Stretch-activated non-selective cation channels in the antiluminal membrane of porcine cerebral capillaries

Alterations in brain fluid physiology during the early stages of development of ischaemic oedema

Oedema occurs when higher than normal amounts of solutes and water accumulate in tissues. In brain parenchymal tissue, vasogenic oedema arises from changes in blood-brain barrier permeability, e.g. in peritumoral oedema. Cytotoxic oedema arises from excess accumulation of solutes within cells, e.g. ischaemic oedema following stroke. This type of oedema is initiated when blood flow in the affected core region falls sufficiently to deprive brain cells of the ATP needed to maintain ion gradients. As a consequence, there is: depolarization of neurons; neural uptake of Na+ and Cl- and loss of K+; neuronal swelling; astrocytic uptake of Na+, K+ and anions; swelling of astrocytes; and reduction in ISF volume by fluid uptake into neurons and astrocytes. There is increased parenchymal solute content due to metabolic osmolyte production and solute influx from CSF and blood. The greatly increased [K+]isf triggers spreading depolarizations into the surrounding penumbra increasing metabolic load leading to increased size of the ischaemic core. Water enters the parenchyma primarily from blood, some passing into astrocyte endfeet via AQP4. In the medium term, e.g. after three hours, NaCl permeability and swelling rate increase with partial opening of tight junctions between blood-brain barrier endothelial cells and opening of SUR1-TPRM4 channels. Swelling is then driven by a Donnan-like effect. Longer term, there is gross failure of the blood-brain barrier. Oedema resolution is slower than its formation. Fluids without colloid, e.g. infused mock CSF, can be reabsorbed across the blood-brain barrier by a Starling-like mechanism whereas infused serum with its colloids must be removed by even slower extravascular means. Large scale oedema can increase intracranial pressure (ICP) sufficiently to cause fatal brain herniation. The potentially lethal increase in ICP can be avoided by craniectomy or by aspiration of the osmotically active infarcted region. However, the only satisfactory treatment resulting in retention of function is restoration of blood flow, providing this can be achieved relatively quickly. One important objective of current research is to find treatments that increase the time during which reperfusion is successful. Questions still to be resolved are discussed.

Mechanosensitive Piezo1 channel in physiology and pathophysiology of the central nervous system

Since the discovery of the mechanosensitive Piezo1 channel in 2010, there has been a significant amount of research conducted to explore its regulatory role in the physiology and pathology of various organ systems. Recently, a growing body of compelling evidence has emerged linking the activity of the mechanosensitive Piezo1 channel to health and disease of the central nervous system. However, the exact mechanisms underlying these associations remain inadequately comprehended. This review systematically summarizes the current research on the mechanosensitive Piezo1 channel and its implications for central nervous system mechanobiology, retrospects the results demonstrating the regulatory role of the mechanosensitive Piezo1 channel on various cell types within the central nervous system, including neural stem cells, neurons, oligodendrocytes, microglia, astrocytes, and brain endothelial cells. Furthermore, the review discusses the current understanding of the involvement of the Piezo1 channel in central nervous system disorders, such as Alzheimer's disease, multiple sclerosis, glaucoma, stroke, and glioma.

Ion channels in capillary endothelium

Vascular beds are anatomically and functionally compartmentalized into arteries, capillaries, and veins. The bulk of the vasculature consists of the dense, anastomosing capillary network, composed of capillary endothelial cells (cECs) that are intimately associated with the parenchyma. Despite their abundance, the ion channel expression and function and Ca²⁺ signaling behaviors of capillaries have only recently begun to be explored in detail. Here, we discuss the established and emerging roles of ion channels and Ca²⁺ signaling in cECs. By mining a publicly available RNA-seq dataset, we outline the wide variety of ion channel genes that are expressed in these cells, which potentially imbue capillaries with a broad range of sensing and signal transduction capabilities. We also underscore subtle but critical differences between cEC and arteriolar EC ion channel expression that likely underlie key functional differences in ECs at these different levels of the vascular tree. We focus our discussion on the cerebral vasculature, but the findings and principles being elucidated in this area likely generalize to other vascular beds.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Cardiac Mechano-Gated Ion Channels and Arrhythmias

Mechanical forces will have been omnipresent since the origin of life, and living organisms have evolved mechanisms to sense, interpret, and respond to mechanical stimuli. The cardiovascular system in general, and the heart in particular, is exposed to constantly changing mechanical signals, including stretch, compression, bending, and shear. The heart adjusts its performance to the mechanical environment, modifying electrical, mechanical, metabolic, and structural properties over a range of time scales. Many of the underlying regulatory processes are encoded intracardially and are, thus, maintained even in heart transplant recipients. Although mechanosensitivity of heart rhythm has been described in the medical literature for over a century, its molecular mechanisms are incompletely understood. Thanks to modern biophysical and molecular technologies, the roles of mechanical forces in cardiac biology are being explored in more detail, and detailed mechanisms of mechanotransduction have started to emerge. Mechano-gated ion channels are cardiac mechanoreceptors. They give rise to mechano-electric feedback, thought to contribute to normal function, disease development, and, potentially, therapeutic interventions. In this review, we focus on acute mechanical effects on cardiac electrophysiology, explore molecular candidates underlying observed responses, and discuss their pharmaceutical regulation. From this, we identify open research questions and highlight emerging technologies that may help in addressing them.

Swelling-activated and arachidonic acid-induced currents are TREK-1 in rat bladder smooth muscle cells

Using the perforated patch voltage clamp, we investigated swelling-activated ionic channels (SACs) in rat urinary bladder smooth muscle cells.　Hypo-osmotic (60%) bath solution increased a membrane current which was inhibited by the SAC inhibitor, gadolinium.　The reversal potential of the hypotonicity-induced current shifted in the positive direction by increasing external K(+) concentration.　The hypotonicity-induced current was inhibited by extracellular acidic pH, phorbol ester and forskolin.　These pharmacological properties are identical to those of arachidonic acid-induced current present in these cells, suggesting the presence of TREK-1, a four-transmembrane two pore domain K(+) channel.　Using RT-PCR we screened rat bladder smooth muscles and cerebellum for expression of TREK-1, TREK-2 and TRAAK mRNAs.　Only TREK-1 mRNA was expressed in the bladder, while all three were expressed in the cerebellum.　We conclude that a mechanosensitive K(+) channel is present in rat bladder myocytes, which is activated by arachidonic acid and most likely is TREK-1.　This K(+) channel may have an important role in the regulation of bladder smooth muscle tone during urine storage.

Blood-brain barrier Na transporters in ischemic stroke

Blood-brain barrier (BBB) endothelial cells form a barrier that is highly restrictive to passage of solutes between blood and brain. Many BBB transport mechanisms have been described that mediate transcellular movement of solutes across the barrier either into or out of the brain. One class of BBB transporters that is all too often overlooked is that of the ion transporters. The BBB has a rich array of ion transporters and channels that carry Na, K, Cl, HCO3, Ca, and other ions. Many of these are asymmetrically distributed between the luminal and abluminal membranes, giving BBB endothelial cells the ability to perform vectorial transport of ions across the barrier between blood and brain. In this manner, the BBB performs the important function of regulating the volume and composition of brain interstitial fluid. Through functional coupling of luminal and abluminal transporters and channels, the BBB carries Na, Cl, and other ions from blood into brain, producing up to 30% of brain interstitial fluid in healthy brain. During ischemic stroke cerebral edema forms by processes involving increased activity of BBB luminal Na transporters, resulting in "hypersecretion" of Na, Cl, and water into the brain interstitium. This review discusses the roles of luminal BBB Na transporters in edema formation in stroke, with an emphasis on Na-K-Cl cotransport and Na/H exchange. Evidence that these transporters provide effective therapeutic targets for reduction of edema in stroke is also discussed, as are recent findings regarding signaling pathways responsible for ischemia stimulation of the BBB Na transporters.

Endothelial mechanosensors: the gatekeepers of vascular homeostasis and adaptation under mechanical stress

Endothelial cells (ECs) not only serve as a barrier between blood and extravascular space to modulate the exchange of fluid, macromolecules and cells, but also play a critical role in regulation of vascular homeostasis and adaptation under mechanical stimulus via intrinsic mechanotransduction. Recently, with the dissection of microdomains responsible for cellular responsiveness to mechanical stimulus, a lot of mechanosensing molecules (mechanosensors) and pathways have been identified in ECs. In addition, there is growing evidence that endothelial mechanosensors not only serve as key vascular gatekeepers, but also contribute to the pathogenesis of various vascular disorders. This review focuses on recent findings in endothelial mechanosensors in subcellular microdomains and their roles in regulation of physiological and pathological functions under mechanical stress.

Non-channel mechanosensors working at focal adhesion-stress fiber complex

Mechanosensitive ion channels (MSCs) have long been the only established molecular class of cell mechanosensors; however, in the last decade, a variety of non-channel type mechanosensor molecules have been identified. Many of them are focal adhesion-associated proteins that include integrin, talin, and actin. Mechanosensors must be non-soluble molecules firmly interacting with relatively rigid cellular structures such as membranes (in terms of lateral stiffness), cytoskeletons, and adhesion structures. The partner of MSCs is the membrane in which MSC proteins efficiently transduce changes in the membrane tension into conformational changes that lead to channel opening. By contrast, the integrin, talin, and actin filament form a linear complex of which both ends are typically anchored to the extracellular matrices via integrins. Upon cell deformation by forces, this structure turns out to be a portion that efficiently transduces the generated stress into conformational changes of composite molecules, leading to the activation of integrin (catch bond with extracellular matrices) and talin (unfolding to induce vinculin bindings). Importantly, this structure also serves as an "active" mechanosensor to detect substrate rigidity by pulling the substrate with contraction of actin stress fibers (SFs), which may induce talin unfolding and an activation of MSCs in the vicinity of integrins. A recent study demonstrates that the actin filament acts as a mechanosensor with unique characteristics; the filament behaves as a negative tension sensor in which increased torsional fluctuations by tension decrease accelerate ADF/cofilin binding, leading to filament disruption. Here, we review the latest progress in the study of those non-channel mechanosensors and discuss their activation mechanisms and physiological roles.

Mechanosensitive channels are activated by stress in the actin stress fibres, and could be involved in gravity sensing in plants

Mechanosensitive (MS) channels are expressed in a variety of cells. The molecular and biophysical mechanism involved in the regulation of MS channel activities is a central interest in basic biology. MS channels are thought to play crucial roles in gravity sensing in plant cells. To date, two mechanisms have been proposed for MS channel activation. One is that tension development in the lipid bilayer directly activates MS channels. The second mechanism proposes that the cytoskeleton is involved in the channel activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. We tested whether tension in the cytoskeleton activates MS channels. Mammalian endothelial cells were microinjected with phalloidin-conjugated beads, which bound to stress fibres, and a traction force to the actin cytoskeleton was applied by dragging the beads with optical tweezers. MS channels were activated when the force was applied, demonstrating that a sub-pN force to the actin filaments activates a single MS channel. Plants may use a similar molecular mechanism in gravity sensing, since the cytoplasmic Ca(2+) concentration increase induced by changes in the gravity vector was attenuated by potential MS channel inhibitors, and by actin-disrupting drugs. These results support the idea that the tension increase in actin filaments by gravity-dependent sedimentation of amyloplasts activates MS Ca(2+) -permeable channels, which can be the molecular mechanism of a Ca(2+) concentration increase through gravistimulation. We review recent progress in the study of tension sensing by actin filaments and MS channels using advanced biophysical methods, and discuss their possible roles in gravisensing.

Osmosensory mechanisms in cellular and systemic volume regulation

Perturbations of cellular and systemic osmolarity severely challenge the function of all organisms and are consequently regulated very tightly. Here we outline current evidence on how cells sense volume perturbations, with particular focus on mechanisms relevant to the kidneys and to extracellular osmolarity and whole body volume homeostasis. There are a variety of molecular signals that respond to perturbations in cell volume and osmosensors or volume sensors responding to these signals. The early signals of volume perturbation include integrins, the cytoskeleton, receptor tyrosine kinases, and transient receptor potential channels. We also present current evidence on the localization and function of central and peripheral systemic osmosensors and conclude with a brief look at the still limited evidence on pathophysiological conditions associated with deranged sensing of cell volume.

Modulation of intracellular Ca2+ concentration in brain microvascular endothelial cells in vitro by acoustic cavitation

Localized delivery of therapeutic agents through the blood-brain barrier (BBB) is a clinically significant task that remains challenging. Ultrasound (US) application after intravenous administration of microbubbles has been shown to generate localized BBB opening in animal models but the detailed mechanisms are not yet fully described. The current study investigates the effects of US-stimulated microbubbles on in vitro murine brain microvascular endothelial (bEnd.3) cells by monitoring sonoporation and changes in intracellular calcium concentration ([Ca(2+)](i)) using real-time fluorescence and high-speed brightfield microscopy. Cells seeded in microchannels were exposed to a single US pulse (1.25 MHz, 10 cycles, 0.24 MPa peak negative pressure) in the presence of Definity microbubbles and extracellular calcium concentration [Ca(2+)](o) = 0.9 mM. Disruption of the cell membrane was assessed using propidium iodide (PI) and change in the [Ca(2+)](i) was measured using fura-2. Cells adjacent to a microbubble exhibited immediate [Ca(2+)](i) changes after US pulse with and without PI uptake and the [Ca(2+)](i) changes were twice as large in cells with PI uptake. Cell viability assays showed that sonoporated cells could survive with modulation of [Ca(2+)](i) and uptake of PI. Cells located near sonoporated cells were observed to exhibit changes in [Ca(2+)](i) that were delayed from the time of US application and without PI uptake. These results demonstrate that US-stimulated microbubbles not only directly cause changes in [Ca(2+)](i) in brain endothelial cells in addition to sonoporation but also generate [Ca(2+)](i) transients in cells not directly interacting with microbubbles, thereby affecting cells in larger regions beyond the cells in contact with microbubbles.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Membrane stretching triggers mechanosensitive Ca2+ channel activation in Chara

In order to confirm that mechanosensitive Ca(2+) channels are activated by membrane stretching, we stretched or compressed the plasma membrane of Chara by applying osmotic shrinkage or swelling of the cell by varying the osmotic potential of the bathing medium. Aequorin studies revealed that treatments causing membrane stretching induced a transient but large increase in cytoplasmic concentration of Ca(2+) (Delta[Ca(2+)](c)). However, the observed Delta[Ca(2+)](c) decreased during the treatments, resulting in membrane compression. A second experiment was carried out to study the relationship between changes in membrane potential (DeltaE(m)) and stretching or compression of the plasma membrane. Significant DeltaE(m) values, often accompanied by an action potential, were observed during the initial exchange of the bathing medium from a hypotonic medium to a hypertonic one (plasmolysis). DeltaE(m) appears to be triggered by a partial stretching of the membrane as it was peeled from the cell wall. After plasmolysis, other exchanges from hypertonic to hypotonic media, with their accompanying membrane stretching, always induced large DeltaE(m) values and were often accompanied by an action potential. By contrast, action potentials were scarcely observed during other exchanges from hypotonic to hypertonic solutions (=membrane compression). Thus, we concluded that activation of the mechanosensitive channels is triggered by membrane stretching in Chara.

Actin stress fibers transmit and focus force to activate mechanosensitive channels

Mechanosensitive (MS) channels are expressed in various cells in a wide range of phylogenetic lineages from bacteria to humans. Understanding the molecular and biophysical mechanisms of their activation is an important research pursuit. It is controversial whether eukaryotic MS channels need accessory proteins – typically cytoskeletal structures – for activation, because MS channel activities are modulated by pharmacological treatments that affect the cytoskeleton. Here we demonstrate that direct mechanical stimulation (stretching) of an actin stress fiber using optical tweezers can activate MS channels in cultured human umbilical vein endothelial cells (HUVECs). Furthermore, by using high-speed total internal reflection microscopy, we visualized spots of Ca2+ influx across individual MS channels distributed near focal adhesions in the basal surface of HUVECs. This study provides the first direct evidence that the cytoskeleton works as a force-transmitting and force-focusing molecular device to activate MS channels in eukaryotic cells.

Sequential activation of RhoA and FAK/paxillin leads to ATP release and actin reorganization in human endothelium

We have investigated the cellular mechanisms of mechanical stress-induced immediate responses in human umbilical vein endothelial cells (HUVECs). Hypotonic stress (HTS) induced ATP release, which evoked a Ca(2+) transient, followed by actin reorganization within a few minutes, in HUVECs. Disruption of the actin cytoskeleton did not suppress HTS-induced ATP release, and inhibition of the ATP-mediated Ca(2+) response did not affect actin reorganization, thereby indicating that these two responses are not interrelated. ATP release and actin reorganization were also induced by lysophosphatidic acid (LPA). HTS and LPA induced membrane translocation of RhoA, which occurs when RhoA is activated, and tyrosine phosphorylation of focal adhesion kinase (FAK) and paxillin. Tyrosine kinase inhibitors (herbimycin A or tyrphostin 46) inhibited both HTS- and LPA-induced ATP release and actin reorganization, but did not affect RhoA activation. In contrast, Rho-kinase inhibitor (Y27632) inhibited all of the HTS- and LPA-induced responses. These results indicate that the activation of the RhoA/Rho-kinase pathway followed by tyrosine phosphorylation of FAK and paxillin leads to ATP release and actin reorganization in HUVECs. Furthermore, the fact that HTS and LPA evoke exactly the same intracellular signals and responses suggests that even these immediate mechanosensitive responses are in fact not mechanical stress-specific.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Blood-brain barrier permeability to ammonia in liver failure: a critical reappraisal

In patients with acute liver failure (ALF), hyperammonemia is related to development of cerebral edema and herniation. The present review discusses the mechanisms for the cerebral uptake of ammonia. A mathematical framework is provided to allow a quantitative examination of whether published studies can be explained by the conventional view that cerebral uptake of ammonia is restricted to diffusion of the unprotonated form (NH(3)) (the diffusion hypothesis). An increase in cerebral blood flow (CBF) enhanced ammonia uptake more than expected, possibly due to recruitment or heterogeneity of brain capillaries. Reported effects of pH on ammonia uptake were in the direction predicted by the diffusion hypothesis, but often less pronounced than expected. The published effects of mannitol, cooling, and indomethacin in experimental animals and patients were difficult to explain by the diffusion hypothesis alone, unless dramatic changes of capillary surface area or permeability for ammonia were induced. Therefore we considered the possible role of membrane protein mediated transport of NH(4)(+) across the blood-brain barrier (BBB). Early tracer studies in Rhesus monkeys suggested that NH(4)(+) is responsible for 20% or even more of the transport of ammonia from plasma to brain. In other locations, such as in the thick ascending limb of Hendle's loop and in isolated astrocytes, transport protein mediated translocation of NH(4)(+) is predominant. Many of the ion-transporters involved in renal NH(4)(+) reabsorbtion are also present in brain capillary membranes and could mediate uptake of NH(4)(+). Astrocytic uptake of NH(4)(+) is associated with increased extracellular K(+), which is a potent cerebral vasodilator. Such interference between transport of NH(4)(+) and other cations could be clinically important because increased cerebral blood flow often precedes cerebral herniation in acute liver failure. We suggest that protein mediated transport of NH(4)(+) through the brain capillary wall is a realistic possibility that should be more intensely studied.

Ca(2+)- and voltage-dependent gating of Ca(2+)- and ATP-sensitive cationic channels in brain capillary endothelium

Biophysical properties of the Ca(2+)-activated nonselective cation channel expressed in brain capillaries were studied in inside-out patches from primary cultures of rat brain microvascular endothelial cells. At -40 mV membrane potential, open probability (P(o)) was activated by cytosolic [Ca(2+)] > 1 micro M and was half-maximal at approximately 20 micro M. Increasing [Ca(2+)] stimulated opening rate with little effect on closing rate. At constant [Ca(2+)], P(o) was voltage-dependent, and effective gating charge corresponded to 0.6 +/- 0.1 unitary charges. Depolarization accelerated opening and slowed closing, thereby increasing apparent affinity for Ca(2+). Within approximately 1 min of excision, P(o) declined to a lower steady state with decreased sensitivity toward activating Ca(2+) when studied at a fixed voltage, and toward activating voltage when studied at a fixed [Ca(2+)]. Deactivated channels opened approximately 5-fold slower and closed approximately 10-fold faster. The sulfhydryl-reducing agent dithiotreitol (1 mM) completely reversed acceleration of closing rate but failed to recover opening rate. Single-channel gating was complex; distributions of open and closed dwell times contained at least four and five exponential components, respectively. The longest component of the closed-time distribution was markedly sensitive to both [Ca(2+)] and voltage. We conclude that the biophysical properties of gating of this channel are remarkably similar to those of large-conductance Ca(2+)-activated K(+) channels.

Hypoxia activates a background conductance in freshly isolated pulmonary arterial endothelial cells of the rat

Utilising the patch-clamp recording technique we have demonstrated for the first time the effects of hypoxia on the background current in pulmonary arterial endothelial cells. Electrophysiological studies revealed the presence of a novel oxygen-sensitive, non-selective cation conductance (I(NSC)) in these cells. The inward component of I(NSC) was significantly potentiated by hypoxia. Both the inward and outward components of I(NSC) were inhibited by both La(3+) and Gd(3+). Hypoxic activation of I(NSC) may provide an important Ca(2+) influx pathway essential for the release of a pulmonary-selective vasoconstrictor pivotal to the sustained phase of hypoxic pulmonary vasoconstriction.

A mechanosensitive cation channel in endothelial cells

A mechanosensitive Ca2+-permeable channel is present in vascular endothelial cells. The activity of this channel increases in response to hemodynamic blood flow. Recently, it has been found that the activity of this channel may be regulated by cGMP through a protein kinase G-dependent pathway. Inhibition of the channel by cGMP abolishes the Ca2+ influx elicited by flow. Several inhibitors of the cation channel including Gd3+, Ni2+, and SK&F-96365 also inhibit the Ca2+ influx due to flow stimulation. These data suggest that a mechanosensitive cation channel is the primary pathway mediating the flow-induced Ca2+ entry in vascular endothelial cells.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Stretch-sensitive switching among different channel sublevels of an endothelial cation channel

A mechanosensitive Ca(2+)-permeable cation channel was recorded by patch clamp in isolated rat aortic endothelial cells. A low level of channel activity could be observed after seal formation. The channel displayed some inward rectification and had a conductance for inward current of approx. 32 pS in Ca(2+)-free pipette and bath solutions. Negative suction of -10 to -20 mmHg increased the probability of the channel being open. When the negative pressure in the pipette was raised to -35 to -45 mmHg, the channel underwent an abrupt transition to a large conductance substate that was interrupted occasionally by two other low conductance levels. Under this condition, the overwhelming majority of openings and closings were between a main level of 83 pS and the closed level. Compared to the 32 pS substate, the 83 pS large conductance substate had shorter mean open and closed times. The two channel substates had similar ionic selectivity and both were sensitive to the inhibition of cGMP and protein kinase G. This is the first demonstration showing that mechanostress can change the single channel conductance level of an ion channel in eukaryotic cells.

Flow-activated Na(+)and K(+)Current in cardiac microvascular endothelial cells

The patch-clamp technique was used to investigate the electrical response of cardiac microvascular endothelial cells to fluid stream applied through a microtube. The fluid stream induced a membrane current whose amplitude was dependent on flow rate. The I-V relationship of the flow-activated current showed a strong inward rectification and a reversal potential close to +30 mV, which gave a P(Na)/P(k)=3.47. The inward and outward components of the current nearly disappeared when extracellular Na(+)and internal K(+), respectively, were replaced by NMDG(+). Finally, the flow-activated current was fully blocked by 50 microM Gd(3+)and La(3+). These results show that cardiac microvascular endothelial cells respond to mechanical stimulation associated with flow by increasing their permeability to Na(+)and K(+). This mechanism is suggested to contribute to ionic homeostasis at high heart rates and during post-ischaemic reperfusion.

Volume regulatory decrease in UMR-106.01 cells is mediated by specific alpha1 subunits of L-type calcium channels

An early cellular response of osteoblasts to swelling is plasma membrane depolarization, accompanied by a transient increase in intracellular calcium ([Ca2+]i), which initiates regulatory volume decrease (RVD). The authors have previously demonstrated a hypotonically induced depolarization of the osteoblast plasma membrane, sufficient to open L-type Ca channels and mediate Ca2+ influx. Herein is described the initiation of RVD in UMR-106.01 cells, mediated by hypotonically induced [Ca2+]i transients resulting from the activation of specific isoforms of L-type Ca channels. The authors further demonstrate that substrate interaction determines which specific alpha1 Ca channel subunit isoform predominates and mediates Ca2+ entry and RVD. Swelling-induced [Ca2+]i transients, and RVD in cells grown on a type I collagen matrix, are inhibited by removal of Ca from extracellular solutions, dihydropyridines, and antisense oligodeoxynucleotides directed exclusively to the alpha1C isoform of the L-type Ca channel. Ca2+ transients and RVD in cells grown on untreated glass cover slips were inhibited by similar maneuvers, but only by antisense oligodeoxynucleotides directed to the alpha1S isoform of the L-type Ca channel. This represents the first molecular identification of the Ca channels that transduce the initiation signal for RVD by osteoblastic cells.

Shear stress-mediated extracellular signal-regulated kinase activation is regulated by sodium in endothelial cells. Potential role for a voltage-dependent sodium channel

Fluid shear stress is an important regulator of endothelial cell (EC) function. To determine whether mechanosensitive ion channels participate in the EC response to shear stress, we characterized the role of ion transport in shear stress-mediated extracellular signal-regulated kinase (ERK1/2) stimulation. Replacement of all extracellular Na+ with either N-methyl-D-glucamine or choline chloride increased the ERK1/2 stimulation in response to shear stress by 1.89 +/- 0.1-fold. The Na+ effect was concentration-dependent (maximal effect, </=12.5 mM) and was specific for shear stress-mediated ERK1/2 activation as epidermal growth factor-stimulated ERK1/2 activation was unaffected by removal of extracellular Na+. Shear stress-mediated ERK1/2 activation was potentiated by the voltage-gated sodium channel antagonist, tetrodotoxin (100 nM), to a magnitude similar to that achieved with extracellular Na+ withdrawal. Transfection of Chinese hamster ovary cells with a rat brain type IIa voltage-gated sodium channel completely inhibited shear stress-mediated ERK1/2 activation in these cells. Inhibition was reversed by performing the experiment in sodium-free buffer or by including tetrodotoxin in the buffer. Western blotting of bovine and human EC lysates with SP19 antibody detected a 250-kDa protein consistent with the voltage-gated sodium channel. Degenerate polymerase chain reaction of cDNA from primary human EC yielded transcripts whose sequences were identical to the sodium channel SCN4a and SCN8a alpha subunit genes. These results indicate that shear stress-mediated ERK1/2 activation is regulated by extracellular sodium and demonstrate that ion transport via Na+ channels modulates EC responses to shear stress.

Voltage-dependent K(+) current in capillary endothelial cells isolated from guinea pig heart

Capillary fragments were isolated from guinea pig hearts, and their electrical properties were studied using the perforated-patch and cell-attached mode of the patch-clamp technique. A voltage-dependent K(+) current was discovered that was activated at potentials positive to -20 mV and showed a sigmoid rising phase. For depolarizing voltage steps from -128 to +52 mV, the time to peak was 71 +/- 5 ms (mean +/- SE) and the amplitude of the current was 3.7 +/- 0.5 pA/pF in the presence of 5 mM external K(+). The time course of inactivation was exponential with a time constant of 7.2 +/- 0.5 s at +52 mV. The current was blocked by tetraethylammonium (inhibitory constant approximately 3 mM) but was not affected by charybdotoxin (1 microM) or apamin (1 microM). In the cell-attached mode, depolarization-activated single-channel currents were found that inactivated completely within 30 s; the single-channel conductance was 12.3 +/- 2.4 pS. The depolarization-activated K(+) current described here may play a role in membrane potential oscillations of the endothelium.

Na+-Ca2+ exchange and its implications for calcium homeostasis in primary cultured rat brain microvascular endothelial cells

1. The role of Na+-Ca2+ exchange in the regulation of the cytosolic free Ca2+ concentration ([Ca2+]i) was studied in primary cultured rat brain capillary endothelial cells. [Ca2+]i was measured by digital fluorescence imaging in cells loaded with fura-2. 2. ATP (100 microM) applied for a short period of time (6 s) caused a rise in [Ca2+]i from 127 +/- 3 (n = 290) to 797 +/- 25 nM, which then declined to the resting level, with a t time required for [Ca2+]i to decline to half of peak [Ca2+]i) of 5.4 +/- 0.09 s. This effect was independent of external Ca2+ and could be abolished by previously discharging the Ca2+ pool of the endoplasmic reticulum with thapsigargin (1 microM). 3. Application of thapsigargin (1 microM) or cyclopiazonic acid (10 microM) to inhibit the Ca2+-ATPase of the endoplasmic reticulum 6 s prior to ATP application did not influence the peak [Ca2+]i but greatly reduced the rate of decline of [Ca2+]i, with t values of 15 +/- 1.6 and 23 +/- 3 s, respectively. 4. In the absence of external Na+ (Na+ replaced by Li+ or N-methylglucamine) the basal [Ca2+]i was slightly elevated (152 +/- 6 nM) and the restoration of [Ca2+]i after the ATP stimulation was significantly slower (t , 7.3 +/- 0.46 s in Li+ medium, 8.12 +/- 0.4 s in N-methylglucamine medium). 5. The external Na+-dependent component of the [Ca2+]i sequestration was also demonstrated in cells stimulated by ATP subsequent to addition of cyclopiazonic acid; in a Na+-free medium [Ca2+]i remained at the peak level in 88 % of the cells after stimulation with ATP. 6. Addition of monensin (10 microM) in the presence of external Na+ increased the resting [Ca2+]i to 222 +/- 9 nM over approximately 1 min and subsequent removal of extracellular sodium resulted in a further increase in [Ca2+]i to a peak of 328 +/- 11 nM, which was entirely dependent on external Ca2+. 7. These findings indicate that a functional Na+-Ca2+ exchanger is present at the blood-brain barrier, which plays a significant role in shaping the stimulation-evoked [Ca2+]i signal and is able to work in reverse mode under pharmacological conditions.

Mechanical stress-induced Ca2+ entry and Cl- current in cultured human aortic endothelial cells

A fluid stream through a microtube was applied to cultured human aortic endothelial cells to investigate the endothelial responses of both the ionic currents and intracellular Ca2+ concentration ([Ca2+]i) to mechanical stimulation. The fluid stream induced an increase in [Ca2+]i that was dependent on both the flow rate and the extracellular Ca2+ concentration. Gd3+ and niflumic acid inhibited the fluid stream-induced increase in [Ca2+]i, whereas Ba2+ and tetraethylammonium ion exhibited no effect. The fluid stream-induced [Ca2+]i increase was accompanied by the activation of an inward current at -52.8 mV. The reversal potential of the fluid stream-induced current shifted to positive potentials when the external Cl- concentration was reduced but was not affected by variation of the external Na+ concentration. During the exposure to the fluid stream, [Ca2+]i was voltage dependent, i.e., depolarization decreased [Ca2+]i. We therefore conclude that the fluid stream-induced current is largely carried by Cl- and that the Cl- current may thus play a role in modulating the Ca2+ influx by altering the membrane potential of endothelial cells.

Stretch-activated cation channel in human umbilical vein endothelium in normal pregnancy and in preeclampsia

OBJECTIVE

To determine whether stretch-activated cation channels (SAC) are present in intact human umbilical vein endothelium (HUVE) and in an endothelial cell line (EA.hy) and whether they act as endothelial mechanosensors, and to determine whether endothelial SAC in HUVE from women with pregnancies complicated by preeclampsia undergo functional changes compared with those in HUVE from women with normotensive pregnancies.

METHODS AND RESULTS

By use of the patch-clamp technique we identified a SAC in intact HUVE and in an endothelial cell line. The SAC had mean conductances of 29+/-5 pS (n = 38) for K+ and 12+/-2 pS (n = 4) for Ca2+. Administration of 50 micromol/I gadolinium, a blocker of mechanosensitive ion channels, completely blocked activity of this channel. We found from single-channel recordings that influx of Ca2+ through SAC directly activated high-conductance Ca2+-dependent potassium channels, proving that a significant influx of Ca2+ through SAC occurs at physiologic concentrations of Ca2+. In a comparative study, apparent channel density of SAC (percentage of patches with SAC activity) in HUVE from women with pregnancies complicated by preeclampsia (36.2 +/- 4.3%) was twofold higher than that in HUVE from women with normal pregnancies (17.9+/-2.9%, P< 0.01). Channel conductance and sensitivity to stretching of SAC were not altered by preeclampsia.

CONCLUSIONS

Since SAC are capable of acting as endothelial mechanosensors, the greater than normal density of SAC associated with preeclampsia might reflect an alteration of mechanotransduction.

Signal transduction and regulation of lung endothelial cell permeability. Interaction between calcium and cAMP

Pulmonary endothelium forms a semiselective barrier that regulates fluid balance and leukocyte trafficking. During the course of lung inflammation, neurohumoral mediators and oxidants act on endothelial cells to induce intercellular gaps permissive for transudation of proteinaceous fluid from blood into the interstitium. Intracellular signals activated by neurohumoral mediators and oxidants that evoke intercellular gap formation are incompletely understood. Cytosolic Ca2+ concentration ([Ca2+]i) and cAMP are two signals that importantly dictate cell-cell apposition. Although increased [Ca2+]i promotes disruption of the macrovascular endothelial cell barrier, increased cAMP enhances endothelial barrier function. Furthermore, during the course of inflammation, elevated endothelial cell [Ca2+]i decreases cAMP to facilitate intercellular gap formation. Given the significance of both [Ca2+]i and cAMP in mediating cell-cell apposition, this review addresses potential sites of cross talk between these two intracellular signaling pathways. Emerging data also indicate that endothelial cells derived from different vascular sites within the pulmonary circulation exhibit distinct sensitivities to permeability-inducing stimuli; that is, elevated [Ca2+]i promotes macrovascular but not microvascular barrier disruption. Thus this review also considers the roles of [Ca2+]i and cAMP in mediating site-specific alterations in endothelial permeability.

A cationic nonselective stretch-activated channel in the Reissner's membrane of the guinea pig cochlea

The Reissner's membrane (RM) separates in the mammalian cochlea the K(+)-rich endolymph from the Na(+)-rich perilymph. The patch-clamp technique was used to investigate the transport mechanisms in epithelial cells of RM freshly dissected from the guinea pig cochlea. This study shows a stretch-activated nonselective cationic channel (SA channel) with a linear current-voltage relationship (23 pS) highly selective for cations over anions [K+ approximately Na+ (1) > Ba2+ (0.65) > Ca2+ (0.32) >> Cl- (0.14)] and activated by the intrapipette gradient pressure. The open probability-pressure relationship is best fitted by a Boltzmann distribution (half-maximal pressure = 37.8 mmHg, slope constant = 8.2 mmHg). SA channels exhibit a strong voltage dependency and are insensitive to internal Ca2+, ATP, and fenamates but are blocked by 1 microM GdCl3 in the pipette. They are reversibly activated by in situ superfusion of the cell with hyposmotic solutions. Kinetic studies show that depolarization and mechanical or osmotic stretch modify the closed and open time constants probably by a different mechanism. These channels could participate in pressure-induced modifications of ionic permeability of the RM.

The role of calcium ion in anoxia/reoxygenation damage of cultured brain capillary endothelial cells

Capillary endothelial cells are critical targets in both ischemia and reperfusion of the brain. Arachidonic acids and oxygen free radicals have been shown to cause disruption of blood-brain barrier (BBB) by destruction of capillary endothelial cell membrane. However, the exact mechanism of BBB breakdown by cerebral ischemia/reperfusion remains undetermined. The aim of the present study is to clarify the mechanism of intracellular calcium ion ([Ca2+]i) change in brain capillary endothelial cells under anoxia/reoxygenation. Brains capillary endothelial cells were isolated from ten male Sprague-Dawley rats by a two step enzymatic process. [Ca2+]i was measured by means of a confocal laser scanning microscope using Indo 1-A/M as a calcium indicator. The endothelial cells were subjected to anoxia and reoxygenization under different conditions. [Ca2+]i increased gradually during anoxia and slightly decreased after reoxygenation. Indomethacin and SOD suppressed the elevation of [Ca2+]i during anoxia. NG-nitro-L-arginine methyl ester and catalase moderately suppressed the elevation, however nifedipine did not suppress it at all. In this model, rapid [Ca2+]i change was not observed during the reoxygenation phase. The results indicate that the anoxia induced elevation of [Ca2+]i in the brain capillary endothelial cells depends on superoxide and peroxynitrite generation.

The mechanism of reversible osmotic opening of the blood-brain barrier: role of intracellular calcium ion in capillary endothelial cells

Despite clinical and experimental interest in the osmotic opening of the blood-brain barrier (BBB), the mechanism underlying the phenomenon remain undetermined. The aim of this study is to investigate the mechanism of intracellular Ca2+ change in brain microvascular endothelial cells subjected to hyperosmotic stress. Cultured rat brain capillary endothelial cells were obtained by two-step enzymatic purification. Intracellular Ca2+ was measured by a confocal laser scanning microscope. After exposing the endothelial cells to 1.4 M mannitol for 30 seconds, the change of intracellular Ca2+ concentration was monitored. Intracellular Ca2+ concentration increased rapidly and reached its peak value within 10 seconds after the application of mannitol. The Ca2+ concentration returned to the basal level within 200 seconds. A calcium channel blocker nifedipine (100 microM, 10 microM) did not block the increase. A specific blocker (KB-R7943) of Na+/Ca2+ exchange did not affect the rapid elevation of intracellular Ca2+. However, it blocked the return phase almost completely. The results indicated that the Na+/Ca2+ exchanger pumped out the increased intracellular Ca2+ during the return phase. Reversible osmotic disruption and reconstruction of the BBB is not due to simple mechanical shrinkage of the endothelial cells but is due to the intracellular Ca(2+)-activated complex mechanism. The manipulation of the reconstruction phase, which depends on Na+/Ca2+ exchanger, may have clinical implications.

Stretch-activated nonselective cation, Cl- and K+ channels in apical membrane of epithelial cells of Reissner's membrane

Ion channels on the apical membrane of epithelial cells (the surface facing the endolymph) of acutely isolated Reissner's membrane from guinea-pig cochlea were investigated by using patch-clamp technique in cell-attached and inside-out configurations. Three types of ion channel were identified: namely, a stretch-activated nonselective cation, a chloride and a potassium channel. When the pipette was filled with high-K+ endolymph-like solution, the most significant channel activity was nonselective cation channels (85/110, 77% patches). The current versus voltage relationship was linear with a unitary conductance of 22.1 +/- 0.4 pS and reversal potential (Vr) of 2.3 +/- 0.8 mV (n = 18). The channel exhibited a lower conductance (14.0 +/- 0.6 pS, n = 8) to Ca2+. The open probability was low (NPo approximately 0.1) in cell-attached configuration under +60 mV pipette potential and increased when the membrane was stretched with negative pressure. The channel was blocked by 10 microM extracellular Gd3+. The two other types of channels were a small voltage-sensitive Cl- channel (6.0 +/- 0.3 pS; 91/99, 92% patches) and a K+ channel (approximately 30 pS; 29/191, 15% patches). These channels might play roles in the regulation of cell volume, in balancing the hydrostatic pressure across Reissner's membrane and in maintaining the electrochemical composition of endolymph.

Mechanosensitive cation channels in aortic endothelium of normotensive and hypertensive rats

In response to humoral and hemodynamic stimuli, vascular endothelium regulates vascular tone by releasing endothelium-derived vasoactive factors. Stretch-activated cation channels have been postulated to act as endothelial mechanosensors that respond to changes in hemodynamic forces. We report the presence of a nonselective (n=98) and K+-selective (n=53) stretch-activated channel in rat intact aortic endothelium and isolated aortic endothelial cells. The nonselective channel showed a permeability ratio for Na+, K+, and Ca2+ of 1:0.95:0.23 and was completely blocked by 50 micromol/L gadolinium, a blocker of stretch-activated channels. The K+-selective channel was selectively permeable for K+, with a K+-Na+ permeability ratio of 10.9:1. In whole-cell current recordings, hyposmotic cell swelling induced an increase in cell conductance. The swelling-induced current was completely blocked by 50 micromol/L gadolinium, showing that stretch-activated channels were activated by cell swelling and carry macroscopic cell currents. In a comparative study with normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR), the K+-selective stretch-activated channel was observed in a 4.4-fold higher density in adult SHR compared with WKY. Also, in adult SHR, the stretch sensitivity of the nonselective channel was nearly twice as high as in WKY. In contrast, channel properties were unchanged in young SHR (5 to 6 weeks old) compared with age-matched WKY. These data suggest that stretch-activated channels are regulated in their sensitivity and density when subjected to increased hemodynamic forces such as in hypertension. Since the channels are capable of acting as endothelial mechanosensors, the altered channel properties might contribute to an altered mechanoreception in hypertension.

Ion channels in vascular endothelium

The functional impact of ion channels in vascular endothelial cells (ECs) is still a matter of controversy. This review describes different types of ion channels in ECs and their role in electrogenesis, Ca2+ signaling, vessel permeability, cell-cell communication, mechano-sensor functions, and pH and volume regulation. One major function of ion channels in ECs is the control of Ca2+ influx either by a direct modulation of the Ca2+ influx pathway or by indirect modulation of K+ and Cl- channels, thereby clamping the membrane at a sufficiently negative potential to provide the necessary driving force for a sustained Ca2+ influx. We discuss various mechanisms of Ca2+ influx stimulation: those that activate nonselective, Ca(2+)-permeable cation channels or those that activate Ca(2+)-selective channels, exclusively or partially operated by the filling state of intracellular Ca2+ stores. We also describe the role of various Ca(2+)- and shear stress-activated K+ channels and different types of Cl- channels for the regulation of the membrane potential.

The normal and pathological physiology of brain water

The physicochemical properties of water enable it to act as a solvent for electrolytes, and to influence the molecular configuration and hence the function--enzymatic in particular--of polypeptide chains in biological systems. The association of water with electrolytes determines the osmotic regulation of cell volume and allows the establishment of the transmembrane ion concentration gradients that underlie nerve excitation and impulse conduction. Fluid in the central nervous system is distributed in the intracellular and extracellular spaces (ICS, ECS) of the brain parenchyma, the cerebrospinal fluid, and the vascular compartment--the brain capillaries and small arteries and veins. Regulated exchange of fluid between these various compartments occurs at the blood-brain barrier (BBB), and at the ventricular ependyma and choroid plexus, and, on the brain surface, at the pia mater. The normal BBB is relatively permeable to water, but considerably less so to ions, including the principal electrolytes Brain fluid regulation takes place within the context of systemic fluid volume control, which depends on the mutual interaction of osmo-, volume-, and pressure-receptors in the hypothalamus, heart and kidney, hormones such as vasopressin, renin-angiotensin, aldosterone, atriopeptins, and digitalis-like immunoreactive substance, and their respective sites of action. Evidence for specific transport capabilities of the cerebral capillary endothelium, for example high Na+K(+)-ATPase activity and the presence at the abluminal surface of a Na(+)--H+ antiporter, suggests that cerebral microvessels play a more active part in brain volume regulation and ion homoeostasis than do capillaries in other vascular beds. The normal brain ECS amounts to 12-19% of brain volume, and is markedly reduced in anoxia, ischaemia, metabolic poisoning, spreading depression, and conventional procedures for histological fixation. The asymmetrical distributions of Na+ K+ and Ca2+ between ICS and ECS underlie the roles of these cations in nerve excitation and conduction, and in signal transduction. The relatively large volume of the CSF, and extensive diffusional exchange of many substances between brain ECS and CSF, augment the ion-homeostasing capacity of the ECS. The choroid plexus, in addition to secreting CSF principally by biochemical mechanisms (there is an additional small component from the extracellular fluid), actively transports some substances from the blood (e.g. nucleotides and ascorbic acid), and actively removes others from the CSF. In contrast with CSF secretion, CSF reabsorption is principally a biomechanical process, passively dependent on the CSF-dural sinus pressure gradient. Pathological increases in intracranial water content imply development of an intracranial mass lesion. The additional water may be distributed diffusely within the brain parenchyma as brain oedema, as a cyst, or as increase in ventricular volume due to hydrocephalus. Brain oedema is classified on the basis of pathophysiology into four categories, vasogenic, cytotoxic, osmotic and hydrostatic. The clinical conditions in which brain oedema presents the greatest problems are tumour, ischaemia, and head injury. Peritumoural oedema is predominantly vasogenic and related to BBB dysfunction. Ischaemic oedema is initially cytotoxic, with a shift of Na+ and CI- ions from ECS to ICS, followed by osmotically obliged water, this shift can be detected by diffusion-weighted MRI. Later in the evolution of an ischaemic lesion the oedema becomes vasogenic, with disruption of the BBB. Recent imaging studies in patients with head injury suggest that the development of traumatic brain oedema may follow a biphasic time course similar to that of ischaemic oedema. Hydrocephalus is associated in the great majority of cases with an obstruction to the circulation or drainage of CSF, or, occasionally, with overproduction of CSF by a choroid plexus papilloma. In either case, the consequence is a ris

Ultrastructural study of brain microvessels in patients with traumatic cerebral contusions

Brain tissue from 11 patients with traumatic cerebral contusions submitted to surgery was studied. Control biopsy specimens were obtained from 5 patients undergoing ventriculo-peritoneal shunts for "communicating" hydrocephalus. After collection, the small fragments were fixed by immersion in glutaraldehyde-osmium and embedded in Epon. Semi-thin sections stained with toluidine blue were observed with the light microscope. Thin sections stained with lead citrate and uranyl acetate were observed using a Jeol electron microscope. In tissues from patients with head trauma a clear space most probably corresponding to fluid accumulation was systematically observed around microvessels. Ultrastructurally endothelial cells from these specimens exhibited signs of marked intracellular oedema, tight junctions being intact. Pinocytotic activity was increased, mainly at the abluminal surface. Swelling of astrocytic perivascular processes and the appearance of macrophagic cells with voluminous lysosomes were also observed. The authors conclude that the oedema of endothelial cells probably represent a central fact in the pathophysiology of traumatic brain oedema and speculate on the putative involvement of stretch-activated receptors in this condition.

A transferable, beta-naphthoflavone-inducible, hyperpolarizing factor is synthesized by native and cultured porcine coronary endothelial cells

1. The vascular endothelium releases a hyperpolarizing factor (endothelium-derived hyperpolarizing factor, EDHF) tentatively identified as a cytochrome P450-derived arachidonic acid metabolite. However, there is still controversy concerning its transferability and identity. We designed a bioassay system for assessing EDHF release in which the membrane potential was recorded in cultured vascular smooth muscle cells located downstream from donor endothelial cells. 2. Under combined nitric oxide (NO) synthase and cyclo-oxygenase blockade with NG-nitro-L-arginine (100 mumol l-1) and diclofenac (10 mumol l-1), the superfusate from bradykinin (30 mumol l-1)-stimulated, cultured porcine coronary endothelial cells induced a distinct hyperpolarization followed by a depolarization. Direct application of bradykinin to the smooth muscle cells resulted solely in membrane depolarization. Similar results were obtained using bradykinin-stimulated porcine coronary arteries as donor. 3. Single-channel current measurements suggest that this EDHF-induced hyperpolarization was elicited by the activation of Ca(2+)-dependent K+ channels. 4. Increasing the transmural pressure within the donor segment significantly enhanced the duration, but not the amplitude of the hyperpolarization induced by the effluate from bradykinin-stimulated donor segments. 5. Inhibition of P450 oxygenase activity with clotrimazole (3 mumol l-1) or 17-octadecynoic acid (3 mumol l-1) abolished EDHF release from the coronary endothelium, while the P450-derived arachidonic acid metabolite, 5,6-epoxyeicosatrienoic acid, induced a hyperpolarization of detector smooth muscle cells almost identical to that induced by EDHF. Moreover, induction of P450 activity by beta-naphthoflavone (3 mumol l-1, 48 h), significantly increased the bradykinin-induced release of EDHF. 6. These findings suggest that the vascular endothelium releases a transferable hyperpolarizing factor, chemically distinct from NO and prostacyclin, in response to agonists and mechanical stimulation. This beta-naphthoflavone-inducible EDHF appears to be a cytochrome P450-derived metabolite of arachidonic acid, which elicits hyperpolarization by activation of Ca(2+)-dependent K+ channels.

Mechanotransducing ion channels in C6 glioma cells

Mechanotransducing (MS) ion channels and images of the patch membrane were studied in cell-attached patches in C6 glioma cells. MS channel density was approximately 0.08 to 0.5 channels/microns2, channel conductance was approximately 40 pS (at -40 mV), and the reversal potential was +15 mV. Replacement of NaCl with KCl, CsCl, or Na gluconate in the pipette solution was without substantial effect on the current-voltage relationship. Replacement of NaCl with NMDG (N-Methyl-D-Glucamine) Cl or reducing NaCl decreased the amplitude of inward currents at negative membrane potentials and caused the reversal potential to shift in the negative direction. Rapid application of suction to the back of the pipette usually elicited a fast (< 0.1 s) appearance of channel activity. The peak (phasic) in channel activity was followed by a decrease to a constant (tonic) level of activity. The reduction in channel activity--called adaptation--was reduced at depolarizing membrane potentials and disappeared if too much pressure was applied. Positive pressure caused the patch membrane to curve toward the pipette tip, move in the direction of the tip, and evoke MS channel activity. Removal of the positive pressure caused the patch to move back to the original position. Conversely, negative pressure caused the patch membrane to curve away from the pipette tip, move away from the tip, and elicit MS channel activity. Gigohm seal resistances were always maintained during translational movement of the patch membrane. Tonic MS channel activity was not associated with translational movements of the patch membrane. Phasic and tonic channel activity were independent of the sign of curvature of the patch membrane. C6 glioma cells have rapidly adapting voltage-dependent MS ion channels, which are non-selective for monovalent cations, and belong to the stretch-activating class of mechanosensory ion channels. Adaptation in MS channels may allow the cell to limit the influx of cations in response to mechanical input. The selective loss of adaptation suggests that the MS channel's gate receives input from two sources. A minimal viscoelastic mechanical model of adaptation and two alternative models for translational movement of the patch are presented.

Up-regulation of pressure-activated Ca(2+)-permeable cation channel in intact vascular endothelium of hypertensive rats

In endothelial cells, stretch-activated cation channels have been proposed to act as mechanosensors for changes in hemodynamic forces. We have identified a novel mechanosensitive pressure-activated channel in intact endothelium from rat aorta and mesenteric artery. The 18-pS cation channel responded with a multifold increase in channel activity when positive pressure was applied to the luminal cell surface with the patch pipette and inactivated at negative pipette pressure. Channel permeability ratio for K+, Na+, and Ca2+ ions was 1:0.98:0.23. Ca2+ influx through the channel was sufficient to activate a neighboring Ca2(+)-dependent K+ channel. Hemodynamic forces are chronically disturbed in arterial hypertension. Endothelial cell dysfunction has been implicated in the pathogenesis of arterial hypertension. In two comparative studies, density of the pressure-activated channel was found to be significantly higher in spontaneously hypertensive rats and renovascular hypertensive rats compared with their respective normotensive controls. Channel activity presumably leads to mechanosensitive Ca2+ influx and induces cell hyperpolarization by K+ channel activity. Both Ca2+ influx and hyperpolarization are known to induce a vasodilatory endothelial response by stimulating endothelial nitric oxide (NO) production. Up-regulation of channel density in hypertension could, therefore, represent a counterregulatory mechanism of vascular endothelium.

Hyperosmotic activation of transmitter release from presynaptic terminals onto retinal ganglion cells

A method for evoking neurotransmitter release without light stimulation has been developed and applied to a retinal slice preparation of the tiger salamander (Ambystoma Tigrum). This method utilizes a micropipette containing hyperosmotic levels of sucrose in Ringer, positioned within the inner plexiform layer (IPL) under visual control. Intermittent pressure (between 0.1 and 2 bars) applied to the pipette evoked release of neurotransmitters which were evaluated with whole-cell recording (WCR) technique applied to cells in the ganglion cell layer. Pharmacological studies were used to characterize the properties of the hyperosmotic sucrose-evoked response (HSER) and in some cases, we compared the HSER with synaptic currents evoked by light stimulation. The HSER typically consisted of both inhibitory and excitatory components with a reversal potential in between that for chloride (approximately -60 mV) and non-specific cation channels (approximately 0 mV). Relatively pure inhibition or excitation could be revealed through pharmacological techniques by blocking the inhibition with picrotoxin/strychnine or by blocking the glutamatergic neurotransmission with D-AP7 (D-2-amino-7-phosphonoheptanoate) and NBQX (2,3-dihydroxy-6-nitro-sulfamoyl -benzo (F) quinoxaline). A comparison of light-evoked responses (LER) and the HSER suggested that they activate the same pool of releasable neurotransmitter.

Classification of ion channels in the luminal and abluminal membranes of guinea-pig endocardial endothelial cells

1. The ion channels on both the luminal and abluminal membranes of endocardial endothelial (EE) cells were separately recorded using the patch clamp technique in the guinea-pig heart. 2. The major population consisted of two types of non-selective cation channels, which showed open probabilities of 0.21 and 0.33 at the resting potential, and conductances of 36 and 11 pS, respectively. 3. The next major class was Cl- channels with an ohmic conductance of 409 pS. The channel was quiescent in the cell-attached mode but was activated by strong depolarization after excising the patch membrane. 4. The channels activated by intracellular Ca2+ were mainly K+ channels showing a 34 pS slope conductance and, less frequently, Ca(2+)-dependent K+ channels having a large conductance (210 pS). The inward rectifier K+ channel (32 pS) was also observed. 5. The non-selective cation channels were recorded on the luminal membrane, but scarcely on the abluminal membrane, suggesting an active transport of K+ and Na+ across the endocardium. 6. The resting membrane conductance of the EE cells may be provided mostly by non-selective cation channels and 34 pS Ca(2+)-dependent K+ channels.

Cerebrovascular, neuronal, and behavioral effects of long-term Ca2+ channel blockade in aging normotensive and hypertensive rat strains

The pathogenesis of essential hypertension is not fully understood, but most of the cardiovascular, metabolic, neurogenic, and humoral abnormalities are explained by dysfunctions in the control of intracellular Ca2+ concentrations in the cells of the vascular wall. Most theories of disturbed calcium regulation focus on the calcium concentration within vascular smooth muscle cells. The implications of hypertension for the increased calcium content of aging arteries seem to be clear, but were only studied in the peripheral circulation; hypertension prominently augments the aging-related accumulation of calcium in the vessel wall. Although the contribution of calcium overload in hypertensive cerebrovascular damage is well documented, it is not clear yet if hypertension per se is the main cause of hypertension-associated calcium-dependent cerebral damage. Thus far, the hypotensive effects of most calcium antagonists were extensively described, and their efficacy in stroke prevention was proven. Earlier studies indicated that chronic administration of nimodipine revealed a protective effect in the occurrence of strokes in SHR-SP rats, yielding a decreased mortality rate. Because nimodipine did not lower the extremely high blood pressure of these animals, the mechanisms behind such nimodipine-induced stroke prevention may be attributed to a direct cerebrovascular and/or neuronal action of nimodipine. Hypertension is generally considered a vascular pathologic condition, and most research has been directed towards the influences of hypertension on large peripheral arteries such as the aorta and coronary artery. The influence of the CNS on the regulation of cardiovascular system and blood pressure regulation was described in detail, and the role of the CNS in hypertension also was the subject of study. The increased risk of stroke in hypertensive subjects generated numerous studies on the precise nature of compromised cerebrovascular functioning under hypertensive conditions. Few data are available on Ca2+ alterations in cerebral neurons during hypertension. Honda et al. demonstrated that voltage-dependent Ca2+ uptake was higher in cortical synaptosomes from SHR than form normotensive animals and suggested that an important alteration in Ca2+ channel characteristics may occur in SHR brain synaptosomes. Although the density of L-type calcium channels was shown to be higher in the hippocampus of SHR rats, others reported that the number of L-type calcium channels was significantly lower in the brain of SHR rats than WKY normotensive controls. The latter data suggest that hypertension may be associated with similar alterations in neuronal calcium homeostasis as demonstrated for aging in normotensive subjects.(ABSTRACT TRUNCATED AT 400 WORDS)