muscle of OZR provided independent support for these observations, indicating increased perfusion heterogeneity that was corrected only by combined adrenoreceptor blockade and improved endothelial function. These results suggest that a defining element of PVD in the metabolic syndrome may be an altered γ at microvascular bifurcations, that its contributors are heterogeneous and spatially distinct, and that interventions to rectify this negative outcome must take a new conceptual framework into account.