Calcium entry-dependent oscillations of cytoplasmic calcium concentration in cultured endothelial cell monolayers

A Membrane Potential- and Calpain-Dependent Reversal of Caspase-1 Inhibition Regulates Canonical NLRP3 Inflammasome

The NLRP3 inflammasome senses a range of cellular disturbances, although no consensus exists regarding a common mechanism. Canonical NLRP3 activation is blocked by high extracellular K⁺, regardless of the activating signal. We report here that canonical NLRP3 activation leads to Ca²⁺ flux and increased calpain activity. Activated calpain releases a pool of Caspase-1 sequestered by the cytoskeleton to regulate NLRP3 activation. Using electrophysiological recording, we found that resting-state eukaryotic membrane potential (MP) is required for this calpain activity, and depolarization by high extracellular K⁺ or artificial hyperpolarization results in the inhibition of calpain. Therefore, the MP/Ca²⁺/calpain/ Caspase-1 axis acts as an independent regulatory mechanism for NLRP3 activity. This finding provides mechanistic insight into high K⁺-mediated inhibition of NLRP3 activation, and it offers an alternative model of NLRP3 inflammasome activation that does not involve K⁺ efflux.

Zhang et al. find that, in canonical NLRP inflammasome activation, calpain activity is essential for releasing caspase-1 from flightless-1 and the cytoskeleton. Membrane depolarization, such as under high extracellular K⁺ or hyperpolarization, impairs this activity. This work provides insight into extracellular K⁺ -mediated inhibition of the NLRP3 inflammasome.

Enhancement by Hydrogen Peroxide of Calcium Signals in Endothelial Cells Induced by 5-HT1B and 5-HT2B Receptor Agonists

Hydrogen peroxide, formed in the endothelium, acts as a factor contributing to the relaxation of blood vessels. The reason for this vasodilatory effect could be modulation by H₂O₂ of calcium metabolism, since mobilization of calcium ions in endothelial cells is a trigger of endothelium-dependent relaxation. The aim of this work was to investigate the influence of H₂O₂ on the effects of Ca²⁺-mobilizing agonists in human umbilical vein endothelial cells (HUVEC). We have found that H₂O₂ in concentration range 10-100 μM increases the rise of [Ca²⁺]_i induced by 5-hydroxytryptamine (5-HT) and carbachol and does not affect the calcium signals of ATP, agonist of type 1 protease-activated receptor SFLLRN, histamine and bradykinin. Using specific agonists of 5-HT1B and 5-HT2B receptors CGS12066B and BW723C86, we have demonstrated that H₂O₂ potentiates the effects mediated by these types of 5-HT receptors. Potentiation of the effect of BW723C86 can be produced by the induction of endogenous oxidative stress in HUVEC. We have shown that the activation of 5-HT2B receptor by BW723C86 causes production of reactive oxygen species (ROS). Inhibitor of NADPH oxidases VAS2870 suppressed formation of ROS and partially inhibited [Ca²⁺]_i rise induced by BW723C86. Thus, it can be assumed that vasorelaxation induced by endogenous H₂O₂ in endothelial cells partially occurs due to the potentiation of the agonist-induced calcium signaling.

Pregnancy-adapted uterine artery endothelial cell Ca2+ signaling and its relationship with membrane potential

Pregnancy‐derived uterine artery endothelial cells (P‐UAEC) express P2Y2 receptors and at high cell density show sustained and synchronous [Ca2+]i burst responses in response to ATP. Bursts in turn require coupling of transient receptor potential canonical type3 channel (TRPC3) and inositol 1,4,5‐triphosphate receptor type 2 (IP3R2), which is upregulated in P‐UAEC in a manner dependent on connexin 43 (Cx43) gap junctions. While there is no known direct interaction of TRPC3 with Cx43, early descriptions of TRPC3 function showed it may also be influenced by altered membrane potential (V_m). Herein, we ask if enhanced TRPC3 Ca2+ bursting due to enhanced Cx43 coupling may be coupled via dynamic alterations in V_m in P‐UAEC, as reported in some (HUVEC) but not all endothelial cells. Following basic electrical characterization of UAEC, we employed a high sensitivity cell imaging system to simultaneously monitor cell V_m and [Ca2+]i in real time in continuous monolayers of UAEC. Our findings show that while acute and sustained phase [Ca2+]i bursting occur dose‐dependently in response to ATP, V_m is not coregulated with any periodicity related to [Ca2+]i bursting. Only a small but significant progressive change in V_m is seen, and this is more closely related to overall mobilization of Ca2+. Surprisingly, this is also most apparent in NP‐UAEC > P‐UAEC. In contrast [Ca2+]i bursting is more synchronous in P‐UAEC and even achieves [Ca2+]i waves passing through the P‐UAEC monolayer. The relevance of these findings to mechanisms of pregnancy adaptation and its failure in hypertensive pregnancy are discussed.

Acetylcholine induces intracellular Ca2+ oscillations and nitric oxide release in mouse brain endothelial cells

Basal forebrain neurons increase cortical blood flow by releasing acetylcholine (Ach), which stimulates endothelial cells (ECs) to produce the vasodilating gasotransmitter, nitric oxide (NO). Surprisingly, the mechanism whereby Ach induces NO synthesis in brain microvascular ECs is unknown. An increase in intracellular Ca²⁺ concentration recruits a multitude of endothelial Ca²⁺-dependent pathways, such as Ca²⁺/calmodulin endothelial NO synthase (eNOS). The present investigation sought to investigate the role of intracellular Ca²⁺ signaling in Ach-induced NO production in bEND5 cells, an established model of mouse brain microvascular ECs, by conventional imaging of cells loaded with the Ca²⁺-sensitive dye, Fura-2/AM, and the NO-sensitive fluorophore, DAF-DM diacetate. Ach induced dose-dependent Ca²⁺ oscillations in bEND5 cells, 300 μM being the most effective dose to generate a prolonged Ca²⁺ burst. Pharmacological manipulation revealed that Ach-evoked Ca²⁺ oscillations required metabotropic muscarinic receptor (mAchR) activation and were patterned by a complex interplay between repetitive ER Ca²⁺ release via inositol-1,4,5-trisphosphate receptors (InsP₃Rs) and store-operated Ca²⁺ entry (SOCE). A comprehensive real time-polymerase chain reaction analysis demonstrated the expression of the transcripts encoding for M3-mAChRs, InsP₃R1 and InsP₃R3, Stim1-2 and Orai2. Next, we found that Ach-induced NO production was hindered by L-NAME, a selective NOS inhibitor, and BAPTA, a membrane permeable intracellular Ca²⁺ buffer. Moreover, Ach-elicited NO synthesis was blocked by the pharmacological abrogation of the accompanying Ca²⁺ spikes. Overall, these data shed novel light on the molecular mechanisms whereby neuronally-released Ach controls neurovascular coupling in blood microvessels.

Functional impairment of endothelial cells by the antimycotic amphotericin B

We set out to determine the membrane potential (Vm) of the endothelial cell line EA.hy926 and its sensitivity to the antimycotic amphotericin B (AmB), a commonly used antifungal component in cell culture media. We measured the endothelial Vm under various experimental conditions by patch clamp technique and found that Vm of AmB-treated cells is (-12.1 ± 9.3) mV, while in AmB-untreated (control) cells it is (-57.1 ± 4.1) mV. In AmB-free extracellular solutions, Vm recovered toward control levels and this gain in Vm rapidly dissipated upon re-addition of AmB, demonstrating a rapid and reversible effect of AmB on endothelial Vm. The consequences of AmB dependent alterations in endothelial transmembrane potential were tested at the levels of Ca(2+) signaling, of nucleotide concentrations, and energy metabolism. In AmB-treated cells we found substantially reduced Ca(2+) entry (to about 60% of that in control cells) in response to histamine induced endoplasmic reticulum (ER) Ca(2+) depletion, and diminished the ATP-to-ADP ratio (by >30%). Our data demonstrate a marked and experimentally relevant dependence of basic functional parameters of cultured endothelial cells on the presence of the ionophoric antimycotic AmB. The profound and reversible effects of the widely used culture media component AmB need careful consideration when interpreting experimental data obtained under respective culture conditions.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Statistical platform to discern spatial and temporal coordination of endothelial sprouting

Many biological processes, including angiogenesis, involve intercellular feedback and temporal coordination, but inference of these relations is often drowned in low sample sizes or noisy population data. To address this issue, a methodology was developed to statistically study spatial lateral inhibition and temporal synchronization in one specific biological process, endothelial sprouting mediated by Notch signaling. Notch plays an essential role in the development of organized vasculature, but the effects of Notch on the temporal characteristics of angiogenesis are not well understood. Results from this study showed that Notch lateral inhibition operates at distances less than 31 μm. Furthermore, combining time lapse microscopy with an intraclass correlation model typically used to analyze family data showed intrinsic temporal synchronization among endothelial sprouts originating from the same microcarrier. Such synchronization was reduced with Notch inhibitors, but was enhanced with the addition of Notch ligands. These results indicate that Notch plays a critical role in the temporal regulation of angiogenesis, as well as spatial control, and this method of analysis will be of significant utility in studies of a variety of other biological processes.

Connexin channels provide a target to manipulate brain endothelial calcium dynamics and blood-brain barrier permeability

The cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) is an important factor determining the functional state of blood-brain barrier (BBB) endothelial cells but little is known on the effect of dynamic [Ca(2+)](i) changes on BBB function. We applied different agonists that trigger [Ca(2+)](i) oscillations and determined the involvement of connexin channels and subsequent effects on endothelial permeability in immortalized and primary brain endothelial cells. The inflammatory peptide bradykinin (BK) triggered [Ca(2+)](i) oscillations and increased endothelial permeability. The latter was prevented by buffering [Ca(2+)](i) with BAPTA, indicating that [Ca(2+)](i) oscillations are crucial in the permeability changes. Bradykinin-triggered [Ca(2+)](i) oscillations were inhibited by interfering with connexin channels, making use of carbenoxolone, Gap27, a peptide blocker of connexin channels, and Cx37/43 knockdown. Gap27 inhibition of the oscillations was rapid (within minutes) and work with connexin hemichannel-permeable dyes indicated hemichannel opening and purinergic signaling in response to stimulation with BK. Moreover, Gap27 inhibited the BK-triggered endothelial permeability increase in in vitro and in vivo experiments. By contrast, [Ca(2+)](i) oscillations provoked by exposure to adenosine 5' triphosphate (ATP) were not affected by carbenoxolone or Gap27 and ATP did not disturb endothelial permeability. We conclude that interfering with endothelial connexin hemichannels is a novel approach to limiting BBB-permeability alterations.

Activation of NAD(P)H:quinone oxidoreductase ameliorates spontaneous hypertension in an animal model via modulation of eNOS activity

AIMS

Hypertension is one of the most common human diseases worldwide, and extensive research efforts are focused upon the identification and utilizing of novel therapeutic drug targets. Nitric oxide (NO) produced by endothelial NO synthase (eNOS) is an important regulator of blood pressure (BP). β-Lapachone (βL), a well-known substrate of NAD(P)H:quinone oxidoreductase (NQO1), increases the cellular NAD(+)/NADH ratio via the activation of NQO1. In this study, we evaluated whether βL-induced activation of NQO1 modulates BP in an animal model of hypertension.

METHODS AND RESULTS

Spontaneously hypertensive rats (SHR), primary human aortic endothelial cells (HAEC), and endothelial cell lines were used to investigate the hypotensive effect of βL and its mode of action. βL treatment stimulated endothelium-dependent vascular relaxation in response to acetylcholine in aorta of SHR and dramatically lowered BP in SHR, but the hypotensive effect was completely blocked by eNOS inhibition with ω-nitro-l-arginine methyl ester. Aortic eNOS phosphorylation and eNOS protein expression were significantly increased in βL-treated SHR. In vitro studies revealed that βL treatment elevated the intracellular NAD(+)/NADH ratio and concentration of free Ca(2+) ([Ca(2+)]i), and resulted in Akt/AMP-activated protein kinase/eNOS activation. These effects were abolished by NQO1 siRNA and [Ca(2+)]i inhibition through a ryanodine receptor blockade.

CONCLUSION

This study is the first to demonstrate that NQO1 activation has a hypotensive effect mediated by eNOS activation via cellular NAD(+)/NADH ratio modulation in an animal model. These results provide strong evidence suggesting NQO1 might be a new therapeutic target for hypertension.

Lung vascular cell heterogeneity: endothelium, smooth muscle, and fibroblasts

The pulmonary circulation represents a unique vascular bed, receiving 100% of the cardiac output while maintaining low blood pressure. Multiple different cell types, including endothelium, smooth muscle, and fibroblasts, contribute to normal vascular function, and to the vascular response to injury. Our understanding of the basic cell biology of these various cell types, and the roles they play in vascular homeostasis and disease, remains quite limited despite several decades of study. Recent advances in approaches that enable the mapping of cell origin and the study of the molecular basis of structure and function have resulted in a rapid accumulation of new information that is essential to vascular biology. A recent National Institutes of Health workshop was held to discuss emerging concepts in lung vascular biology. The findings of this workshop are summarized in this article.

Phospholipase C-delta extends intercellular signalling range and responses to injury-released growth factors in non-excitable cells

OBJECTIVES

Intercellular communication in non-excitable cells is restricted to a limited range close to the signal source. Here, we have examined whether modification of the intracellular microenvironment could prolong the spatial proposition of signal generation and could increase cell proliferation.

MATERIAL AND METHODS

Mathematical models and experimental studies of endothelial repair after controlled mechanical injury were used. The models predict the diffusion range of injury-released growth factors and identify important parameters involved in a signalling regenerative mode. Transfected human umbilical vein endothelial cells (HUVECs) were used to validate model results, by examining intercellular calcium signalling range, cell proliferation and wound healing rate.

RESULTS

The models predict that growth factors have a limited capacity of extracellular diffusion and that intercellular signals are specially sensitive to cell phospholipase C-delta (PLCdelta) levels. As basal PLCdelta levels are increased by transfection, a significantly increased intercellular calcium range, enhanced cell proliferation, and faster wound healing rate were observed.

CONCLUSION

Our in silico and in vitro studies demonstrated that non-excitable endothelial cells respond to stimuli in a complex manner, in which intercellular communication is controlled by physicochemical properties of the stimulus and by the cell microenvironment. Such findings may have profound implications for our understanding of the tight nature of autocrine cell growth control, compensation to stress states and response to altered microenvironment, under pathological conditions.

Study of the neuronal effects of ouabain and palytoxin and their binding to Na,K-ATPases using an optical biosensor

The phycotoxin palytoxin (PTX) binds to Na,K-ATPase, inhibiting its activity and converting the pump into a channel. These mechanisms are poorly understood. We examined the effect of PTX on membrane potential (E(m)), intracellular calcium concentration ([Ca2+]i) and intracellular pH (pH(i)) in primary cultures of cerebellar granule cells (CGC) and compared PTX and ouabain actions in the same cellular parameters. In this system, PTX caused depolarization, intracellular calcium increase and acidification. This is similar to the effect of ouabain. Preincubation of the cells with ouabain, before addition of PTX, altered E(m), [Ca2+]i, and pH(i) in a fashion similar to that of ouabain alone. This suggest a direct interaction of PTX with the Na,K-ATPase. Therefore, we used a resonant mirror biosensor to evaluate the binding of PTX and ouabain to immobilized Na,K-ATPase. Ouabain binding to immobilized Na,K-ATPase was concentration-dependent. No binding of PTX to Na,K-ATPase was observed with up to 10 microM, or with PTX addition in the presence of ATP. The fact that ouabain binds to the pump in an immobilized conformation whereas not binding of PTX was observed indicates that PTX and ouabain do not share the same binding site, and PTX binding may require the tridimensional pump structure.

Role of the plasma membrane calcium adenosine triphosphatase on domoate-induced intracellular acidification in primary cultures of cerebelar granule cells

Changes in intracellular pH (pH(i)) and cytosolic calcium concentration ([Ca(2+)](c)) caused by the glutamate agonist domoate (DOM) were studied in single cultured mouse cerebellar granule cells (CGC) by using the fluorescent probes 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) and simultaneous evaluation of cytosolic calcium concentration with the fluorescent dye Fura-2 acetoxymethyl ester (Fura-2 AM). DOM caused a concentration-dependent increase in [Ca(2+)](c) and a concentration-dependent intracellular acidification of CGC. DOM-induced intracellular acidification was completely abolished by the use of Ca(2+)-free medium, suggesting that it was due mostly to an influx of extracellular calcium. The pH(i) decrease caused by DOM was also completely blocked in the presence of the AMPA/kainate receptor antagonist CNQX, indicating that the DOM-induced intracellular acidification was caused by DOM activation of the AMPA/kainate subtype of glutamate receptors. Different mechanisms that could be involved in DOM-induced pH(i) decrease, such as displacement of H(+) by Ca(2+) from a common intracellular binding site, DOM-induced alteration of pH(i) regulation mechanisms, and a possible acidification caused by DOM-induced increase of mitochondrial Ca(2+) uptake, were excluded. DOM-induced intracellular acidification was completely prevented by inhibitors of the plasma membrane calcium adenosine triphosphatase (ATPase) (PMCA), including orthovanadate, lanthanum extracellular pH of 8.5, and the specific PMCA inhibitor caloxin 2A1. Our results therefore indicate that PMCA is involved in DOM-induced intracellular acidification in primary cultures of CGC. Simultaneous recording of [Ca(2+)](c) and pH(i) indicates that the increase in intracellular calcium evoked by DOM will activate the calcium extrusion mechanisms through the calcium pump, which, in turn, will decrease intracellular pH by countertransport of H(+) ions.

Roles of nuclear NPY and NPY receptors in the regulation of the endocardial endothelium and heart functionThis paper is one of a selection of papers published in this Special issue, entitled Second Messengers and Phosphoproteins—12th International Conference

It is now well accepted that the heart is a multifunctional organ in which endothelial cells, and more particularly endocardial endothelial cells (EECs), seem to play an important role in regulating and maintaining cardiac excitation–contraction coupling. Even if major differences exist between vascular endothelial cells (VECs) and EECs, all endothelial cells including EECs release a variety of auto- and paracrine factors such as nitric oxide, endothelin-1, angiotensin II, and neuropeptide Y. All these factors were reported to affect cardiomyocyte contractile performance and rhythmicity. In this review, findings on the morphology of EECs, differences between EECs and other types of endothelial cells, interactions between EECs and the adjacent cardiomyocytes, and effects of NPY on the heart will be presented. We will also show evidence on the presence and localization of NPY and the Y1receptor in the endocardial endothelium and discuss their role in the regulation of cytosolic and nuclear free calcium.

Vasomotion: cellular background for the oscillator and for the synchronization of smooth muscle cells

1. Vasomotion is the oscillation of vascular tone with frequencies in the range from 1 to 20 min(-1) seen in most vascular beds. The oscillation originates in the vessel wall and is seen both in vivo and in vitro. 2. Recently, our ideas on the cellular mechanisms responsible for vasomotion have improved. Three different types of cellular oscillations have been suggested. One model has suggested that oscillatory release of Ca2+ from intracellular stores is important (the oscillation is based on a cytosolic oscillator). A second proposed mechanism is an oscillation originating in the sarcolemma (a membrane oscillator). A third mechanism is based on an oscillation of glycolysis (metabolic oscillator). For the two latter mechanisms, only limited experimental evidence is available. 3. To understand vasomotion, it is important to understand how the cells synchronize. For the cytosolic oscillators synchronization may occur via activation of Ca2+-sensitive ion channels by oscillatory Ca2+ release. The ensuing membrane potential oscillation feeds back on the intracellular Ca2+ stores and causes synchronization of the Ca2+ release. While membrane oscillators in adjacent smooth muscle cells could be synchronized through the same mechanism that sets up the oscillation in the individual cells, a mechanism to synchronize the metabolic-based oscillators has not been suggested. 4. The interpretation of the experimental observations is supported by theoretical modelling of smooth muscle cells behaviour, and the new insight into the mechanisms of vasomotion has the potential to provide tools to investigate the physiological role of vasomotion.

A model of intracellular Ca2+ oscillations based on the activity of the intermediate-conductance Ca2+-activated K+ channels

Intracellular Ca2+ oscillations are observed in a large number of non-excitable cells. While most appear to reflect an intermittent Ca2+ release from intracellular stores, in some instances intracellular Ca2+ oscillations strongly depend on Ca2+ influx, and are coupled to oscillations of the membrane potential, suggesting that a plasma membrane-based mechanism may be involved. We have developed a theoretical model for the latter type of intracellular Ca2+ oscillations based on the Ca2+-dependent modulation of the intermediate-conductance, Ca2+-activated K+ (IKCa) channel. The functioning of this model relies on the Ca2+-dependent activation, and the much slower Ca2+-dependent rundown of this channel. We have shown that Ca2+-dependent activation of the IKCa channels, the consequent membrane hyperpolarization and the resulting increase in Ca2+ influx may confer the positive feedback mechanism required for the ascending phase of the oscillation. The much slower Ca2+-dependent rundown process will conversely halt this positive loop, and establish the descending phase of the intracellular Ca2+ oscillation. We found that this simple model gives rise to intracellular Ca2+ oscillations when using physiologically reasonable parameters, suggesting that IKCa channels could participate in the generation of intracellular Ca2+ oscillations.

Coordinate regulation of membrane cAMP by Ca2+-inhibited adenylyl cyclase and phosphodiesterase activities

Activation of store-operated Ca(2+) entry inhibits type 6 adenylyl cyclase (EC; AC(6); Yoshimura M and Cooper DM. Proc Natl Acad Sci USA 89: 6712-6720, 1992) activity in pulmonary artery endothelial cells. However, in lung microvascular endothelial cells (PMVEC), which express AC(6) and turn over cAMP at a rapid rate, inhibition of global (whole cell) cAMP is not resolved after direct activation of store-operated Ca(2+) entry using thapsigargin. Present studies sought to determine whether the high constitutive phosphodiesterase activity in PMVECs rapidly hydrolyzes cAMP so that Ca(2+) inhibition of AC(6) is difficult to resolve. Direct stimulation of adenylyl cyclase using forskolin and inhibition of type 4 phosphodiesterases using rolipram increased cAMP and revealed Ca(2+) inhibition of AC(6). Enzyme activity was assessed using PMVEC membranes, where Ca(2+) and cAMP concentrations were independently controlled. Endogenous AC(6) activity exhibited high- and low-affinity Ca(2+) inhibition, similar to that observed in C6-2B cells, which predominantly express AC(6). Ca(2+) inhibition of AC(6) in PMVEC membranes was observed after enzyme activation and inhibition of phosphodiesterase activity and was independent of the free cAMP concentration. Thus, under basal conditions, the constitutive type 4 phosphodiesterase activity rapidly hydrolyzes cAMP so that Ca(2+) inhibition of AC(6) is difficult to resolve, indicating that high phosphodiesterase activity works coordinately with AC(6) to regulate membrane-delimited cAMP concentrations, which is important for control of cell-cell apposition.

Mechanisms of heterogeneous endothelial cytoplasmic calcium increases in venular microvessels

OBJECTIVE

Localized inflammatory leaky sites form at regions of the microvessel wall with the largest increase in endothelial cell cytoplasmic calcium concentration, [Ca(2+)](i). We investigated the mechanisms that modulate localized increases in [Ca(2+)](i) in individual endothelial cells of microvessels after exposure to ATP.

METHODS

[Ca(2+)](i) was measured by using digital fluorescence microscopy and fura-2 in the endothelial cells forming the walls of individually perfused frog mesenteric microvessels. The spread of [Ca(2+)](i) from a localized mechanical stimulus was also measured.

RESULTS

The peak [Ca(2+)](i) after ATP showed marked heterogeneity, ranging from 227 to 1469 nM from resting values of 69 +/- 5 nM. After depolarization with high-potassium solutions, the endothelial cells with the largest peak increase in [Ca(2+)](i) had the largest fractional reduction. Localized increases in [Ca(2+)](i) due to mechanical stimulus did not spread.

CONCLUSION

The key mechanism regulating the heterogeneity in initial peak increase in [Ca(2+)](i) is a calcium-dependent process regulated by the calcium influx itself. One such mechanism, the calcium-dependent opening of additional potassium channels leading to membrane hyperpolarization and increased driving force for calcium entry through passive conductance pathways, accounts for a significant amount of the heterogeneity of [Ca(2+)](i) in our experiments. Further investigations of both localized calcium influx and membrane potentials in the endothelial cells of intact microvessels in both frog and mammals using the imaging methods developed for these investigations are needed to understand the formation of localized leaky sites in inflamed microvessels.

Calcium signaling: a tale for all seasons

An experiment performed in London nearly 120 years ago, which by today's standards would be considered unacceptably sloppy, marked the beginning of the calcium (Ca(2+)) signaling saga. Sidney Ringer [Ringer, S. (1883) J. Physiol. 4, 29-43] was studying the contraction of isolated rat hearts. In earlier experiments, Ringer had suspended them in a saline medium for which he admitted to having used London tap water, which is hard: The hearts contracted beautifully. When he proceeded to replace the tap water with distilled water, he made a startling finding: The beating of the hearts became progressively weaker, and stopped altogether after about 20 min. To maintain contraction, he found it necessary to add Ca(2+) salts to the suspension medium. Thus, Ringer had serendipitously discovered that Ca(2+), hitherto exclusively considered as a structural element, was active in a tissue that has nothing to do with bone or teeth, and performed there a completely novel function: It carried the signal that initiated heart contraction. It was a landmark observation, which should have immediately aroused wide interest. Unexpectedly, however, for decades it attracted no particular attention. Occasionally, farsighted pioneers argued forcefully for a messenger role of Ca(2+), offering compelling experimental evidence. Among them, one could quote L. V. Heilbrunn [Heilbrunn, L. V. (1940) Physiol. Zool. 13, 88-94], who contracted frog muscle fibers by applying Ca(2+) salts to their cut ends, but not to their surfaces. Heilbrunn correctly concluded that Ca(2+) had diffused from the cut ends to the internal contractile elements to elicit their contraction. One could also quote K. Bailey [Bailey, K. (1942) Biochem. J. 36, 121-139], who showed that the ATPase activity of myosin was strongly activated by Ca(2+) (but not by Mg(2+)), and concluded that the liberation of Ca(2+) in the neighborhood of the myosin controlled muscle contraction. Clearly, enough evidence was there, but only a handful of people had the vision to see it and to foresee its far-reaching implications. Perhaps no better example of clairvoyance can be offered than the quip by O. Loewy in 1959: "Ja Kalzium, das ist alles!"

Repolarization of the plasma membrane shapes NMDA-induced cytosolic [Ca2+ ] transients

After inactivation of NMDA receptors, restoration of basal cytosolic [Ca2+] ([Ca2+]c) is delayed. This may be caused by Ca2+ influx via reverse Na/Ca exchange or voltage-gated Ca2+ channels, and/or by Ca2+ efflux from internal stores. Monitoring of [Na+]c, [Ca2+]c, and plasma membrane potential in cultured cerebellar granule cells showed that repolarization of the plasma membrane and inactivation of voltage-gated Ca channels plays the most critical role in restoration of low [Ca2+]c following NMDA receptor inactivation. During NMDA receptor activation, however, an Na-dependent mechanism enhanced NMDA-induced elevation in [Ca2+]c. This mechanism did not involve Na,K-ATPase activation by Na+, because it operated even when Na,K-ATPase was inhibited.

Ruled by waves? Intracellular and intercellular calcium signalling

The field of calcium signalling has evolved rapidly the last 20 years. Physiologists had worked with cytosolic Ca2+ as the coupler of excitation and contraction of muscles and as a secretory signal in exocrine glands and in the synapses of the brain for several decades before the discovery of cellular calcium as a second messenger. Development of powerful techniques for measuring the concentration of cytosolic free calcium ions in cell suspensions and later in single cells and even in different cellular compartments, has resulted in an upsurge in the knowledge of the cellular machinery involved in intracellular calcium signalling. However, the focus on intracellular mechanisms might have led this field of study away from physiology. During the last few years there is an increasing evidence for an important role of calcium also as an intercellular signal. Via gap junctions calcium is able to co-ordinate cell populations and even organs like the liver. Here we will give an overview of the general mechanisms of intracellular calcium signalling, and then review the recent data on intercellular calcium signals. A functional coupling of cells in different tissues and organs by the way of calcium might be an important mechanism for controlling and synchronizing physiological responses

ATP-sensitive potassium channels in capillaries isolated from guinea-pig heart

The full-length cDNAs of two different alpha-subunits (Kir6.1 and Kir6.2) and partial cDNAs of three different beta-subunits (SUR1, SUR2A and SUR2B) of ATP-sensitive potassium (KATP) channels of the guinea-pig (gp) were obtained by screening a cDNA library from the ventricle of guinea-pig heart. Cell-specific reverse-transcriptase PCR with gene-specific intron-spanning primers showed that gpKir6.1, gpKir6.2 and gpSUR2B were expressed in a purified fraction of capillary endothelial cells. In cardiomyocytes, gpKir6.1, gpKir6.2, gpSUR1 and gpSUR2A were detected. Patch-clamp measurements were carried out in isolated capillary fragments consisting of 3-15 endothelial cells. The membrane capacitance measured in the whole-cell mode was 19.9 +/- 1.0 pF and was independent of the length of the capillary fragment, which suggests that the endothelial cells were not electrically coupled under our experimental conditions. The perforated-patch technique was used to measure the steady-state current-voltage relation of capillary endothelial cells. Application of K+ channel openers (rilmakalim or diazoxide) or metabolic inhibition (250 microM 2,4-dinitrophenol plus 10 mM deoxyglucose) induced a current that reversed near the calculated K+ equilibrium potential. Rilmakalim (1 microM), diazoxide (300 microM) and metabolic inhibition increased the slope conductance measured at -55 mV by a factor of 9.0 (+/-1.8), 2.5 (+/-0.2) and 3.9 (+/-1.7), respectively. The effects were reversed by glibenclamide (1 microM). Our results suggest that capillary endothelial cells from guinea-pig heart express KATP channels composed of SUR2B and Kir6.1 and/or Kir6.2 subunits. The hyperpolarization elicited by the opening of KATP channels may lead to an increase in free cytosolic Ca2+, and thus modulate the synthesis of NO and the permeability of the capillary wall.

Histamine-induced Ca2+ oscillations in a human endothelial cell line depend on transmembrane ion flux, ryanodine receptors and endoplasmic reticulum Ca2+-ATPase

Using single cell microfluorometry to monitor changes in bulk Ca2+ concentration ([Ca2+]bulk) and the whole-cell configuration of the patch clamp technique to measure K+ currents (voltage clamp) and membrane potential (current clamp), the mechanisms of histamine-induced Ca2+ oscillations in the umbilical vein endothelial cell-derived cell line EA.hy926 were studied. In single cells, histamine (10 microM) evoked sinusoidal Ca2+ oscillations in low extracellular Ca2+ concentrations ([Ca2+]o = 10-30 microM). In contrast, histamine did not initiate Ca2+ oscillations either in the absence of extracellular Ca2+ (10 microM EGTA) or in the presence of 2.5 mM extracellular Ca2+. Ca2+ oscillations were accompanied by rhythmic activation of Ca2+-activated K+ (KCa) channels and membrane hyperpolarization of 18.1 +/- 3.9 mV. Hence, cell depolarization with 70 mM extracellular K+ or the inhibition of non-selective cation channels (NSCCs) and KCa channels by 10 microM Loe 908 and 10 mM tetrabutylammonium prevented histamine-evoked Ca2+ oscillations. Preventing Na+-Ca2+ exchange (NCX) by 10 microM 2', 4'-dichlorobenzamil, or removal of extracellular Na+, abolished histamine-induced Ca2+ oscillations. Lowering the extracellular Na+ concentration and thus promoting the reversed mode of NCX (3Na+ out and 1Ca2+ in) increased the amplitude and frequency of histamine-induced Ca2+ oscillations by 25 and 13 %, respectively. Hence, in the absence of extracellular Ca2+, 10 microM histamine induced an elevation of intracellular Na+ concentration in certain subplasmalemmal domains. The inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 2,5-di-tert-butyl-1, 4-benzo-hydroquinone (15 microM) prevented histamine-induced Ca2+ oscillations. In addition, blockage of ryanodine-sensitive Ca2+ release (RsCR) by 25 microM ryanodine blunted Ca2+ oscillations. In endothelial cells that were treated for 16 h with 10 microM nocodazole to collapse the superficial endoplasmic reticulum (sER), no histamine-induced Ca2+ oscillations were found. We conclude that in low [Ca2+]o conditions histamine-induced Ca2+ oscillations depend on transmembrane Na+ loading through NSCCs that leads to Ca2+ entry via NCX. Cation influx is controlled by KCa channel activity that triggers membrane hyperpolarization and, thus, provides the driving force for cation influx. Hence, the Ca2+ entering needs to be sequestrated via SERCA into sER to become released by RsCR to evoke Ca2+ spiking. These data further support our previous work on localized Ca2+ signalling as a key phenomenon in endothelial Ca2+ homeostasis.

N-methyl-D-aspartate excitotoxicity: relationships among plasma membrane potential, Na(+)/Ca(2+) exchange, mitochondrial Ca(2+) overload, and cytoplasmic concentrations of Ca(2+), H(+), and K(+)

A high cytoplasmic Na(+) concentration may contribute to N-methyl-D-aspartate (NMDA)-induced excitotoxicity by promoting Ca(2+) influx via reverse operation of the Na(+)/Ca(2+) exchanger (NaCaX), but may simultaneously decrease the electrochemical Ca(2+) driving force by depolarizing the plasma membrane (PM). Digital fluorescence microscopy was used to compare the effects of Na(+) versus ions that do not support the NaCaX operation, i.e., N-methyl-D-glucamine(+) or Li(+), on: PM potential; cytoplasmic concentrations of Ca(2+), H(+), and K(+); mitochondrial Ca(2+) storage; and viability of primary cultures of cerebellar granule cells exposed to NMDA receptor agonists. In the presence of Na(+) or Li(+), NMDA depolarized the PM and decreased cytoplasmic pH (pH(C)); in the presence of Li(+), Ca(2+) influx was reduced, mitochondrial Ca(2+) overload did not occur, and the cytoplasm became more acidified than in the presence of Na(+). In the presence of N-methyl-D-glucamine(+), NMDA instantly hyperpolarized the PM, but further changes in PM potential and pH(C) were Ca-dependent. In the absence of Ca(2+), hyperpolarization persisted, pH(C) was decreasing very slowly, K(+) was retained in the cytoplasm, and cerebellar granule cells survived the challenge; in the presence of Ca(2+), pH(C) dropped rapidly, the K(+) concentration gradient across the PM began to collapse as the PM began to depolarize, and Ca(2+) influx and excitotoxicity greatly increased. These results indicate that the dominant, very likely excitotoxic, component of NMDA-induced Ca(2+) influx is mediated by reverse NaCaX and that direct Ca(2+) influx via NMDA channels is curtailed by Na-dependent PM depolarization.

1-Ethyl-2-benzimidazolinone stimulates endothelial K(Ca) channels and nitric oxide formation in rat mesenteric vessels

Hyperpolarization of most blood vessels occurs by the opening of K(Ca) channels. 1-Ethyl-2-benzimidazolinone (1-EBIO) is a direct activator of K(Ca) channels in epithelial cells and is potentially valuable for studying cellular hyperpolarization. This study reports the effects of 1-EBIO on isolated rat mesenteric beds perfused with normal (4.7 mM), or high (20 or 80 mM) K+ physiological salt solution (PSS) and constricted with an alpha1-adrenoceptor agonist, cirazoline (0.3-1 microM). Arterial perfusion pressures were decreased by 1-EBIO (0.1-30 nmol) in a dose- and endothelium-dependent manner. Infusion of penitrem A (100 nM), a maxi-K+ channel blocker, or apamin (0.5 microM), a small-conductance (SK(Ca)) K+ channel blocker, produced significant increases in cirazoline-mediated tone (mm Hg): 103.3 +/- 8.7 (control) vs. 156.3 +/- 14.3 (penitrem A); or 93.0 +/- 15.8 (control) vs. 114.0 +/- 15.4 (apamin). 1-EBIO relaxations were attenuated by penitrem A, while apamin, dendrotoxin (50 nM; a Kv channel antagonist), or ouabain (100 microM; a sodium pump blocker) failed to alter the responses. I-EBIO-mediated relaxations decreased significantly with increasing extracellular [K+]: relaxations to 30 nmol were 89.3% +/- 3.2% (4.7 mM K+, normal PSS) vs. 59.5% +/- 3.4% and 19.0% +/- 3.9% for 20 and 80 mM K+ PSS, respectively. Nomega-nitro-L-arginine-methyl ester (L-NAME; 100 microM), and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM), selective inhibitors of nitric oxide synthase, and nitric oxide-sensitive guanylate cyclase, respectively, abolished 1-EBIO relaxations in vessels perfused with 20 or 80 mM K+ PSS. We conclude that: (1) maxi-K+ and SK(Ca) channels are present in rat mesenteric arterial vessels and actively contribute to vascular tone, (2) vasodilator action of 1-EBIO involves the opening of endothelial maxi-K+ channels and nitric oxide synthesis.

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

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

Ceramide inhibits inwardly rectifying K+ currents via a Ras- and Raf-1-dependent pathway in cultured oligodendrocytes

Ceramide is a lipid mediator implicated in apoptosis induced by proinflammatory cytokines in many cell types, including oligodendrocytes (OLGs). To determine whether ceramide modulates transmembrane signaling events in OLGs, we studied its effect on intracellular Ca2+ (Cai), resting membrane potential and inwardly rectifying K+ currents (IKir) in cultured neonatal rat OLGs. We report here that (1) exposure to C2-ceramide (cer) rarely increases OLG Cai, whereas sphingosine elicits sustained increase in Cai; (2) cer causes OLG depolarization, an effect mimicked by sphingosine-1-phosphate but not by sphingosine; and (3) cer, but not its inactive analog dihydroceramide, inhibits OLG IKir. The cer effect is attenuated by Ras antibody Y13-259, by protein kinase C inhibitory peptide (19-36), and by suppression of c-Raf-1 expression with antisense raf-1 oligonucleotides. We conclude that cer-induced OLG depolarization is mediated via inhibition of IKir by a Ras- and raf-1-dependent pathway, which results in the phosphorylation of the inward rectifier K+ channel protein.

Ceramide inhibits inwardly rectifying K+ currents via a Ras- and Raf-1-dependent pathway in cultured oligodendrocytes

Ceramide is a lipid mediator implicated in apoptosis induced by proinflammatory cytokines in many cell types, including oligodendrocytes (OLGs). To determine whether ceramide modulates transmembrane signaling events in OLGs, we studied its effect on intracellular Ca2+ (Cai), resting membrane potential and inwardly rectifying K+ currents (IKir) in cultured neonatal rat OLGs. We report here that (1) exposure to C2-ceramide (cer) rarely increases OLG Cai, whereas sphingosine elicits sustained increase in Cai; (2) cer causes OLG depolarization, an effect mimicked by sphingosine-1-phosphate but not by sphingosine; and (3) cer, but not its inactive analog dihydroceramide, inhibits OLG IKir. The cer effect is attenuated by Ras antibody Y13-259, by protein kinase C inhibitory peptide (19-36), and by suppression of c-Raf-1 expression with antisense raf-1 oligonucleotides. We conclude that cer-induced OLG depolarization is mediated via inhibition of IKir by a Ras- and raf-1-dependent pathway, which results in the phosphorylation of the inward rectifier K+ channel protein.

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Outward potassium current oscillations in macrophage polykaryons: extracellular calcium entry and calcium-induced calcium release

Outward current oscillations associated with transient membrane hyperpolarizations were induced in murine macrophage polykaryons by membrane depolarization in the absence of external Na+. Oscillations corresponded to a cyclic activation of Ca(2+)-dependent K+ currents (IKCa) probably correlated with variations in intracellular Ca2+ concentration. Addition of external Na+ (8 mM) immediately abolished the outward current oscillations, suggesting that the absence of the cation is necessary not only for their induction but also for their maintenance. Oscillations were completely blocked by nisoldipine. Ruthenium red and ryanodine reduced the number of outward current cycles in each episode, whereas quercetin prolonged the hyperpolarization 2- to 15-fold. Neither low molecular weight heparin nor the absence of a Na+ gradient across the membrane had any influence on oscillations. The evidence suggests that Ca2+ entry through a pathway sensitive to Ca2+ channel blockers is elicited by membrane depolarization in Na(+)-free medium and is essential to initiate oscillations, which are also dependent on the cyclic release of Ca2+ from intracellular Ca(2+)-sensitive stores; Ca2+ ATPase acts by reducing intracellular Ca2+, thus allowing slow deactivation of IKCa. Evidence is presented that neither a Na+/Ca2+ antiporter nor Ca2+ release from IP3-sensitive Ca2+ stores participate directly in the mechanism of oscillation.

Hyperpolarization of isolated capillaries from guinea-pig heart induced by K+ channel openers and glucose deprivation

1. The present study was designed to test if microvascular coronary endothelial cells express ATP-sensitive K+ channels (KATP channels). We performed microfluorometric measurements of the membrane potential of freshly isolated guinea-pig coronary capillaries equilibrated with the voltage-sensitive dye bis-oxonol (bis-[1,3-dibutylbarbituric acid] trimethineoxonol, [DiBAC4(3)]). 2. The resting membrane potential of capillaries in physiological salt solution was -46 +/- 4.2 mV (n = 8) at room temperature (22 degrees C) as determined after calibration of the fluorescence using the Na(+)-K+ ionophore gramicidin in the presence of different K+ concentrations. Spontaneous membrane potential fluctuations of 10-20 mV amplitude were often observed. 3. A reversible, sustained hyperpolarization to a new membrane potential close to the K+ equilibrium potential (EK) could be induced by application of the K+ channel openers HOE 234 (100 nM to 1 microM), diazoxide (10 PM to 100 nM) or pinacidil (100 nM). Subsequent addition of glibenclamide (200 nM to 2 microM) reversed this hyperpolarization. 4. A glibenclamide-sensitive hyperpolarization of coronary capillaries to values near EK was also observed upon omission of D-glucose (10 mM) from the superfusing solution or by substituting L-glucose for D-glucose. Maximum hyperpolarization was reached in less than 10 min. 5. Our results suggest that microvascular coronary endothelial cells express KATP channels which may be activated during hypoglycaemia.

Separate analysis of nuclear and cytosolic Ca2+ concentrations in human umbilical vein endothelial cells

Ca2+ concentration inside human umbilical vein endothelial cells was studied separately in cytosol and nucleus by a confocal laser scanning microscopy using fluo-3. The in vivo calibration curve for cytosol and nucleus showed good linearity between fluorescence intensity and Ca2+ concentration in cytosol ([Ca2+]i) and nuclei ([Ca2+]n). After calibration, [Ca2+]n was constantly higher than [Ca2+]i before and after the chelation of extracellular Ca2+ suggesting an active Ca2+ accumulation system on nuclear membrane. [Ca2+]n was also constantly higher than [Ca2+]i after the stimulation of thrombin (0.05 U/ml), FCS (10%), and thapsigargin (Tsg, 1 microM). The temporal change of [Ca2+]n and [Ca2+]i was identical, and [Ca2+]i gradient towards the nucleus and peripheral or central [Ca2+]n rise was observed after these stimulations. From these results, [Ca2+]n is not only regulated by the active Ca2+ accumulation system on nuclear membrane at rest but also the generation of inositol-triphosphate. FCS caused heterogeneous [Ca2+]n or [Ca2+]i rise from cell to cell; single spike or oscillatory change of [Ca2+]n and [Ca2+]i was observed in about 56% of cells, which were completely abolished by the chelation of extracellular Ca2+, suggesting that FCS stimulated [Ca2+]n and [Ca2+]i rise solely depending on Ca2+ influx from extracellular medium. The higher concentration of [Ca2+]n and heterogeneous [Ca2+]n rise may have important roles in nuclear-specific cellular responses.

Endothelial cell oxidant generation during K(+)-induced membrane depolarization

We tested the hypothesis that membrane depolarization may initiate oxidant generation in the endothelial cell. Depolarization was produced in bovine pulmonary arterial endothelial cells (BPAEC) in monolayer culture with varying external K+, or with glyburide (10 microM), tetraethylammonium (TEA, 10 mM), gramicidin (1 microM), or nigericin (2 microM). Evaluation of bisoxonol fluorescence of BPAEC indicated concentration-dependent depolarization by high K+ (2% change in fluorescence/mV change in membrane potential in the 5.9-48 mM range of K+) and essentially complete depolarization with glyburide. Generation of oxidants was assessed with o-phenylenediamine dihydrochloride (o-PD) oxidation in the presence of horseradish peroxidase (HRP). There was a time-dependent increase in o-PD oxidation with 24 mM K+, nigericin, and gramicidin over 2 hours compared with control. In 1 hour o-PD oxidation increased 2.8-fold for 24 mM and 3.7-fold for 48 mM K+ compared with control. Catalase reduced 24 mM K(+)-induced o-PD oxidation by 50%, while Cu/Zn-superoxide dismutase (SOD) abolished the increase. Oxidation of o-PD was reduced by 57% in the absence of HRP in the system. With K+ channel blockade, o-PD oxidation increased 3.8-fold with glyburide and 4.6-fold with TEA compared with control. These data indicate formation of H2O2 and possibly other oxidants with depolarization and suggest involvement of K(+)-channels in this process.

Cytosolic calcium concentration in resting and stimulated endothelium of excised intact rat aorta

1. Optical fibres were used to excite and record fluorescence from the lumenal face of rat aorta or tail artery loaded with fura-2. 2. Acetylcholine (ACh) evoked an endothelium-dependent rise in the fura-2 340/380 nm excitation ratio in both vessels. High [K+] or phenylephrine evoked an endothelium-independent rise in ratio in tail artery but failed to increase the ratio in aorta. These observations indicate that fura-2 fluorescence and therefore cytosolic calcium concentration ([Ca2+]i) may be selectively recorded from the endothelium of intact rat aorta. 3. In aortic endothelium, resting [Ca2+]i was 95 +/- 8 nM (n = 44). ACh evoked a monophasic rise in [Ca2+]i which was temporally coincident with a membrane hyperpolarization. 4. ATP in most (22/35) preparations evoked a rise in [Ca2+]i which declined towards resting and was followed by a secondary rise. The biphasic [Ca2+]i responses were accompanied by biphasic electrical responses of initial hyperpolarization followed by depolarization above the resting potential and subsequent restoration towards rest. In the presence of high [K+] or the K+ ionophore valinomycin, ATP did not evoke changes in membrane potential and only monophasic rises in [Ca2+]i were observed. In some (7/35) preparations, ATP evoked oscillations in [Ca2+]i, with membrane potential oscillating in antiphase. 5. These data suggest interplay between [Ca2+]i and membrane potential in the generation of agonist-evoked responses in native endothelium in situ. The observed oscillations in [Ca2+]i imply spatio-temporal synchronization of Ca2+ signalling in large groups of endothelial cells in intact rat aorta.

T cell receptor-mediated Ca2+ signaling: release and influx are independent events linked to different Ca2+ entry pathways in the plasma membrane

In this study, we showed that cross-linking CD3 molecules on the T cell surface resulted in Ca2+ release from the intracellular stores followed by a sustained Ca2+ influx. Inhibition of release with TMB-8 did not block the influx. However, inhibition of phospholipase C activity suppressed both Ca2+ release and influx. Once activated, the influx pathway remained open in the absence of further hydrolysis of PIP2. Thapsigargin, a microsomal Ca(2+)-ATPase inhibitor, stimulated Ca2+ entry into the cells by a mechanism other than emptying Ca2+ stores. In addition, Ca2+ entry into the Ca(2+)-depleted cells was stimulated by low basal level of cytosolic Ca2+, not by the emptying of intracellular Ca2+ stores. Both the Ca2+ release and influx were dependent on high and low concentrations of extracellular Ca2+. At low concentrations, Mn2+ entered the cell through the Ca2+ influx pathway and quenched the sustained phase of fluorescence; whereas, at higher Mn2+ concentration both the transient and the sustained phases of fluorescence were quenched. Moreover, Ca2+ release was inhibited by low concentrations of Ni2+, La3+, and EGTA, while Ca2+ influx was inhibited by high concentrations. Thus, in T cells Ca2+ influx occurs independently of IP3-dependent Ca2+ release. However, some other PIP2 hydrolysis-dependent event was involved in prolonged activation of Ca2+ influx. Extracellular Ca2+ influenced Ca2+ release and influx through the action of two plasma membrane Ca2+ entry pathways with different pharmacological and biochemical properties.

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

In a freshly isolated endothelial cell preparation from rabbit aorta, the regulation of the acetylcholine (ACh)-sensitive intracellular Ca2+ store and the effects of the Ca(2+)-induced Ca2+ release agonists ryanodine and caffeine were studied using fura 2 imaging fluorescence microscopy. ACh (10 mumol/L) caused a transient release of Ca2+ from an intracellular store, presumably via an inositol tris-phosphate-sensitive mechanism. This ACh response could be repeated in the presence of extracellular Ca2+ but was obtained only once in Ca(2+)-free bathing solution, which shows that a depleted intracellular Ca2+ store can be rapidly refilled from the extracellular space. Refilling can be prevented by the endoplasmic reticulum Ca(2+)-ATPase inhibitor cyclopiazonic acid (10 mumol/L), implying that Ca2+ enters the cytoplasm before accumulation in the endoplasmic reticulum. Ionomycin (10 mumol/L) caused a large Ca2+ release even after the ACh-releasable store had been emptied, indicating the existence of other ACh-insensitive stores, perhaps including the mitochondria. In one third of the cells studied, ACh induced oscillations in [Ca2+]i that were dependent on extracellular Ca2+. Also investigated were the effects of caffeine and ryanodine. In this cell preparation neither caffeine nor ryanodine induced a Ca2+ transient but instead slowly increased [Ca2+]i. It was observed that both caffeine and ryanodine were able to slowly deplete the ACh-sensitive store. These results indicate the presence of functional ryanodine receptors in native endothelial cells and demonstrate overlap between the caffeine and agonist-sensitive Ca2+ stores. We also found that caffeine was able to directly inhibit the process of ACh-induced Ca2+ release.(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of human factor VIIa to tissue factor induces cytosolic Ca2+ signals in J82 cells, transfected COS-1 cells, Madin-Darby canine kidney cells and in human endothelial cells induced to synthesize tissue factor

Tissue factor (TF) is the most potent trigger of blood clotting known. It activates factor VII (FVII) thereby initiating a cascade of proteolytic reactions resulting in thrombin production. The cloning of TF revealed its structural characteristics to be those of a receptor related to the class 2 cytokine receptor superfamily, but until now no intracellular signal has been discovered related to binding of the ligand (FVIIa) to the putative receptor. We have studied possible intracellular signaling effects of the FVIIa-TF interaction by measuring cytosolic free Ca2+ in single fura-2-loaded cells and found that 200 nM FVIIa caused Ca2+ transients in about 30% of human umbilical vein endothelial cells treated with interleukin-1 beta to express TF, compared to below 5% in uninduced cells. A gradual increase of the basal Ca2+ level was also caused by binding of FVIIa. In the human bladder carcinoma cell line J82, which has a high constitutive TF activity, similar results were found. An antibody neutralizing TF activity decreased the response rate to control levels. COS-1 cells which do not make TF did not respond to FVIIa as opposed to COS-1 cells expressing TF after transfection with a human TF cDNA construct. The canine kidney cell line MDCK, a constitutive TF producer, responded especially well; up to 100% of the cells examined showed Ca2+ oscillations which were dose dependent with regard to frequency, latency, maximal amplitude, and recruitment of responding cells. The frequency was reduced by inhibition of Ca2+ influx with 100 microM LaCl3. In confluent MDCK cells the Ca2+ oscillations were synchronous, constituting the first evidence of a synchronous cytosolic Ca2+ oscillator generated by global application of agonist. Thus, TF mediates a cytosolic Ca2+ signal upon interaction with its ligand FVIIa, thereby suggesting a more complex biological role for TF.

Prostaglandin-stimulated second messenger signaling in bone-derived endothelial cells is dependent on confluency in culture

New bone formation is associated with an increase in blood flow by the invasion of capillaries. Endothelial cells that line the capillaries can produce paracrine factors that affect bone growth and development, and in turn, could be affected by products produced by bone cells, in particular the osteoblasts. Since osteoblasts produce prostaglandins E2 and F2 alpha (PGE2, PGF2 alpha), it was investigated if these PGs were agonists to bone-derived endothelial cells (BBE) by assessing changes in cAMP and free cytosolic calcium concentration ([Ca2+]i) second messenger generation. We found that confluent cultures of BBE cells, a clonal endothelial cell line derived from bovine sternal bone, responded to 1 microM PGE2 by an increase in cAMP. PGF2 alpha at the same concentration was less potent in stimulating an increase in cAMP production in confluent BBE cells. Subconfluent cells with a morphology similar to that of fibroblastic cells were not as sensitive to PGE2-stimulated cAMP generation. PGF2 alpha failed to elicit any cAMP production in subconfluent cultures. PGE2 and PGF2 alpha both stimulated an increase in [Ca2+]i concentration in a dose-dependent manner. The potency of PGE2 was similar to that of PGF2 alpha in stimulating an increase in [Ca2+]i. The Ca2+ response was mostly independent of extracellular Ca+, was unchanged even with prior indomethacin treatment, was unaffected by caffeine pretreatment, but was abolished subsequent to thapsigargin pretreatment. The PG-induced increase in [Ca2+]i was also dependent on the confluency of the cells. In a subconfluent state, the responses to PGE2, or PGF2 alpha were either negligible, or only small increases in [Ca2+]i were noted with high concentrations of these two PGs. Consistent, dose-dependent increases in [Ca2+]i were stimulated by these PGs only when the cells were confluent and had a cobblestoned appearance. Since it was previously demonstrated that BBE cells respond to parathyroid hormone (PTH) by the production of cAMP, we tested if bovine PTH(1-34) amide ]bPTH(1-34) also increased [Ca2+]i in these cells. No change in [Ca2+]i was found in response to bPTH (1-34), although bPTH (1-34) stimulated a nine to tenfold increase in cAMP. We conclude that BBE cells respond to PGE2 and PGF2 alpha but not to bPTH(1-34) by an increase in [Ca2+]i probably secondary to stimulation of phospholipase C and that the cAMP and [Ca2+]i second messenger responses in BBE cells are dependent on the state of confluency of the cells.

Simultaneous oscillations in the membrane potential of pig coronary artery endothelial and smooth muscle cells

1. The effects of tetrabutylammonium (TBA) on the mechanical tension and on the electrical behaviour of endothelial and smooth muscle cells were studied in intact porcine coronary artery strips. 2. Superfusion of strips with TBA (2-20 mM) induced mechanical oscillations, leading to an increase in tonic isometric tension. 3. TBA-induced mechanical oscillations were correlated with fluctuations of the membrane potential of endothelial cells, which were identified by iontophoretic injection of Lucifer Yellow. 4. The endothelial cell membrane potential fluctuations appeared as action potentials or smaller amplitude slow waves, and were synchronized with electrical membrane potential fluctuations of the underlying coronary smooth muscle cells. 5. Oscillations induced by TBA in smooth muscle cells were not affected by removal of the endothelium, and depended on the presence of calcium in the external medium. 6. To our knowledge, this is the first description of action potential-like fluctuations in the endothelium. It is concluded that the oscillations were generated in the smooth muscle and that they propagate to the endothelium. The question of the mode of propagation of the signal is discussed.

Vectorial Ca2+ flux from the extracellular space to the endoplasmic reticulum via a restricted cytoplasmic compartment regulates inositol 1,4,5-trisphosphate-stimulated Ca2+ release from internal stores in vascular endothelial cells

Depletion of the Ins(1,4,5)P3-sensitive intracellular Ca2+ store of vascular endothelial cells after selective inhibition of the endoplasmic-reticulum (ER) Ca2+ pump by thapsigargin or 2,5-di-t-butylhydroquinone (BHQ) increases Ca2+ influx from the extracellular space in the absence of phosphoinositide hydrolysis. One model to account for these results suggests a close association between the internal store and the plasmalemma, allowing for the vectorial movement of Ca2+ from the extracellular space to the ER. Furthermore, recent evidence suggests that Ins(1,4,5)P3-induced Ca2+ release from intracellular stores is regulated by the free cytosolic Ca2+ concentration ([Ca2+]i). Thus agonist-induced Ca2+ entry may directly regulate Ca2+ release from internal stores. To test these hypotheses, we examined the effect of 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole (SKF 96365), an inhibitor of Ca2+ influx, on unidirectional 45Ca2+ efflux (i.e. retrograde radioisotope flux via the influx pathway) and on [Ca2+]i as measured by fura-2. Bradykinin produced a transient increase in [Ca2+]i, reflecting release of Ca2+ from internal stores, and a sustained increase indicative of Ca2+ influx. In the absence of agonist, 45Ca2+ efflux was slow and monoexponential with time. Addition of BK dramatically increased 45Ca2+ efflux; 50-60% of the 45Ca2+ associated with the cell monolayer was released within 2 min after addition of bradykinin. Both the bradykinin-induced change in [Ca2+]i and the stimulation of 45Ca2+ efflux was completely blocked by loading the cells with the Ca2+ chelator BAPTA. At a supermaximal concentration of bradykinin (50 nM), SKF 96365 (50 microM) inhibited the rise in [Ca2+]i attributed to influx without affecting release from internal stores. At a threshold concentration of bradykinin (2 nM), SKF 96365 blocked influx, but stimulated Ca2+ release from internal stores, as indicated by increases in both the transient component of the fura-2 response and 45Ca2+ efflux. Thapsigargin (200 nM) and BHQ (10 microM) produced an increase in 45Ca2+ efflux that was completely blocked by SKF 96365 or by cytosolic loading with BAPTA. These results suggest the existence of a restricted sub-plasmalemmal space that is defined by an area of surface membrane which contains the Ca(2+)-influx pathway but is devoid of Ca2+ pumps, and by a section of ER that is rich in thapsigargin-sensitive Ca(2+)-pump units.(ABSTRACT TRUNCATED AT 400 WORDS)

Spontaneous oscillations of cytoplasmic free calcium ion concentration in cultured smooth muscle cells from guinea pig ileum

The cytoplasmic free calcium ion concentration ([Ca2+]i) of cultured guinea pig ileum longitudinal muscle cells loaded with a fluorescent [Ca2+]i indicator, fura-2, was measured by digital ratio imaging microscopy. Spontaneous [Ca2+]i oscillations were observed in 25% to 80% of the cells, which differed with the batches of the cultured cells after 5 to 8 days in culture. The frequency and amplitude of the [Ca2+]i oscillations in each individual cell were usually regular, but heterogeneity between neighboring cells was observed. The spontaneous [Ca2+]i oscillations were also observed even after incubation of the cells under a serum-free condition for 72 hr. Exchange of extracellular solution to Ca(2+)-free solution containing EGTA or BAPTA immediately stopped the [Ca2+]i oscillations. The ratio of the oscillating cells was dependent on the extracellular calcium ion concentration ([Ca2+]o); and heterogeneity in the range of the [Ca2+]o to generate the [Ca2+]i oscillations was observed. An inorganic Ca(2+)-antagonist, LaCl3, immediately suppressed the [Ca2+]i oscillations, but the treatment with verapamil or nicardipine, Ca(2+)-channel blockers, did not have any effect on the [Ca2+]i oscillations. An inhibitor of the intracellular Ca2+ pump, thapsigargin, induced a transient increase in [Ca2+]i and then inhibited the spontaneous [Ca2+]i oscillations. Neomycin, a compound known to inhibit phosphoinositide turnover, inhibited the [Ca2+]i oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Inositol trisphosphate-mediated Ca2+ influx into Xenopus oocytes triggers Ca2+ liberation from intracellular stores

1. Inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger by liberating Ca2+ from intracellular stores and by promoting influx of extracellular Ca2+. We examined whether Ca2+ influx modulates intracellular Ca2+ liberation in Xenopus oocytes by fluorescence monitoring of cytosolic free Ca2+ together with voltage clamp recording of Ca(2+)-activated Cl- membrane currents. Sustained activation of membrane Ca2+ permeability was induced by intracellular injections of a non-metabolizable InsP3 analogue, 3-deoxy-3-fluoro-D-myo-inositol 1,4,5-trisphosphate (3-F-InsP3), and Ca2+ influx was controlled by applying step changes in membrane potential to alter the driving force for Ca2+ entry. 2. Negative-going potential steps evoked intracellular Ca2+ signals comprising two components; an initial transient peak followed by a slower rise. The initial transient grew steeply over a narrow (ca 40 mV) voltage range but then increased little with further polarization, whereas the second component showed a nearly linear voltage dependence. 3. The transient Ca2+ signal continued to rise almost unchanged when Ca2+ influx was interrupted by stepping the potential to more positive values after brief hyperpolarization. In contrast, Ca2+ levels declined monotonically when positive-going steps were applied after longer intervals during the second component of the Ca2+ signal. 4. Large Ca(2+)-dependent transient inward (T(in)) membrane currents were evoked during the rising phase of the initial Ca2+ signal, but little current was associated with the second component of the Ca2+ signal. 5. The T(in) currents evoked by hyperpolarization were mimicked at fixed clamp potential by re-admitting Ca2+ to the bathing solution, and by flash photolysis of caged Ca2+ loaded into the oocyte. 6. T(in) currents were strongly inhibited by prior release of Ca2+ from InsP3-sensitive intracellular stores, and vice versa. Experiments with paired hyperpolarizing pulses and paired photorelease of InsP3 showed that responses to both stimuli recovered with similar time courses. 7. We conclude that the transient Ca2+ signal and associated T(in) current evoked by hyperpolarization arise because Ca2+ entering the oocyte triggers regenerative release of Ca2+ from InsP3-sensitive intracellular stores. Since membrane currents evoked by liberated Ca2+ were much greater than those evoked by Ca2+ entry per se, a major function of InsP3-mediated Ca2+ entry may be to modulate the activity of intracellular Ca2+ stores.

Electrical properties of resting and acetylcholine-stimulated endothelium in intact rat aorta

1. The passive electrical properties and the effects of acetylcholine on the membrane potential of the endothelium of intact rat aorta were investigated using the whole cell mode of the patch clamp technique. 2. Unstimulated endothelium had a membrane potential of -58 +/- 8 mV (S.E.M., n = 193; range -47 to -76 mV). The input resistance was 43 +/- 13 M omega (S.E.M., n = 8; range 26-64 M omega). KCl and BaCl2, but not tetraethylammonium (2 mM), 4-aminopyridine (5 mM) or 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 100 microM) depolarized the endothelium. 3. Acetylcholine (0.2-4 microM) evoked in most preparations a biphasic response with a transient hyperpolarization to a value close to the K+ reversal potential, followed by depolarization beyond the resting potential. In 46% of recordings, the depolarization was followed by oscillations in membrane potential. The duration of the hyperpolarization and magnitude of the depolarization was similar in all recordings from a given aorta, but varied greatly between different preparations. 4. Hyperpolarization of the endothelium below the K+ reversal potential reversed the direction of the first phase of the acetylcholine-evoked response, which was unaffected by tetraethylammonium, 4-aminopyridine, or DIDS. 5. The removal of extracellular Ca2+ evoked a depolarization of the endothelium from -61 +/- 3 to -34 +/- 3 mV (S.E.M., n = 9) over 2-15 min. Restoration of external Ca2+ evoked a transient hyperpolarization. 6. ACh applied in nominally Ca(2+)-free medium shortly after Ca2+ removal evoked only a transient hyperpolarization. After the establishment of a stable membrane potential in Ca(2+)-free medium, acetylcholine was without effect. 7. NiCl2 (2 mM) evoked a small depolarization of the endothelium (6 +/- 2 mV; S.E.M., n = 7). The subsequent removal of Ni2+ evoked a transient hyperpolarization. 8. In the presence of Ni2+, acetylcholine evoked a short-lived hyperpolarization. Both the application of Ni2+ and the removal of extracellular Ca2+ immediately blocked oscillations in membrane potential evoked by acetylcholine. 9. The blockers of voltage-operated Ca2+ channels, nifedipine (1-10 microM) and verapamil (20 microM) were without effect on the biphasic acetylcholine-evoked responses. 10. In preparations in which acetylcholine evoked large (20-45 mV) oscillations in membrane potential, depolarization of the endothelium alone, by current injection or application of KCl, did not evoke oscillations. 11. The activator of protein kinase C, phorbol 12, 13-dibutyrate (200 nM) depolarized and greatly increased the input resistance of the endothelium, presumably due to an effect on gap junctions.(ABSTRACT TRUNCATED AT 400 WORDS)

Anti-ryanodine receptor antibody binding sites in vascular and endocardial endothelium

The ryanodine receptor (RyR) functions as the calcium release channel of the sarcoplasmic reticulum activated by electromechanical coupling in skeletal and cardiac muscles. In smooth muscle, inositol trisphosphate releases calcium from internal stores during pharmacomechanical coupling, but these cells also contain ryanodine-sensitive calcium stores. In this study, we establish the presence of anti-RyR antibody binding sites in vascular and endocardial endothelium. Both types of endothelia also contain messenger RNA, which hybridizes to a cardiac RyR isoform cDNA probe. Western blots of endothelial cell homogenates demonstrate the presence of a single, high molecular weight band of protein that corresponds to the cardiac RyR isoform. Confocal micrographs of endothelial cells labeled with a specific anti-RyR antibody reveal an intense fluorescent signal surrounding the nucleus and distributed in a nonhomogeneous pattern throughout the cytoplasm. This pattern of fluorescence is consistent with the electron microscopic distribution of the endoplasmic reticulum. The pattern of immunofluorescence seen with the anti-RyR antibody is distinctly different from that seen with the mitochondrial fluorophore rhodamine 123. Our findings suggest that the RyR plays a role in endothelial signaling.