Acetylcholine-sensitive intracellular Ca2+ store in fresh endothelial cells and evidence for ryanodine receptors

Non-neuronal cell-derived acetylcholine, a key modulator of the vascular endothelial function in health and disease

Vascular endothelial cells play an important role in regulating peripheral circulation by modulating arterial tone in the microvasculature. Elevated intracellular Ca2+ levels are required in endothelial cells to induce smooth muscle relaxation via endothelium-dependent mechanisms such as nitric oxide production, prostacyclin, and endothelial cell hyperpolarization. It is well established that exogenous administration of acetylcholine can increase intracellular Ca2+ concentrations, followed by endothelium-dependent vasodilation. Although endogenous acetylcholine's regulation of vascular tone remains debatable, recent studies have reported that endogenously derived acetylcholine, but not neuronal cell-derived acetylcholine, is a key modulator of endothelial cell function. In this minireview, we summarize the current knowledge of the non-neuronal cholinergic system (NNCS) in vascular function, particularly vascular endothelial cell function, which contributes to blood pressure regulation. We also discuss the possible pathophysiological impact of endothelial NNCS, which may induce the development of vascular diseases due to endothelial dysfunction, and the potential of endothelial NNCS as a novel therapeutic target for endothelial dysfunction in the early stages of metabolic syndrome, diabetes, and hypertension.

WNK kinase is a vasoactive chloride sensor in endothelial cells

Endothelial cells (ECs) line the wall of blood vessels and regulate arterial contractility to tune regional organ blood flow and systemic pressure. Chloride (Cl-) is the most abundant anion in ECs and the Cl- sensitive With-No-Lysine (WNK) kinase is expressed in this cell type. Whether intracellular Cl- signaling and WNK kinase regulate EC function to alter arterial contractility is unclear. Here, we tested the hypothesis that intracellular Cl- signaling in ECs regulates arterial contractility and examined the signaling mechanisms involved, including the participation of WNK kinase. Our data obtained using two-photon microscopy and cell-specific inducible knockout mice indicated that acetylcholine, a prototypical vasodilator, stimulated a rapid reduction in intracellular Cl- concentration ([Cl-]i) due to the activation of TMEM16A, a Cl- channel, in ECs of resistance-size arteries. TMEM16A channel-mediated Cl- signaling activated WNK kinase, which phosphorylated its substrate proteins SPAK and OSR1 in ECs. OSR1 potentiated transient receptor potential vanilloid 4 (TRPV4) currents in a kinase-dependent manner and required a conserved binding motif located in the channel C terminus. Intracellular Ca2+ signaling was measured in four dimensions in ECs using a high-speed lightsheet microscope. WNK kinase-dependent activation of TRPV4 channels increased local intracellular Ca2+ signaling in ECs and produced vasodilation. In summary, we show that TMEM16A channel activation reduces [Cl-]i, which activates WNK kinase in ECs. WNK kinase phosphorylates OSR1 which then stimulates TRPV4 channels to produce vasodilation. Thus, TMEM16A channels regulate intracellular Cl- signaling and WNK kinase activity in ECs to control arterial contractility.

Reciprocality Between Estrogen Biology and Calcium Signaling in the Cardiovascular System

17β-Estradiol (E₂) is the main estrogenic hormone in the body and exerts many cardiovascular protective effects. Via three receptors known to date, including estrogen receptors α (ERα) and β (ERβ) and the G protein-coupled estrogen receptor 1 (GPER, aka GPR30), E₂ regulates numerous calcium-dependent activities in cardiovascular tissues. Nevertheless, effects of E₂ and its receptors on components of the calcium signaling machinery (CSM), the underlying mechanisms, and the linked functional impact are only beginning to be elucidated. A picture is emerging of the reciprocality between estrogen biology and Ca²⁺ signaling. Therein, E₂ and GPER, via both E₂-dependent and E₂-independent actions, moderate Ca²⁺-dependent activities; in turn, ERα and GPER are regulated by Ca²⁺ at the receptor level and downstream signaling via a feedforward loop. This article reviews current understanding of the effects of E₂ and its receptors on the cardiovascular CSM and vice versa with a focus on mechanisms and combined functional impact. An overview of the main CSM components in cardiovascular tissues will be first provided, followed by a brief review of estrogen receptors and their Ca²⁺-dependent regulation. The effects of estrogenic agonists to stimulate acute Ca²⁺ signals will then be reviewed. Subsequently, E₂-dependent and E₂-independent effects of GPER on components of the Ca²⁺ signals triggered by other stimuli will be discussed. Finally, a case study will illustrate how the many mechanisms are coordinated to moderate Ca²⁺-dependent activities in the cardiovascular system.

Calcium signals that determine vascular resistance

Small arteries in the body control vascular resistance, and therefore, blood pressure and blood flow. Endothelial and smooth muscle cells in the arterial walls respond to various stimuli by altering the vascular resistance on a moment to moment basis. Smooth muscle cells can directly influence arterial diameter by contracting or relaxing, whereas endothelial cells that line the inner walls of the arteries modulate the contractile state of surrounding smooth muscle cells. Cytosolic calcium is a key driver of endothelial and smooth muscle cell functions. Cytosolic calcium can be increased either by calcium release from intracellular stores through IP3 or ryanodine receptors, or the influx of extracellular calcium through ion channels at the cell membrane. Depending on the cell type, spatial localization, source of a calcium signal, and the calcium-sensitive target activated, a particular calcium signal can dilate or constrict the arteries. Calcium signals in the vasculature can be classified into several types based on their source, kinetics, and spatial and temporal properties. The calcium signaling mechanisms in smooth muscle and endothelial cells have been extensively studied in the native or freshly isolated cells, therefore, this review is limited to the discussions of studies in native or freshly isolated cells. This article is categorized under: Biological Mechanisms > Cell Signaling Laboratory Methods and Technologies > Imaging Models of Systems Properties and Processes > Mechanistic Models.

Transient Receptor Potential Channels and Endothelial Cell Calcium Signaling

The vascular endothelium is a broadly distributed and highly specialized organ. The endothelium has a number of functions including the control of blood vessels diameter through the production and release of potent vasoactive substances or direct electrical communication with underlying smooth muscle cells, regulates the permeability of the vascular barrier, stimulates the formation of new blood vessels, and influences inflammatory and thrombotic processes. Endothelial cells that make up the endothelium express a variety of cell-surface receptors and ion channels on the plasma membrane that are capable of detecting circulating hormones, neurotransmitters, oxygen tension, and shear stress across the vascular wall. Changes in these stimuli activate signaling cascades that initiate an appropriate physiological response. Increases in the global intracellular Ca²⁺ concentration and localized Ca²⁺ signals that occur within specialized subcellular microdomains are fundamentally important components of many signaling pathways in the endothelium. The transient receptor potential (TRP) channels are a superfamily of cation-permeable ion channels that act as a primary means of increasing cytosolic Ca²⁺ in endothelial cells. Consequently, TRP channels are vitally important for the major functions of the endothelium. In this review, we provide an in-depth discussion of Ca²⁺ -permeable TRP channels in the endothelium and their role in vascular regulation. © 2019 American Physiological Society. Compr Physiol 9:1249-1277, 2019.

Atherosclerosis differentially affects calcium signalling in endothelial cells from aortic arch and thoracic aorta in Apolipoprotein E knockout mice

Apolipoprotein‐E knockout (ApoE^−/−) mice develop hypercholesterolemia and are a useful model of atherosclerosis. Hypercholesterolemia alters intracellular Ca²⁺ signalling in vascular endothelial cells but our understanding of these changes, especially in the early stages of the disease process, is limited. We therefore determined whether carbachol‐mediated endothelial Ca²⁺ signals differ in plaque‐prone aortic arch compared to plaque‐resistant thoracic aorta, of wild‐type and ApoE^−/− mice, and how this is affected by age and the presence of hypercholesterolemia. The extent of plaque development was determined using en‐face staining with Sudan IV. Tissues were obtained from wild‐type and ApoE^−/− mice at 10 weeks (pre‐plaques) and 24 weeks (established plaques). We found that even before development of plaques, significantly increased Ca²⁺ responses were observed in arch endothelial cells. Even with aging and plaque formation, ApoE^−/− thoracic responses were little changed, however a significantly enhanced Ca²⁺ response was observed in arch, both adjacent to and away from lesions. In wild‐type mice of any age, 1–2% of cells had oscillatory Ca²⁺ responses. In young ApoE^−/− and plaque‐free regions of older ApoE^−/−, this is unchanged. However a significant increase in oscillations (~13–15%) occurred in thoracic and arch cells adjacent to lesions in older mice. Our data suggest that Ca²⁺ signals in endothelial cells show specific changes both before and with plaque formation, that these changes are greatest in plaque‐prone aortic arch cells, and that these changes will contribute to the reported deterioration of endothelium in atherosclerosis.

We have investigated aortic endothelial cell calcium signalling changes in the Apolipoprotein E knockout mouse model of atherosclerosis. Our data show that calcium signals in endothelial cells undergo specific changes both before and with plaque formation, that these changes are greater in plaque‐prone aortic arch than in plaque‐resistant thoracic aorta, and that these changes will contribute to the reported deterioration of endothelium in atherosclerosis.

Acidic intracellular Ca(2+) stores and caveolae in Ca(2+) signaling and diabetes

Acidic Ca(2+) stores, particularly lysosomes, are newly discovered players in the well-orchestrated arena of Ca(2+) signaling and we are at the verge of understanding how lysosomes accumulate Ca(2+) and how they release it in response to different chemical, such as NAADP, and physical signals. Additionally, it is now clear that lysosomes play a key role in autophagy, a process that allows cells to recycle components or to eliminate damaged structures to ensure cellular well-being. Moreover, lysosomes are being unraveled as hubs that coordinate both anabolism via insulin signaling and catabolism via AMPK. These acidic vesicles have close contact with the ER and there is a bidirectional movement of information between these two organelles that exquisitely regulates cell survival. Lysosomes also connect with plasma membrane where caveolae are located as specialized regions involved in Ca(2+) and insulin signaling. Alterations of all these signaling pathways are at the core of insulin resistance and diabetes.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.

Benefit of SERCA2a gene transfer to vascular endothelial and smooth muscle cells: a new aspect in therapy of cardiovascular diseases

Despite the great progress in cardiovascular health and clinical care along with marked decline in morbidity and mortality, cardiovascular diseases remain the leading causes of death and disability in the developed world. New therapeutic approaches, targeting not only systematic but also causal dysfunction, are ultimately needed to provide a valuable alternative for treatment of complex cardiovascular diseases. In heart failure, there are currently a number of trials that have been either completed or are ongoing targeting the sarcoplasmic reticulum calcium ATPase pump (SERCA2a) gene transfer in the context of heart failure. Recently, a phase 2 trial was completed, demonstrating safety and suggested benefit of adeno-associated virus type 1/SERCA2a gene transfer in advanced heart failure, supporting larger confirmatory trials. The experimental and clinical data suggest that, when administrated through perfusion, virus vector carrying SERCA2a can also transduce vascular endothelial and smooth muscle cells (EC and SMC) thereby improving the clinical benefit of gene therapy. Indeed, recent advances in understanding the molecular basis of vascular dysfunction point towards a reduction of sarcoplasmic reticulum Ca2+ uptake and an impairment of Ca2+ cycling in vascular EC and SMC from patients and preclinical models with cardiac diseases or with cardiovascular risk factors such as diabetes, hypercholesterolemia, coronary artery diseases, as well as other conditions such as pulmonary hypertension. In recent years, several studies have established that SERCA2a gene-based therapy could be an efficient option to treat vascular dysfunction. This review focuses on the recent finding showing the beneficial effects of SERCA2a gene transfer in vascular EC and SMC.

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.

Caffeine's Vascular Mechanisms of Action

Caffeine is the most widely consumed stimulating substance in the world. It is found in coffee, tea, soft drinks, chocolate, and many medications. Caffeine is a xanthine with various effects and mechanisms of action in vascular tissue. In endothelial cells, it increases intracellular calcium stimulating the production of nitric oxide through the expression of the endothelial nitric oxide synthase enzyme. Nitric oxide is diffused to the vascular smooth muscle cell to produce vasodilation. In vascular smooth muscle cells its effect is predominantly a competitive inhibition of phosphodiesterase, producing an accumulation of cAMP and vasodilation. In addition, it blocks the adenosine receptors present in the vascular tissue to produce vasoconstriction. In this paper the main mechanisms of action of caffeine on the vascular tissue are described, in which it is shown that caffeine has some cardiovascular properties and effects which could be considered beneficial.

Effects of exercise training on cellular mechanisms of endothelial nitric oxide synthase regulation in coronary arteries after chronic occlusion

Exercise training enhances agonist-mediated relaxation in both control and collateral-dependent coronary arteries of hearts subjected to chronic occlusion, an enhancement that is mediated in part by nitric oxide. The purpose of the present study was to elucidate exercise training-induced adaptations in specific cellular mechanisms involved in the regulation of endothelial nitric oxide synthase (eNOS) in coronary arteries of ischemic hearts. Ameroid constrictors were surgically placed around the proximal left circumflex coronary artery (LCX) of adult female Yucatan miniature swine. Eight weeks postoperatively, animals were randomized into sedentary (pen-confined) or exercise training (treadmill run; 5 days/wk; 14 wk) protocols. Coronary artery segments ( approximately 1.0 mm luminal diameter) were isolated from collateral-dependent (LCX) and control (nonoccluded left anterior descending) arteries 22 wk after ameroid placement. Endothelial cells were enzymatically dissociated, and intracellular Ca(2+) responses (fura 2) to bradykinin stimulation were studied. Immunofluorescence and laser scanning confocal microscopy were used to quantify endothelial cell eNOS and caveolin-1 cellular distribution under basal and bradykinin-stimulated conditions. Immunoblot analysis was used to determine eNOS, phosphorylated (p)-eNOS, protein kinase B (Akt), pAkt, and caveolin-1 protein levels. Bradykinin-stimulated nitrite plus nitrate (NOx; nitric oxide metabolites) levels were assessed via HPLC. Exercise training resulted in significantly enhanced bradykinin-mediated increases in endothelial Ca(2+) levels, NOx levels, and the distribution of eNOS-to-caveolin-1 ratio at the plasma membrane in endothelial cells of control and collateral-dependent arteries. Exercise training also significantly increased total eNOS and phosphorylated levels of eNOS (pSer(1179)) in collateral-dependent arteries. Total eNOS protein levels were also significantly increased in collateral-dependent arteries of sedentary animals. These data provide new insights into exercise training-induced adaptations in cellular mechanisms of nitric oxide regulation in collateral-dependent coronary arteries of chronically occluded hearts that contribute to enhanced nitric oxide production.

Calcium-independent phospholipase A(2) plays a key role in the endothelium-dependent contractions to acetylcholine in the aorta of the spontaneously hypertensive rat

Phospholipase A(2) (PLA(2)), a regulatory enzyme found in most mammalian cells, catalyzes the breakdown of membrane phospholipids to arachidonic acid. There are two major cytosolic types of the enzyme, calcium-dependent (cPLA(2)) and calcium-independent (iPLA(2)) PLA(2). The present study investigated whether or not iPLA(2) plays a role in the endothelium-dependent contractions of the aorta of the spontaneously hypertensive rat and its normotensive counterpart, the Wistar-Kyoto rat. The presence of iPLA(2) in the endothelial cells was identified by using immunochemistry and immunoblotting. Aortic rings with and without the endothelium were suspended in organ chambers for isometric tension recording. The production of prostanoids was measured by using enzyme immunoassay kits. iPLA(2) was densely distributed in endothelial cells of the aorta of both strains. At 3 x 10(-6) M, the selective iPLA(2) inhibitor, bromoenol lactone (BEL), abrogated endothelium-dependent contractions induced by acetylcholine but not those evoked by the calcium ionophore A-23187. The effects of BEL were similar in the aortae of Wistar-Kyoto and spontaneously hypertensive rats. The nonselective PLA(2) inhibitor quinacrine abolished the contractions triggered by both acetylcholine and A-23187, whereas the store-operated calcium channel inhibitor SKF-96365 prevented only the acetylcholine-induced contraction. The acetylcholine- but not the A-23187-induced release of 6-keto prostaglandin F(1alpha) was inhibited by BEL. The release of thromboxane B(2) by either acetylcholine or A-23187 was not affected by BEL. In conclusion, iPLA(2) plays a substantial role in the generation of endothelium-derived contracting factor evoked by acetylcholine.

Effect of uncoupling endothelial nitric oxide synthase on calcium homeostasis in aged porcine endothelial cells

AIMS

The requirement of endothelial NO synthase (NOS3) calcium to produce NO is well described, although the effect of NO on intracellular calcium levels [Ca(2+)](i) is still confusing. Therefore, NO and [Ca(2+)](i) cross-talk were studied in parallel in endothelial cells possessing a functional or a dysfunctional NO pathway.

METHODS AND RESULTS

Dysfunctional porcine endothelial cells were obtained either in vitro by successive passages or in vivo from regenerated endothelium 1 month after coronary angioplasty. Activity of NOS3 was characterized by conversion of arginine to citrulline, BH(4) intracellular availability, cGMP, and superoxide anion production. Imaging of the Ca(2+) indicator FURA 2-AM was recorded and sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) pump activity was analysed by (45)Ca(2+) uptake into cells. In endothelial cells with a functional NO pathway, NOS3 inhibition increased [Ca(2+)](i) and, conversely, an NO donor decreased it. In aged cells with an uncoupled NOS3 as shown by the reduced BH(4) level, the increase in superoxide anion and the lower production of cGMP and the decrease in NO bioavailability were linearly correlated with the increase in basal [Ca(2+)](i). Moreover, when stimulated by bradykinin, the calcium response was reduced while its decay was slowed down. These effects on the calcium signalling were abolished in calcium-free buffer and were similarly induced by SERCA inhibitors. In aged cells, NO improved the reduced SERCA activity and tended to normalize the agonist calcium response.

CONCLUSION

In control endothelial cells, NO exerts a negative feedback on cytosolic Ca(2+) homeostasis. In aged cells, uncoupled NOS3 produced NO that was insufficient to control the [Ca(2+)](i). Consequently, under resting conditions, SERCA activity decreased and [Ca(2+)](i) increased. These alterations were reversible as exogenous NO, in a cGMP-independent way, refilled intracellular calcium stores, reduced calcium influx, and improved the agonist-evoked calcium response. Therefore, prevention of the decrease in NO in dysfunctional endothelium would normalize the calcium-dependent functions.

Effects on kidney filtration rate by agmatine requires activation of ryanodine channels for nitric oxide generation

Agmatine, decarboxylated arginine, is produced in the kidney and can increase nephron and kidney filtration rate via renal vasodilatation and increases in plasma flow. This increase in filtration rate after agmatine is prevented by administration of nitric oxide synthase (NOS) inhibitors. In endothelial cells, agmatine-stimulated nitrite production is accompanied by induction of cytosolic calcium. NOS activity requires calcium for activation; however, the source of this calcium remains unknown. Ryanodine receptor (RyR) calcium-activated calcium release channels are present in the kidney cortex, and we evaluated if RyR contributes to the agmatine response. Agmatine microperfused into Bowman's space reversibly increases nephron filtration rate (SNGFR) by approximately 30%. cADP-ribose (cADPR) regulates RyR channel activity. Concurrent infusion of agmatine with the cADPR blocker 8-bromo-cADPR (2 microM) prevents the increase in filtration rate. Furthermore, direct activation of the RyR channel with ryanodine at agonist concentrations (5 microM) increases SNGFR, and, like agmatine, this increase is prevented by administration of N(G)-monomethyl-l-arginine, a nonselective NOS blocker. We demonstrate that agmatine does not elicit ADPR cyclase activity in vascular smooth muscle membranes and does not directly affect RyR calcium channel responses using sea urchin egg homogenates. These results imply interplay between endothelial cell cADPR/RyR/Ca(2+)/NO and the cADPR/RyR/Ca(2+) pathways in vascular smooth muscle cells in arterioles in the regulation of kidney filtration rate. In conclusion, we show that agmatine-induced effects require activation of cADPR and RyR calcium release channels for NO generation, vasodilation, and increased filtration rate.

Endothelial dysfunction and compromised eNOS/Akt signaling in the thoracic aorta during the progression of Marfan syndrome

BACKGROUND AND PURPOSE

Aortic complications account for the major mortality in Marfan syndrome (MFS), a connective tissue disorder caused by mutations in FBN1 encoding fibrillin-1. We hypothesized that MFS impaired endothelial function and nitric oxide (NO) production in the aorta.

EXPERIMENTAL APPROACH

Mice (at 3, 6, 9 and 12 months of age) heterozygous for the Fbn1 allele encoding a cysteine substitution (Fbn1 (C1039G/+), Marfan mice, n=75), the most common class of mutation in MFS, were compared with age-matched control littermates (n=75). Thoracic and abdominal aortas from the two groups were studied.

KEY RESULTS

Isometric force measurements revealed that relaxation to ACh (but not to sodium nitroprusside) was diminished in the phenylephrine-precontracted Marfan thoracic aorta at 6 months of age (pEC(50)=6.12+/-0.22; maximal response, E(max)=52.7+/-6.8%; control: pEC(50)=7.34+/-0.19; E(max)=84.8+/-2.2%). At one year, both inhibition of NO production with N(omega)-nitro-L-arginine methyl ester, or denudation of endothelium increased the phenylephrine-stimulated contraction in the control thoracic aorta by 35%, but had no effect in the Marfan aorta, indicating a loss of basal NO production in the Marfan vessel. From 6 months, a reduced phosphorylation of endothelial NOS (eNOS)(Ser1177) and Akt(Thr308) detected by Western blotting was observed in the Marfan thoracic aorta, which was accompanied by decreased levels of cGMP. Expressions of Akt and eNOS in the abdominal aorta were not different between the two groups.

CONCLUSIONS AND IMPLICATIONS

MFS impairs endothelial function and signaling of NO production in the thoracic aorta, suggesting the importance of NO in the age-related progression of thoracic aortic manifestations.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

Calcium release from presynaptic ryanodine-sensitive stores is required for long-term depression at hippocampal CA3-CA3 pyramidal neuron synapses

Although Ca2+ release from internal stores has been proposed to be important for the induction of long-term synaptic plasticity, the importance of Ca2+ stores localized in presynaptic terminals remains unclear. Here, we have selectively applied pharmacological antagonists to either the presynaptic or postsynaptic cell in paired whole-cell recordings from hippocampal CA3 pyramidal neurons in slice culture. We demonstrate directly the necessary role of presynaptic, but not postsynaptic, ryanodine-sensitive Ca2+ stores in the induction of NMDA receptor (NMDAR)-dependent long-term depression (LTD). Using two-photon laser scanning microscopy, we further find that release from the ryanodine-sensitive stores during prolonged synaptic stimulation generates a slowly rising Ca2+ signal in the presynaptic terminal that is required for the induction of LTD. Moreover, this form of LTD has a significant presynaptic component of expression because it causes a marked decrease in the rate of release from CA3 neuron presynaptic terminals of FM 1-43, a fluorescent probe of synaptic vesicle cycling. Thus, Ca2+ release from presynaptic ryanodine-sensitive stores is critical in the induction of a presynaptic component of NMDAR-dependent LTD.

Inositol 1,4,5-trisphosphate and ryanodine receptors mobilize calcium from a common functional pool in human U373 MG cells

This investigation concentrates on the change in Ca(2+) concentration ([Ca(2+)]) caused by ryanodine in U373 MG cells. This cell type from a human astrocytoma is a unique cellular model because it only expresses the type 3 ryanodine receptor (RyR3), which is generally the least abundant isoform. In the presence of physiological [Ca(2+)] in the extracellular medium, U373 MG cells are caffeine-insensitive, even after forskolin treatment, and ryanodine-sensitive only when an unusually high concentration (30 microM) is applied. Xestospongin C behaves like thapsigargin and therefore cannot be used as a selective antagonist of inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). After ryanodine challenge, addition of an analog of Substance P (SP), which should deplete InsP(3)-sensitive stores, has no effect on [Ca(2+)](i). After thapsigargin treatment, which unmasks the calcium leak from intracellular stores, neither ryanodine nor SP change [Ca(2+)](i), suggesting that thapsigargin completely depletes the ryanodine-sensitive and the InsP(3)-sensitive stores of U373 MG cells. Finally, in experiments monitoring the [Ca(2+)] in intracellular stores, InsP(3) stimulation of permeabilized cells causes a decrease in [Ca(2+)] that is not affected by subsequent ryanodine treatment. Our results support the conclusion that U373 MG cells express both InsP(3)Rs and RyRs that can individually or in combination mobilize only one functional Ca(2+) pool.

Vectorial Ca2+ release via ryanodine receptors contributes to Ca2+ extrusion from freshly isolated rabbit aortic endothelial cells

In this study, we identified ryanodine receptors (RyRs) as a component of a cytosolic Ca(2+) removal pathway in freshly isolated rabbit aortic endothelial cells. In an earlier article, we reported that the sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) and Na(+)/Ca(2+) exchanger (NCX) function in series to extrude cytosolic Ca(2+) to the extracellular space. Here we employed caffeine and ryanodine as modulators of RyR and showed that they act as the linkage between SERCA and NCX in removing Ca(2+) from the cytoplasm. Our data indicate that both 15 mM caffeine and 1 microM ryanodine facilitated Ca(2+) extrusion by activating RyRs while 100 microM ryanodine had the opposite effect by blocking RyRs. A further attempt to investigate RyR pharmacology revealed that in the absence of extracellular Ca(2+), ryanodine at 1 microM, but not 100 microM, stimulated Ca(2+) loss from the endoplasmic reticulum (ER). Blockade of RyR had no effect on the Ca(2+) removal rate when NCX had been previously blocked. In addition, the localization of RyR was determined using confocal microscopy of BODIPY TR-X fluorescent staining. Taken together, our findings suggest that in freshly isolated endothelial cells Ca(2+) is removed in part by transport through SERCA, RyR, and eventually NCX, and that RyR and NCX are in close functional proximity near the plasma membrane. After blockade of this component, Ca(2+) extrusion could be further inhibited by carboxyeosin, indicating a parallel contribution by the plasmalemmal Ca(2+)-ATPase (PMCA).

Altered effect of cyclopiazonic acid on endothelium-dependent relaxation in femoral arteries from hypertensive rats

The function of endoplasmic reticulum in hypertensive vascular endothelium has not been intensively studied. The current study was designed to investigate a role of intracellular Ca2+ stores in endothelium-dependent relaxations to acetylcholine using femoral arteries obtained from Wistar-Kyoto (WKYs) and spontaneously hypertensive rats (SHRs). Rings were prepared from the femoral arteries and changes in isometric tension were recorded. Endothelium-dependent relaxations induced by acetylcholine in rings contracted with serotonin were identical in WKYs and SHRs. Cyclopiazonic acid (CPA) inhibited the relaxation in SHRs but not in WKYs. In WKYs, acetylcholine evoked smaller relaxations in rings contracted with KCl than in those contracted with serotonin, whereas in SHRs the relaxation was not affected by the contractile agonists used. The relaxation in rings contracted with KCl was abolished by Nw-nitro-l-arginine methyl ester (l-NAME) and was reduced by CPA to a similar extent in both strains. In rings contracted with serotonin, l-NAME abolished the relaxation in SHRs, but the inhibitor only partially reduced the relaxation in WKYs. CPA did not alter the relaxation in the presence of l-NAME. Endothelium-independent relaxations to sodium nitroprusside were not affected by CPA. These results suggest that acetylcholine relaxes rat femoral arteries by releasing both nitric oxide and endothelium-derived hyperpolarizing factor (EDHF). In SHRs, the relaxation is preserved, but the release of EDHF is absent. CPA inhibits the relaxation mediated by nitric oxide, but not EDHF and, thus, inhibits the relaxation in SHRs but not in WKYs. Functional alteration of endoplasmic reticulum in the hypertensive endothelium cannot be detected.

Ca(2+) removal mechanisms in freshly isolated rabbit aortic endothelial cells

Calcium removal from the cytoplasm was investigated in freshly isolated aortic endothelial cells by monitoring changes in intracellular calcium ([Ca(2+)](i)) using ratiometric fura-2 fluorimetry. Blockade of the Na(+)/Ca(2+) exchanger (NCX) by replacement of external sodium with equi-molar N-methyl-D-glutamine (0Na PSS) decreased the removal rate by 52%. Blockade of the sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) by cyclopiazonic acid (CPA) decreased the removal rate by 50%. Simultaneous application of CPA and 0Na PSS did not reduce the removal rate any further (53%). The lack of additivity of these two procedures, suggests that SERCA and the NCX function in series to lower [Ca(2+)](i). In addition, in the absence of extracellular Ca(2+), removal of external Na(+) markedly reduced the rate of loss of Ca(2+) from the ER further supporting the hypothesis that NCX is functionally linked to ER calcium release channels, and thus, plays an important role in ER calcium unloading. To investigate the mechanism for the coupling of NCX and SERCA, the same protocols as described above were repeated after treating the cells with cytochalasin D, which disrupts the cytoskeleton. This treatment uncoupled the NCX from SERCA, as evidenced by the resulting additive inhibitory effects of application of CPA and removal of extracellular Na(+) on the rate of Ca(2+) removal from the cytoplasm. These data suggest that in endothelial cells NCX and SERCA function in series to remove about half of the free Ca(2+) from the cytosol, while PMCA contributes to the other half of the Ca(2+) removal process.

EDHF mediates the relaxation of stretched canine femoral arteries to acetylcholine

To test the hypothesis that mechanically stretched arteries relax to endothelium-derived vasodilators, we challenged endothelium-intact dog femoral artery rings stretched from 1 to 16 g total initial tension (active force and passive elastic) with 10–6 M acetylcholine (ACh), an endothelium-dependent dilator. The relaxation to 10–6 M sodium nitroprusside (SNP), an endothelium-independent dilator, increased with the total initial tension. The relaxation to ACh averaged approximately 65% of the relaxation to SNP at total initial tensions of 4 to 16 g. To determine the nature of the endothelial-derived products involved, we compared the ACh-induced relaxation of stretched rings (6.5 ± 0.2 g total initial tension) with rings chemically contracted with phenylephrine (Phe, 10–7 to 10–5 M) (6.5 ± 0.3 g total initial tension). ACh-induced relaxation was evaluated before and after the inhibition of the synthesis of eicosanoids [cyclooxygenase (10–5 M indomethacin) and lipoxygenase (10–5 M nordihydroguariaretic acid)] and nitric oxide [nitric oxide synthase (10–5 M Nw-nitro-L-arginine)]. The contribution of endothelium-derived hyperpolarizing factor (EDHF) was identified by blocking calcium-activated potassium channels (10–8 M iberiotoxin). SNP (10–6 M) relaxed stretched rings by 1.7 ± 0.1 g and chemically-activated rings by 4.8 ± 0.2 g. ACh relaxed stretched rings to 73 ± 3% of the SNP relaxation and this was only attenuated in the presence of iberiotoxin. ACh relaxed Phe-activated rings to 60 ± 3% of the SNP relaxation. This relaxation was attenuated by inhibition of the synthesis of nitric oxide and (or) eicosanoids. Therefore, ACh relaxed stretched rings through the release of EDHF whereas the relaxation of chemically activated rings to ACh involved multiple endothelium-derived vasodilators.Key words: endothelium-derived relaxing factor (EDRF), endothelium-derived hyperpolarizing factor (EDHF), arachidonic acid metabolites.

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.

Functionally separate intracellular Ca2+ stores in smooth muscle

In smooth muscle, release via the inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)R) and ryanodine receptors (RyR) on the sarcoplasmic reticulum (SR) controls oscillatory and steady-state cytosolic Ca(2+) concentrations ([Ca(2+)](c)). The interplay between the two receptors, itself determined by their organization on the SR, establishes the time course and spatial arrangement of the Ca(2+) signal. Whether or not the receptors are co-localized or distanced from each other on the same store or whether they exist on separate stores will significantly affect the Ca(2+) signal produced by the SR. To date these matters remain unresolved. The functional arrangement of the RyR and Ins(1,4,5)P(3)R on the SR has now been examined in isolated single voltage-clamped colonic myocytes. Depletion of the ryanodine-sensitive store, by repeated application of caffeine, in the presence of ryanodine, abolished the response to Ins(1,4,5)P(3), suggesting that Ins(1,4,5)P(3)R and RyR share a common Ca(2+) store. Ca(2+) release from the Ins(1,4,5)P(3)R did not activate Ca(2+)-induced Ca(2+) release at the RyR. Depletion of the Ins(1,4,5)P(3)-sensitive store, by the removal of external Ca(2+), on the other hand, caused only a small decrease ( approximately 26%) in caffeine-evoked Ca(2+) transients, suggesting that not all RyR exist on the common store shared with Ins(1,4,5)P(3)R. Dependence of the stores on external Ca(2+) for replenishment also differed; removal of external Ca(2+) depleted the Ins(1,4,5)P(3)-sensitive store but caused only a slight reduction in caffeine-evoked transients mediated at RyR. Different mechanisms are presumably responsible for the refilling of each store. Refilling of both Ins(1,4,5)P(3)-sensitive and caffeine-sensitive Ca(2+) stores was inhibited by each of the SR Ca(2+) ATPase inhibitors thapsigargin and cyclopiazonic acid. These results may be explained by the existence of two functionally distinct Ca(2+) stores; the first expressing only RyR and refilled from [Ca(2+)](c), the second expressing both Ins(1,4,5)P(3)R and RyR and dependent upon external Ca(2+) for refilling.

Endothelin-1-induced contraction of mesenteric small arteries is mediated by ryanodine receptor Ca2+ channels and cyclic ADP-ribose

Contraction of vascular smooth muscle by endothelin-1 is dependent on extracellular and intracellular Ca2+. However, the role of ryanodine-sensitive Ca2+ stores in endothelin-1-induced contraction is unknown. Vascular contraction was measured in mesenteric small arteries (200-300 microm intraluminal diameter) isolated from Sprague-Dawley rats and maintained at a constant intraluminal pressure of 40 mm Hg. The presence of functional ryanodine receptor Ca2+ release channels (RyRC) was demonstrated by the finding that ryanodine (10 microM), which locks the RyRC in a subconductance state, produced significant contraction of small arteries in the presence of 15 mM KCl. This effect was inhibited by dantrolene (10 microM), a RyRC inhibitor. Dantrolene significantly reduced the ET(A) receptor-mediated contraction to endothelin-1 (10(-11)-10(-9) M). The ability of dantrolene to reverse contraction induced by endothelin-1 was also determined. Dantrolene (1-10 microM) produced concentration-dependent relaxation of vessels precontracted to 38+/-3% of resting diameter with endothelin-1 but had no effect in vessels precontracted to a similar degree with phenylephrine or KCl. Because activation of RyRC may be dependent on production of cyclic ADP-ribose, the effect of nicotinamide (2 mM), an inhibitor of ADP-ribosyl cyclase, on contraction to endothelin-1 was determined. Nicotinamide had an inhibitory effect similar to that produced by dantrolene. A combination of nicotinamide and dantrolene had no greater effect than either agent alone, suggesting a common pathway for cyclic ADP-ribose and RyRC. In summary, endothelin-1 induces contraction of small mesenteric arteries through ET(A) receptor-mediated production of cyclic ADP-ribose and activation of RyRC.

In human hypercholesterolemia increased reactivity of vascular smooth muscle cells is due to altered subcellular Ca(2+) distribution

There is evidence that, besides an attenuated endothelium-dependent relaxation, functional changes in smooth muscle contractility occur in experimental hypercholesterolemic animals. Unfortunately, little is known of the situation in human arteries, and the intracellular mechanisms involved in the modulation of vascular smooth muscle function in human hypercholesterolemia are still unclear. Thus, besides acetylcholine-induced endothelium-dependent relaxation, smooth muscle reactivity to KCl, norepinephrine (NE) and phenylephrine (PE) was evaluated in uterine arteries from 34 control individuals (CI) and 22 hypercholesterolemic patients (HC). Contractions to KCl, norepinephrine and phenylephrine were enhanced by 1.3-, 2.1- and 3.5-fold in vessels from HC. Furthermore, the Ca(2+) signaling in the perinuclear cytosol, which promotes cell contraction, and that of the subplasmalemmal region, which contributes to smooth muscle relaxation, were examined in freshly isolated smooth muscle cells. In cells from HC, increases in perinuclear Ca(2+) concentration ([Ca(2+)](peri)) in response to 30 mM KCl and 300 nM NE were increased by 67 and 93%, respectively. In contrast, the increase in the subplasmalemmal Ca(2+) concentration ([Ca(2+)](sub)) to 10 microM NE was reduced in cells from HC by 33%. No further differences in perinuclear and subplasmalemmal Ca(2+) signaling were found in cultured smooth muscle cells from CI and HC (primary culture 4-6 weeks after isolation). These data indicate a significant change in the subcellular Ca(2+) distribution in smooth muscle cells from HC. In addition, production of superoxide anions (O(2)(-)) was increased 3.8-fold in uterine arteries from HC. Treatment of smooth muscle cells with the O(2)(-)-generating mixture xanthine oxidase/hypoxanthine mimicked hypercholesterolemia on smooth muscle Ca(2+) signaling. From these findings, we conclude that during hypercholesterolemia, besides a reduced endothelium-dependent relaxation, changes in smooth muscle reactivity take place. Thereby, smooth muscle contractility is increased possibly due to the observed changes in subcellular Ca(2+) signaling. The observed increased O(2)(-) production in HC might play a crucial role in the alteration of smooth muscle function in hypercholesterolemia.

High K(+)-induced membrane depolarization attenuates endothelium-dependent pulmonary vasodilation

Impairment of endothelium-dependent pulmonary vasodilation has been implicated in the development of pulmonary hypertension. Pulmonary vascular smooth muscle cells and endothelial cells communicate electrically through gap junctions; thus, membrane depolarization in smooth muscle cells would depolarize endothelial cells. In this study, we examined the effect of prolonged membrane depolarization induced by high K(+) on the endothelium-dependent pulmonary vasodilation. Isometric tension was measured in isolated pulmonary arteries (PA) from Sprague-Dawley rats, and membrane potential was measured in single PA smooth muscle cells. Increase in extracellular K(+) concentration from 4.7 to 25 mM significantly depolarized PA smooth muscle cells. The 25 mM K(+)-mediated depolarization was characterized by an initial transient depolarization (5-15 s) followed by a sustained depolarization that could last for up to 3 h. In endothelium-intact PA rings, ACh (2 microM), levcromakalim (10 microM), and nitroprusside (10 microM) reversibly inhibited the 25 mM K(+)-mediated contraction. Functional removal of endothelium abolished the ACh-mediated relaxation but had no effect on the levcromakalim- or the nitroprusside-mediated pulmonary vasodilation. Prolonged ( approximately 3 h) membrane depolarization by 25 mM K(+) significantly inhibited the ACh-mediated PA relaxation (-55 +/- 4 vs. -29 +/- 2%, P < 0.001), negligibly affected the levcromakalim-mediated pulmonary vasodilation (-92 +/- 4 vs. -95 +/- 5%), and slightly but significantly increased the nitroprusside-mediated PA relaxation (-80 +/- 2 vs. 90 +/- 3%, P < 0. 05). These data indicate that membrane depolarization by prolonged exposure to high K(+) concentration selectively inhibited endothelium-dependent pulmonary vasodilation, suggesting that membrane depolarization plays a role in the impairment of pulmonary endothelial function in pulmonary hypertension.

Depolarization-mediated inhibition of Ca(2+) entry in endothelial cells

The effect of extracellular Cl(-) in regulating ACh-induced Ca(2+) entry into freshly isolated rabbit aortic endothelial cells was studied using Ca(2+)-sensitive fluorescence microscopy and patch-clamp electrophysiology. After ACh caused transient Ca(2+) release in Ca(2+)-free medium, readdition of 3 mM Ca(2+) to the bath maintained Ca(2+) entry. Removal of extracellular Cl(-) abolished the plateau phase in Ca(2+) signal and inhibited divalent cation entry. However, in the presence of the K(+) ionophore valinomycin, removal of Cl(-) had no effect on the Ca(2+) plateau. Under current-clamp conditions, substitution of gluconate for Cl(-) induced membrane depolarization. Under voltage clamp, with CsCl in the pipette, ACh activated a slowly developing Cl(-) current, which was blocked by SITS and 5-nitro-2-(3-phenylpropylamino)benzoic acid. Varying the membrane potential by utilizing different extracellular K(+) concentrations in the presence of 5 microM valinomycin demonstrated that depolarization blocked ACh-stimulated Mn(2+) entry. These data suggest that ACh-induced Ca(2+) entry in freshly isolated endothelial cells requires the presence of extracellular Cl(-) to maintain a polarized membrane potential.

Stealth ryanodine-sensitive Ca2+ release contributes to activity of capacitative Ca2+ entry and nitric oxide synthase in bovine endothelial cells

1. The involvement of ryanodine-sensitive Ca2+ release (RsCR) in bradykinin (Bk)-induced Ca2+ release, capacitative Ca2+ entry (CCE) and nitric oxide synthase (NOS) activation was assessed in freshly isolated bovine coronary artery endothelial cells. 2. Using deconvolution microscopy fura-2 was found throughout the whole cytosol, while the cell membrane impermeable dye FFP-18 was exclusively in the cell membrane. Thus, perinuclear ([Ca2+]pn) and subplasmalemmal Ca2+ concentration ([Ca2+]sp) were monitored using fura-2 and FFP-18. 3. Inhibition of Na+-Ca2+ exchange by lowering extracellular Na+ concentration augmented the Bk-induced [Ca2+]pn signal in Ca2+-free solution. This effect was abolished when RsCR was prevented with 25 micromol l-1 ryanodine, while inhibition of RsCR had no effect on Bk-induced increase in [Ca2+]pn without inhibition of Na+-Ca2+ exchange. 4. Initiating RsCR by 200 nmol l-1 ryanodine increased [Ca2+]sp, while [Ca2+]pn remained constant. However, when Na+-Ca2+ exchange was prevented, ryanodine was also able to elevate [Ca2+]pn. 5. Blockage of RsCR diminished Ca2+ extrusion in response to stimulation with Bk in normal Na+-containing solution. 6. Inhibition of RsCR blunted Bk-activated CCE, while inhibition of Na+-Ca2+ exchange during stimulation enhanced CCE. 7. Although direct activation of RsCR failed to activate NOS, inhibition of RsCR diminished the effect of ATP and Bk on NOS, while the effect of thapsigargin remained unchanged. 8. These data suggest that during stimulation subplasmalemmal RsCR occurs, which contributes to the activities of CCE and NOS. Thus, the function of the subplasmalemmal Ca2+ control unit must be extended as a regulator for CCE and NOS.

ACh- and caffeine-induced Ca2+ mobilization and current activation in rabbit arterial endothelial cells

Fura 2 microfluorometry and perforated-patch whole cell recording were carried out simultaneously to investigate the relationship between intracellular free Ca2+ concentration ([Ca2+]i) and membrane current activation in response to ACh and caffeine in freshly dissociated arterial endothelial cells. ACh and caffeine evoked transient increases in [Ca2+]i. The initial increase in [Ca2+]i was accompanied by a transient outward current, which caused membrane hyperpolarization. The amplitudes of the [Ca2+]i transient and outward current were dependent on caffeine concentration (EC50 approximately 1 mM). Cyclopiazonic acid raised resting [Ca2+]i levels by >/=50 nM and failed to completely block caffeine- or ACh-induced [Ca2+]i transients but slowed [Ca2+]i recovery fourfold. The reversal potential of caffeine-induced currents was dependent on external K+ and Cl- concentrations. Caffeine-induced current amplitudes, but not [Ca2+]i responses, were attenuated by external tetraethylammonium, Zn2+, and La3+. A consistent temporal relationship between agonist-activated membrane current and [Ca2+]i increases was not observed, and, in some cases, time differences were greater than expected for simple diffusion of Ca2+ throughout the cell. These results suggest that Ca2+-dependent current activation monitors local [Ca2+]i changes adjacent to the plasmalemma, whereas single-cell photometry provides a measure of global changes in [Ca2+]i.

The responsiveness of isolated human hand vein endothelial cells in normal pregnancy and in pre-eclampsia

1. Human hand vein endothelial cells were isolated from blood obtained by traumatic venepuncture. Cells were identified as endothelial by staining with endothelium-specific antibodies. The subject groups studied were (i) non-pregnant, (ii) pregnant (mean, 35 weeks gestation) and (iii) pre-eclamptic women (mean, 36 weeks gestation). 2. Fura-2 was used to measure agonist-induced responses in intracellular Ca2+ in single endothelial cells isolated and maintained in vitro. All of the cells examined responded to adenosine triphosphate (ATP) with a large transient increase in Ca2+ followed by a sustained plateau. 3. The responses to ATP were significantly larger in the cells from pregnant women than in those from non-pregnant and pre-eclamptic women, but no other differences were observed. The amplitudes of the responses to ATP were (means +/- s.e.m.) 0.56 +/- 0.04, 1.42 +/- 0.24 and 0.65 +/- 0.09 fura-2 ratio units for cells from non-pregnant, pregnant and pre-eclamptic subjects, respectively. 4. In cells isolated from non-pregnant subjects, the amplitude of the responses to carbachol, histamine and bradykinin were all smaller than those activated by ATP: 5.1, 13.9 and 4.4 %, respectively. Not all cells responded to these agonists: 25 % responded to carbachol, 70.5 % responded to histamine and 12.5 % responded to bradykinin. Sixty-five per cent of the cells from normotensive pregnant subjects responded to bradykinin compared with 25 % in the non-pregnant and 13.9 % in the pre-eclamptic subjects. 5. These data suggest that there may be differences in the responsiveness of venous endothelial cells in pregnancy and that pre-eclamptic cells behave differently.

Submaximal stimulation of porcine endothelial cells causes focal Ca2+ elevation beneath the cell membrane

1. Endothelial cell activation is correlated with increased cytosolic Ca2+ concentration, often monitored with cytoplasmic Ca2+ dyes, such as fura-2 and Calcium Green-1. We tested the hypothesis that during weak stimulation of porcine coronary artery endothelial cells, focal, subplasmalemmal Ca2+ elevations occur which are controlled by cell membrane Na(+)-Ca2+ exchange near mitochondrial membrane and superficial endoplasmic reticulum (SER). 2. Bulk Ca2+ concentration ([Ca2+]b) was monitored using fura-2 or Calcium Green-1 and subplasmalemmal Ca2+ concentration ([Ca2+]sp) was determined with FFP-18. The distribution of the SER network was estimated using laser scanning and deconvolution microscopy. 3. Sodium fluoride (10 mmol l-1) and submaximal concentrations of bradykinin (Bk; 1 nmol l-1) stimulated Ca2+ entry with no increase in [Ca2+]b. Although inositol 1,4,5-trisphosphate formation and intracellular Ca2+ release in response to both stimuli were similar, Ca2+ entry in response to NaF exceeded that in response to 1 nmol l-1 BK by fourfold, suggesting additional effects of NaF on Ca+ entry pathways but stimulation via intracellular Ca2+ release. 4. Prevention of Na(+)-Ca2+ exchange activity by decreasing extracellular Na+ unmasked intracellular Ca2+ release in response to NaF and 1 nmol l-1 Bk, indicated by an increase in [Ca2+]b. Thereby, NaF depleted Bk-releasable Ca2+ pools, while mitochondrial Ca2+ content (released with FCCP or oligomycin) and the amount of Ca2+ stored within the cells (released with ionomycin) was increased compared with cells treated with NaF under normal Na+ conditions. The NaF-initiated increase in [Ca2+]b and depletion of Bk-releasable Ca2+ pool(s) in the low-Na+ condition was diminished by 25 mumol l-1 ryanodine, indicating the involvement of Ca(2+)-induced Ca2+ release (CICR). 5. In simultaneous recordings of [Ca2+]sp (with FFP-18) and [Ca2+]b (with Calcium Green-1), 1 nmol l-1 Bk or 10 mmol l-1 NaF yielded focal [Ca2+] elevation in the subplasmalemmal region with no increase in the perinuclear area. 6. Treatment with 10 mumol-1 nocodazole caused the SER to collapse and unmasked Ca2+ release in response to 1 nmol l-1 Bk and 10 mmol l-1 NaF, similar to low-Na+ conditions, while the effect of thapsigargin was not changed. 7. These data show that in endothelial cells, focal, subplasmalemmal Ca2+ elevations in response to small or slow IP3 formation occur due to vectorial Ca2+ release from the SER towards the plasmalemma followed by Ca2+ extrusion by Na(+)-Ca2+ exchange. While these local Ca2+ elevations are not detectable with Ca2+ dyes for the determination of [Ca2+]b, prevention of Ca2+ extrusion or SER disruption yields increases in [Ca2+]b partially due to CICR. 8. All of the data support our hypothesis that in weakly stimulated endothelial cells, intracellular Ca2+ release and [Ca2+] elevation are limited to the subplasmalemmal region. We propose that the SER co-operates with associated parts of the plasma membrane to control Ca2+ homeostasis, Ca2+ distribution and Ca2+ entry. The existence of such a subplasmalemmal Ca2+ control unit (SCCU) needs to be considered in discussions of Ca2+ signalling, especially when cytoplasmic Ca2+ dyes, such as fura-2 or Calcium Green-1, are used.

Defective endothelium-dependent relaxation of vascular smooth muscle and endothelial cell Ca2+ signaling in mice lacking sarco(endo)plasmic reticulum Ca2+-ATPase isoform 3

Sarco(endo)plasmic reticulum Ca2+ ATPase isoform 3 (SERCA3) is one of two Ca2+ pumps serving intracellular Ca2+ signaling pools in non-muscle tissues; however, unlike the ubiquitous SERCA2b, it exhibits a restricted cell-type distribution. Gene targeting was used to generate a mouse with a null mutation in the SERCA3 gene. Homozygous mutant mice were viable, fertile, and did not exhibit an overt disease phenotype. Because SERCA3 is expressed in arterial endothelial cells, aortic ring preparations were analyzed to determine whether it is involved in the regulation of vascular tone. Contraction-isometric force relations in response to phenylephrine or KCl, as well as relaxation produced by exposure to a nitric oxide donor, were similar in wild-type and null mutant aortas. Acetylcholine-induced endothelium-dependent relaxation of aortas after precontraction with phenylephrine was significantly reduced in homozygous mutants (61.3 +/- 5.6% in wild type, 35.4 +/- 7.3% in mutants). Ca2+ imaging of cultured aortic endothelial cells demonstrated that the acetylcholine-induced intracellular Ca2+ signal is sharply diminished in SERCA3-deficient cells and also indicated that replenishment of the acetylcholine-responsive Ca2+ stores is severely impaired. These results indicate that SERCA3 plays a critical role in endothelial cell Ca2+ signaling events involved in nitric oxide-mediated relaxation of vascular smooth muscle.

Tunicamycin increases intracellular calcium levels in bovine aortic endothelial cells

Tunicamycin is a nucleoside antibiotic that inhibits protein glycosylation and palmitoylation. The therapeutic use of tunicamycin is limited in animals because of its toxic effects, particularly in cerebral vasculature. Tunicamycin decreases palmitoylation of the endothelial isoform of nitric oxide synthase, stimulates nitric oxide synthesis, and increases the concentration of intracellular calcium ([Ca2+]i) in bovine aortic endothelial cells (B. J. Buckley and A. R. Whorton. FASEB J. 11: A110, 1997). In the present study, we investigated the mechanism by which tunicamycin alters [Ca2+]i using the Ca2+-sensitive dye fura 2. We found that tunicamycin increased [Ca2+]i without increasing levels of inositol phosphates. When cells were incubated in the absence of extracellular Ca2+, [Ca2+]i rapidly rose in response to tunicamycin, although a full response was not achieved. The pool of intracellular Ca2+ mobilized by tunicamycin overlapped with that mobilized by thapsigargin. Extracellular nickel blocked a full response to tunicamycin when cells were incubated in the presence of extracellular Ca2+. The effects of tunicamycin on [Ca2+]i were partially reversed by washing out the drug, and the remainder of the response was inhibited by removing extracellular Ca2+. These results indicate that tunicamycin mobilizes Ca2+ from intracellular stores in a manner independent of phospholipase C activation and increases the influx of Ca2+ across the plasma membrane.

Central role of intracellular calcium stores in acute flow- and agonist-evoked endothelial nitric oxide release

1. We have used a cascade bioassay system and isolated arterial ring preparations to investigate the contribution of Ca2+ release from endothelial intracellular stores to nitric oxide (NO) production evoked by increases in shear stress and by acetylcholine in rabbit aorta. 2. Experiments were performed before and following incubation with either the endoplasmic reticulum Ca(2+)-ATPase inhibitors cyclopiazonic acid (CPA, 10 microM) and thapsigargin (TSG, 1 microM) or ryanodine (30, 100 microM) which binds to a specific endoplasmic reticulum Ca(2+)-release channel. 3. In cascade bioassay all three agents induced relaxations of the recipient ring (CPA, 24.4 +/- 3.8%; TSG, 51.5 +/- 10.6%; ryanodine, 17.4 +/- 1.6%) which were significantly attenuated by preincubation of the donor with 100 microM NG-nitro-L-arginine methyl ester (L-NAME). However, in isolated rings, only CPA and TSG induced L-NAME-sensitive relaxations (CPA 52.7 +/- 6.5%; TSG 61.3 +/- 7%). 4. Addition of superoxide dismutase (SOD) to the donor perfusate evoked relaxations of the recipient ring in cascade bioassay (13.3 +/- 1.4%, n = 22). Prior administration of SOD attenuated relaxations to TSG (23.2 +/- 3.8% n = 4) and ryanodine (1.7 +/- 0.8%, n = 4), and pre-incubation with TSG and ryanodine blunted SOD-induced responses (4 +/- 1.5%, n = 4 and 8.9 +/- 1.1%, n = 4, respectively). By contrast, no interaction was observed between the relaxations evoked by SOD and CPA. In isolated rings, SOD exerted no direct relaxant and did not modulate relaxations to CPA, TSG or ryanodine. 5. In cascade bioassay studies time-averaged shear stress was manipulated with dextran (1-4% w/v, 800000 MW) to increase perfusate viscosity. NO-dependent relaxation of the recipient ring induced by increased perfusate viscosity was significantly attenuated by CPA (P < 0.01; n = 6) and TSG (P < 0.05; n = 7), but not by ryanodine (n = 6). 6. Endothelium-dependent relaxations to acetylcholine (0.1-30 microM) in cascade bioassay and in isolated aortic ring preparations were markedly attenuated by pretreatment with CPA and TSG, but were unaffected by ryanodine. Ryanodine and CPA caused only a small attenuation of endothelium-independent relaxations to sodium nitroprusside (0.001-10 microM), whereas TSG had no effect. 7. We conclude that release of Ca2+ from CPA- and TSG-sensitive endothelial stores is necessary for NO release evoked by acute flow changes and agonists in rabbit abdominal aorta. Ca(2+)-induced Ca2+ release via the ryanodine-sensitive release channel plays no direct role in these responses. Free radical interactions may complicate the interpretation of findings in cascade bioassay compared with isolated ring preparations.

Nitric oxide induces transient Ca2+ changes in endothelial cells independent of cGMP

Ca2+ changes induced by nitric oxide (NO.) were investigated in cultured human endothelial cells. Sodium nitroprusside (SNP) (1-100 mumol/L) and S-Nitroso-N-acetylpenicillamine (SNAP) (100 mumol/L) were used as NO. donors. The cytoplasmatic Ca2+ concentration was calculated using ratiometric FURA2 fluorescence measurements. Both NO. donors caused transient oscillatory Ca2+ changes, which were not detectable in the presence of oxyhemoglobin (50 mumol/L). Digital ratio imaging revealed initiation sites within cells where Ca2+ increases started spreading, which indicates that nonuniformly distributed targets might be involved in these reactions. Calcium was released from intracellular stores as indicated by experiments performed in Ca(2+)-free buffer. L-type Ca(2+)-channel blocker diltiazem (100 mumol/L) was not able to block these responses. NO.-induced Ca2+ release from intracellular stores caused capacitative Ca2+ entry. Both thapsigargin (1 mumol/L) and cyclopiazonic acid (10 mumol/L) inhibited the SNP response completely, whereas neither ryanodine (up to 100 mumol/L) nor dantrolene (100 mumol/L) was able to inhibit Ca2+ changes induced by SNP, indicating that primarily inositol 1,4,5-triphosphate (IP3)-dependent stores are released upon stimulation with NO.. A small inhibitory effect of ATP- and SNP-induced peak [Ca2+]i increase was measured in the presence of both caffeine (20 mmol/L) and procaine (1 mmol/L). Evidence is presented that cGMP is not involved in NO.-induced Ca2+ signals, as neither inhibitors of guanylate cyclase (methylene blue and LY 83583) nor cell permeant analogues of cGMP altered or simulated [Ca2+] changes. An inhibitor of cGMP-dependent protein kinase was also ineffective. We therefore propose that endothelial cells have specific targets proximal or at IP3 receptors to induce Ca2+ changes in endothelial cells stimulated with NO..

Antioxidants prevent high-D-glucose-enhanced endothelial Ca2+/cGMP response by scavenging superoxide anions

Very recently we proposed that hyperactivity of endothelial Ca2+/cGMP signaling under hyperglycemic conditions is due to superoxide anion (O2-) release. The present study was designed to investigate changes in endothelial glutathione (GSH) levels in response to high D-glucose and possible prevention of the high-D-glucose-initiated changes in Ca2+/cGMP signal by antioxidants. Under hyperglycemic conditions, GSH content increased by 29% within 4 h. Co-incubation with 10 mM GSH during high-D-glucose treatment normalized the Ca2+/cGMP response associated with an increase in GSH content by 222%. Vitamin C (250 microM) markedly diminished the high-D-glucose-mediated hyperreactivity of endothelial Ca2+ entry (by 40%) and Ca2+ release (by 52%). Similar to GSH, co-incubation with vitamin E (alpha-tocopherol; 50 micrograms/ml) and probucol (50 microM) completely prevented the high-D-glucose-initiated hyperreactivity of the endothelial Ca2+/cGMP response. Vitamin E, probucol, GSH and vitamin C diminished the high-D-glucose-mediated O2- release by 78, 65, 89 and 46%, respectively. These data suggest that antioxidants prevent high-D-glucose-initiated changes in endothelial Ca2+/cGMP response by scavenging the overshoot of O2-.

Temperature dependence of agonist-stimulated Ca2+ signaling in cultured endothelial cells

In cultured endothelial cells, the temperature dependence of bradykinin-initiated Ca2+ signaling was studied using Fura-2 technique. Initially, the temperature dependence of the dissociation constant of Fura-2 for Ca2+ was investigated. Temperature-initiated changes in the apparent dissociation constant (K'D) using the ratio (F340/F380) were due to a hypsochromic shift in excitation wavelengths and changes in the effective dissociation constant of Fura-2 for Ca2+ (K"D). Equations were provided to correct the dissociation constant for Fura-2, either for using the common ratio (F340/F380) or the shift corrected ratio (F340-delta lambda/F380-delta lambda). In a simple experimental protocol, the temperature dependence of the transient increase in free intracellular Ca2+ to bradykinin (i.e. Ca2+ release, sequestration and extrusion) and Ca2+/Mn2+ entry through a Ca2+ store-operated Ca2+ entry pathway (SOCP) were determined. While the temperature dependence of intracellular Ca2+ release, sequestration and extrusion (i.e. enzymatically controlled phenomena) were found to follow the same exponential function [t = A x e(-B x T); t, reaction time; A, B, constants; T, experimental temperature in K; K = degree C + 273], Ca2+/Mn2+ entry upon ion application to pre-stimulated cells strictly followed Fick's law of diffusion [t = A x (1/T) x e(B/T); t, reaction time; A, B, constants; T, experimental temperature in K]. In contrast to the temperature dependence of bradykinin-stimulated Ca2+/Mn2+ entry, the temperature dependence of Mn2+ entry on addition of agonist did not correlate with Fick's law of diffusion, but followed the same exponential function obtained for Ca2+ release, sequestration and extrusion. In conclusion, these data suggest that activation of SOCP by autacoid is due to enzymatic mechanism(s), while Ca2+ entry through SOCP, once activated, is due to a diffusion-like phenomenon.