Skeletal muscle capillary function: contemporary observations and novel hypotheses

Making the case for resistance training in improving vascular function and skeletal muscle capillarization

Through decades of empirical data, it has become evident that resistance training (RT) can improve strength/power and skeletal muscle hypertrophy. Yet, until recently, vascular outcomes have historically been underemphasized in RT studies, which is underscored by several exercise-related reviews supporting the benefits of endurance training on vascular measures. Several lines of evidence suggest large artery diameter and blood flow velocity increase after a single bout of resistance exercise, and these events are mediated by vasoactive substances released from endothelial cells and myofibers (e.g., nitric oxide). Weeks to months of RT can also improve basal limb blood flow and arterial diameter while lowering blood pressure. Although several older investigations suggested RT reduces skeletal muscle capillary density, this is likely due to most of these studies being cross-sectional in nature. Critically, newer evidence from longitudinal studies contradicts these findings, and a growing body of mechanistic rodent and human data suggest skeletal muscle capillarity is related to mechanical overload-induced skeletal muscle hypertrophy. In this review, we will discuss methods used by our laboratories and others to assess large artery size/function and skeletal muscle capillary characteristics. Next, we will discuss data by our groups and others examining large artery and capillary responses to a single bout of resistance exercise and chronic RT paradigms. Finally, we will discuss RT-induced mechanisms associated with acute and chronic vascular outcomes.

Contraction intensity affects NIRS-derived skeletal muscle oxidative capacity but not its relationships to mitochondrial protein content or aerobic fitness

To further refine the near-infrared spectroscopy (NIRS)-derived measure of skeletal muscle oxidative capacity in humans, we sought to determine whether the exercise stimulus intensity affected the τ value and/or influenced the magnitude of correlations with in vitro measures of mitochondrial content and in vivo indices of exercise performance. Males (n = 12) and females (n = 12), matched for maximal aerobic fitness per fat-free mass, completed NIRS-derived skeletal muscle oxidative capacity tests for the vastus lateralis following repeated contractions at 40% (τ40) and 100% (τ100) of maximum voluntary contraction, underwent a skeletal muscle biopsy of the same muscle, and performed multiple intermittent isometric knee extension tests to task failure to establish critical torque (CT). The value of τ100 (34.4 ± 7.0 s) was greater than τ40 (24.2 ± 6.9 s, P < 0.001), but the values were correlated (r = 0.688; P < 0.001). The values of τ40 (r = -0.692, P < 0.001) and τ100 (r = -0.488, P = 0.016) correlated with myosin heavy chain I percentage and several markers of mitochondrial content, including COX II protein content in whole muscle (τ40: r = -0.547, P = 0.006; τ100: r = -0.466, P = 0.022), type I pooled fibers (τ40: r = -0.547, P = 0.006; τ100: r = -0.547, P = 0.006), and type II pooled fibers (τ40: r = -0.516, P = 0.009; τ100: r = -0.635, P = 0.001). The value of τ40 (r = -0.702, P < 0.001), but not τ100 (r = -0.378, P = 0.083) correlated with critical torque (CT); however, neither value correlated with W' (τ40: r = 0.071, P = 0.753; τ100: r = 0.054, P = 0.812). Overall, the NIRS method of assessing skeletal muscle oxidative capacity is sensitive to the intensity of skeletal muscle contraction but maintains relationships to whole body fitness, isolated limb critical intensity, and mitochondrial content regardless of intensity.NEW & NOTEWORTHY Skeletal muscle oxidative capacity measured using near-infrared spectroscopy (NIRS) was lower following high-intensity compared with low-intensity isometric knee extension contractions. At both intensities, skeletal muscle oxidative capacity was correlated with protein markers of mitochondrial content (in whole muscle and pooled type I and type II muscle fibers) and critical torque. These findings highlight the importance of standardizing contraction intensity while using the NIRS method with isometric contractions and further demonstrate its validity.

Toward a Better Understanding of Muscle Microvascular Perfusion During Exercise in Patients With Peripheral Artery Disease: The Effect of Lower-Limb Revascularization

PURPOSE

Leg muscle microvascular blood flow (perfusion) is impaired in response to maximal exercise in patients with peripheral artery disease (PAD); however, during submaximal exercise, microvascular perfusion is maintained due to a greater increase in microvascular blood volume compared with that seen in healthy adults. It is unclear whether this submaximal exercise response reflects a microvascular impairment, or whether it is a compensatory response for the limited conduit artery flow in PAD. Therefore, to clarify the role of conduit artery blood flow, we compared whole-limb blood flow and skeletal muscle microvascular perfusion responses with exercise in patients with PAD (n=9; 60±7 years) prior to, and following, lower-limb endovascular revascularization.

MATERIALS AND METHODS

Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after a 5 minute bout of submaximal intermittent isometric plantar-flexion exercise using contrast-enhanced ultrasound imaging. Exercise contraction-by-contraction whole-leg blood flow and vascular conductance were measured using strain-gauge plethysmography.

RESULTS

With revascularization there was a significant increase in whole-leg blood flow and conductance during exercise (p<0.05). Exercise-induced muscle microvascular perfusion response did not change with revascularization (pre-revascularization: 3.19±2.32; post-revascularization: 3.89±1.67 aU.s-1; p=0.38). However, the parameters that determine microvascular perfusion changed, with a reduction in the microvascular volume response to exercise (pre-revascularization: 6.76±3.56; post-revascularization: 2.42±0.69 aU; p<0.01) and an increase in microvascular flow velocity (pre-revascularization: 0.25±0.13; post-revascularization: 0.59±0.25 s-1; p=0.02).

CONCLUSION

These findings suggest that patients with PAD compensate for the conduit artery blood flow impairment with an increase in microvascular blood volume to maintain muscle perfusion during submaximal exercise.

CLINICAL IMPACT

The findings from this study support the notion that the impairment in conduit artery blood flow in patients with PAD leads to compensatory changes in microvascular blood volume and flow velocity to maintain muscle microvascular perfusion during submaximal leg exercise. Moreover, this study demonstrates that these microvascular changes are reversed and become normalized with successful lower-limb endovascular revascularization.

Differences in peripheral microcirculatory blood flow regulation in chronic kidney disease based on wavelet analysis of resting near-infrared spectroscopy

Vascular impairment is closely related to increased mortality in chronic kidney disease (CKD). The objective of this study was to assess impairments in the regulation of peripheral microvascular perfusion in patients with CKD based on time-frequency spectral analysis of resting near-infrared spectroscopy (NIRS) signals. Total hemoglobin (tHb) concentration and tissue saturation index (TSI) signals were collected using NIRS for a continuous 5 mins at 10 Hz from the forearm of 55 participants (34 CKD including 5 with end-stage renal disease, and 21 age-matched control). Continuous wavelet transform-based spectral analysis was used to quantify the spectral amplitude within five pre-defined frequency intervals (I, 0.0095-0.021 Hz; II, 0.021-0.052 Hz; III, 0.052-0.145 Hz; IV, 0.145-0.6 Hz and V, 0.6-2.0 Hz), representing endothelial, neurogenic, myogenic, respiratory and heartbeat activity, respectively. CKD patients showed lower tHb average spectral amplitude within the neurogenic frequency interval compared with controls (p = 0.014), consistent with an increased sympathetic outflow observed in CKD. CKD patients also showed lower TSI average spectral amplitude within the endothelial frequency interval compared with controls (p = 0.046), consistent with a reduced endothelial function in CKD. These findings demonstrate the potential of wavelet analysis of NIRS to provide complementary information on peripheral microvascular regulation in CKD.

Relationship between muscle venous blood oxygenation and near-infrared spectroscopy: quantitative analysis of the Hb and Mb contributions

A linear relationship between skeletal muscle venous ([Formula: see text]) and oxygenated (ΔHbMbO2,N) or deoxygenated (ΔHHbMbN) near-infrared spectroscopy (NIRS) signals suggest a main hemoglobin (Hb) contribution to the NIRS signal. However, experimental, and computational evidence supports a significant contribution of myoglobin (Mb) to the NIRS. Venous and NIRS measurements from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48(10):2013-2020, 2016) were integrated into a computational model of muscle O2 transport and utilization to evaluate whether the relationship between venous and NIRS oxygenation can be affected by a significant Mb contribution to the NIRS signals. The mathematical model predicted well the measure of the changes of [Formula: see text] and NIRS signals for different O2 delivery conditions (blood flow, arterial O2 content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O2 delivery, Mb contribution to NIRS signals was significant (20%-30%) even in the presence of a linear [Formula: see text]-NIRS relationship; for a reduced O2 delivery the nonlinearity of the [Formula: see text]-NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O2 delivery is reduced from 1.3 to 0.7 L kg-1·min-1 ([Formula: see text] < 10 mLO2 100 mL-1) and Mb saturation decreased from 85% to 40% corresponding to an increase of the Mb contribution to ΔHHbMbN from 15% to 50% and the contribution to ΔHbMbO2,N from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (ΔHHbMbN and ΔHbMbO2,N are significantly affected by Hb and Mb oxygenation changes.NEW & NOTEWORTHY Within the near-infrared spectroscopy (NIRS) signal, the contribution from hemoglobin is indistinguishable from that of myoglobin. A computation analysis indicates that a linear relationship between muscle venous oxygen content and NIRS signals does not necessarily indicate a negligible myoglobin contribution to the NIRS signal. A reduced oxygen delivery increases the myoglobin contribution to the NIRS signal. The integrative approach proposed is a powerful way to assist in interpreting the elements from which the NIRS signals are derived.

Endothelial structure contributes to heterogeneity in brain capillary diameter

The high metabolic demand of brain tissue is supported by a constant supply of blood through dense microvascular networks. Capillaries are the smallest class of vessels and vary in diameter between ∼2 to 5 μm in the brain. This diameter range plays a significant role in the optimization of blood flow resistance, blood cell distribution, and oxygen extraction. The control of capillary diameter has largely been ascribed to pericyte contractility, but it remains unclear if endothelial wall architecture also contributes to capillary diameter heterogeneity. Here, we use public, large-scale volume electron microscopy data from mouse cortex (MICrONS Explorer, Cortical MM^3) to examine how endothelial cell number, endothelial cell thickness, and pericyte coverage relates to microvascular lumen size. We find that transitional vessels near the penetrating arteriole and ascending venule are composed of 2 to 5 interlocked endothelial cells, while the numerous capillary segments intervening these zones are composed of either 1 or 2 endothelial cells, with roughly equal proportions. The luminal area and diameter is on average slightly larger with capillary segments composed of 2 interlocked endothelial cells versus 1 endothelial cell. However, this difference is insufficient to explain the full range of capillary diameters seen in vivo. This suggests that both endothelial structure and other influences, such as pericyte tone, contribute to the basal diameter and optimized perfusion of brain capillaries.

A transcriptome atlas of leg muscles from healthy human volunteers reveals molecular and cellular signatures associated with muscle location

Skeletal muscles support the stability and mobility of the skeleton but differ in biomechanical properties and physiological functions. The intrinsic factors that regulate muscle-specific characteristics are poorly understood. To study these, we constructed a large atlas of RNA-seq profiles from six leg muscles and two locations from one muscle, using biopsies from 20 healthy young males. We identified differential expression patterns and cellular composition across the seven tissues using three bioinformatics approaches confirmed by large-scale newly developed quantitative immune-histology procedures. With all three procedures, the muscle samples clustered into three groups congruent with their anatomical location. Concomitant with genes marking oxidative metabolism, genes marking fast- or slow-twitch myofibers differed between the three groups. The groups of muscles with higher expression of slow-twitch genes were enriched in endothelial cells and showed higher capillary content. In addition, expression profiles of Homeobox (HOX) transcription factors differed between the three groups and were confirmed by spatial RNA hybridization. We created an open-source graphical interface to explore and visualize the leg muscle atlas (https://tabbassidaloii.shinyapps.io/muscleAtlasShinyApp/). Our study reveals the molecular specialization of human leg muscles, and provides a novel resource to study muscle-specific molecular features, which could be linked with (patho)physiological processes.

Endothelial structure contributes to heterogeneity in brain capillary diameter

The high metabolic demand of brain tissue is supported by a constant supply of blood flow through dense microvascular networks. Capillaries are the smallest class of vessels in the brain and their lumens vary in diameter between ~2 and 5 μm. This diameter range plays a significant role in optimizing blood flow resistance, blood cell distribution, and oxygen extraction. The control of capillary diameter has largely been ascribed to pericyte contractility, but it remains unclear if the architecture of the endothelial wall also contributes to capillary diameter. Here, we use public, large-scale volume electron microscopy data from mouse cortex (MICrONS Explorer, Cortical mm3) to examine how endothelial cell number, endothelial cell thickness, and pericyte coverage relates to microvascular lumen size. We find that transitional vessels near the penetrating arteriole and ascending venule are composed of two to six interlocked endothelial cells, while the capillaries intervening these zones are composed of either one or two endothelial cells, with roughly equal proportions. The luminal area and diameter are on average slightly larger with capillary segments composed of two interlocked endothelial cells vs one endothelial cell. However, this difference is insufficient to explain the full range of capillary diameters seen in vivo. This suggests that both endothelial structure and other influences, including pericyte tone, contribute to the basal diameter and optimized perfusion of brain capillaries.

Muscle transcriptomic circuits linked to periarticular physiology in end-stage osteoarthritis

The ability of individuals with end-stage osteoarthritis (OA) to functionally recover from total joint arthroplasty is highly inconsistent. The molecular mechanisms driving this heterogeneity have yet to be elucidated. Furthermore, OA disproportionately impacts females, suggesting a need for identifying female-specific therapeutic targets. We profiled the skeletal muscle transcriptome in females with end-stage OA (n = 20) undergoing total knee or hip arthroplasty using RNA-Seq. Single-gene differential expression (DE) analyses tested for DE genes between skeletal muscle overlaying the surgical (SX) joint and muscle from the contralateral (CTRL) leg. Network analyses were performed using Pathway-Level Information ExtractoR (PLIER) to summarize genes into latent variables (LVs), i.e., gene circuits, and link them to biological pathways. LV differences in SX versus CTRL muscle and across sources of muscle tissue (vastus medialis, vastus lateralis, or tensor fascia latae) were determined with ANOVA. Linear models tested for associations between LVs and muscle phenotype on the SX side (inflammation, function, and integrity). DE analysis revealed 360 DE genes (|Log2 fold-difference| ≥ 1, FDR ≤ 0.05) between the SX and CTRL limbs, many associated with inflammation and lipid metabolism. PLIER analyses revealed circuits associated with protein degradation and fibro-adipogenic cell gene expression. Muscle inflammation and function were linked to an LV associated with endothelial cell gene expression highlighting a potential regulatory role of endothelial cells within skeletal muscle. These findings may provide insight into potential therapeutic targets to improve OA rehabilitation before and/or following total joint replacement.

Muscle Oxygenation Measured with Near-Infrared Spectroscopy Following Different Intermittent Training Protocols in a World-Class Kayaker-A Case Study

Training elite kayakers at a distance of 1000 m is associated with aerobic and anaerobic metabolism, while intermittent training, in a variety of forms, is one of the effective ways to improve cardiorespiratory and metabolic function. Thus, this study aimed to investigate muscle oxygenation responses during repetition training (RT), interval training (IT), and sprint interval training (SIT). Near-infrared spectroscopy (NIRS) monitors were placed on the latissimus dorsi (LD), pectoralis major (PM), and vastus lateralis (VL) of a world-class kayaker during their preparatory period. The intensity of work, relief, and recovery intervals were the independent variables that were manipulated using three different training protocols. The inferential analysis between intermittent training protocols showed significant differences for all variables except total the hemoglobin (tHb) index in LD during bout 2 (F = 2.83, p = 0.1, ηp2 = 0.205); bout 3 (F = 2.7, p = 0.125, ηp2 = 0.193); bout 4 (F = 1.8, p = 0.202, ηp2 = 0.141); and bout 6 (F = 1.1, p = 0.327, ηp2 = 0.092). During the rest bouts, all training protocols showed significant differences for all variables except muscle oxygen saturation (SmO₂) in the VL during bout 5 (F = 4.4, p = 0.053, ηp2 = 0.286) and tHb in VL during bout 1 (F = 2.28, p = 0.132, ηp2 = 0.172); bout 2 (F = 0.564, p = 0.561, ηp2 = 0.049); bout 3 (F = 1.752, p = 0.205, ηp2 = 0.137); bout 4 (F = 1.216, p = 0.301, ηp2 = 0.1); and bout 6 (F = 4.146, p = 0.053, ηp2 = 0.274). The comparison between IT protocols RT and SIT presented similar results. All variables presented higher values during SIT, except HR results. Finally, the comparison between IT and SIT showed significant differences in several variables, and a clear trend was identified. The results of this study suggest that the application of different intermittent exercise protocols promotes distinct and significant changes in the peripheral effect of muscle oxygenation in response to training stimuli and may be internal predictors of hemodynamic and metabolic changes.

Optical method to determine in vivo capillary hematocrit, hemoglobin concentration, and 3-D network geometry in skeletal muscle

OBJECTIVE

The aim of this study was to develop a tool to visualize and quantify hemodynamic information, such as hemoglobin concentration and hematocrit, within microvascular networks recorded in vivo using intravital video microscopy. Additionally, we aimed to facilitate the 3-D reconstruction of the microvascular networks.

METHODS

Digital images taken from an intravital video microscopy preparation of the extensor digitorum longus muscle in rats for 25 capillary segments were used. The developed algorithm was used to delineate capillaries of interest, calculate the optical density for each pixel in the image, and reconstruct the 3-D capillary geometry using the calculated light path-lengths. Subsequently, the mean corpuscular hemoglobin concentration (MCHC), hemoglobin concentration, and hematocrit for these capillaries were calculated. We evaluated the hematocrit values determined by our methodology by comparing them to those obtained using a previously published method.

RESULTS

The hematocrit values from the proposed optical method were strongly correlated with those calculated using published methods r² (25) = .92, p < .001, and demonstrated excellent agreement with a mean difference of 1.3% and a coefficient of variation (CV) of 11%. The average MCHC, hemoglobin concentration, and light path-lengths were 23.83 g/dl, 8.06 g/dl, and 3.92 µm, respectively.

CONCLUSION

The proposed methodology can quantify hemodynamic measurements and produce functional images for visualization of the microcirculation in vivo.

Tissue Drug Concentration

Blood flow enables the delivery of oxygen and nutrients to the different tissues of the human body. Drugs follow the same route as oxygen and nutrients; thus, drug concentrations in tissues are highly dependent on the blood flow fraction delivered to each of these tissues. Although the free drug concentration in blood is considered to correlate with pharmacodynamics, the pharmacodynamics of a drug is actually primarily commanded by the concentrations of drug in the aqueous spaces of bodily tissues. However, the concentrations of drug are not homogeneous throughout the tissues, and they rarely reflect the free drug concentration in the blood. This heterogeneity is due to differences in the blood flow fraction delivered to the tissues and also due to membrane transporters, efflux pumps, and metabolic enzymes. The rate of drug elimination from the body (systemic elimination) depends more on the driving force of drug elimination than on the free concentration of drug at the site from which the drug is being eliminated. In fact, the actual free drug concentration in the tissues results from the balance between the input and output rates. In the present paper, we develop a theoretical concept regarding solute partition between intravascular and extravascular spaces; discuss experimental research on aqueous/non-aqueous solute partitioning and clinical research on microdialysis; and present hypotheses to predict in-vivo elimination using parameters of in-vitro metabolism.

Capillary Endothelial Insulin Transport: The Rate-limiting Step for Insulin-stimulated Glucose Uptake

The rate-limiting step for skeletal muscle glucose uptake is transport from microcirculation to muscle interstitium. Capillary endothelium poses a barrier that delays the onset of muscle insulin action. Defining physiological barriers that control insulin access to interstitial space is difficult because of technical challenges that confront study of microscopic events in an integrated physiological system. Two physiological variables determine muscle insulin access. These are the number of perfused capillaries and the permeability of capillary walls to insulin. Disease states associated with capillary rarefaction are closely linked to insulin resistance. Insulin permeability through highly resistant capillary walls of muscle poses a significant barrier to insulin access. Insulin may traverse the endothelium through narrow intercellular junctions or vesicular trafficking across the endothelial cell. Insulin is large compared with intercellular junctions, making this an unlikely route. Transport by endothelial vesicular trafficking is likely the primary route of transit. Studies in vivo show movement of insulin is not insulin receptor dependent. This aligns with single-cell transcriptomics that show the insulin receptor is not expressed in muscle capillaries. Work in cultured endothelial cell lines suggest that insulin receptor activation is necessary for endothelial insulin transit. Controversies remain in the understanding of transendothelial insulin transit to muscle. These controversies closely align with experimental approaches. Control of circulating insulin accessibility to skeletal muscle is an area that remains ripe for discovery. Factors that impede insulin access to muscle may contribute to disease and factors that accelerate access may be of therapeutic value for insulin resistance.

Dissociation between exercise intensity thresholds: mechanistic insights from supine exercise

This study tested the hypothesis that the respiratory compensation point (RCP) and breakpoint in deoxygenated [heme] [deoxy[heme]_BP, assessed via near-infrared spectroscopy (NIRS)] during ramp incremental exercise would occur at the same metabolic rate in the upright (U) and supine (S) body positions. Eleven healthy men completed ramp incremental exercise tests in U and S. Gas exchange was measured breath-by-breath and time-resolved-NIRS was used to measure deoxy[heme] in the vastus lateralis (VL) and rectus femoris (RF). RCP (S: 2.56 ± 0.39, U: 2.86 ± 0.40 L·min^-1, P = 0.02) differed from deoxy[heme]_BP in the VL in U (3.10 ± 0.44 L·min^-1, P = 0.002), but was not different in S in the VL (2.70 ± 0.50 L·min^-1, P = 0.15). RCP was not different from the deoxy[heme]_BP in the RF for either position (S: 2.34 ± 0.48 L·min^-1, U: 2.76 ± 0.53 L·min^-1, P > 0.05). However, the deoxy[heme]_BP differed between muscles in both positions (P < 0.05), and changes in deoxy[heme]_BP did not relate to ΔRCP between positions (VL: r = 0.55, P = 0.080, RF: r = 0.26, P = 0.44). The deoxy[heme]_BP was consistently preceded by a breakpoint in total[heme], and was, in turn, itself preceded by a breakpoint in muscle surface electromyography (EMG). RCP and the deoxy[heme]_BP can be dissociated across muscles and different body positions and, therefore, do not represent the same underlying physiological phenomenon. The deoxy[heme]_BP may, however, be mechanistically related to breakpoints in total[heme] and muscle activity.

Blood volume versus deoxygenated NIRS signal: computational analysis of the effects muscle O2 delivery and blood volume on the NIRS signals

Near-infrared spectroscopy (NIRS) signals quantify the oxygenated (ΔHbMbO₂) and deoxygenated (ΔHHbMb) heme group concentrations. ΔHHbMb has been preferred to ΔHbMbO₂ in evaluating skeletal muscle oxygen extraction because it is assumed to be less sensitive to blood volume (BV) changes, but uncertainties exist on this assumption. To analyze this assumption, a computational model of oxygen transport and metabolism is used to quantify the effect of O₂ delivery and BV changes on the NIRS signals from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48: 2013-2020, 2016). The computational analysis accounts for microvascular (ΔHbO₂, ΔHHb) and extravascular (ΔMbO₂, ΔHMb) oxygenated and deoxygenated forms. Simulations predicted muscle oxygen uptake and NIRS signal changes well for blood flows ranging from resting to contracting muscle. Additional NIRS signal simulations were obtained in the absence or presence of BV changes corresponding to a heme groups concentration changes (ΔHbMb = 0-48 µM). Under normal delivery (Q = 1.0 L·kg^-1·min^-1) in contracting muscle, capillary oxygen saturation (So₂) was 62% with capillary ΔHbO₂ and ΔHHb of ± 41 µΜ for ΔHbMb = 0. An increase of BV (ΔHbMb = 24 µΜ) caused a ΔHbO₂ decrease (16µΜ) almost twice as much as the increase observed for ΔHHb (9 µΜ). When So₂ increased to more than 80%, only ΔHbO₂ was significantly affected by BV changes. The analysis indicates that microvascular So₂ is a key factor in determining the sensitivity of ΔHbMbO₂ and deoxygenated ΔHHbMb to BV changes. Contrary to a common assumption, the ΔHHbMb is affected by BV changes in normal contracting muscle and even more in the presence of impaired O₂ delivery.NEW & NOTEWORTHY Deoxygenated is preferred to the oxygenated near-infrared spectroscopy signal in evaluating skeletal muscle oxygen extraction because it is assumed to be insensitive to blood volume changes. The quantitative analysis proposed in this study indicates that even in absence of skin blood flow effects, both NIRS signals in presence of either normal or reduced oxygen delivery are affected by blood volume changes. These changes should be considered to properly quantify muscle oxygen extraction by NIRS methods.

Muscle Oxygenation Level Might Trigger the Regulation of Capillary Venous Blood Filling during Fatiguing Isometric Muscle Actions

The regulation of oxygen and blood supply during isometric muscle actions is still unclear. Recently, two behavioral types of oxygen saturation (SvO2) and relative hemoglobin amount (rHb) in venous microvessels were described during a fatiguing holding isometric muscle action (HIMA) (type I: nearly parallel behavior of SvO2 and rHb; type II: partly inverse behavior). The study aimed to ascertain an explanation of these two regulative behaviors. Twelve subjects performed one fatiguing HIMA trial with each arm by weight holding at 60% of the maximal voluntary isometric contraction (MVIC) in a 90° elbow flexion. Six subjects additionally executed one fatiguing PIMA trial by pulling on an immovable resistance with 60% of the MVIC with each side and same position. Both regulative types mentioned were found during HIMA (I: n = 7, II: n = 17) and PIMA (I: n = 3, II: n = 9). During the fatiguing measurements, rHb decreased initially and started to increase in type II at an average SvO2-level of 58.75 ± 2.14%. In type I, SvO2 never reached that specific value during loading. This might indicate the existence of a threshold around 59% which seems to trigger the increase in rHb and could explain the two behavioral types. An approach is discussed to meet the apparent incompatibility of an increased capillary blood filling (rHb) despite high intramuscular pressures which were found by other research groups during isometric muscle actions.

Fluid and protein exchange in microvascular networks: Importance of modelling heterogeneity in geometrical and biophysical properties

KEY POINTS

Microvascular network architecture defines coupling of fluid and protein exchange. Network arrangements markedly reduce capillary hydrostatic pressures and resting fluid movement at the same time as increasing the capacity for change The presence of vascular remodelling or angiogenesis puts constraints of network behaviour The sites of fluid and protein exchange can be segregated to different portions of the network Although there is a net filtration of fluid from a network of exchange vessels, there are specific areas where fluid moves into the circulation (reabsorption) and, when protein is moving into tissue, the amount is insufficient under basal conditions to result in changes in oncotic pressure.

ABSTRACT

Integration of functional results obtained across scales, from chemical signalling to the whole organism, is a daunting task requiring the marriage of experimental data with mathematical modelling. In the present study, a novel coupled computational fluid dynamics model is developed incorporating fluid and protein transport using measurements in an in vivo frog (Rana pipiens) mesenteric microvascular network. The influences of network architecture and exchange are explored systematically under the common assumptions of structurally and functionally identical microvessels (Homogeneous Scenario) or microvessels classified by position in flow (Class Uniform Scenario), which are compared with realistic microvascular network components (Heterogeneous Scenario). The model incorporates ten quantities that vary within a microvessel; pressure boundary conditions are calibrated against experimental measurements. The Homogeneous Scenario standard model showed that assuming a single 'typical' capillary hides the influence of vessels arranged into a network architecture, where capillary hydrostatic pressures (pT ) are reduced, resulting in both a nonuniform distribution of blood flow and reduced volume flow rate (Jf,T ). In the Class Uniform Scenario pT was further attenuated to produce a ∼60% reduction in Jf,T . Finally, the Heterogeneous Scenario, incorporating measures of individual vessel surface area, demonstrates additional lowering of pT from inlet values favouring a >70% reduction of Jf,T in the face of a ∼120% increase in protein movement into the tissues relative to the Homogeneous Scenario. Beyond the impacts of network architecture, an unanticipated finding was the influence of a blind-end microvessel on model convergence, indicating a profound influence of the largely unexplored dynamics of vascular remodelling on tissue perfusion.

Skeletal muscle microvascular perfusion responses to cuff occlusion and submaximal exercise assessed by contrast-enhanced ultrasound: The effect of age

Impairments in skeletal muscle microvascular function are frequently reported in patients with various cardiometabolic conditions for which older age is a risk factor. Whether aging per se predisposes the skeletal muscle to microvascular dysfunction is unclear. We used contrast‐enhanced ultrasound (CEU) to compare skeletal muscle microvascular perfusion responses to cuff occlusion and leg exercise between healthy young (n = 12, 26 ± 3 years) and older (n = 12, 68 ± 7 years) adults. Test–retest reliability of CEU perfusion parameters was also assessed. Microvascular perfusion (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after: (a) 5‐min of thigh‐cuff occlusion, and (b) 5‐min of submaximal intermittent isometric plantar‐flexion exercise (400 N) using CEU. Whole‐leg blood flow was measured using strain‐gauge plethysmography. Repeated measures were obtained with a 15‐min interval, and averaged responses were used for comparisons between age groups. There were no differences in post‐occlusion whole‐leg blood flow and muscle microvascular perfusion between young and older participants (p > .05). Similarly, total whole‐leg blood flow during exercise and post‐exercise peak muscle microvascular perfusion did not differ between groups (p > .05). The overall level of agreement between the test–retest measures of calf muscle perfusion was excellent for measurements taken at rest (intraclass correlation coefficient [ICC] 0.85), and in response to cuff occlusion (ICC 0.89) and exercise (ICC 0.95). Our findings suggest that healthy aging does not affect muscle perfusion responses to cuff‐occlusion and submaximal leg exercise. CEU muscle perfusion parameters measured in response to these provocation tests are highly reproducible in both young and older adults.

Intravoxel Incoherent Motion Magnetic Resonance Imaging in Skeletal Muscle: Review and Future Directions

Throughout the body, muscle structure and function can be interrogated using a variety of noninvasive magnetic resonance imaging (MRI) methods. Recently, intravoxel incoherent motion (IVIM) MRI has gained momentum as a method to evaluate components of blood flow and tissue diffusion simultaneously. Much of the prior research has focused on highly vascularized organs, including the brain, kidney, and liver. Unique aspects of skeletal muscle, including the relatively low perfusion at rest and large dynamic range of perfusion between resting and maximal hyperemic states, may influence the acquisition, postprocessing, and interpretation of IVIM data. Here, we introduce several of those unique features of skeletal muscle; review existing studies of IVIM in skeletal muscle at rest, in response to exercise, and in disease states; and consider possible confounds that should be addressed for muscle-specific evaluations. Most studies used segmented nonlinear least squares fitting with a b-value threshold of 200 sec/mm² to obtain IVIM parameters of perfusion fraction (f), pseudo-diffusion coefficient (D*), and diffusion coefficient (D). In healthy individuals, across all muscles, the average ± standard deviation of D was 1.46 ± 0.30 × 10^-3  mm² /sec, D* was 29.7 ± 38.1 × 10^-3  mm² /sec, and f was 11.1 ± 6.7%. Comparisons of reported IVIM parameters in muscles of the back, thigh, and leg of healthy individuals showed no significant difference between anatomic locations. Throughout the body, exercise elicited a positive change of all IVIM parameters. Future directions including advanced postprocessing models and potential sequence modifications are discussed. LEVEL OF EVIDENCE: 2 TECHNICAL EFFICACY: Stage 2.

Fractional calculus tracer kinetic compartment model for quantification of microvascular perfusion

Objective. We evaluate a tracer kinetic model for quantification of physiological perfusion and microvascular residue time kurtosis (RTK) in skeletal muscle vasculature with first pass bolus experiments in dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI).Approach. A decreasing stretched Mittag-Leffler function (f1C model) was obtained as the impulse response solution of a rate equation of real-valued ('fractional') derivation order. The method was validated in skeletal muscle in the lower limb of seven female pigs examined by DCE-MRI. Dynamic imaging during blood pool contrast agent elimination was performed using a 3D gradient echo sequence with k-space sharing. Blood flow was augmented by continuous infusion of the vasodilator adenosine into the femoral artery increasing blood flow up to four times. Blood flow measured by a Doppler flow probe placed at the femoral artery served as ground truth.Main results. Goodness of fit and correlation with the Doppler measurements,r= 0.80 (P< 0.001), of the 4-parameter f1C model was comparable with the results obtained with a previously tested 6-parameter two-compartment (2C) model. The derivation orderαof the f1C model can be interpreted as a measure of microvascular RTK. With increasing blood flow,αdropped significantly, leading to an increase in RTK.Significance. The f1C model is a practical approach based on hemodynamic principles to quantify physiological microvascular perfusion but it is impaired due to its compartmental nature.

Development and validation of a novel microfluidic device for the manipulation of skeletal muscle microvascular blood flow in vivo

OBJECTIVE

To develop and validate a novel liquid microfluidic approach to deliver drugs to microscale regions of tissue while simultaneously allowing for visualization and quantification of microvascular blood flow.

METHODS

Microfluidic devices were fabricated using soft lithographic techniques, molded in polydimethylsiloxane, and bound to a coverslip with a 600 × 300 μm micro-outlet. Sprague-Dawley rats, anesthetized with pentobarbital, were instrumented to monitor systemic parameters. The extensor digitorum longus muscle was dissected, externalized, and reflected across the device mounted on the stage of an inverted microscope. Doses (10^-8 to 10^-3  M) of adenosine triphosphate (ATP), acetylcholine, and phenylephrine (PE) were administered to the muscle via perfusion through the device. Microvascular blood flow directly overlying the micro-outlet was recorded at multiple focal depths. Red blood cell (RBC) velocity, supply rate, and hematocrit were measured from recordings.

RESULTS

ATP significantly increased RBC velocity and supply rate. Increasing concentrations of PE caused a decrease in RBC velocity and supply rate. Perfusion changes were restricted to areas directly overlying the micro-outlet and within 500 μm.

CONCLUSIONS

This novel microfluidic device allows for a controlled delivery of dissolved substances to constrained regions of microvasculature while simultaneously allowing for visualization and measurement of blood flow within discrete vessels and networks.

Identification of novel sublingual parameters to analyze and diagnose microvascular dysfunction in sepsis: the NOSTRADAMUS study

Background

The availability of handheld, noninvasive sublingual video-microscopes allows for visualization of the microcirculation in critically ill patients. Recent studies demonstrate that reduced numbers of blood-perfused microvessels and increased penetration of erythrocytes into the endothelial glycocalyx are essential components of microvascular dysfunction. The aim of this study was to identify novel microvascular variables to determine the level of microvascular dysfunction in sepsis and its relationship with clinical variables.

Methods

This observational, prospective, cross-sectional study included 51 participants, of which 34 critically ill sepsis patients were recruited from intensive care units of a university hospital. Seventeen healthy volunteers served as controls. All participants underwent sublingual videomicroscopy by sidestream darkfield imaging. A new developed version of the Glycocheck™ software was used to quantify vascular density, perfused boundary region (PBR-an inverse variable of endothelial glycocalyx dimensions), red blood cell (RBC) velocity, RBC content, and blood flow in sublingual microvessels with diameters between 4 and 25 µm.

Results

A detailed analysis of adjacent diameter classes (1 µm each) of vessels between 4 and 25 µm revealed a severe reduction of vascular density in very small capillaries (5–7 µm), which correlated with markers of sepsis severity. Analysis of RBC velocity (V_RBC) revealed a strong dependency between capillary and feed vessel V_RBC in sepsis patients (R² = 0.63, p < 0.0001) but not in healthy controls (R² = 0.04, p = 0.43), indicating impaired capillary (de-)recruitment in sepsis. This finding enabled the calculation of capillary recruitment and dynamic capillary blood volume (CBV_dynamic). Moreover, adjustment of PBR to feed vessel V_RBC further improved discrimination between sepsis patients and controls by about 50%. By combining these dynamic microvascular and glycocalyx variables, we developed the microvascular health score (MVHS_dynamic™), which decreased from 7.4 [4.6–8.7] in controls to 1.8 [1.4–2.7] in sepsis patients (p < 0.0001) and correlated with sepsis severity.

Conclusion

We introduce new important diameter-specific quantification and differentiated analysis of RBC kinetics, a key to understand microvascular dysfunction in sepsis. MVHS_dynamic, which has a broad bandwidth to detect microvascular (dys-) function, might serve as a valuable tool to detect microvascular impairment in critically ill patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13054-021-03520-w.

The capillary fascicle in skeletal muscle: Structural and functional physiology of RBC distribution in capillary networks

KEY POINTS

The capillary module, consisting of parallel capillaries from arteriole to venule, is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres. Using intravital video microscopy of resting extensor digitorum longus muscle in rats, we demonstrated the capillary fascicle as a series of interconnected modules forming continuous columns that align naturally with the dimensions of the muscle fascicle. We observed structural heterogeneity for module topology, and functional heterogeneity in space and time for capillary-red blood cell (RBC) haemodynamics within a module and between modules. We found that module RBC haemodynamics were independent of module resistance, providing direct evidence for microvascular flow regulation at the level of the capillary module. The capillary fascicle is an updated paradigm for characterizing blood flow and RBC distribution in skeletal muscle capillary networks.

ABSTRACT

Capillary networks are the fundamental site of oxygen exchange in the microcirculation. The capillary module (CM), consisting of parallel capillaries from terminal arteriole (TA) to post-capillary venule (PCV), is classically considered as the building block of complex capillary networks. In skeletal muscle, this structure fails to address how blood flow is regulated along the entire length of the synchronously contracting muscle fibres, requiring co-ordination from numerous modules. It has previously been recognized that TAs and PCVs interact with multiple CMs, creating interconnected networks. Using label-free intravital video microscopy of resting extensor digitorum longus muscle in rats, we found that these networks form continuous columns of linked CMs spanning thousands of microns, herein denoted as the capillary fascicle (CF); this structure aligns naturally with the dimensions of the muscle fascicle. We measured capillary-red blood cell (RBC) haemodynamics and module topology (n = 9 networks, 327 modules, 1491 capillary segments). The average module had length 481 μm, width 157 μm and 9.51 parallel capillaries. We observed structural heterogeneity for CM topology, and functional heterogeneity in space and time for capillary-RBC haemodynamics within a module and between modules. There was no correlation between capillary RBC velocity and lineal density. A passive inverse relationship between module length and haemodynamics was remarkably absent, providing direct evidence for microvascular flow regulation at the level of the CM. In summary, the CF is an updated paradigm for characterizing RBC distribution in skeletal muscle, and strengthens the theory of capillary networks as major contributors to the signal that regulates capillary perfusion.

Voluntary exercise does not improve muscular properties or functional capacity during C26-induced cancer cachexia in mice

Exercise training is considered as a potential intervention to counteract muscle degeneration in cancer cachexia. However, evidence to support such intervention is equivocal. Therefore, we investigated the effect of exercise training, i.e. voluntary wheel running, on muscle wasting, functional capacity, fiber type composition and vascularization during experimental cancer cachexia in mice. Balb/c mice were injected with PBS (CON) or C26 colon carcinoma cells to induce cancer cachexia (C26). Mice had free access to a running wheel in their home cage (CON_EX and C26_EX, n = 8-9) or were sedentary (CON_S and C26_S, n = 8-9). Mice were sacrificed 18 days upon tumor cell injection. Immunohistochemical analyes were performed on m. gastrocnemius and quadriceps, and ex vivo contractile properties were assessed in m. soleus and extensor digitorum longus (EDL). Compared with CON, C26 mice exhibited body weight loss (~ 20 %), muscle atrophy (~ 25 %), reduced grip strength (~ 25 %), and lower twitch and tetanic force (~ 20 %) production in EDL but not in m. soleus. Furthermore, muscle of C26 mice were characterizd by a slow-to-fast fiber type shift (type IIx fibers: +57 %) and increased capillary density (~ 30 %). In C26 mice, wheel running affect neither body weight loss, nor muscle atrophy or functional capacity, nor inhibited tumor growth. However, wheel running induced a type IIb to type IIa fiber shift in m. quadriceps from both CON and C26, but not in m. gastrocnemius. Wheel running does not exacerbate muscular degeneration in cachexic mice, but, when voluntary, is insufficient to improve the muscle phenotype.

The role of vascular function on exercise capacity in health and disease

Three sentinel parameters of aerobic performance are the maximal oxygen uptake ( V ̇ O 2 max ), critical power (CP) and speed of the V ̇ O 2 kinetics following exercise onset. Of these, the latter is, perhaps, the cardinal test of integrated function along the O2 transport pathway from lungs to skeletal muscle mitochondria. Fast V ̇ O 2 kinetics demands that the cardiovascular system distributes exercise-induced blood flow elevations among and within those vascular beds subserving the contracting muscle(s). Ideally, this process must occur at least as rapidly as mitochondrial metabolism elevates V ̇ O 2 . Chronic disease and ageing create an O2 delivery (i.e. blood flow × arterial [O2 ], Q ̇ O 2 ) dependency that slows V ̇ O 2 kinetics, decreasing CP and V ̇ O 2 max , increasing the O2 deficit and sowing the seeds of exercise intolerance. Exercise training, in contrast, does the opposite. Within the context of these three parameters (see Graphical Abstract), this brief review examines the training-induced plasticity of key elements in the O2 transport pathway. It asks how structural and functional vascular adaptations accelerate and redistribute muscle Q ̇ O 2 and thus defend microvascular O2 partial pressures and capillary blood-myocyte O2 diffusion across a ∼100-fold range of muscle V ̇ O 2 values. Recent discoveries, especially in the muscle microcirculation and Q ̇ O 2 -to- V ̇ O 2 heterogeneity, are integrated with the O2 transport pathway to appreciate how local and systemic vascular control helps defend V ̇ O 2 kinetics and determine CP and V ̇ O 2 max in health and how vascular dysfunction in disease predicates exercise intolerance. Finally, the latest evidence that nitrate supplementation improves vascular and therefore aerobic function in health and disease is presented.

Exercise training in COPD: muscle O2 transport plasticity

Both convective oxygen (O₂) transport to, and diffusive transport within, skeletal muscle are markedly diminished in patients with COPD. However, it is unknown how these determinants of peak muscle O₂ uptake (V'_mO2peak) respond to exercise training in patients with COPD. Therefore, the purpose of this study was to assess the plasticity of skeletal muscle O₂ transport determinants of V'_mO2peak in patients with COPD.Adaptations to 8 weeks of single-leg knee-extensor exercise training were measured in eight patients with severe COPD (mean±sem forced expiratory volume in 1 s (FEV₁) 0.9±0.1 L) and eight healthy, well-matched controls. Femoral arterial and venous blood samples, and thermodilution-assessed leg blood flow were used to determine muscle O₂ transport and utilisation at maximal exercise pre- and post-training.Training increased V'_mO2peak in both COPD (by ∼26% from 271±29 to 342±35 mL·min^-1) and controls (by ∼32% from 418±37 to 553±41 mL·min^-1), restoring V'_mO2peak in COPD to only ∼80% of pre-training control V'_mO2peak Muscle diffusive O₂ transport increased similarly in both COPD (by ∼38% from 6.6±0.9 to 9.1±0.9 mL·min^-1·mmHg^-1) and controls (by ∼36% from 10.4±0.7 to 14.1±0.8 mL·min^-1·mmHg^-1), with the patients reaching ∼90% of pre-training control values. In contrast, muscle convective O₂ transport increased significantly only in controls (by ∼26% from 688±57 to 865±69 mL·min^-1), leaving patients with COPD (438±45 versus 491±51 mL·min^-1) at ∼70% of pre-training control values.While muscle diffusive O₂ transport in COPD was largely restored by exercise training, V'_mO2peak remained constrained by limited plasticity in muscle convective O₂ transport.

Cardiovascular Responses to Muscle Stretching: A Systematic Review and Meta-analysis

The aim of this study will be to review the current body of literature to understand the effects of stretching on the responses of the cardiovascular system. A literature search was performed using the following databases: Scopus, NLM Pubmed and ScienceDirect. Studies regarding the effects of stretching on responses of the cardiovascular system were investigated. Outcomes regarded heart rate(HR), blood pressure, pulse wave velocity (PWV of which baPWV for brachial-ankle and cfPWV for carotid-femoral waveforms), heart rate variability and endothelial vascular function. Subsequently, the effects of each outcome were quantitatively synthetized using meta-analytic synthesis with random-effect models. A total of 16 studies were considered eligible and included in the quantitative synthesis. Groups were also stratified according to cross-sectional or longitudinal stretching interventions. Quality assessment through the NHLBI tools observed a "fair-to-good" quality of the studies. The meta-analytic synthesis showed a significant effect of d=0.38 concerning HR, d=2.04 regarding baPWV and d=0.46 for cfPWV. Stretching significantly reduces arterial stiffness and HR. The qualitative description of the studies was also supported by the meta-analytic synthesis. No adverse effects were reported, after stretching, in patients affected by cardiovascular disease on blood pressure. There is a lack of studies regarding vascular adaptations to stretching.

Rapid vasodilation within contracted skeletal muscle in humans: new insight from concurrent use of diffuse correlation spectroscopy and Doppler ultrasound

Previous studies showed that conduit artery blood flow rapidly increases after even a brief contraction of muscles within the dependent limb. Whether this rapid hyperemia occurs within contracted skeletal muscle in humans has yet to be confirmed, however. We therefore used diffuse correlation spectroscopy (DCS) to characterize the rapid hyperemia and vasodilatory responses within the muscle microvasculature induced by single muscle contractions in humans. Twenty-five healthy male volunteers performed single 1-s isometric handgrips at 20%, 40%, 60%, and 80% of maximum voluntary contraction. DCS probes were placed on the flexor digitorum superficialis muscle, and a skeletal muscle blood flow index (SMBFI) was derived continuously. At the same time, brachial artery blood flow (BABF) responses were measured using Doppler ultrasound. Single muscle contractions evoked rapid, monophasic increases in both SMBFI and BABF that occurred within 3 s after release of contraction. The initial and peak responses increased with increases in contraction intensity and were greater for BABF than for SMBFI at all intensities. BABF reached its peak within 5 to 8 s after the end of contraction. The SMBFI continued to increase after the BABF passed its peak and was decreasing toward the resting level and peaked about 10 to 15 s after completion of the contraction. We conclude that single muscle contractions induce rapid, intensity-dependent hyperemia within the contracted skeletal muscle microvasculature. Moreover, the characteristics of the rapid hyperemia and vasodilatory responses of skeletal muscle microvessels differ from those simultaneously evaluated in the upstream conduit artery.NEW & NOTEWORTHY Through the concurrent use of diffuse correlation spectroscopy and Doppler ultrasound, we provide the first evidence in humans that a single brief muscle contraction evokes rapid, intensity-dependent hyperemia within the contracted skeletal muscle microvasculature and the upstream conduit artery. We also show that the magnitude and time course of the contraction-induced rapid hyperemia and vasodilatory responses within skeletal muscle microvessels significantly differ from those in the conduit artery.

Vascular ATP-sensitive K+ channels support maximal aerobic capacity and critical speed via convective and diffusive O2 transport

KEY POINTS

Oral sulphonylureas, widely prescribed for diabetes, inhibit pancreatic ATP-sensitive K+ (KATP ) channels to increase insulin release. However, KATP channels are also located within vascular (endothelium and smooth muscle) and muscle (cardiac and skeletal) tissue. We evaluated left ventricular function at rest, maximal aerobic capacity ( V ̇ O2 max) and submaximal exercise tolerance (i.e. speed-duration relationship) during treadmill running in rats, before and after systemic KATP channel inhibition via glibenclamide. Glibenclamide impaired critical speed proportionally more than V ̇ O2 max but did not alter resting cardiac output. Vascular KATP channel function (topical glibenclamide superfused onto hindlimb skeletal muscle) resolved a decreased blood flow and interstitial PO2 during twitch contractions reflecting impaired O2 delivery-to-utilization matching. Our findings demonstrate that systemic KATP channel inhibition reduces V ̇ O2 max and critical speed during treadmill running in rats due, in part, to impaired convective and diffusive O2 delivery, and thus V ̇ O2 , especially within fast-twitch oxidative skeletal muscle.

ABSTRACT

Vascular ATP-sensitive K+ (KATP ) channels support skeletal muscle blood flow and microvascular oxygen delivery-to-utilization matching during exercise. However, oral sulphonylurea treatment for diabetes inhibits pancreatic KATP channels to enhance insulin release. Herein we tested the hypotheses that: i) systemic KATP channel inhibition via glibenclamide (GLI; 10 mg kg-1 i.p.) would decrease cardiac output at rest (echocardiography), maximal aerobic capacity ( V ̇ O2 max) and the speed-duration relationship (i.e. lower critical speed (CS)) during treadmill running; and ii) local KATP channel inhibition (5 mg kg-1 GLI superfusion) would decrease blood flow (15 µm microspheres), interstitial space oxygen pressures (PO2 is; phosphorescence quenching) and convective and diffusive O2 transport ( Q ̇ O2 and DO2 , respectively; Fick Principle and Law of Diffusion) in contracting fast-twitch oxidative mixed gastrocnemius muscle (MG: 9% type I+IIa fibres). At rest, GLI slowed left ventricular relaxation (2.11 ± 0.59 vs. 1.70 ± 0.23 cm s-1 ) and decreased heart rate (321 ± 23 vs. 304 ± 22 bpm, both P < 0.05) while cardiac output remained unaltered (219 ± 64 vs. 197 ± 39 ml min-1 , P > 0.05). During exercise, GLI reduced V ̇ O2 max (71.5 ± 3.1 vs. 67.9 ± 4.8 ml kg-1 min-1 ) and CS (35.9 ± 2.4 vs. 31.9 ± 3.1 m min-1 , both P < 0.05). Local KATP channel inhibition decreased MG blood flow (52 ± 25 vs. 34 ± 13 ml min-1 100 g tissue-1 ) and PO2 isnadir (5.9 ± 0.9 vs. 4.7 ± 1.1 mmHg) during twitch contractions. Furthermore, MG V ̇ O2 was reduced via impaired Q ̇ O2 and DO2 (P < 0.05 for each). Collectively, these data support that vascular KATP channels help sustain submaximal exercise tolerance in healthy rats. For patients taking sulfonylureas, KATP channel inhibition may exacerbate exercise intolerance.

Blood flow, capillary transit times, and tissue oxygenation: the centennial of capillary recruitment

The transport of oxygen between blood and tissue is limited by blood's capillary transit time, understood as the time available for diffusion exchange before blood returns to the heart. If all capillaries contribute equally to tissue oxygenation at all times, this physical limitation would render vasodilation and increased blood flow insufficient means to meet increased metabolic demands in the heart, muscle, and other organs. In 1920, Danish physiologist August Krogh was awarded the Nobel Prize in Physiology or Medicine for his mathematical and quantitative, experimental demonstration of a solution to this conceptual problem: capillary recruitment, the active opening of previously closed capillaries to meet metabolic demands. Today, capillary recruitment is still mentioned in textbooks. When we suspect symptoms might represent hypoxia of a vascular origin, however, we search for relevant, flow-limiting conditions in our patients and rarely ascribe hypoxia or hypoxemia to short capillary transit times. This review describes how natural changes in capillary transit-time heterogeneity (CTH) and capillary hematocrit (HCT) across open capillaries during blood flow increases can account for a match of oxygen availability to metabolic demands in normal tissue. CTH and HCT depend on a number of factors: on blood properties, including plasma viscosity, the number, size, and deformability of blood cells, and blood cell interactions with capillary endothelium; on anatomical factors including glycocalyx, endothelial cells, basement membrane, and pericytes that affect the capillary diameter; and on any external compression. The review describes how risk factor- and disease-related changes in CTH and HCT interfere with flow-metabolism coupling and tissue oxygenation and discusses whether such capillary dysfunction contributes to vascular disease pathology.

August Krogh's theory of muscle microvascular control and oxygen delivery: a paradigm shift based on new data

August Krogh twice won the prestigious international Steegen Prize, for nitrogen metabolism (1906) and overturning the concept of active transport of gases across the pulmonary epithelium (1910). Despite this, at the beginning of 1920, the consummate experimentalist was relatively unknown worldwide and even among his own University of Copenhagen faculty. But, in early 1919, he had submitted three papers to Dr Langley, then editor of The Journal of Physiology in England. These papers coalesced anatomical observations of skeletal muscle capillary numbers with O₂ diffusion theory to propose a novel active role for capillaries that explained the prodigious increase in blood-muscle O₂ flux from rest to exercise. Despite his own appraisal of the first two papers as "rather dull" to his friend, the eminent Cambridge respiratory physiologist, Joseph Barcroft, Krogh believed that the third one, dealing with O₂ supply and capillary regulation, was"interesting". These papers, which won Krogh an unopposed Nobel Prize for Physiology or Medicine in 1920, form the foundation for this review. They single-handedly transformed the role of capillaries from passive conduit and exchange vessels, functioning at the mercy of their upstream arterioles, into independent contractile units that were predominantly closed at rest and opened actively during muscle contractions in a process he termed 'capillary recruitment'. Herein we examine Krogh's findings and some of the experimental difficulties he faced. In particular, the boundary conditions selected for his model (e.g. heavily anaesthetized animals, negligible intramyocyte O₂ partial pressure, binary open-closed capillary function) have not withstood the test of time. Subsequently, we update the reader with intervening discoveries that underpin our current understanding of muscle microcirculatory control and place a retrospectroscope on Krogh's discoveries. The perspective is presented that the imprimatur of the Nobel Prize, in this instance, may have led scientists to discount compelling evidence. Much as he and Marie Krogh demonstrated that active transport of gases across the blood-gas barrier was unnecessary in the lung, capillaries in skeletal muscle do not open and close spontaneously or actively, nor is this necessary to account for the increase in blood-muscle O₂ flux during exercise. Thus, a contemporary model of capillary function features most muscle capillaries supporting blood flow at rest, and, rather than capillaries actively vasodilating from rest to exercise, increased blood-myocyte O₂ flux occurs predominantly via elevating red blood cell and plasma flux in already flowing capillaries. Krogh is lauded for his brilliance as an experimentalist and for raising scientific questions that led to fertile avenues of investigation, including the study of microvascular function.

The effect of dietary nitrate supplementation on the speed-duration relationship in mice with sickle cell disease

Sickle cell disease (SCD) causes exercise intolerance likely due to impaired skeletal muscle function and low nitric oxide (NO) bioavailability. Dietary nitrate improves hemodynamic and metabolic control during exercise in humans and animals. The purpose of this investigation was to assess the impact of nitrate supplementation on exercise capacity as measured by the running speed to exercise duration relationship [critical speed (CS)]in mice with SCD. We tested the hypothesis that nitrate supplementation via beetroot juice (BR) would attenuate the exercise intolerance observed in mice with SCD. Ten wild-type (WT) and 18 Berkley sickle-cell mice (BERK) received water (WT: n = 10, BERK: n = 10) or nitrate-rich BR (BERK+BR: n = 8, nitrate dose 1 mmol/kg/day) for 5 days. Following the supplementation period, all mice performed 3-5 constant-speed treadmill tests that resulted in exhaustion within 1.5 to 20 min. Time to exhaustion vs. treadmill speed was fit to a hyperbolic model to determine CS. CS was significantly lower in BERK vs. WT and BERK+BR with no significant difference between WT and BERK+BR (WT: 36.6 ± 1.6, BERK: 23.8 ± 1.5, BERK+BR: 31.1 ± 2.1 m/min, P < 0.05). Exercise tolerance, measured via CS, was significantly lower in BERK mice relative to WT. However, BERK mice receiving 5 days of nitrate supplementation exhibited no difference in exercise tolerance when compared with WT. These results support the potential utility of a dietary nitrate intervention to improve functionality in SCD patients.NEW & NOTEWORTHY Sickle cell disease compromises muscle O₂ delivery resulting in exercise intolerance. Dietary nitrate supplementation increases skeletal muscle blood flow during exercise and may improve exercise capacity in a mouse model of sickle cell disease. We investigated the effects of dietary nitrate supplementation on exercise tolerance in a mouse model of sickle cell disease using the treadmill speed-duration relationship (critical speed). Mice with sickle cell disease provided with a dietary nitrate supplement had a critical speed not significantly different from healthy wild-type mice.

Development of a flow standard to enable highly reproducible measurements of deformability of stored red blood cells in a microfluidic device

BACKGROUND

Great deformability allows red blood cells (RBCs) to flow through narrow capillaries in tissues. A number of microfluidic devices with capillary-like microchannels have been developed to monitor storage-related impairment of RBC deformability during blood banking operations. This proof-of-concept study describes a new method to standardize and improve reproducibility of the RBC deformability measurements using one of these devices.

STUDY DESIGN AND METHODS

The rate of RBC flow through the microfluidic capillary network of the microvascular analyzer (MVA) device made of polydimethylsiloxane was measured to assess RBC deformability. A suspension of microbeads in a solution of glycerol in phosphate-buffered saline was developed to be used as an internal flow rate reference alongside RBC samples in the same device. RBC deformability and other in vitro quality markers were assessed weekly in six leukoreduced RBC concentrates (RCCs) dispersed in saline-adenine-glucose-mannitol additive solution and stored over 42 days at 4°C.

RESULTS

The use of flow reference reduced device-to-device measurement variability from 10% to 2%. Repeated-measure analysis using the generalized estimating equation (GEE) method showed a significant monotonic decrease in relative RBC flow rate with storage from Week 0. By the end of storage, relative RBC flow rate decreased by 22 ± 6% on average.

CONCLUSIONS

The suspension of microbeads was successfully used as a flow reference to increase reproducibility of RBC deformability measurements using the MVA. Deformability results suggest an early and late aging phase for stored RCCs, with significant decreases between successive weeks suggesting a highly sensitive measurement method.

Effect of differential muscle activation patterns on muscle deoxygenation and microvascular haemoglobin regulation

NEW FINDINGS

What is the central question of this study? Does the presence and extent of heterogeneity in the ratio of O2 delivery to uptake across human muscles relate specifically to different muscle activation patterns? What is the main finding and its importance? During ramp incremental knee-extension and cycling exercise, the profiles of muscle deoxygenation (deoxy[haemoglobin + myoglobin]) and diffusive O2 potential (total[haemoglobin + myoglobin]) in the vastus lateralis corresponded to different muscle activation strategies. However, this was not the case for the rectus femoris, where muscle activation and deoxygenation profiles were dissociated and might therefore be determined by other structural and/or functional attributes (e.g. arteriolar vascular regulation and control of red blood cell flux).

ABSTRACT

Near-infrared spectroscopy has revealed considerable heterogeneity in the ratio of O2 delivery to uptake as identified by disparate deoxygenation {deoxy[haemoglobin + myoglobin] (deoxy[Hb + Mb])} values in the exercising quadriceps. However, whether this represents a recruitment phenomenon or contrasting vascular and metabolic control, as seen among fibre types, has not been established. We used knee-extension (KE) and cycling (CE) incremental exercise protocols to examine whether differential muscle activation profiles could account for the heterogeneity of deoxy[Hb + Mb] and microvascular haemoconcentration (i.e. total[Hb + Mb]). Using time-resolved near-infrared spectroscopy for the quadriceps femoris (vastus lateralis and rectus femoris) during exhaustive ramp exercise in eight participants, we tested the following hypotheses: (i) the deoxy[Hb + Mb] (i.e. fractional O2 extraction) would relate to muscle activation levels across exercise protocols; and (ii) KE would induce greater total[Hb + Mb] (i.e. diffusive O2 potential) at task failure (i.e. peak O2 uptake) than CE irrespective of muscle site. At a given level of muscle activation, as assessed by the relative integrated EMG normalized to maximal voluntary contraction (%iEMGmax ), the vastus lateralis deoxy[Hb + Mb] profile was not different between exercise protocols. However, at peak O2 uptake and until 20% iEMGmax for CE, rectus femoris exhibited a lower deoxy[Hb + Mb] (83.2 ± 15.5 versus 98.2 ± 19.4 μm) for KE than for CE (P < 0.05). The total[Hb + Mb] at peak O2 uptake was not different between exercise protocols for either muscle site. These data support the hypothesis that the contrasting patterns of convective and diffusive O2 transport correspond to different muscle activation patterns in vastus lateralis but not rectus femoris. Thus, the differential deoxygenation profiles for rectus femoris across exercise protocols might be dependent upon specific facets of muscle architecture and functional haemodynamic events.

Are transient changes in capillary surface area required to explain peritoneal transport in renal failure patients?

BACKGROUND

Waniewski postulated a transient increase in peritoneal capillary surface area to fit their model predictions to experimental data of Heimburger measured in renal failure (RF) patients undergoing peritoneal dialysis (PD) but with only a 3.86% glucose dialysis fluid. The present aim is to propose a new mathematical model of the patient PD procedure that could closely fit the complete Heimburger measurement set without this postulate.

METHODS

The three-pore model of Rippe was used to describe transient changes in peritoneal volume and solute concentrations during a PD dwell. The predialysis, RF patient, plasma solute concentrations were assumed to remain constant during the dwell. The model was validated using the 3.86% glucose Heimburger measurements. Permeability surface area product parameters were chosen to match only the end-dwell peritoneal fluid glucose concentration and the end-dwell amounts of urea, creatinine, and Na⁺ removed from this simulated patient group. Then, this model was used to predict additional measurements by Heimburger on two other patient groups dialyzed with glucose concentrations of 2.27% and 1.36%, respectively. Parameters were unchanged when simulating these other patient groups.

RESULTS

To match the shape of the transient changes in drained volume and dialysis fluid glucose concentration for the 3.86% glucose group, it was necessary for only one parameter, the effective radius of glucose, to vary linearly in proportion to the dialysis fluid glucose concentration. This description was unchanged in the other two groups.

CONCLUSION

Postulated transient increases in peritoneal capillary surface area were unnecessary to predict the entire Heimburger measurements.

Krogh’s capillary recruitment hypothesis, 100 years on: Is the opening of previously closed capillaries necessary to ensure muscle oxygenation during exercise?

In 1919, August Krogh published his seminal work on skeletal muscle oxygenation. Krogh’s observations indicated that muscle capillary diameter is actively regulated, rather than a passive result of arterial blood flow regulation. Indeed, combining a mathematical model with the number of ink-filled capillaries he observed in muscle cross sections taken at different workloads, Krogh was able to account for muscle tissue’s remarkably efficient oxygen extraction during exercise in terms of passive diffusion from nearby capillaries. Krogh was awarded the 1920 Nobel Prize for his account of muscle oxygenation. Today, his observations are engrained in the notion of capillary recruitment: the opening of previously closed capillaries. While the binary distinction between “closed” and “open” was key to Krogh’s model argument, he did in fact report a continuum of capillary diameters, degrees of erythrocyte deformation, and perfusion states. Indeed, modern observations question the presence of closed muscle capillaries. We therefore examined whether changes in capillary flow patterns and hematocrit among open capillaries can account for oxygen extraction in muscle across orders-of-magnitude changes in blood flow. Our four-compartment model of oxygen extraction in muscle confirms this notion and provides a framework for quantifying the impact of changes in microvascular function on muscle oxygenation in health and disease. Our results underscore the importance of capillary function for oxygen extraction in muscle tissue as first proposed by Krogh. While Krogh’s model calculations still hold, our model predictions support that capillary recruitment can be viewed in the context of continuous, rather than binary, erythrocyte distributions among capillaries.

NEW & NOTEWORTHY Oxygen extraction in working muscle is extremely efficient in view of single capillaries properties. The underlying mechanisms have been widely debated. Here, we develop a four-compartment model to quantify the influence of each of the hypothesized mechanisms on muscle oxygenation. Our results show that changes in capillary flow pattern and hematocrit can account for the high oxygen extraction observed in working muscle, while capillary recruitment is not required to account for these extraction properties.

Perfusion controls muscle glucose uptake by altering the rate of glucose dispersion in vivo

These studies test, using intravital microscopy (IVM), the hypotheses that perfusion effects on insulin-stimulated muscle glucose uptake (MGU) are 1) capillary recruitment independent and 2) mediated through the dispersion of glucose rather than insulin. For experiment 1, capillary perfusion was visualized before and after intravenous insulin. No capillary recruitment was observed. For experiment 2, mice were treated with vasoactive compounds (sodium nitroprusside, hyaluronidase, and lipopolysaccharide), and dispersion of fluorophores approximating insulin size (10-kDa dextran) and glucose (2-NBDG) was measured using IVM. Subsequently, insulin and 2[¹⁴C]deoxyglucose were injected and muscle phospho-2[¹⁴C]deoxyglucose (2[C¹⁴]DG) accumulation was used as an index of MGU. Flow velocity and 2-NBDG dispersion, but not perfused surface area or 10-kDa dextran dispersion, predicted phospho-2[¹⁴C]DG accumulation. For experiment 3, microspheres of the same size and number as are used for contrast-enhanced ultrasound (CEU) studies of capillary recruitment were visualized using IVM. Due to their low concentration, microspheres were present in only a small fraction of blood-perfused capillaries. Microsphere-perfused blood volume correlated to flow velocity. These findings suggest that 1) flow velocity rather than capillary recruitment controls microvascular contributions to MGU, 2) glucose dispersion is more predictive of MGU than dispersion of insulin-sized molecules, and 3) CEU measures regional flow velocity rather than capillary recruitment.

The vascular endothelium plays a role in insulin action

The endocrine system relies on the vasculature for delivery of hormones throughout the body, and the capillary microvasculature is the site where the hormones cross from the blood into the target tissue. Once considered an inert wall, various studies have now highlighted the functions of the capillary endothelium to regulate transport and therefore affect or maintain the interstitial environment. The role of the capillary may be clear in areas where there is a continuous endothelium, yet there also appears to be a role of endothelial cells in tissues with a sinusoidal structure. Here we focused on the most common endocrine disorder, diabetes, and several of the target organs associated with the disease, including skeletal muscle, liver and pancreas. However, it is important to note that the ability of hormones to cross the endothelium to reach their target tissue is a component of all endocrine functions. It is also a consideration in organs throughout the body and may have greater impact for larger hormones with target tissues containing a continuous endothelium. We noted that the blood levels do not always equal interstitial levels, which is what the cells are exposed to, and discussed how this may change in diseases such as obesity and insulin resistance. The capillary endothelium is, therefore, an essential and understudied aspect of endocrinology and metabolism that can be altered in disease, which may be an appropriate target for treatment.

Determinants of skeletal muscle oxygen consumption assessed by near-infrared diffuse correlation spectroscopy during incremental handgrip exercise

Near-infrared diffuse correlation spectroscopy (DCS) is a rapidly evolving optical imaging technique for the assessment of skeletal muscle O₂ utilization (mVO₂). We compared DCS-derived determinants of mVO₂ with conventional measures [blood flow by brachial artery Doppler ultrasound and venous O₂ saturation (SVO2)] in eight volunteers at rest and during incremental handgrip exercise. Brachial artery blood flow and DCS-derived blood flow index (BFI) were linearly related (R² = 0.57) and increased with each workload, whereas SVO2 decreased from 65.3 ± 2.5% (rest) to 39.9 ± 3.0% (light exercise; P < 0.01) with no change thereafter. In contrast, DCS-derived tissue O₂ saturation decreased progressively with each incremental stage (P < 0.01), driven almost entirely by an initial steep rise in deoxyhemoglobin/myoglobin, followed by a linear increase thereafter. Whereas seemingly disparate at first glance, we believe these two approaches provide similar information. Indeed, by plotting the mean convective O₂ delivery and diffusive O₂ conductance, we show that the initial increase in mVO₂ during the transition from rest to exercise was achieved by a greater increase in diffusive O₂ conductance versus convective O₂ delivery (10-fold vs. 4-fold increase, respectively), explaining the initial decline in SVO2. In contrast, the increase in mVO₂ from light to heavy exercise was achieved by equal increases (1.8-fold) in convective O₂ delivery and diffusive O₂ conductance, explaining the plateau in SVO2. That DCS-derived BFI and deoxyhemoglobin/myoglobin (surrogate measure of O₂ extraction) share the same general biphasic pattern suggests that both DCS and conventional approaches provide complementary information regarding the determinants of mVO₂.NEW & NOTEWORTHY Near-infrared diffuse correlation spectroscopy (DCS) is an emerging optical imaging technique for quantifying skeletal muscle O₂ delivery and utilization at the microvascular level. Here, we show that DCS provides complementary insight into the determinants of muscle O₂ consumption across a wide range of exercise intensities, further establishing the utility of DCS.

Edward F. Adolph Distinguished Lecture. Contemporary model of muscle microcirculation: gateway to function and dysfunction

This review strikes at the very heart of how the microcirculation functions to facilitate blood-tissue oxygen, substrate, and metabolite fluxes in skeletal muscle. Contemporary evidence, marshalled from animals and humans using the latest techniques, challenges iconic perspectives that have changed little over the past century. Those perspectives include the following: the presence of contractile or collapsible capillaries in muscle, unitary control by precapillary sphincters, capillary recruitment at the onset of contractions, and the notion of capillary-to-mitochondrial diffusion distances as limiting O₂ delivery. Today a wealth of physiological, morphological, and intravital microscopy evidence presents a completely different picture of microcirculatory control. Specifically, capillary red blood cell (RBC) and plasma flux is controlled primarily at the arteriolar level with most capillaries, in healthy muscle, supporting at least some flow at rest. In healthy skeletal muscle, this permits substrate access (whether carried in RBCs or plasma) to a prodigious total capillary surface area. Pathologies such as heart failure or diabetes decrease access to that exchange surface by reducing the proportion of flowing capillaries at rest and during exercise. Capillary morphology and function vary disparately among tissues. The contemporary model of capillary function explains how, following the onset of exercise, muscle O₂ uptake kinetics can be extremely fast in health but slowed in heart failure and diabetes impairing contractile function and exercise tolerance. It is argued that adoption of this model is fundamental for understanding microvascular function and dysfunction and, as such, to the design and evaluation of effective therapeutic strategies to improve exercise tolerance and decrease morbidity and mortality in disease.

Unaltered V̇o2 kinetics despite greater muscle oxygenation during heavy-intensity two-legged knee extension versus cycle exercise in humans

Relative perfusion of active muscles is greater during knee extension ergometry (KE) than cycle ergometry (CE). This provides the opportunity to investigate the effects of increased O₂ delivery (Q̇o₂) on deoxygenation heterogeneity among quadriceps muscles and pulmonary oxygen uptake (V̇o₂) kinetics. Using time-resolved near-infrared spectroscopy, we hypothesized that compared with CE the superficial vastus lateralis (VL), superficial rectus femoris, and deep VL in KE would have 1) a smaller amplitude of the exercise-induced increase in deoxy[Hb + Mb] (related to the balance between V̇o₂ and Q̇o₂); 2) a greater amplitude of total[Hb + Mb] (related to the diffusive O₂ conductance); 3) a greater homogeneity of regional muscle deoxy[Hb + Mb]; and 4) no difference in pulmonary V̇o₂ kinetics. Eight participants performed square-wave KE and CE exercise from 20 W to heavy work rates. Deoxy[Hb + Mb] amplitude was less for all muscle regions in KE (P < 0.05: superficial, KE 17-24 vs. CE 19-40; deep, KE 19 vs. CE 26 μM). Furthermore, the amplitude of total[Hb + Mb] was greater for KE than CE at all muscle sites (P < 0.05: superficial, KE, 7-21 vs. CE, 1-16; deep, KE, 11 vs. CE, -3 μM). Although the amplitude and heterogeneity of deoxy[Hb + Mb] were significantly lower in KE than CE during the first minute of exercise, the pulmonary V̇o₂ kinetics was not different for KE and CE. These data show that the microvascular Q̇o₂ to V̇o₂ ratio, and thus tissue oxygenation, was greater in KE than CE. This suggests that pulmonary and muscle V̇o₂ kinetics in young healthy humans are not limited by Q̇o₂ during heavy-intensity cycling.

Spatial and Temporal Heterogeneities of Capillary Hemodynamics and Its Functional Coupling During Neural Activation

The cerebral vascular system provides a means to meet the constant metabolic needs of neuronal activities in the brain. Within the cerebral capillary bed, the interactions of spatial and temporal hemodynamics play a deterministic role in oxygen diffusion, however, the progression of which remains unclear. Taking the advantages of high-spatiotemporal resolution of optical coherence tomography capillary velocimetry designed with the eigen-decomposition statistical analysis, we investigated intrinsic red blood cell (RBC) velocities and their spatiotemporal adjustment within the capillaries permeating mouse cerebral cortex during electrical stimulation of contralateral hind paw. We found that the mean capillary transit velocity (mCTV) is increased and its temporal fluctuation bandwidth (TFB) is broadened within hind-paw somatosensory cortex. In addition, the degree to which the mCTV is increased negatively correlates with resting state mCTV, and the degree to which the TFB is increased negatively correlates with both the resting state mCTV and the TFB. In order to confirm the changes are due to hemodynamic regulation, we performed angiographic analyses and found that the vessel density remains almost constant, suggesting the observed functional activation does not involve recruitment of reserved capillaries. To further differentiate the contributions of the mCTV and the TFB to the spatiotemporally coupled hemodynamics, changes in the mCTV and TBF of the capillary flow were modeled and investigated through a Monte Carlo simulation. The results suggest that neural activation evokes the spatial transit time homogenization within the capillary bed, which is regulated via both the heterogeneous acceleration of RBC flow and the heterogeneous increase of temporal RBC fluctuation, ensuring sufficient oxygenation during functional hyperemia.

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.

Walking performances and muscle oxygen desaturation are increased after supervised exercise training in Takayasu arteritis: a case report and a review of the literature

Background

Takayasu arteritis (TAK) is a rare chronic inflammatory vasculitis predominantly affecting the aorta and its main branches. Takayasu arteritis has been shown to increase cardiovascular risk. Supervised exercise training (SET) is a well-recognized and effective therapeutic tool improving walking performances in patients with chronic atherosclerotic disease; however, the effects of SET, and the underlying mechanisms, remain poorly documented in TAK patients.

Case summary

We reviewed the literature and investigated the effects of a 12-week SET programme on walking performances, physical function, and calf muscle oxygen saturation (StO₂; assessed by near-infrared spectroscopy) during exercise in a 28-year-old man with TAK and symptoms of arterial lower limb claudication. The literature review evidences only two recent publications suggesting that exercise training is effective and well-tolerated in patients with arteritis. The treadmill pain-free (+22%) and maximal (+273%) walking distance, 6-min walking distance (+66%), and physical function of lower extremities (+20%) following SET were significantly improved in our patient. Moreover, we observed a greater muscle oxygen desaturation (increased oxygen extraction) during exercise.

Discussion

Following SET, the increased oxygen extraction may be related to improved microvascular milieu leading to a better match between muscle oxygen supply and demand during exercise. These new results may contribute to mechanistic insights in peripheral adaptations following exercise training in TAK patients and may help to explain, at least partly, the increased walking performances. Although more studies are needed to better explore the impact of exercise training, these results suggest that exercise should be recommended in TAK patients.

Leg blood flow and skeletal muscle microvascular perfusion responses to submaximal exercise in peripheral arterial disease

Peripheral arterial disease (PAD) is characterized by stenosis and occlusion of the lower limb arteries. Although leg blood flow is limited in PAD, it remains unclear whether skeletal muscle microvascular perfusion is affected. We compared whole leg blood flow and calf muscle microvascular perfusion after cuff occlusion and submaximal leg exercise between patients with PAD ( n = 12, 69 ± 9 yr) and healthy age-matched control participants ( n = 12, 68 ± 7 yr). Microvascular blood flow (microvascular volume × flow velocity) of the medial gastrocnemius muscle was measured before and immediately after the following: 1) 5 min of thigh-cuff occlusion, and 2) a 5-min bout of intermittent isometric plantar-flexion exercise (400 N) using real-time contrast-enhanced ultrasound. Whole leg blood flow was measured after thigh-cuff occlusion and during submaximal plantar-flexion exercise using strain-gauge plethysmography. Postocclusion whole leg blood flow and calf muscle microvascular perfusion were lower in patients with PAD than control participants, and these parameters were strongly correlated ( r = 0.84, P < 0.01). During submaximal exercise, total whole leg blood flow and vascular conductance were not different between groups. There were also no group differences in postexercise calf muscle microvascular perfusion, although microvascular blood volume was higher in patients with PAD than control participants (12.41 ± 6.98 vs. 6.34 ± 4.98 arbitrary units, P = 0.03). This study demonstrates that the impaired muscle perfusion of patients with PAD during postocclusion hyperemia is strongly correlated with disease severity and is likely mainly determined by the limited conduit artery flow. In response to submaximal leg exercise, microvascular flow volume was elevated in patients with PAD, which may reflect a compensatory mechanism to maintain muscle perfusion and oxygen delivery during recovery from exercise.

NEW & NOTEWORTHY This study suggests that peripheral arterial disease (PAD) has different effects on the microvascular perfusion responses to cuff occlusion and submaximal leg exercise. Patients with PAD have impaired microvascular perfusion after cuff occlusion, similar to that previously reported after maximal exercise. In response to submaximal exercise, however, the microvascular flow volume response was elevated in patients with PAD compared with control. This finding may reflect a compensatory mechanism to maintain perfusion and oxygen delivery during recovery from exercise.

On the Physics Underlying Longitudinal Capillary Recruitment

Numerous researchers have found that capillary vessel haematocrit depends on the vasodilatory state of the arterioles. At rest, vessel haematocrit is down to 15 %, suggesting a red blood cell velocity three times higher than the plasma velocity. This finding is analysed in the context of present understanding of propulsion of red blood cells (RBCs) and plasma by means of the arteriovenous pressure gradient. Interfacial forces between the red blood cells and the plasma are proposed as a rational explanation of the observed red blood cell velocities. While the arteriovenous pressure gradient across the capillaries propels the red blood cell and the plasma jointly, interfacial forces along the red blood cell membrane can propel RBCs at the cost of the plasma. Different options are explored for the physical origin of these interfacial forces and oxygen gradients are found to be the most probable source.