Capillary oxygen regulates demand-supply coupling by triggering connexin40-mediated conduction: Rethinking the metabolic hypothesis.

Coupling red blood cell (RBC) supply to O2 demand is an intricate process requiring O2 sensing, generation of a stimulus, and signal transduction that alters upstream arteriolar tone. Although actively debated, this process has been theorized to be induced by hypoxia and to involve activation of endothelial inwardly rectifying K+ channels (KIR) 2.1 by elevated extracellular K+ to trigger conducted hyperpolarization via connexin40 (Cx40) gap junctions to upstream resistors. This concept was tested in resting healthy skeletal muscle of Cx40-/- and endothelial KIR2.1-/- mice using state-of-the-art live animal imaging where the local tissue O2 environment was manipulated using a custom gas chamber. Second-by-second capillary RBC flow responses were recorded as O2 was altered. A stepwise drop in PO2 at the muscle surface increased RBC supply in capillaries of control animals while elevated O2 elicited the opposite response; capillaries were confirmed to express Cx40. The RBC flow responses were rapid and tightly coupled to O2; computer simulations did not support hypoxia as a driving factor. In contrast, RBC flow responses were significantly diminished in Cx40-/- mice. Endothelial KIR2.1-/- mice, on the other hand, reacted normally to O2 changes, even when the O2 challenge was targeted to a smaller area of tissue with fewer capillaries. Conclusively, microvascular O2 responses depend on coordinated electrical signaling via Cx40 gap junctions, and endothelial KIR2.1 channels do not initiate the event. These findings reconceptualize the paradigm of blood flow regulation in skeletal muscle and how O2 triggers this process in capillaries independent of extracellular K+.

Intriguing Roles for Endothelial ADAM10/Notch Signaling in the Development of Organ-Specific Vascular Beds.

The vasculature is a remarkably interesting, complex, and interconnected organ. It provides a conduit for oxygen and nutrients, filtration of waste products, and rapid communication between organs. Much remains to be learned about the specialized vascular beds that fulfill these diverse, yet vital functions. This review was prompted by the discovery that Notch signaling in mouse endothelial cells is crucial for the development of specialized vascular beds found in the heart, kidneys, liver, intestines, and bone. We will address the intriguing questions raised by the role of Notch signaling and that of its regulator, the metalloprotease ADAM10, in the development of specialized vascular beds. We will cover fundamentals of ADAM10/Notch signaling, the concept of Notch-dependent cell fate decisions, and how these might govern the development of organ-specific vascular beds through angiogenesis or vasculogenesis. We will also consider common features of the affected vessels, including the presence of fenestra or sinusoids and their occurrence in portal systems with two consecutive capillary beds. We hope to stimulate further discussion and study of the role of ADAM10/Notch signaling in the development of specialized vascular structures, which might help uncover new targets for the repair of vascular beds damaged in conditions like coronary artery disease and glomerulonephritis.

Clonal Proliferation and Stochastic Pruning Orchestrate Lymph Node Vasculature Remodeling.

Lymph node (LN) expansion during an immune response relies on the transient remodeling of its vasculature. Although the mechanisms driving LN endothelial cell division are beginning to be understood, a comprehensive view of LN endothelial cell dynamics at the single-cell level is lacking. Here, we used multicolored fluorescent fate-mapping models to track the behavior of blood endothelial cells during LN expansion upon inflammation and subsequent return to homeostasis. We found that expansion of the LN vasculature relied on the sequential assembly of endothelial cell proliferative units. This segmented growth was sustained by the clonal proliferation of high endothelial venule (HEV) cells, which act as local progenitors to create capillaries and HEV neo-vessels at the periphery of the LN. Return to homeostasis was accompanied by the stochastic death of pre-existing and neo-synthesized LN endothelial cells. Thus, our fate-mapping studies unravel-at a single-cell level-the complex dynamics of vascular-tree remodeling during LN expansion and contraction.

Impact of capillary and sarcolemmal proximity on mitochondrial structure and energetic function in skeletal muscle.

Mitochondria within skeletal muscle cells are located either between the muscle contractile apparatus (interfibrillar mitochondria, IFM) or beneath the cell membrane (subsarcolemmal mitochondria, SSM), with several structural and functional differences reported between IFM and SSM. However, recent 3D imaging studies demonstrate that mitochondria are particularly concentrated in the proximity of capillaries embedded in sarcolemmal grooves rather than in proximity to the sarcolemma itself (paravascular mitochondria, PVM). To evaluate the impact of capillary vs. sarcolemmal proximity, we compared the structure and function of skeletal muscle mitochondria located either lateral to embedded capillaries (PVM), adjacent to the sarcolemma but not in PVM pools (SSM) or interspersed between sarcomeres (IFM). Mitochondrial morphology and interactions were assessed by 3D electron microscopy coupled with machine learning segmentation, whereas mitochondrial energy conversion was assessed by two-photon microscopy of mitochondrial membrane potential, content, calcium, NADH redox and flux in live, intact cells. Structurally, although PVM and SSM were similarly larger than IFM, PVM were larger, rounder and had more physical connections to neighbouring mitochondria compared to both IFM and SSM. Functionally, PVM had similar or greater basal NADH flux compared to SSM and IFM, respectively, despite a more oxidized NADH pool and a greater membrane potential, signifying a greater activation of the electron transport chain in PVM. Together, these data indicate that proximity to capillaries has a greater impact on resting mitochondrial energy conversion and distribution in skeletal muscle than the sarcolemma alone. KEY POINTS: Capillaries have a greater impact on mitochondrial energy conversion in skeletal muscle than the sarcolemma. Paravascular mitochondria are larger, and the outer mitochondrial membrane is more connected with neighbouring mitochondria. Interfibrillar mitochondria are longer and have greater contact sites with other organelles (i.e. sarcoplasmic reticulum and lipid droplets). Paravascular mitochondria have greater activation of oxidative phosphorylation than interfibrillar mitochondria at rest, although this is not regulated by calcium.

Precapillary sphincters and pericytes at first-order capillaries as key regulators for brain capillary perfusion.

Rises in local neural activity trigger local increases of cerebral blood flow, which is essential to match local energy demands. However, the specific location of microvascular flow control is incompletely understood. Here, we used two-photon microscopy to observe brain microvasculature in vivo. Small spatial movement of a three-dimensional (3D) vasculature makes it challenging to precisely measure vessel diameter at a single x-y plane. To overcome this problem, we carried out four-dimensional (x-y-z-t) imaging of brain microvessels during exposure to vasoactive molecules in order to constrain the impact of brain movements on the recordings. We demonstrate that rises in synaptic activity, acetylcholine, nitric oxide, cyclic guanosine monophosphate, ATP-sensitive potassium channels, and endothelin-1 exert far greater effects on brain precapillary sphincters and first-order capillaries than on penetrating arterioles or downstream capillaries, but with similar kinetics. The high level of responsiveness at precapillary sphincters and first-order capillaries was matched by a higher level of α-smooth muscle actin in pericytes as compared to penetrating arterioles and downstream capillaries. Mathematical modeling based on 3D vasculature reconstruction showed that precapillary sphincters predominantly regulate capillary blood flow and pressure as compared to penetrating arterioles and downstream capillaries. Our results confirm a key role for precapillary sphincters and pericytes on first-order capillaries as sensors and effectors of endothelium- or brain-derived vascular signals.

The post-arteriole transitional zone: a specialized capillary region that regulates blood flow within the CNS microvasculature.

The brain is an energy hog, consuming available energy supplies at a rate out of all proportion to its relatively small size. This outsized demand, largely reflecting the unique computational activity of the brain, is met by an ensemble of neurovascular coupling mechanisms that link neuronal activity with local increases in blood delivery. This just-in-time replenishment strategy, made necessary by the limited energy-storage capacity of neurons, complicates the nutrient-delivery task of the cerebral vasculature, layering on a temporo-spatial requirement that invites - and challenges - mechanistic interpretation. The centre of gravity of research efforts to disentangle these mechanisms has shifted from an initial emphasis on astrocyte-arteriole-level processes to mechanisms that operate on the capillary level, a shift that has brought into sharp focus questions regarding the fine control of blood distribution to active neurons. As these investigations have drilled down into finer reaches of the microvasculature, they have revealed an arteriole-proximate subregion of CNS capillary networks that serves a regulatory function in directing blood flow into and within downstream capillaries. They have also illuminated differences in researchers' perspectives on the vascular structures and identity of mural cells in this region that impart the vasomodulatory effects that control blood distribution. In this review, we highlight the regulatory role of a variably named region of the microvasculature, referred to here as the post-arteriole transition zone, in channeling blood flow within CNS capillary networks, and underscore the contribution of dynamically contractile perivascular mural cell - generally, but not universally, recognized as pericytes - to this function.

A tight squeeze: how do we make sense of small changes in microvascular diameter?

The brain is an energetically demanding tissue which, to function adequately, requires constant fine tuning of its supporting blood flow, and hence energy supply. Whilst blood flow was traditionally believed to be regulated only by vascular smooth muscle cells on arteries and arterioles supplying the brain, recent work has suggested a critical role for capillary pericytes, which are also contractile. This concept has evoked some controversy, especially over the relative contributions of arterioles and capillaries to the control of cerebral blood flow. Here we outline why pericytes are in a privileged position to control cerebral blood flow. First we discuss the evidence, and fundamental equations, which describe how the small starting diameter of capillaries, compared to upstream arterioles, confers a potentially greater control by capillary pericytes than by arterioles over total cerebral vascular resistance. Then we suggest that the faster time frame over which low branch order capillary pericytes dilate in response to local energy demands provides a niche role for pericytes to regulate blood flow compared to slower responding arterioles. Finally, we discuss the role of pericytes in capillary stalling, whereby pericyte contraction appears to facilitate a transient stall of circulating blood cells, exacerbating the effect of pericytes upon cerebral blood flow.

Estimation of shear stress heterogeneity along capillary segments in angiogenic rat mesenteric microvascular networks.

OBJECTIVE: Fluid shear stress is thought to be a regulator of endothelial cell behavior during angiogenesis. The link, however, requires an understanding of stress values at the capillary level in angiogenic microvascular networks. Critical questions remain. What are the stresses? Do capillaries experience similar stress magnitudes? Can variations explain vessel-specific behavior? The objective of this study was to estimate segment-specific shear stresses in angiogenic networks. METHODS: Images of angiogenic networks characterized by increased vascular density were obtained from rat mesenteric tissues stimulated by compound 48/80-induced mast cell degranulation. Vessels were identified by perfusion of a 40 kDa fixable dextran prior to harvesting and immunolabeling for PECAM. Using a network flow-based segment model with physiologically relevant parameters, stresses were computed per vessel for regions across multiple networks. RESULTS: Stresses ranged from 0.003 to 2328.1 dyne/cm2 and varied dramatically at the capillary level. For all regions, the maximum segmental shear stresses were for capillary segments. Stresses along proximal capillaries branching from arteriole inlets were increased compared to stresses along capillaries in more distal regions. CONCLUSIONS: The results highlight the variability of shear stresses along angiogenic capillaries and motivate new discussions on how endothelial cells may respond in vivo to segment-specific microenvironment during angiogenesis.

Hematoloechus sp. attachment shifts endothelium in vivo from pro- to anti-inflammatory profile in Rana pipiens: evidence from systemic and capillary physiology.

This prospective, descriptive study focused on lung flukes (Hematoloechus sp., H) and their impact on systemic and individual capillary variables measured in pithed Rana pipiens, a long-standing model for studies of capillary physiology. Three groups were identified based on Hematoloechus attachment: no Hematoloechus (No H), Hematoloechus not attached (H Not Att), and Hematoloechus attached (H Att). Among 38 descriptive, cardiovascular, and immunological variables, 18 changed significantly with H. Symptoms of H included weight loss, elevated immune cells, heart rate variability, faster coagulation, lower hematocrit, and fluid accumulation. Important capillary function discoveries included median baselines for hydraulic conductivity (Lp) of 7.0 (No H), 12.4 (H Not Att), and 4.2 (H Att) × 10-7 cm·s-1·cmH2O-1 (P < 0.0001) plus seasonal adaptation of sigma delta pi [σ(πc-πi), P = 0.03]. Pro- and anti-inflammatory phases were revealed for Lp and plasma nitrite/nitrate concentration ([NOx]) in both H Not Att and H Att, whereas capillary wall tensile strength increased in the H Att. H attachment was advantageous for the host due to lower edema and for the parasite via a sustained food source illustrating an excellent example of natural symbiosis. However, H attachment also resulted in host weight loss: in time, a conundrum for the highly dependent parasite. The study increases overall knowledge of Rana pipiens by revealing intriguing effects of H and previously unknown, naturally occurring seasonal changes in many variables. The data improve Rana pipiens as a general scientific and capillary physiology model. Diseases of inflammation and stroke are among the clinical applications.

Skeletal muscle adaptation to indirect electrical stimulation: divergence between microvascular and metabolic adaptations.

NEW FINDINGS: What is the central question of this study? Can we manipulate muscle recruitment to differentially enhance skeletal muscle fatigue resistance? What is the main finding and its importance? Through manipulation of muscle activation patterns, it is possible to promote distinct microvascular growth. Enhancement of fatigue resistance is closely associated with the distribution of the capillaries within the muscle, not necessarily with quantity. Additionally, at the acute stages of remodelling in response to indirect electrical stimulation, the improvement in fatigue resistance appears to be primarily driven by vascular remodelling, with metabolic adaptation of secondary importance. ABSTRACT: Exercise involves a complex interaction of factors influencing muscle performance, where variations in recruitment pattern (e.g., endurance vs. resistance training) may differentially modulate the local tissue environment (i.e., oxygenation, blood flow, fuel utilization). These exercise stimuli are potent drivers of vascular and metabolic change. However, their relative contribution to adaptive remodelling of skeletal muscle and subsequent performance is unclear. Using implantable devices, indirect electrical stimulation (ES) of locomotor muscles of rat at different pacing frequencies (4, 10 and 40 Hz) was used to differentially recruit hindlimb blood flow and modulate fuel utilization. After 7 days, ES promoted significant remodelling of microvascular composition, increasing capillary density in the cortex of the tibialis anterior by 73%, 110% and 55% for the 4 Hz, 10 and 40 Hz groups, respectively. Additionally, there was remodelling of the whole muscle metabolome, including significantly elevated amino acid turnover, with muscle kynurenic acid levels doubled by pacing at 10 Hz (P < 0.05). Interestingly, the fatigue index of skeletal muscle was only significantly elevated in 10 Hz (58% increase) and 40 Hz (73% increase) ES groups, apparently linked to improved capillary distribution. These data demonstrate that manipulation of muscle recruitment pattern may be used to differentially expand the capillary network prior to altering the metabolome, emphasising the importance of local capillary supply in promoting exercise tolerance.

Endothelial k-RasV12 Expression Induces Capillary Deficiency Attributable to Marked Tube Network Expansion Coupled to Reduced Pericytes and Basement Membranes.

OBJECTIVE: We sought to determine how endothelial cell (EC) expression of the activating k-Ras (kirsten rat sarcoma 2 viral oncogene homolog) mutation, k-RasV12, affects their ability to form lumens and tubes and interact with pericytes during capillary assembly Approach and Results: Using defined bioassays where human ECs undergo observable tubulogenesis, sprouting behavior, pericyte recruitment to EC-lined tubes, and pericyte-induced EC basement membrane deposition, we assessed the impact of EC k-RasV12 expression on these critical processes that are necessary for proper capillary network formation. This mutation, which is frequently seen in human ECs within brain arteriovenous malformations, was found to markedly accentuate EC lumen formation mechanisms, with strongly accelerated intracellular vacuole formation, vacuole fusion, and lumen expansion and with reduced sprouting behavior, leading to excessively widened tube networks compared with control ECs. These abnormal tubes demonstrate strong reductions in pericyte recruitment and pericyte-induced EC basement membranes compared with controls, with deficiencies in fibronectin, collagen type IV, and perlecan deposition. Analyses of signaling during tube formation from these k-RasV12 ECs reveals strong enhancement of Src (Src proto-oncogene, non-receptor tyrosine kinase), Pak2 (P21 [RAC1 (Rac family small GTPase 1)] activated kinase 2), b-Raf (v-raf murine sarcoma viral oncogene homolog B1), Erk (extracellular signal-related kinase), and Akt (AK strain transforming) activation and increased expression of PKCε (protein kinase C epsilon), MT1-MMP (membrane-type 1 matrix metalloproteinase), acetylated tubulin and CDCP1 (CUB domain-containing protein 1; most are known EC lumen regulators). Pharmacological blockade of MT1-MMP, Src, Pak, Raf, Mek (mitogen-activated protein kinase) kinases, Cdc42 (cell division cycle 42)/Rac1, and Notch markedly interferes with lumen and tube formation from these ECs. CONCLUSIONS: Overall, this novel work demonstrates that EC expression of k-RasV12 disrupts capillary assembly due to markedly excessive lumen formation coupled with strongly reduced pericyte recruitment and basement membrane deposition, which are critical pathogenic features predisposing the vasculature to develop arteriovenous malformations.

Tissue Oxygenation Around Capillaries: Effects of Hematocrit and Arteriole Oxygen Condition.

Oxygen transfer in the microvasculature is a complex phenomenon that involves multiple physical and chemical processes and multiple media. Hematocrit, the volume fraction of red blood cells (RBCs) in blood, has direct influences on the blood flow as well as the oxygen supply in the microcirculation. On the one hand, a higher hematocrit means that more RBCs present in capillaries, and thus, more oxygen is available at the source end. On the other hand, the flow resistance increases with hematocrit, and therefore, the RBC motion becomes slower, which in turn reduces the influx of oxygen-rich RBCs entering capillaries. Such double roles of hematocrit have not been investigated adequately. Moreover, the oxygen-hemoglobin dissociation rate depends on the oxygen tension and hemoglobin saturation of the cytoplasm inside RBCs, and the dissociation kinetics exhibits a nonlinear fashion at different oxygen tensions. To understand how these factors and mechanisms interplay in the oxygen transport process, computational modeling and simulations are favorite since we have a good control of the system parameters and also we can access to the detailed information during the transport process. In this study, we conduct numerical simulations for the blood flow and RBC deformation along a capillary and the oxygen transfer from RBCs to the surrounding tissue. Different values for the hematocrit, arteriole oxygen tension, tissue metabolism rate and hemoglobin concentration and affinity are considered, and the simulated spatial and temporal variations of oxygen concentration are analyzed in conjunction with the nonlinear oxygen-hemoglobin reaction kinetics. Our results show that there are two competing mechanisms for the tissue oxygenation response to a hematocrit increases: the favorite effect of the higher RBC density and the negative effect of the slower RBC motion. Moreover, in the low oxygen situations with RBC oxygen tension less than 50 mmHg at capillary inlet, the reduced RBC velocity effect dominates, resulting in a decrease in tissue oxygenation at higher hematocrit. On the opposite, for RBC oxygen tension higher than 50 mmHg when entering the capillary, a higher hematocrit is beneficial to the tissue oxygenation. More interestingly, the pivoting arteriole oxygen tension at which the two competing mechanisms switch dominance on tissue oxygenation becomes lower for higher oxygen-hemoglobin affinity and lower hemoglobin concentration. This observation has also been analyzed based on the oxygen supply from RBCs and the oxygen-hemoglobin reaction kinetics. The results and discussions presented in this article could be helpful for a better understanding of oxygen transport in microcirculation.

Contractile pericytes determine the direction of blood flow at capillary junctions.

The essential function of the circulatory system is to continuously and efficiently supply the O2 and nutrients necessary to meet the metabolic demands of every cell in the body, a function in which vast capillary networks play a key role. Capillary networks serve an additional important function in the central nervous system: acting as a sensory network, they detect neuronal activity in the form of elevated extracellular K+ and initiate a retrograde, propagating, hyperpolarizing signal that dilates upstream arterioles to rapidly increase local blood flow. Yet, little is known about how blood entering this network is distributed on a branch-to-branch basis to reach specific neurons in need. Here, we demonstrate that capillary-enwrapping projections of junctional, contractile pericytes within a postarteriole transitional region differentially constrict to structurally and dynamically determine the morphology of capillary junctions and thereby regulate branch-specific blood flow. We further found that these contractile pericytes are capable of receiving propagating K+-induced hyperpolarizing signals propagating through the capillary network and dynamically channeling red blood cells toward the initiating signal. By controlling blood flow at junctions, contractile pericytes within a functionally distinct postarteriole transitional region maintain the efficiency and effectiveness of the capillary network, enabling optimal perfusion of the brain.

Mechanical adaptation of monocytes in model lung capillary networks.

Proper circulation of white blood cells (WBCs) in the pulmonary vascular bed is crucial for an effective immune response. In this branched vascular network, WBCs have to strongly deform to pass through the narrowest capillaries and bifurcations. Although it is known that this process depends on the cell mechanical properties, it is still poorly understood due to the lack of a comprehensive model of cell mechanics and of physiologically relevant experiments. Here, using an in-house microfluidic device mimicking the pulmonary capillary bed, we show that the dynamics of THP1 monocytes evolves along successive capillary-like channels, from a nonstationary slow motion with hops to a fast and smooth efficient one. We used actin cytoskeleton drugs to modify the traffic dynamics. This led us to propose a simple mechanical model that shows that a very finely tuned cortical tension combined with a high cell viscosity governs the fast transit through the network while preserving cell integrity. We finally highlight that the cortical tension controls the steady-state cell velocity via the viscous friction between the cell and the channel walls.

[Capillary blood flow morphofunctional readjustments in northerners of different ages.]

Morphofunctional characteristics of capillary blood flow can be considered one of the most sensitive markers of ontogenetic desadaptive changes both within a local area of the capillary network and in the entire cardiovascular system. The study involved two hundred seventy-seven male northerners aged 18-66 to identify the age dynamics of the capillary bed morphofunctional characteristics. Subjects exhibited a reduction in diameters of arterial and transitional sections of the microcapillary bed testifying to a significant age dynamics from young men to the elderly. The remodeling coefficient increasingly came up in workable and elderly men, which indicated the risk of reducing in effectiveness of substances diffusion. We could see the average capillary length increase in this age vector and considered that as a compensatory growth of the contact surface of the vessel aimed at maintaining the necessary level of transcapillary exchange. We clearly observed microcirculatory changes with subjective age and considered them to be adaptive changes of microangioarchitectonics which may indicate the formation of a pattern characteristic of northerners' ontogenetic readjustments.

[Vascular biology and hemophilia].

By carrying a systemic circulation, hematopoietic and vascular systems coordinately govern the functional organ connections in the body. Blood vessels play an important role in the development, regeneration, and maintenance of organs by acting as conduits for environmental factors in the blood to tissues and secreting organ-specific cytokines as angiocrine signals. Recently, it has become clear that vascular endothelial cells, which are the main constituent cells of the blood vessels and play a role in homeostasis, are diverse. It has also been established that the cells of stem cell fraction exist in endothelial cells. The vascular endothelial cells in various organs are functionally different. For example, it has been discovered that sinusoidal blood vessels in the liver produce coagulation factor VIII as an organ-specific vascular function. Determining how such tissue-/organ-specific function of the endothelial cells is induced is a topic of interest in the vascular field of study.

Capillary module haemodynamics and mechanisms of blood flow regulation in skeletal muscle capillary networks: Experimental and computational analysis.

The capillary module (CM), consisting of parallel capillaries from terminal arteriole to post-capillary venule, is classically considered to be the building block of complex capillary networks. In skeletal muscle, CMs form interconnected columns spanning thousands of microns, which we recently described as the capillary fascicle. However, detailed evaluation of CM haemodynamics has not been described, and may provide insight into mechanisms of blood flow regulation in the microcirculation. We used intravital videomicroscopy from resting extensor digitorum longus muscle in rats (n = 9 networks, 112 capillary modules), as well as dual-phase computational modelling of blood flow in simulated CM geometries. We found that the mean driving pressure across CMs was 3.236 ± 1.833 mmHg. Red blood cell (RBC) flow was independent of CM resistance, and the ratio of blood flow in adjacent modules was not correlated with their ratio of resistances. In simulated CM geometries, increases to driving pressure produced a direct linear increase to RBC and plasma flow, with no changes to RBC distribution; increases to arteriolar inflow haematocrit resulted in increased RBC flow, but with viscosity-dependent increases to CM resistance. CM RBC flow heterogeneity was higher than plasma flow heterogeneity in experimental data, in contrast to simulated geometries, suggesting that time-dependent flow variability may have important consequences for RBC distribution. In summary, these findings suggest that CMs are active participants in microvascular flow regulation, likely achieved through adjustments to CM driving pressure using pre- and post-capillary loci of flow control. Increases to CM viscosity may be important during the regulation of functional hyperaemia. KEY POINTS: The capillary module (CM), consisting of parallel capillaries from the arteriole to venule, is classically considered to be the building block of capillary networks in skeletal muscle. A detailed evaluation of module haemodynamics may provide insight into mechanisms of blood flow regulation in the microcirculation. Using experimental data from resting skeletal muscle in rats, as well as dual-phase computational models of blood flow, we analysed haemodynamic relationships and the impact of variations to boundary conditions on red blood cell and plasma distribution. We showed that driving pressure across CMs is low, and that simulated increases to inflow haematocrit have important viscosity-dependent effects on module resistance. We found that red blood cell flow was independent from module resistance, which strongly suggests the regulation of driving pressure at the level of the capillary module using pre- and post-capillary loci of flow control. These findings place CMs as central participants in microvascular flow regulation, with important consequences for disease and functional hyperaemia.

Capillary communication: the role of capillaries in sensing the tissue environment, coordinating the microvascular, and controlling blood flow.

Historically, capillaries have been viewed as the microvascular site for flux of nutrients to cells and removal of waste products. Capillaries are the most numerous blood vessel segment within the tissue, whose vascular wall consists of only a single layer of endothelial cells and are situated within microns of each cell of the tissue, all of which optimizes capillaries for the exchange of nutrients between the blood compartment and the interstitial space of tissues. There is, however, a growing body of evidence to support that capillaries play an important role in sensing the tissue environment, coordinating microvascular network responses, and controlling blood flow. Much of our growing understanding of capillaries stems from work in skeletal muscle and more recent work in the brain, where capillaries can be stimulated by products released from cells of the tissue during increased activity and are able to communicate with upstream and downstream vascular segments, enabling capillaries to sense the activity levels of the tissue and send signals to the microvascular network to coordinate the blood flow response. This review will focus on the emerging role that capillaries play in communication between cells of the tissue and the vascular network required to direct blood flow to active cells in skeletal muscle and the brain. We will also highlight the emerging central role that disruptions in capillary communication may play in blood flow dysregulation, pathophysiology, and disease.

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.

Estimation of shear stress values along endothelial tip cells past the lumen of capillary sprouts.

Shear stress is recognized as a regulator of angiogenesis. However, the shear stress experienced by the endothelial cells of capillary sprouts remains unknown. The objective of this study was to estimate shear stress due to local interstitial flow along endothelial tip cells at the end of the capillary sprout lumen. Computational fluid dynamics were used to model flow within a blind-ended vessel, transendothelial flow across the vessel wall, and flow within the surrounding perivascular/interstitial space. Shear stress along the wall of the tip cells was calculated while varying sprout length, perivascular space channel width, and vessel wall hydraulic conductivity. Increasing sprout length, increasing wall hydraulic conductivity, and decreasing perivascular space width increased shear stress magnitude. Wall shear stress magnitude within the lumen ranged from 0.015 to 0.55 dyne/cm2 at the sprout entrance and linearly decreased to near zero at the base of the tip cells. Tip cell wall shear stress magnitude due to interstitial flow ranged from 0.009 to 4.65 dyne/cm2. In 3 out of 8 cases, shear stress magnitude was above 1 dyne/cm2 and considered physiologically relevant. The results provide a framework for discussing the role of local mechanical cues in regulating endothelial cell dynamics involved in angiogenesis. Mainly, interstitial flows may generate physiologically relevant shear stresses on tip cells in certain scenarios. This source of tip cell shear stress has not been previously considered or modeled.