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

Temperature effects on morphological integrity and Ca²⁺ signaling in freshly isolated murine feed artery endothelial cell tubes

To study Ca(2+) signaling in the endothelium of murine feed arteries, we determined the in vitro stability of endothelial cell (EC) tubes freshly isolated from abdominal muscle feed arteries of male and female C57BL/6 mice (5-9 mo, 25-35 g). We tested the hypothesis that intracellular Ca(2+) concentration ([Ca(2+)](i)) responses to muscarinic receptor activation would increase with temperature. Intact EC tubes (length: 1-2 mm, width: 65-80 μm) were isolated using gentle enzymatic digestion with trituration to remove smooth muscle cells. A freshly isolated EC tube was secured in a chamber and superfused at 24 (room temperature), 32, or 37°C. Using fura-2 dye, [Ca(2+)](i) was monitored (ratio of fluorescence at 340- to 380-nm wavelength) at rest and in response to bolus doses of ACh (20 nmol to 200 μmol). The morphological integrity of EC tubes was preserved at 24 and 32°C. Based on the Ca(2+) K(d) values we determined for fura-2 (174 nM at 24°C and 146 nM at 32°C), resting [Ca(2+)](i) remained stable for 180 min at both 24 and 32°C (27 ± 4 and 34 ± 2 nM, respectively), with peak responses to ACh (20 μmol) increasing from ∼220 nM at 24°C to ∼500 nM at 32°C (P < 0.05). There was no difference in responses to ACh between EC tubes from male versus female mice. When EC tubes were maintained at 37°C (typical in vivo temperature), resting [Ca(2+)](i) increased by ∼30% within 15 min, and gaps formed between individual ECs as they retracted and extruded dye, precluding further study. We conclude that EC tubes enable Ca(2+) signaling to be evaluated in the freshly isolated endothelium of murine feed arteries. While Ca(2+) responses are enhanced by approximately twofold at 32 versus 24°C, the instability of EC tubes at 37°C precludes their study at typical body temperature.

Temperature-dependent STIM1 activation induces Ca²+ influx and modulates gene expression

Intracellular Ca²⁺ is essential for diverse cellular functions. Ca²⁺ entry into many cell types including immune cells is triggered by depleting endoplasmic reticulum (ER) Ca²⁺, a process termed store-operated Ca²⁺ entry (SOCE). STIM1 is an ER Ca²⁺ sensor. Upon Ca²⁺ store depletion, STIM1 clusters at ER-plasma membrane junctions where it interacts with and gates Ca²⁺-permeable Orai1 ion channels. Here we show that STIM1 is also activated by temperature. Heating cells caused clustering of STIM1 at temperatures above 35°C without depleting Ca²⁺ stores, and led to STIM1/Orai1-mediated Ca²⁺ influx as a heat off-response (response after cooling). Interestingly, the functional coupling of STIM1 and Orai1 is prevented at high temperatures, potentially explaining the heat off-response. Importantly, physiologically-relevant temperature shifts modulates STIM1-dependent gene expression in Jurkat T-cells. Therefore, temperature is an important regulator of STIM1 function.

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

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

Role of STIM1 in regulation of store-operated Ca2+ influx in pheochromocytoma cells

Changes in the local environment such as pH (acidosis/alkalosis), temperature (hypothermia/hyperthermia), and agonist (glutamate) can adversely affect neuronal function, and are important factors in clinical situations such as anesthesia and intensive care. Regulation of intracellular Ca2+ ([Ca2+](i)) is key to neuronal function. Stromal interaction molecule (STIM1) has been recently recognized to trigger store-operated Ca2+ entry (SOCE), an important component of [Ca2+](i) regulation. Using differentiated, fura-2 loaded rat pheochromocytoma (PC12) cells transfected with small interference RNA for STIM1 (or vehicle), we examined the role of STIM1 in SOCE sensitivity to temperature, pH, and glutamate. SOCE was triggered following endoplasmic reticulum depletion. Cells were washed and exposed to altered pH (6.0-8.0), altered temperature (34-40 degrees C), or to glutamate. In non-transfected cells, SOCE was inhibited by acidosis or hypothermia, but increased with alkalosis and hyperthermia. Increasing glutamate concentrations progressively stimulated SOCE. STIM1 siRNA decreased SOCE at normal temperature and pH, and substantially decreased sensitivity to acidosis and hypothermia, eliminating the concentration-dependence to glutamate. Sensitivity of SOCE to these environmental parameters was less altered by decreased extracellular Ca2+ alone (with STIM1 intact). We conclude that STIM1 mediates exquisite susceptibility of SOCE to pH, temperature, and glutamate: factors that can adversely affect neuronal function under pathological conditions.

Stimulus-specific modulation of the cation channel TRPV4 by PACSIN 3

TRPV4, a member of the vanilloid subfamily of the transient receptor potential (TRP) channels, is activated by a variety of stimuli, including cell swelling, moderate heat, and chemical compounds such as synthetic 4alpha-phorbol esters. TRPV4 displays a widespread expression in various cells and tissues and has been implicated in diverse physiological processes, including osmotic homeostasis, thermo- and mechanosensation, vasorelaxation, tuning of neuronal excitability, and bladder voiding. The mechanisms that regulate TRPV4 in these different physiological settings are currently poorly understood. We have recently shown that the relative amount of TRPV4 in the plasma membrane is enhanced by interaction with the SH3 domain of PACSIN 3, a member of the PACSIN family of proteins involved in synaptic vesicular membrane trafficking and endocytosis. Here we demonstrate that PACSIN 3 strongly inhibits the basal activity of TRPV4 and its activation by cell swelling and heat, while leaving channel gating induced by the synthetic ligand 4alpha-phorbol 12,13-didecanoate unaffected. A single proline mutation in the SH3 domain of PACSIN 3 abolishes its inhibitory effect on TRPV4, indicating that PACSIN 3 must bind to the channel to modulate its function. In line herewith, mutations at specific proline residues in the N terminus of TRPV4 abolish binding of PACSIN 3 and render the channel insensitive to PACSIN 3-induced inhibition. Taken together, these data suggest that PACSIN 3 acts as an auxiliary protein of TRPV4 channel that not only affects the channel's subcellular localization but also modulates its function in a stimulus-specific manner.

Determinants of 4 alpha-phorbol sensitivity in transmembrane domains 3 and 4 of the cation channel TRPV4

TRPV4, a Ca(2+)-permeable member of the vanilloid subgroup of the transient receptor potential (TRP) channels, is activated by cell swelling and moderate heat (>27 degrees C) as well as by diverse chemical compounds including synthetic 4 alpha-phorbol esters, the plant extract bisandrographolide A, and endogenous epoxyeicosatrienoic acids (EETs; 5,6-EET and 8,9-EET). Previous work identified a tyrosine residue located in the first half of putative transmembrane segment 3 (TM3) as a crucial determinant for the activation of TRPV4 by its most specific agonist 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), suggesting that 4 alpha-PDD interacts with the channel through its transmembrane segments. To obtain insight in the 4 alpha-PDD-binding site and in the mechanism of ligand-dependent TRPV4 activation, we investigated the consequences of specific point mutations in TM4 on the sensitivity of the channel to different chemical and physical stimuli. Mutations of two hydrophobic residues in the central part of TM4 (Leu(584) and Trp(586)) caused a severe reduction of the sensitivity of the channel to 4 alpha-PDD, bisandrographolide A, and heat, whereas responses to cell swelling, arachidonic acid, and 5,6-EET remained unaffected. In contrast, mutations of two residues in the C-terminal part of TM4 (Tyr(591) and Arg(594)) affected channel activation of TRPV4 by all stimuli, suggesting an involvement in channel gating rather than in interaction with agonists. Based on a comparison of the responses of WT and mutant TRPV4 to 4 alpha-PDD and different 4 alpha-phorbol esters, we conclude that the length of the fatty acid moiety determines the ligand binding affinity and propose a model for the interaction between 4 alpha-phorbol esters and the TM3/4 region of TRPV4.

TRPM8-independent Menthol-induced Ca2+ Release from Endoplasmic Reticulum and Golgi

Menthol, a secondary alcohol produced by the peppermint herb, Mentha piperita, is widely used in the food and pharmaceutical industries as a cooling/soothing compound and odorant. It induces Ca2+ influx in a subset of sensory neurons from dorsal root and trigeminal ganglia, due to activation of TRPM8, a Ca2+-permeable, cold-activated member of the TRP superfamily of cation channels. Menthol also induces Ca2+ release from intracellular stores in several TRPM8-expressing cell types, which has led to the suggestion that TRPM8 can function as an intracellular Ca2+-release channel. Here we show that menthol induces Ca2+ release from intracellular stores in four widely used cell lines (HEK293, lymph node carcinoma of the prostate (LNCaP), Chinese hamster ovary (CHO), and COS), and provide several lines of evidence indicating that this release pathway is TRPM8-independent: 1) menthol-induced Ca2+ release was potentiated at higher temperatures, which contrasts to the cold activation of TRPM8; 2) overexpression of TRPM8 did not enhance the menthol-induced Ca2+) release; 3) menthol-induced Ca2+ release was mimicked by geraniol and linalool, which are structurally related to menthol, but not by the more potent TRPM8 agonists icilin or eucalyptol; and 4) TRPM8 expression in HEK293 cells was undetectable at the protein and mRNA levels. Moreover, using a novel TRPM8-specific antibody we demonstrate that both heterologously expressed TRPM8 (in HEK293 cells) and endogenous TRPM8 (in LNCaP cells) are mainly localized in the plasma membrane, which contrast to previous localization studies using commercial anti-TRPM8 antibodies. Finally, aequorin-based measurements demonstrate that the TRPM8-independent menthol-induced Ca2+ release originates from both endoplasmic reticulum and Golgi compartments.

PACSINs bind to the TRPV4 cation channel. PACSIN 3 modulates the subcellular localization of TRPV4

TRPV4 is a cation channel that responds to a variety of stimuli including mechanical forces, temperature, and ligand binding. We set out to identify TRPV4-interacting proteins by performing yeast two-hybrid screens, and we isolated with the avian TRPV4 amino terminus the chicken orthologues of mammalian PACSINs 1 and 3. The PACSINs are a protein family consisting of three members that have been implicated in synaptic vesicular membrane trafficking and regulation of dynamin-mediated endocytotic processes. In biochemical interaction assays we found that all three murine PACSIN isoforms can bind to the amino terminus of rodent TRPV4. No member of the PACSIN protein family was able to biochemically interact with TRPV1 and TRPV2. Co-expression of PACSIN 3, but not PACSINs 1 and 2, shifted the ratio of plasma membrane-associated versus cytosolic TRPV4 toward an apparent increase of plasma membrane-associated TRPV4 protein. A similar shift was also observable when we blocked dynamin-mediated endocytotic processes, suggesting that PACSIN 3 specifically affects the endocytosis of TRPV4, thereby modulating the subcellular localization of the ion channel. Mutational analysis shows that the interaction of the two proteins requires both a TRPV4-specific proline-rich domain upstream of the ankyrin repeats of the channel and the carboxyl-terminal Src homology 3 domain of PACSIN 3. Such a functional interaction could be important in cell types that show distribution of both proteins to the same subcellular regions such as renal tubule cells where the proteins are associated with the luminal plasma membrane.

TRPV channels and modulation by hepatocyte growth factor/scatter factor in human hepatoblastoma (HepG2) cells

Using patch clamp and Ca(2+) imaging techniques, we have studied Ca(2+) entry pathways in human hepatoblastoma (HepG2) cells. These cells express the mRNA of TRPV1, TRPV2, TRPV3 and TRPV4 channels, but not those of TRPV5 and TRPV6. Functional assessment showed that capsaicin (10 microM), 4alpha-phorbol-12,13-didecanoate (4alphaPDD, 1 microM), arachidonic acid (10 microM), hypotonic stress, and heat all stimulated increases in [Ca(2+)](i) within minutes. The increase in [Ca(2+)](i) depended on extracellular Ca(2+) and on the transmembrane potential, which indicated that both driving forces affected Ca(2+) entry. Capsaicin also stimulated an increase in [Ca(2+)](i) in nominally Ca(2+)-free solutions, which was compatible with the receptor functioning as a Ca(2+) release channel. Hepatocyte growth factor/scatter factor (HGF/SF) modulated Ca(2+) entry. Ca(2+) influx was greater in HepG2 cells incubated with HGF/SF (20 ng/ml for 20 h) compared with non-stimulated cells, but this occurred only in those cells with a migrating phenotype as determined by presence of a lamellipodium and trailing footplate. The effect of capsaicin on [Ca(2+)](i) was greater in migrating HGF/SF-treated cells, and this was inhibited by capsazepine. The difference between control and HGF/SF-treated cells was not found in Ca(2+)-free solutions. 4alphaPDD also had no greater effect on HGF/SF-treated cells. We conclude that TRPV1 and TRPV4 channels provide Ca(2+) entry pathways in HepG2 cells. HGF/SF increases Ca(2+) entry via TRPV1, but not via TRPV4. This rise in [Ca(2+)](i) may constitute an early response of a signalling cascade that gives rise to cell locomotion and the migratory phenotype.

Cell swelling, heat, and chemical agonists use distinct pathways for the activation of the cation channel TRPV4

TRPV4 is a Ca(2+)- and Mg(2+)-permeable cation channel within the vanilloid receptor subgroup of the transient receptor potential (TRP) family, and it has been implicated in Ca(2+)-dependent signal transduction in several tissues, including brain and vascular endothelium. TRPV4-activating stimuli include osmotic cell swelling, heat, phorbol ester compounds, and 5',6'-epoxyeicosatrienoic acid, a cytochrome p450 epoxygenase metabolite of arachidonic acid (AA). It is presently unknown how these distinct activators converge on opening of the channel. Here, we demonstrate that blockers of phospholipase A(2) (PLA(2)) and cytochrome p450 epoxygenase inhibit activation of TRPV4 by osmotic cell swelling but not by heat and 4alpha-phorbol 12,13-didecanoate. Mutating a tyrosine residue (Tyr-555) in the N-terminal part of the third transmembrane domain to an alanine strongly impairs activation of TRPV4 by 4alpha-phorbol 12,13-didecanoate and heat but has no effect on activation by cell swelling or AA. We conclude that TRPV4-activating stimuli promote channel opening by means of distinct pathways. Cell swelling activates TRPV4 by means of the PLA(2)-dependent formation of AA, and its subsequent metabolization to 5',6'-epoxyeicosatrienoic acid by means of a cytochrome p450 epoxygenase-dependent pathway. Phorbol esters and heat operate by means of a distinct, PLA(2)- and cytochrome p450 epoxygenase-independent pathway, which critically depends on an aromatic residue at the N terminus of the third transmembrane domain.

Measurement of cytoplasmic calcium concentration in the rods of wild-type and transducin knock-out mice

A 10 microm spot of argon laser light was focused onto the outer segments of intact mouse rods loaded with fluo-3, fluo-4 or fluo-5F, to estimate dark, resting free Ca(2+) concentration ([Ca(2+)](i)) and changes in [Ca(2+)](i) upon illumination. Dye concentration was adjusted to preserve the normal physiology of the rod, and the laser intensity was selected to minimise bleaching of the fluorescent dye. Wild-type mouse rods illuminated continuously with laser light showed a progressive decrease in fluorescence well fitted by two exponentials with mean time constants of 154 and 540 ms. Rods from transducin alpha-subunit knock-out (Tralpha-/-) animals showed no light-dependent decline in fluorescence but exhibited an initial rapid component of fluorescence increase which could be fitted with a single exponential (tau~1-4 ms). This fluorescence increase was triggered by rhodopsin bleaching, since its amplitude was reduced by pre-exposure to bright bleaching light and its time constant decreased with increasing laser intensity. The rapid component was however unaffected by incorporation of the calcium chelator BAPTA and seemed therefore not to reflect an actual increase in [Ca(2+)](i). A similar rapid increase in fluorescence was also seen in the rods of wild-type mice just preceding the fall in fluorescence produced by the light-dependent decrease in [Ca(2+)](i). Dissociation constants were measured in vitro for fluo-3, fluo-4 and fluo-5F with and without 1 mM Mg(2+) from 20 to 37 degrees C. All three dyes showed a strong temperature dependence, with the dissociation constant changing by a factor of 3-4 over this range. Values at 37 degrees C were used to estimate absolute levels of rod [Ca(2+)](i). All three dyes gave similar values for [Ca(2+)](i) in wild-type rods of 250 +/- 20 nM in darkness and 23 +/- 2 nM after exposure to saturating light. There was no significant difference in dark [Ca(2+)](i) between wild-type and Tralpha-/- animals.

Effects of temperature on calcium-sensitive fluorescent probes

The effect of temperature on the binding equilibria of calcium-sensing dyes has been extensively studied, but there are also important temperature-related changes in the photophysics of the dyes that have been largely ignored. We conducted a systematic study of thermal effects on five calcium-sensing dyes under calcium-saturated and calcium-free conditions. Quin-2, chlortetracycline, calcium green dextran, Indo-1, and Fura-2 all show temperature-dependent effects on fluorescence in all or part of the range tested (5-40 degrees C). Specifically, the intensity of the single-wavelength dyes increased at low temperature. The ratiometric dyes, because of variable effects at the two wavelengths, showed, in general, a reduction in the fluorescence ratio as temperature decreased. Changes in viscosity, pH, oxygen quenching, or fluorescence maxima could not fully explain the effects of temperature on fluorescence. The excited-state lifetimes of the dyes were determined, in both the presence and absence of calcium, using multifrequency phase-modulation fluorimetry. In most cases, low temperature led to prolonged fluorescence lifetimes. The increase in lifetimes at reduced temperature is probably largely responsible for the effects of temperature on the physical properties of the calcium-sensing dyes. Clearly, these temperature effects can influence reported calcium concentrations and must therefore be taken into consideration during any investigation involving variable temperatures.

Histamine evoked sustained elevations of cytosolic Ca2+ in bovine adrenal chromaffin cells independently of Ca2+ entry

Whole-cell patch-clamp experiments and optical measurements with the Ca2+ fluorescent dye fura-2 were performed to examine histamine induced cytosolic Ca2+ changes in bovine adrenal chromaffin cells. The purpose of this study was to find out whether the sustained plateau phase, which followed the rapid transient increase, was due to Ca2+ influx. The extracellular Ca2+ dependence appeared to be minor, because substitution of Ca2+ with EGTA or BAPTA did not cause obvious changes in the biphasic Ca2+ response. Application of histamine in a Mn2+ containing external solution did not quench the fura-2 signal. It was neither possible to detect a histamine induced depolarisation, nor a Ca2+ permeable current. Changing the driving force for Ca2+ during the plateau phase did not result in a correlating fura-2 signal. Metal ions like Cd2+, La3+ and Co2+ which are known to block Ca2+ influx were unable to abolish the typical histamine induced Ca2+ response. These results suggest that primarily intracellular Ca2+ was responsible for generating the characteristic biphasic Ca2+ response due to histamine in bovine adrenal chromaffin cells.

The effect of pH and temperature on the dissociation constant for fura-2 and their effects on [Ca(2+)](i) in enterocytes from a poikilothermic animal, Atlantic cod (Gadus morhua)

In this study, we investigated the validity of the fluorescent probe fura-2 in determinations of intracellular Ca(2+) concentrations ([Ca(2+)](i)), at physiological temperatures, in poikilothermic animals living at low temperatures. The K(d) for fura-2 was found to decrease with increasing temperature (5-37 degrees C) and DeltaH, in the Van't Hoff isochore equation, was determined to be 11.03 kJ/mol, when pH was corrected to 7.2 for all temperatures tested. The absorption maxima (340 nm) and isobestic point (360 nm) for the UV spectra of fura-2 were not affected by temperature. Thus, if pH- and temperature-dependent changes in K(d) are corrected for, fura-2 is a suitable tool for measurements of [Ca(2+)](i) at temperatures of 5-37 degrees C. The present study demonstrates that Atlantic cod enterocytes, acclimated to 37 degrees C, show a lower basal [Ca(2+)](i) (65 +/- 8 nM) compared to enterocytes acclimated to 10 degrees C (161 +/- 6 nM). Furthermore, addition of 10 mM Ca(2+) increases the [Ca(2+)](i) by 526%, when compared to basal [Ca(2+)](i), in cells at 37 degrees C but only by 36%, in cells kept at 10 degrees C. Thus, performing experiments at unphysiological temperatures results in cellular responses that would not be observed under physiological conditions.

Effects of P2 purinoceptor agonists on membrane potential and intracellular Ca2+ of human cardiac endothelial cells

Vasoactive agonists like adenosine-5'-triphosphate (ATP) increase intracellular Ca2+ ([Ca2+]i) in vascular endothelial cells with an initial peak due to inositol 1,4,5-triphosphate-mediated Ca2+ release from intracellular stores followed by a sustained plateau that is dependent on the presence of extracellular Ca2+, thus leading to an increased synthesis and release of prostacyclin and nitric oxide. We studied the effects of nucleotides on membrane potential and [Ca2+]i in confluent human microvascular cardiac endothelial cells obtained from patients with dilated cardiomyopathy. The whole-cell configuration of the patch-clamp technique and a confocal laser scanning microscope employing fluo-3 as a Ca2+ indicator were used. Both uridine-5'-triphosphate (UTP) and 2-methylthioadenosine-5'-triphosphate (2MeSATP) induced depolarizations in human microvascular cardiac endothelial cells and increased [Ca2+]i with a rank order of potency 2MeSATP>ATP=UTP (EC50 values (in microM) were 0.084 2MeSATP, 0.67 ATP and 1.1 UTP). This suggests that both P2u and P2y purinoceptors are present on human microvascular cardiac endothelial cells. Maximal [Ca2+]i responses of confluent human microvascular cardiac endothelial cell monolayers to UTP were lower when compared to 2MeSATP. Nucleotide-induced increases in [Ca2+]i consisted of a transient peak, which was also observed in the absence of extracellular Ca2+, and a sustained [Ca2+]i plateau. This plateau, which was not observed in all monolayers studied, was not markedly influenced by increasing extracellular [K+]. Previous incubation with thapsigargin abolished ATP-induced increases of [Ca2+]i. It is concluded that human microvascular cardiac endothelial cells express both P2y and P2u purinoceptors. P2 purinoceptor agonists release Ca2+ from intracellular thapsigargin-sensitive stores and stimulate capacitative Ca2+ influx pathways. K+ efflux through Ca2+-dependent K+ (K(Ca)) channels does not play a major role in the regulation of nucleotide-induced Ca2+ influx in human microvascular cardiac endothelial cells, which might be related to an impaired function of the cells.

Marked Temperature Dependence of the Platelet Calcium Signal Induced by Human von Willebrand Factor

Interaction of von Willebrand factor (vWF) with the platelet is essential to hemostasis when vascular injury occurs. This interaction elevates the intracellular free calcium concentration ([Ca2+]i) and promotes platelet activation. The present study investigated the temperature dependence of vWF-induced [Ca2+]i signaling in human platelets. The influence of temperature can provide invaluable insight into the underlying mechanism. Platelet [Ca2+]i was monitored with Fura-PE3. Ristocetin-mediated binding of vWF induced a transient platelet [Ca2+]i increase at 37°C, but no response at lower temperatures (20°C to 25°C). This temperature dependence could not be attributed to a reduction in vWF binding, as ristocetin-mediated platelet aggregation and agglutination were essentially unaffected by temperature. Most other platelet agonists (U-46619, -thrombin, and adenosine 5′-diphosphate [ADP]) induced a [Ca2+]isignal whose amplitude did not diminish at lower temperatures. The [Ca2+]i signal in response to arachidonic acid, however, showed similar temperature dependence to that seen with vWF. Assessment of thromboxane A2 production showed a strong temperature dependence for metabolism of arachidonic acid by the cyclo-oxygenase pathway. vWF induced thromboxane A2production in the platelet. Aspirin treatment abolished the vWF-induced [Ca2+]i signal. These observations suggest that release of arachidonic acid and its conversion to thromboxane A2 play a central role in vWF-mediated [Ca2+]i signaling in the platelet at physiological temperatures.

Potentiation of Ca2+ signaling in endothelial cells by 11,12-epoxyeicosatrienoic acid

Incubation of endothelium with an increased epoxyeicosatrienoic acid (EET) concentration specifically augments the endothelium-dependent relaxation ascribed to endothelium-derived hyperpolarizing factor in porcine coronary arteries (Weintraub et al., Circ Res 1997;81:258-267). Experiments were designed to test whether such sustained increased levels of EETs in the environment of endothelial cells alters Ca2+ signaling. Changes in cytosolic Ca2+ were monitored in cultured porcine aortic endothelial cells (PAECs) and in the human endothelial EA.hy926 cell line after incubation (or not) with 5 microM 11,12-epoxyeicosatrienoic acid (EET). Although the mobilization of intracellular Ca2+ induced by 2 microM thapsigargin was not affected significantly, EET treatment augmented the capacitative Ca2+ entry evoked by the Ca(2+)-ATPase) inhibitor in both cell types. Similar observations were obtained by using histamine as a stimulant in EA.hy926 cells. As assessed in PAECs, 2 micrograms/ml triacsin C, a known inhibitor of the incorporation of EETs into phospholipids, did not significantly affect the potentiating action of EETs on Ca2+ signaling in response to thapsigargin. However, in solvent-control cells, triacsin C significantly reduced both the mobilization of Ca2+ from intracellular stores and the capacitative Ca2+ entry provoked by thapsigargin. Thus the EET-potentiating effect overcomes the inhibitory action of triacsin C on Ca2+ signaling in endothelial cells. Taken together, these results demonstrate that sustained increases in EETs may amplify Ca2+ signaling. However, contrary to the EET-induced augmentation of endothelium-dependent relaxation in the porcine coronary artery, resistance of this novel action of EETs to triacsin C suggests that the mechanism involved does not depend on incorporation into phospholipids.

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.

Origin and function of epoxyeicosatrienoic acids in vascular endothelial cells: more than just endothelium-derived hyperpolarizing factor?

1. In addition to their contribution to endothelium-derived hyperpolarization, our understanding of the physiological function of epoxyeicosatrienoic acids (EET) within the vascular wall and the actual enzymes involved in the formation of the EET in endothelial cells is very limited. In the present study, the expression of potential cytochrome P450 (CYP) mono/epoxygenases was assessed in endothelial cells isolated from porcine and bovine aortas as well as in the human umbilical vein-derived cell lines EA.hy926 and ECV304. 2. Expression of CYP2B1, CYP2E1 and CYP3A could be found. The latter were inducible by dexamethasone/clofibrate for 72 h, a procedure that also enhanced CYP epoxygenase activity in endothelial cells. 3. Enzyme induction yielded increases in capacitative Ca2+ entry and membrane hyperpolarization in response to autacoids, such as bradykinin and thapsigargin. Thiopentone sodium, an inhibitor of endothelial CYP mono/epoxygenase(s), diminished autacoid-induced capacitative Ca2+ entry and membrane hyperpolarization, while the effect of EET remained unchanged. 4. Epoxyeicosatrienoic acids activated endothelial tyrosine kinase activity in a concentration-dependent manner. Arachidonic acid, at 20-fold higher concentrations, also increased tyrosine kinase activity. Because only the effect of arachidonic acid was inhibited by thiopentone sodium, an inhibitor of CYP mono/epoxygenases, these data suggest that arachidonic acid needs to be converted to the EET in order to stimulate tyrosine kinase. 5. All these data provide clear evidence that the CYP epoxygenase-derived arachidonic acid metabolites (EET) not only serve as potential endothelium-derived hyperpolarizing factors but also constitute highly active intracellular messengers with a physiological role including the control of Ca2+ signalling, membrane potential and tyrosine kinase activity.

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.

Activation of microsomal cytochrome P450 mono-oxygenase by Ca2+ store depletion and its contribution to Ca2+ entry in porcine aortic endothelial cells

1. We investigated how microsomal cytochrome P450 mono-oxygenase (Cyp450 MO) is regulated in cultured porcine aortic endothelial cells. The hypothesis that a Cyp450 MO-derived metabolite links Ca2+ store depletion and Ca2+ entry was studied further. 2. Microsomal Cyp450 MO was monitored fluorometrically by dealkylation of 1-ethoxypyrene-3,6,8-tris-(dimethyl-sulphonamide; EPSA) in saponin permeabilized cells or in subcellular compartments. Endothelial Ca2+ signalling was measured by a standard fura-2 technique, membrane potential was determined with the potential-sensitive fluorescence dye, bis-(1,3-dibutylbarbituric acid) pentamethine oxonol (DiBAC4(5)) and tyrosine kinase was quantified by measuring the phosphorylation of a immobilized substrate with a horseradish peroxidase labelled phosphotyrosine specific antibody. 3. Depletion of cellular Ca2+ pools with inositol 1,4,5-trisphosphate (IP3), thapsigargin or cyclopiazonic acid activated microsomal Cyp450 MO. Similar to direct Ca2+ store depletion, chelating of intramicrosomal Ca2+ with oxalate stimulated Cyp450 MO activity, while changing cytosolic free Ca2+ failed to influence Cyp450 MO activity. These data indicate that microsomal Cyp450 MO is activated by depletion of IP3-sensitive stores. 4. Besides the common cytochrome P450 inhibitors, econazole, proadifen and miconazole, thiopentone sodium and methohexitone inhibited Cyp450 MO in a concentration-dependent manner. The physiological substrate of Cyp450 MO, arachidonic acid, inhibited EPSA dealkylation. In contrast to most other cytochrome P450 inhibitors used in this study, thiopentone sodium did not directly interfere with Ca2+ entry pathways, membrane hyperpolarization due to K+ channel activation or tyrosine kinase activity. 5. Inhibition of Cyp450 MO by thiopentone sodium diminished Ca2+/Mn2+ entry to Ca2+ store depletion by 43%, while it did not interfere with intracellular Ca2+ release by IP3 or thapsigargin. 6. Cyp450 MO inhibition with thiopentone sodium diminished autacoid-induced membrane hyperpolarization. 7. Induction of Cyp450 MO with dexamethasone/clofibrate for 72 h yielded increases in thapsigargin-induced Cyp450 MO activity (by 35%), Ca2+/Mn2+ entry (by 105%) and membrane hyperpolarization (by 40%). 8. The Cyp450 MO-derived compounds, 11,12 and 5,6-epoxyeicosatrienoic acids (EETs) yielded membrane hyperpolarization, insensitive to thiopentone sodium. 9. These data demonstrate that endothelial Cyp450 MO is activated by Ca2+ store depletion and Cyp450 MO produced compounds that hyperpolarize endothelial cells. 10. The data presented and our previous findings indicate that Cyp450 MO plays a crucial role in the regulation of store-operated Ca2+ influx. We propose that Cyp450 MO-derived EETs constitute a signal for Ca2+ entry activation and increase the driving force for Ca2+ entry by membrane hyperpolarization in porcine aortic endothelial cells.