Ca2+ entry channels involved in endothelin-1-induced contractions of vascular smooth muscle cells

Endothelin-1 induces lipolysis through activation of the GC/cGMP/Ca2+/ERK/CaMKIII pathway in 3T3-L1 adipocytes

Endothelin-1 (ET-1) is a potent vasoconstrictive peptide produced and secreted mainly by endothelial cells. Recent studies indicate that ET-1 can regulate lipid metabolism, which may increase the risk of insulin resistance. Our previous studies revealed that ET-1 induced lipolysis in adipocytes, but the underlying mechanisms were unclear. 3T3-L1 adipocytes were used to investigate the effect of ET-1 on lipolysis and the underlying mechanisms. Glycerol levels in the incubation medium and hormone-sensitive lipase (HSL) phosphorylation were used as indices for lipolysis. ET-1 significantly increased HSL phosphorylation and lipolysis, which were completely inhibited by ERK inhibitor (PD98059) and guanylyl cyclase (GC) inhibitor (LY83583). LY83583 reduced ET-1-induced ERK phosphorylation. A Ca2+-free medium and PLC inhibitor caused significant decreases in ET-1-induced lipolysis as well as ERK and HSL phosphorylation, and IP3 receptor activator (D-IP3) increased lipolysis. ET-1 increased cGMP production, which was not affected by depletion of extracellular Ca2+. On the other hand, LY83583 diminished the ET-1-induced Ca2+ influx. Transient receptor potential vanilloid-1 (TRPV-1) antagonist and shRNA partially inhibited ET-1-induced lipolysis. ET-1-induced lipolysis was completely suppressed by CaMKIII inhibitor (NH-125). These results indicate that ET-1 stimulates extracellular Ca2+ entry and activates the intracellular PLC/IP3/Ca2+ pathway through a cGMP-dependent pathway. The increased cytosolic Ca2+ that results from ET-1 treatment stimulates ERK and HSL phosphorylation, which subsequently induces lipolysis. ET-1 induces HSL phosphorylation and lipolysis via the GC/cGMP/Ca2+/ERK/CaMKIII signaling pathway in 3T3-L1 adipocytes.

Endothelin-1: Biosynthesis, Signaling and Vasoreactivity

Endothelin-1 (ET-1) is an extremely potent vasoconstrictor peptide originally isolated from endothelial cells. Its synthesis, mainly regulated at the gene transcription level, involves processing of a precursor by a furin-type proprotein convertase to an inactive intermediate, big ET-1. The latter peptide can then be cleaved directly by an endothelin-converting enzyme (ECE) into ET-1 or reach the active metabolite through a two-step process involving chymase hydrolyzing big ET-1 to ET-1 (1-31), itself needing conversion to ET-1 by neprilysin (NEP) to exert physiological activity. ET-1 signals through two G protein-coupled receptors, endothelin receptor A (ETA) and endothelin receptor B (ETB). Both receptors induce an increase in intracellular Ca(2+), mainly from the extracellular space through voltage-independent mechanisms, the receptor-operated channels and store-operated channels. ET-1 also induces signaling through epidermal growth factor receptor transactivation, oxidative stress induction, rho-kinase, and the activation (ETA) or inhibition (ETB) of the adenylate cyclase/cyclic adenosine monophosphate pathway. Arterial vasoconstriction is mediated mainly by the ETA receptor. ET-1, via endothelium-located ETB, relaxes arteries or constricts vessels following activation of the same receptor type on the smooth muscle, where it can interact with ETA. In addition, ETB-dependent vasoconstriction seems more prominent in the venous vasculature. A better understanding of how ET-1 is synthesized and how ETA and ETB receptors interact could help design better pharmacological agents in the treatment of cardiovascular diseases where targeting the ET-1 system is indicated.

Adaptive regulation of endothelin receptor type-A and type-B in vascular smooth muscle cells during pregnancy in rats

Normal pregnancy is associated with systemic vasodilation and decreased vascular contraction, partly due to increased release of endothelium-derived vasodilator substances. Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor acting via endothelin receptor type A (ETA R) and possibly type B (ETB R) in vascular smooth muscle cells (VSMCs), with additional vasodilator effects via endothelial ETB R. However, the role of ET-1 receptor subtypes in the regulation of vascular function during pregnancy is unclear. We investigated whether the decreased vascular contraction during pregnancy reflects changes in the expression/activity of ETAR and ETBR. Contraction was measured in single aortic VSMCs isolated from virgin, mid-pregnant (mid-Preg, day 12), and late-Preg (day 19) Sprague-Dawley rats, and the mRNA expression, protein amount, tissue and cellular distribution of ETAR and ETBR were examined using RT-PCR, Western blots, immunohistochemistry, and immunofluorescence. Phenylephrine (Phe, 10(-5)  M), KCl (51 mM), and ET-1 (10(-6)  M) caused VSMC contraction that was in late-Preg < mid-Preg and virgin rats. In VSMCs treated with ETB R antagonist BQ788, ET-1 caused significant contraction that was still in late-Preg < mid-Preg and virgin rats. In VSMCs treated with the ETAR antagonist BQ123, ET-1 caused a small contraction; and the ETBR agonists IRL-1620 and sarafotoxin 6c (S6c) caused similar contraction that was in late-Preg < mid-Preg and virgin rats. RT-PCR revealed similar ETAR, but greater ETBR mRNA expression in pregnant versus virgin rats. Western blots revealed similar ETAR, and greater protein amount of ETBR in endothelium-intact vessels, but reduced ETBR in endothelium-denuded vessels of pregnant versus virgin rats. Immunohistochemistry revealed prominent ETBR staining in the intima, but reduced ETAR and ETBR in the aortic media of pregnant rats. Immunofluorescence signal for ETAR and ETBR was less in VSMCs of pregnant versus virgin rats. The pregnancy-associated decrease in ETAR- and ETBR-mediated VSMC contraction appears to involve downregulation of ETAR and ETBR expression/activity in VSM, and may play a role in the adaptive vasodilation during pregnancy.

Functional transient receptor potential canonical type 1 channels in human atrial myocytes

Transient receptor potential (TRP) channels are not well understood in human atrium, and the present study was therefore designed to investigate whether TRPC channels would mediate the nonselective cation current reported previously and are involved in the formation of store-operated Ca(2+) entry (SOCE) channels in human atrial myocytes using approaches of whole-cell patch voltage-clamp, RT-PCR, Western blotting, co-immunoprecipitation, and confocal scanning approaches, etc. We found that a nonselective cation current was recorded under K(+)-free conditions in human atrial myocytes, and the current was inhibited by the TRP channel blocker La(3+). Thapsigargin enhanced the current, and its effect was suppressed by La(3+) and prevented by pipette inclusion of anti-TRPC1 antibody. Endothlin-1 and angiotensin II enhanced the current that could be inhibited by La(3+). Gene and protein expression of TRPC1 channels were abundant in human atria. In addition, mRNA and protein of STIM1 and Orai1, components of SOCE channels, were abundantly expressed in human atria. Co-immunoprecipitation analysis demonstrated an interaction of TRPC1 with STIM1 and/or Orai1. Ca(2+) signaling mediated by SOCE channels was detected by a confocal microscopy technique. These results demonstrate the novel evidence that TRPC1 channels not only mediate the nonselective cation current, but also form SOCE channels in human atria as a component. TRPC1 channels can be activated by endothelin-1 or angiotensin II, which may be involved in the atrial electrical remodeling in patients with atrial fibrillation.

Hypoxic depression of PKG-mediated inhibition of serotonergic contraction in ovine carotid arteries

Chronic hypoxia attenuates soluble guanylate cyclase-induced vasorelaxation in serotonin (5-HT)-contracted ovine carotid arteries. Because protein kinase G (PKG) mediates many effects of soluble guanylate cyclase activation through phosphorylation of multiple kinase targets in vascular smooth muscle, we tested the hypothesis that chronic hypoxia reduces the ability of PKG to phosphorylate its target proteins, which attenuates the ability of PKG to induce vasorelaxation. We also tested the hypothesis that hypoxia attenuates PKG expression and/or activity. Arteries from normoxic and chronically hypoxic (altitude of 3,820 m for 110 days) fetal and adult sheep were denuded of endothelium and equilibrated with 95% O2-5% CO2 in the presence of nitro-l-arginine methyl ester (l-NAME) and N(G)-nitro-l-arginine (l-NNA) to inhibit residual endothelial nitric oxide synthase. Concentration-response relations for 5-HT were determined in the presence of prazosin to minimize activation of α-adrenergic receptors. The PKG activator 8-(p-chlorophenylthio)-guanosine 3',5'-cyclic monophosphate (8-pCTP-cGMP) reduced agonist binding affinity of the 5-HT receptor in a concentration-dependent manner that was attenuated by hypoxia. Expression and activity of PKG-I was not significantly affected by chronic hypoxia in either fetal or adult arteries, although PKG-I abundance was greater in fetal arteries. Pretreatment with the large conductance calcium-sensitive potassium channel (BK) inhibitor iberiotoxin attenuated the vasorelaxation induced by 8-pCPT-cGMP in normoxic but not chronically hypoxic arteries. These results support the hypothesis that hypoxia attenuates the vasorelaxant effects of PKG through suppression of the ability of PKG to activate large conductance calcium-sensitive potassium channels in arterial smooth muscle. The results also reveal that this hypoxic effect is greater in fetal than adult arteries and that chronic maternal hypoxia can profoundly affect fetal vascular function.

Vascular Smooth Muscle Cell Signaling Mechanisms for Contraction to Angiotensin II and Endothelin-1

Vasoactive peptides, such as endothelin-1 and angiotensin II are recognized by specific receptor proteins located in the cell membrane of target cells. Following receptor recognition, the specificity of the cellular response is achieved by G-protein coupling of ligand binding to the regulation of intracellular effectors. These intracellular effectors will be the subject of this brief review on contractile activity initiated by endothelin-1 and angiotensin II.Activation of receptors by endothelin-1 and angiotensin II in smooth muscle cells results in phopholipase C (PLC) activation leading to the generation of the second messengers insitol trisphosphate (IP(3)) and diacylglycerol (DAG). IP(3) stimulates intracellular Ca(2+) release from the sarcoplasmic reticulum and DAG causes protein kinase C (PKC) activation. Additionally, different Ca(2+) entry channels, such as voltage-operated (VOC), receptor-operated (ROC), and store-operated (SOC) Ca(2+) channels, as well as Ca(2+)-permeable nonselective cation channels (NSCC), are involved in the elevation of intracellular Ca(2+) concentration. The elevation in intracellular Ca(2+) is transient and initiates contractile activity by a Ca(2+)-calmodulin interaction, stimulating myosin light chain (MLC) phosphorylation. When the Ca(2+) concentration begins to decline, Ca(2+)-sensitization of the contractile proteins is signaled by the RhoA/Rho-kinase pathway to inhibit the dephosphorylation of MLC phosphatase (MLCP) thereby maintaining force generation. Removal of Ca(2+) from the cytosol and stimulation of MLCP initiates the process of smooth muscle relaxation. In pathological conditions such as hypertension, alterations in these cellular signaling components can lead to an over stimulated state causing maintained vasoconstriction and blood pressure elevation.

Pharmacological profile of phosphatidylinositol 3-kinases and related phosphatidylinositols mediating endothelin(A) receptor-operated native TRPC channels in rabbit coronary artery myocytes

BACKGROUND AND PURPOSE

Endothelin(A) (ET(A) ) receptor-operated canonical transient receptor potential (TRPC) channels mediate Ca²⁺ influx pathways, which are important in coronary artery function. Biochemical pathways linking ET(A) receptor stimulation to TRPC channel opening are unknown. We investigated the involvement of phosphatidylinositol 3-kinases (PI3K) in ET(A) receptor activation of native heteromeric TRPC1/C5/C6 and TRPC3/C7 channels in rabbit coronary artery vascular smooth muscle cells (VSMCs).

EXPERIMENTAL APPROACH

A pharmacological profile of PI3K was created by studying the effect of pan-PI3K, pan-Class I PI3K and Class I PI3K isoform-selective inhibitors on ET(A) receptor-evoked single TRPC1/C5/C6 and TRPC3/C7 channel activities in cell-attached patches from rabbit freshly isolated coronary artery VSMCs. The action of phosphatidylinositol 3-phosphate- [PI(3)P], 4-phosphate- [PI(4)P] and 5-phosphate- [PI(5)P] containing molecules involved in PI3K-mediated reactions were studied in inside-out patches. Expression of PI3K family members in coronary artery tissue lysates were analysed using quantitative PCR.

KEY RESULTS

ET(A) receptor-operated TRPC1/C5/C6 and TRPC3/C7 channel activities were inhibited by wortmannin. However, ZSTK474 and AS252424 reduced ET(A) receptor-evoked TRPC1/C5/C6 channel activity but potentiated TRPC3/C7 channel activity. All the PI(3)P-, PI(4)P- and PI(5)P-containing molecules tested induced TRPC1/C5/C6 channel activation, whereas only PI(3)P stimulated TRPC3/C7 channels.

CONCLUSIONS AND IMPLICATIONS

ET(A) receptor-operated native TRPC1/C5/C6 and TRPC3/C7 channel activities are likely to be mediated by Class I PI3Kγ and Class II/III PI3K isoforms, respectively. ET(A) receptor-evoked and constitutively active PI3Kγ-mediated pathways inhibit TRPC3/C7 channel activation. PI3K-mediated pathways are novel regulators of native TRPC channels in VSMCs, and these signalling cascades are potential pharmacological targets for coronary artery disease.

Potential effect of resistin on the ET-1-increased reactions of blood pressure in rats and Ca2+ signaling in vascular smooth muscle cells

Resistin and endothelin-1 (ET-1) are upregulated in people with type II diabetes mellitus, central obesity, and hypertension. ET-1 signaling is involved in Ca(2+)-contraction coupling and related to blood pressure regulation. The aim of this study is to investigate the role of resistin on ET-1-increased blood pressure and Ca(2+) signaling. The blood pressure and cytosolic Ca(2+) of vascular smooth muscle cells (VSMCs) of Sprague-Dawley rats were detected. The data demonstrated that resistin accelerated and prolonged ET-1-induced increases in blood pressure and had significant effects on ET-1-increased Ca(2+) reactions. Resistin-enhanced ET-1-increased Ca(2+) reactions were reversed by blockers of store-operated Ca(2+) entry (SOCE) and extracellular-signal-regulated kinase (ERK). The endogenous expression of Orai and stromal interaction molecular (STIM) were characterized in the VSMCs. Furthermore, resistin-enhanced ET-1 Ca(2+) reactions and the resistin-dependent activation of SOCE were abolished under STIM1-siRNA treatment, indicating that STIM1 plays an important role in resistin-enhanced ET-1 Ca(2+) reactions in VSMCs. Resistin appears to exert effects on ET-1-induced Ca(2+) increases by enhancing the activity of ERK-dependent SOCE (STIM1-partcipated), and may accelerate and prolong ET-1-increased blood pressure via the same pathway.

GPER regulates endothelin-dependent vascular tone and intracellular calcium

AIMS

An increase in intracellular vascular smooth muscle cell calcium concentration (VSMC [Ca(2+)](i)) is essential for endothelin-1 (ET-1)-induced vasoconstriction. Based on previous findings that activation of the G protein-coupled estrogen receptor (GPER) inhibits vasoconstriction in response to ET-1 and regulates [Ca(2+)](i) in cultured VSMC, we investigated whether endogenous GPER regulates ET-1-induced changes in VSMC [Ca(2+)](i) and constriction of intact arteries.

MAIN METHODS

Pressurized carotid arteries of GPER-deficient (GPER(0)) and wildtype (WT) mice were loaded with the calcium indicator fura 2-AM. Arteries were stimulated with the GPER-selective agonist G-1 or solvent followed by exposure to ET-1. Changes in arterial diameter and VSMC [Ca(2+)](i) were recorded simultaneously. Vascular gene expression levels of ET(A) and ET(B) receptors were determined by qPCR.

KEY FINDINGS

ET-1-dependent vasoconstriction was increased in arteries from GPER(0) compared to arteries from WT mice. Despite the more potent vasoconstriction to ET-1, GPER deficiency was associated with a marked reduction in the ET-1-stimulated VSMC [Ca(2+)](i) increase, suggesting an increase in myofilament force sensitivity to [Ca(2+)](i). Activation of GPER by G-1 had no effect on vasoconstriction or VSMC [Ca(2+)](i) responses to ET-1, and expression levels of ET(A) or ET(B) receptor were unaffected by GPER deficiency.

SIGNIFICANCE

These results demonstrate that endogenous GPER inhibits ET-1-induced vasoconstriction, an effect that may be associated with reduced VSMC Ca(2+) sensitivity. This represents a potential mechanism through which GPER could contribute to protective effects of endogenous estrogen in the cardiovascular system.

Function and regulation of endothelin type A receptor-operated transient receptor potential canonical channels

The purpose of this study is to identify transient receptor potential canonical (TRPC) channels responsible for receptor-operated Ca(2+) entry (ROCE) triggered by activation of endothelin type A receptor (ET(A)R) and to clarify the importance of calmodulin (CaM) / inositol 1,4,5-trisphosphate (IP(3)) receptor binding (CIRB) domain at the C terminus of TRPC channels in ET(A)R-activated channel regulation. In HEK293 cells coexpressing ET(A)R and one of seven TRPC isoforms, ET(A)R stimulation induced ROCE through TRPC3, TRPC5, TRPC6, and TRPC7. The TRPC3- and TRPC6-mediated ROCE was inhibited by selective inhibitors of G(q) protein, phospholipase C (PLC), and CaM. The CIRB domain deletion mutants of TRPC3 and TRPC6 failed to induce ET(A)R-mediated ROCE. Either deletion of the CIRB domain or pharmacological inhibition of CaM did not inhibit the targeting of these channels to the plasma membrane. These results suggest that 1) TRPC3, TRPC5, TRPC6, and TRPC7 can function as ET(A)R-operated Ca(2+) channels; 2) G(q) protein, PLC, and CaM are involved in TRPC3- and TRPC6-mediated ROCE; 3) ET(A)R-mediated activation of TRPC3 and TRPC6 requires the CIRB domain; and 4) abolition of ET(A)R-induced ROCE by CIRB domain deletion and CaM inhibition is due to loss of CaM binding to the channels but not loss of cell surface TRPC3 and TRPC6.

Endothelin-dependent vasoconstriction in human uterine artery: application to preeclampsia

BACKGROUND

Reduced uteroplacental perfusion, the initiating event in preeclampsia, is associated with enhanced endothelin-1 (ET-1) production which feeds the vasoconstriction of uterine artery. Whether the treatments of preeclampsia were effective on ET-1 induced contraction and could reverse placental ischemia is the question addressed in this study. We investigated the effect of antihypertensive drugs used in preeclampsia and of ET receptor antagonists on the contractile response to ET-1 on human uterine arteries.

METHODOLOGY/PRINCIPAL FINDINGS

Experiments were performed, ex vivo, on human uterine artery samples obtained after hysterectomy. We studied variations in isometric tension of arterial rings in response to the vasoconstrictor ET-1 and evaluated the effects of various vasodilators and ET-receptor antagonists on this response. Among antihypertensive drugs, only dihydropyridines were effective in blocking and reversing the ET-1 contractile response. Their efficiency, independent of the concentration of ET-1, was only partial. Hydralazine, alpha-methyldopa and labetalol had no effect on ET-1 induced contraction which is mediated by both ET(A) and ET(B) receptors in uterine artery. ET receptors antagonists, BQ-123 and BQ-788, slightly reduced the amplitude of the response to ET-1. Combination of both antagonists was more efficient, but it was not possible to reverse the maximal ET-1-induced contraction with antagonists used alone or in combination.

CONCLUSION

Pharmacological drugs currently used in the context of preeclampsia, do not reverse ET-1 induced contraction. Only dihydropyridines, which partially relax uterine artery previously contracted with ET-1, might offer interesting perspectives to improve placental perfusion.

Endothelin type B receptor-induced sustained Ca2+ influx involves G(q/11)/phospholipase C-independent, p38 mitogen-activated protein kinase-dependent activation of Na+/H+ exchanger

The mechanism for sustained Ca2+ influx activated by G protein-coupled receptors was examined. In Chinese hamster ovary cells expressing recombinant human endothelin type B receptor (ET(B)R) and endogenous P2Y receptor (P2Y-R), endothelin-1 elicited a sustained Ca2+ influx depending on G(q/11 )protein, phospholipase C (PLC), Na+/H+ exchanger (NHE), and p38 mitogen-activated protein kinase (p38MAPK), whereas P2Y-R-induced sustained Ca2+ influx was negligible. Functional studies showed that NHE activation by ET(B)R was mediated via p38MAPK but not G(q/11)/PLC, while that by P2Y-R involves only G(q/11)/PLC/p38MAPK. These results suggest that G(q/11)/PLC-independent NHE activation via p38MAPK plays an important role in ET(B)R- mediated sustained Ca2+ influx.

Endothelin evokes distinct calcium transients in neuronal and non-neuronal cells of rat olfactory mucosa primary cultures

The olfactory system is regulated by several nervous and hormonal factors, and there is a growing body of evidence that some of these modulations already take place in the olfactory mucosa (OM). We recently suggested that, among others, vasoactive peptides might play multifaceted roles in different OM cells. Here we studied the effect of the vasoconstrictive peptide endothelin (ET) in the rat OM. We identified different components of the ET system both in the olfactory mucosa and in long-term primary culture of OM cells, composed of olfactory sensory neurons (OSNs) lying on a blend of non-neuronal OM cells (nNCs). We demonstrated that ET receptors are differentially expressed on OM cells, and that ET might be locally matured by the endothelin-converting enzyme ECE-1 located in OSNs. Using calcium imaging, we showed that ET triggers robust dose-dependent Ca(2+) responses in most OM cells, which consist of a transient phase, followed, in nNCs, by a sustained plateau phase. All transient responses depended on intracellular calcium release, while the sustained plateau phase also depended on subsequent external calcium entry. Using both pharmacology and spotting lethal (sl/sl) mutant rats, lacking functional ET(B) receptors, we finally demonstrated that these effects of ET are mediated through ET(B) receptors in OSNs and ET(A) receptors in nNCs.The present study therefore identifies endothelin as a potent endogenous modulator of the olfactory mucosa; specific endothelin-mediated Ca(2+) signals may serve distinct signaling functions, and thereby suggest differential functional roles of endothelin in both neuronal and non-neuronal OM cells.

Differential coupling of human endothelin type A receptor to G(q/11) and G(12) proteins: the functional significance of receptor expression level in generating multiple receptor signaling

This study examines the influence of receptor expression level on signaling pathways activated via endothelin type A receptor (ET(A)R) expressed in Chinese hamster ovary cells at 32,100 (ET(A)R-high-CHO) and 893 (ET(A)R-low-CHO) fmolmg protein(-1). Endothelin-1 (ET-1) elicited a sustained increase in intracellular Ca(2+) concentration ([Ca(2+)](i)), which was dependent on G(q/11) protein, phospholipase C (PLC), Na(+)/H(+) exchanger (NHE), and p38 mitogen-activated protein kinase (p38MAPK) in ET(A)R-high-CHO, whereas the sustained [Ca(2+)](i) increase was negligible in ET(A)R-low-CHO. Functional study with Cytosensor(TM) microphysiometer showed that ET-1 evoked an NHE1-mediated increase in extracellular acidification rate (ECAR) in ET(A)R-high-CHO and ET(A)R-low-CHO. In ET(A)R-high-CHO, the ECAR response at 30 min after ET-1 stimulation was insensitive to G(q/11) and PLC inhibitors, but sensitive to the p38MAPK inhibitor. In ET(A)R-low-CHO, the ECAR response at 30 min was sensitive to these inhibitors. Western blot analysis demonstrated that ET-1-induced p38MAPK phosphorylation in ET(A)R-low-CHO but not in ET(A)R-high-CHO was mediated via G(q/11) and PLC. The G(q/11)/PLC-independent p38MAPK phosphorylation in ET(A)R-high-CHO was suppressed by expression of the C terminus of G(alpha12) protein to disrupt receptor-G(12) protein coupling. These results provide evidence for multiple signaling pathways of ET(A)R that were activated via at least the G(q/11)/PLC/NHE, G(12)/p38MAPK/NHE, and G(q/11)/PLC/p38MAPK/NHE cascades in an expression level-dependent manner.

Activation of native TRPC1/C5/C6 channels by endothelin-1 is mediated by both PIP3 and PIP2 in rabbit coronary artery myocytes

We investigate activation mechanisms of native TRPC1/C5/C6 channels (termed TRPC1 channels) by stimulation of endothelin-1 (ET-1) receptor subtypes in freshly dispersed rabbit coronary artery myocytes using single channel recording and immunoprecipitation techniques. ET-1 evoked non-selective cation channel currents with a unitary conductance of 2.6 pS which were not inhibited by either ET(A) or ET(B) receptor antagonists, respectively BQ-123 and BQ788, when administered separately. However, in the presence of both antagonists, ET-1-evoked channel activity was abolished indicating that both ET(A) and ET(B) receptor stimulation activate this conductance. Stimulation of both ET(A) and ET(B) receptors evoked channel activity which was inhibited by the protein kinase C (PKC) inhibitor chelerythrine and by anti-TRPC1 antibodies indicating that activation of both receptor subtypes causes TRPC1 channel activation by a PKC-dependent mechanism. ET(A) receptor-mediated TRPC1 channel activity was selectively inhibited by phosphoinositol-3-kinase (PI-3-kinase) inhibitors wortmannin (50 nM) and PI-828 and by antibodies raised against phosphoinositol-3,4,5-trisphosphate (PIP(3)), the product of PI-3-kinase-mediated phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Moreover, exogenous application of diC8-PIP(3) stimulated PKC-dependent TRPC1 channel activity. These results indicate that stimulation of ET(A) receptors evokes PKC-dependent TRPC1 channel activity through activation of PI-3-kinase and generation of PIP(3). In contrast, ET(B) receptor-mediated TRPC1 channel activity was inhibited by the PI-phospholipase C (PI-PLC) inhibitor U73122. 1-Oleoyl-2-acetyl-sn-glycerol (OAG), an analogue of diacylglycerol (DAG), which is a product of PI-PLC, also activated PKC-dependent TRPC1 channel activity. OAG-induced TRPC1 channel activity was inhibited by anti-phosphoinositol-4,5-bisphosphate (PIP(2)) antibodies and high concentrations of wortmannin (20 microM) which depleted tissue PIP(2) levels. In addition exogenous application of diC8-PIP(2) activated PKC-dependent TRPC1 channel activity. These data indicate that stimulation of ET(B) receptors evokes PKC-dependent TRPC1 activity through PI-PLC-mediated generation of DAG and requires a permissive role of PIP(2). In conclusion, we provide the first evidence that stimulation of ET(A) and ET(B) receptors activate native PKC-dependent TRPC1 channels through two distinct phospholipids pathways involving a novel action of PIP(3), in addition to PIP(2), in rabbit coronary artery myocytes.

Agonist-evoked calcium entry in vascular smooth muscle cells requires IP3 receptor-mediated activation of TRPC1

Transient receptor potential canonical (TRPC) proteins have been proposed to function as plasma membrane Ca2+ channels activated by store depletion and/or by receptor stimulation. However, their role in the increase in cytosolic Ca2+ activated by contractile agonists in vascular smooth muscle is not yet elucidated. The present study was designed to investigate the functional and molecular properties of the Ca2+ entry pathway activated by endothelin-1 in primary cultured aortic smooth muscle cells. Measurement of the Ca2+ signal in fura-2-loaded cells allowed to characterize endothelin-1-evoked Ca2+ entry, which was resistant to dihydropyridine, and was blocked by 2-aminoethoxydiphenylborate (2-APB) and micromolar concentration of Gd3+. It was not activated by store depletion, but was inhibited by the endothelin ETA receptor antagonist BQ-123, and by heparin. On the opposite, thapsigargin-induced store depletion activated a Ca2+ entry pathway that was not affected by 2-APB, BQ-123 or heparin, and was less sensitive to Gd3+ than was endothelin-1-evoked Ca2+ entry. Investigation of the gene expression of TRPC isoforms by real-time RT-PCR revealed that TRPC1 was the most abundant. In cells transfected with TRPC1 small interfering RNA sequence, TRPC1 mRNA and protein expression were decreased by 72+/-3% and 86+/-2%, respectively, while TRPC6 expression was unaffected. In TRPC1 knockdown cells, both endothelin-1-evoked Ca2+ entry and store-operated Ca2+ entry evoked by thapsigargin were blunted. These results indicate that in aortic smooth muscle cells, TRPC1 is not only involved in Ca2+ entry activated by store depletion but also in receptor-operated Ca2+ entry, which requires inositol (1,4,5) triphosphate receptor activation.

Novel mechanism of endothelin-1-induced vasospasm after subarachnoid hemorrhage

Cerebral vasospasm is a major cause of morbidity and mortality after aneurysmal subarachnoid hemorrhage (SAH). It is a sustained constriction of the cerebral arteries that can be reduced by endothelin (ET) receptor antagonists. Voltage-gated Ca(2+) channel antagonists such as nimodipine are relatively less effective. Endothelin-1 is not increased enough after SAH to directly cause the constriction, so we sought alternate mechanisms by which ET-1 might mediate vasospasm. Vasospasm was created in dogs, and the smooth muscle cells were studied molecularly, electrophysiologically, and by isometric tension. During vasospasm, ET-1, 10 nmol/L, induced a nonselective cation current carried by Ca(2+) in 64% of cells compared with in only 7% of control cells. Nimodipine and 2-aminoethoxydiphenylborate (a specific antagonist of store-operated channels) had no effect, whereas SKF96365 (a nonspecific antagonist of nonselective cation channels) decreased this current in vasospastic smooth muscle cells. Transient receptor potential (TRP) proteins may mediate entry of Ca(2+) through nonselective cationic pathways. We tested their role by incubating smooth muscle cells with anti-TRPC1 or TRPC4, both of which blocked ET-1-induced currents in SAH cells. Anti-TRPC5 had no effect. Anti-TRPC1 also inhibited ET-1 contraction of SAH arteries in vitro. Quantitative polymerase chain reaction and Western blotting of seven TRPC isoforms found increased expression of TRPC4 and a novel splice variant of TRPC1 and increased protein expression of TRPC4 and TRPC1. Taken together, the results support a novel mechanism whereby ET-1 significantly increases Ca(2+) influx mediated by TRPC1 and TRPC4 or their heteromers in smooth muscle cells, which promotes development of vasospasm after SAH.

Characterization of noradrenaline-induced increases in intracellular Ca2+ levels in Chinese hamster ovary cells stably expressing human alpha1A-adrenoceptor

The mechanism for noradrenaline (NA)-induced increases in intracellular Ca(2+) concentration ([Ca(2+)](i)) and physiological significance of Na(+) influx through receptor-operated channels (ROCs) and store-operated channels (SOCs) were studied in Chinese hamster ovary (CHO) cells stably expressing human alpha(1A)-adrenoceptor (alpha(1A)-AR). [Ca(2+)](i) was measured using the Ca(2+) indicator fura-2. NA (1 microM) elicited transient and subsequent sustained [Ca(2+)](i) increases, which were inhibited by YM-254890 (G(alphaq/11) inhibitor), U-73122 (phospholipase C (PLC) inhibitor), and bisindolylmaleimide I (protein kinase C (PKC) inhibitor), suggesting their dependence on G(alphaq/11)/PLC/PKC. Both phases were suppressed by extracellular Ca(2+) removal, SK&F 96365 (inhibitor of SOC and nonselective cation channel type-2 (NSCC-2)), LOE 908 (inhibitor of NSCC-1 and NSCC-2), and La(3+) (inhibitor of transient receptor potential canonical (TRPC) channel). Reduction of extracellular Na(+) and pretreatment with KB-R7943, a Na(+)/Ca(2+) exchanger (NCX) inhibitor, inhibited both phases of [Ca(2+)](i) increases. These results suggest that 1) stimulation of alpha(1A)-AR with NA elicits the transient and sustained increases in [Ca(2+)](i) mediated through NSCC-2 that belongs to a TRPC family; 2) Na(+) influx through these channels drives NCX in the reverse mode, causing Ca(2+) influx in exchange for Na(+) efflux; and 3) the G(alphaq/11)/PLC/PKC-dependent pathway plays an important role in the increases in [Ca(2+)](i).

Endothelin-1-mediated wave-like [Ca2+]i oscillations in intact rabbit inferior vena cava

Endothelin-1 (ET1) is an endogenous vasoconstrictor released by the vascular system to regulate the contractility of vascular smooth muscle cells (VSMC). It is implicated in the pathogenesis of hypertension and diabetic vasculopathy. In rabbit inferior vena cava (IVC), 10 nM ET1 induces tonic contraction mainly via type A endothelin receptor activation. Using confocal imaging of Fluo-3 loaded in thein situ VSMC within the intact IVC, we found that ET1 elicited [Ca2+]i oscillations with an average frequency of 0.31 +/- 0.01 Hz. These [Ca2+]i oscillations occurred as repetitive Ca2+ waves traveling along the longitudinal axis of the cells with an average velocity of 29 +/- 3 microm/s. The Ca2+ waves were not synchronized between neighboring VSMC nor were they propagated between them. Nifedipine (10 microM) inhibited the tonic contraction by 27.0 +/- 5.0% while SKF96365 (50 microM) abolished the remaining contraction. In a parallel Ca2+ study, nifedipine reduced the frequency of the oscillations to 0.22 +/- 0.01 Hz while SKF96365 abolished the remaining [Ca2+]i oscillations. Subsequent application of 25 mM caffeine elicited no further Ca2+ signal. Thus, we conclude that ET1 stimulates tonic contraction in the rabbit IVC by inducing [Ca2+]i oscillations and that stimulated Ca2+ entry through both the L-type voltage-gated Ca2+ channels and a nifedipine-resistant and SKF96365-sensitive pathway is crucial for the maintenance of [Ca2+]i oscillations and tonic contraction.

Endothelin-1 decreases [Ca2+]i via Na+/Ca2+ exchanger in CHO cells stably expressing endothelin ETA receptor

Endothelin ET(A) receptor couples to Gq/11 protein that transduces a variety of receptor signals to modulate diverse cellular responses including Ca2+ mobilization. Stimulation of endothelin ETA receptor with endothelin-1 is generally believed to induce an increase in intracellular Ca2+ concentration ([Ca2+]i) via Gq/11 protein. Here we provide the first convincing evidence that endothelin-1 elicited Gq/11 protein-dependent and -independent 'decrease' in [Ca2+]i via Na+/Ca2+ exchanger (NCX) in Chinese hamster ovary (CHO) cells stably expressing human endothelin ETA receptor. In the cells treated with 1 microM thapsigargin, an inhibitor of endoplasmic Ca2+ pump, that induces an increase in [Ca2+]i via capacitative Ca2+ entry, endothelin-1 induced a decrease in [Ca2+]i which was partially inhibited by YM-254890, a specific inhibitor of Gq/11, indicating that Gq/11-dependent and independent pathways are involved in the decrease. The endothelin-1-induced decrease in [Ca2+]i was markedly suppressed by 3',4'-dichlorobenzamil hydrochloride, a potent NCX inhibitor, and also by a replacement of extracellular Na+ with Li+, which was not transported by NCX, indicating a major role of NCX operating in the forward mode in the endothelin-1-induced decrease in [Ca2+]i. Molecular approach with RT-PCR demonstrated the expression of mRNA for NCX1, NCX2 and NCX3. These results suggest that stimulation of endothelin ETA receptor with endothelin-1 activates the forward mode NCX through Gq/11-dependent and -independent mechanisms: the NCX exports Ca2+ out of the cell depending on Na+ gradient across the cell membrane, resulting in the decrease in [Ca2+]i.

Endothelin-1, endothelin A and B receptor expression and their pharmacological properties in GFAP negative human lamina cribrosa cells

Primary open angle glaucoma (POAG) is a progressive optic neuropathy, characterized, in part by extensive extra cellular matrix remodeling and collapse of the lamina cribrosa (LC). Endothelin-1 (ET-1), a potent vasoactive peptide and its receptors, endothelin receptor A (ET(A)) and endothelin receptor B (ET(B)), have been implicated in glaucomatous optic neuropathy. In this study we examined the expression of ET-1 and its receptors in GFAP negative LC cells. RT-PCR analysis revealed that LC cells express both ET(A), ET(B) receptors and prepro- ET-1, the primary gene transcript of ET-1. A dose-dependent increase in intra-cellular calcium concentrations was observed in the presence of 1, 10 and 100nM ET-1. Increased intracellular calcium concentrations were blocked by the ET(A) selective antagonist BQ610 but not by the ET(B) specific antagonist BQ788. Desensitization to ET(A)-mediated increase in intracellular calcium was observed in LC cells following pre-treatment with ET-1 for 24h. Western blot analysis of LC cells treated with ET-1 for 24h revealed a decreased expression of ET(A) receptor protein at 1, 10 and 100nM concentrations, while a dose dependent increase in the ET(B) receptor was observed with a significant increase at 100nM. Quantitative PCR showed a dose-dependent decrease in ET(A) receptor mRNA levels and an increase in the mRNA levels of ET(B) receptors. A Griess colorimetric assay was used to measure the NO released from LC cells and ET-1 induced a dose-dependent increase in NO release which was significant at 100nM concentration. ET-1 induced NO release was significantly blocked by BQ788, an ET(B) selective antagonist, and as well as BQ610, an ET(A) selective antagonist. These results suggested that human lamina cribrosa cells expressed functional ET(A) and ET(B) receptors and their expression and function was altered in response to prolong exposure to ET-1. This may have an implication in the normal physiology of LC cells and in POAG subjects where elevated levels of ET-1 could impact LC function.

Endothelin-1 activates a Ca2+-permeable cation channel with TRPC3 and TRPC7 properties in rabbit coronary artery myocytes

In the present work we used patch pipette techniques to study the properties of a novel Ca(2+)-permeable cation channel activated by the potent coronary vasoconstrictor endothelin-1 (ET-1) in freshly dispersed rabbit coronary artery myocytes. With cell-attached recording bath application of 10 nm ET-1 evoked cation channel currents (I(cat)) with subconductance states of about 18, 34 and 51 and 68 pS, and a reversal potential of 0 mV. ET-1 evoked channel activity when extracellular Ca(2+) was the charge carrier, illustrating significant Ca(2+) permeability. ET-1-induced responses were inhibited by the ET(A) receptor antagonist BQ123 and the phospholipase C (PLC) inhibitor U73122. The diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) also stimulated I(cat), but the protein kinase C (PKC) inhibitor chelerythrine did not inhibit either the OAG- or ET-1-induced I(cat). Inositol 1,4,5-trisphosphate (IP(3)) did not activate I(cat), but greatly potentiated the response to OAG and this effect was blocked by heparin. Bath application of anti-TRPC3 and anti-TRPC7 antibodies to inside-out patches markedly inhibited ET-1-evoked I(cat), but antibodies to TRPC1, C4, C5 and C6 had no effect. Immunocytochemical studies demonstrated preferential TRPC7 expression in the plasmalemma, whereas TRPC3 was distributed throughout the myocyte, and moreover co-localization of TRPC3 and TRPC7 signals was observed at, or close to, the plasma membrane. Flufenamic acid, Gd(3+), La(3+) and extracellular Ca(2+) inhibited I(cat) with IC(50) values of 2.45 microm, 3.8 microm, 7.36 microm and 22 microm, respectively. These results suggest that in rabbit coronary artery myocytes ET-1 evokes a Ca(2+)-permeable non-selective cation channel with properties similar to TRPC3 and TRPC7, and indicates that these proteins may be important components of this conductance.