Modulation of cerebral arteriolar diameter by intraluminal flow and pressure

A role for astrocytic insulin-like growth factor I receptors in the response to ischemic insult

Increased neurotrophic support, including insulin-like growth factor I (IGF-I), is an important aspect of the adaptive response to ischemic insult. However, recent findings indicate that the IGF-I receptor (IGF-IR) in neurons plays a detrimental role in the response to stroke. Thus, we investigated the role of astrocytic IGF-IR on ischemic insults using tamoxifen-regulated Cre deletion of IGF-IR in glial fibrillary acidic protein (GFAP) astrocytes, a major cellular component in the response to injury. Ablation of IGF-IR in astrocytes (GFAP-IGF-IR KO mice) resulted in larger ischemic lesions, greater blood-brain-barrier disruption and more deteriorated sensorimotor coordination. RNAseq detected increases in inflammatory, cell adhesion and angiogenic pathways, while the expression of various classical biomarkers of response to ischemic lesion were significantly increased at the lesion site compared to control littermates. While serum IGF-I levels after injury were decreased in both control and GFAP-IR KO mice, brain IGF-I mRNA expression show larger increases in the latter. Further, greater damage was also accompanied by altered glial reactivity as reflected by changes in the morphology of GFAP astrocytes, and relative abundance of ionized calcium binding adaptor molecule 1 (Iba 1) microglia. These results suggest a protective role for astrocytic IGF-IR in the response to ischemic injury.

The fluidic memristor as a collective phenomenon in elastohydrodynamic networks

Fluid flow networks are ubiquitous and can be found in a broad range of contexts, from human-made systems such as water supply networks to living systems like animal and plant vasculature. In many cases, the elements forming these networks exhibit a highly non-linear pressure-flow relationship. Although we understand how these elements work individually, their collective behavior remains poorly understood. In this work, we combine experiments, theory, and numerical simulations to understand the main mechanisms underlying the collective behavior of soft flow networks with elements that exhibit negative differential resistance. Strikingly, our theoretical analysis and experiments reveal that a minimal network of nonlinear resistors, which we have termed a 'fluidic memristor', displays history-dependent resistance. This new class of element can be understood as a collection of hysteresis loops that allows this fluidic system to store information, and it can be directly used as a tunable resistor in fluidic setups. Our results provide insights that can inform other applications of fluid flow networks in soft materials science, biomedical settings, and soft robotics, and may also motivate new understanding of the flow networks involved in animal and plant physiology.

Evaluation of Barrier Integrity Using a Two-Layered Microfluidic Device Mimicking the Blood-Brain Barrier

The blood-brain barrier (BBB) plays an essential role in maintaining the homeostasis of the brain microenvironment by controlling the influx and efflux of biological substances that are necessary to sustain the neuronal metabolic activity and functions. This barrier is established at the blood-brain interface of the brain microcapillaries by different cells. These include microvascular endothelial cells, astrocytes, and pericytes besides other components such as microglia, basal membrane, and neuronal cells forming together what is commonly referred to as the neurovascular unit; different in vivo and in vitro platforms are available to study the BBB where each system provides specific benefits and drawbacks. Recently, organ-on-a-chip platforms combine the elegance of microengineering technology with the complexity of biological systems to create near-ideal experimental models for various diseases and organs. These microfluidic devices with micron-sized channels allow the cells to be grown in a more biologically relevant environment, enabling cell to cell communications with continuous bathing in biological fluids in a tissue-like fashion. They also closely represent tissue and organ functionality by recapitulating mechanical forces as well as vascular perfusion. Here, we describe the use of humanized BBB model created with microfluidic organ-on-a-chip technology where human brain microvascular endothelial cells (BMECs) are cocultured with primary human pericytes and astrocytes. We thoroughly described the method to assess BBB integrity using a microfluidic chip and various sizes of labeled dextran as permeability markers. In addition, we provide a detailed protocol on how to microscopically investigate the tight junction proteins expression between hBMECs.

Pericytes and the Control of Blood Flow in Brain and Heart

Pericytes, attached to the surface of capillaries, play an important role in regulating local blood flow. Using optogenetic tools and genetically encoded reporters in conjunction with confocal and multiphoton imaging techniques, the 3D structure, anatomical organization, and physiology of pericytes have recently been the subject of detailed examination. This work has revealed novel functions of pericytes and morphological features such as tunneling nanotubes in brain and tunneling microtubes in heart. Here, we discuss the state of our current understanding of the roles of pericytes in blood flow control in brain and heart, where functions may differ due to the distinct spatiotemporal metabolic requirements of these tissues. We also outline the novel concept of electro-metabolic signaling, a universal mechanistic framework that links tissue metabolic state with blood flow regulation by pericytes and vascular smooth muscle cells, with capillary K_ATP and Kir2.1 channels as primary sensors. Finally, we present major unresolved questions and outline how they can be addressed.

3D printed multi-growth factor delivery patches fabricated using dual-crosslinked decellularized extracellular matrix-based hybrid inks to promote cerebral angiogenesis

Generally, brain angiogenesis is a tightly regulated process, which scarcely occurred in the absence of specific pathological conditions. Delivery of exogenous angiogenic factors enables the induction of desired angiogenesis by stimulating neovasculature formation. However, effective strategies of mimicking the angiogenesis process with exogenous factors have not yet been fully explored. Herein, we develop a 3D printed spatiotemporally compartmentalized cerebral angiogenesis inducing (SCAI) hydrogel patch, releasing dual angiogenic growth factors (GFs), using extracellular matrix-based hybrid inks. We introduce a new hybrid biomaterial-based ink for printing patches through dual crosslinking mechanisms: Chemical crosslinking with aza-Michael addition reaction with combining methacrylated hyaluronic acid (HAMA) and vascular-tissue-derived decellularized extracellular matrix (VdECM), and thermal crosslinking of VdECM. 3D printing technology, a useful approach with fabrication versatility with customizable systems and multiple biomaterials, is adopted to print three-layered hydrogel patch with spatially separated dual GFs as outer- and inner-layers that provide tunable release profiles of multiple GFs and fabrication versatility. Consequently, these layers of the patch spatiotemporally separated with dual GFs induce excellent neovascularization in the brain area, monitored by label-free photoacoustic microscopy in vivo. The developed multi-GFs releasing patch may offer a promising therapeutic approach of spatiotemporal drugs releasing such as cerebral ischemia, ischemic heart diseases, diabetes, and even use as vaccines. STATEMENT OF SIGNIFICANCE: Effective strategies of mimicking the angiogenesis process with exogenous factors have not yet been fully explored. In this study, we develop a 3D printed spatiotemporally compartmentalized cerebral angiogenesis inducing (SCAI) hydrogel patch, releasing dual angiogenic growth factors (GFs) using extracellular matrix-based hybrid inks. We introduce a new hybrid biomaterial-based ink through dual crosslinking mechanisms: Chemical crosslinking with aza-Michael addition, and thermal crosslinking. 3D printing technology is adopted to print three-layered hydrogel patch with spatially separated dual GFs as outer- and inner-layers that provide tunable release profiles of multiple GFs and fabrication versatility. Consequently, these layers of the patch spatiotemporally separated with dual GFs induce excellent neovascularization in the brain area, monitored by photoacoustic microscopy in vivo.

Multi-compartmental model of glymphatic clearance of solutes in brain tissue

The glymphatic system is the subject of numerous pieces of research in biology. Mathematical modelling plays a considerable role in this field since it can indicate the possible physical effects of this system and validate the biologists' hypotheses. The available mathematical models that describe the system at the scale of the brain (i.e. the macroscopic scale) are often solely based on the diffusion equation and do not consider the fine structures formed by the perivascular spaces. We therefore propose a mathematical model representing the time and space evolution of a mixture flowing through multiple compartments of the brain. We adopt a macroscopic point of view in which the compartments are all present at any point in space. The equations system is composed of two coupled equations for each compartment: One equation for the pressure of a fluid and one for the mass concentration of a solute. The fluid and solute can move from one compartment to another according to certain membrane conditions modelled by transfer functions. We propose to apply this new modelling framework to the clearance of 14C-inulin from the rat brain.

The role of nitric oxide in flow-induced and myogenic responses in 1A, 2A, and 3A branches of the porcine middle cerebral artery

Myogenic and flow-induced reactivity contribute to cerebral autoregulation, with potentially divergent roles for smaller versus larger arteries. The present study tested the hypotheses that compared with first-order (1A) branches of the middle cerebral artery, second- and third-order branches (2A and 3A, respectively) exhibit greater myogenic reactivity but reduced flow-induced constriction. Furthermore, nitric oxide synthase (NOS) inhibition may amplify myogenic reactivity and abolish instances of flow-induced dilation. Isolated porcine cerebral arteries mounted in a pressure myograph were exposed to incremental increases in intraluminal pressure (40-120 mmHg; n = 41) or flow (1-1,170 µL/min; n = 31). Intraluminal flows were adjusted to achieve 5, 10, 20, and 40 dyn/cm2 of wall shear stress at 60 mmHg. Myogenic tone was greater in 3A versus 1A arteries (P < 0.05). There was an inverse relationship between myogenic reactivity and passive arterial diameter (P < 0.01). NOS inhibition increased basal tone to a lesser extent in 3A versus 1A arteries (P < 0.01) but did not influence myogenic reactivity (P = 0.49). Increasing flow decreased luminal diameter (P ≤ 0.01), with increased vasoconstriction at 10-40 dyn/cm2 of shear stress (P < 0.01). However, relative responses were similar between 1A, 2A, and 3A arteries (P = 0.40) with and without NOS inhibition conditions (P ≥ 0.29). Whereas NOS inhibition increases basal myogenic tone, and myogenic reactivity was less in smaller versus larger arteries (range = ∼100-550 µM), neither NOS inhibition nor luminal diameter influences flow-induced constriction in porcine cerebral arteries.NEW & NOTEWORTHY This study demonstrated size-dependent heterogeneity in myogenic reactivity in porcine cerebral arteries. Smaller branches of the middle cerebral artery exhibited increased myogenic reactivity, but attenuated NOS-dependent increases in myogenic tone compared with larger branches. Flow-dependent regulation does not exhibit the same variation; diameter-independent flow-induced vasoconstrictions occur across all branch orders and are not affected by NOS inhibition. Conceptually, flow-induced vasoconstriction contributes to cerebral autoregulation, particularly in larger arteries with low myogenic tone.

Endothelial Cells and the Cerebral Circulation

Endothelial cells form the innermost layer of all blood vessels and are the only vascular component that remains throughout all vascular segments. The cerebral vasculature has several unique properties not found in the peripheral circulation; this requires that the cerebral endothelium be considered as a unique entity. Cerebral endothelial cells perform several functions vital for brain health. The cerebral vasculature is responsible for protecting the brain from external threats carried in the blood. The endothelial cells are central to this requirement as they form the basis of the blood-brain barrier. The endothelium also regulates fibrinolysis, thrombosis, platelet activation, vascular permeability, metabolism, catabolism, inflammation, and white cell trafficking. Endothelial cells regulate the changes in vascular structure caused by angiogenesis and artery remodeling. Further, the endothelium contributes to vascular tone, allowing proper perfusion of the brain which has high energy demands and no energy stores. In this article, we discuss the basic anatomy and physiology of the cerebral endothelium. Where appropriate, we discuss the detrimental effects of high blood pressure on the cerebral endothelium and the contribution of cerebrovascular disease endothelial dysfunction and dementia. © 2022 American Physiological Society. Compr Physiol 12:3449-3508, 2022.

Inward Rectifier Potassium Channels: Membrane Lipid-Dependent Mechanosensitive Gates in Brain Vascular Cells

Cerebral arteries contain two primary and interacting cell types, smooth muscle (SMCs) and endothelial cells (ECs), which are each capable of sensing particular hemodynamic forces to set basal tone and brain perfusion. These biomechanical stimuli help confer tone within arterial networks upon which local neurovascular stimuli function. Tone development is intimately tied to arterial membrane potential (VM) and changes in intracellular [Ca2+] driven by voltage-gated Ca2+ channels (VGCCs). Arterial VM is in turn set by the dynamic interplay among ion channel species, the strongly inward rectifying K+ (Kir) channel being of special interest. Kir2 channels possess a unique biophysical signature in that they strongly rectify, display negative slope conductance, respond to elevated extracellular K+ and are blocked by micromolar Ba2+. While functional Kir2 channels are expressed in both smooth muscle and endothelium, they lack classic regulatory control, thus are often viewed as a simple background conductance. Recent literature has provided new insight, with two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol, noted to (1) stabilize Kir2 channels in a preferred open or closed state, respectively, and (2) confer, in association with the cytoskeleton, caveolin-1 (Cav1) and syntrophin, hemodynamic sensitivity. It is these aspects of vascular Kir2 channels that will be the primary focus of this review.

Mechanisms of Flow-Mediated Dilation of Pial Collaterals and the Effect of Hypertension

Leptomeningeal anastomoses are small distal anastomotic vessels also known as pial collaterals in the brain. These vessels redirect blood flow during an occlusion and are important for stroke treatment and outcome. Pial collaterals have unique hemodynamic forces and experience significantly increased luminal flow and shear stress after the onset of ischemic stroke. However, there is limited knowledge of how pial collaterals respond to flow and shear stress, and whether this response is altered in chronic hypertension. Using an in vitro system, pial collaterals from normotensive and hypertensive rats (n=6-8/group) were isolated and luminal flow was induced with intravascular pressure maintained at 40 mm Hg. Collateral lumen diameter was measured following each flow rate in the absence or presence of pharmacological inhibitors and activators. Collaterals from male and female Wistar rats dilated similarly to increased flow (2 µL/minute: 58.4±18.7% versus 67.9±7.4%; P=0.275), and this response was prevented by inhibition of the transient receptor potential vanilloid type 4 channel, as well as inhibitors of nitric oxide and intermediate-conductance calcium-activated potassium channels, suggesting shear stress-induced activation of this pathway was involved. However, the vasodilation was significantly impaired in hypertensive rats (2 µL/minute: 17.7±7.7%), which was restored by inhibitors of reactive oxygen species and mimicked by angiotensin II. Thus, flow- and shear stress-induced vasodilation of pial collaterals appears to be an important stimulus for increasing collateral flow during large vessel occlusion. Impairment of this response during chronic hypertension may be related to poorly engaged pial collaterals during ischemic stroke in hypertensive subjects.

Quantitative 3D analysis of tissue damage in a rat model of microembolization

There is a discrepancy between successful recanalization and good clinical outcome after endovascular treatment (EVT) in acute ischemic stroke patients. During removal of a thrombus, a shower of microemboli may release and lodge to the distal circulation. The objective of this study was to determine the extent of damage on brain tissue caused by microemboli. In a rat model of microembolization, a mixture of microsphere (MS) sizes (15, 25 and 50 µm diameter) was injected via the left internal carotid artery. A 3D image of the left hemisphere was reconstructed and a point-pattern spatial analysis was applied based on G- and K-functions to unravel the spatial correlation between MS and the induced hypoxia or infarction. We show a spatial correlation between MS and hypoxia or infarction spreading up to a distance of 1000-1500 µm. These results imply that microemboli, which individually may not always be harmful, can interact and result in local areas of hypoxia or even infarction when lodged in large numbers.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Alterations in arterial CO2 rather than pH affect the kinetics of neurovascular coupling in humans

KEY POINTS

We investigated the influence of arterial P C O 2 ( P aC O 2 ) with and without acute experimental metabolic alkalosis on neurovascular coupling (NVC). We assessed stepwise iso-oxic alterations in P aC O 2 prior to and following intravenous NaHCO₃ to acutely elevate arterial pH and [HCO₃ ^- ]. The NVC response was not altered following NaHCO₃ between stepwise P aC O 2 stages; therefore, NVC is acutely mediated by P aC O 2 rather than the prevailing arterial [H⁺ ]/pH. The NVC response was attenuated by 27-38% with -10 mmHg P aC O 2 and the absolute peak change was reduced by -19% with +10 mmHg P aC O 2 irrespective of acutely elevated arterial pH/[HCO₃ ^- ]. The NVC kinetics (i.e. time to peak) were markedly slower with hypercapnia versus hypocapnia (24 ± 5 vs. 7 ± 5 s, respectively) likely indicating an influence of resting cerebrovascular tone on NVC responsiveness.

ABSTRACT

Elevations in cerebral metabolism necessitate appropriate coordinated and localized increases in cerebral blood flow (i.e. neurovascular coupling; NVC). Recent pre-clinical work indicates that arterial P C O 2 ( P aC O 2 ) mediates NVC independently of arterial/extracellular pH; this has yet to be experimentally tested in humans. The goal of this study was to investigate the hypotheses that: (1) the NVC response would be unaffected by acute experimentally elevated arterial pH; rather, P aC O 2 would regulate any changes in NVC; and (2) stepwise respiratory alkalosis and acidosis would each progressively reduce the NVC response. Ten healthy males completed a standardized visual stimulus-evoked NVC test during matched stepwise iso-oxic alterations in P aC O 2 (hypocapnia: -5, -10 mmHg; hypercapnia: +5, +10 mmHg) prior to and following intravenous NaHCO₃ (8.4%, 50 mEq/50 ml) that elevated arterial pH (7.406 ± 0.019 vs. 7.457 ± 0.029; P < 0.001) and [HCO₃ ^- ] (26.2 ± 1.5 vs. 29.3 ± 0.9 mEq/l; P < 0.001). Although the NVC response was collectively attenuated by 27-38% with -10 mmHg P aC O 2 (stage post hoc: all P < 0.05), this response was unaltered following NaHCO₃ (all P > 0.05) irrespective of the higher pH (P = 0.002) at each matched stage of P aC O 2 (P = 0.417). The absolute peak change was reduced by -19 ± 41% with +10 mmHg P aC O 2 irrespective of acutely elevated arterial pH/[HCO₃ ^- ] (stage post hoc: P = 0.022). The NVC kinetics (i.e. time to peak) were markedly slower with hypercapnia versus hypocapnia (24 ± 5 vs. 7 ± 5 s, respectively; stage effect: P < 0.001). Overall, these findings indicate that temporal patterns in NVC are acutely regulated by P aC O 2 rather than arterial pH per se in the setting of acute metabolic alkalosis in humans.

Emergent dynamics in excitable flow systems

Flow networks can describe many natural and artificial systems. We present a model for a flow system that allows for volume accumulation, includes conduits with a nonlinear relation between current and pressure difference, and can be applied to networks of arbitrary topology. The model displays complex dynamics, including self-sustained oscillations in the absence of any dynamics in the inputs and outputs. In this work we analytically show the origin of self-sustained oscillations for the one-dimensional case. We numerically study the behavior of systems of arbitrary topology under different conditions: we discuss their excitability, the effect of different boundary conditions, and wave propagation when the network has regions of conduits with linear conductance.

Recovery of Hypoxic Regions in a Rat Model of Microembolism

OBJECTIVES

Endovascular treatment (EVT) has become the standard of care for acute ischemic stroke. Despite successful recanalization, a limited subset of patients benefits from the new treatment. Human MRI studies have shown that during removal of the thrombus, a shower of microclots is released from the initial thrombus, possibly causing new ischemic lesions. The aim of the current study is to quantify tissue damage following microembolism.

MATERIALS AND METHODS

In a rat model, microembolism was generated by injection of a mixture of polystyrene fluorescent microspheres (15, 25 and 50 µm in diameter). The animals were killed at three time-points: day 1, 3 or 7. AMIRA and IMARIS software was used for 3D reconstruction of brain structure and damage, respectively.

CONCLUSIONS

Microembolism induces ischemia, hypoxia and infarction. Infarcted areas persist, but hypoxic regions recover over time suggesting that repair processes in the brain rescue the regions at risk.

Cerebral blood flow, cerebrovascular reactivity and their influence on ventilatory sensitivity

NEW FINDINGS

What is the topic of this review? Cerebrovascular reactivity to CO₂ , which is a principal factor in determining ventilatory responses to CO₂ through the role reactivity plays in determining cerebral extra- and intracellular pH. What advances does it highlight? Recent animal evidence suggests central chemoreceptor vasculature may demonstrate regionally heterogeneous cerebrovascular reactivity to CO₂ , potentially as a protective mechanism against excessive CO₂ washout from the central chemoreceptors, thereby allowing ventilation to reflect the systemic acid-base balance needs (respiratory changes in P aC O 2 ) rather than solely the cerebral needs. Ventilation per se does not influence cerebrovascular reactivity independent of changes in P aC O 2 .

ABSTRACT

Alveolar ventilation and cerebral blood flow are both predominantly regulated by arterial blood gases, especially arterial P C O 2 , and so are intricately entwined. In this review, the fundamental mechanisms underlying cerebrovascular reactivity and central chemoreceptor control of breathing are covered. We discuss the interaction of cerebral blood flow and its reactivity with the control of ventilation and ventilatory responsiveness to changes in P C O 2 , as well as the lack of influence of ventilation itself on cerebrovascular reactivity. We briefly summarize the effects of arterial hypoxaemia on the relationship between ventilatory and cerebrovascular response to both P C O 2 and P O 2 . We then highlight key methodological considerations regarding the interaction of reactivity and ventilatory sensitivity, including the following: regional heterogeneity of cerebrovascular reactivity; a pharmacological approach for the reduction of cerebral blood flow; reactivity assessment techniques; the influence of mean arterial blood pressure; and sex-related differences. Finally, we discuss ventilatory and cerebrovascular control in the context of high altitude and congestive heart failure. Future research directions and pertinent questions of interest are highlighted throughout.

A Brief Review of Brain's Blood Flow-Metabolism Coupling and Pressure Autoregulation

BACKGROUND

 The human brain, depending on aerobic glycolysis to cover its metabolic needs and having no energy reserves whatsoever, relies on a constant and closely regulated blood supply to maintain its structural and functional integrity. Cerebral autoregulation, that is, the brain's intrinsic ability to regulate its own blood flow independently from the systemic blood pressure and cardiac output, is an important physiological mechanism that offers protection from hypoperfusion injury.

DISCUSSION

 Two major independent mechanisms are known to be involved in cerebral autoregulation: (1) flow-metabolism coupling and (2) myogenic responses of cerebral blood vessels to changes in transmural/arterial pressure. A third, less prominent component of cerebral autoregulation comes in the form of neurogenic influences on cerebral vasculature.

CONCLUSION

 Although fragmentation of cerebral autoregulation in separate and distinct from each other mechanisms is somewhat arbitrary, such a scheme is useful for reasons of simplification and to better understand their overall effect. Comprehension of cerebral autoregulation is imperative for clinicians in order for them to mitigate consequences of its impairment in the context of traumatic brain injury, subarachnoid hemorrhage, stroke, or other pathological conditions.

A blood-brain barrier overview on structure, function, impairment, and biomarkers of integrity

The blood–brain barrier is playing a critical role in controlling the influx and efflux of biological substances essential for the brain’s metabolic activity as well as neuronal function. Thus, the functional and structural integrity of the BBB is pivotal to maintain the homeostasis of the brain microenvironment. The different cells and structures contributing to developing this barrier are summarized along with the different functions that BBB plays at the brain–blood interface. We also explained the role of shear stress in maintaining BBB integrity. Furthermore, we elaborated on the clinical aspects that correlate between BBB disruption and different neurological and pathological conditions. Finally, we discussed several biomarkers that can help to assess the BBB permeability and integrity in-vitro or in-vivo and briefly explain their advantages and disadvantages.

Compromised regulation of the rat brain parenchymal arterioles in vasopressin-associated acute hyponatremia

OBJECTIVE

In this study, we examined the effect of acute hyponatremia associated with vasopressin (AVP) on the responses of the isolated rat's MCAs and PAs to acidosis, nitric oxide donor (SNAP) and to endothelium-dependent vasodilator ATP.

METHODS

The studies were performed on isolated, perfused and pressurized MCAs and PAs in control conditions and during AVP-associated hyponatremia. Hyponatremia was induced in vitro by lowering Na⁺ concentration from 144 to 121 mmol/L in intra- and extravascular fluid in the presence of AVP.

RESULTS

Parenchymal arterioles showed greater response to an increase in H⁺ and K⁺ ions concentration and to ATP in comparison with MCAs in control normonatremic conditions. Both PAs and MCAs constricted in response to acute hyponatremia associated with AVP. Interestingly, disordered regulation of vascular tone was observed in PAs but not in MCAs. The abnormalities in the regulation comprised a significant reduction of PA response to acidosis and the absence of the response to the administration of SNAP or ATP.

CONCLUSIONS

Arginine vasopressin-associated hyponatremia leads to constriction and dysregulation of PAs which may impair neurovascular coupling.

Deconstructing the principles of ductal network formation in the pancreas

The mammalian pancreas is a branched organ that does not exhibit stereotypic branching patterns, similarly to most other glands. Inside branches, it contains a network of ducts that undergo a transition from unconnected microlumen to a mesh of interconnected ducts and finally to a treelike structure. This ductal remodeling is poorly understood, both on a microscopic and macroscopic level. In this article, we quantify the network properties at different developmental stages. We find that the pancreatic network exhibits stereotypic traits at each stage and that the network properties change with time toward the most economical and optimized delivery of exocrine products into the duodenum. Using in silico modeling, we show how steps of pancreatic network development can be deconstructed into two simple rules likely to be conserved for many other glands. The early stage of the network is explained by noisy, redundant duct connection as new microlumens form. The later transition is attributed to pruning of the network based on the flux of fluid running through the pancreatic network into the duodenum.

In the pancreas of mammals, digestive enzymes are transported from their production site in acini (clusters of cells that secrete the enzymes) to the intestine via a network of ducts. During organ development in fetuses, the ducts initially form by the coordinated polarization of cells to form small holes, which will connect and fuse, to constitute a meshwork. This hyperconnected network further develops into a treelike structure by the time of birth. In this article, we use methods originally developed to analyze road, rail, web, or river networks to quantify the network properties at different developmental stages. We find that the pancreatic network properties are similar between individuals at specific time points but eventually change to achieve the most economical and optimized structure to deliver pancreatic juice into the duodenum. Using in silico modeling, we show how the stages of pancreatic network development follow two simple rules, which are likely to be conserved for the development of other glands. The early stage of the network is explained by noisy, redundant duct connection as new small ductal holes form. Later on, the secretion of fluid that runs through the pancreatic network into the duodenum leads to the widening of ducts with the greatest flow, while nonnecessary ducts are eliminated, akin to how river beds are formed.

The Role of Endothelial Ca2+ Signaling in Neurovascular Coupling: A View from the Lumen

BACKGROUND

Neurovascular coupling (NVC) is the mechanism whereby an increase in neuronal activity (NA) leads to local elevation in cerebral blood flow (CBF) to match the metabolic requirements of firing neurons. Following synaptic activity, an increase in neuronal and/or astrocyte Ca2+ concentration leads to the synthesis of multiple vasoactive messengers. Curiously, the role of endothelial Ca2+ signaling in NVC has been rather neglected, although endothelial cells are known to control the vascular tone in a Ca2+-dependent manner throughout peripheral vasculature.

METHODS

We analyzed the literature in search of the most recent updates on the potential role of endothelial Ca2+ signaling in NVC.

RESULTS

We found that several neurotransmitters (i.e., glutamate and acetylcholine) and neuromodulators (e.g., ATP) can induce dilation of cerebral vessels by inducing an increase in endothelial Ca2+ concentration. This, in turn, results in nitric oxide or prostaglandin E2 release or activate intermediate and small-conductance Ca2+-activated K⁺ channels, which are responsible for endothelial-dependent hyperpolarization (EDH). In addition, brain endothelial cells express multiple transient receptor potential (TRP) channels (i.e., TRPC3, TRPV3, TRPV4, TRPA1), which induce vasodilation by activating EDH.

CONCLUSIONS

It is possible to conclude that endothelial Ca2+ signaling is an emerging pathway in the control of NVC.

Pulse pressure-dependent cerebrovascular eNOS regulation in mice

Arterial blood pressure is oscillatory; whether pulse pressure (PP) regulates cerebral artery myogenic tone (MT) and endothelial function is currently unknown. To test the impact of PP on MT and dilation to flow (FMD) or to acetylcholine (Ach), isolated pressurized mouse posterior cerebral arteries were subjected to either static pressure (SP) or a physiological PP (amplitude: 30 mm Hg; frequency: 550 bpm). Under PP, MT was significantly higher than in SP conditions ( p < 0.05) and was not affected by eNOS inhibition. In contrast, under SP, eNOS inhibition increased ( p < 0.05) MT to levels observed under PP, suggesting that PP may inhibit eNOS. At a shear stress of 20 dyn/cm², FMD was lower ( p < 0.05) under SP than PP. Under SP, eNOS-dependent [Formula: see text] production contributed to FMD, while under PP, eNOS-dependent NO was responsible for FMD, indicating that PP favours eNOS coupling. Differences in FMD between pressure conditions were abolished after NOX2 inhibition. In contrast to FMD, Ach-induced dilations were higher ( p < 0.05) under SP than PP. Reactive oxygen species scavenging reduced ( p < 0.05) Ach-dependent dilations under SP, but increased ( p < 0.05) them under PP; hence, under PP, Ach promotes ROS production and limits eNOS-derived NO activity. In conclusion, PP finely regulates eNOS, controlling cerebral artery reactivity.

Vasculo-Neuronal Coupling: Retrograde Vascular Communication to Brain Neurons

Continuous cerebral blood flow is essential for neuronal survival, but whether vascular tone influences resting neuronal function is not known. Using a multidisciplinary approach in both rat and mice brain slices, we determined whether flow/pressure-evoked increases or decreases in parenchymal arteriole vascular tone, which result in arteriole constriction and dilation, respectively, altered resting cortical pyramidal neuron activity. We present evidence for intercellular communication in the brain involving a flow of information from vessel to astrocyte to neuron, a direction opposite to that of classic neurovascular coupling and referred to here as vasculo-neuronal coupling (VNC). Flow/pressure increases within parenchymal arterioles increased vascular tone and simultaneously decreased resting pyramidal neuron firing activity. On the other hand, flow/pressure decreases evoke parenchymal arteriole dilation and increased resting pyramidal neuron firing activity. In GLAST-CreERT2; R26-lsl-GCaMP3 mice, we demonstrate that increased parenchymal arteriole tone significantly increased intracellular calcium in perivascular astrocyte processes, the onset of astrocyte calcium changes preceded the inhibition of cortical pyramidal neuronal firing activity. During increases in parenchymal arteriole tone, the pyramidal neuron response was unaffected by blockers of nitric oxide, GABA_A, glutamate, or ecto-ATPase. However, VNC was abrogated by TRPV4 channel, GABA_B, as well as an adenosine A₁ receptor blocker. Differently to pyramidal neuron responses, increases in flow/pressure within parenchymal arterioles increased the firing activity of a subtype of interneuron. Together, these data suggest that VNC is a complex constitutive active process that enables neurons to efficiently adjust their resting activity according to brain perfusion levels, thus safeguarding cellular homeostasis by preventing mismatches between energy supply and demand.

SIGNIFICANCE STATEMENT

We present evidence for vessel-to-neuron communication in the brain slice defined here as vasculo-neuronal coupling. We showed that, in response to increases in parenchymal arteriole tone, astrocyte intracellular Ca²⁺ increased and cortical neuronal activity decreased. On the other hand, decreasing parenchymal arteriole tone increased resting cortical pyramidal neuron activity. Vasculo-neuronal coupling was partly mediated by TRPV4 channels as genetic ablation, or pharmacological blockade impaired increased flow/pressure-evoked neuronal inhibition. Increased flow/pressure-evoked neuronal inhibition was blocked in the presence of adenosine A1 receptor and GABA_B receptor blockade. Results provide evidence for the concept of vasculo-neuronal coupling and highlight the importance of understanding the interplay between basal CBF and resting neuronal activity.

Glioblastoma Induces Vascular Dysregulation in Nonenhancing Peritumoral Regions in Humans

OBJECTIVE

Glioblastoma is an invasive primary brain malignancy that typically infiltrates the surrounding tissue with malignant cells. It disrupts cerebral blood flow through a variety of biomechanical and biochemical mechanisms. Thus, neuroimaging focused on identifying regions of vascular dysregulation may reveal a marker of tumor spread. The purpose of this study was to use blood oxygenation level-dependent (BOLD) functional MRI (fMRI) to compare the temporal dynamics of the enhancing portion of a tumor with those of brain regions without apparent tumors.

MATERIALS AND METHODS

Patients with pathologically proven glioblastoma underwent preoperative resting-state BOLD fMRI, T1-weighted contrast-enhanced MRI, and FLAIR MRI. The contralesional control hemisphere, contrast-enhancing tumor, and peritu-moral edema were segmented by use of structural images and were used to extract the time series of these respective regions. The parameter estimates (beta values) for the two regressors and resulting z-statistic images were used as a metric to compare the similarity of the tumor dynamics to those of other brain regions.

RESULTS

The time course of the contrast-enhancing tumor was significantly different from that of the rest of the brain (p < 0.05). Similarly, the control signal intensity was significantly different from the tumor signal intensity (p < 0.05). Notably, the temporal dynamics in the peritumoral edema, which did not contain enhancing tumor, were most similar to the those of enhancing tumor than to those of control regions.

CONCLUSION

The findings show that the disruption in vascular regulation induced by a glioblastoma can be detected with BOLD fMRI and that the spatial distribution of these disruptions is localized to the immediate vicinity of the tumor and peritumoral edema.

Nitric oxide and the cardiovascular system

Nitric oxide (NO) generated by endothelial cells to relax vascular smooth muscle is one of the most intensely studied molecules in the past 25 years. Much of what is known about NO regulation of NO is based on blockade of its generation and analysis of changes in vascular regulation. This approach has been useful to demonstrate the importance of NO in large scale forms of regulation but provides less information on the nuances of NO regulation. However, there is a growing body of studies on multiple types of in vivo measurement of NO in normal and pathological conditions. This discussion will focus on in vivo studies and how they are reshaping the understanding of NO's role in vascular resistance regulation and the pathologies of hypertension and diabetes mellitus. The role of microelectrode measurements in the measurement of [NO] will be considered because much of the controversy about what NO does and at what concentration depends upon the measurement methodology. For those studies where the technology has been tested and found to be well founded, the concept evolving is that the stresses imposed on the vasculature in the form of flow-mediated stimulation, chemicals within the tissue, and oxygen tension can cause rapid and large changes in the NO concentration to affect vascular regulation. All these functions are compromised in both animal and human forms of hypertension and diabetes mellitus due to altered regulation of endothelial cells and formation of oxidants that both damage endothelial cells and change the regulation of endothelial nitric oxide synthase.

Methamphetamine induces the release of endothelin

Methamphetamine is a potent psychostimulant drug of abuse that increases release and blocks reuptake of dopamine, producing intense euphoria, factors that may contribute to its widespread abuse. It also produces severe neurotoxicity resulting from oxidative stress, DNA damage, blood-brain barrier disruption, microgliosis, and mitochondrial dysfunction. Intracerebral hemorrhagic and ischemic stroke have been reported after intravenous and oral abuse of methamphetamine. Several studies have shown that methamphetamine causes vasoconstriction of vessels. This study investigates the effect of methamphetamine on endothelin-1 (ET-1) release in mouse brain endothelial cells by ELISA. ET-1 transcription as well as endothelial nitric oxide synthase (eNOS) activation and transcription were measured following methamphetamine treatment. We also examine the effect of methamphetamine on isolated cerebral arteriolar vessels from C57BL/6 mice. Penetrating middle cerebral arterioles were cannulated at both ends with a micropipette system. Methamphetamine was applied extraluminally, and the vascular response was investigated. Methamphetamine treatment of mouse brain endothelial cells resulted in ET-1 release and a transient increase in ET-1 message. The activity and transcription of eNOS were only slightly enhanced after 24 hr of treatment with methamphetamine. In addition, methamphetamine caused significant vasoconstriction of isolated mouse intracerebral arterioles. The vasoconstrictive effect of methamphetamine was attenuated by coapplication of the endothelin receptor antagonist PD145065. These findings suggest that vasoconstriction induced by methamphetamine is mediated through the endothelin receptor and may involve an endothelin-dependent pathway.

A critical role for the vascular endothelium in functional neurovascular coupling in the brain

Background

The functional modulation of blood flow in the brain is critical for brain health and is the basis of contrast in functional magnetic resonance imaging. There is evident coupling between increases in neuronal activity and increases in local blood flow; however, many aspects of this neurovascular coupling remain unexplained by current models. Based on the rapid dilation of distant pial arteries during cortical functional hyperemia, we hypothesized that endothelial signaling may play a key role in the long‐range propagation of vasodilation during functional hyperemia in the brain. Although well characterized in the peripheral vasculature, endothelial involvement in functional neurovascular coupling has not been demonstrated.

Methods and Results

We combined in vivo exposed‐cortex multispectral optical intrinsic signal imaging (MS‐OISI) with a novel in vivo implementation of the light‐dye technique to record the cortical hemodynamic response to somatosensory stimulus in rats before and after spatially selective endothelial disruption. We demonstrate that discrete interruption of endothelial signaling halts propagation of stimulus‐evoked vasodilation in pial arteries, and that wide‐field endothelial disruption in pial arteries significantly attenuates the hemodynamic response to stimulus, particularly the early, rapid increase and peak in hyperemia.

Conclusions

Involvement of endothelial pathways in functional neurovascular coupling provides new explanations for the spatial and temporal features of the hemodynamic response to stimulus and could explain previous results that were interpreted as evidence for astrocyte‐mediated control of functional hyperemia. Our results unify many aspects of blood flow regulation in the brain and body and prompt new investigation of direct links between systemic cardiovascular disease and neural deficits.

Potassium buffering in the neurovascular unit: models and sensitivity analysis

Astrocytes are critical regulators of neural and neurovascular network communication. Potassium transport is a central mechanism behind their many functions. Astrocytes encircle synapses with their distal processes, which express two potassium pumps (Na-K and NKCC) and an inward rectifying potassium channel (Kir), whereas the vessel-adjacent endfeet express Kir and BK potassium channels. We provide a detailed model of potassium flow throughout the neurovascular unit (synaptic region, astrocytes, and arteriole) for the cortex of the young brain. Our model reproduces several phenomena observed experimentally: functional hyperemia, in which neural activity triggers astrocytic potassium release at the perivascular endfoot, inducing arteriole dilation; K(+) undershoot in the synaptic space after periods of neural activity; neurally induced astrocyte hyperpolarization during Kir blockade. Our results suggest that the dynamics of the vascular response during functional hyperemia are governed by astrocytic Kir for the fast onset and astrocytic BK for maintaining dilation. The model supports the hypothesis that K(+) undershoot is caused by excessive astrocytic uptake through Na-K and NKCC pumps, whereas the effect is balanced by Kir. We address parametric uncertainty using high-dimensional stochastic sensitivity analysis and identify possible model limitations.

Model-based physiomarkers of cerebral hemodynamics in patients with mild cognitive impairment

In our previous studies, we have introduced model-based "functional biomarkers" or "physiomarkers" of cerebral hemodynamics that hold promise for improved diagnosis of early-stage Alzheimer's disease (AD). The advocated methodology utilizes subject-specific data-based dynamic nonlinear models of cerebral hemodynamics to compute indices (serving as possible diagnostic physiomarkers) that quantify the state of cerebral blood flow autoregulation to pressure-changes (CFAP) and cerebral CO2 vasomotor reactivity (CVMR) in each subject. The model is estimated from beat-to-beat measurements of mean arterial blood pressure, mean cerebral blood flow velocity and end-tidal CO2, which can be made reliably and non-invasively under resting conditions. In a previous study, it was found that a CVMR index quantifying the impairment in CO2 vasomotor reactivity correlates with clinical indications of early AD, offering the prospect of a potentially useful diagnostic tool. In this paper, we explore the use of the same model-based indices for patients with amnestic Mild Cognitive Impairment (MCI), a preclinical stage of AD, relative to a control subjects and clinical cognitive assessments. It was found that the model-based CVMR values were lower for MCI patients relative to the control subjects.

The neurovascular unit - concept review

The cerebral hyperaemia is one of the fundamental mechanisms for the central nervous system homeostasis. Due also to this mechanism, oxygen and nutrients are maintained in satisfactory levels, through vasodilation and vasoconstriction. The brain hyperaemia, or coupling, is accomplished by a group of cells, closely related to each other; called neurovascular unit (NVU). The neurovascular unit is composed by neurones, astrocytes, endothelial cells of blood-brain barrier (BBB), myocytes, pericytes and extracellular matrix components. These cells, through their intimate anatomical and chemical relationship, detect the needs of neuronal supply and trigger necessary responses (vasodilation or vasoconstriction) for such demands. Here, we review the concepts of NVU, the coupling mechanisms and research strategies.

Brain ischemia in patients with intracranial hemorrhage: pathophysiological reasoning for aggressive diagnostic management

Patients with intracranial hemorrhage have to be managed aggressively to avoid or minimize secondary brain damage due to ischemia, which contributes to high morbidity and mortality. The risk of brain ischemia, however, is not the same in every patient. The risk of complications associated with an aggressive prophylactic therapy in patients with a low risk of brain ischemia can outweigh the benefits of therapy. Accurate and timely identification of patients at highest risk is a diagnostic challenge. Despite the availability of many diagnostic tools, stroke is common in this population, mostly because the pathogenesis of stroke is frequently multifactorial whereas diagnosticians tend to focus on one or two risk factors. The pathophysiological mechanisms of brain ischemia in patients with intracranial hemorrhage are not yet fully elucidated and there are several important areas of ongoing research. Therefore, this review describes physiological and pathophysiological aspects associated with the development of brain ischemia such as the mechanism of oxygen and carbon dioxide effects on the cerebrovascular system, neurovascular coupling and respiratory and cardiovascular factors influencing cerebral hemodynamics. Consequently, we review investigations of cerebral blood flow disturbances relevant to various hemodynamic states associated with high intracranial pressure, cerebral embolism, and cerebral vasospasm along with current treatment options.

Diversity in mechanisms of endothelium-dependent vasodilation in health and disease

Small arterioles (40-150 μm) contribute to the majority of vascular resistance within organs and tissues. Under resting conditions, the basal tone of these vessels is determined by a delicate balance between vasodilator and vasoconstrictor influences. Cardiovascular homeostasis and regional tissue perfusion is largely a function of the ability of these small blood vessels to constrict or dilate in response to the changing metabolic demands of specific tissues. The endothelial cell layer of these microvessels is a key modulator of vasodilation through the synthesis and release of vasoactive substances. Beyond their vasomotor properties, these compounds importantly modulate vascular cell proliferation, inflammation, and thrombosis. Thus, the balance between local regulation of vascular tone and vascular pathophysiology can vary depending upon which factors are released from the endothelium. This review will focus on the dynamic nature of the endothelial released dilator factors depending on species, anatomic site, and presence of disease, with a focus on the human coronary microcirculation. Knowledge how endothelial signaling changes with disease may provide insights into the early stages of developing vascular inflammation and atherosclerosis, or related vascular pathologies.

Astrocyte regulation of cerebral vascular tone

Cerebral blood flow is controlled by two crucial processes, cerebral autoregulation (CA) and neurovascular coupling (NVC) or functional hyperemia. Whereas CA ensures constant blood flow over a wide range of systemic pressures, NVC ensures rapid spatial and temporal increases in cerebral blood flow in response to neuronal activation. The focus of this review is to discuss the cellular mechanisms by which astrocytes contribute to the regulation of vascular tone in terms of their participation in NVC and, to a lesser extent, CA. We discuss evidence for the various signaling modalities by which astrocytic activation leads to vasodilation and vasoconstriction of parenchymal arterioles. Moreover, we provide a rationale for the contribution of astrocytes to pressure-induced increases in vascular tone via the vasoconstrictor 20-HETE (a downstream metabolite of arachidonic acid). Along these lines, we highlight the importance of the transient receptor potential channel of the vanilloid family (TRPV4) as a key molecular determinant in the regulation of vascular tone in cerebral arterioles. Finally, we discuss current advances in the technical tools available to study NVC mechanisms in the brain as it relates to the participation of astrocytes.

Endothelium-dependent control of cerebrovascular functions through age: exercise for healthy cerebrovascular aging

Cognitive performances are tightly associated with the maximal aerobic exercise capacity, both of which decline with age. The benefits on mental health of regular exercise, which slows the age-dependent decline in maximal aerobic exercise capacity, have been established for centuries. In addition, the maintenance of an optimal cerebrovascular endothelial function through regular exercise, part of a healthy lifestyle, emerges as one of the key and primary elements of successful brain aging. Physical exercise requires the activation of specific brain areas that trigger a local increase in cerebral blood flow to match neuronal metabolic needs. In this review, we propose three ways by which exercise could maintain the cerebrovascular endothelial function, a premise to a healthy cerebrovascular function and an optimal regulation of cerebral blood flow. First, exercise increases blood flow locally and increases shear stress temporarily, a known stimulus for endothelial cell maintenance of Akt-dependent expression of endothelial nitric oxide synthase, nitric oxide generation, and the expression of antioxidant defenses. Second, the rise in circulating catecholamines during exercise not only facilitates adequate blood and nutrient delivery by stimulating heart function and mobilizing energy supplies but also enhances endothelial repair mechanisms and angiogenesis. Third, in the long term, regular exercise sustains a low resting heart rate that reduces the mechanical stress imposed to the endothelium of cerebral arteries by the cardiac cycle. Any chronic variation from a healthy environment will perturb metabolism and thus hasten endothelial damage, favoring hypoperfusion and neuronal stress.

Assessment of leptomeningeal collaterals using dynamic CT angiography in patients with acute ischemic stroke

Whole-brain dynamic time-resolved computed tomography angiography (CTA) is a technique developed on the new 320-detector row CT scanner capable of generating time-resolved cerebral angiograms from skull base to vertex. Unlike a conventional cerebral angiogram, this technique visualizes pial arterial filling in all vascular territories, thereby providing additional hemodynamic information. Ours was a retrospective study of consecutive patients with ischemic stroke and M1 middle cerebral artery +/- intracranial internal carotid artery occlusions presenting to our center from June 2010 and undergoing dynamic time-resolved CTA and perfusion CT within 6 hours of symptom onset. Leptomeningeal collateral status was assessed by determining relative prominence of pial arteries in the ischemic region, rate and extent of retrograde flow, and various topographical patterns of pial arterial filling. Twenty-five patients were included in the study. We demonstrate the existence of the following novel properties of leptomeningeal collaterals in humans: (a) posterior (posterior cerebral artery (PCA)-MCA) dominant collateralization, (b) intra-territorial 'within MCA region' leptomeningeal collaterals, and (c) significant variability in size, extent, and retrograde filling time in pial arteries. We also describe a simple and reliable collateral grading template that, for the first time on dynamic CTA, incorporates back-filling time as well as size and extent of collateral filling.

Role of dual-specificity protein phosphatase-5 in modulating the myogenic response in rat cerebral arteries

The present study examined the role of the dual-specificity protein phosphatase-5 (DUSP-5) in the pressure-induced myogenic responses of organ-cultured cerebral arterial segments. In these studies, we initially compared freshly isolated and organ-cultured cerebral arterial segments with respect to responses to step increases in intravascular pressure, vasodilator and vasoconstrictor stimuli, activities of the large-conductance arterial Ca(2+)-activated K(+) (K(Ca)) single-channel current, and stable protein expression of DUSP-5 enzyme. The results demonstrate maintained pressure-dependent myogenic vasoconstriction, DUSP-5 protein expression, endothelium-dependent and -independent dilations, agonist-induced constriction, and unitary K(Ca) channel conductance in organ-cultured cerebral arterial segments similar to that in freshly isolated cerebral arteries. Furthermore, using a permeabilization transfection technique in organ-cultured cerebral arterial segments, gene-specific small interfering RNA (siRNA) induced knockdown of DUSP-5 mRNA and protein, which were associated with enhanced pressure-dependent cerebral arterial myogenic constriction and increased phosphorylation of PKC-βII. In addition, siRNA knockdown of DUSP-5 reduced levels of phosphorylated ROCK and ERK1 with no change in the level of phosphorylated ERK2. Pharmacological inhibition of ERK1/2 phosphorylation significantly attenuated pressure-induced myogenic constriction in cerebral arteries. The findings within the present studies illustrate that DUSP-5, native in cerebral arterial muscle cells, appears to regulate signaling of pressure-dependent myogenic cerebral arterial constriction, which is crucial for the maintenance of constant cerebral blood flow to the brain.

A bidirectional model for communication in the neurovascular unit

The neurovascular unit is a coordinated and interactional system of neurons, astrocytes, and microvessels in the brain. A central autoregulation mechanism observed in this unit is functional hyperemia, in which the microvasculature dilates in response to local neural activity in order to meet the increased demand for blood flow and oxygen. We have developed the first interactional model of bidirectional signaling in the neurovascular unit. The vascular model includes a description of vasomotion, the vascular oscillatory response to transmural pressure, observed in vivo. The communication mechanisms in the model include neural synaptic glutamate and potassium signaling to the astrocytes, potassium signaling from the astrocyte to the microvasculature, and astrocytic mechanosensation of vascular changes. The model response of the astrocyte to arteriolar dilation is validated with recent in vivo and in vitro experimental results. The model reproduces for the first time the in vitro observed phenomenon in which arteriole radius and Ca(2+) oscillations, "vasomotion," are damped due to neural induced astrocytic signaling.

A lumped parameter model of cerebral blood flow control combining cerebral autoregulation and neurovascular coupling

Cerebral blood flow regulation is based on a variety of different mechanisms, of which the relative regulatory role remains largely unknown. The cerebral regulatory system expresses two regulatory properties: cerebral autoregulation and neurovascular coupling. Since partly the same mechanisms play a role in cerebral autoregulation and neurovascular coupling, this study aimed to develop a physiologically based mathematical model of cerebral blood flow regulation combining these properties. A lumped parameter model of the P2 segment of the posterior cerebral artery and its distal vessels was constructed. Blood flow regulation is exerted at the arteriolar level by vascular smooth muscle and implements myogenic, shear stress based, neurogenic, and metabolic mechanisms. In eight healthy subjects, cerebral autoregulation and neurovascular coupling were challenged by squat-stand maneuvers and visual stimulation using a checkerboard pattern, respectively. Cerebral blood flow velocity was measured using transcranial Doppler, whereas blood pressure was measured by finger volume clamping. In seven subjects, the model proposed fits autoregulation and neurovascular coupling measurement data well. Myogenic regulation is found to dominate the autoregulatory response. Neurogenic regulation, although only implemented as a first-order mechanism, describes neurovascular coupling responses to a great extent. It is concluded that our single, integrated model of cerebral blood flow control may be used to identify the main mechanisms affecting cerebral blood flow regulation in individual subjects.

Contribution of flow-dependent vasomotor mechanisms to the autoregulation of cerebral blood flow

Regulation of cerebral blood flow (CBF) is the result of multilevel mechanisms to maintain the appropriate blood supply to the brain while having to comply with the limited space available in the cranium. The latter requirement is ensured by the autoregulation of CBF, in which the pressure-sensitive myogenic response is known to play a pivotal role. However, in vivo increases in pressure are accompanied by increases in flow; yet the effects of flow on the vasomotor tone of cerebral vessels are less known. Earlier studies showed flow-sensitive dilation and/or constriction or both, but no clear picture emerged. Recently, the important role of flow-sensitive mechanism(s) eliciting the constriction of cerebral vessels has been demonstrated. This review focuses on the effect of hemodynamic forces (especially intraluminal flow) on the vasomotor tone of cerebral vessels and the underlying cellular and molecular mechanisms. A novel concept of autoregulation of CBF is proposed, suggesting that (in certain areas of the cerebrovascular tree) pressure- and flow-induced constrictions together maintain an effective autoregulation, and that alterations in these mechanisms may contribute to the development of cerebrovascular disorders. Future studies are warranted to explore the signals, the details of signaling processes and the in vivo importance of these mechanisms.

Advanced in vitro approach to study neurovascular coupling mechanisms in the brain microcirculation

An understanding of the signalling events underlying neurovascular coupling mechanisms in the brain is a crucial step in the development of novel therapeutic approaches for the treatment of cerebrovascular-associated disorders. In this study we present an enhanced in vitro brain slice preparation from male Wistar rat cortical slices that incorporates haemodynamic variables (flow and pressure) into parenchymal arterioles resulting in the development of myogenic tone (28% from maximum dilatation). Moreover, we characterized flow-induced vascular responses, resulting in various degrees of vasoconstrictions and the response to 10 mM K(+) or astrocytic activation with the mGluR agonist, t-ACPD (100 μM), resulting in vasodilatations of 33.6±4.7% and 38.6±4.6%, respectively. Using fluorescence recovery, we determined perfusate velocity to calculate diameter changes under different experimental pH conditions. Using this approach, we demonstrate no significant differences between diameter changes measured using videomicroscopy or predicted from the velocity values obtained using fluorescence recovery after photobleaching. The model is further validated by demonstrating our ability to cannulate arterioles in two brain regions (cortex and supraoptic nucleus of the hypothalamus). Altogether, we believe this is the first study demonstrating successful cannulation and perfusion of parenchymal arterioles while monitoring/estimating luminal diameter and pressure under conditions where flow rates are controlled.

Isolated human and rat cerebral arteries constrict to increases in flow: role of 20-HETE and TP receptors

Elevation of intraluminal pressure increases vasomotor tone, which thought to have a substantial role in regulation of cerebral blood flow (CBF). Interestingly, responses of cerebral vessels to increases in flow varied and have not been studied in human cerebral arteries. We hypothesized that increases in flow elicit constrictions of isolated human and rat cerebral arteries and aimed to elucidate the underlying mechanisms. Human cerebral arteries and rat middle cerebral arteries constricted to increases in flow (P<0.05). Simultaneous increase in intraluminal flow+pressure further reduced the diameter compared with pressure-induced changes (P<0.05), leading to constant estimated CBF. Flow-induced constrictions were abolished by HET0016 (inhibitor of synthesis of 20-hydroxyeicosatetraenoic acid (20-HETE) or inhibition of COXs or blocking TP (thromboxane A(2)/prostaglandin H(2), receptors and attenuated by scavenging reactive oxygen species (ROS). Flow-enhanced ROS formation was significantly reduced by HET0016. In conclusion, in human and rat cerebral arteries (1) increases in flow elicit constrictions, (2) signaling mechanism of flow-induced constriction of cerebral arteries involves enhanced production of ROS, COX activity, and mediated by 20-HETE via TP receptors, and (3) we propose that simultaneous operation of pressure- and flow-induced constrictions is necessary to provide an effective autoregulation of CBF.

Protecting against vascular disease in brain

Endothelial cells exert an enormous influence on blood vessels throughout the circulation, but their impact is particularly pronounced in the brain. New concepts have emerged recently regarding the role of this cell type and mechanisms that contribute to endothelial dysfunction and vascular disease. Activation of the renin-angiotensin system plays a prominent role in producing these abnormalities. Both oxidative stress and local inflammation are key mechanisms that underlie vascular disease of diverse etiology. Endogenous mechanisms of vascular protection are also present, including antioxidants, anti-inflammatory molecules, and peroxisome proliferator-activated receptor-γ. Despite their clear importance, studies of mechanisms that underlie cerebrovascular disease continue to lag behind studies of vascular biology in general. Identification of endogenous molecules and pathways that protect the vasculature may result in targeted approaches to prevent or slow the progression of vascular disease that causes stroke and contributes to the vascular component of dementia and Alzheimer's disease.

Integration of in silico and in vitro platforms for pharmacokinetic-pharmacodynamic modeling

IMPORTANCE OF THE FIELD

Pharmacokinetic-pharmacodynamic (PK-PD) modeling enables quantitative prediction of the dose-response relationship. Recent advances in microscale technology enabled researchers to create in vitro systems that mimic biological systems more closely. Combination of mathematical modeling and microscale technology offers the possibility of faster, cheaper and more accurate prediction of the drug's effect with a reduced need for animal or human subjects.

AREAS COVERED IN THIS REVIEW

This article discusses combining in vitro microscale systems and PK-PD models for improved prediction of drug's efficacy and toxicity. First, we describe the concept of PK-PD modeling and its applications. Different classes of PK-PD models are described. Microscale technology offers an opportunity for building physical systems that mimic PK-PD models. Recent progress in this approach during the last decade is summarized.

WHAT THE READER WILL GAIN

This article is intended to review how microscale technology combined with cell cultures, also known as 'cells-on-a-chip', can confer a novel aspect to current PK-PD modeling. Readers will gain a comprehensive knowledge of PK-PD modeling and 'cells-on-a-chip' technology, with the prospect of how they may be combined for synergistic effect.

TAKE HOME MESSAGE

The combination of microscale technology and PK-PD modeling should contribute to the development of a novel in vitro/in silico platform for more physiologically-realistic drug screening.

Limitations of collateral flow after occlusion of a single cortical penetrating arteriole

Occlusions of penetrating arterioles, which plunge into cortex and feed capillary beds, cause severe decreases in blood flow and are potential causes of ischemic microlesions. However, surrounding arterioles and capillary beds remain flowing and might provide collateral flow around the occlusion. We used femtosecond laser ablation to trigger clotting in single penetrating arterioles in rat cortex and two-photon microscopy to measure changes in microvessel diameter and red blood cell speed after the clot. We found that after occlusion of a single penetrating arteriole, nearby penetrating and surface arterioles did not dilate, suggesting that alternate blood flow routes are not actively recruited. In contrast, capillaries showed two types of reactions. Capillaries directly downstream from the occluded arteriole dilated after the clot, but other capillaries in the same vicinity did not dilate. This heterogeneity in capillary response suggests that signals for vasodilation are vascular rather than parenchymal in origin. Although both neighboring arterioles and capillaries dilated in response to topically applied acetylcholine after the occlusion, the flow in the territory of the occluded arteriole did not improve. Collateral flow from neighboring penetrating arterioles is neither actively recruited nor effective in improving blood flow after the occlusion of a single penetrating arteriole.

Vascular tone and neurovascular coupling: considerations toward an improved in vitro model

Neurovascular research has made significant strides toward understanding how the brain neurovascular unit accomplishes rapid and spatial increases in blood flow following neuronal activation. Among the experimental models used, the in vitro brain slice preparation provides unique information revealing the potential signals and cellular mechanisms involved in functional hyperemia. The most crucial limitation of this model, however, is the lack of intraluminal pressure and flow in the vessels being studied. Moreover, differences in basal vascular tone have led to varied interpretations regarding the polarity of vascular responses following neuron-to-glial stimulation. Given the complexity of astrocyte-induced neurovascular responses, we propose the use of a modified in vitro brain slice preparation, where intraluminal arteriolar pressure and flow are retained. Throughout this review, we discuss the advantages and disadvantages to be considered when using brain slices for neurovascular studies. Potential ways to overcome the current limitations are proposed.

Impaired cerebral autoregulation during acute alcohol withdrawal

Heavy alcohol consumption increases the risk for all major types of stroke and is associated with autonomic dysfunction during alcohol withdrawal syndrome (AWS). Cerebral autoregulation is the mechanism by which cerebral perfusion is maintained stable, representing an intrinsic protective system of the cerebral circulation. Here, we aimed to analyze the influence of acute AWS on cerebral hemodynamics in alcohol-dependent patients. We investigated 20 men in the unmedicated acute state of AWS and repeated the investigation 24h after initiation of clomethiazole treatment. Dynamic cerebral autoregulation (dCA) was assessed by the correlation coefficient index and transfer function analysis (phase and gain) from oscillations of arterial blood pressure and cerebral blood flow velocity (CBFV). The vasomotor reserve (VMR) was measured by the CO(2)-reactivity test. In addition, we assessed autonomic modulation by means of heart rate variability and baroreflex sensitivity. We observed impaired dynamic autoregulation as shown by a multivariate analysis of variance (p<0.038) including all parameters of dCA. Similar results were found for VMR at admission (p<0.05). Pair-wise comparison between baseline and treatment with clomethiazole revealed a significant improvement for the systolic correlation coefficient index (Sx; p<0.001). Furthermore, we found a strong association of autonomic dysfunction and impaired autoregulation indicated by a correlation between the LF/HF ratio and Sx (p<0.001). In conclusion, cerebral autoregulation and VMR are disturbed during acute AWS. Influences of autonomic dysbalance and mental state during withdrawal are suggested. The finding of an affected autoregulation during acute withdrawal might indicate an increased risk for cerebro-vascular disease.

Impairment of intracerebral arteriole dilation responses after subarachnoid hemorrhage. Laboratory investigation

OBJECT

Cerebrovascular dysfunction after subarachnoid hemorrhage (SAH) may contribute to ischemia, but little is known about the contribution of intracerebral arterioles. In this study, the authors tested the hypothesis that SAH inhibits the vascular reactivity of intracerebral arterioles and documented the time course of this dysfunction.

METHODS

Subarachnoid hemorrhage was induced using an endovascular filament model in halothane-anesthetized male Sprague-Dawley rats. Penetrating intracerebral arterioles were harvested 2, 4, 7, or 14 days postinsult, cannulated using a micropipette system that allowed luminal perfusion and control of luminal pressure, and evaluated for reactivity to vasodilator agents.

RESULTS

Spontaneous tone developed in all pressurized (60 mm Hg) intracerebral arterioles harvested in this study (from 66 rats), with similar results in the sham and SAH groups. Subarachnoid hemorrhage did not affect dilation responses to acidic pH (6.8) but led to a persistent impairment of endothelium-dependent dilation responses to adenosine triphosphate (p < 0.01), as well as a transient attenuation (p < 0.05) of vascular smooth muscle-dependent dilation responses to adenosine, sodium nitroprusside, and 8-Br-cyclic guanosine monophosphate (cGMP). Impairment of NO-mediated dilation was more sustained than adenosine- and 8-Br-cGMP-induced responses (up to 7 days postinsult compared with 2 days). All smooth muscle-dependent responses returned to sham levels by 14 days after SAH.

CONCLUSIONS

Subarachnoid hemorrhage led to a persistent impairment of endothelium-dependent dilation and a transient attenuation of vascular smooth muscle-dependent dilation responses to adenosine. Impairment of NO-mediated dilation occurred when the response to cGMP was intact, suggesting a change in cGMP levels rather than an alteration in intracellular mechanisms downstream from cGMP.

The role of adenosine in hypercarbic hyperemia: in vivo and in vitro studies in adenosine 2(A) receptor knockout and wild-type mice

OBJECT

The authors tested the hypothesis that adenosine, acting through the A(2A) receptor, is not involved in hypercarbic hyperemia by assessing the effects of increased PaCO(2) on cerebral blood flow (CBF) in vivo in wild-type and A(2A) receptor knockout mice. In addition, they evaluated the effect of abluminal pH changes in vitro on the diameter of isolated perfused penetrating arterioles harvested from wild-type and A(2A) receptor knockout mice.

METHODS

The authors evaluated in a blinded fashion the CBF response during transient (60-second) hypercapnic (7% CO(2)) hypercarbia in anesthetized, ventilated C57Bl/6 wild-type and adenosine A(2A) receptor knockout mice. They also evaluated the hypercarbic response in the absence and presence of the nonselective and selective adenosine antagonists.

RESULTS

Cerebral blood flow was measured using laser Doppler flowmetry. There were no differences between the CBF responses to hypercarbia in the wild-type and the knockout mice. Moreover, the hypercarbic hyperemia response was not affected by the adenosine receptor antagonists. The authors also tested the response to alteration in abluminal pH in isolated perfused, pressurized, penetrating arterioles (average diameter 63.3 +/- 3.6 microm) harvested from wild-type (6 mice) and knockout (5 mice) animals. Arteriolar dilation in response to a decrease in abluminal pH, simulating the change in vivo during hypercarbia, was similar in wild-type (15.9 +/- 2.6%) and A(2A) receptor knockout (17.7 +/- 1.3%) mice. With abluminal application of CGS 21680 (10(-6) M), an A(2A) receptor agonist, wild-type arterioles dilated in an expected manner (9.8 +/- 0.7%), whereas A(2A) receptor knockout vessels had minimal response.

CONCLUSIONS

The results of the in vivo and in vitro studies in wild-type and A(2A) receptor knockout mice support the authors' hypothesis that hypercarbic vasodilation does not involve an adenosine A(2A) receptor-related mechanism.