Evidence for a receptor-activated Ca2+ entry pathway independent from Ca2) store depletion in endothelial cells

Endothelial Dysfunction Following Enhanced TMEM16A Activity in Human Pulmonary Arteries

Endothelial dysfunction is one of the hallmarks of different vascular diseases, including pulmonary arterial hypertension (PAH). Ion channelome changes have long been connected to vascular remodeling in PAH, yet only recently has the focus shifted towards Ca²⁺-activated Cl⁻ channels (CaCC). The most prominent member of the CaCC TMEM16A has been shown to contribute to the pathogenesis of idiopathic PAH (IPAH) in pulmonary arterial smooth muscle cells, however its role in the homeostasis of healthy human pulmonary arterial endothelial cells (PAECs) and in the development of endothelial dysfunction remains underrepresented. Here we report enhanced TMEM16A activity in IPAH PAECs by whole-cell patch-clamp recordings. Using adenoviral-mediated TMEM16A increase in healthy primary human PAECs in vitro and in human pulmonary arteries ex vivo, we demonstrate the functional consequences of the augmented TMEM16A activity: alterations of Ca²⁺ dynamics and eNOS activity as well as decreased NO production, PAECs proliferation, wound healing, tube formation and acetylcholine-mediated relaxation of human pulmonary arteries. We propose that the ERK1/2 pathway is specifically affected by elevated TMEM16A activity, leading to these pathological changes. With this work we introduce increased TMEM16A activity in the cell membrane of human PAECs for the development of endothelial dysfunction in PAH.

Tissue Specificity: SOCE: Implications for Ca2+ Handling in Endothelial Cells

Many cellular functions of the vascular endothelium are regulated by fine-tuned global and local, microdomain-confined changes of cytosolic free Ca²⁺ ([Ca²⁺]_i). Vasoactive agonist-induced stimulation of vascular endothelial cells (VECs) typically induces Ca²⁺ release through IP₃ receptor Ca²⁺ release channels embedded in the membrane of the endoplasmic reticulum (ER) Ca²⁺ store, followed by Ca²⁺ entry from the extracellular space elicited by Ca²⁺ store depletion and referred to as capacitative or store-operated Ca²⁺ entry (SOCE). In vascular endothelial cells, SOCE is graded with the degree of store depletion and controlled locally in the subcellular microdomain where depletion occurs. SOCE provides distinct Ca²⁺ signals that selectively control specific endothelial functions: in calf pulmonary artery endothelial cells, the SOCE Ca²⁺ signal drives nitric oxide (an endothelium-derived relaxing factor of the vascular smooth muscle) production and controls activation and nuclear translocation of the transcription factor NFAT. Both cellular events are not affected by Ca²⁺ signals of comparable magnitude arising directly from Ca²⁺ release from intracellular stores, clearly indicating that SOCE regulates specific Ca²⁺-dependent cellular tasks by a unique and exclusive mechanism. This review discusses the mechanisms of intracellular Ca²⁺ regulation in vascular endothelial cells and the role of store-operated Ca²⁺ entry for endothelium-dependent smooth muscle relaxation and nitric oxide signaling, endothelial oxidative stress response, and excitation-transcription coupling in the vascular endothelium.

N-Arachidonoyl glycine suppresses Na⁺/Ca²⁺ exchanger-mediated Ca²⁺ entry into endothelial cells and activates BK(Ca) channels independently of GPCRs

Background and Purpose

N-arachidonoyl glycine (NAGly) is a lipoamino acid with vasorelaxant properties. We aimed to explore the mechanisms of NAGly's action on unstimulated and agonist-stimulated endothelial cells.

Experimental Approach

The effects of NAGly on endothelial electrical signalling were studied in combination with vascular reactivity.

Key Results

In EA.hy926 cells, the sustained hyperpolarization to histamine was inhibited by the non-selective Na⁺/Ca²⁺ exchanger (NCX) inhibitor bepridil and by an inhibitor of reversed mode NCX, KB-R7943. In cells dialysed with Cs⁺-based Na⁺-containing solution, the outwardly rectifying current with typical characteristics of NCX was augmented following histamine exposure, further increased upon external Na⁺ withdrawal and inhibited by bepridil. NAGly (0.3–30 μM) suppressed NCX currents in a URB597- and guanosine 5′-O-(2-thiodiphosphate) (GDPβS)-insensitive manner, [Ca²⁺]_i elevation evoked by Na⁺ removal and the hyperpolarization to histamine. In rat aorta, NAGly opposed the endothelial hyperpolarization and relaxation response to ACh. In unstimulated EA.hy926 cells, NAGly potentiated the whole-cell current attributable to large-conductance Ca²⁺-activated K⁺ (BK_Ca) channels in a GDPβS-insensitive, paxilline-sensitive manner and produced a sustained hyperpolarization. In cell-free inside-out patches, NAGly stimulated single BK_Ca channel activity.

Conclusion and Implications

Our data showed that NCX is a Ca²⁺ entry pathway in endothelial cells and that NAGly is a potent G-protein-independent modulator of endothelial electrical signalling and has a dual effect on endothelial electrical responses. In agonist pre-stimulated cells, NAGly opposes hyperpolarization and relaxation via inhibition of NCX-mediated Ca²⁺ entry, while in unstimulated cells, it promotes hyperpolarization via receptor-independent activation of BK_Ca channels.

The endocannabinoid N-arachidonoyl glycine (NAGly) inhibits store-operated Ca2+ entry by preventing STIM1-Orai1 interaction

The endocannabiniod anandamide (AEA) and its derivate N-arachidonoyl glycine (NAGly) have a broad spectrum of physiological effects, which are induced by both binding to receptors and receptor-independent modulations of ion channels and transporters. The impact of AEA and NAGly on store-operated Ca(2+) entry (SOCE), a ubiquitous Ca(2+) entry pathway regulating many cellular functions, is unknown. Here we show that NAGly, but not AEA reversibly hinders SOCE in a time- and concentration-dependent manner. The inhibitory effect of NAGly on SOCE was found in the human endothelial cell line EA.hy926, the rat pancreatic β-cell line INS-1 832/13, and the rat basophilic leukemia cell line RBL-2H3. NAGly diminished SOCE independently from the mode of Ca(2+) depletion of the endoplasmic reticulum, whereas it had no effect on Ca(2+) entry through L-type voltage-gated Ca(2+) channels. Enhanced Ca(2+) entry was effectively hampered by NAGly in cells overexpressing the key molecular constituents of SOCE, stromal interacting molecule 1 (STIM1) and the pore-forming subunit of SOCE channels, Orai1. Fluorescence microscopy revealed that NAGly did not affect STIM1 oligomerization, STIM1 clustering, or the colocalization of STIM1 with Orai1, which were induced by Ca(2+) depletion of the endoplasmic reticulum. In contrast, independently from its slow depolarizing effect on mitochondria, NAGly instantly and strongly diminished the interaction of STIM1 with Orai1, indicating that NAGly inhibits SOCE primarily by uncoupling STIM1 from Orai1. In summary, our findings revealed the STIM1-Orai1-mediated SOCE machinery as a molecular target of NAGly, which might have many implications in cell physiology.

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.

Transient receptor potential canonical channels are required for in vitro endothelial tube formation

In endothelial cells Ca(2+) entry is an essential component of the Ca(2+) signal that takes place during processes such as cell proliferation or angiogenesis. Ca(2+) influx occurs via the store-operated Ca(2+) entry pathway, involving stromal interaction molecule-1 (STIM1) and Orai1, but also through channels gated by second messengers like the transient receptor potential canonical (TRPC) channels. The human umbilical vein-derived endothelial cell line EA.hy926 expressed STIM1 and Orai1 as well as several TRPC channels. By invalidating each of these molecules, we showed that TRPC3, TRPC4, and TRPC5 are essential for the formation of tubular structures observed after EA.hy926 cells were plated on Matrigel. On the contrary, the silencing of STIM1 or Orai1 did not prevent tubulogenesis. Soon after being plated on Matrigel, the cells displayed spontaneous Ca(2+) oscillations that were strongly reduced by treatment with siRNA against TRPC3, TRPC4, or TRPC5, but not siRNA against STIM1 or Orai1. Furthermore, we showed that cell proliferation was reduced upon siRNA treatment against TRPC3, TRPC5, and Orai1 channels, whereas the knockdown of STIM1 had no effect. On primary human umbilical vein endothelial cells, TRPC1, TRPC4, and STIM1 are involved in tube formation, whereas Orai1 has no effect. These data showed that TRPC channels are essential for in vitro tubulogenesis, both on endothelial cell line and on primary endothelial cells.

Uncoupling protein 3 (UCP3) modulates the activity of Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) by decreasing mitochondrial ATP production

The uncoupling proteins UCP2 and UCP3 have been postulated to catalyze Ca(2+) entry across the inner membrane of mitochondria, but this proposal is disputed, and other, unrelated proteins have since been identified as the mitochondrial Ca(2+) uniporter. To clarify the role of UCPs in mitochondrial Ca(2+) handling, we down-regulated the expression of the only uncoupling protein of HeLa cells, UCP3, and measured Ca(2+) and ATP levels in the cytosol and in organelles with genetically encoded probes. UCP3 silencing did not alter mitochondrial Ca(2+) uptake in permeabilized cells. In intact cells, however, UCP3 depletion increased mitochondrial ATP production and strongly reduced the cytosolic and mitochondrial Ca(2+) elevations evoked by histamine. The reduced Ca(2+) elevations were due to inhibition of store-operated Ca(2+) entry and reduced depletion of endoplasmic reticulum (ER) Ca(2+) stores. UCP3 depletion accelerated the ER Ca(2+) refilling kinetics, indicating that the activity of sarco/endoplasmic reticulum Ca(2+) (SERCA) pumps was increased. Accordingly, SERCA inhibitors reversed the effects of UCP3 depletion on cytosolic, ER, and mitochondrial Ca(2+) responses. Our results indicate that UCP3 is not a mitochondrial Ca(2+) uniporter and that it instead negatively modulates the activity of SERCA by limiting mitochondrial ATP production. The effects of UCP3 on mitochondrial Ca(2+) thus reflect metabolic alterations that impact on cellular Ca(2+) homeostasis. The sensitivity of SERCA to mitochondrial ATP production suggests that mitochondria control the local ATP availability at ER Ca(2+) uptake and release sites.

Thapsigargin activates Ca²+ entry both by store-dependent, STIM1/Orai1-mediated, and store-independent, TRPC3/PLC/PKC-mediated pathways in human endothelial cells

The ER Ca²+ sensor STIM1 and the Ca²+ channel Orai1 are key players in store-operated Ca²+ entry (SOCE). In addition, channels from the TRPC family were also shown to be engaged during SOCE, while their precise implication remains controversial. In this study, we investigated the molecular players involved in SOCE triggered by the SERCA pump inhibitor thapsigargin in an endothelial cell line, the EA.hy926. siRNA directed against STIM1 or Orai1 reduced Ca²+ entry by about 50-60%, showing that a large part of the entry is independent from these proteins. Blocking the PLC or the PKC pathway completely abolished thapsigargin-induced Ca²+ entry in cells depleted from STIM1 and/or Orai1. The phorbol ester PMA or the DAG analog OAG restored the Ca²+ entry inhibited by PLC blockers, showing an involvement of PLC/PKC pathway in SOCE. Using pharmacological inhibitors or siRNA revealed that the PKCeta is required for Ca²+ entry, and pharmacological inhibition of the tyrosine kinase Src also reduced Ca²+ entry. TRPC3 silencing diminished the entry by 45%, while the double STIM1/TRPC3 invalidation reduced Ca²+ entry by more than 85%. Hence, in EA.hy926 cells, TG-induced Ca²+ entry results from the activation of the STIM1/Orai1 machinery, and from the activation of TRPC3.

Molecular mechanisms underlying Ca2+-mediated motility of human pancreatic duct cells

We recently reported that transforming growth factor-β (TGF-β) induces an increase in cytosolic Ca(2+) ([Ca(2+)](cyt)) in pancreatic cancer cells, but the mechanisms by which TGF-β mediates [Ca(2+)](cyt) homeostasis in these cells are currently unknown. Transient receptor potential (TRP) channels and Na(+)/Ca(2+) exchangers (NCX) are plasma membrane proteins that play prominent roles in controlling [Ca(2+)](cyt) homeostasis in normal mammalian cells, but little is known regarding their roles in the regulation of [Ca(2+)](cyt) in pancreatic cancer cells and pancreatic cancer development. Expression and function of NCX1 and TRPC1 proteins were characterized in BxPc3 pancreatic cancer cells. TGF-β induced both intracellular Ca(2+) release and extracellular Ca(2+) entry in these cells; however, 2-aminoethoxydiphenyl borate [2-APB; a blocker for both inositol 1,4,5-trisphosphate (IP(3)) receptor and TRPC], LaCl(3) (a selective TRPC blocker), or KB-R7943 (a selective inhibitor for the Ca(2+) entry mode of NCX) markedly inhibited the TGF-β-induced increase in [Ca(2+)](cyt). 2-APB or KB-R7943 treatment was able to dose-dependently reverse membrane translocation of PKCα induced by TGF-β. Transfection with small interfering RNA (siRNA) against NCX1 almost completely abolished NCX1 expression in BxPc3 cells and also inhibited PKCα serine phosphorylation induced by TGF-β. Knockdown of NCX1 or TRPC1 by specific siRNA transfection reversed TGF-β-induced pancreatic cancer cell motility. Therefore, TGF-β induces Ca(2+) entry likely via TRPC1 and NCX1 and raises [Ca(2+)](cyt) in pancreatic cancer cells, which is essential for PKCα activation and subsequent tumor cell invasion. Our data suggest that TRPC1 and NCX1 may be among the potential therapeutic targets for pancreatic cancer.

Mitochondrial Ca2+ uptake and not mitochondrial motility is required for STIM1-Orai1-dependent store-operated Ca2+ entry

Store-operated Ca2+ entry (SOCE) is established by formation of subplasmalemmal clusters of the endoplasmic reticulum (ER) protein, stromal interacting molecule 1 (STIM1) upon ER Ca2+ depletion. Thereby, STIM1 couples to plasma membrane channels such as Orai1. Thus, a close proximity of ER domains to the plasma membrane is a prerequisite for SOCE activation, challenging the concept of local Ca2+ buffering by mitochondria as being essential for SOCE. This study assesses the impact of mitochondrial Ca2+ handling and motility on STIM1–Orai1-dependent SOCE. High-resolution microscopy showed only 10% of subplasmalemmal STIM1 clusters to be colocalized with mitochondria. Impairments of mitochondrial Ca2+ handling by inhibition of mitochondrial Na+-Ca2+ exchanger (NCXmito) or depolarization only partially suppressed Ca2+ entry in cells overexpressing STIM1-Orai1. However, SOCE was completely abolished when both NCXmito was inhibited and the inner mitochondrial membrane was depolarized, in STIM1- and Orai1-overexpressing cells. Immobilization of mitochondria by expression of mAKAP-RFP-CAAX, a construct that physically links mitochondria to the plasma membrane, affected the Ca2+ handling of the organelles but not the activity of SOCE. Our observations indicate that mitochondrial Ca2+ uptake, including reversal of NCXmito, is fundamental for STIM1–Orai1-dependent SOCE, whereas the proximity of mitochondria to STIM1-Orai1 SOCE units and their motility is not required.

Electrophysiological characterization of store-operated and agonist-induced Ca2+ entry pathways in endothelial cells

In endothelial cells, agonist-induced Ca(2+) entry takes place via the store-operated Ca(2+) entry pathway and/or via channel(s) gated by second messengers. As cell stimulation leads to both a partial Ca(2+) store depletion as well as the production of second messengers, these two pathways are problematic to distinguish. We showed that passive endoplasmic reticulum (ER) depletion by thapsigargin or cell stimulation by histamine activated a similar Ca(2+)-release-activated Ca(2+) current (CRAC)-like current when 10 mM Ba(2+)/2 mM Ca(2+) was present in the extracellular solution. Importantly, during voltage clamp recordings, histamine stimulation largely depleted the ER Ca(2+) store, explaining the activation of a CRAC-like current (due to store depletion) upon histamine in Ba(2+) medium. On the contrary, in the presence of 10 mM Ca(2+), the ER Ca(2+) content remained elevated, and histamine induced an outward rectifying current that was inhibited by Ni(2+) and KB-R7943, two blockers of the Na(+)/Ca(2+) exchanger (NCX). Both blockers also reduced histamine-induced cytosolic Ca(2+) elevation. In addition, removing extracellular Na(+) increased the current amplitude which is in line with a current supported by the NCX. These data are consistent with the involvement of the NCX working in reverse mode (Na(+) out/Ca(2+) in) during agonist-induced Ca(2+) entry in endothelial cells.

Calcium signals activated by ghrelin and D-Lys(3)-GHRP-6 ghrelin antagonist in developing dorsal root ganglion glial cells

Ghrelin is a hormone regulating energy homeostasis via interaction with its receptor, GHSR-1a. Ghrelin activities in dorsal root ganglia (DRG) cells are unknown. Herein we show that ghrelin induces a change of cytosolic calcium concentration in both glia and neurons of embryonic chick DRG. Both RT-PCR and binding studies performed with fluorescent ghrelin in the presence of either unlabeled ghrelin or GHSR-1a antagonist D-Lys(3)-GHRP-6, indicate that DRG cells express GHSR-1a. In glial cells the response is characterized by a rapid transient rise in [Ca(2+)](i) followed by a long lasting rise. The calcium elevation is dependent on calcium release from thapsigargin-sensitive intracellular stores and on activation of two distinct Ca(2+) entry pathways, a receptor activated calcium entry and a store operated calcium entry. Surprisingly, D-Lys(3)-GHRP-6 exerts several activities in the absence of exogenous ghrelin: (i) it activates calcium release from thapsigargin-sensitive intracellular stores and calcium entry via voltage-operated channels in non-neuronal cells; (ii) it inhibits calcium oscillations in non-neuronal cells exhibiting spontaneous Ca(2+) activity and iii) it promotes apoptosis of DRG cells, both neurons and glia. In summary, we provide the first evidence for ghrelin activity in DRG, and we also demonstrate that the widely used D-Lys(3)-GHRP-6 ghrelin antagonist features ghrelin independent activities.

Regulation of plasma membrane calcium fluxes by mitochondria

The role of mitochondria in cell signaling is becoming increasingly apparent, to an extent that the signaling role of mitochondria appears to have stolen the spotlight from their primary function as energy producers. In this chapter, we will review the ionic basis of calcium handling by mitochondria and discuss the mechanisms that these organelles use to regulate the activity of plasma membrane calcium channels and transporters.

Mitochondrial regulation of sarcoplasmic reticulum Ca2+ content in vascular smooth muscle cells

Subplasmalemmal ion fluxes have global effects on Ca(2+) signaling in vascular smooth muscle. Measuring cytoplasmic and mitochondrial [Ca(2+)]and [Na(+)], we previously showed that mitochondria buffer both subplasmalemmal cytosolic [Ca(2+)] and [Na(+)] in vascular smooth muscle cells. We have now directly measured sarcoplasmic reticulum [Ca(2+)] in aortic smooth muscle cells, revealing that mitochondrial Na(+)/Ca(2+) exchanger inhibition with CGP-37157 impairs sarcoplasmic reticulum Ca(2+) refilling during purinergic stimulation. By overexpressing hFis1 to remove mitochondria from the subplasmalemmal space, we show that the rate and extent of sarcoplasmic reticulum refilling is augmented by a subpopulation of peripheral mitochondria. In ATP-stimulated cells, hFis-1-mediated relocalization of mitochondria impaired the sarcoplasmic reticulum refilling process and reduced mitochondrial [Ca(2+)] elevations, despite increased cytosolic [Ca(2+)] elevations. Reversal of plasmalemmal Na(+)/Ca(2+) exchange was the primary Ca(2+) entry mechanism following ATP stimulation, based on the effects of KB-R7943. We propose that subplasmalemmal mitochondria ensure efficient sarcoplasmic reticulum refilling by cooperating with the plasmalemmal Na(+)/Ca(2+) exchanger to funnel Ca(2+) into the sarcoplasmic reticulum and minimize cytosolic [Ca(2+)] elevations that might otherwise contribute to hypertensive or proliferative vasculopathies.

Stimulation of histamine H2 receptors activates TRPC3 channels through both phospholipase C and phospholipase D

Histamine plays an important role in many physiological functions; and a change in cytosolic Ca(2+) ([Ca(2+)](i)) may be an early signal in these processes. In the present study, we investigated the activation mechanism of TRPC3, the Canonical Transient Receptors Potential 3 Channels, by histamine via a non-capacitative Ca(2+) entry pathway. TRPC3 was transfected into HEK293 cells and the cells were treated with thapsigargin to deplete the intracellular Ca(2+) stores; re-addition of Ca(2+) initiated a capacitative Ca(2+) entry (CCE). A subsequent application of histamine evoked another Ca(2+) influx on top of the CCE signal only in the TRPC3-transfected HEK293 cells, indicating that histamine can activate TRPC3 via a non-capacitative Ca(2+) entry pathway (non-CCE). This histamine-induced non-CCE was abolished by cimitidine, a histamine H(2) receptors antagonist, but not by histamine H(1) receptor antagonists pyrilamine and diphenhydramine. KT5720, a protein kinase A (PKA) inhibitor, had no effect on the histamine-induced non-CCE. This histamine-induced non-CCE was partially reduced by U73122, a phospholipase C (PLC) inhibitor, and by butan-1-ol, a phospholipase D (PLD) inhibitor. When both PLC and PLD inhibitors were simultaneously applied, the non-CCE signal was completely abolished. Taken together, our results showed, for the first time, that histamine could activate TRPC3 via histamine H(2) receptors, and both PLC and PLD participated in this process.

Cytosolic Ca2+ prevents the subplasmalemmal clustering of STIM1: an intrinsic mechanism to avoid Ca2+ overload

The stromal interacting molecule (STIM1) is pivotal for store-operated Ca(2+) entry (SOC). STIM1 proteins sense the Ca(2+) concentration within the lumen of the endoplasmic reticulum (ER) via an EF-hand domain. Dissociation of Ca(2+) from this domain allows fast oligomerization of STIM1 and the formation of spatially discrete clusters close to the plasma membrane. By lifetime-imaging of STIM1 interaction, the rearrangement of STIM1, ER Ca(2+) concentration ([Ca(2+)](ER)) and cytosolic Ca(2+) signals ([Ca(2+)](cyto)) we show that [Ca(2+)](cyto) affects the subcellular distribution of STIM1 oligomers and prevents subplasmalemmal STIM clustering, even if the ER is depleted. These data indicate that [Ca(2+)](cyto), independently of the ER Ca(2+) filling state, crucially tunes the formation and disassembly of subplasmalemmal STIM1 clusters, and, thus, protects cells against Ca(2+) overload resulting from excessive SOC activity.

Ca2+ signaling in injured in situ endothelium of rat aorta

The inner wall of excised rat aorta was scraped by a microelectrode and Ca2+ signals were investigated by fluorescence microscopy in endothelial cells (ECs) directly coupled with injured cells. The injury caused an immediate increase in the intracellular Ca2+ concentration ([Ca2+]i), followed by a long-lasting decay phase due to Ca2+ influx from extracellular space. The immediate response was mainly due to activation of purinergic receptors, as shown by the effect of P2X and P2Y receptors agonists and antagonists, such as suramin, alpha,beta-MeATP, MRS-2179 and 2-MeSAMP. Inhibition of store-operated Ca2+ influx did not affect either the peak response or the decay phase. Furthermore, the latter was: (i) insensitive to phospholipase C inhibition, (ii) sensitive to the gap junction blockers, palmitoleic acid, heptanol, octanol and oleamide, and (iii) sensitive to La3+ and Ni2+, but not to Gd3+. Finally, ethidium bromide or Lucifer Yellow did not enter ECs facing the scraped area. These results suggest that endothelium scraping: (i) causes a short-lasting stimulation of healthy ECs by extracellular nucleotides released from damaged cells and (ii) uncouples the hemichannels of the ECs facing the injury site; these hemichannels do not fully close and allow a long-lasting Ca2+ entry.

Evidence for a receptor-activated Ca2+ entry pathway independent from Ca2) store depletion in endothelial cells

Ca(2+) entry in endothelial cells is a key signaling event as it prolongs the Ca(2+) signal activated by a receptor agonist, and thus allows an adequate production of a variety of compounds. The possible routes that lead to Ca(2+) entry in non-excitable cells include the receptor-activated Ca(2+) entry (RACE), which requires the presence of an agonist to be activated, and the store-operated Ca(2+) entry (SOCE) pathway, whose activation requires the depletion of the ER Ca(2+) store. However, the relative importance of these two influx pathways during physiological stimulation is not known. In the present study we experimentally differentiated these two types of influxes and determined under which circumstances they are activated. We show that La(3+) (at 10 microM) is a discriminating compound that efficiently blocks SOCE but is almost without effect on histamine-induced Ca(2+) entry (RACE). In line with this, histamine does not induce massive store depletion when performed in the presence of extracellular Ca(2+). In addition, inhibition of mitochondrial respiration significantly reduces SOCE but modestly affects RACE. Thus, agonist-induced Ca(2+) entry is insensitive to La(3+), and only modestly affected by mitochondrial depolarization. These data shows that agonist relies almost exclusively on RACE for sustained Ca(2+) signaling in endothelial cells.