Heterogeneity of caffeine- and bradykinin-sensitive Ca2+ stores in vascular endothelial cells

Calcium-Dependent Ion Channels and the Regulation of Arteriolar Myogenic Tone

Arterioles in the peripheral microcirculation regulate blood flow to and within tissues and organs, control capillary blood pressure and microvascular fluid exchange, govern peripheral vascular resistance, and contribute to the regulation of blood pressure. These important microvessels display pressure-dependent myogenic tone, the steady state level of contractile activity of vascular smooth muscle cells (VSMCs) that sets resting arteriolar internal diameter such that arterioles can both dilate and constrict to meet the blood flow and pressure needs of the tissues and organs that they perfuse. This perspective will focus on the Ca²⁺-dependent ion channels in the plasma and endoplasmic reticulum membranes of arteriolar VSMCs and endothelial cells (ECs) that regulate arteriolar tone. In VSMCs, Ca²⁺-dependent negative feedback regulation of myogenic tone is mediated by Ca²⁺-activated K⁺ (BK_Ca) channels and also Ca²⁺-dependent inactivation of voltage-gated Ca²⁺ channels (VGCC). Transient receptor potential subfamily M, member 4 channels (TRPM4); Ca²⁺-activated Cl⁻ channels (CaCCs; TMEM16A/ANO1), Ca²⁺-dependent inhibition of voltage-gated K⁺ (K_V) and ATP-sensitive K⁺ (K_ATP) channels; and Ca²⁺-induced-Ca²⁺ release through inositol 1,4,5-trisphosphate receptors (IP₃Rs) participate in Ca²⁺-dependent positive-feedback regulation of myogenic tone. Calcium release from VSMC ryanodine receptors (RyRs) provide negative-feedback through Ca²⁺-spark-mediated control of BK_Ca channel activity, or positive-feedback regulation in cooperation with IP₃Rs or CaCCs. In some arterioles, VSMC RyRs are silent. In ECs, transient receptor potential vanilloid subfamily, member 4 (TRPV4) channels produce Ca²⁺ sparklets that activate IP₃Rs and intermediate and small conductance Ca²⁺ activated K⁺ (IK_Ca and sK_Ca) channels causing membrane hyperpolarization that is conducted to overlying VSMCs producing endothelium-dependent hyperpolarization and vasodilation. Endothelial IP₃Rs produce Ca²⁺ pulsars, Ca²⁺ wavelets, Ca²⁺ waves and increased global Ca²⁺ levels activating EC sK_Ca and IK_Ca channels and causing Ca²⁺-dependent production of endothelial vasodilator autacoids such as NO, prostaglandin I₂ and epoxides of arachidonic acid that mediate negative-feedback regulation of myogenic tone. Thus, Ca²⁺-dependent ion channels importantly contribute to many aspects of the regulation of myogenic tone in arterioles in the microcirculation.

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.

Towards the maturation and characterization of smooth muscle cells derived from human embryonic stem cells

In this study we demonstrate that CD34(+) cells derived from human embryonic stem cells (hESCs) have higher smooth muscle cell (SMC) potential than CD34(-) cells. We report that from all inductive signals tested, retinoic acid (RA) and platelet derived growth factor (PDGF(BB)) are the most effective agents in guiding the differentiation of CD34(+) cells into smooth muscle progenitor cells (SMPCs) characterized by the expression of SMC genes and proteins, secretion of SMC-related cytokines, contraction in response to depolarization agents and vasoactive peptides and expression of SMC-related genes in a 3D environment. These cells are also characterized by a low organization of the contractile proteins and the contractility response is mediated by Ca(2+), which involves the activation of Rho A/Rho kinase- and Ca(2+)/calmodulin (CaM)/myosin light chain kinase (MLCK)-dependent pathways. We further show that SMPCs obtained from the differentiation of CD34(+) cells with RA, but not with PDGF(BB,) can be maturated in medium supplemented with endothelin-1 showing at the end individualized contractile filaments. Overall the hESC-derived SMCs presented in this work might be an unlimited source of SMCs for tissue engineering and regenerative medicine.

Exercise training decreases store-operated Ca2+entry associated with metabolic syndrome and coronary atherosclerosis

AIMS

Stenting attenuates restenosis, but accelerated coronary artery disease (CAD) adjacent to the stent (peri-stent CAD) remains a concern in metabolic syndrome (MetS). Smooth muscle cell proliferation, a major mechanism of CAD, is mediated partly by myoplasmic Ca2+ dysregulation and store-operated Ca2+ entry (SOCE) via canonical transient receptor potential 1 (TRPC1) channels is proposed to play a key role. Exercise is known to prevent Ca2+ dysregulation in CAD. We tested the hypothesis that MetS increases SOCE and peri-stent CAD and exercise attenuates these events.

METHODS AND RESULTS

Groups (n = 9 pigs each) were (i) healthy lean Ossabaw swine fed standard chow, (ii) excess calorie atherogenic diet fed (MetS), and (iii) aerobically exercise trained starting after 50 weeks of development of MetS (XMetS). Bare metal stents were placed after 54 weeks on diets, and CAD and SOCE were assessed 4 weeks later. Coronary cells were dispersed proximal to the stent (peri-stent) and from non-stent segments, and fura-2 fluorescence was used to assess SOCE, which was verified by Ni2+ blockade and insensitivity to nifedipine. XMetS pigs had increased physical work capacity and decreased LDL/HDL (P < 0.05), but no attenuation of robust insulin resistance, glucose intolerance, hypertriglyceridaemia, or hypertension. CAD was greater in peri-stented vs. non-stented artery segments. MetS had the greatest CAD, SOCE, and TRPC1 and STIM1 mRNA and protein expression, which were all attenuated in XMetS.

CONCLUSION

This is the first report of the protective effect of exercise on native CAD, peri-stent CAD, SOCE, and molecular expression of TRPC1, STIM1, and Orai1 in MetS.

Ca2+ refilling of the endoplasmic reticulum is largely preserved albeit reduced Ca2+ entry in endothelial cells

In this study the relationship between the efficiency of endoplasmic reticulum (ER) Ca2+ refilling and the extent of Ca2+ entry was investigated in endothelial cells. ER and mitochondrial Ca2+ concentration were measured using genetically encoded Ca2+ sensors, while the amount of entering Ca2+ was controlled by varying either the extracellular Ca2+ or the electrical driving force for Ca2+ by changing the plasma membrane potential. In the absence of an agonist, ER Ca2+ replenishment was fully accomplished even if the Ca2+ concentration applied was reduced from 2 to 0.5mM. A similar strong efficiency of ER Ca2+ refilling was obtained under condition of plasma membrane depolarization. However, in the presence of histamine, ER Ca2+ refilling depended on mitochondrial Ca2+ transport and was more susceptible to membrane depolarization. Store-operated Ca2+ entry (SOCE), was strongly reduced under low Ca2+ and depolarizing conditions but increased if ER Ca2+ uptake was blocked or if ER Ca2+ was released continuously by IP(3). A correlation of the kinetics of ER Ca2+refilling with cytosolic Ca2+ signals revealed that termination of SOCE is a rapid event that is not delayed compared to ER refilling. Our data indicate that ER refilling occurs in priority to, and independently from the cytosolic Ca2+ elevation upon Ca2+ entry and that this important process is widely achieved even under conditions of diminished Ca2+entry.

Impaired endothelium-derived hyperpolarizing factor-mediated relaxation in porcine pulmonary microarteries after cold storage with Euro-Collins and University of Wisconsin solutions

BACKGROUND

Endothelium plays an important role in mediating the function of transplanted organs. The widely used University of Wisconsin solution impairs the endothelium-derived hyperpolarizing factor-mediated relaxation in coronary arteries, but little is known about effects of lung preservation on endothelium-derived hyperpolarizing factor-mediated endothelial function. This study examined the effect of organ preservation solutions on the endothelium-derived hyperpolarizing factor-mediated relaxation in the pulmonary microarteries (diameter 200 to 450 microm).

METHODS

Two segments (1 as control) from the same microartery were allocated in 2 chambers of a myograph. After incubation with hyperkalemia (potassium 115 mmol/L), University of Wisconsin, or Euro-Collins solution (at 4 degrees C for 4 hours), the endothelium-derived hyperpolarizing factor-mediated relaxation was induced by bradykinin (-10 to -6.5 log M, n = 8) or calcium ionophore (A(23187), -9 to -5.5 log M, n = 7) in U(46619) (-7.5 log M) precontracted rings in the presence of indomethacin (7 micromol/L), N(G)-nitro-L-arginine (300 micromol/L), and oxyhemoglobin (20 micromol/L).

RESULTS

Exposure to hyperkalemia and storage with Euro-Collins or University of Wisconsin solution significantly decreased the relaxation to bradykinin (51.9 +/- 8.4% vs 60.3 +/- 6.1%, P =.02 or 49.3 +/- 7.3% vs 65.2 +/- 3.5%, P =.04) or A(23187) (12.5 +/- 0.02% vs 33.8 +/- 0.07%, P =.02 or 13.2 +/- 0.03% vs 31.0 +/- 0.05%, P =.03%).

CONCLUSIONS

Endothelium-derived hyperpolarizing factor plays an important role in porcine pulmonary microarteries, and the endothelium-derived hyperpolarizing factor-mediated relaxation is impaired when the lung is preserved with University of Wisconsin or Euro-Collins solution. This impairment may affect the lung function during the reperfusion period after lung transplantation.

SERCA pump isoform expression in endothelium of veins and arteries: every endothelium is not the same

Endothelium from rat aorta expresses sarco/endoplasmic reticulum Ca2+(SERCA) pump gene SERCA3 where as the smooth muscle expresses SERCA2. This has led to the postulate that vascular endothelium expresses SERCA3. To test this postulate, we examined the SERCA2 and SERCA3 mRNA expression in endothelium and smooth muscle dissected from coronary artery, coronary vein, aorta and vena cava of pig. Smooth muscle from all arteries and veins expressed only the SERCA2 mRNA. Endothelium from coronary artery, coronary vein and aorta expressed both SERCA2 and SERCA3 mRNA but the endothelium from vena cava did not express SERCA3 mRNA although it expressed SERCA2. These observations support the postulate that vascular endothelium expresses SERCA3 but the affirmation is equivocal because vena cava endothelium does not express SERCA3.

Anandamide-induced mobilization of cytosolic Ca2+ in endothelial cells

1. Experiments were designed to determine whether anandamide affects cytosolic Ca2+ concentrations in endothelial cells and, if so, whether CB1 cannabinoid receptors are involved. To this effect, human umbilical vein-derived EA.hy926 endothelial cells were loaded with fura-2 to monitor changes in cytosolic Ca2+ using conventional fluorescence spectrometry methods. 2. Anandamide induced an increase in Ca2+ in endothelial cells which, in contrast to histamine, developed slowly and was transient. Anandamide caused a concentration-dependent release of Ca2+ from intracellular stores without triggering capacitative Ca2+ entry, contrary to histamine or the endoplasmic reticulum Ca2+ -ATPase inhibitor thapsigargin. 3. Anandamide pretreatment slightly reduced the mobilization of Ca2+ from intracellular stores that was evoked by histamine. The mobilization of Ca2+ from intracellular stores evoked by anandamide was impaired by 10 mM caffeine. 4. Anandamide and histamine each significantly increased NO synthase activity in EA.hy926 cells, as determined by the enhanced conversion of L-[3H]-arginine to L-[3H]-citruline. 5. The CB1 cannabinoid receptor antagonist SR141716A (1 microM) only produced a marginal reduction of the mobilization of Ca2+ produced by 5 microM anandamide. However, at 5 microM SR141716A elicited the release of Ca2+ from intracellular stores. This concentration strongly impaired the mobilization of cytosolic Ca2+ evoked by either anandamide, histamine or thapsigargin. 6. Pretreatment of the cells with either 200 microM phenylmethylsulphonyl fluoride (to inhibit the conversion of anandamide into arachidonic acid) or 400 ng ml(-1) pertussis toxin (to uncouple CB1 cannabinoid receptors from Gi/o proteins) had no significant effect on the mobilization of cytosolic Ca2+ evoked by either anandamide, or histamine. 7. Taken together the results demonstrate that anandamide mobilizes Ca2+ from a caffeine-sensitive intracellular Ca2+ store that functionally overlaps in part with the internal stores mobilized by histamine. However, a classical CB1 cannabinoid receptor-mediated and pertussis toxin-sensitive mechanism does not mediate this novel effect of anandamide in endothelial cells. 8. The mobilization of cytosolic Ca2+ in endothelial cells may account for the endothelium-dependent and NO-mediated vasodilator actions of anandamide. Due to its non-specific inhibition of Ca2+ signalling in endothelial cells, SR141716A may not be used to assess the physiological involvement of endogenous cannabinoids to endothelium-dependent control of vascular smooth muscle tone.

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.

Effects of superoxide anions on endothelial Ca2+ signaling pathways

Although the involvement of free radicals in the development of endothelial dysfunction under pathological conditions, like diabetes and hypercholesterolemia, has been proposed frequently, there is limited knowledge as to how superoxide anions (O2-) might affect endothelial signal transduction. In this study, we investigated the effects of preincubation with the O2(-)-generating system xanthine oxidase/hypoxanthine (XO/HX) on mechanisms for Ca2+ signaling in cultured porcine aortic endothelial cells. Incubation of cells with XO/HX yielded increased intracellular Ca2+ release and capacitative Ca2+ entry in response to bradykinin and ATP in a time- and concentration-dependent manner. This effect was prevented by superoxide dismutase but not by the tyrosine kinase inhibitor tyrphostin A48. In addition, capacitative Ca2+ entry induced by the receptor-independent stimulus 2,5-di-(tert-butyl)-1,4-benzohydroquinone or thapsigargin was enhanced in O2(-)-exposed cells (+38% and +32%, respectively). Increased Ca2+ release in response to bradykinin in XO/HX-pretreated cells might be due to enhanced formation of inositol-1,4,5-trisphosphate (+140%). Exposure to XO/HX also affected other signal transduction mechanisms involved in endothelial Ca2+ signaling, such as microsomal cytochrome P450 epoxygenase and membrane hyperpolarization to Ca2+ store depletion with thapsigargin (+103% and +48%, respectively) and tyrosine kinase activity (+97%). A comparison of bradykinin-initiated intracellular Ca2+ release and thapsigargin-induced hyperpolarization with membrane viscosity modulated by XO/HX (decrease in viscosity) or cholesterol (increase in viscosity) reflected a negative correlation between bradykinin-initiated Ca2+ release and membrane viscosity. Because intracellular Ca2+ is a main regulator of endothelial vascular function, our data suggest that O2- anions are involved in regulation of the vascular endothelium.

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

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

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.

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.

Peroxide resistance of ER Ca2+ pump in endothelium: implications to coronary artery function

We examined the effects of peroxide on the sarco(endo)plasmic reticulum Ca2+ (SERCA) pump in pig coronary artery endothelium and smooth muscle at three organizational levels: Ca2+ transport in permeabilized cells, cytosolic Ca2+ concentration in intact cells, and contractile function of artery rings. We monitored the ATP-dependent, azide-insensitive, oxalate-stimulated 45Ca2+ uptake by saponin-permeabilized cultured cells. Low concentrations of peroxide inhibited the uptake less effectively in endothelium than in smooth muscle whether we added the peroxide directly to the Ca2+ uptake solution or treated intact cells with peroxide and washed them before the permeabilization. An acylphosphate formation assay confirmed the greater resistance of the SERCA pump in endothelial cells than in smooth muscle cells. Pretreating smooth muscle cells with 300 microM peroxide inhibited (by 77 +/- 2%) the cyclopiazonic acid (CPA)-induced increase in cytosolic Ca2+ concentration in a Ca2+-free solution, but it did not affect the endothelial cells. Peroxide pretreatment inhibited the CPA-induced contraction in deendothelialized arteries with a 50% inhibitory concentration of 97 +/- 13 microM, but up to 500 microM peroxide did not affect the endothelium-dependent, CPA-induced relaxation. Similarly, 500 microM peroxide inhibited the angiotensin-induced contractions in deendothelialized arteries by 93 +/- 2%, but it inhibited the bradykinin-induced, endothelium-dependent relaxation by only 40 +/- 13%. The greater resistance of the endothelium to reactive oxygen may be important during ischemia-reperfusion or in the postinfection immune response.

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..

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.

Evidence for the existence of inositol (1,4,5)-trisphosphate- and ryanodine-sensitive pools in bovine endothelial cells. Ca2+ releases in cells with different basal level of intracellular Ca2+

In single bovine aortic endothelial (BAE) cells pre-loaded with Fura-2, Ca2+ transients in a Ca2+-free medium have been revealed, which evidently reflects Ca2+ release from intracellular stores. In cells with different levels of resting basal cytoplasmic Ca2+ ([Ca2+]i) from about 50 to 110 nM, a biphasic dependence of the Ca2+ transients on resting [Ca2+]i was shown and spontaneous Ca2+ oscillations were observed. At a [Ca2+]i level over 110 nM, a pronounced rise in Ca2+ transients occurred and only single transients were observed. Ryanodine (10 microM) produced a transient [Ca2+]i elevation, suggesting the presence of ryanodine receptors in intracellular store membranes. The results imply that both inositol 1,4,5-trisphosphate-sensitive Ca2+ release (IICR) and Ca2+-sensitive Ca2+ release (CICR) take place in BAE cells. Only IICR seems to be sufficient for generating baseline Ca2+ oscillations in BAE cells, whereas the ATP-induced (5-100 microM) Ca2+ response involves the CICR set in motion by an oscillatory IICR of high frequency. The completion of both the spontaneous and ATP-induced Ca2+ transients was associated with a [Ca2+]i decrease to a level below the initial resting [Ca2+]i (undershoot). Its depth biphasically depended on the resting [Ca2+]i from 50 to 110 nM, suggesting that the lack of a Ca2+ leak from inositol 1,4,5-trisphosphate-sensitive stores is responsible for the undershoot in this range. The Ca2+ leak is concluded to play a key role in the initiation and termination of regenerative IICR both in spontaneous oscillations and in ATP-induced transients.

Ca2+ signaling in endothelial cells stimulated by bradykinin: Ca2+ measurement in the mitochondria and the cytosol by confocal microscopy

In this study we have monitored the change of intracellular Ca2+ concentrations in the cytosol ([Ca2+]c) and the mitochondria ([Ca2+]m) of single bovine endothelial cells following treatment with bradykinin (BK). Using laser scanning confocal microscopy, we have found that the Ca2+ indicator, Fluo-3, is compartmentalized in the mitochondria of endothelial cells loaded with Fluo-3/AM. After BK stimulation, the pattern of Ca2+ increase in the cytosol is different from that in the mitochondria. The amplitude of the Ca2+ rise in the mitochondria is higher than that in the cytosol. Further analysis using rapid scanning measurements indicates that the [Ca2+]c increase is very fast after BK addition and reaches a maxima level within 400 ms. In contrast, the [Ca2+]m increase appears to be biphasic with an initial rapid increase (concomitant with the [Ca2+]c increase) followed by a slower [Ca2+]m increase before reaching a maximal level (within 5 s of BK treatment). The differential Ca2+ signaling pattern between the cytosol and the mitochondria suggests that the intracellular Ca2+ concentrations needed to regulate various Ca(2+)-dependent enzymes located in these two compartments are different during BK-induced endothelial cell activation.

Ca(2+)-dependent non-selective cation and potassium channels activated by bradykinin in pig coronary artery endothelial cells

1. Using the cell-attached and inside-out modes of the patch-clamp technique, we studied the Ca(2+)-dependent ionic channels activated by bradykinin in cultured pig coronary artery endothelial cells to further understand electrophysiological events underlying cellular activation. 2. In the cell-attached mode, bradykinin (94 nM) activated two types of Ca(2+)-dependent channels: a high conductance K+ channel (285 pS in high symmetrical K+), whose open state probability was increased by depolarization, and a lower conductance inwardly rectifying non-selective cation channel (44 pS in high symmetrical K+). 3. The 285 pS K+ channel was half-maximally activated by cytosolic Ca2+ levels of 1.6 and 4.5 microM at +10 and -30 mV, respectively. Such local concentrations should be reached in the presence of bradykinin, which induces a mean maximal cytosolic Ca2+ rise of 1.3 microM. 4. The 285 pS K+ channel was inhibited by d-tubocurarine, which acted by reducing the mean open time duration (flickering pattern), finally reducing the channel conductance. 5. Divalent cations such as Ca2+ could flow through the 44 pS non-selective cation channel, with nearly the same permeability (P) as monovalent cations (PK: PNa: PCa = 1:1:0.7). 6. The cation channel appeared to be more sensitive to Ca2+ than the K+ channel, with a half-maximal open probability induced by 0.7 microM Ca2+ on the intracellular side of the membrane. 7. In contrast to the K+ channel, the cation channel mean open time was clearly increased by bradykinin. This effect was delayed compared with the increase in the channel open state probability and was rapidly lost in the inside-out configuration. Caffeine also activated the cation channel but more transiently than bradykinin and without any effect on the open duration. 8. In the absence of extracellular Ca2+, the bradykinin-induced increase in cytosolic free Ca2+ was shortened temporally by 52% and reduced in amplitude by 88%, whereas the bradykinin-induced hyperpolarization was not significantly reduced in amplitude but was shortened by 70%, thus illustrating the major role of Ca2+ influx in endothelial cell activation by bradykinin. 9. We conclude that bradykinin activates two types of Ca(2+)-dependent channels in coronary endothelial cells: a high conductance K+ channel regulated by membrane potential, and an inwardly rectifying cation channel allowing Ca2+ entry, the cation channel being about 6 times more sensitive to Ca2+ than the K+ channel. The increase in cation channel open state probability involves an increase in open number, like the K+ channel, but also involves a rise in channel open duration. Ca2+ entry via cation channels could contribute to increase the cytoplasmic Ca2+ level, activate Ca(2+)-dependent K+ channels, thus triggering membrane hyperpolarization when the endothelial cell is stimulated by a vasoactive agonist such as bradykinin.

Depletion of ryanodine-sensitive Ca2+ store activates Ca2+ entry in rat submandibular gland acinar cells

The existence of ryanodine-sensitive Ca2+ stores and their role in the Ca2+ entry mechanism were examined in the rat submandibular gland acinar cells, using the microfluorimetry of intracellular Ca2+ concentration ([Ca2+]i). In the presence of thapsigargin, a Ca(2+)-ATPase inhibitor of inositol (1, 4, 5) triphosphate (InsP3)-sensitive Ca2+ stores, caffeine caused an increase in [Ca2+]i, which was inhibited by treatment with ryanodine (a ligand to the Ca(2+)-induced Ca2+ release channels). In the cells treated with ryanodine, 1 mM Ca2+ addition to a Ca(2+)-free solution caused a marked increase in [Ca2+]i, which was eliminated by application of Ni2+ or SK & F 96365, suggesting a Ca2+ entry triggered by ryanodine. The maximal change in the net increase in [Ca2+]i caused by the ryanodine-coupled Ca2+ entry, was 104.0 +/- 16.0 nM, which intense was caused by 10 microM ryanodine. Emptying the InsP3-sensitive stores by treatment with thapsigargin also caused Ca2+ entry, which maximally changed [Ca2+]i by 349.6 +/- 15.1 nM. Ten mumol/liter ryanodine was confirmed to cause a release of 45Ca2+ from the parotidic microsomal fraction enriched in endopalsmic reticulum. We propose that ryanodine-sensitive Ca2+ stores are present in rat submandibular gland acinar cells. We further propose that release of Ca2+ from the ryanodine-sensitive stores, which means eventually depletion of the ryanodine-sensitive Ca2+ stores, can activate the Ca2+ entry. The ability for Ca2+ entry coupled with the ryanodine-sensitive Ca2+ stores seems to be about 30% of the ability for Ca2+ entry coupled with the thapsigargin-sensitive Ca2+ stores.

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)

Caffeine-evoked, calcium-sensitive membrane currents in rabbit aortic endothelial cells

1. Single cell photometry and whole-cell patch clamp recording were used to study caffeine-induced intracellular Ca2+ signals and membrane currents, respectively, in endothelial cells freshly dissociated from rabbit aorta. 2. Caffeine (5 mM) evoked a transient increase in [Ca2+]i in fura-2-loaded endothelial cells. Pretreatment of cells with 10 microM ryanodine did not alter resting [Ca2+]i but irreversibly inhibited the caffeine-induced rise in [Ca2+]i. The caffeine-induced increase in [Ca2+]i was not attenuated by the removal of extracellular Ca2+ and did not stimulate the rate of Mn2+ quench of fura-2 fluorescence. 3. Bath application of caffeine evoked a dose- and voltage-dependent outward current. The rate of onset and amplitude of the caffeine-evoked outward current increased with higher caffeine concentrations and membrane depolarization. The relationship between caffeine-evoked current amplitude and membrane potential was non linear, suggesting that the channels underlying the current are voltage-sensitive. 4. In the absence of extracellular Ca2+, the amplitude of the caffeine-evoked outward current was reduced by approximately 50% but the duration of the current was prolonged compared to that observed in the presence of external Ca2+. Ca(2+)-free external solutions produced an unexpected increase in both the frequency and amplitude of spontaneous transient outward currents (STOCs). 5. Inclusion of heparin (10 micrograms ml-1) in the patch pipette abolished the acetylcholine (ACh)-induced outward current but failed to inhibit either STOCs or the caffeine-evoked outward current in native endothelial cells. In the absence of extracellular Ca2+, heparin did not affect either STOCs or the caffeine-induced outward current. 6. Externally applied tetraethylammonium ions (TEA, 3-10mM) reversibly inhibited unitary Ca2+-activated K+ currents and STOCs in endothelial cells but failed to inhibit completely the outward current evoked by 20 mM caffeine.7. Bath application of 0.1 mM zinc ion (Zn2+), a chloride channel blocker, did not affect unitary currents or STOCs but reduced the amplitude of the caffeine-evoked current by >75% compared to control. Replacement of extracellular NaCl with Na gluconate also reduced the amplitude of the caffeine-induced outward current. Bath application of 0.1 mM Zn2+ and 10 mM TEA completely blocked the caffeine-evoked outward current in endothelial cells.8. Caffeine-induced Ca2+ release from intracellular stores evokes a transient rise in [Ca2+1, which is correlated with a large, transient outward current. The ionic dependence and inhibition of the caffeine sensitive current by TEA and Zn2+ suggests that Ca2+-activated K+ and Cl- conductances contribute to the caffeine response in rabbit aortic endothelial cells.

Cytochrome P450 mono-oxygenase-regulated signalling of Ca2+ entry in human and bovine endothelial cells

1. We tested the hypothesis that agonist-stimulated Ca2+ entry, and thus formation of endothelium-derived nitric oxide (EDNO) in vascular endothelial cells, is related to activation of microsomal P450 mono-oxygenase (P450 MO) and the biosynthesis of 5,6-epoxyeicosatrienoic acid (5,6-EET). 2. Several P450 inhibitors diminished the sustained [Ca2+]i plateau response to agonist or intracellular Ca2+ store depletion with ATPase inhibitors by 31-69% (fura-2 technique). Mn2+ influx stimulated by agonists or ATPase inhibitors was prevented by P450 inhibitors. 3. Histamine- or ATPase inhibitor-stimulated formation of EDNO was strongly attenuated (50-83%) by P450 inhibitors, without any effect on EDNO formation by the Ca2+ ionophore A23187, indicating that decreased EDNO synthesis is due specifically to the inhibition of Ca2+ entry by these compounds. 4. Induction of P450 MO by beta-naphthoflavone potentiated agonist-induced Ca2+ and Mn2+ influx by 60 and 53%, respectively. Intracellular Ca2+ release remained unchanged. 5. The P450 MO product, 5,6-EET (< 156 nmol l-1), activated Ca2+/Mn2+ entry without any depletion of intracellular Ca2+ stores. The 5,6-EET-stimulated Ca2+/Mn2+ entry was not affected by P450 inhibitors. 6. As with the bradykinin-stimulated Ca2+ entry pathway, the 5,6-EET-activated Ca2+ entry pathway was permeable to Mn2+ and Ba2+, sensitive to Ni2+, La3+ and membrane depolarization, and insensitive to the removal of extracellular Na+ or the organic Ca2+ antagonist, nitrendipine. 7. In the presence of 5,6-EET, stimulation with bradykinin only transiently increased [Ca2+]i. Vice versa, 5,6-EET failed to increase [Ca2+]i further in bradykinin-stimulated cells. The sustained [Ca2+]i plateau phase induced by a co-stimulation with bradykinin and 5,6-EET was identical to that observed with bradykinin or 5,6-EET alone. 8. These results demonstrate that Ca2+ entry induced by the P450 MO product, 5,6-EET, is indistinguishable to that observed by stimulation with bradykinin. 9. All data support our hypothesis that depletion of endothelial Ca2+ stores activates microsomal P450 MO which in turn synthesizes 5,6-EET. We propose that the arachidonic acid metabolite 5,6-EET or one of its metabolites is a second messenger for activation of endothelial Ca2+ entry.

Histamine-induced Ca2+ entry precedes Ca2+ mobilization in bovine adrenal chromaffin cells

The relationship between histamine-induced Ca2+ mobilization and Ca2+ entry in bovine adrenal chromaffin cells has been investigated. Stopped-flow fluorimetry of fura-2-loaded chromaffin cell populations revealed that 10 microM histamine promoted entry of Ca2+ or Mn2+ without measurable delay (< or = 20 ms), through a pathway that was insensitive to the dihydropyridine antagonist nifedipine. In the absence of extracellular Ca2+, or in the presence of 100 microM La3+, a blocker of receptor-mediated Ca2+ entry, 10 microM histamine triggered an elevation in intracellular calcium concentration ([Ca2+]i), but only after a delay of approx. 200 ms, which presumably represented the time required to mobilize intracellular Ca2+. These data suggested that histamine-induced bivalent-cation entry precedes extensive Ca2+ mobilization in chromaffin cells. In order to confirm that histamine can promote Ca2+ entry largely independently of mobilizing intracellular Ca2+, the ability of histamine to promote Ca2+ entry into cells whose intracellular Ca2+ store had been largely depleted was assessed. Fura-2-loaded chromaffin cells were treated with 10 microM ryanodine together with 40 mM caffeine, to deplete the hormone-sensitive Ca2+ store. This resulted in an approx. 95% inhibition of histamine-induced Ca2+ release. Under these conditions, histamine was still able to promote an entry of Ca2+ that was essentially indistinguishable from that promoted in control cells. In single cells, introduction of heparin (100 mg/ml), but not de-N-sulphated heparin (100 mg/ml), abolished the histamine-induced rise in [Ca2+]i. All these data suggest that histamine can induce G-protein- or inositol phosphate-dependent rapid (< or = 20 ms) Ca2+ entry without an extensive intracellular mobilization response in chromaffin cells, which points to activation of an entry mechanism distinct from the Ca(2+)-release-activated Ca2+ channel found in non-excitable cells.