Muscle Microcirculatory O2 Exchange in Health and Disease. In: Takahashi E, Bruley DF, editors. Oxygen Transport to Tissue XXXI. Boston: Springer US; 2010. pp. 301-7. [DOI: 10.1007/978-1-4419-1241-1_43]

Shifted vascular optimization: the emergence of a new arteriolar behaviour with chronic metabolic disease

NEW FINDINGS

What is the topic of this review? Altered perfusion distribution at skeletal muscle arteriolar bifurcations and how this is modified by development of chronic metabolic disease. What advances does it highlight? The outcome created is a distribution of erythrocytes in the distal microcirculation that is characterized by increased spatial heterogeneity and reduced flexibility such that mass transport/exchange within the network is impaired, with limited ability to respond to imposed challenges. This advances our understanding of how altered vascular structure and function with metabolic disease impairs perfusion to skeletal muscle at a level of resolution that would not be identified through bulk flow responses.

ABSTRACT

This review is based on the presentation 'Shifted vascular optimization: the emergence of a new arteriolar behaviour with chronic metabolic disease', given at the Symposium 'Understanding Complex Behaviours in the Microcirculation: from Blood Flow to Oxygenation' during the Annual Meeting of the Physiological Society at the Aberdeen Exhibition and Conference Centre in Aberdeen, UK in July 2019. The past years of dedicated investigation on linkages between vascular (dys)function under conditions of elevated cardiovascular disease risk and tissue/organ performance have produced results and insights that frequently suffer from limited correlation and causation. Reaching out from this challenge, it was proposed that this may reflect a 'level of resolution' argument and that altered haemodynamic behaviour in vascular networks could be a stronger predictor of functional outcomes than higher resolution measures. Using this approach, we have determined that an attractor that describes the spatial and temporal shift in perfusion distribution at successive arteriolar bifurcations within the skeletal muscle is a strong predictor of functional outcomes within animals and provides novel insight into fundamental mechanistic contributors to altered patterns of intra-muscular perfusion. This article focuses on the applicability and utility of the attractor in models of cardiovascular and metabolic disease risk of increasing severity. We will also discuss the utility of the attractor in terms of understanding the effectiveness of aggressive interventions for reversing established vasculopathy and perfusion impairments.

Influence of priming exercise on oxygen uptake and muscle deoxygenation kinetics during moderate-intensity cycling in type 2 diabetes

The pulmonary oxygen uptake (V̇o2) kinetics during the transition to moderate-intensity exercise is slowed in individuals with type 2 diabetes (T2D), at least in part because of limitations in O2 delivery. The present study tested the hypothesis that a prior heavy-intensity warm-up or “priming” exercise (PE) bout would accelerate V̇o2 kinetics in T2D, because of a better matching of O2 delivery to utilization. Twelve middle-aged individuals with T2D and 12 healthy controls (ND) completed moderate-intensity constant-load cycling bouts either without (Mod A) or with (Mod B) prior PE. The rates of muscle deoxygenation (i.e., deoxygenated hemoglobin and myoglobin concentration, [HHb+Mb]) and oxygenation (i.e., tissue oxygenation index) were continuously measured by near-infrared spectroscopy at the vastus lateralis muscle. The local matching of O2 delivery to O2 utilization was assessed by the Δ[HHb+Mb]-to-ΔV̇o2 ratio. Both groups demonstrated an accelerated V̇O2 kinetics response during Mod B compared with Mod A (T2D, 32 ± 9 vs. 42 ± 12 s; ND, 28 ± 9 vs. 34 ± 8 s; means ± SD) and an elevated muscle oxygenation throughout Mod B, whereas the [HHb+Mb] amplitude was greater during Mod B only in individuals with T2D. The [HHb+Mb] kinetics remained unchanged in both groups. In T2D, Mod B was associated with a decrease in the “overshoot” relative to steady state in the Δ[HHb+Mb]-to-ΔV̇o2 ratio (1.17 ± 0.17 vs. 1.05 ± 0.15), whereas no overshoot was observed in the control group before (1.04 ± 0.12) or after (1.01 ± 0.12) PE. Our findings support a favorable priming-induced acceleration of the V̇o2 kinetics response in middle-aged individuals with uncomplicated T2D attributed to an enhanced matching of microvascular O2 delivery to utilization.

NEW & NOTEWORTHY Heavy-intensity “priming” exercise (PE) elicited faster pulmonary oxygen uptake (V̇o2) kinetics during moderate-intensity cycling exercise in middle-aged individuals with type 2 diabetes (T2D). This was accompanied by greater near-infrared spectroscopy-derived muscle deoxygenation (i.e., deoxygenated hemoglobin and myoglobin concentration, [HHb+Mb]) responses and a reduced Δ[HHb+Mb]-to-ΔV̇o2 ratio. This suggests that the PE-induced acceleration in oxidative metabolism in T2D is a result of greater O2 extraction and better matching between O2 delivery and utilization.

Beneficial Pleiotropic Antidepressive Effects of Cardiovascular Disease Risk Factor Interventions in the Metabolic Syndrome

BACKGROUND

Although the increased prevalence and severity of clinical depression and elevated cardiovascular disease risk represent 2 vexing public health issues, the growing awareness of their combined presentation compounds the challenge. The obese Zucker rat, a model of the metabolic syndrome, spontaneously develops significant depressive symptoms in parallel with the progression of the metabolic syndrome and, thus, represents a compelling model for study. The primary objective was to assess the impact on both cardiovascular outcomes, specifically vascular structure and function, and depressive symptoms in obese Zucker rats after aggressive treatment for cardiovascular disease risk factors with long-term exercise or targeted pharmacological interventions.

METHODS AND RESULTS

We chronically treated obese Zucker rats with clinically relevant interventions against cardiovascular disease risk factors to determine impacts on vascular outcomes and depressive symptom severity. While most of the interventions (chronic exercise, anti-hypertensive, the interventions (long-term exercise, antihypertensive, antidyslipidemia, and antidiabetic) were differentially effective at improving vascular outcomes, only those that also resulted in a significant improvement to oxidant stress, inflammation, arachidonic acid metabolism (prostacyclin versus thromboxane A2), and their associated sequelae were effective at also blunting depressive symptom severity. Using multivariable analyses, discrimination between the effectiveness of treatment groups to maintain behavioral outcomes appeared to be dependent on breaking the cycle of inflammation and oxidant stress, with the associated outcomes of improving endothelial metabolism and both cerebral and peripheral vascular structure and function.

CONCLUSIONS

This initial study provides a compelling framework from which to further interrogate the links between cardiovascular disease risk factors and depressive symptoms and suggests mechanistic links and potentially effective avenues for intervention.

Decreases in maximal oxygen uptake following long-duration spaceflight: Role of convective and diffusive O2 transport mechanisms

We have previously predicted that the decrease in maximal oxygen uptake (V̇o_2max) that accompanies time in microgravity reflects decrements in both convective and diffusive O₂ transport to the mitochondria of the contracting myocytes. The aim of this investigation was therefore to quantify the relative changes in convective O₂ transport (Q̇o₂) and O₂ diffusing capacity (Do₂) following long-duration spaceflight. In nine astronauts, resting hemoglobin concentration ([Hb]), V̇o_2max, maximal cardiac output (Q̇_Tmax), and differences in arterial and venous O₂ contents ([Formula: see text]-[Formula: see text]) were obtained retrospectively for International Space Station Increments 19-33 (April 2009-November 2012). Q̇o₂ and Do₂ were calculated from these variables via integration of Fick's Principle of Mass Conservation and Fick's Law of Diffusion. V̇o_2max significantly decreased from pre- to postflight (-53.9 ± 45.5%, P = 0.008). The significant decrease in Q̇_Tmax (-7.8 ± 9.1%, P = 0.05), despite an unchanged [Hb], resulted in a significantly decreased Q̇o₂ (-11.4 ± 10.5%, P = 0.02). Do₂ significantly decreased from pre- to postflight by -27.5 ± 24.5% (P = 0.04), as did the peak [Formula: see text]-[Formula: see text] (-9.2 ± 7.5%, P = 0.007). With the use of linear regression analysis, changes in V̇o_2max were significantly correlated with changes in Do₂ (R² = 0.47; P = 0.04). These data suggest that spaceflight decreases both convective and diffusive O₂ transport. These results have practical implications for future long-duration space missions and highlight the need to resolve the specific mechanisms underlying these spaceflight-induced changes along the O₂ transport pathway.NEW & NOTEWORTHY Long-duration spaceflight elicited a significant decrease in maximal oxygen uptake. Given the adverse physiological adaptations to microgravity along the O₂ transport pathway that have been reported, an integrative approach to the determinants of postflight maximal oxygen uptake is needed. We demonstrate that both convective and diffusive oxygen transport are decreased following ~6 mo International Space Station missions.

VO(2max) and Microgravity Exposure: Convective versus Diffusive O(2) Transport

Exposure to a microgravity environment decreases the maximal rate of O2 uptake (VO(2max)) in healthy individuals returning to a gravitational environment. The magnitude of this decrease in VO(2max) is, in part, dependent on the duration of microgravity exposure, such that long exposure may result in up to a 38% decrease in VO(2max). This review identifies the components within the O(2) transport pathway that determine the decrease in postmicrogravity VO(2max) and highlights the potential contributing physiological mechanisms. A retrospective analysis revealed that the decline in VO(2max) is initially mediated by a decrease in convective and diffusive O(2) transport that occurs as the duration of microgravity exposure is extended. Mechanistically, the attenuation of O(2) transport is the combined result of a deconditioning across multiple organ systems including decreases in total blood volume, red blood cell mass, cardiac function and mass, vascular function, skeletal muscle mass, and, potentially, capillary hemodynamics, which become evident during exercise upon re-exposure to the head-to-foot gravitational forces of upright posture on Earth. In summary, VO(2max) is determined by the integration of central and peripheral O(2) transport mechanisms, which, if not maintained during microgravity, will have a substantial long-term detrimental impact on space mission performance and astronaut health.

Superficial blood flow of the superior labrum associated with rotator cuff tear using laser Doppler flowmeter

INTRODUCTION

Superior labrum anterior and posterior (SLAP) lesions due to overuse by repeated pitching motion and trauma are often noted and usually. However, shoulder disorders with a chronic course are often accompanied by SLAP lesions. SLAP lesions are frequently observed during arthroscopic surgery of the shoulder joint for rotator cuff tear in middle-aged and elderly individuals, suggesting the involvement of factors other than pitching motion and traumatic events in the pathogenesis. We considered that blood flow in the labrum is altered. The purpose of this study was to clarify the superficial blood flow in the superior labrum during arthroscopic surgery of the rotator cuff tear and investigate whether it is altered with labrum injury.

MATERIALS AND METHODS

We evaluated 33 subjects with a mean age of 55.1 years who underwent arthroscopic surgery for partial or complete rotator cuff tears. The blood flow measurement site for the superior labrum was set at the supraglenoid tubercle attachment site, and blood flow was measured twice using laser Doppler flowmeter. The mean was calculated and compared among the normal labrum and type 1 SLAP lesions.

RESULTS

The morphology of the labrum was normal in 16 subjects, and 17 subjects had type 1 SLAP lesions. The mean blood flow was 1.75 ml/min/100 g in subjects with a normal labrum; however, in subjects with type 1 SLAP lesions, blood flow was 1.32 ml/min/100 g, showing significant differences (P < 0.01).

CONCLUSION

Superficial blood flow in the superior labrum of the shoulder with rotator cuff tear was measured using laser Doppler flowmetry. The blood flow in type 1 SLAP lesions was lower than that in the normal labrum.

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Relation of pulse pressure to long-distance gait speed in community-dwelling older adults: findings from the LIFE-P study

Background

Reduced gait speed is associated with falls, late-life disability, hospitalization/institutionalization and cardiovascular morbidity and mortality. Aging is also accompanied by a widening of pulse pressure (PP) that contributes to ventricular-vascular uncoupling. The purpose of this study was to test the hypothesis that PP is associated with long-distance gait speed in community-dwelling older adults in the Lifestyle Interventions and Independence for Elders Pilot (LIFE-P) study.

Methods

Brachial blood pressure and 400-meter gait speed (average speed maintained over a 400-meter walk at “usual” pace) were assessed in 424 older adults between the ages of 70–89 yrs at risk for mobility disability (mean age = 77 yrs; 31% male). PP was calculated as systolic blood pressure (BP) – diastolic BP.

Results

Patients with a history of heart failure and stroke (n = 42) were excluded leaving 382 participants for final analysis. When categorized into tertiles of PP, participants within the highest PP tertile had significantly slower gait speed than those within the lowest PP tertile (p<0.05). Following stepwise multiple regression, PP was significantly and inversely associated with 400-meter gait speed (p<0.05). Other significant predictors of gait speed included: handgrip strength, body weight, age and history of diabetes mellitus (p<0.05). Mean arterial pressure, systolic BP and diastolic BP were not predictors of gait speed.

Conclusions

Pulse pressure is associated long-distance gait speed in community-dwelling older adults. Vascular senescence and altered ventricular-vascular coupling may be associated with the deterioration of mobility and physical function in older adults.

Pulse pressure is associated with walking impairment in multiple sclerosis

Persons with multiple sclerosis (MS) have reduced gait performance and this is associated with disability and disease progression. The current study sought to test the hypothesis that higher central (aortic and carotid) and peripheral (brachial) pulse pressure (PP), manifestations of ventricular-vascular uncoupling related to increased arterial stiffness and pressure from wave reflections, would be associated with reduced gait performance in persons with MS. Participants consisted of 33 individuals with MS and 33 age/sex matched controls. Central blood pressure (BP) was assessed via applanation tonometry. Brachial BP was measured using an automated oscillometric cuff. PP was defined as systolic BP--diastolic BP. Gait performance was measured as 6-minute walk (6 MW) distance. Within the sample with MS, the 6 MW distance was significantly associated with brachial (r = -.49, p<.005), aortic (r = -.52, p<.001), and carotid (r = -.57, p<.001) pulse pressure. There was no association between any PP measure and 6 MW distance in controls (p>0.05 for all). In conclusion, PP is a predictor of gait performance in persons with MS. These findings suggest that vascular senescence and altered ventricular-vascular coupling may contribute, in part, to the deterioration of physical function in persons with MS.

Dynamics of muscle microcirculatory and blood-myocyte O(2) flux during contractions

The O(2) requirements of contracting skeletal muscle may increase 100-fold above rest. In 1919, August Krogh's brilliant insights recognized the capillary as the principal site for this increased blood-myocyte O(2) flux. Based on the premise that most capillaries did not sustain RBC flux at rest, Krogh proposed that capillary recruitment [i.e. initiation of red blood cell (RBC) flux in previously non-flowing capillaries] increased the capillary surface area available for O(2) flux and reduced mean capillary-to-mitochondrial diffusion distances. More modern experimental approaches reveal that most muscle capillaries may support RBC flux at rest. Thus, rather than contraction-induced capillary recruitment per se, increased RBC flux and haematocrit within already-flowing capillaries probably elevate perfusive and diffusive O(2) conductances and hence blood-myocyte O(2) flux. Additional surface area for O(2) exchange is recruited but, crucially, this may occur along the length of already-flowing capillaries (i.e. longitudinal recruitment). Today, the capillary is still considered the principal site for O(2) and substrate delivery to contracting skeletal muscle. Indeed, the presence of very low intramyocyte O(2) partial pressures (PO(2)s) and the absence of intramyocyte PO(2) gradients, whilst refuting the relevance of diffusion distances, place an even greater importance on capillary hemodynamics. This emergent picture calls for a paradigm-shift in our understanding of the function of capillaries by de-emphasizing de novo'capillary recruitment'. Diseases such as heart failure impair blood-myocyte O(2) flux, in part, by decreasing the proportion of RBC-flowing capillaries. Knowledge of capillary function in healthy muscle is requisite for identification of pathology and efficient design of therapeutic treatments.