Interactions between inositol trisphosphate and Ca2+ dependent Ca2+ release mechanisms on the endoplasmic reticulum of permeabilised bovine aortic endothelial cells

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.

Ouabain modulation of endothelial calcium signaling in descending vasa recta

Using fura 2-loaded vessels, we tested whether ouabain modulates endothelial cytoplasmic calcium concentration ([Ca(2+)](CYT)) in rat descending vasa recta (DVR). Over a broad range between 10(-10) and 10(-4) M, ouabain elicited biphasic peak and plateau [Ca(2+)](CYT) elevations. Blockade of voltage-gated Ca(2+) entry with nifedipine did not affect the response to ouabain mitigating against a role for myo-endothelial gap junctions. Reduction of extracellular Na(+) concentration ([Na(+)](o)) or Na(+)/Ca(2+) exchanger (NCX) inhibition with SEA-0400 (10(-6) M) elevated [Ca(2+)](CYT), supporting a role for NCX in the setting of basal [Ca(2+)](CYT). SEA-0400 abolished the [Ca(2+)](CYT) response to ouabain implicating NCX as a mediator. The transient peak phase of [Ca(2+)](CYT) elevation that followed either ouabain or reduction of [Na(+)](o) was abolished by 2-aminoethoxydiphenyl borate (5 x 10(-5) M). Cation channel blockade with La(3+) (10 muM) or SKF-96365 (10 muM) also attenuated the ouabain-induced [Ca(2+)](CYT) response. Ouabain pretreatment increased the [Ca(2+)](CYT) elevation elicited by bradykinin (10(-7) M). We conclude that inhibition of ouabain-sensitive Na(+)-K(+)-ATPase enhances DVR endothelial Ca(2+) store loading and modulates [Ca(2+)](CYT) signaling through mechanisms that involve NCX, Ca(2+) release, and cation channel activation.

Signaling pathways mediating VEGF165-induced calcium transients and membrane depolarization in human endothelial cells

Cytosolic calcium and membrane potential were monitored simultaneously in quiescent human umbilical vein endothelial cells (HUVEC) exposed to vascular endothelial growth factor (VEGF)165 using the fluorescent indicators indo-1 AM and DiSBAC2(3), respectively. Application of VEGF165 to cells elicits a rapid rise in cytosolic calcium followed by a slower decline toward control values. Peak calcium is associated with a slight membrane hyperpolarization; however, as calcium falls toward control, a strong depolarization develops and is sustained throughout a 10-min period of VEGF165 stimulation. Both the VEGF165-mediated rise in cytosolic calcium and membrane depolarization are eliminated by inhibitors of VEGFR-2, tyrosine kinase, src kinase and inositol-1,4,5 triphosphate-operated calcium channels. Calcium entry, which is initially facilitated by transient hyperpolarization, is restricted by a substantial, sustained depolarization that developed during the downstroke of the calcium spike. Inhibition of plasmalemmal calcium channels diminished the magnitude and duration of the calcium spike, suggesting that extracellular calcium influx, secondary to stores release, is a significant component of the calcium transient. Inhibition of chloride channels substantially reduced membrane depolarization. In addition, the depolarization is modulated by PI3 kinase in a ras-independent manner. In summary, intracellular calcium and membrane potential are influenced by several key signaling cascades of VEGFR-2 activation in HUVEC.

Transmitter release from Rana pipiens vestibular hair cells via mGluRs: a role for intracellular Ca(++) release

The response of the semicircular canal (SCC) to the group I mGluR-selective agonist dihydroxyphenylglycine (DHPG; 300 microM) - facilitation of afferent discharge rate - was dose-dependently reduced by the phospholipase C inhibitor U-73122 (1-100 microM; IC(50): 22 microM), the smooth endoplasmic reticulum Ca(++) ATPase inhibitor thapsigargin (100 nM-3 microM; IC(50): 500 nM), and xestospongin C (100 pM-1 microM; IC(50): 11 nM), an inositol trisphosphate receptor (IP(3)R) antagonist. Ryanodine, a modulator of Ca(++)-induced Ca(++) release, biphasically facilitated, then suppressed this response (1 nM-1 mM; approximate IC(50): 50 microM). 5 mM caffeine increased the amplitude (34.6+/-13.4%) and duration (453+/-169.8%; n=4) of the response of the SCC to DHPG, while 50 mM caffeine eliminated this response (n=2). The protein kinase C inhibitor bisindolylmaleimide I-HCl (10-100 microM; n=3) and the cyclic-ADP ribose antagonist 8-Br-cyclic-ADP ribose (1-10 microM; n=3) had no effect on the response of the SCC to DHPG. These data suggest that the increase in transmitter release following activation of group I mGluRs on vestibular hair cells is associated with intracellular Ca(++) release from both IP(3)-sensitive and ryanodine/caffeine-sensitive intracellular Ca(++) stores. Such positive feedback on transmitter release may serve to enhance the contrast between the spontaneous and stimulus-evoked modes of hair cell transmitter release, thereby optimizing signal discrimination at the synapse between hair cells and vestibular afferent fibers.

Red wine polyphenols increase calcium in bovine aortic endothelial cells: a basis to elucidate signalling pathways leading to nitric oxide production

1. The present study investigates the mechanisms by which polyphenolic compounds from red wine elicit Ca(2+) mobilization in bovine aortic endothelial cells (BAECs). Two polyphenol-containing red wine extracts, red wine polyphenolic compounds (RWPC) and Provinols, and delphinidin, an anthocyanin were used. 2. RWPC stimulated a Ca(2+)-dependent release of nitric oxide (NO) from BAECs accounting for the relaxation of endothelium-denuded rat aortic rings as shown by cascade bioassay. 3. RWPC, Provinols and delphinidin increased cytosolic free calcium ([Ca(2+)](i)), by releasing Ca(2+) from intracellular stores and by increasing Ca(2+) entry. 4. The RWPC-induced increase in [Ca(2+)](i) was decreased by exposure to ryanodine (30 microM), whereas Provinols and delphinidin-induced increases in [Ca(2+)](i) were decreased by bradykinin (0.1 microM) and thapsigargin (1 microM) pre-treatment. 5. RWPC, Provinols and delphinidin-induced increases in [Ca(2+)](i) were sensitive to inhibitors of phospholipase C (neomycin, 3 mM; U73122, 3 microM) and tyrosine kinase (herbimycin A, 1 microM). 6. RWPC, Provinols and delphinidin induced herbimycin A (1 microM)-sensitive tyrosine phosphorylation of several intracellular proteins. 7. Provinols released Ca(2+) via both a cholera (CTX) and pertussis toxins (PTX)-sensitive pathway, whereas delphinidin released Ca(2+) only via a PTX-sensitive mechanism. 8. Our data contribute in defining the mechanisms of endothelial NO production caused by wine polyphenols including the increase in [Ca(2+)](i) and the activation of tyrosine kinases. Furthermore, RWPC, Provinols and delphinidin display differences in the process leading to [Ca(2+)](i) increases in endothelial cells illustrating multiple cellular targets of natural dietary polyphenolic compounds.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Simvastatin and Ca(2+) signaling in endothelial cells: involvement of rho protein

The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor simvastatin is able to produce endothelium-dependent relaxation in addition to its lipid-lowering properties. The underlying mechanisms were investigated in bovine aortic endothelial cells (BAEC). Simvastatin induced an increase in cytosolic calcium ([Ca(2+)](i)) in BAEC, by releasing Ca(2+) from intracellular stores sensitive to thapsigargin and ryanodine, and increasing Ca(2+) entry. Simvastatin response was not altered by the phospholipase A(2) inhibitor ONO-RS-082, or the combination of superoxide dismutase plus catalase. However, the response to simvastatin was reduced by the product of HMG-CoA reductase, mevalonate or by the inhibitor of small G proteins of the Rho family, Clostridium botulinum C3 toxin. Thus, increase in [Ca(2+)](i) involving the activation of Rho protein through mevalonate-dependent pathway is essential for the action of simvastatin and might contribute to its beneficial effects against vascular diseases. This study helps elucidate the mechanisms of endothelial factor generation by simvastatin in BAEC.

Mechanism of endothelial nitric oxide-dependent vasorelaxation induced by wine polyphenols in rat thoracic aorta

The mechanisms by which red wine polyphenolic compounds (RWPCs) induced endothelium-dependent relaxation were investigated in rat thoracic aorta rings with endothelium. RWPCs produced relaxation that was prevented by the nitric oxide (NO) synthase inhibitor, N(omega)-nitro-L-arginine-methyl-ester. This relaxation was abolished in the absence of extracellular calcium in the medium or in the presence of the Ca2+ entry blocker, La3+, but it was not affected by the nonselective K+ channels blocker, tetrabutylammonium. N-Ethyl-maleimide (NEM), a sulfhydryl alkylating agent, abolished vasorelaxation produced by RWPCs and acetylcholine but not that produced either by the sarcoendoplasmic reticulum Ca2+-adenosine triphosphatase (ATPase) pump inhibitor, cyclopyazonic acid (CPA) or the calcium ionophore, ionomycin. Neither pertussis toxin (PTX) nor cholera toxin (CTX) inhibited the vasorelaxant effect of RWPC. The effect of RWPC was not affected by the phospholipase C (PLC) blocker, L-alpha-glycerophospho-D-myo-inositol 4-monophosphate (Gro-pip), and the phospholipase A2 pathway blockers, quinacrine and ONO-RS-082. Finally, the protein kinase C (PKC) inhibitor, GF 109203X, and tyrosine kinase inhibitors, tyrphostin A-23 and genistein, did not impair the response to RWPCs. These results suggest that RWPCs produce endothelium-NO-derived vasorelaxation through an extracellular Ca2+-dependent mechanism via an NEM-sensitive pathway. They also show that PTX- or CTX-sensitive G proteins, activation of PLC or PLA2 pathways, PKC, or tyrosine kinase may not be involved.

Evidence for mitochondrial Ca(2+)-induced Ca2+ release in permeabilised endothelial cells

Generally most intracellular Ca2+ is stored in the endoplasmic reticulum (ER) and mitochondria. Recently a mitochondrial Ca(2+)-induced Ca2+ release (mCICR) mechanism, unconnected with ryanodine receptors (RyR's), has been shown in tumour cells. The existence of a mitochondrial Ca2+ release mechanism in BAE cells was investigated using saponin-permeabilised BAE cells. When buffered intracellular solution were 'stepped' from 10 nM to 10 microM free Ca2+, the mitochondrial inhibitors CN (2 mM), FCCP (1 microM), and RR (20 microM) significantly reduced total CICR by approximately 25%. The ER Ca(2+)-ATPase inhibitor thapsigargin (100 nM) had no effect. Furthermore, cyclosporin A (200 nM), an inhibitor of the mitochondrial permeability transition pore (PTP), abolished total CICR. Therefore, the novel ryanodine-caffeine insensitive CICR mechanism previously reported in BAE cells involves mitochondrial Ca2 release. It is proposed that in BAE cells, mCICR occurs via the mitochondrial PTP and may be physiologically important in endothelial cell Ca2+ signalling.

Inositol 1,4,5-trisphosphate- and ryanodine-sensitive Ca2+ release channel-dependent Ca2+ signalling in rat portal vein myocytes

Ca2+ signalling events were analyzed in single myocytes from rat portal vein by using a laser confocal microscope combined with the patch-clamp technique. Increase in inositol 1,4,5-trisphosphate (InsP3) concentration was obtained by photorelease from a caged precursor or intracellular dialysis of 3F-InsP3. Low InsP3 concentrations activated either small elevations of [Ca2+]i or localized Ca2+ transients whereas high InsP3 concentrations activated either homogeneous Ca2+ responses or propagated Ca2+ waves. The InsP3-evoked localized Ca2+ transients had spatio-temporal properties characteristic of Ca2+ sparks. In addition, compounds that blocked Ca2+ sparks and Ca2+ responses activated by Ca2+ jumps reduced the global InsP3-activated Ca2+ responses and suppressed the Ca2+ transients. In contrast, Ca2+ responses evoked by flash-photolytic Ca2+ jumps or caffeine were not affected by heparin (an InsP3 receptor antagonist). These results suggest that the absence of elementary Ca2+ events evoked by InsP3 may be related to the lack of clustered InsP3 receptor units in these cells, as confirmed by immunocytochemistry. Cooperativity between InsP3- and ryanodine-sensitive Ca2+ channels may represent a novel mechanism to amplify Ca2+ release from the same intracellular store and give rise to propagated Ca2+ waves.

Inositol 1,4,5-trisphosphate and basal Ca2+ release is affected by the cytoplasmic concentration of Cl- in endothelial cells

The effects of varying Cl- concentration in the intracellular bathing medium, on IP3-induced 45Ca2+ release from internal stores, were examined in saponin-permeabilised bovine aortic endothelial (BAE) cells. Results from this study show that the release of Ca2+ from the internal stores is affected by the cytoplasmic concentration of Cl- ions. Complete replacement of Cl- with gluconate augmented IP3 (3 microM)-induced 45Ca2+ release by 33 +/- 8%. Replacement of both Cl- and K+ with gluconate and NMG, respectively, had no significant effect on 45Ca2+ release. However, resting levels of internal 45Ca2+ were found to be affected by Cl- removal. These data suggest that in BAE cells, IP3 and also basal 45Ca2+ release may be regulated by the physiological intracellular Cl- concentration.