Thapsigargin discharges intracellular calcium stores and induces transmembrane currents in human endothelial cells

The Molecular Heterogeneity of Store-Operated Ca2+ Entry in Vascular Endothelial Cells: The Different roles of Orai1 and TRPC1/TRPC4 Channels in the Transition from Ca2+-Selective to Non-Selective Cation Currents

Store-operated Ca²⁺ entry (SOCE) is activated in response to the inositol-1,4,5-trisphosphate (InsP₃)-dependent depletion of the endoplasmic reticulum (ER) Ca²⁺ store and represents a ubiquitous mode of Ca²⁺ influx. In vascular endothelial cells, SOCE regulates a plethora of functions that maintain cardiovascular homeostasis, such as angiogenesis, vascular tone, vascular permeability, platelet aggregation, and monocyte adhesion. The molecular mechanisms responsible for SOCE activation in vascular endothelial cells have engendered a long-lasting controversy. Traditionally, it has been assumed that the endothelial SOCE is mediated by two distinct ion channel signalplexes, i.e., STIM1/Orai1 and STIM1/Transient Receptor Potential Canonical 1(TRPC1)/TRPC4. However, recent evidence has shown that Orai1 can assemble with TRPC1 and TRPC4 to form a non-selective cation channel with intermediate electrophysiological features. Herein, we aim at bringing order to the distinct mechanisms that mediate endothelial SOCE in the vascular tree from multiple species (e.g., human, mouse, rat, and bovine). We propose that three distinct currents can mediate SOCE in vascular endothelial cells: (1) the Ca²⁺-selective Ca²⁺-release activated Ca²⁺ current (I_CRAC), which is mediated by STIM1 and Orai1; (2) the store-operated non-selective current (I_SOC), which is mediated by STIM1, TRPC1, and TRPC4; and (3) the moderately Ca²⁺-selective, I_CRAC-like current, which is mediated by STIM1, TRPC1, TRPC4, and Orai1.

Patch Clamp: The First Four Decades of a Technique That Revolutionized Electrophysiology and Beyond

Forty years ago, the introduction of a new electrophysiological technique, the patch clamp, revolutionized the fields of Cellular Physiology and Biophysics, providing for the first time the possibility of describing the behavior of a single protein, an ion-permeable channel of the cell plasma membrane, in its physiological environment. The new approach was actually much more potent and versatile than initially envisaged, and it has evolved into several different modalities that have radically changed our knowledge of how cells (not only the classical "electrically excitable "ones, such as nerves and muscles) use electrical signaling to modulate and organize their activity. This review aims at telling the history of the background from which the new technique evolved and at analyzing some of its more recent developments.

Deep insights into the response of human cervical carcinoma cells to a new cyano enone-bearing triterpenoid soloxolone methyl: a transcriptome analysis

Semisynthetic triterpenoids, bearing cyano enone functionality in ring A, are considered now as novel promising anti-tumor agents. However, despite the large-scale studies, their effects on cervical carcinoma cells and, moreover, mechanisms underlying cell death activation by such compounds in this cell type have not been fully elucidated. In this work, we attempted to reconstitute the key pathways and master regulators involved in the response of human cervical carcinoma KB-3-1 cells to the novel glycyrrhetinic acid derivative soloxolone methyl (SM) by a transcriptomic approach. Functional annotation of differentially expressed genes, analysis of their cis^- regulatory sequences and protein-protein interaction network clearly indicated that stress of endoplasmic reticulum (ER) is the central event triggered by SM in the cells. A range of key ER stress sensors and transcription factor AP-1 were identified as upstream transcriptional regulators, controlling the response of the cells to SM. Additionally, by using Gene Expression Omnibus data, we showed the ability of SM to modulate the expression of key genes involved in regulation of the high proliferative rate of cervical carcinoma cells. Further Connectivity Map analysis revealed similarity of SM's effects with known ER stress inducers thapsigargin and geldanamycin, targeting SERCA and Grp94, respectively. According to the molecular docking study, SM could snugly fit into the active sites of these proteins in the positions very close to that of both inhibitors. Taken together, our findings provide a basis for the better understanding of the intracellular processes in tumor cells switched on in response to cyano enone-bearing triterpenoids.

Tissue Specificity: SOCE: Implications for Ca2+ Handling in Endothelial Cells

Many cellular functions of the vascular endothelium are regulated by fine-tuned global and local, microdomain-confined changes of cytosolic free Ca²⁺ ([Ca²⁺]_i). Vasoactive agonist-induced stimulation of vascular endothelial cells (VECs) typically induces Ca²⁺ release through IP₃ receptor Ca²⁺ release channels embedded in the membrane of the endoplasmic reticulum (ER) Ca²⁺ store, followed by Ca²⁺ entry from the extracellular space elicited by Ca²⁺ store depletion and referred to as capacitative or store-operated Ca²⁺ entry (SOCE). In vascular endothelial cells, SOCE is graded with the degree of store depletion and controlled locally in the subcellular microdomain where depletion occurs. SOCE provides distinct Ca²⁺ signals that selectively control specific endothelial functions: in calf pulmonary artery endothelial cells, the SOCE Ca²⁺ signal drives nitric oxide (an endothelium-derived relaxing factor of the vascular smooth muscle) production and controls activation and nuclear translocation of the transcription factor NFAT. Both cellular events are not affected by Ca²⁺ signals of comparable magnitude arising directly from Ca²⁺ release from intracellular stores, clearly indicating that SOCE regulates specific Ca²⁺-dependent cellular tasks by a unique and exclusive mechanism. This review discusses the mechanisms of intracellular Ca²⁺ regulation in vascular endothelial cells and the role of store-operated Ca²⁺ entry for endothelium-dependent smooth muscle relaxation and nitric oxide signaling, endothelial oxidative stress response, and excitation-transcription coupling in the vascular endothelium.

Purinergic transmission and transglial signaling between neuron somata in the dorsal root ganglion

Most dorsal root ganglion neuronal somata (NS) are isolated from their neighbours by a satellite glial cell (SGC) sheath. However, some NS are associated in pairs, separated solely by the membrane septum of a common SGC to form a neuron-glial cell-neuron (NGlN) trimer. We reported that stimulation of one NS evokes a delayed, noisy and long-duration inward current in both itself and its passive partner that was blocked by suramin, a general purinergic antagonist. Here we test the hypothesis that NGlN transmission involves purinergic activation of the SGC. Stimulation of the NS triggered a sustained current noise in the SGC. Block of transmission through the NGlN by reactive blue 2 or thapsigargin, a Ca(2+) store-depletion agent, implicated a Ca(2+) store discharge-linked P2Y receptor. P2Y2 was identified by simulation of the NGlN-like transmission by puffing UTP onto the SGC and by immunocytochemical localization to the SGC membrane septum. Block of the UTP effect by BAPTA, an intracellular Ca(2+) scavenger, supported the involvement of SGC Ca(2+) stores in the signaling pathway. We infer that transmission through the NGlN trimer involves secretion of ATP from the NS and triggering of SGC Ca(2+) store discharge via P2Y2 receptors. Presumably, cytoplasmic Ca(2+) elevation leads to the release of an as-yet unidentified second transmitter from the glial cell to complete transmission. Thus, the two NS of the NGlN trimer communicate via a 'sandwich synapse' transglial pathway, a novel signaling mechanism that may contribute to information transfer in other regions of the nervous system.

Oxytocin hyperpolarizes cultured duodenum myenteric intrinsic primary afferent neurons by opening BK(Ca) channels through IP₃ pathway

Oxytocin (OT) is clinically important in gut motility and constitutively reduces duodenum contractility. Intrinsic primary afferent neurons (IPANs), whose physiological classification is as AH cells, are the 1st neurons of the peristaltic reflex pathway. We set out to investigate if this inhibitory effect is mediated by IPANs and to identify the ion channel(s) and intracellular signal transduction pathway that are involved in this effect. Myenteric neurons were isolated from the longitudinal muscle myenteric plexus (LMMP) preparation of rat duodenum and cultured for 16-24 h before electrophysiological recording in whole cell mode and AH cells identified by their electrophysiological characteristics. The cytoplasmic Ca²⁺ concentration ([Ca²⁺](i) ) of isolated neurons was measured using calcium imaging. The concentration of IP(3) in the LMMP and the OT secreted from the LMMP were measured using ELISA. The oxytocin receptor (OTR) and large-conductance calcium-activated potassium (BK(Ca)) channels, as well as the expression of OT and the IPAN marker calbindin 28 K, on the myenteric plexus neurons were localized using double-immunostaining techniques. We found that administration of OT (10⁻⁷ to 10⁻⁵ M) dose dependently hyperpolarized the resting membrane potential and increased the total outward current. The OTR antagonist atosiban or the BK(Ca) channel blocker iberiotoxin (IbTX) blocked the effects of OT suggesting that the increased outward current resulted from BK(Ca) channel opening. OTR and the BK(Ca) α subunit were co-expressed on a subset of myenteric neurons at the LMMP. NS1619 (10⁻⁵ M, a BK(Ca) channel activator) increased the outward current similar to the effect of OT. OT administration also increased [Ca²⁺](i) and the OT-evoked outward current was significantly attenuated by thapsigargin (10⁻⁶ M) or CdCl₂. The effect of OT on the BK(Ca) current was also blocked by pre-treatment with the IP₃ receptor antagonist 2-APB (10⁻⁴ M) or the PLC inhibitor U73122 (10⁻⁵ M). OT (10⁻⁶ M) also increased the IP₃ concentration within the LMMP. Both of the spontaneous and KCl-induced secretion of OT was enhanced by atosiban. Most of OT-immunoreactive cells are also immunoreactive for calbindin 28 K. In summary, we concluded that OT hyperpolarized myenteric IPANs by activating BK(Ca) channels via the OTR-PLC-IP₃-Ca²⁺ signal pathway. OT might modulate IPANs mediated ENS reflex by an autocrine and negative feedback manner.

Cysteinyl leukotriene 2 receptor-mediated vascular permeability via transendothelial vesicle transport

Cysteinyl leukotrienes (CysLTs) are potent mediators of inflammation synthesized by the concerted actions of 5-lipoxygenase (5-LO), 5-LO-activating protein (FLAP), leukotriene C(4) synthase, and additional downstream enzymes, starting with arachidonic acid substrate. CysLTs produced by macrophages, eosinophils, mast cells, and other inflammatory cells activate 3 different high-affinity CysLT receptors: CysLT(1)R, CysLT(2)R, and GPR 17. We sought to investigate vascular sites of CysLT(2)R expression and the role and mechanism of this receptor in mediating vascular permeability events. Vascular expression of CysLT(2)R was investigated by reporter gene expression in a novel CysLT(2)R deficient-LacZ mouse model. CysLT(2)R was expressed in small, but not large, vessels in mouse brain, bladder, skin, and cremaster muscle. Intravital, in addition to confocal and electron, microscopy investigations using FITC-labeled albumin in cremaster postcapillary venule preparations indicated rapid CysLT-mediated permeability, which was blocked by application of BAY-u9773, a dual CysLT(1)R/CysLT(2)R antagonist or by CysLT(2)R deficiency. Endothelial human CysLT(2)R overexpression in mice exacerbated vascular leakage even in the absence of exogenous ligand. The enhanced vascular permeability mediated by CysLT(2)R takes place via a transendothelial vesicle transport mechanism as opposed to a paracellular route and is controlled via Ca(2+) signaling. Our results reveal that CysLT(2)R can mediate inflammatory reactions in a vascular bed-specific manner by altering transendothelial vesicle transport-based vascular permeability.

Calcium signalling in the endothelium

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

Protein kinase Calpha phosphorylates the TRPC1 channel and regulates store-operated Ca2+ entry in endothelial cells

The TRPC1 (transient receptor potential canonical-1) channel is a constituent of the nonselective cation channel that mediates Ca2+ entry through store-operated channels (SOCs) in human endothelial cells. We investigated the role of protein kinase Calpha (PKCalpha) phosphorylation of TRPC1 in regulating the opening of SOCs. Thrombin or thapsigargin added to the external medium activated Ca2+ entry after Ca2+ store depletion, which we monitored by changes in cellular Fura 2 fluorescence. Internal application of the metabolism-resistant analog of inositol 1,4,5-trisphosphate (IP3) activated an inward cationic current within 1 min, which we recorded using the whole cell patch clamp technique. La3+ or Gd3+ abolished the current, consistent with the known properties of SOCs. Pharmacological (Gö6976) or genetic (kinase-defective mutant) inhibition of PKCalpha markedly inhibited IP3-induced activation of the current. Thrombin or thapsigargin also activated La3+-sensitive Ca2+ entry in a PKCalpha-dependent manner. We determined the effects of a specific antibody directed against an extracellular epitope of TRPC1 to address the functional importance of TRPC1. External application of the antibody blocked thrombin- or IP3-induced Ca2+ entry. In addition, we showed that addithrombin or thapsigargin induced phosphorylation of TRPC1 within 1 min. Thrombin failed to induce TRPC1 phosphorylation in the absence of PKCalpha activation. Phosphorylation of TRPC1 and the resulting Ca2+ entry were essential for the increase in permeability induced by thrombin in confluent endothelial monolayers. These results demonstrate that PKCalpha phosphorylation of TRPC1 is an important determinant of Ca2+ entry in human endothelial cells.

Endothelium-dependent noradrenergic hyperresponsiveness induced by thapsigargin in human saphenous veins: role of thromboxane and calcium

To further investigate the mechanisms which regulate sympathetic vascular tone, we studied the effects of the sarcoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin, on the vasoconstriction induced by transmural nerve stimulation and noradrenaline in superfused human saphenous vein rings. The contractions induced by both transmural nerve stimulation and noradrenaline were potentiated by thapsigargin in endothelium-intact, but not in endothelium-denuded vessels. This potentiation was unaffected by the non-selective endothelin ET(A/B) receptor antagonist, Ro 47-0203 (4-tert-Butyyl-N-[6-(2-hydroxy-ethoxy)-5-(2-methoxy-phenoxy)-2,2'-bipyrimidin-4yl]benzene sulfonamide), or by the nitric oxide (NO) synthase inhibitor, L-NNA (N(omega)-nitro-L-arginine), but was inhibited by the thromboxane A(2) receptor antagonist, Bay u3405 (3(R)-[[(4-flurophenyl) sulphonyl]amino-1,2,3,4-tetrahydro-9H-carbazole-9-propanoic acid]) or by the thromboxane A(2) synthase inhibitor, UK 38485 (3-(1H-imidazol-1-yl-methyl)-2-methyl-1H-indole-1-propanoic acid). Moreover, the thapsigargin-induced noradrenergic hyperresponsiveness, as well as that produced by subthreshold concentrations of the thromboxane A(2) mimetic, U 46619, were blocked by the Ca(2+) channel antagonist, verapamil. In conclusion, our results indicate that thapsigargin enhances the contractions produced by sympathetic nerve stimulation in human saphenous vein rings through the endothelial release of thromboxane A(2) that potentiates the vasoconstriction induced by the noradrenergic mediator with a verapamil-sensitive mechanism.

Role of endothelial Ca2+ stores in the regulation of hydraulic conductivity of Rana microvessels in vivo

Vascular permeability is regulated by endothelial cytosolic Ca(2+) concentration ([Ca(2+)](i)). To determine whether vascular permeability is dependent on extracellular Ca(2+) influx or release of Ca(2+) from stores, hydraulic conductivity (L(p)) was measured in single perfused frog mesenteric microvessels in the presence and absence of Ca(2+) influx and store depletion. Prevention of Ca(2+) reuptake into stores by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibition increased L(p) in the absence of extracellular Ca(2+) influx. L(p) was further increased when Ca(2+) influx was restored. Depletion of the Ca(2+) stores with ionomycin and SERCA inhibition increased L(p) in the presence and the absence of extracellular Ca(2+) influx. However, store depletion in itself did not significantly increase L(p) in the absence of active Ca(2+) release from stores into the cytoplasm. There was a significant positive correlation between baseline permeability and the magnitude of the responses to both Ca(2+) store release and Ca(2+) influx, indicating that the Ca(2+) regulating properties of the endothelial cells may regulate the baseline L(p). To investigate the role of Ca(2+) stores in regulation of L(p), the relationship between SERCA inhibition and store release was studied. The magnitude of the L(p) increase during SERCA inhibition significantly and inversely correlated with that during store release by Ca(2+) ionophore, implying that the degree of store depletion regulates the size of the increase on L(p). These data show that microvascular permeability in vivo can be increased by agents that release Ca(2+) from stores in the absence of Ca(2+) influx. They also show that capacitative Ca(2+) entry results in increased L(p) and that the size of the permeability increase can be regulated by the degree of Ca(2+) release.

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.

Effects of SK&F 96365 and mefenamic acid on Ca2+ influx in stimulated endothelial cells and on endothelium-derived hyperpolarizing factor-mediated arterial hyperpolarization and relaxation

This study was undertaken to assess how Ca2+ influx into endothelial cells via Ca2+-permeable nonselective cation channels (NSCCs) is important in vascular responses mediated by endothelium-derived hyperpolarizing factor (EDHF). In cultured porcine aortic endothelial cells, the sustained increases in the intracellular Ca2+ concentration ([Ca2+]i) elicited by bradykinin and cyclopiazonic acid, which were strongly dependent on the presence of extracellular Ca2+, were suppressed by the NSCC blockers, SK&F 96365 and mefenamic acid. In porcine coronary artery with intact endothelium, bradykinin elicited a rapid fall in the membrane potential, followed by sustained hyperpolarization with a slow decay. In the presence of SK&F 96365 or mefenamic acid, the peak amplitude was severely reduced and the decay phase of hyperpolarization to bradykinin was greatly accelerated, which was apparently similar to the response obtained in Ca2+-free medium. Cyclopiazonic acid caused sustained hyperpolarization in an extracellular Ca2+-dependent manner, an effect which was markedly diminished by SK&F 96365 and mefenamic acid. In rings of coronary artery precontracted with U46619, bradykinin and cyclopiazonic acid produced endothelium-dependent relaxations even in the presence of N(G)-nitro-L-arginine and indomethacin. SK&F 96365 and mefenamic acid significantly attenuated the relaxant responses. These results indicate that the increase in [Ca2+]i of endothelial cells due to Ca2+ entry via NSCCs plays a crucial role in the maintenance of the EDHF-mediated vascular responses.

Bradykinin evokes a Ca2+-activated chloride current in non-neuronal cells isolated from neonatal rat dorsal root ganglia

We have studied the effect of bradykinin (Bk) on fibroblast-like satellite (FLS) cells isolated from cultures of neonatal rat dorsal root ganglia (DRG). In voltage-clamped FLS cells Bk evoked an inward current response that was concentration dependent with a half-maximal concentration of 2 nM. In indo-1 AM-loaded FLS cells Bk evoked a rise in intracellular Ca2+ that was concentration dependent with a half-maximal concentration of 1 nM. The FLS cells still produced an inward current in response to Bk in the absence of extracellular Ca2+ but the response was inhibited if the intracellular concentration of EGTA was increased from 0.5 to 5 mM, which suggests that the inward current was dependent on the release and subsequent rise of intracellular Ca2+. The reversal potential of the Bk-induced inward current was consistent with the current being due to an increase in Cl- conductance and shifted in a Nernstian manner when the intracellular Cl- concentration was reduced. The inward current response to Bk was blocked by the B2 receptor antagonist HOE-140, which indicates that the response was due to activation of B2 receptors. The data suggest that Bk evokes a rise in intracellular Ca2+ and activation of a Ca2+-activated Cl- conductance in the FLS cells and raise the possibility that FLS cells contribute to the pro-inflammatory effects of Bk in DRG.

VEGF and ATP act by different mechanisms to increase microvascular permeability and endothelial [Ca(2+)](i)

Vascular endothelial growth factor (VEGF) increases hydraulic conductivity (L(p)) by stimulating Ca(2+) influx into endothelial cells. To determine whether VEGF-mediated Ca(2+) influx is stimulated by release of Ca(2+) from intracellular stores, we measured the effect of Ca(2+) store depletion on VEGF-mediated increased L(p) and endothelial intracellular Ca(2+) concentration ([Ca(2+)](i)) of frog mesenteric microvessels. Inhibition of Ca(2+) influx by perfusion with NiCl(2) significantly attenuated VEGF-mediated increased [Ca(2+)](i). Depletion of Ca(2+) stores by perfusion of vessels with thapsigargin did not affect the VEGF-mediated increased [Ca(2+)](i) or the increase in L(p). In contrast, ATP-mediated increases in both [Ca(2+)](i) and L(p) were inhibited by thapsigargin perfusion, demonstrating that ATP stimulated store-mediated Ca(2+) influx. VEGF also increased Mn(2+) influx after perfusion with thapsigargin, whereas ATP did not. These data showed that VEGF increased [Ca(2+)](i) and L(p) even when Ca(2+) stores were depleted and under conditions that prevented ATP-mediated increases in [Ca(2+)](i) and L(p). This suggests that VEGF acts through a Ca(2+) store-independent mechanism, whereas ATP acts through Ca(2+) store-mediated Ca(2+) influx.

Role of endothelium in thapsigargin-induced arterial responses in rat aorta

We assessed the role of endothelium in the arterial response to thapsigargin, the Ca(2+)-ATPase inhibitor of the endoplasmic reticulum, in rat isolated aortic rings. Thapsigargin induced an endothelium-dependent relaxation of phenylephrine-contracted aortic rings with an EC(50) of 2.6+/-0.4 nM and a 75% maximum relaxation, while it was less effective against 30 mM K(+)-induced contraction. Pretreatment of aortic rings with N(G)-nitro-L-arginine methyl ester (30 microM) or methylene blue (1 microM) reduced thapsigargin-induced relaxation by approximately 85%. Thapsigargin failed to relax the endothelium-denuded rings. L-Arginine (3 mM) partially, but significantly, antagonized the effect of 30 microM N(G)-nitro-L-arginine methyl ester. Pretreatment with indomethacin (3 microM), glibenclamide (1 microM) or iberiotoxin (100 nM) did not alter the thapsigargin-induced relaxation. In contrast, pretreatment with tetrapentylammonium ions (TPA(+), 1-3 microM) or with 300 microM Ba(2+) suppressed the relaxant response to thapsigargin. TPA(+) (3 microM) also attenuated acetylcholine-induced relaxation. Thapsigargin-induced endothelium-dependent relaxation was primarily dependent on the presence of extracellular Ca(2+). Interestingly, when the tissues were exposed to very low concentrations of thapsigargin (1-3 nM) the nitric oxide-dependent relaxation induced by acetylcholine or A23187 was markedly reduced. While thapsigargin (3 nM) did not influence the relaxation induced by endothelium-independent dilators, sodium nitroprusside and verapamil. These results indicate that thapsigargin produced complex vascular effects primarily by acting on the endothelial cells. Thapsigargin causes an endothelial nitric oxide-dependent relaxation; on the other hand, it inhibits nitric oxide-mediated relaxation at the similar concentrations. Activation of TPA(+)- and Ba(2+)-sensitive but not Ca(2+)-activated or ATP-sensitive K(+) channels may be also involved in thapsigargin-induced relaxation of rat isolated aortic rings.

Na(+)/Ca(2+) exchange facilitates Ca(2+)-dependent activation of endothelial nitric-oxide synthase

Recent evidence suggests the expression of a Na(+)/Ca(2+) exchanger (NCX) in vascular endothelial cells. To elucidate the functional role of endothelial NCX, we studied Ca(2+) signaling and Ca(2+)-dependent activation of endothelial nitric-oxide synthase (eNOS) at normal, physiological Na(+) gradients and after loading of endothelial cells with Na(+) ions using the ionophore monensin. Monensin-induced Na(+) loading markedly reduced Ca(2+) entry and, thus, steady-state levels of intracellular free Ca(2+) ([Ca(2+)](i)) in thapsigargin-stimulated endothelial cells due to membrane depolarization. Despite this reduction of overall [Ca(2+)](i), Ca(2+)-dependent activation of eNOS was facilitated as indicated by a pronounced leftward shift of the Ca(2+) concentration response curve in monensin-treated cells. This facilitation of Ca(2+)-dependent activation of eNOS was strictly dependent on the presence of Na(+) ions during treatment of the cells with monensin. Na(+)-induced facilitation of eNOS activation was not due to a direct effect of Na(+) ions on the Ca(2+) sensitivity of the enzyme. Moreover, the effect of Na(+) was not related to Na(+) entry-induced membrane depolarization or suppression of Ca(2+) entry, since neither elevation of extracellular K(+) nor the Ca(2+) entry blocker 1-(beta-[3-(4-methoxyphenyl)-propoxy]-4-methoxyphenethyl)-1H-imidazol e hydrochloride (SK&F 96365) mimicked the effects of Na(+) loading. The effects of monensin were completely blocked by 3', 4'-dichlorobenzamil, a potent and selective inhibitor of NCX, whereas the structural analog amiloride, which barely affects Na(+)/Ca(2+) exchange, was ineffective. Consistent with a pivotal role of Na(+)/Ca(2+) exchange in Ca(2+)-dependent activation of eNOS, an NCX protein was detected in caveolin-rich membrane fractions containing both eNOS and caveolin-1. These results demonstrate for the first time a crucial role of cellular Na(+) gradients in regulation of eNOS activity and suggest that a tight functional interaction between endothelial NCX and eNOS may take place in caveolae.

Properties of heterologously expressed hTRP3 channels in bovine pulmonary artery endothelial cells

1. We combined patch clamp and fura-2 fluorescence methods to characterize human TRP3 (hTRP3) channels heterologously expressed in cultured bovine pulmonary artery endothelial (CPAE) cells, which do not express the bovine trp3 isoform (btrp3) but express btrp1 and btrp4. 2. ATP, bradykinin and intracellular InsP3 activated a non-selective cation current (IhTRP3) in htrp3-transfected CPAE cells but not in non-transfected wild-type cells. During agonist stimulation, the sustained rise in [Ca2+]i was significantly higher in htrp3-transfected cells than in control CPAE cells. 3. The permeability for monovalent cations was PNa > PCs approximately PK >> PNMDG and the ratio PCa/PNa was 1.62 +/- 0.27 (n = 11). Removal of extracellular Ca2+ enhanced the amplitude of the agonist-activated IhTRP3 as well as that of the basal current The trivalent cations La3+ and Gd3+ were potent blockers of IhTRP3 (the IC50 for La3+ was 24.4 +/- 0.7 microM). 4. The single-channel conductance of the channels activated by ATP, assessed by noise analysis, was 23 pS. 5. Thapsigargin and 2,5-di-tert-butyl-1, 4-benzohydroquinone (BHQ), inhibitors of the organellar Ca2+-ATPase, failed to activate IhTRP3. U-73122, a phospholipase C blocker, inhibited IhTRP3 that had been activated by ATP and bradykinin. Thimerosal, an InsP3 receptor-sensitizing compound, enhanced IhTRP3, but calmidazolium, a calmodulin antagonist, did not affect IhTRP3. 6. It is concluded that hTRP3 forms non-selective plasmalemmal cation channels that function as a pathway for agonist-induced Ca2+ influx.

Possible involvement of Ca2+ entry and its pharmacological characteristics responsible for endothelium-dependent, NO-mediated relaxation induced by thapsigargin in guinea-pig aorta

Thapsigargin, a specific inhibitor of Ca(2+)-pump Ca(2+)-ATPase in the sarcoplasmic/endoplasmic reticulum (SR/ER), produces an endothelium-dependent vascular relaxation. In the present study, pharmacological features of thapsigargin-induced endothelium-dependent relaxation were functionally characterized in the isolated guinea-pig aorta especially focusing on the Ca2+ mobilization mechanisms in endothelial cells. Thapsigargin-induced endothelium-dependent vascular relaxation was markedly suppressed by N(G)-nitro-L-arginine (L-NNA) and calmidazolium, suggesting that the vascular relaxation to thapsigargin is largely attributable to endothelium-derived nitric oxide (NO) produced as a result of the activation of Ca2+, calmodulin-dependent NO synthase (NOS). Removal of Ca2+ from the external solution abolished the endothelium-dependent relaxation of guinea-pig aorta in response to thapsigargin. Thapsigargin-induced endothelium-dependent relaxation was inhibited more strongly compared with the endothelium-independent relaxation to an NO donor, SIN-1 (3-(4-morpholinyl)-sydnonimine), when the artery preparation was preconstricted with a high concentration (80 mM) of KCl instead of agonistic stimulation. Endothelium-dependent relaxation induced by thapsigargin was not affected by diltiazem, a blocker of L-type voltage-gated Ca2+ channels. SK&F96365 (1-[beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl]-1 H-imidazole) and Ni2+, both of which block capacitative Ca(2+) entry, did not show any appreciable inhibitory effects on the endothelium-dependent relaxation to thapsigargin. These findings suggest that in guinea-pig aorta, endothelium-dependent NO-mediated relaxation induced by thapsigargin is preceded by the increase in the cytosolic free Ca2+ concentrations ([Ca2+]cyt) following the depletion of stored Ca2+ in thapsigargin-sensitive store sites in endothelial cells. Although the increase in [Ca2+]cyt responsible for the activation of endothelium NOS leading to thapsigargin-induced vascular relaxation may be ascribed to the capacitative Ca2+ entry from extracellular space, the Ca2+ entry mechanism stimulated with thapsigargin is deficient in sensitivity to SK&F96365 and Ni2+ in the endothelium of guinea-pig aorta.

Evidence against a major role for Ca2+ in hypoxia-induced gene expression in human hepatoma cells (Hep3B)

1. The human hepatoma cell line Hep3B is a widely used model for studies of hypoxia-related gene expression. Cytosolic free calcium concentration ([Ca2+]i) has been implicated in cellular oxygen-sensing processes. We investigated whether calcium ions have a significant impact on the production of erythropoietin (EPO) and vascular endothelial growth factor (VEGF). 2. We found that the calcium ionophore ionomycin induced a rapid and sustained increase of [Ca2+]i while thapsigargin, an inhibitor of endoplasmic reticulum calcium ATPase, only caused a 20 % elevation of [Ca2+]i within 10 min after application. However, the calcium content of intracellular stores was considerably reduced by thapsigargin after an incubation period of 24 h. 3. Variations in [Ca2+]o did not result in altered EPO or VEGF secretion rates. Ionomycin decreased EPO production while the lowering of VEGF production was not statistically significant. In the presence of extracellular Ca2+ the membrane permeant calcium chelator BAPTA-AM stimulated the production of EPO (P < 0.05) but not of VEGF while EGTA-AM, a closely related agent, affected neither EPO nor VEGF formation under these conditions. Incubation with thapsigargin resulted in decreased EPO synthesis (P < 0.05) but stimulated VEGF secretion (P < 0.05). 4. In the absence of extracellular calcium, EGTA-AM led to an accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha). This treatment significantly stimulated VEGF synthesis but also decreased EPO secretion (P < 0.05). 5. Our data suggest that the calcium transient and the cytosolic Ca2+ concentration do not play a key role in hypoxia-induced EPO and VEGF production in Hep3B cells.

Agonist-stimulated calcium entry in primary cultures of human cerebral microvascular endothelial cells

Primary cultures of human cerebral microvascular endothelial cells (HCMEC) were loaded with fura-2. The intracellular free Ca2+ concentration ([Ca2+]i) was measured by digital imaging microscopy. Agonists ATP (100 micro), thrombin (10 units/ml), and histamine (25 microM) induced a transient [Ca2+]i increase. Histamine (100 microM) induced a biphasic [Ca2+]i increase with an initial [Ca2+]i peak followed by a [Ca2+]i plateau. The [Ca2+]i plateau was blocked by the receptor-operated Ca2+ channel (ROC) blockers SK&F 96365 and NCDC, indicating a contribution by Ca2+ influx through ROC to the [Ca2+]i plateau. However, this [Ca2+]i plateau was not blocked by the voltage-gated Ca2+ channel (VGC) blocker diltiazem (DTZ). Depolarization with 80K+ or application of the VGC agonist BAY K 8644 did not alter the resting [Ca2+]i; but 80K+ reduced the histamine (100 microM) induced [Ca2+]i plateau. These results show that HCMEC are devoid of functional VGC. Thus the membrane potential (Em) regulates Ca2+ entry mainly by enhancing the electrochemical Ca2+ gradient, such that hyperpolarization increases while depolarization decreases [Ca2+]i. Blockade of sarcoplasmic/endoplasmic reticulum Ca2+-ATPase (SERCA) by CPA increased [Ca2+]i. This effect was dependent on extracellular Ca2+ and reduced by iberiotoxin (IBTX) blockade of Ca2+-activated K+ channels (Kca), suggesting a role for Kca in regulating Ca2+ influx. Ca2+ is the principal activator of endothelial nitric oxide synthase (eNOS), which stimulates cyclic GMP production. The final result that the eNOS inhibitor L-NAME enhanced the histamine (100 microM) induced [Ca2+]i plateau suggests a negative feedback loop (via cGMP) of endothelial NO on its own synthesis in the regulation of endothelial [Ca2+]i signal.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Relationship between NaF- and thapsigargin-induced endothelium-dependent hyperpolarization in rat mesenteric artery

1. In isolated rat mesenteric artery with endothelium, NaF caused slowly developing hyperpolarization. The hyperpolarizing effect was unchanged in the presence of N(G)-nitro-L-arginine (L-NOARG) and indomethacin, but was markedly reduced by high K+. In Ca2+ -free medium or in the presence of Ni2+, NaF failed to produce hyperpolarization. 2. NaF-induced hyperpolarization was substantially unaffected by deferoxamine, an Al3+ chelator, okadaic acid and calyculin A, phosphatase inhibitors, and preincubation with pertussis toxin, suggesting that neither the action of fluoroaluminates as a G protein activator nor inhibition of phosphatase activity contributes to the hyperpolarizing effect. 3. The selective inhibitors of the Ca2+ -pump ATPase of endoplasmic reticulum, thapsigargin and cyclopiazonic acid, elicited hyperpolarization, whose properties were very similar to those of NaF. When intracellular Ca2+ stores had been depleted with these inhibitors, NaF no longer generated hyperpolarization. 4. In Ca2+ -free medium, NaF (or thapsigargin) caused a transient increase in the cytosolic Ca2+ concentration ([Ca2+]i) in cultured porcine aortic endothelial cells, and subsequent application of thapsigargin (or NaF) failed to increase [Ca2+]i. 5. In arterial rings precontracted with phenylephrine, NaF produced endothelium-dependent relaxation followed by sustained contraction even in the presence of L-NOARG and indomethacin. The relaxant response was abolished by high K+ or cyclopiazonic acid. 6. These results indicate that NaF causes endothelium-dependent hyperpolarization, thereby leading to smooth muscle relaxation of rat mesenteric artery. This action appears to be mediated by the promotion of Ca2+ influx into endothelial cells that can be triggered by the emptying of intracellular Ca2+ stores, as proposed for those of thapsigargin and cyclopiazonic acid.

Differential modulation of the phases of a Ca2+ spike by the store Ca2+-ATPase in human umbilical vein endothelial cells

1. Histamine-stimulated cytosolic free Ca2+ ([Ca2+]i) oscillations in human umbilical vein endothelial cells (HUVECs) comprise repetitive spikes generated by pulsatile release from stores. We have investigated the roles of the store Ca2+-ATPases in regulating both the upstroke and downstroke of a Ca2+ spike. 2. The sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) inhibitor cyclopiazonic acid (CPA) dramatically affected oscillations whereas inhibition of the plasma membrane Ca2+-ATPase (PMCA) with La3+ had little effect. This and other evidence suggested that the downstroke of a spike is predominantly mediated by SERCA. 3. Artificial [Ca2+]i spiking generated by repetitive pulsatile application of 0.3 microM histamine in Ca2+-free medium did not cause net loss of Ca2+ from the cell whereas repetitive pulsatile application of 1 and 10 microM histamine did, with the higher concentration being more effective. We conclude that there is an inverse relationship between stimulus intensity and relative SERCA activity. 4. For a Ca2+ transient, the initiation of release was suppressed by SERCA during either the lag phase or the interspike period (ISP) since: (i) the ISP was shortened by low CPA concentrations, (ii) higher concentrations of CPA stimulated an explosive Ca2+ release when applied during the ISP but not when applied in the absence of agonist, and (iii) CPA synchronized the initial Ca2+ response to a low histamine dose (even recruiting silent, histamine-unresponsive cells). 5. Two aspects of the regenerative upstroke of a spike were differently affected by SERCA inhibition: Ca2+ wave velocity was entirely unaffected by CPA whereas the local rate of rise was increased. 6. The [Ca2+]i at which a Ca2+ spike terminated depended on SERCA since CPA dose dependently enhanced the peak [Ca2+]i. 7. We conclude that SERCA plays a powerful and dynamic role in regulating [Ca2+]i oscillations in HUVECs. SERCA differentially modulates the phases of Ca2+ release in addition to bringing about the falling phase of a Ca2+ spike.

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.

Mass spectrometric evidence that agents that cause loss of Ca2+ from intracellular compartments induce hydrolysis of arachidonic acid from pancreatic islet membrane phospholipids by a mechanism that does not require a rise in cytosolic Ca2+ concentration

Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may amplify the glucose-induced Ca2+ entry into islet beta-cells that triggers insulin secretion. Ca2+ loss from beta-cell intracellular compartments has been proposed to induce both Ca2+ entry and events dependent on arachidonate metabolism. We examine here effects of inducing Ca2+ loss from intracellular sequestration sites with ionophore A23187 and thapsigargin on arachidonate hydrolysis from islet phospholipids. A23187 induces a decline in islet arachidonate-containing phospholipids and release of nonesterified arachidonate. A23187-induced arachidonate release is of similar magnitude when islets are stimulated in Ca2+-replete or in Ca2+-free media or when islets loaded with the intracellular Ca2+ chelator BAPTA are stimulated in Ca2+-free medium, a condition in which A23187 induces no rise in beta-cell cytosolic [Ca2+]. Thapsigargin also induces islet arachidonate release under these conditions. A23187- or thapsigargin-induced arachidonate release is prevented by a bromoenol lactone (BEL) inhibitor of a beta-cell phospholipase A2 (iPLA2), which does not require Ca2+ for catalytic activity and which is negatively modulated by and physically interacts with calmodulin by Ca2+-dependent mechanisms. Agents that cause Ca2+ loss from islet intracellular compartments thus induce arachidonate hydrolysis from phospholipids by a BEL-sensitive mechanism that does not require a rise in cytosolic [Ca2+], and a BEL-sensitive enzyme-like iPLA2 or a related membranous activity may participate in sensing Ca2+ compartment content.

Blockade by 18beta-glycyrrhetinic acid of intercellular electrical coupling in guinea-pig arterioles

1. Intercellular electrical communication between smooth muscle and endothelial cells was examined in guinea-pig mesenteric arterioles using the whole-cell patch-clamp method. The time course of the current required to impose a 10 mV voltage clamp step was used to determine the extent of electrical coupling between them. Currents recorded from both smooth muscle and endothelial cells relaxed in a multi-exponential manner, indicating the existence of electrical coupling between cells. 2. 18beta-Glycyrrhetinic acid, a gap junction blocker, quickly blocked electrical communication at 40 microM, while neither heptanol nor octanol did so at concentrations of up to 1 mM. 3. In the current clamp mode, repetitive spikes, induced by 10 mM Ba2+ solutions, could be recorded from both kinds of cells. After blocking gap junctions, spikes could only be recorded from the smooth muscle cell layer, indicating that they had been conducted through myoendothelial junctions. 4. In endothelial cells, acetylcholine (ACh, 3 microM) induced hyperpolarizing responses, which had two phases (an initial fast and a second slower phase) in the current clamp condition. This ACh response persisted in the presence of 18beta-glycyrrhetinic acid, although this compound seemed to make the membrane slightly leaky. 5. After blocking gap junctions, the membrane potential of a single cell in a multicellular preparation could be well clamped. Thus, 18beta-glycyrrhetinic acid may be useful in studying the function of both arteriolar smooth muscle and endothelial cells while they remain located within a multicellular preparation.

Store-operated calcium entry promotes shape change in pulmonary endothelial cells expressing Trp1

Activation of Ca2+ entry is known to produce endothelial cell shape change, leading to increased permeability, leukocyte migration, and initiation of angiogenesis in conduit-vessel endothelial cells. The mode of Ca2+ entry regulating cell shape is unknown. We hypothesized that activation of store-operated Ca2+ channels (SOCs) is sufficient to promote cell shape change necessary for these processes. SOC activation in rat pulmonary arterial endothelial cells increased free cytosolic Ca2+ that was dependent on a membrane current having a net inward component of 5.45 +/- 0.90 pA/pF at -80 mV. Changes in endothelial cell shape accompanied SOC activation and were dependent on Ca2+ entry-induced reconfiguration of peripheral (cortical) filamentous actin (F-actin). Because the identity of pulmonary endothelial SOCs is unknown, but mammalian homologues of the Drosophila melanogaster transient receptor potential (trp) gene have been proposed to form Ca2+ entry channels in nonexcitable cells, we performed RT-PCR using Trp oligonucleotide primers in both rat and human pulmonary arterial endothelial cells. Both cell types were found to express Trp1, but neither expressed Trp3 nor Trp6. Our study indicates that 1) Ca2+ entry in pulmonary endothelial cells through SOCs produces cell shape change that is dependent on site-specific rearrangement of the microfilamentous cytoskeleton and 2) Trp1 may be a component of pulmonary endothelial SOCs.

Depletion of intracellular Ca2+ stores or lowering extracellular calcium alters intracellular Ca2+ changes during cerebral energy deprivation

Cytoplasmatic calcium concentrations are elevated two to three fold during cerebral ischemia. In order to determine the role of calcium-release from intracellular stores vs. calcium entry from the extracellular space, intracellular stores were depleted by the use of thapsigargin and calcium was removed from the incubation fluid prior to energy deprivation (ED). CA 1 pyramidal neurons in hippocampal rat slices were filled with a 1:2 mixture of Fluo-3 and Fura Red by intracellular injection. The neurons were visualized in a Confocal Laser Scanning Microscope (CLSM) and the fluorescence ratio from the probe mixture was used to quantify the calcium concentration. Intracellular calcium concentration was monitored before and during ED. The intracellular calcium concentration was 55 nM prior to ED and increased to 25 microM during ED. The resting levels were the same in the experimental groups, but the increase during ED was significantly lower in the intervention groups. The increase in the calcium free group was to 1 microM and in the thapsigargin group to 5 microM. In the last experimental group, thapsigargin treatment and removal of extracellular calcium, the intracellular calcium increased to 630 nM. These results demonstrate that the increased intracellular calcium seen during ED originates from several sources. Calcium-release from intracellular stores may be of major importance in calcium-related neuronal injury during cerebral ischemia.

The tachykinin NK1 receptor antagonist, RP67580, inhibits the bradykinin-induced rise in intracellular Ca2+ concentration in bovine pulmonary artery endothelial cells

The bradykinin-induced rise in intracellular Ca2+ concentration ([Ca2+]i) and the bradykinin receptor involved in this response were characterized in bovine pulmonary artery endothelial cells. It was found that bradykinin induces an intracellular biphasic Ca2+ response, consisting of a transient peak followed by an elevated plateau phase. Both bradykinin and the bradykinin B1 receptor agonist, des-Arg9-bradykinin, induced a concentration-dependent increase in [Ca2+]i, but the bradykinin-induced rise was much greater. Moreover, the bradykinin-induced [Ca2+]i rise could be inhibited by the bradykinin B2 receptor antagonists, D-Arg0[Hyp3, Thi(5,8), D-Phe7]bradykinin and Hoe 140 (D-Arg[Hyp3, Thi5, D-Tic7, Oic8]bradykinin), but not by the bradykinin B1 receptor antagonist, des-Arg9-[Leu8]bradykinin. From these results it can be concluded that a bradykinin B2 receptor is involved in this response. Furthermore, we found that the tachykinin NK1 receptor antagonist, RP67580 ([imino 1 (methoxy-2-phenyl)-2 ethyl]-2 diphenyl 7,7 perhydroisoindolone-4 (3aR, 7aR)), and its negative enantiomer, RP68651 (2-[1-imino 2-(2 methoxy phenyl) ethyl] 7,7 diphenyl 4-perhydroisoindolone (3aS-7aS)), could inhibit the bradykinin-induced [Ca2+]i response, although no functional tachykinin NK1 receptors were found. Binding studies evidenced no binding of RP67580 or RP68651 to the bradykinin receptor. We conclude that RP67580 inhibits the bradykinin-induced rise in [Ca2+]i via a bradykinin B2 receptor-independent mechanism.

Regulation of bradykinin-stimulated cation entry into endothelial cells by tyrosine kinase

In endothelial cells, bradykinin stimulates the release of intracellular Ca2+, which is followed by the entry of extracellular Ca2+ into the cells. However, the mechanism underlying the Ca2+ entry is not well understood. To investigate the possible implication of tyrosine kinases in bradykinin-mediated Ca2+ signaling in endothelial cells, cultured porcine aortic endothelial cells were loaded with fura-2/AM, and Mn2+ influx into the cells was determined by the quenching of fluorescence intensity of fura-2 at 360 nm excitation. The tyrosine kinase inhibitors genistein and herbimycin A attenuated not only Ca2+ influx but also Mn2+ influx from the extracellular space without affecting the release of Ca2+ from internal stores in bradykinin-treated cells. In contrast to tyrosine kinase inhibitors, the tyrosine phosphatase inhibitor vanadate stimulated Ca2+ influx as well as Mn2+ influx. On the other hand, both an inactive analog of genistein, daidzein, and an inhibitor of diacylglycerol (DAG) kinase, ethylene glycol dioctanoate, were without effect on [Ca2+]i following the stimulation of agonist. These findings suggest that tyrosine kinase is involved in the regulation of cation influx in endothelial cells.

Capacitative Ca2+ influx in glial cells is inhibited by glycolytic inhibitors

In non-excitable cells, stimulation of phosphoinositide (PI) turnover and inhibition of the endoplasmic reticulum (ER) Ca2+-ATPase are methods commonly used to deplete calcium stores, resulting in a capacitative Ca2+ influx (i.e., Ca2+ release-activated Ca2+ influx). Since this Ca2+ influx in glial cells has not been thoroughly investigated, we have used C6 glioma cells as a glial cell model to study this phenomenon. On adding cyclopiazonic acid (CPA) or thapsigargin (TG) (two ER Ca2+-ATPase inhibitors) in Ca2+-free medium, only a small transient increase in intracellular Ca2+ was seen. After depletion of the stored Ca2+, a marked Ca2+ influx, followed by a prolonged plateau, was seen on re-addition of extracellular Ca2+ ions (2 mM), i.e., capacitative Ca2+ influx. A similar effect was seen on adding ATP, known to deplete the inositol triphosphate (IP3)-sensitive Ca2+ store in C6 cells. After various degrees of store depletion, the amplitude of the capacitative Ca2+ influx was found to be highly dependent on the amount of Ca2+ remaining in the store. This Ca2+ influx was markedly inhibited by (1) La3+ and Ni2+, (2) SK&F 96365, econazole, and miconazole, and (3) membrane depolarization, clearly showing that this Ca2+ influx after store depletion in C6 cells is a capacitative mechanism. Interestingly, the capacitative Ca2+ influx can be inhibited by a reduction in intracellular ATP (ATPi) levels in glial cells. The role of ATPi in the capacitative Ca2+ influx is discussed.

Involvement of calcium and G proteins in the acute release of tissue-type plasminogen activator and von Willebrand factor from cultured human endothelial cells

In this study, we investigated the role of Ca2+ and G proteins in thrombin-induced acute release (regulated secretion) of tissue-type plasminogen activator (TPA) and von Willebrand factor (vWF), using a previously described system of primary human umbilical vein endothelial cells (HUVECs). The acute release of TPA and vWF, as induced by alpha-thrombin, was almost zero after chelation of Ca2+i, showing that an increase in [Ca2+]i was required. It did not matter whether the increase in [Ca2+]i came from an intracellular or extracellular Ca2+ source. Thrombin-induced release of TPA and vWF already started at low [Ca2+]i, around 100 nmol/L. Half-maximal release was found at a [Ca2+]i, of 261 nmol/L for TPA and at 222 nmol/L for vWF. The Ca2+ signal was transduced to calmodulin, as calmodulin inhibitors inhibited TPA and vWF release. The Ca2+ ionophore ionomycin dose dependently released vWF; half-maximal vWF release occurred at a [Ca2+]i of 311 nmol/L. In contrast, no TPA release was found at all below a [Ca2+]i of 500 nmol/L. Thus, below 500 nmol/L [Ca2+]i, an increase in [Ca2+]i alone was sufficient to induce vWF release but not sufficient to induce TPA release. Protein kinase C did not appear to be involved in TPA or vWF release, as neither an activator nor an inhibitor of protein kinase C significantly influenced release. Inhibition of phospholipase A2 also did not reduce thrombin-induced TPA and vWF release. The involvement of G proteins was studied by using both saponin-permeabilized and intact cells. GDP-beta-S, which inhibits heterotrimeric and small G proteins, significantly inhibited thrombin-induced vWF and TPA release from permeabilized cells. AlF-4, which activates heterotrimeric G proteins, induced TPA and vWF release in both intact and permeabilized HUVECs. Preincubation of HUVECs with pertussis toxin significantly inhibited thrombin-induced vWF release, due to inhibition of thrombin-induced Ca2+ influx. Pertussis toxin did not affect ionomycin-induced release. The inhibitory effect of pertussis toxin was less obvious in thrombin-induced TPA release, because it was counterbalanced by a positive effect of the toxin on TPA release. Thus, both inhibitory and stimulatory (pertussis toxin-sensitive) G proteins were involved in TPA release. Therefore, thrombin-induced acute release of TPA and vWF differed in two respects. First, below a [Ca2+]i of 500 nmol/L, an increase in Ca2+ was sufficient for vWF release but not for TPA release. Second, pertussis toxin-sensitive G proteins were differentially involved in acute TPA and vWF release.

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..

Energy turnover of vascular endothelial cells

Two noninvasive methods, calorimetry and 31P nuclear magnetic resonance (NMR), were used to further define energy-consuming and energy-providing reactions in endothelial cells. With 31P-NMR, cellular ATP content was measured; with calorimetry, heat flux as a result of ATP turnover was measured. For these measurements, pig aortic endothelial cells were cultured on microcarrier beads and perfused in a column at constant flow rate. Pig aortic endothelial cells synthesize ATP mainly through glycolysis and, as determined by NMR, contain no phosphocreatine. In such a system, calorimetry-measured heat flux reflects rate of cellular ATP turnover. By use of inhibitors of ATP-dependent processes, the following changes in basal heat flux (231 +/- 65.5 microW/mg protein) were obtained: 18% for 2,3-butanedione monoxime (inhibitor of actomyosin-ATPase), 17% for wortmannin (inhibitor of myosin light chain kinase), 10% for cytochalasin D (inhibitor of actin polymerization), 23% for cycloheximide (inhibitor of protein synthesis), 11% for thapsigargin (inhibitor of endoplasmic reticulum Ca(2+)-ATPase), and 6% for bafilomycin A1 (inhibitor of lysosomal H(+)-ATPase). Cytochalasin D, 2,3-butanedione monoxime, wortmannin, and thapsigargin caused changes in F-actin distribution, as revealed by rhodamine-phalloidin cytochemistry. In a separate experimental series, when cells were perfused with a medium containing no glucose, heat flux decreased by 40% while cellular ATP remained unchanged. Inhibition of glycolysis with 2-deoxy-D-glucose decreased heat flux by 73%, and ATP was no longer visible with 31P-NMR. Despite this massive ATP depletion, which was maintained for 3 h, cells fully recovered heat flux and ATP when 2-deoxy-D-glucose was removed. The results, together with previously published data for Na(+)-K(+)-ATPase [M. L. H. Gruwel, C. Alves, and J. Schrader. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H351-H358, 1995], demonstrate that > 70% of total ATP-consuming processes of endothelial cells can be attributed to specific cellular processes. Actomyosin-ATPase (18%) and protein synthesis (23%) comprise the largest fraction. At least three-fourths of ATP synthesized is provided by glycolysis. Endothelial cells exhibit the remarkable ability to coordinate downregulation of ATP synthesis and consumption when glycolysis is inhibited.

Transient Ca2+ changes in endothelial cells induced by low doses of reactive oxygen species: role of hydrogen peroxide

Cultured human and rat endothelial cells were used to study cellular toxicity and Ca2+ signalling upon exposure to reactive oxygen species. Superoxide and hydrogen peroxide (O2.-/H2O2) were produced by the hypoxanthine/xanthine oxidase system (HX/XO) and caused intracellular Ca2+ concentration ([Ca2+]i) to rise steadily when activities above 2 mU/ml were used. These Ca2+ increases were also measured when the glucose/glucose oxidase (G/GO) system above 5 mU/ml was used to produce hydrogen peroxide (H2O2). Gross morphological changes appeared to parallel elevated [Ca2+]i levels preceding cell death. However, when HX/XO or G/GO were used at non toxic doses rapid and transient changes in [Ca2+]i were measured. These treatments did not alter subsequent receptor mediated Ca2+ signalling induced by ATP (10 microM) or histamine (100 microM). Superoxide dismutase (50 U/ml), which dismutates O2.- into H2O2 also had no influence, whereas catalase (50 U/ml), which removes H2O2, completely diminished transient [Ca2+]i responses. H2O2 added directly was able to induce similar Ca2+ transients when concentrations of at least 500 microM were used. Buffering trace amounts of iron (o-phenanthroline; 200 microM) in order to inhibit .OH radical formation was not effective to alter Ca2+ changes. Experiments performed in Ca(2+)-free buffer showed a similar rise in [Ca2+]i and readdition of Ca2+ to the extracellular medium indicated the activation of store operated Ca2+ entry. Blocking Ca(2+)-ATPases of the endoplasmatic reticulum with thapsigargin (1 microM) inhibited ROS induced transient increases and cells preincubated with pertussis toxin (200 nM) showed unchanged Ca2+ transients after exposure to both enzyme systems. Phospholipase C inhibitor U73122 (2 microM) effectively reduced hydrogen peroxide induced emptying of intracellular stores. Taken together, we demonstrate that enzymatically produced non-toxic H2O2 rather than O2.- or .OH causes calcium signalling from thapsigargin sensitive stores, and activates store operated Ca2+ entry at least partially by activating phospholipase C. These changes clearly differ from pathological 'oxidative stress' associated with a progressive increase in [Ca2+]i.

Cellular uptake of lead is activated by depletion of intracellular calcium stores

The mechanisms of cellular lead uptake were characterized using a fluorescence method in cells loaded with indo-1. Pb2+ bound to intracellular indo-1 with much higher affinity than Ca2+ and quenched fluorescence at all wavelengths. Pb2+ uptake into pituitary GH3 cells, glial C6 cells, and a subclone of HEK293 cells was assessed by fluorescence quench at a Ca2+-insensitive emission wavelength. Pb2+ uptake was concentration- and time-dependent. Pb2+ uptake in all three cell types occurred at a much faster rate when intracellular Ca2+ stores were depleted by two different methods: addition of drugs that inhibit the endoplasmic reticulum Ca2+ pump (thapsigargin, cyclopiazonic acid, and tert-butylhydroquinone), and prolonged incubation of cells in Ca2+-free media. Application of receptor agonists, which deplete intracellular Ca2+ stores via inositol trisphosphate-sensitive channels, did not activate Pb2+ uptake. Agonists were just as effective as thapsigargin in stimulating uptake of Ca2+ but less so in stimulating uptake of Mn2+. Basal and stimulated Pb2+ uptake were partially reduced by 1 mM extracellular Ca2+ and strongly inhibited by 10 mM Ca2+. Pb2+ entry in GH3 cells was inhibited by two drugs that block capacitative Ca2+ entry, La3+ and SK&F 96365. Depolarization of electrically excitable GH3 cells increased the initial rate of Pb2+ uptake 1.6-fold, whereas thapsigargin increased uptake 12-fold. In conclusion, Pb2+ crosses the plasma membrane of GH3, C6, and HEK293 cells via channels that are activated by profound depletion of intracellular Ca2+ stores.

Sources of Ca2+ in relation to generation of acetylcholine-induced endothelium-dependent hyperpolarization in rat mesenteric artery

1. The aim of the present study was to identify the sources of Ca2+ contributing to acetylcholine (ACh)-induced release of endothelium-derived hyperpolarizing factor (EDHF) from endothelial cells of rat mesenteric artery and to assess the pathway involved. The changes in membrane potentials of smooth muscles by ACh measured with the microelectrode technique were evaluated as a marker for EDHF release. 2. ACh elicited membrane hyperpolarization of smooth muscle cells in an endothelium-dependent manner. The hyperpolarizing response was not affected by treatment with 10 microM indomethacin, 300 microM NG-nitro-L-arginine or 10 microM oxyhaemoglobin, thereby indicating that the hyperpolarization is not mediated by prostanoids or nitric oxide but is presumably by EDHF. 3. In the presence of extracellular Ca2+, 1 microM ACh generated a hyperpolarization composed of the transient and sustained components. By contrast, in Ca(2+)-free medium, ACh produced only transient hyperpolarization. 4. Pretreatment with 100 nM thapsigargin and 3 microM cyclopiazonic acid, endoplasmic reticulum Ca(2+)-ATPase inhibitors, completely abolished ACh-induced hyperpolarization. Pretreatment with 20 mM caffeine also markedly attenuated ACh-induced hyperpolarization. However, the overall pattern and peak amplitude of hyperpolarization were unaffected by pretreatment with 1 microM ryanodine. 5. In the presence of 5 mM Ni2+ or 3 mM Mn2+, the hyperpolarizing response to ACh was transient, and the sustained component of hyperpolarization was not observed. On the other hand, 1 microM nifedipine had no effect on ACh-induced hyperpolarization. 6. ACh-induced hyperpolarization was nearly completely eliminated by 500 nM U-73122 or 200 microM 2-nitro-4-carboxyphenyl-N, N-diphenylcarbamate, inhibitors of phospholipase C, but was unchanged by 500 nM U-73343, an inactive form of U-73122. Pretreatment with 20 nM staurosporine, an inhibitor of protein kinase C, did not modify ACh-induced hyperpolarization. 7. These results indicate that the ACh-induced release of EDHF from endothelial cells of rat mesenteric artery is possibly initiated by Ca2+ release from inositol 1,4,5-trisphosphate (IP3)-sensitive Ca2+ pool as a consequence of stimulation of phospholipid hydrolysis due to phospholipase C activation, and maintained by Ca2+ influx via a Ni(2+)- and Mn(2+)-sensitive pathway distinct from L-type Ca2+ channels. The Ca(2+)-influx mechanism seems to be activated following IP3-induced depletion of the pool.

Ion channels in vascular endothelium

The functional impact of ion channels in vascular endothelial cells (ECs) is still a matter of controversy. This review describes different types of ion channels in ECs and their role in electrogenesis, Ca2+ signaling, vessel permeability, cell-cell communication, mechano-sensor functions, and pH and volume regulation. One major function of ion channels in ECs is the control of Ca2+ influx either by a direct modulation of the Ca2+ influx pathway or by indirect modulation of K+ and Cl- channels, thereby clamping the membrane at a sufficiently negative potential to provide the necessary driving force for a sustained Ca2+ influx. We discuss various mechanisms of Ca2+ influx stimulation: those that activate nonselective, Ca(2+)-permeable cation channels or those that activate Ca(2+)-selective channels, exclusively or partially operated by the filling state of intracellular Ca2+ stores. We also describe the role of various Ca(2+)- and shear stress-activated K+ channels and different types of Cl- channels for the regulation of the membrane potential.

Ca2+ influx does more than provide releasable Ca2+ to maintain repetitive spiking in human umbilical vein endothelial cells

We investigated why oscillations of intracellular Ca2+ concentrations ([Ca2+]i) in endothelial cells challenged by sub-maximal histamine run down in Ca(2+)-free medium despite stores retaining most of their Ca2+. One explantation is that only a small subpopulation of the Ca2+ stores oscillate and are completely emptied of Ca2+. To investigate if influx refills an empty store subpopulation, we differentiated between cations entering the cell and those released from internal stores by using extracellular Sr2+ as a Ca2+ surrogate; we distinguished between [Sr2+]i and [Ca2+]i by using the larger effect of Sr2+ on fura 2 fluorescence at 360 nm (F360). Ca2+ was still available for release when oscillations had run down since oscillations promptly reappeared on addition of Sr2+o and these were predominantly of Ca2+ (indicated by F360 changes). Also, totally depleting Ca2+ stores inhibited Sr(2+)-induced oscillations, suggesting that Sr2+ entry leads to Ca2+ release. In contrast, Ba2+o was unable to stimulate oscillations. Finally, oscillations generated by photolytic release of inositol trisphosphate (IP3) analogues were similarly sensitive to extracellular Ca2+ and Sr2+. We conclude that stores (or a sub-population) are not completely depleted of Ca2+ when oscillations run down in Ca(2+)-free medium. Bivalent cation entry therefore maintains sensitivity to IP3, possibly by maintaining luminal bivalent cation levels.

Inhibition of a store-operated Ca2+ entry pathway in human endothelial cells by the isoquinoline derivative LOE 908

1. The novel cation channel blocker, LOE 908, was tested for its effects on Ca2+ entry and membrane currents activated by depletion of intracellular Ca2+ stores in human endothelial cells. 2. LOE 908 inhibited store-operated Ca2+ entry induced by direct depletion of Ca2+ stores with 100 nM thapsigargin or 100 nM ionomycin with an EC50 of 2 microM and 4 microM, respectively. 3. LOE 908 did not affect thapsigargin- or ionomycin-induced Ca2+ release from intracellular stores up to concentrations of 3 microM. 4. LOE 908 reversibly suppressed thapsigargin- as well as ionomycin-induced whole-cell membrane currents. 5. The LOE 908-sensitive membrane conductance corresponded to a cation permeability of 5.5 and 6.9 fold selectivity for Ca2+ over K+ in the presence of thapsigargin and ionomycin, respectively. 6. Our results suggest that the isoquinoline, LOE 908 is a novel, potent inhibitor of the store-operated (capacitive) Ca2+ entry pathway in endothelial cells.

The effect of the calcium-antagonist nitrendipine on intracellular calcium concentration in endothelial cells

1. Nitrendipine induces NO-release from coronary vascular endothelium presumably by activating endothelial NO-synthase. We have investigated whether this effect may be mediated by an influence on the intracellular calcium in endothelial cells. 2. Bovine aortic endothelial cells (BAEC) were incubated with Fura-2/AM (1 microM) for 30 min and Fura-2 fluorescence was measured at 510 nm in response to chopped excitation with both 340 and 380 nm. The ratio 340/380 nm (known to reflect changes in intracellular calcium) was calculated from these data. 3. Nitrendipine (0.1 to 100 microM) led to a significant, concentration-dependent, monophasic increase in [Ca2+]i in suspended BAEC by 11 +/- 2 nM (0.1 microM), 23 +/- 3 nM (1 microM), 34 +/- 4 nM (10 microM) and by 47 +/- 5 nM (100 microM) from a control levels of 118 +/- 10 nM. 4. This elevation of intracellular calcium was prevented by pretreatment of BAECs with gadolinium (100 microM) or by incubation with calcium free saline solution. In contrast, the application of 0.3 microM thapsigargin did not abolish the nitrendipine-induced calcium signal. In additional experiments it was shown that the nitrendipine-induced NO-release (as measured with the oxy-haemoglobin-method could also be inhibited by gadolinium and was absent in calcium-free solution. 5. Thus, nitrendipine elevates intracellular calcium in suspended BAECs in a concentration-dependent manner. This elevation is mainly due to a gadolinium-sensitive calcium influx from the extracellular space rather than a calcium release from intracellular stores.

Effects of cyclopiazonic acid and thapsigargin on electromechanical activities and intracellular Ca2+ in smooth muscle of carotid artery of hypertensive rats

1. The effects of cyclopiazonic acid (CPA) and thapsigargin (TG), both of which are known to inhibit sarcoplasmic reticular Ca(2+)-ATPase, on the mechanical activities, intracellular Ca2+ level and electrical activities of smooth muscle of the carotid artery of stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY) were compared. 2. Both CPA and TG induced elevation of tension of the smooth muscle, which was composed of a phasic and a tonic component. The level of tension attained, especially the tonic component, was greater in the preparation from SHRSP. 3. The elevation of tension was associated with an increased intracellular Ca2+ level. Both the elevation of tension and the increase in intracellular Ca2+ were diminished by the removal of extracellular Ca2+ or by the application of verapamil. 4. The resting membrane potential of the preparations from SHRSP were depolarized to a greater extent than those from WKY.CPA depolarized the smooth muscle from both SHRSP and WKY, and the final level was also more depolarized in the preparation from SHRSP. 5. These results indicate that the elevation of tension induced by these drugs is mainly due to increased Ca2+ influx through voltage-dependent Ca2+ channels, and the difference in the action between the preparation from SHRSP and that from WKY can be explained mainly by the changes in the channels. 6. Thus, differences in the action of these drugs on the tension of smooth muscle between preparations from WKY and SHRSP can mainly be explained by the difference in the membrane potential which is related to the difference in voltage-dependent Ca2+ influx.

Inhibition by SK&F96365 of NO-mediated relaxation induced by Ca2(+) -ATPase inhibitors in rat thoracic aorta

1. We investigated the effect of SK&F96365, a putative inhibitor of receptor-operated Ca2+ entry, on the endothelium-dependent, NO-mediated relaxation and cyclic GMP formation induced by Ca2(+)-ATPase inhibitors in rat thoracic aorta. 2. SK&F96365 inhibited cyclopiazonic acid or thapsigargin-induced relaxation and cyclic GMP formation mediated by a constitutive NO synthase, which is known to be activated by the Ca2+ that enters into the endothelial cells via plasma membrane Ca2+ channels subsequent to depletion of stored Ca2+ by Ca2(+)-ATPase inhibitors. 3. SK&F96365 also inhibited relaxation and cyclic GMP formation induced by acetylcholine, without affecting those induced by nitroprusside and A23187. 4. Ni2+ attenuated relaxation and cyclic GMP formation induced by cyclopiazonic acid and acetylcholine. 5. In contrast, the voltage-dependent Ca2+ channel blocker, nifedipine, did not affect the relaxation caused by Ca2(+)-ATPase inhibitors. 6. These results suggest that endothelium-dependent, NO-mediated relaxation of the arteries induced by Ca2(+)-ATPase inhibitors is triggered by the Ca2+ that enters into endothelial cells via receptor-operated channels (SK&F96365-sensitive channels) subsequent to depletion of stored Ca2+ as a result of inhibition of the Ca2(+)-ATPase (Ca2+ pump) of the stores.

Two distinct membrane currents activated by cyclopiazonic acid-induced calcium store depletion in single smooth muscle cells of the mouse anococcygeus

1. By use of the whole-cell configuration of the patch-clamp technique, membrane currents induced by cyclopiazonic acid (CPA; an inhibitor of the sarcoplasmic reticulum (SR) calcium-ATPase) were investigated in single smooth muscle cells freshly dispersed from the mouse anococcygeus. Voltage-dependent calcium currents were blocked with extracellular nifedipine and caesium and tetraethylammonium chloride were used to block voltage-dependent potassium currents. 2. At a holding potential of -40 mV, CPA (10 microM) activated an inward current that consisted of two distinct components. The first was an initial transient current with an amplitude of 19.6 +/- 1.9 pA while the second was sustained and had an amplitude of 3.5 +/- 0.3 pA. 3. The current-voltage (I-V) relationship for the transient current showed marked outward rectification. The current had a reversal potential of 9.1 +/- 1.1 mV which was shifted to 29.0 +/- 4.2 mV when the extracellular chloride concentration was lowered from 148.4 to 58.4 mM. The sustained current had a near-linear I-V relationship and a reversal potential of 31.0 +/- 2.7 mV. Removal of extracellular calcium had no effect on the transient current, but shifted the reversal potential of the sustained current to 18.2 +/- 5.7 mV. 3. The initial transient current was abolished in cells bathed in extracellular solutions containing the chloride channel blockers, 4,4' diisothiocyanato-stilbene-2,2'-disulphonic acid (DIDS; 1 mM) or anthracene-9-carboxylic acid (A-9-C; 1 mM), and was absent in cells containing the calcium buffers EGTA (1 to 5 mM) or BAPTA (10 mM). The second sustained current was unaffected by either the chloride channel blockers or the intracellular calcium buffers. 4. Treatment of the cells with caffeine (10 mM) produced similar inward currents to those produced by CPA. In the presence of caffeine, CPA (10 microM) induced no further inward current. 5. In organ bath studies, CPA (10 microM)-induced contractions of the mouse anococcygeus were inhibited by cadmium and nickel (both 50-400 microM) and the general calcium entry blocker, SKF 96365 (10 microM); lanthanum and gadolinium had no effect at concentrations up to 400 microM. The pharmacology of the CPA-induced non-selective cation current mirrored that of the CPA-induced whole muscle contraction being reversed by cadmium (100 microM) and SKF 96365 (10 microM), but unaffected by lanthanum (400 microM). The initial chloride conductance was unaffected by cadmium, SKF 96365 or lanthanum. 6. It is concluded that CPA activates a transient calcium-dependent chloride current as a consequence of calcium release from intracellular stores; this current would result in depolarization and opening of voltage-operated calcium channels, which mediate the nifedipine-sensitive component of muscle contraction. In addition, as a result of emptying the SR, CPA activates a non-selective cation conductance which may underlie the nifedipine-insensitive calcium entry process utilised during sustained contraction.

Muscarinic regulation of phospholipase D and its role in arachidonic acid release in rat submandibular acinar cells

The characteristics of muscarinic cholinergic-induced phospholipase D (PLD) activation, and the involvement of the enzyme in the release of arachidonic acid were examined in rat submandibular acinar cells. Carbachol produced a dose-related activation of PLD to around fivefold control values at 100 microM agonist concentration. This was associated with the appearance of free choline, phosphatidic acid and arachidonic acid, indicating that the PLD substrate was phosphatidylcholine. The response to carbachol was inhibited by 60% by U73122, a blocker of a phospholipase C (PLC) specific to phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], suggesting that the cleavage of phosphatidylcholine by PLD was, at least in part, secondary to agonist-coupled hydrolysis of PtdIns(4,5)P2 by PLC. Consistent with this, PLD was also activated to levels comparable to those induced by carbachol, by the phorbol ester, 12-O-tetradecanoylphorbol-13-acetate (TPA), and the Ca2+ mobilizer, thapsigargin, two agents that respectively mimic the activation of protein kinase C (PKC) by diacylglycerol and the elevation of cytosolic Ca2+ by inositol 1,4,5-triphosphate [Ins(1,4,5)P3] in the phosphoinositide effect. The cell-permeant Ca2+ chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetraacetoxymethyl ester (BAPTA/AM) abolished the thapsigargin-induced activation of PLD and inhibited the responses of PLD to carbachol and TPA by 60%. The PKC inhibitor, Ro-31-8220, also inhibited the activation of PLD by carbacol and TPA to a level of approximately double control values, but had no effect on the thapsigargin-induced elevation of PLD. A role for both the PKC-associated and Ca(2+)-mobilizing arms of the PtdIns(4,5)P2-PLC pathway in PLD regulation is thus suggested. Pretreatment of cells with the phosphatidate phosphohydrolase blocker, propranolol, significantly enhanced the carbachol-induced elevation of phosphatidic acid, but decreased agonist-stimulated production of diacylglycerol and arachidonic acid, indicating that phosphatidlycholine was the likely source of arachidonic acid. We therefore propose that, in submandibular mucous acinar cells, muscarinic activation of the PtdIns(4,5)P2-PLC pathway regulates phosphatidylcholine-specific PLD through both the PKC- and Ca(2+)-mobilizing arms of the phosphoinositide response, and that diacylglycerol, derived from phosphatidylcholine via phosphatidic acid, is a source of free arachidonic acid.

Role of mitogen-induced calcium influx in the control of the cell cycle in Balb-c 3T3 fibroblasts

The role of mitogen-activated calcium influx from the extracellular medium in the control of cell proliferation was studied in Balb-c 3T3 fibroblasts. Stimulation of serum-deprived, quiescent cells with 10% foetal calf serum (FCS) induced a long-lasting (up to 70 min) elevation of intracellular free calcium concentration ([Ca2+]i). Both the sustained [Ca2+]i increase and the related inward current, described in a previous paper [Lovisolo D. Munaron L. Baccino FM. Bonelli G. (1992) Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts. Biochim. Biophys. Acta, 1104, 73-82], could be abolished either by chelation of extracellular calcium with EGTA or by SK&F 96365, an imidazole derivative that can block receptor-activated calcium channels. The effect of the abolition of these ionic signals on FCS-induced proliferation was investigated by adding either EGTA or SK&F 96365 to the culture medium during the first hours of stimulation of quiescent cells with 10% FCS. As measured after 24 h, a 22% inhibition of growth was observed when SK&F 96365 was added for the first hour, and stronger inhibitions, up to 56%, were obtained by adding the blocker for the first 2 or 4 h. Similar effects were observed with addition of 3 mM EGTA, though the inhibition was less marked for the 4 h treatment. By contrast, incubation with either substance in the next 4 h of serum stimulation did not influence cell growth, except for a slight inhibition observed when SK&F 96365 was applied from the 4th to the 8th hour. The reduction in growth resulting from the abolition of the early calcium influx was paralleled by an accumulation of cells in the G2/M phase. Both growth inhibition and G2/M accumulation were reversible, since after further 24 h in 10% FCS cells had fully recovered the exponential growth. These data indicate that the early calcium influx seen in response to mitogen stimulation develops on a timescale long enough to play a significant role in cell cycle progression, and that its block in the early G1 phase can lead to a reduction of proliferation by arresting cells in later stages of the cycle.