P2y1 and P2y2 receptor-operated Ca2+ signals in primary cultures of cardiac microvascular endothelial cells

The Molecular Heterogeneity of Store-Operated Ca2+ Entry in Vascular Endothelial Cells: The Different roles of Orai1 and TRPC1/TRPC4 Channels in the Transition from Ca2+-Selective to Non-Selective Cation Currents

Store-operated Ca²⁺ entry (SOCE) is activated in response to the inositol-1,4,5-trisphosphate (InsP₃)-dependent depletion of the endoplasmic reticulum (ER) Ca²⁺ store and represents a ubiquitous mode of Ca²⁺ influx. In vascular endothelial cells, SOCE regulates a plethora of functions that maintain cardiovascular homeostasis, such as angiogenesis, vascular tone, vascular permeability, platelet aggregation, and monocyte adhesion. The molecular mechanisms responsible for SOCE activation in vascular endothelial cells have engendered a long-lasting controversy. Traditionally, it has been assumed that the endothelial SOCE is mediated by two distinct ion channel signalplexes, i.e., STIM1/Orai1 and STIM1/Transient Receptor Potential Canonical 1(TRPC1)/TRPC4. However, recent evidence has shown that Orai1 can assemble with TRPC1 and TRPC4 to form a non-selective cation channel with intermediate electrophysiological features. Herein, we aim at bringing order to the distinct mechanisms that mediate endothelial SOCE in the vascular tree from multiple species (e.g., human, mouse, rat, and bovine). We propose that three distinct currents can mediate SOCE in vascular endothelial cells: (1) the Ca²⁺-selective Ca²⁺-release activated Ca²⁺ current (I_CRAC), which is mediated by STIM1 and Orai1; (2) the store-operated non-selective current (I_SOC), which is mediated by STIM1, TRPC1, and TRPC4; and (3) the moderately Ca²⁺-selective, I_CRAC-like current, which is mediated by STIM1, TRPC1, TRPC4, and Orai1.

Ca2+ dysregulation in cardiac stromal cells sustains fibro-adipose remodeling in Arrhythmogenic Cardiomyopathy and can be modulated by flecainide

BACKGROUND

Cardiac mesenchymal stromal cells (C-MSC) were recently shown to differentiate into adipocytes and myofibroblasts to promote the aberrant remodeling of cardiac tissue that characterizes arrhythmogenic cardiomyopathy (ACM). A calcium (Ca2+) signaling dysfunction, mainly demonstrated in mouse models, is recognized as a mechanism impacting arrhythmic risk in ACM cardiomyocytes. Whether similar mechanisms influence ACM C-MSC fate is still unknown. Thus, we aim to ascertain whether intracellular Ca2+ oscillations and the Ca2+ toolkit are altered in human C-MSC obtained from ACM patients, and to assess their link with C-MSC-specific ACM phenotypes.

METHODS AND RESULTS

ACM C-MSC show enhanced spontaneous Ca2+ oscillations and concomitant increased Ca2+/Calmodulin dependent kinase II (CaMKII) activation compared to control cells. This is manly linked to a constitutive activation of Store-Operated Ca2+ Entry (SOCE), which leads to enhanced Ca2+ release from the endoplasmic reticulum through inositol-1,4,5-trisphosphate receptors. By targeting the Ca2+ handling machinery or CaMKII activity, we demonstrated a causative link between Ca2+ oscillations and fibro-adipogenic differentiation of ACM C-MSC. Genetic silencing of the desmosomal gene PKP2 mimics the remodelling of the Ca2+ signalling machinery occurring in ACM C-MSC. The anti-arrhythmic drug flecainide inhibits intracellular Ca2+ oscillations and fibro-adipogenic differentiation by selectively targeting SOCE.

CONCLUSIONS

Altogether, our results extend the knowledge of Ca2+ dysregulation in ACM to the stromal compartment, as an etiologic mechanism of C-MSC-related ACM phenotypes. A new mode of action of flecainide on a novel mechanistic target is unveiled against the fibro-adipose accumulation in ACM.

Optical excitation of organic semiconductors as a highly selective strategy to induce vascular regeneration and tissue repair

Therapeutic neovascularization represents a promising strategy to rescue the vascular network and restore organ function in cardiovascular disorders (CVDs), including acute myocardial infarction, heart failure, peripheral artery disease, and brain stroke. Endothelial colony forming cells (ECFCs), which are mobilized in circulation upon an ischemic insult, are commonly regarded as the most suitable cellular tool to achieve therapeutic neovascularization. ECFCs can be genetically or pharmacologically manipulated to enhance their vasoreparative potential by boosting specific pro-angiogenic signalling pathways. However, optical stimulation represents the most reliable approach to control cellular activity because of its high selectivity and unprecedented spatio-temporal resolution. Herein, we discuss a novel strategy to drive ECFC angiogenic activity in ischemic tissues by combining geneless optical excitation with photosensitive organic semiconductors. We describe how photoexcitation of the conducting polymer poly(3-hexylthiophene-2,5-diyl), also known as P3HT, stimulates extracellular Ca²⁺ entry through Transient Receptor Potential Vanilloid 1 (TRPV1) channels upon the production of hydrogen peroxide (H₂O₂) in the cleft between the nanomaterial and the cell membrane. H₂O₂-induced TRPV1-dependent Ca²⁺ entry stimulates ECFC proliferation and tube formation, thereby providing the proof-of-concept that photoexcitation of organic semiconductors may offer a reliable strategy to stimulate ECFCs-dependent neovascularization in CVDs.

Reactive Oxygen Species and Endothelial Ca2+ Signaling: Brothers in Arms or Partners in Crime?

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) controls virtually all endothelial cell functions and is, therefore, crucial to maintain cardiovascular homeostasis. An aberrant elevation in endothelial can indeed lead to severe cardiovascular disorders. Likewise, moderate amounts of reactive oxygen species (ROS) induce intracellular Ca²⁺ signals to regulate vascular functions, while excessive ROS production may exploit dysregulated Ca²⁺ dynamics to induce endothelial injury. Herein, we survey how ROS induce endothelial Ca²⁺ signals to regulate vascular functions and, vice versa, how aberrant ROS generation may exploit the Ca²⁺ handling machinery to promote endothelial dysfunction. ROS elicit endothelial Ca²⁺ signals by regulating inositol-1,4,5-trisphosphate receptors, sarco-endoplasmic reticulum Ca²⁺-ATPase 2B, two-pore channels, store-operated Ca²⁺ entry (SOCE), and multiple isoforms of transient receptor potential (TRP) channels. ROS-induced endothelial Ca²⁺ signals regulate endothelial permeability, angiogenesis, and generation of vasorelaxing mediators and can be exploited to induce therapeutic angiogenesis, rescue neurovascular coupling, and induce cancer regression. However, an increase in endothelial [Ca²⁺]_i induced by aberrant ROS formation may result in endothelial dysfunction, inflammatory diseases, metabolic disorders, and pulmonary artery hypertension. This information could pave the way to design alternative treatments to interfere with the life-threatening interconnection between endothelial ROS and Ca²⁺ signaling under multiple pathological conditions.

G protein-coupled purinergic P2Y receptor oligomerization: Pharmacological changes and dynamic regulation

P2Y receptors (P2YRs) are a δ group of rhodopsin-like G protein-coupled receptors (GPCRs) with many essential functions in physiology and pathology, such as platelet aggregation, immune responses, neuroprotective effects, inflammation, and cellular proliferation. Thus, they are among the most researched therapeutic targets used for the clinical treatment of diseases (e.g., the antithrombotic drug clopidogrel and the dry eye treatment drug diquafosol). GPCRs transmit signals as dimers to increase the diversity of signalling pathways and pharmacological activities. Many studies have frequently confirmed dimerization between P2YRs and other GPCRs due to their functions in cardiovascular and cerebrovascular processes in vivo and in vitro. Recently, some P2YR dimers that dynamically balance physiological functions in the body were shown to be involved in effective signal transduction and exert pathological responses. In this review, we summarize the types, pharmacological changes, and active regulators of P2YR-related dimerization, and delineate new functions and pharmacological activities of P2YR-related dimers, which may be a novel direction to improve the effectiveness of medications.

Endolysosomal Ca2+ signaling in cardiovascular health and disease

An increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) regulates a plethora of functions in the cardiovascular (CV) system, including contraction in cardiomyocytes and vascular smooth muscle cells (VSMCs), and angiogenesis in vascular endothelial cells and endothelial colony forming cells. The sarco/endoplasmic reticulum (SR/ER) represents the largest endogenous Ca²⁺ store, which releases Ca²⁺ through ryanodine receptors (RyRs) and/or inositol-1,4,5-trisphosphate receptors (InsP₃Rs) upon extracellular stimulation. The acidic vesicles of the endolysosomal (EL) compartment represent an additional endogenous Ca²⁺ store, which is targeted by several second messengers, including nicotinic acid adenine dinucleotide phosphate (NAADP) and phosphatidylinositol 3,5-bisphosphate [PI(3,5)P₂], and may release intraluminal Ca²⁺ through multiple Ca²⁺ permeable channels, including two-pore channels 1 and 2 (TPC1-2) and Transient Receptor Potential Mucolipin 1 (TRPML1). Herein, we discuss the emerging, pathophysiological role of EL Ca²⁺ signaling in the CV system. We describe the role of cardiac TPCs in β-adrenoceptor stimulation, arrhythmia, hypertrophy, and ischemia-reperfusion injury. We then illustrate the role of EL Ca²⁺ signaling in VSMCs, where TPCs promote vasoconstriction and contribute to pulmonary artery hypertension and atherosclerosis, whereas TRPML1 sustains vasodilation and is also involved in atherosclerosis. Subsequently, we describe the mechanisms whereby endothelial TPCs promote vasodilation, contribute to neurovascular coupling in the brain and stimulate angiogenesis and vasculogenesis. Finally, we discuss about the possibility to target TPCs, which are likely to mediate CV cell infection by the Severe Acute Respiratory Disease-Coronavirus-2, with Food and Drug Administration-approved drugs to alleviate the detrimental effects of Coronavirus Disease-19 on the CV system.

Polarized Proteins in Endothelium and Their Contribution to Function

Protein localization in endothelial cells is tightly regulated to create distinct signaling domains within their tight spatial restrictions including luminal membranes, abluminal membranes, and interendothelial junctions, as well as caveolae and calcium signaling domains. Protein localization in endothelial cells is also determined in part by the vascular bed, with differences between arteries and veins and between large and small arteries. Specific protein polarity and localization is essential for endothelial cells in responding to various extracellular stimuli. In this review, we examine protein localization in the endothelium of resistance arteries, with occasional references to other vessels for contrast, and how that polarization contributes to endothelial function and ultimately whole organism physiology. We highlight the protein localization on the luminal surface, discussing important physiological receptors and the glycocalyx. The protein polarization to the abluminal membrane is especially unique in small resistance arteries with the presence of the myoendothelial junction, a signaling microdomain that regulates vasodilation, feedback to smooth muscle cells, and ultimately total peripheral resistance. We also discuss the interendothelial junction, where tight junctions, adherens junctions, and gap junctions all convene and regulate endothelial function. Finally, we address planar cell polarity, or axial polarity, and how this is regulated by mechanosensory signals like blood flow.

Towards Novel Geneless Approaches for Therapeutic Angiogenesis

Cardiovascular diseases are the leading cause of mortality worldwide. Such a widespread diffusion makes the conditions affecting the heart and blood vessels a primary medical and economic burden. It, therefore, becomes mandatory to identify effective treatments that can alleviate this global problem. Among the different solutions brought to the attention of the medical-scientific community, therapeutic angiogenesis is one of the most promising. However, this approach, which aims to treat cardiovascular diseases by generating new blood vessels in ischemic tissues, has so far led to inadequate results due to several issues. In this perspective, we will discuss cutting-edge approaches and future perspectives to alleviate the potentially lethal impact of cardiovascular diseases. We will focus on the consolidated role of resident endothelial progenitor cells, particularly endothelial colony forming cells, as suitable candidates for cell-based therapy demonstrating the importance of targeting intracellular Ca²⁺ signaling to boost their regenerative outcome. Moreover, we will elucidate the advantages of physical stimuli over traditional approaches. In particular, we will critically discuss recent results obtained by using optical stimulation, as a novel strategy to drive endothelial colony forming cells fate and its potential in the treatment of cardiovascular diseases.

Targeting Purinergic Signaling and Cell Therapy in Cardiovascular and Neurodegenerative Diseases

Extracellular purines exert several functions in physiological and pathophysiological mechanisms. ATP acts through P2 receptors as a neurotransmitter and neuromodulator and modulates heart contractility, while adenosine participates in neurotransmission, blood pressure, and many other mechanisms. Because of their capability to differentiate into mature cell types, they provide a unique therapeutic strategy for regenerating damaged tissue, such as in cardiovascular and neurodegenerative diseases. Purinergic signaling is pivotal for controlling stem cell differentiation and phenotype determination. Proliferation, differentiation, and apoptosis of stem cells of various origins are regulated by purinergic receptors. In this chapter, we selected neurodegenerative and cardiovascular diseases with clinical trials using cell therapy and purinergic receptor targeting. We discuss these approaches as therapeutic alternatives to neurodegenerative and cardiovascular diseases. For instance, promising results were demonstrated in the utilization of mesenchymal stem cells and bone marrow mononuclear cells in vascular regeneration. Regarding neurodegenerative diseases, in general, P2X7 and A_2A receptors mostly worsen the degenerative state. Stem cell-based therapy, mainly through mesenchymal and hematopoietic stem cells, showed promising results in improving symptoms caused by neurodegeneration. We propose that purinergic receptor activity regulation combined with stem cells could enhance proliferative and differentiation rates as well as cell engraftment.

Purinergic Calcium Signals in Tumor-Derived Endothelium

Tumor microenvironment is particularly enriched with extracellular ATP (eATP), but conflicting evidence has been provided on its functional effects on tumor growth and vascular remodeling. We have previously shown that high eATP concentrations exert a strong anti-migratory, antiangiogenic and normalizing activity on human tumor-derived endothelial cells (TECs). Since both metabotropic and ionotropic purinergic receptors trigger cytosolic calcium increase ([Ca2+]c), the present work investigated the properties of [Ca2+]c events elicited by high eATP in TECs and their role in anti-migratory activity. In particular, the quantitative and kinetic properties of purinergic-induced Ca2+ release from intracellular stores and Ca2+ entry from extracellular medium were investigated. The main conclusions are: (1) stimulation of TECs with high eATP triggers [Ca2+]c signals which include Ca2+ mobilization from intracellular stores (mainly ER) and Ca2+ entry through the plasma membrane; (2) the long-lasting Ca2+ influx phase requires both store-operated Ca2+ entry (SOCE) and non-SOCE components; (3) SOCE is not significantly involved in the antimigratory effect of high ATP stimulation; (4) ER is the main source for intracellular Ca2+ release by eATP: it is required for the constitutive migratory potential of TECs but is not the only determinant for the inhibitory effect of high eATP; (5) a complex interplay occurs among ER, mitochondria and lysosomes upon purinergic stimulation; (6) high eUTP is unable to inhibit TEC migration and evokes [Ca2+]c signals very similar to those described for eATP. The potential role played by store-independent Ca2+ entry and Ca2+-independent events in the regulation of TEC migration by high purinergic stimula deserves future investigation.

Glutamate triggers intracellular Ca2+ oscillations and nitric oxide release by inducing NAADP- and InsP3 -dependent Ca2+ release in mouse brain endothelial cells

The neurotransmitter glutamate increases cerebral blood flow by activating postsynaptic neurons and presynaptic glial cells within the neurovascular unit. Glutamate does so by causing an increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) in the target cells, which activates the Ca²⁺ /Calmodulin-dependent nitric oxide (NO) synthase to release NO. It is unclear whether brain endothelial cells also sense glutamate through an elevation in [Ca²⁺ ]_i and NO production. The current study assessed whether and how glutamate drives Ca²⁺ -dependent NO release in bEND5 cells, an established model of brain endothelial cells. We found that glutamate induced a dose-dependent oscillatory increase in [Ca²⁺ ]_i , which was maximally activated at 200 μM and inhibited by α-methyl-4-carboxyphenylglycine, a selective blocker of Group 1 metabotropic glutamate receptors. Glutamate-induced intracellular Ca²⁺ oscillations were triggered by rhythmic endogenous Ca²⁺ mobilization and maintained over time by extracellular Ca²⁺ entry. Pharmacological manipulation revealed that glutamate-induced endogenous Ca²⁺ release was mediated by InsP₃ -sensitive receptors and nicotinic acid adenine dinucleotide phosphate (NAADP) gated two-pore channel 1. Constitutive store-operated Ca²⁺ entry mediated Ca²⁺ entry during ongoing Ca²⁺ oscillations. Finally, glutamate evoked a robust, although delayed increase in NO levels, which was blocked by pharmacologically inhibition of the accompanying intracellular Ca²⁺ signals. Of note, glutamate induced Ca²⁺ -dependent NO release also in hCMEC/D3 cells, an established model of human brain microvascular endothelial cells. This investigation demonstrates for the first time that metabotropic glutamate-induced intracellular Ca²⁺ oscillations and NO release have the potential to impact on neurovascular coupling in the brain.

Muscarinic M5 receptors trigger acetylcholine-induced Ca2+ signals and nitric oxide release in human brain microvascular endothelial cells

Basal forebrain neurons control cerebral blood flow (CBF) by releasing acetylcholine (Ach), which binds to endothelial muscarinic receptors to induce nitric (NO) release and vasodilation in intraparenchymal arterioles. Nevertheless, the mechanism whereby Ach stimulates human brain microvascular endothelial cells to produce NO is still unknown. Herein, we sought to assess whether Ach stimulates NO production in a Ca²⁺ -dependent manner in hCMEC/D3 cells, a widespread model of human brain microvascular endothelial cells. Ach induced a dose-dependent increase in intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ) that was prevented by the genetic blockade of M5 muscarinic receptors (M5-mAchRs), which was the only mAchR isoform coupled to phospholipase Cβ (PLCβ) present in hCMEC/D3 cells. A comprehensive real-time polymerase chain reaction analysis revealed the expression of the transcripts encoding for type 3 inositol-1,4,5-trisphosphate receptors (InsP₃ R3), two-pore channels 1 and 2 (TPC1-2), Stim2, Orai1-3. Pharmacological manipulation showed that the Ca²⁺ response to Ach was mediated by InsP₃ R3, TPC1-2, and store-operated Ca²⁺ entry (SOCE). Ach-induced NO release, in turn, was inhibited in cells deficient of M5-mAchRs. Likewise, Ach failed to increase NO levels in the presence of l-NAME, a selective NOS inhibitor, or BAPTA, a membrane-permeant intracellular Ca²⁺ buffer. Moreover, the pharmacological blockade of the Ca²⁺ response to Ach also inhibited the accompanying NO production. These data demonstrate for the first time that synaptically released Ach may trigger NO release in human brain microvascular endothelial cells by stimulating a Ca²⁺ signal via M5-mAchRs.

Connexin 43 Hemichannel Activity Promoted by Pro-Inflammatory Cytokines and High Glucose Alters Endothelial Cell Function

The present work was done to elucidate whether hemichannels of a cell line derived from endothelial cells are affected by pro-inflammatory conditions (high glucose and IL-1β/TNF-α) known to lead to vascular dysfunction. We used EAhy 926 cells treated with high glucose and IL-1β/TNF-α. The hemichannel activity was evaluated with the dye uptake method and was abrogated with selective inhibitors or knocking down of hemichannel protein subunits with siRNA. Western blot analysis, cell surface biotinylation, and confocal microscopy were used to evaluate total and plasma membrane amounts of specific proteins and their cellular distribution, respectively. Changes in intracellular Ca²⁺ and nitric oxide (NO) signals were estimated by measuring FURA-2 and DAF-FM probes, respectively. High glucose concentration was found to elevate dye uptake, a response that was enhanced by IL-1β/TNF-α. High glucose plus IL-1β/TNF-α-induced dye uptake was abrogated by connexin 43 (Cx43) but not pannexin1 knockdown. Furthermore, Cx43 hemichannel activity was associated with enhanced ATP release and activation of p38 MAPK, inducible NO synthase, COX₂, PGE₂ receptor EP₁, and P2X₇/P2Y₁ receptors. Inhibition of the above pathways prevented completely the increase in Cx43 hemichannel activity of cells treated high glucose and IL-1β/TNF-α. Both synthetic and endogenous cannabinoids (CBs) also prevented the increment in Cx43 hemichannel opening, as well as the subsequent generation and release of ATP and NO induced by pro-inflammatory conditions. The counteracting action of CBs also was extended to other endothelial alterations evoked by IL-1β/TNF-α and high glucose, including increased ATP-dependent Ca²⁺ dynamics and insulin-induced NO production. Finally, inhibition of Cx43 hemichannels also prevented the ATP release from endothelial cells treated with IL-1β/TNF-α and high glucose. Therefore, we propose that reduction of hemichannel activity could represent a strategy against the activation of deleterious pathways that lead to endothelial dysfunction and possibly cell damage evoked by high glucose and pro-inflammatory conditions during cardiovascular diseases.

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.

Activation of P2X7 and P2Y11 purinergic receptors inhibits migration and normalizes tumor-derived endothelial cells via cAMP signaling

Purinergic signaling is involved in inflammation and cancer. Extracellular ATP accumulates in tumor interstitium, reaching hundreds micromolar concentrations, but its functional role on tumor vasculature and endothelium is unknown. Here we show that high ATP doses (>20 μM) strongly inhibit migration of endothelial cells from human breast carcinoma (BTEC), but not of normal human microvascular EC. Lower doses (1–10 mm result ineffective. The anti-migratory activity is associated with cytoskeleton remodeling and is significantly prevented by hypoxia. Pharmacological and molecular evidences suggest a major role for P2X7R and P2Y11R in ATP-mediated inhibition of TEC migration: selective activation of these purinergic receptors by BzATP mimics the anti-migratory effect of ATP, which is in turn impaired by their pharmacological or molecular silencing. Downstream pathway includes calcium-dependent Adenilyl Cyclase 10 (AC10) recruitment, cAMP release and EPAC-1 activation. Notably, high ATP enhances TEC-mediated attraction of human pericytes, leading to a decrease of endothelial permeability, a hallmark of vessel normalization. Finally, we provide the first evidence of in vivo P2X7R expression in blood vessels of murine and human breast carcinoma. In conclusion, we have identified a purinergic pathway selectively acting as an antiangiogenic and normalizing signal for human tumor-derived vascular endothelium.

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

How to utilize Ca²⁺ signals to rejuvenate the repairative phenotype of senescent endothelial progenitor cells in elderly patients affected by cardiovascular diseases: a useful therapeutic support of surgical approach?

Endothelial dysfunction or loss is the early event that leads to a host of severe cardiovascular diseases, such as atherosclerosis, hypertension, brain stroke, myocardial infarction, and peripheral artery disease. Ageing is regarded among the most detrimental risk factor for vascular endothelium and predisposes the subject to atheroscleorosis and inflammatory states even in absence of traditional comorbid conditions. Standard treatment to restore blood perfusion through stenotic arteries are surgical or endovascular revascularization. Unfortunately, ageing patients are not the most amenable candidates for such interventions, due to high operative risk or unfavourable vascular involvement. It has recently been suggested that the transplantation of autologous bone marrow-derived endothelial progenitor cells (EPCs) might constitute an alternative and viable therapeutic option for these individuals. Albeit pre-clinical studies demonstrated the feasibility of EPC-based therapy to recapitulate the diseased vasculature of young and healthy animals, clinical studies provided less impressive results in old ischemic human patients. One hurdle associated to this kind of approach is the senescence of autologous EPCs, which are less abundant in peripheral blood and display a reduced pro-angiogenic activity. Conversely, umbilical cord blood (UCB)-derived EPCs are more suitable for cellular therapeutics due to their higher frequency and sensitivity to growth factors, such as vascular endothelial growth factor (VEGF). An increase in intracellular Ca²⁺ concentration is central to EPC activation by VEGF. We have recently demonstrated that the Ca²⁺ signalling machinery driving the oscillatory Ca²⁺ response to this important growth factor is different in UCB-derived EPCs as compared to their peripheral counterparts. In particular, we focussed on the so-called endothelial colony forming cells (ECFCs), which are the only EPC population belonging to the endothelial lineage and able to form capillary-like structures in vitro and stably integrate with host vasculature in vivo. The present review provides a brief description of how exploiting the Ca²⁺ toolkit of juvenile EPCs to restore the repairative phenotype of senescent EPCs to enhance their regenerative outcome in therapeutic settings.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Diabetes alters intracellular calcium transients in cardiac endothelial cells

Diabetic cardiomyopathy (DCM) is a diabetic complication, which results in myocardial dysfunction independent of other etiological factors. Abnormal intracellular calcium ([Ca²⁺]_i) homeostasis has been implicated in DCM and may precede clinical manifestation. Studies in cardiomyocytes have shown that diabetes results in impaired [Ca²⁺]_i homeostasis due to altered sarcoplasmic reticulum Ca²⁺ ATPase (SERCA) and sodium-calcium exchanger (NCX) activity. Importantly, altered calcium homeostasis may also be involved in diabetes-associated endothelial dysfunction, including impaired endothelium-dependent relaxation and a diminished capacity to generate nitric oxide (NO), elevated cell adhesion molecules, and decreased angiogenic growth factors. However, the effect of diabetes on Ca²⁺ regulatory mechanisms in cardiac endothelial cells (CECs) remains unknown. The objective of this study was to determine the effect of diabetes on [Ca²⁺]_i homeostasis in CECs in the rat model (streptozotocin-induced) of DCM. DCM-associated cardiac fibrosis was confirmed using picrosirius red staining of the myocardium. CECs isolated from the myocardium of diabetic and wild-type rats were loaded with Fura-2, and UTP-evoked [Ca²⁺]_i transients were compared under various combinations of SERCA, sarcoplasmic reticulum Ca²⁺ ATPase (PMCA) and NCX inhibitors. Diabetes resulted in significant alterations in SERCA and NCX activities in CECs during [Ca²⁺]_i sequestration and efflux, respectively, while no difference in PMCA activity between diabetic and wild-type cells was observed. These results improve our understanding of how diabetes affects calcium regulation in CECs, and may contribute to the development of new therapies for DCM treatment.

Functional contribution of P2Y1 receptors to the control of coronary blood flow

Activation of ADP-sensitive P2Y(1) receptors has been proposed as an integral step in the putative "nucleotide axis" regulating coronary blood flow. However, the specific mechanism(s) and overall contribution of P2Y(1) receptors to the control of coronary blood flow have not been clearly defined. Using vertically integrative studies in isolated coronary arterioles and open-chest anesthetized dogs, we examined the hypothesis that P2Y(1) receptors induce coronary vasodilation via an endothelium-dependent mechanism and contribute to coronary pressure-flow autoregulation and/or ischemic coronary vasodilation. Immunohistochemistry revealed P2Y(1) receptor expression in coronary arteriolar endothelial and vascular smooth muscle cells. The ADP analog 2-methylthio-ADP induced arteriolar dilation in vitro and in vivo that was abolished by the selective P2Y(1) antagonist MRS-2179 and the nitric oxide synthase inhibitor N(G)-nitro-l-arginine methyl ester. MRS-2179 did not alter baseline coronary flow in vivo but significantly attenuated coronary vasodilation to ATP in vitro and in vivo and the nonhydrolyzable ATP analog ATPγS in vitro. Coronary blood flow responses to alterations in coronary perfusion pressure (40-100 mmHg) or to a brief 15-s coronary artery occlusion were unaffected by MRS-2179. Our data reveal that P2Y(1) receptors are functionally expressed in the coronary circulation and that activation produces coronary vasodilation via an endothelium/nitric oxide-dependent mechanism. Although these receptors represent a critical component of purinergic coronary vasodilation, our findings indicate that P2Y(1) receptor activation is not required for coronary pressure-flow autoregulation or reactive hyperemia.

P2Y2 receptor desensitization on single endothelial cells

Receptor desensitization, or decreased responsiveness of a receptor to agonist stimulation, represents a regulatory process with the potential to have a significant impact on cell behavior. P2Y(2), a G-protein-coupled receptor activated by extracellular nucleotides, undergoes desensitization at many tissues, including the vascular endothelium. Endothelial cells from a variety of vascular beds are normally exposed to extracellular nucleotides released from damaged cells and activated platelets. The purpose of the present study was to compare P2Y(2) receptor desensitization observed in endothelial cells derived from bovine retina, a model of microvascular endothelium, and human umbilical vein endothelial cells (HUVECs), a model of a large blood vessel endothelium. P2Y(2) receptor desensitization was monitored by following changes in UTP-stimulated intracellular free Ca(2 +) in single cells using fura-2 microfluorometry. Both endothelial cell models exhibited desensitization of the P2Y(2) receptor after stimulation with UTP. However, the cells differed in the rate, dependence on agonist concentration, and percentage of maximal desensitization. These results suggest differential mechanisms of P2Y(2) receptor desensitization and favors heterogeneity in extracellular nucleotide activity in endothelial cells according to its vascular bed origin.

Effects of external ATP on Ca(2+) signalling in endothelial cells isolated from mouse islets

External ATP is believed to initiate and propagate Ca(2+) signals co-ordinating the insulin release pulses within and among the different islets in the pancreas. The possibility that islet endothelial cells participate in this process was evaluated by comparing the effects on [Ca(2+)](i) of purinoceptor activation in these cells with those in beta-cells. beta-Cell-rich pancreatic islets were isolated from ob/ob mice and dispersed into single cells/aggregates. After culture with or without endothelial cell growth supplement (ECGS), the cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) was measured with ratiometric fura-2 technique. Presence of ECGS or prolongation of culture (>5 days) resulted in proliferation of endothelial cells and altered their phenotype from rounded to elongated. Endothelial cells, preliminarily identified by attachment of Dynabeads coated with the Bandeiraea simplicifolia 1 lectin (BS-1), responded in a similar way as those stained with CD31 antibodies after measurements of [Ca(2+)](i). Spontaneous transients and oscillations of [Ca(2+)](i )were seen in beta-cells, but not in endothelial cells exposed to 20 mM glucose. Addition of ATP (10 microM) resulted in pronounced and more extended rise of [Ca(2+)](i) in endothelial cells than in beta-cells. The endothelial cells differed from the beta-cells by also responding with a rise of [Ca(2+)](i) to 10 microM UTP, but not to equimolar ADP and acetylcholine. The results support the idea of mutual interactions between islet endothelium and beta-cells based on ATP-induced Ca(2+) signals. It is suggested that the endothelial cells have a tonic inhibitory action on beta-cell P2 purinoceptors resulting in impaired synchronization of the insulin release pulses.

Melatonin inhibits nitric oxide production by microvascular endothelial cells in vivo and in vitro

BACKGROUND AND PURPOSE

We have previously shown that melatonin inhibits bradykinin-induced NO production by endothelial cells in vitro. The purpose of this investigation was to extend this observation to an in vivo condition and to explore the mechanism of action of melatonin.

EXPERIMENTAL APPROACH

RT-PCR assays were performed with rat cultured endothelial cells. The putative effect of melatonin upon arteriolar tone was investigated by intravital microscopy while NO production by endothelial cells in vitro was assayed by fluorimetry, and intracellular Ca(2+) measurements were assayed by confocal microscopy.

KEY RESULTS

No expression of the mRNA for the melatonin synthesizing enzymes, arylalkylamine N-acetyltransferase and hydroxyindole-O-methyltransferase, or for the melatonin MT(2) receptor was detected in microvascular endothelial cells. Melatonin fully inhibited L-NAME-sensitive bradykinin-induced vasodilation and also inhibited NO production induced by histamine, carbachol and 2-methylthio ATP, but did not inhibit NO production induced by ATP or alpha, beta-methylene ATP. None of its inhibitory effects was prevented by the melatonin receptor antagonist, luzindole. In nominally Ca(2+)-free solution, melatonin reduced intracellular Ca(2+) mobilization induced by bradykinin (40%) and 2-methylthio ATP (62%) but not Ca(2+) mobilization induced by ATP.

CONCLUSIONS AND IMPLICATIONS

We have confirmed that melatonin inhibited NO production both in vivo and in vitro. In addition, the melatonin effect was selective for some G protein-coupled receptors and most probably reflects an inhibition of Ca(2+) mobilization from intracellular stores.

The duration and amplitude of the plateau phase of ATP- and ADP-evoked Ca(2+) signals are modulated by ectonucleotidases in in situ endothelial cells of rat aorta

ATP has a long-lasting vasodilatory effect, possibly due to its capability to induce a prolonged increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in endothelial cells (EC) and activate constitutive nitric oxide synthase. However, contradictory data have been reported regarding the time course of ATP-evoked Ca(2+) signals in in situ EC. In particular, short-duration Ca(2+) signals have been reported, which might be thought to be unable to sustain a prolonged, NO-induced vasodilation. The current experiments were therefore performed in in situ EC of rat aorta in order to more fully define the time course of ATP-evoked Ca(2+) signals. 20 microM ATP evoked a short-lasting Ca(2+) signal. However, medium stirring, high agonist concentrations, inhibition of ectonucleotidases and application of a poorly hydrolyzable agonist evoked long-lasting Ca(2+) signals (up to 20 min at 37 degrees C). These studies suggest that ATP is able to sustain a prolonged [Ca(2+)](i) increase, unless ectonucleotidase activity reduces the agonist concentration near the EC surface to subthreshold values, quickly cutting the Ca(2+) signal. Furthermore, the amplitude of the long-lasting phase of the Ca(2+) signal depended on the balance between agonist degradation by ectonucleotidases and agonist transport, by diffusion and convection, from bulk solution to the EC surface.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Expression and function of neuronal nicotinic ACh receptors in rat microvascular endothelial cells

The expression and function of nicotinic ACh receptors (nAChRs) in rat coronary microvascular endothelial cells (CMECs) were examined using RT-PCR and whole cell patch-clamp recording methods. RT-PCR revealed expression of mRNA encoding for the subunits alpha(2), alpha(3), alpha(4), alpha(5), alpha(7), beta(2), and beta(4) but not beta(3). Focal application of ACh evoked an inward current in isolated CMECs voltage clamped at negative membrane potentials. The current-voltage relationship of the ACh-induced current exhibited marked inward rectification and a reversal potential (E(rev)) close to 0 mV. The cholinergic agonists nicotine, epibatidine, and cytisine activated membrane currents similar to those evoked by ACh. The nicotine-induced current was abolished by the neuronal nAChR antagonist mecamylamine. The direction and magnitude of the shift in E(rev) of nicotine-induced current as a function of extracellular Na(+) concentration indicate that the nAChR channel is cation selective and follows that predicted by the Goldman-Hodgkin-Katz equation assuming K(+)/Na(+) permeability ratio of 1.11. In fura-2-loaded CMECs, application of ACh, but not of nicotine, elicited a transient increase in intracellular free Ca(2+) concentration. Taken together, these results demonstrate that neuronal nAChR activation by cholinergic agonists evokes an inward current in CMECs carried primarily by Na(+), which may contribute to the plasma nicotine-induced changes in microvascular permeability and reactivity induced by elevations in plasma nicotine.

Basal nonselective cation permeability in rat cardiac microvascular endothelial cells

The presence of a basal nonselective cation permeability was mainly investigated in primary cultures of rat cardiac microvascular endothelial cells (CMEC) by applying both the patch-clamp technique and Fura-2 microfluorimetry. With low EGTA in the pipette solution, the resting membrane potential of CMEC was -21.2 +/- 1.1 mV, and a Ca(2+)-activated Cl(-) conductance was present. When the intracellular Ca(2+) was buffered with high EGTA, the membrane potential decreased to 5.5 +/- 1.2 mV. In this condition, full or partial substitution of external Na(+) by NMDG(+) proportionally reduced the inward component of the basal I-V relationship. This current was dependent on extracellular monovalent cations with a permeability sequence of K(+) > Cs(+) > Na(+) > Li(+) and was inhibited by Ca(2+), La(3+), Gd(3+), and amiloride. The K(+)/Na(+) permeability ratio, determined using the Goldman-Hodgkin-Katz equation, was 2.01. The outward component of the basal I-V relationship was reduced when intracellular K(+) was replaced by NMDG(+), but was not sensitive to substitution by Cs(+). Finally, microfluorimetric experiments indicated the existence of a basal Ca(2+) entry pathway, inhibited by La(3+) and Gd(3+). The basal nonselective cation permeability in CMEC could be involved both in the control of myocardial ionic homeostasis, according to the model of the blood-heart barrier, and in the modulation of Ca(2+)-dependent processes.

Ca2+ uptake by the endoplasmic reticulum Ca2+-ATPase in rat microvascular endothelial cells

In non-excitable cells, many agonists increase the intracellular Ca(2+) concentration ([Ca(2+)](i)) by inducing an inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release from the intracellular stores. Ca(2+) influx from the extracellular medium may then sustain the Ca(2+) signal. [Ca(2+)](i) recovers its resting level as a consequence of Ca(2+)-removing mechanisms, i.e. plasma-membrane Ca(2+)-ATPase (PMCA) pump, Na(+)/Ca(2+) exchanger (NCX) and sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) pump. In a study performed in pancreatic acinar cells, evidence has been provided suggesting that, during the decay phase of the agonist-evoked Ca(2+) transients, the Ca(2+) concentration within the intracellular stores remains essentially constant [Mogami, Tepikin and Petersen (1998) EMBO J. 17, 435-442]. It was therefore hypothesized that, in such a situation, intracellular Ca(2+) is not only picked up by the SERCA pump, but is also newly released through IP(3)-sensitive Ca(2+) channels, with the balance between these two processes being approximately null. The main aim of the present work was to test this hypothesis by a different experimental approach. Using cardiac microvascular endothelial cells, we found that inhibition of the SERCA pump has no effect on the time course of agonist-evoked Ca(2+) transients. This result was not due to a low capacity of the SERCA pump since, after agonist removal, this pump proved to be very powerful in clearing the excess of intracellular Ca(2+). We showed further that: (i) in order to avoid a rapid removal of Ca(2+) by the SERCA pump, continuous IP(3) production appears to be required throughout all of the decay phase of the Ca(2+) transient; and (ii) Ca(2+) picked up by the SERCA pump can be fully and immediately released by agonist application. All these results support the model of Mogami, Tepikin and Petersen [(1998) EMBO J. 17, 435-442]. Since the SERCA pump did not appear to be involved in shaping the decay phase of the agonist-evoked Ca(2+) transient, we inhibited the PMCA pump with carboxyeosin, and NCX with benzamil and by removing extracellular Na(+). The results indicate that, during the decay phase of the agonist-evoked Ca(2+) transient, the intracellular Ca(2+) is removed by both the PMCA pump and NCX. Finally, we provide evidence indicating that mitochondria have no role in clearing intracellular Ca(2+) during agonist-evoked Ca(2+) transients.