Role of erythrocyte-released ATP in the regulation of microvascular oxygen supply in skeletal muscle

The role of pannexin/purinergic signaling in intervascular communication from capillaries during skeletal muscle contraction in male Golden hamsters

We sought to determine the physiological relevance of pannexin/purinergic-dependent signaling in mediating conducted vasodilation elicited by capillary stimulation through skeletal muscle contraction. Using hamster cremaster muscle and intravital microscopy we stimulated capillaries through local muscle contraction while observing the associated upstream arteriole. Capillaries were stimulated with muscle contraction at low and high contraction (6 and 60CPM) and stimulus frequencies (4 and 40 Hz) in the absence and presence of pannexin blocker mefloquine (MEF; 10-5 M), purinergic receptor antagonist suramin (SUR 10-5 M) and gap-junction uncoupler halothane (HALO, 0.07%) applied between the capillary stimulation site and the upstream arteriolar observation site. Conducted vasodilations elicited at 6CPM were inhibited by HALO while vasodilations at 60CPM were inhibited by MEF and SUR. The conducted response elicited at 4 Hz was inhibited by MEF while the vasodilation at 40 Hz was unaffected by any blocker. Therefore, upstream vasodilations resulting from capillary stimulation via muscle contraction are dependent upon a pannexin/purinergic-dependent pathway that appears to be stimulation parameter-dependent. Our data highlight a physiological importance of the pannexin/purinergic pathway in facilitating communication between capillaries and upstream arteriolar microvasculature and, consequently, indicating that this pathway may play a crucial role in regulating blood flow in response to skeletal muscle contraction.

Sphingosine-1-phosphate Decreases Erythrocyte Dysfunction Induced by β-Amyloid

Amyloid beta peptides (Aβ) have been identified as the main pathogenic agents in Alzheimer's disease (AD). Soluble Aβ oligomers, rather than monomer or insoluble amyloid fibrils, show red blood cell (RBC) membrane-binding capacity and trigger several morphological and functional alterations in RBCs that can result in impaired oxygen transport and delivery. Since bioactive lipids have been recently proposed as potent protective agents against Aβ toxicity, we investigated the role of sphingosine-1-phosphate (S1P) in signaling pathways involved in the mechanism underlying ATP release in Ab-treated RBCs. In RBCs following different treatments, the ATP, 2,3 DPG and cAMP levels and caspase 3 activity were determined by spectrophotometric and immunoassay. S1P rescued the inhibition of ATP release from RBCs triggered by Ab, through a mechanism involving caspase-3 and restoring 2,3 DPG and cAMP levels within the cell. These findings reveal the molecular basis of S1P protection against Aβ in RBCs and suggest new therapeutic avenues in AD.

The relationship between mean corpuscular hemoglobin concentration and mortality in hypertensive individuals: A population-based cohort study

INTRODUCTION

Hematology is an essential field for investigating the prognostic outcomes of cardiovascular diseases (CVDs). Recent research has suggested that mean corpuscular hemoglobin concentration (MCHC) is associated with a poor prognosis in several CVDs. There is no evidence of a correlation between MCHC and hypertension. Therefore, our study aimed to analyze the association of MCHC with all-cause and cardiovascular mortality in hypertensive patients.

METHODS

We used cohort data from U.S. adults who participated in the National Health and Nutrition Examination Survey from 1999-2014. COX regression was applied to analyze the relationship between MCHC and all-cause and cardiovascular mortality. In addition, three models were adjusted to reduce confounding factors. We reanalyzed the data after propensity score matching (PSM) to inspect the stability of the results. Stratified analysis was additionally adopted to investigate the results of each subgroup.

RESULTS

Our research included 15,154 individuals. During a mean follow-up period of 129 months, 30.6% of the hypertensive population succumbed to mortality. Based on previous studies, we categorized patients with MCHC ≤33mg/dl as the hypochromia group and those with >33mg/dl as the non-hypochromia group. After PSM, the hypochromia group had higher all-cause mortality (adjusted hazard ratio [HR]:1.26, 95% confidence interval [95%CI]:1.11-1.43) and cardiovascular mortality (adjusted HR:1.42, 95%CI:1.12-1.80) than the non-hypochromia group. The results of the COX regression remain stable after matching. Stratified analyses before PSM revealed an interaction of anemia in the relationship between MCHC and mortality, whereas there was no significant interaction after matching.

CONCLUSION

In hypertensive individuals, low MCHC was correlated with a poor prognosis. Further studies on MCHC are necessary to analyze the potential mechanisms of its poor prognosis in hypertensive populations.

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+.

Toxic copper level increases erythrocyte glycolytic rate, glutathione production and alters electrolyte balance in male Wistar rats

BACKGROUND

Copper is a micronutrient vital to several cellular energy metabolic processes and drives erythropoiesis. However, it disrupts cellular biological activities and causes oxidative damage when in excess of cellular needs. This study investigated the effects of copper toxicity on erythrocyte energy metabolism in male Wistar rats.

METHODS

Ten Wistar rats (150-170 g) were randomly divided into 2 groups: control (given 0.1 ml distilled water) and copper toxic (given 100 mg/kg copper sulphate). Rats were orally treated for 30 days. Blood, collected retro-orbitally after sodium thiopentone anaesthesia (50 mg/kg i.p.) into fluoride oxalate and EDTA bottles, was subjected to blood lactate assay and extraction of red blood cell respectively. Red blood cell nitric oxide (RBC NO), glutathione (RBC GSH), adenosine triphosphate (RBC ATP) levels, RBC hexokinase, glucose-6-phosphate (RBC G6P), glucose-6-phosphate dehydrogenase (RBC G6PDH), and lactate dehydrogenase (RBC LDH) activity was estimated spectrophotometrically. Values (Mean±SEM, n = 5) were compared by Student's unpaired T-test at p < 0.05.

RESULTS AND CONCLUSION

Copper toxicity significantly increased RBC hexokinase (23.41 ± 2.80 µM), G6P (0.48 ± 0.03 µM), G6PDH (71.03 ± 4.76nmol/min/ml) activities, ATP (624.70 ± 57.36 µmol/gHb) and GSH (3.08 ± 0.37 µM) level compared to control (15.28 ± 1.37 µM, 0.35 ± 0.02 µM, 330.30 ± 49.58 µmol/gHb, 54.41 ± 3.01nmol/min/ml and 2.05 ± 0.14 µM respectively, p < 0.05). Also, RBC LDH activity (145.00 ± 19.88mU/ml), NO (3.45 ± 0.25 µM) and blood lactate (31.64 ± 0.91 mg/dl) level were lowered significantly compared to control (467.90 ± 94.23mU/ml, 4.48 ± 0.18 µM and 36.12 ± 1.06 mg/dl respectively). This study shows that copper toxicity increases erythrocyte glycolytic rate and glutathione production. This increase could be connected to a compensatory mechanism for cellular hypoxia and increased free radical generation.

The role of the microcirculation and integrative cardiovascular physiology in the pathogenesis of ICU-acquired weakness

Skeletal muscle dysfunction after critical illness, defined as ICU-acquired weakness (ICU-AW), is a complex and multifactorial syndrome that contributes significantly to long-term morbidity and reduced quality of life for ICU survivors and caregivers. Historically, research in this field has focused on pathological changes within the muscle itself, without much consideration for their in vivo physiological environment. Skeletal muscle has the widest range of oxygen metabolism of any organ, and regulation of oxygen supply with tissue demand is a fundamental requirement for locomotion and muscle function. During exercise, this process is exquisitely controlled and coordinated by the cardiovascular, respiratory, and autonomic systems, and also within the skeletal muscle microcirculation and mitochondria as the terminal site of oxygen exchange and utilization. This review highlights the potential contribution of the microcirculation and integrative cardiovascular physiology to the pathogenesis of ICU-AW. An overview of skeletal muscle microvascular structure and function is provided, as well as our understanding of microvascular dysfunction during the acute phase of critical illness; whether microvascular dysfunction persists after ICU discharge is currently not known. Molecular mechanisms that regulate crosstalk between endothelial cells and myocytes are discussed, including the role of the microcirculation in skeletal muscle atrophy, oxidative stress, and satellite cell biology. The concept of integrated control of oxygen delivery and utilization during exercise is introduced, with evidence of physiological dysfunction throughout the oxygen delivery pathway - from mouth to mitochondria - causing reduced exercise capacity in patients with chronic disease (e.g., heart failure, COPD). We suggest that objective and perceived weakness after critical illness represents a physiological failure of oxygen supply-demand matching - both globally throughout the body and locally within skeletal muscle. Lastly, we highlight the value of standardized cardiopulmonary exercise testing protocols for evaluating fitness in ICU survivors, and the application of near-infrared spectroscopy for directly measuring skeletal muscle oxygenation, representing potential advancements in ICU-AW research and rehabilitation.

The Pleiotropic Role of Extracellular ATP in Myocardial Remodelling

Myocardial remodelling is a molecular, cellular, and interstitial adaptation of the heart in response to altered environmental demands. The heart undergoes reversible physiological remodelling in response to changes in mechanical loading or irreversible pathological remodelling induced by neurohumoral factors and chronic stress, leading to heart failure. Adenosine triphosphate (ATP) is one of the potent mediators in cardiovascular signalling that act on the ligand-gated (P2X) and G-protein-coupled (P2Y) purinoceptors via the autocrine or paracrine manners. These activations mediate numerous intracellular communications by modulating the production of other messengers, including calcium, growth factors, cytokines, and nitric oxide. ATP is known to play a pleiotropic role in cardiovascular pathophysiology, making it a reliable biomarker for cardiac protection. This review outlines the sources of ATP released under physiological and pathological stress and its cell-specific mechanism of action. We further highlight a series of cardiovascular cell-to-cell communications of extracellular ATP signalling cascades in cardiac remodelling, which can be seen in hypertension, ischemia/reperfusion injury, fibrosis, hypertrophy, and atrophy. Finally, we summarize current pharmacological intervention using the ATP network as a target for cardiac protection. A better understanding of ATP communication in myocardial remodelling could be worthwhile for future drug development and repurposing and the management of cardiovascular diseases.

Intravenous whole blood transfusion results in faster recovery of vascular integrity and increased survival in experimental cerebral malaria

BACKGROUND

Cerebral malaria is a lethal complication of Plasmodium falciparum infections in need of better therapies. Previous work in murine experimental cerebral malaria (ECM) indicated that the combination of artemether plus intraperitoneal whole blood improved vascular integrity and increased survival compared to artemether alone. However, the effects of blood or plasma transfusion administered via the intravenous route have not previously been evaluated in ECM.

OBJECTIVES

To evaluate the effects of intravenous whole blood compared to intravenous plasma on hematological parameters, vascular integrity, and survival in artemether-treated ECM.

METHODS

Mice with late-stage ECM received artemether alone or in combination with whole blood or plasma administered via the jugular vein. The outcome measures were hematocrit and platelets; plasma angiopoietin 1, angiopoietin 2, and haptoglobin; blood-brain barrier permeability; and survival.

FINDINGS

Survival increased from 54% with artemether alone to 90% with the combination of artemether and intravenous whole blood. Intravenous plasma lowered survival to 18%. Intravenous transfusion provided fast and pronounced recoveries of hematocrit, platelets, angiopoietins levels and blood brain barrier integrity.

MAIN CONCLUSIONS

The outcome of artemether-treated ECM was improved by intravenous whole blood but worsened by intravenous plasma. Compared to prior studies of transfusion via the intraperitoneal route, intravenous administration was more efficacious.

Modeling Reactive Hyperemia to Better Understand and Assess Microvascular Function: A Review of Techniques

Reactive hyperemia is a well-established technique for the non-invasive evaluation of the peripheral microcirculatory function, measured as the magnitude of limb re-perfusion after a brief period of ischemia. Despite widespread adoption by researchers and clinicians alike, many uncertainties remain surrounding interpretation, compounded by patient-specific confounding factors (such as blood pressure or the metabolic rate of the ischemic limb). Mathematical modeling can accelerate our understanding of the physiology underlying the reactive hyperemia response and guide in the estimation of quantities which are difficult to measure experimentally. In this work, we aim to provide a comprehensive guide for mathematical modeling techniques that can be used for describing the key phenomena involved in the reactive hyperemia response, alongside their limitations and advantages. The reported methodologies can be used for investigating specific reactive hyperemia aspects alone, or can be combined into a computational framework to be used in (pre-)clinical settings.

Dynamics of capillary blood flow responses to acute local changes in oxygen and carbon dioxide concentrations

Objectives: We aimed to quantify the magnitude and time transients of capillary blood flow responses to acute changes in local oxygen concentration ([O₂]), and carbon dioxide concentration ([CO₂]) in skeletal muscle. Additionally, we sought to quantify the combined response to both low [O₂] and high [CO₂] to mimic muscle microenvironment changes at the onset of exercise. Methods: 13 Sprague Dawley rats were anaesthetized, mechanically ventilated, and instrumented with indwelling catheters for systemic monitoring. The extensor digitorum longus muscle was blunt dissected, and reflected over a microfluidic gas exchange chamber in the stage of an inverted microscope. Four O₂ challenges, four CO₂ challenges, and a combined low O₂ (7-2%) and high CO₂ (5-10%) challenges were delivered to the surface with simultaneous visualization of capillary blood flow responses. Recordings were made for each challenge over a 1-min baseline period followed by a 2-min step change. The combined challenge employed a 1-min [O₂] challenge followed by a 2-min change in [CO₂]. Mean data for each sequence were fit using least-squared non-linear exponential models to determine the dynamics of each response. Results: 7-2% [O₂] challenges decreased capillary RBC saturation within 2 s following the step change (46.53 ± 19.56% vs. 48.51 ± 19.02%, p < 0.0001, τ = 1.44 s), increased RBC velocity within 3 s (228.53 ± 190.39 μm/s vs. 235.74 ± 193.52 μm/s, p < 0.0003, τ = 35.54 s) with a 52% peak increase by the end of the challenge, hematocrit and supply rate show similar dynamics. 5-10% [CO₂] challenges increased RBC velocity within 2 s following the step change (273.40 ± 218.06 μm/s vs. 276.75 ± 215.94 μm/s, p = 0.007, τ = 79.34s), with a 58% peak increase by the end of the challenge, supply rate and hematocrit show similar dynamics. Combined [O₂] and [CO₂] challenges resulted in additive responses to all microvascular hemodynamic measures with a 103% peak velocity increase by the end of the collection period. Data for mean responses and exponential fitting parameters are reported for all challenges. Conclusion: Microvascular level changes in muscle [O₂] and [CO₂] provoked capillary hemodynamic responses with differing time transients. Simulating exercise via combined [O₂] and [CO₂] challenges demonstrated the independent and additive nature of local blood flow responses to these agents.

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.

Homeostasis of extracellular ATP in uninfected RBCs from a Plasmodium falciparum culture and derived microparticles

Plasmodium falciparum, a dangerous parasitic agent causing malaria, invades human red blood cells (RBCs), causing hemolysis and microvascular obstruction. These and other pathological processes of malaria patients are due to metabolic and structural changes occurring in uninfected RBCs. In addition, infection activates the production of microparticles (MPs). ATP and byproducts are important extracellular ligands modulating purinergic signaling within the intravascular space. Here, we analyzed the contribution of uninfected RBCs and MPs to the regulation of extracellular ATP (eATP) of RBCs, which depends on the balance between ATP release by specific transporters and eATP hydrolysis by ectonucleotidases. RBCs were cultured with P. falciparum for 24-48 h prior to experiments, from which uninfected RBCs and MPs were purified. On-line luminometry was used to quantify the kinetics of ATP release. Luminometry, colorimetry and radioactive methods were used to assess the rate of eATP hydrolysis by ectonucleotidases. Rates of ATP release and eATP hydrolysis were also evaluated in MPs. Uninfected RBCs challenged by different stimuli displayed a strong and transient activation of ATP release, together with an elevated rate of eATP hydrolysis. MPs contained ATP in their lumen, which was released upon vesicle rupture, and were able to hydrolyze eATP. Results suggest that uninfected RBCs and MPs can act as important determinants of eATP regulation of RBCs during malaria. The comparison of eATP homeostasis in infected RBCs, ui-RBCs, and MPs allowed us to speculate on the impact of P. falciparum infection on intravascular purinergic signaling and the control of the vascular caliber by RBCs.

Effects of physical exercise on the functionality of human nucleotidases: A systematic review

Nucleotidases contribute to the regulation of inflammation, coagulation, and cardiovascular activity. Exercise promotes biological adaptations, but its effects on nucleotidase activities and expression are unclear. The objective of this study was to review systematically the effects of exercise on nucleotidase functionality in healthy and unhealthy subjects. The MEDLINE, EMBASE, Cochrane Library, and Web of Science databases were searched to identify, randomized clinical trials, non-randomized clinical trials, uncontrolled clinical trials, quasi-experimental, pre-, and post-interventional studies that evaluated the effects of exercise on nucleotidases in humans, and was not limited by language and date. Two independent reviewers performed the study selection, data extraction, and assessment of risk of bias. Of the 203 articles identified, 12 were included in this review. Eight studies reported that acute exercise, in healthy and unhealthy subjects, elevated the activities or expression of nucleotidases. Four studies evaluated the effects of chronic training on nucleotidase activities in the platelets and lymphocytes of patients with metabolic syndrome, chronic kidney disease, and hypertension and found a decrease in nucleotidase activities in these conditions. Acute and chronic exercise was able to modify the blood plasma and serum levels of nucleotides and nucleosides. Our results suggest that short- and long-term exercise modulate nucleotidase functionality. As such, purinergic signaling may represent a novel molecular adaptation in inflammatory, thrombotic, and vascular responses to exercise.

Simulated hypoxia modulates P2X7 receptor function in mice peritoneal macrophages

The inflammatory focus is similar to the tumor microenvironment, which contains a complex milieu with immune cells and macrophages. The accumulation of cells promotes local pH and O₂ tension decline (hypoxia). Local O₂ tension decline activates hypoxia-inducible factor α and β (HIF-1α and HIF-1β adenosine triphosphate (ATP) release. ATP activates the P2X7 receptor and modulates ischemic/hypoxic conditions. Similarly, α1α may regulate P2X7 receptor expression in the hypoxic microenvironment. Therefore, we investigated P2X7 receptor function under simulated hypoxic conditions by pretreating peritoneal macrophages with mitochondrial electron transport chain complex inhibitors (simulated hypoxia). Treatment with mitochondrial electron transport chain complex inhibitors until three hours of exposure did not cause LDH release. Additionally, mitochondrial electron transport chain complex inhibitors increased ATP-induced P2X7 receptor function without being able to directly activate this receptor. Other P2 receptor subtypes do not appear to participate in this mechanism. Simulated hypoxia augmented HIF-1α levels and suppressed HIF-1α and P2X7 receptor antagonists. Similarly, simulated hypoxia increased ATP-induced dye uptake and inhibited HIF-1α antagonists. Another factor activated in simulated hypoxic conditions was the intracellular P2X7 receptor regulator PIP2. Treatment with HIF-1α agonists increased PIP2 levels and reversed the effects of HIF-1α and P2X7 receptor antagonists. Additionally, the improved ATP-induced dye uptake caused by the simulated hypoxia stimulus was inhibited by P2X7 receptor and PIP2 antagonists. Therefore, simulated hypoxia may augment P2X7 receptor activity for a pathway dependent on HIF-1α and PIP2 activation.

Matching of O2 Utilization and O2 Delivery in Contracting Skeletal Muscle in Health, Aging, and Heart Failure

Skeletal muscle is one of the most dynamic metabolic organs as evidenced by increases in metabolic rate of >150-fold from rest to maximal contractile activity. Because of limited intracellular stores of ATP, activation of metabolic pathways is required to maintain the necessary rates of ATP re-synthesis during sustained contractions. During the very early phase, phosphocreatine hydrolysis and anaerobic glycolysis prevails but as activity extends beyond ∼1 min, oxidative phosphorylation becomes the major ATP-generating pathway. Oxidative metabolism of macronutrients is highly dependent on the cardiovascular system to deliver O₂ to the contracting muscle fibres, which is ensured through a tight coupling between skeletal muscle O₂ utilization and O₂ delivery. However, to what extent O₂ delivery is ideal in terms of enabling optimal metabolic and contractile function is context-dependent and determined by a complex interaction of several regulatory systems. The first part of the review focuses on local and systemic mechanisms involved in the regulation of O₂ delivery and how integration of these influences the matching of skeletal muscle O₂ demand and O₂ delivery. In the second part, alterations in cardiovascular function and structure associated with aging and heart failure, and how these impact metabolic and contractile function, will be addressed. Where applicable, the potential of exercise training to offset/reverse age- and disease-related cardiovascular declines will be highlighted in the context of skeletal muscle metabolic function. The review focuses on human data but also covers animal observations.

Functional implications of microvascular heterogeneity for oxygen uptake and utilization

In the vascular system, an extensive network structure provides convective and diffusive transport of oxygen to tissue. In the microcirculation, parameters describing network structure, blood flow, and oxygen transport are highly heterogeneous. This heterogeneity can strongly affect oxygen supply and organ function, including reduced oxygen uptake in the lung and decreased oxygen delivery to tissue. The causes of heterogeneity can be classified as extrinsic or intrinsic. Extrinsic heterogeneity refers to variations in oxygen demand in the systemic circulation or oxygen supply in the lungs. Intrinsic heterogeneity refers to structural heterogeneity due to stochastic growth of blood vessels and variability in flow pathways due to geometric constraints, and resulting variations in blood flow and hematocrit. Mechanisms have evolved to compensate for heterogeneity and thereby improve oxygen uptake in the lung and delivery to tissue. These mechanisms, which involve long-term structural adaptation and short-term flow regulation, depend on upstream responses conducted along vessel walls, and work to redistribute flow and maintain blood and tissue oxygenation. Mathematically, the variance of a functional quantity such as oxygen delivery that depends on two or more heterogeneous variables can be reduced if one of the underlying variables is controlled by an appropriate compensatory mechanism. Ineffective regulatory mechanisms can result in poor oxygen delivery even in the presence of adequate overall tissue perfusion. Restoration of endothelial function, and specifically conducted responses, should be considered when addressing tissue hypoxemia and organ failure in clinical settings.

Post-occlusive reactive hyperemia and skeletal muscle capillary hemodynamics

Post-occlusive reactive hyperemia (PORH) is an accepted diagnostic tool for assessing peripheral macrovascular function. While conduit artery hemodynamics have been well defined, the impact of PORH on capillary hemodynamics remains unknown, despite the microvasculature being the dominant site of vascular control. Therefore, the purpose of this investigation was to determine the effects of 5 min of feed artery occlusion on capillary hemodynamics in skeletal muscle. We tested the hypothesis that, upon release of arterial occlusion, there would be: 1) an increased red blood cell flux (fRBC) and red blood cell velocity (VRBC), and 2) a decreased proportion of capillaries supporting RBC flow compared to the pre-occlusion condition.

METHODS

In female Sprague-Dawley rats (n = 6), the spinotrapezius muscle was exteriorized for evaluation of capillary hemodynamics pre-occlusion, 5 min of feed artery occlusion (Occ), and 5 min of reperfusion (Post-Occ).

RESULTS

There were no differences in mean arterial pressure (MAP) or capillary diameter (Dc) between pre-occlusion and post-occlusion (P > 0.05). During 30 s of PORH, capillary fRBC was increased (pre: 59 ± 4 vs. 30 s-post: 77 ± 2 cells/s; P < 0.05) and VRBC was not changed (pre: 300 ± 24 vs. 30 s post: 322 ± 25 μm/s; P > 0.05). Capillary hematocrit (Hctcap) was unchanged across the pre- to post-occlusion conditions (P > 0.05). Following occlusion, there was a 20-30% decrease in the number of capillaries supporting RBC flow at 30 s and 300 s-post occlusion (pre: 92 ± 2%; 30 s-post: 66 ± 3%; 300 s-post: 72 ± 6%; both P < 0.05).

CONCLUSION

Short-term feed artery occlusion (i.e. 5 min) resulted in a more heterogeneous capillary flow profile with the presence of capillary no-reflow, decreasing the percentage of capillaries supporting RBC flow. A complex interaction between myogenic and metabolic mechanisms at the arteriolar level may play a role in the capillary no-reflow with PORH. Measurements at the level of the conduit artery mask significant alterations in blood flow distribution in the microcirculation.

Erythrocytes from patients with ST-elevation myocardial infarction induce cardioprotection through the purinergic P2Y13 receptor and nitric oxide signaling

Red blood cells (RBCs) are suggested to play a role in cardiovascular regulation by exporting nitric oxide (NO) bioactivity and ATP under hypoxia. It remains unknown whether such beneficial effects of RBCs are protective in patients with acute myocardial infarction. We investigated whether RBCs from patients with ST-elevation myocardial infarction (STEMI) protect against myocardial ischemia-reperfusion injury and whether such effect involves NO and purinergic signaling in the RBCs. RBCs from patients with STEMI undergoing primary coronary intervention and healthy controls were administered to isolated rat hearts subjected to global ischemia and reperfusion. Compared to RBCs from healthy controls, RBCs from STEMI patients reduced myocardial infarct size (30 ± 12% RBC healthy vs. 11 ± 5% RBC STEMI patients, P < 0.001), improved recovery of left-ventricular developed pressure and dP/dt and reduced left-ventricular end-diastolic pressure in hearts subjected to ischemia-reperfusion. Inhibition of RBC NO synthase with L-NAME or soluble guanylyl cyclase (sGC) with ODQ, and inhibition of cardiac protein kinase G (PKG) abolished the cardioprotective effect. Furthermore, the non-selective purinergic P2 receptor antagonist PPADS but not the P1 receptor antagonist 8PT attenuated the cardioprotection induced by RBCs from STEMI patients. The P2Y₁₃ receptor was expressed in RBCs and the cardioprotection was abolished by the P2Y₁₃ receptor antagonist MRS2211. By contrast, perfusion with PPADS, L-NAME, or ODQ prior to RBCs administration failed to block the cardioprotection induced by RBCs from STEMI patients. Administration of RBCs from healthy subjects following pre-incubation with an ATP analog reduced infarct size from 20 ± 6 to 7 ± 2% (P < 0.001), and this effect was abolished by ODQ and MRS2211. This study demonstrates a novel function of RBCs in STEMI patients providing protection against myocardial ischemia-reperfusion injury through the P2Y₁₃ receptor and the NO-sGC-PKG pathway.

Hypoxic vasodilatory defect and pulmonary hypertension in mice lacking hemoglobin β-cysteine93 S-nitrosylation

Systemic hypoxia is characterized by peripheral vasodilation and pulmonary vasoconstriction. However, the system-wide mechanism for signaling hypoxia remains unknown. Accumulating evidence suggests that hemoglobin (Hb) in RBCs may serve as an O₂ sensor and O₂-responsive NO signal transducer to regulate systemic and pulmonary vascular tone, but this remains unexamined at the integrated system level. One residue invariant in mammalian Hbs, β-globin cysteine93 (βCys93), carries NO as vasorelaxant S-nitrosothiol (SNO) to autoregulate blood flow during O₂ delivery. βCys93Ala mutant mice thus exhibit systemic hypoxia despite transporting O₂ normally. Here, we show that βCys93Ala mutant mice had reduced S-nitrosohemoglobin (SNO-Hb) at baseline and upon targeted SNO repletion and that hypoxic vasodilation by RBCs was impaired in vitro and in vivo, recapitulating hypoxic pathophysiology. Notably, βCys93Ala mutant mice showed marked impairment of hypoxic peripheral vasodilation and developed signs of pulmonary hypertension with age. Mutant mice also died prematurely with cor pulmonale (pulmonary hypertension with right ventricular dysfunction) when living under low O₂. Altogether, we identify a major role for RBC SNO in clinically relevant vasodilatory responses attributed previously to endothelial NO. We conclude that SNO-Hb transduces the integrated, system-wide response to hypoxia in the mammalian respiratory cycle, expanding a core physiological principle.

Red blood cell ATP release correlates with red blood cell hemolysis

The precise matching of blood flow to skeletal muscle during exercise remains an important area of investigation. Release of adenosine triphosphate (ATP) from red blood cells (RBCs) is postulated as a mediator of peripheral vascular tone in response to shear stress, hypoxia, and mechanical deformation. We tested the following hypotheses: 1) RBCs of different densities contain different quantities of ATP; 2) hypoxia is a stimulus for ATP release from RBCs; and 3) hypoxic ATP release from RBCs is related to RBC lysis. Human blood was drawn from male and female volunteers (n = 11); the RBCs were isolated and washed. A Percoll gradient was used to separate RBCs based on cellular density. Density groups were then resuspended to 4% hematocrit and exposed to normoxia or hypoxia in a tonometer. Equilibrated samples were drawn and centrifuged; paired analyses of ATP (luminescence via a luciferase-catalyzed reaction) and hemolysis (Harboe spectrophotometric absorbance assay) were measured in the supernatant. ATP release was not different among low-density cells versus middle-density versus high-density cells. Similarly, hemoglobin (Hb) release was not different among the red blood cell subsets. No difference was found for either ATP release or Hb release following matched exposure to normoxic or hypoxic gas. The concentrations of ATP and Hb for all subsets combined were linearly correlated (r = 0.59, P ≤ 0.001). With simultaneous probing for Hb and ATP in the supernatant of each sample, we conclude that ATP release from RBCs can be explained by hemolysis and that hypoxia per se does not stimulate either ATP release or Hb release from RBCs.

Erythrocyte flow through the interendothelial slits of the splenic venous sinus

The flow patterns of red blood cells through the spleen are intimately linked to clearance of senescent RBCs, with clearance principally occurring within the open flow through the red pulp and slits of the venous sinus system that exists in humans, rats, and dogs. Passage through interendothelial slits (IESs) of the sinus has been shown by MacDonald et al. (Microvasc Res 33:118-134, 1987) to be mediated by the caliber, i.e., slit opening width, of these slits. IES caliber within a given slit of a given sinus section has been shown to operate in an asynchronous manner. Here, we describe a model and simulation results that demonstrate how the supporting forces exerted on the sinus by the reticular meshwork of the red pulp, combined with asymmetrical contractility of stress fibers within the endothelial cells comprising the sinus, describe this vital and intriguing behavior. These results shed light on the function of the sinus slits in species such as humans, rats, and dogs that possess sinusoidal sinuses. Instead of assuming a passive mechanical filtering mechanism of the IESs, our proposed model provides a mechanically consistent explanation for the dynamically modulated IES opening/filtering mechanism observed in vivo. The overall perspective provided is also consistent with the view that IES passage serves as a self-protective mechanism in RBC vesiculation and inclusion removal.

ATP and acetylcholine interact to modulate vascular tone and α1-adrenergic vasoconstriction in humans

The vascular endothelium senses and integrates numerous inputs to regulate vascular tone. Recent evidence reveals complex signal processing within the endothelium, yet little is known about how endothelium-dependent stimuli interact to regulate blood flow. We tested the hypothesis that combined stimulation of the endothelium with adenosine triphosphate (ATP) and acetylcholine (ACh) elicits greater vasodilation and attenuates α₁-adrenergic vasoconstriction compared with combination of ATP or ACh with the endothelium-independent dilator sodium nitroprusside (SNP). We assessed forearm vascular conductance (FVC) in young adults (6 women, 7 men) during local intra-arterial infusion of ATP, ACh, or SNP alone and in the following combinations: ATP + ACh, SNP + ACh, and ATP + SNP, wherein the second dilator was coinfused after attaining steady state with the first dilator. By design, each dilator evoked a similar response when infused separately (ΔFVC, ATP: 48 ± 4; ACh: 57 ± 6; SNP: 53 ± 6 mL·min^-1·100 mmHg^-1; P ≥ 0.62). Combined infusion of the endothelium-dependent dilators evoked greater vasodilation than combination of either dilator with SNP (ΔFVC from first dilator, ATP + ACh: 45 ± 9 vs. SNP + ACh: 18 ± 7 and ATP + SNP: 26 ± 4 mL·min^-1·100 mmHg^-1, P < 0.05). Phenylephrine was subsequently infused to evaluate α₁-adrenergic vasoconstriction. Phenylephrine elicited less vasoconstriction during infusion of ATP or ACh versus SNP (ΔFVC, -25 ± 3 and -29 ± 4 vs. -48 ± 3%; P < 0.05). The vasoconstrictor response to phenylephrine was further diminished during combined infusion of ATP + ACh (-13 ± 3%; P < 0.05 vs. ATP or ACh alone) and was less than that observed when either dilator was combined with SNP (SNP + ACh: -26 ± 3%; ATP + SNP: -31 ± 4%; both P < 0.05 vs. ATP + ACh). We conclude that endothelium-dependent agonists interact to elicit vasodilation and limit α₁-adrenergic vasoconstriction in humans.NEW & NOTEWORTHY The results of this study highlight the vascular endothelium as a critical site for integration of vasomotor signals in humans. To our knowledge, this is the first study to demonstrate that combined stimulation of the endothelium with ATP and ACh results in enhanced vasodilation compared with combination of either ATP or ACh with an endothelium-independent dilator. Furthermore, we show that ATP and ACh interact to modulate α₁-adrenergic vasoconstriction in human skeletal muscle in vivo.

Hemoglobin catalyzes ATP-synthesis in human erythrocytes: a murburn model

Blood hemoglobin (Hb), known to transport oxygen, is the most abundant globular protein in humans. Erythrocytes have ∼10^-3 M concentration of ATP in steady-state and we estimate that this high amounts cannot be formed from 10^-4 - 10^-7 M levels of precursors via substrate-level phosphorylation of glycolysis. To account for this discrepancy, we propose that Hb serves as a 'murzyme' (a redox enzyme working along the principles of murburn concept), catalyzing the synthesis of the major amounts of ATP found in erythrocytes. This proposal is along the lines of our earlier works demonstrating DROS (diffusible reactive oxygen species) mediated ATP-synthesis as a thermodynamically and kinetically viable mechanism for physiological oxidative phosphorylation. We support the new hypothesis for Hb with theoretical arguments, experimental findings of reputed peers and in silico explorations. Using in silico methods, we demonstrate that adenosine nucleotide and 2,3-bisphosphoglycerate (2,3-BPG) binding sites are located suitably on the monomer/tetramer, thereby availing facile access to the superoxide emanating from the heme center. Our proposal explains earlier reported in situ experimental findings/suggestions of 2,3-BPG and ADP binding at the same locus on Hb. The binding energy is in the order of 2,3-BPG > NADH > ATP > ADP > AMP and agrees with earlier reports, potentially explaining the bioenergetic physiology of erythrocytes. Also, the newly discovered site for 2,3-BPG shows lower affinity in fetal Hb (as compared to adults) explaining oxygen transfer from mother to embryo. The findings pose significant implications in routine physiology and pathologies like sickle cell anemia and thalassemia.Communicated by Ramaswamy H. Sarma.

Evidence for role of capillaries in regulation of skeletal muscle oxygen supply

How oxygen (O₂ ) supply to capillaries is regulated to match the tissue's demand is unknown. Erythrocytes have been proposed as sensors in this regulatory mechanism since they release ATP, a vasodilator, in an oxygen saturation (SO₂ )-dependent manner. ATP causes hyperpolarization of endothelial cells resulting in conducted vasodilation to arterioles.

OBJECTIVE

We propose individual capillary units can regulate their own O₂ supply by direct communication to upstream arterioles via electrically coupled endothelium.

METHODS

To test this hypothesis, we developed a transparent micro-exchange device for localized O₂ exchange with surface capillaries of intact tissue. The device was fabricated with an O₂ permeable micro-outlet 0.2 × 1.0 mm. Experiments were performed on rat extensor digitorum longus (EDL) muscle using dual wavelength video microscopy to measure capillary hemodynamics and erythrocyte SO₂ . Responses to local O₂ perturbations were measured with only capillaries positioned over the micro-outlet.

RESULTS

Step changes in the gas mixture %O₂ caused physiological changes in erythrocyte SO₂ , and appropriate changes in flow to offset the O₂ challenge if at least 3-4 capillaries were stimulated.

CONCLUSION

These results support our hypothesis that individual capillary units play a role in regulating their erythrocyte supply in response to a changing O₂ environment.

Specific Binding of Leptin to Red Blood Cells Delivers a Pancreatic Hormone and Stimulates ATP Release

Since its discovery in 1994, leptin continues to have new potential physiological roles uncovered, including a role in the regulation of blood flow. Leptin's role in regulating blood flow is not completely understood. Red blood cell (RBC)-derived ATP is a recognized stimulus of blood flow, and multiple studies suggest that C-peptide, a hormone secreted in equimolar amounts with insulin from the pancreatic β-cells, can stimulate that release when delivered by albumin and in combination with Zn²⁺. Here, we report leptin delivers C-peptide and Zn²⁺ to RBCs in a saturable and specific manner. We labeled leptin with technetium-99 m (^99mTc) to perform binding studies while using albumin to block the specific binding of ^99mTc-leptin in the presence or absence of C-peptide. Our results suggest that leptin has a saturable and specific binding site on the RBC ((K_d = 1.79 ± 0.46) × 10^-7 M) that is statistically equal to the binding affinity in the presence of 20 nM C-peptide ((K_d = 2.05 ± 0.20) × 10^-7 M). While the binding affinity between leptin and the RBC did not change with C-peptide, the moles of bound leptin did increase with C-peptide, suggesting a separate binding site on the cell for a leptin/C-peptide complex. The RBC-derived ATP increased in the presence of a leptin/C-peptide/Zn²⁺ addition, in a concentration-dependent manner. Control RBCs ATP release increased (71 ± 5.6%) in the presence of C-peptide and Zn²⁺, which increased further to (94 ± 5.6%) in the presence of Zn²⁺, C-peptide, and leptin.

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.

Effects of impaired microvascular flow regulation on metabolism-perfusion matching and organ function

Impaired tissue oxygen delivery is a major cause of organ damage and failure in critically ill patients, which can occur even when systemic parameters, including cardiac output and arterial hemoglobin saturation, are close to normal. This review addresses oxygen transport mechanisms at the microcirculatory scale, and how hypoxia may occur in spite of adequate convective oxygen supply. The structure of the microcirculation is intrinsically heterogeneous, with wide variations in vessel diameters and flow pathway lengths, and consequently also in blood flow rates and oxygen levels. The dynamic processes of structural adaptation and flow regulation continually adjust microvessel diameters to compensate for heterogeneity, redistributing flow according to metabolic needs to ensure adequate tissue oxygenation. A key role in flow regulation is played by conducted responses, which are generated and propagated by endothelial cells and signal upstream arterioles to dilate in response to local hypoxia. Several pathophysiological conditions can impair local flow regulation, causing hypoxia and tissue damage leading to organ failure. Therapeutic measures targeted to systemic parameters may not address or may even worsen tissue oxygenation at the microvascular level. Restoration of tissue oxygenation in critically ill patients may depend on restoration of endothelial cell function, including conducted responses.

Adenosine metabolism in the vascular system

The concept of extracellular purinergic signaling was first proposed by Geoffrey Burnstock in the early 1970s. Since then, extracellular ATP and its metabolites ADP and adenosine have attracted an enormous amount of attention in terms of their involvement in a wide range of immunomodulatory, thromboregulatory, angiogenic, vasoactive and other pathophysiological activities in different organs and tissues, including the vascular system. In addition to significant progress in understanding the properties of nucleotide- and adenosine-selective receptors, recent studies have begun to uncover the complexity of regulatory mechanisms governing the duration and magnitude of the purinergic signaling cascade. This knowledge has led to the development of new paradigms in understanding the entire purinome by taking into account the multitude of signaling and metabolic pathways involved in biological effects of ATP and adenosine and compartmentalization of the adenosine system. Along with the "canonical route" of ATP breakdown to adenosine via sequential ecto-nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39) and ecto-5'-nucleotidase/CD73 activities, it has now become clear that purine metabolism is the result of concerted effort between ATP release, its metabolism through redundant nucleotide-inactivating and counteracting ATP-regenerating ectoenzymatic pathways, as well as cellular nucleoside uptake and phosphorylation of adenosine to ATP through complex phosphotransfer reactions. In this review I provide an overview of key enzymes involved in adenosine metabolic network, with special emphasis on the emerging roles of purine-converting ectoenzymes as novel targets for cancer and vascular therapies.

History of Geoff Burnstock's research on P2 receptors

Geoffrey Burnstock is a purinergic signalling legend who's discoveries and conceptualisation created and shaped the field. His scientific achievements were extraordinary and sustained. They included his demonstration that ATP can act as a neurotransmitter and hence extracellular signalling molecule, which he championed despite considerable initial opposition to his proposal that ATP acts outside of its role as an energy source inside cells. He led on purine receptor classification: initially of the P1 and P2 receptor families, then the P2X and P2Y receptor families, and then subtypes of P2X and P2Y receptors. This was achieved across several decades as he conceptualised and made sense of the emerging and growing evidence that there were multiple receptor subtypes for ATP and other nucleotides. He made discoveries about short term and long term/trophic purinergic signalling. He was a leader in the field for over 50 years. He inspired many and was a great colleague and mentor. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, University College London. This review is a personal perspective of some of Geoff's research on P2 receptors carried out during that time. It is a tribute to Geoff who I regarded with enormous respect and admiration.

Purinergic signalling in the cardiovascular system-a tribute to Geoffrey Burnstock

Geoffrey Burnstock made groundbreaking discoveries on the physiological roles of purinergic receptors and led on P2 purinergic receptor classification. His knowledge, vision and leadership inspired and influenced the international scientific community. I had the privilege of spending over 10 years (from 1985) with Geoff at the Department of Anatomy and Developmental Biology, initially as a PhD student and then as a postdoctoral research fellow. I regarded him with enormous admiration and affection. This review on purinergic signalling in the cardiovascular system is a tribute to Geoff. It includes some personal recollections of Geoff.

Environmental stress influences mitochondrial metabolism in vascular cells: consequences for angiogenesis

While the important and varied roles that vascular cells play in both health and disease is well recognised, the focus on potential therapeutic targets continually shifts as new players emerge. Here, we outline how mitochondria may be viewed as more than simply energy-generating organelles, but instead as important sentinels of metabolic health and effectors of appropriate responses to physiological challenges.

Erythrocytes as markers of oxidative stress related pathologies

Erythrocytes are deeply sensitive cells and important health indicators. During inflammatory response RBC, as a part of haematological system, are exposed to circulating inflammatory mediators and related oxidative stress. They present a highly specialized and organized cell membrane that interacts with inflammatory mediators and oxidative agents, leading to a variety of structural changes that promptly signal an abnormal situation. This review is aimed to provide an overview on erythrocyte involvement in physiological and pathological processes related to oxidative stress, such as aging, Down syndrome, neurodegenerative diseases, for instance Alzheimer Disease, erectile dysfunction and cardiovascular diseases. In particular this review will focus on the effects of oxidative stress on structural changes in the cell membrane and also on in the activity of erythrocyte enzymes such as membrane-bound, cytosolic glycohydrolases and RBC-eNOS. This review also underlines the potential clinical application of erythrocyte specific related parameters, which can be important tools not only for the study but also for the monitoring of several oxidative stress related diseases.

Red blood cell dysfunction: a new player in cardiovascular disease

The primary role of red blood cells (RBCs) is to transport oxygen to the tissues and carbon dioxide to the lungs. However, emerging evidence suggests an important role of the RBC beyond being just a passive carrier of the respiratory gases. The RBCs are of importance for redox balance and are actively involved in the regulation of vascular tone, especially during hypoxic and ischaemic conditions by the release of nitric oxide (NO) bioactivity and adenosine triphosphate. The role of the RBC has gained further interest after recent discoveries demonstrating a markedly altered function of the cell in several pathological conditions. Such alterations include increased adhesion capability, increased formation of reactive oxygen species as well as altered protein content and enzymatic activities. Beyond signalling increased oxidative stress, the altered function of RBCs is characterized by reduced export of NO bioactivity regulated by increased arginase activity. Of further importance, the altered function of RBCs has important implications for several cardiovascular disease conditions. RBCs have been shown to induce endothelial dysfunction and to increase cardiac injury during ischaemia-reperfusion in diabetes mellitus. Finally, this new knowledge has led to novel therapeutic possibilities to intervene against cardiovascular disease by targeting signalling in the RBC. These novel data open up an entirely new view on the underlying pathophysiological mechanisms behind the cardiovascular disease processes in diabetes mellitus mediated by the RBC. This review highlights the current knowledge regarding the role of RBCs in cardiovascular regulation with focus on their importance for cardiovascular dysfunction in pathological conditions and therapeutic possibilities for targeting RBCs in cardiovascular disease.

A Computer Model of Oxygen Dynamics in the Cortex of the Rat Kidney at the Cell-Tissue Level

The renal cortex drives renal function. Hypoxia/reoxygenation are primary factors in ischemia-reperfusion (IR) injuries, but renal oxygenation per se is complex and awaits full elucidation. Few mathematical models address this issue: none captures cortical tissue heterogeneity. Using agent-based modeling, we develop the first model of cortical oxygenation at the cell-tissue level (RCM), based on first principles and careful bibliographical analysis. Entirely parameterized with Rat data, RCM is a morphometrically equivalent 2D-slice of cortical tissue, featuring peritubular capillaries (PTC), tubules and interstitium. It implements hemoglobin/O2 binding-release, oxygen diffusion, and consumption, as well as capillary and tubular flows. Inputs are renal blood flow RBF and PO2 feeds; output is average tissue PO2 (tPO2). After verification and sensitivity analysis, RCM was validated at steady-state (tPO2 37.7 ± 2.2 vs. 36.9 ± 6 mmHg) and under transients (ischemic oxygen half-time: 4.5 ± 2.5 vs. 2.3 ± 0.5 s in situ). Simulations confirm that PO2 is largely independent of RBF, except at low values. They suggest that, at least in the proximal tubule, the luminal flow dominantly contributes to oxygen delivery, while the contribution of capillaries increases under partial ischemia. Before addressing IR-induced injuries, upcoming developments include ATP production, adaptation to minutes-hours scale, and segmental and regional specification.

Contribution of prostaglandins to exercise hyperaemia: workload, ethnicity and sex matter!

The contribution of prostaglandins (PGs) to exercise hyperaemia is controversial. In this review, we argue this is partly explained by differences in exercise intensity between studies. The effects of cyclooxygenase (COX) inhibition and PG assays indicate that PGs contribute more at moderate to heavy than at light workloads and are mainly released by low tissue O₂ . But, the release and actions of PGs also depend on other O₂ -dependent dilators including ATP, adenosine and NO. K⁺ may inhibit the action of PGs and other mediators by causing hyperpolarization, but contributes to the hyperaemia. Thus, at lighter loads, the influence of PGs may be blunted by K⁺ , while COX inhibition leads to compensatory increases in other O₂ -dependent dilators. In addition, we show that other sources of variability are sex and ethnicity. Our findings indicate that exercise hyperaemia following rhythmic contractions at 60% maximum voluntary contraction, is smaller in young black African (BA) men and women than in their white European (WE) counterparts, but larger in men than in women of both ethnicities. We propose the larger absolute force in men causes greater vascular occlusion and accumulation of dilators, while blunted hyperaemia in BAs may reflect lower oxidative capacity and O₂ requirement. Nevertheless, COX inhibition attenuated peak hyperaemia by ∼30% in WE, BA men and WE women, indicating PGs make a substantial contribution in all three groups. There was no effect in BA women. Lack of PG involvement may provide early evidence of endothelial dysfunction, consistent in BA women with their greater risk of cardiovascular disease.

Dietary Fatty Acids Affect Red Blood Cell Membrane Composition and Red Blood Cell ATP Release in Dairy Cows

Diets of dairy cows are often based on maize silage (MS), delivering lower amounts of n-3 fatty acids (FA) compared to grass silage-based diets. The fatty acid composition of the cell membrane can affect the cell function. We evaluated the effects of an MS-based diet on bovine red blood cell (RBC) membrane FA composition and dietary effects on controlled ATP release of RBC. In trial 1, German Holstein cows were fed an MS-based total mixed ration for 24 weeks. The FA composition of RBC membranes from repeatedly taken blood samples was analysed in addition to the abundance of the RBC membrane protein flotillin-1, which is involved in, for example, cell signalling. In trial 2, four rumen fistulated MS-fed cows were abomasally infused in a 4 × 4 Latin square model with three successively increasing lipid dosages (coconut oil, linseed–safflower oil mix (EFA; rich in n-3 FA), Lutalin®, providing conjugated linoleic acids (CLA) or the combination of the supplements, EFA + CLA) for six weeks, followed by a three-week washout period. In trial 2, we analysed RBC ATP release, flotillin-1, and the membrane protein abundance of pannexin-1, which is involved in ATP release as the last part of a signalling cascade. In trial 1, the total amount of n-3 FA in RBC membranes decreased and the flotillin-1 abundance increased over time. In trial 2, the RBC n-3 FA amount was higher after the six-week infusion period of EFA or EFA + CLA. Furthermore, depending on the dosage of FA, the ATP release from RBC increased. The abundance of flotillin-1 and pannexin-1 was not affected in trial 2. It is concluded that changes of the membrane FA composition influence the RBC function, leading to altered ATP release from intact bovine RBC.

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.

Regulation of extracellular ATP of human erythrocytes treated with α-hemolysin. Effects of cell volume, morphology, rheology and hemolysis

Alpha-hemolysin (HlyA) of uropathogenic strains of Escherichia coli irreversibly binds to human erythrocytes (RBCs) and triggers activation of ATP release and metabolic changes ultimately leading to hemolysis. We studied the regulation of extracellular ATP (ATPe) of RBCs exposed to HlyA. Luminometry was used to assess ATP release and ATPe hydrolysis, whereas changes in cell volume and morphology were determined by electrical impedance, ektacytometry and aggregometry. Exposure of RBCs to HlyA induced a strong increase of [ATPe] (3-36-fold) and hemolysis (1-44-fold), partially compensated by [ATPe] hydrolysis by ectoATPases and intracellular ATPases released by dead cells. Carbenoxolone, a pannexin 1 inhibitor, partially inhibited ATP release (43-67%). The un-acylated toxin ProHlyA and the deletion analog HlyA∆914-936 were unable to induce ATP release or hemolysis. For HlyA treated RBCs, a data driven mathematical model showed that simultaneous lytic and non-lytic release mainly governed ATPe kinetics, while ATPe hydrolysis became important after prolonged toxin exposure. HlyA induced a 1.5-fold swelling, while blocking this swelling reduced ATP release by 77%. Blocking ATPe activation of purinergic P2X receptors reduced swelling by 60-80%. HlyA-RBCs showed an acute 1.3-2.2-fold increase of Ca²⁺i, increased crenation and externalization of phosphatidylserine. Perfusion of HlyA-RBCs through adhesion platforms showed strong adhesion to activated HMEC cells, followed by rapid detachment. HlyA exposed RBCs exhibited increased sphericity under osmotic stress, reduced elongation under shear stress, and very low aggregation in viscous media. Overall results showed that HlyA-RBCs displayed activated ATP release, high but weak adhesivity, low deformability and aggregability and high sphericity.

Skeletal muscle interstitial O2 pressures: bridging the gap between the capillary and myocyte

The oxygen transport pathway from air to mitochondria involves a series of transfer steps within closely integrated systems (pulmonary, cardiovascular, and tissue metabolic). Small and finite O₂ stores in most mammalian species require exquisitely controlled changes in O₂ flux rates to support elevated ATP turnover. This is especially true for the contracting skeletal muscle where O₂ requirements may increase two orders of magnitude above rest. This brief review focuses on the mechanistic bases for increased microvascular blood-myocyte O₂ flux (V̇O₂ ) from rest to contractions. Fick's law dictates that V̇O₂ elevations driven by muscle contractions are produced by commensurate changes in driving force (ie, O₂ pressure gradients; ΔPO₂ ) and/or effective diffusing capacity (DO₂ ). While previous evidence indicates that increased DO₂ helps modulate contracting muscle O₂ flux, up until recently the role of the dynamic ΔPO₂ across the capillary wall was unknown. Recent phosphorescence quenching investigations of both microvascular and novel interstitial PO₂ kinetics in health have resolved an important step in the O₂ cascade between the capillary and myocyte. Specifically, the significant transmural ΔPO₂ at rest was sustained (but not increased) during submaximal contractions. This supports the contention that the blood-myocyte interface provides a substantial effective resistance to O₂ diffusion and underscores that modulations in erythrocyte hemodynamics and distribution (DO₂ ) are crucial to preserve the driving force for O₂ flux across the capillary wall (ΔPO₂ ) during contractions. Investigation of the O₂ transport pathway close to muscle mitochondria is key to identifying disease mechanisms and develop therapeutic approaches to ameliorate dysfunction and exercise intolerance.

Validation of a LC/MSMS method for simultaneous quantification of 9 nucleotides in biological matrices

Nucleotides play a role in inflammation processes: cAMP and cGMP in the endothelial barrier function, ADP in platelet aggregation, ATP and UTP in vasodilatation and/or vasoconstriction of blood vessels, UDP in macrophages activation. The aim of this study is to develop and validate a LC/MS-MS method able to quantify simultaneously nine nucleotides (AMP, cAMP, ADP, ATP, GMP, cGMP, UMP, UDP and UTP) in biological matrixes (cells and plasma). The method we developed, has lower LOQ's than others and has the main advantage to quantify all nucleotides within one single injection in less than 10 min. The measured nucleotides concentrations obtained with this method are similar to those obtained with assay kits commercially available. Analysis of plasma and red blood cells from healthy donors permits to estimate the physiological concentration of those nucleotides in human plasma and red blood cells, such information being poorly available in the literature. Furthermore, the protocol presented in this paper allowed us to observe that AMP, ADP, ATP concentrations are modified in human red blood cells and plasma after a venous stasis of 4 min compared to physiological blood circulation. Therefore, this specific method enables future studies on nucleotides implications in chronic inflammatory diseases but also in other pathologies where nucleotides are implicated in.

Inhibition of Na+ /K+ -ATPase and KIR channels abolishes hypoxic hyperaemia in resting but not contracting skeletal muscle of humans

KEY POINTS

Increasing blood flow (hyperaemia) to exercising muscle helps match oxygen delivery and metabolic demand. During exercise in hypoxia, there is a compensatory increase in muscle hyperaemia that maintains oxygen delivery and tissue oxygen consumption. Nitric oxide (NO) and prostaglandins (PGs) contribute to around half of the augmented hyperaemia during hypoxic exercise, although the contributors to the remaining response are unknown. In the present study, inhibiting NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels did not blunt augmented hyperaemia during hypoxic exercise beyond previous observations with NO/PG block alone. Furthermore, although inhibition of only Na⁺ /K⁺ -ATPase and K_IR channels abolished hyperaemia during hypoxia at rest, it had no effect on augmented hyperaemia during hypoxic exercise. This is the first study in humans to demonstrate that Na⁺ /K⁺ -ATPase and K_IR channel activation is required for augmented muscle hyperaemia during hypoxia at rest but not during hypoxic exercise, thus providing new insight into vascular control.

ABSTRACT

Exercise hyperaemia in hypoxia is augmented relative to the same exercise intensity in normoxia. During moderate-intensity handgrip exercise, endothelium-derived nitric oxide (NO) and vasodilating prostaglandins (PGs) contribute to ∼50% of the augmented forearm blood flow (FBF) response to hypoxic exercise (HypEx), although the mechanism(s) underlying the remaining response are unclear. We hypothesized that combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and inwardly rectifying potassium (K_IR ) channels would abolish the augmented hyperaemic response in HypEx. In healthy young adults, FBF responses were measured (Doppler ultrasound) and forearm vascular conductance was calculated during 5 min of rhythmic handgrip exercise at 20% maximum voluntary contraction under regional sympathoadrenal inhibition in normoxia and isocapnic HypEx (O₂ saturation ∼80%). Compared to control, combined inhibition of NO, PGs, Na⁺ /K⁺ -ATPase and K_IR channels (l-NMMA + ketorolac + ouabain + BaCl_2; Protocol 1; n = 10) blunted the compensatory increase in FBF during HypEx by ∼50% (29 ± 6 mL min^-1 vs. 62 ± 8 mL min^-1 , respectively, P < 0.05). By contrast, ouabain + BaCl₂ alone (Protocol 2; n = 10) did not affect this augmented hyperaemic response (50 ± 11 mL min^-1 vs. 60 ± 13 mL min^-1 , respectively, P > 0.05). However, the blocked condition in both protocols abolished the hyperaemic response to hypoxia at rest (P < 0.05). We conclude that activation of Na⁺ /K⁺ -ATPase and K_IR channels is involved in the hyperaemic response to hypoxia at rest, although it does not contribute to the augmented exercise hyperaemia during hypoxia in humans.

Pyruvate as a novel carrier of hydroxyethyl starch 130/0.4 may protect kidney in rats subjected to severe burns

BACKGROUND

The carrier of hydroxyethyl starch (HES) may play a critical role in kidney injury in fluid resuscitation. This study aimed mainly to compare effects of pyruvate-enriched saline with normal saline (NS) and acetate Ringer's (AR) solution as a carrier in HES130/0.4 on kidney function in rats subjected to severe burns.

METHODS

Using a lethal burn model, 140 rats were randomly allocated in seven groups (n = 20): sham group (group S); no fluid after burn (group N); burn resuscitated with NS (group NS); burn resuscitated with pyruvate saline (group PS); burn resuscitated with AR plus pyruvate-HES (group SP); burn resuscitated with AR plus acetate-HES (group SA), and burn resuscitated with AR plus NS-HES (group SN). A low volume (18.75 mL·kg^-1 during 12 h) of HES130/0.4 was infused with the ratio of 1:1 to crystalloids. Renal surface blood flow, blood creatinine and blood urea nitrogen, early sensitive indicators of kidney function: alpha-1 microglobulin, cystatin-C, and neutrophil gelatinase-associated lipocalin in blood and urine, and kidney tissue water contents were determined. Renal histopathological alterations with Paller scores were also measured at 8 h and 24 h after burn (n = 10), respectively.

RESULTS

The results showed in a comparable manner that group SP was the best in three HES groups and group PS was superior to group NS in renal preservation; group SP appeared significantly beneficial compared with group PS in renal surface blood flow, cystatin-C, neutrophil gelatinase-associated lipocalin, water contents, and Paller scores at 8-h or both time points after burn, respectively (all P < 0.05).

CONCLUSIONS

The carrier of HES130/0.4 played a crucial role in kidney injury in fluid resuscitation of rats subjected to severe burns. Pyruvate-enriched HES130/0.4 was superior and HES130/0.4, per se, might be not renocytotoxic, but renoprotective. Further studies are warranted.