Signal transduction pathways and gating mechanisms of native TRP-like cation channels in vascular myocytes

Vascular smooth muscle ion channels in essential hypertension

Hypertension is a highly prevalent chronic disease and the major risk factor for cardiovascular diseases, the leading cause of death worldwide. Hypertension is characterized by an increased vascular tone determined by the contractile state of vascular smooth muscle cells that depends on intracellular calcium levels. The interplay of ion channels determine VSMCs membrane potential and thus intracellular calcium that controls the degree of contraction, vascular tone and blood pressure. Changes in ion channels expression and function have been linked to hypertension, but the mechanisms and molecular entities involved are not completely clear. Furthermore, the literature shows discrepancies regarding the contribution of different ion channels to hypertension probably due to differences both in the vascular preparation and in the model of hypertension employed. Animal models are essential to study this multifactorial disease but it is also critical to know their characteristics to interpret properly the results obtained. In this review we summarize previous studies, using the hypertensive mouse (BPH) and its normotensive control (BPN), focused on the identified changes in the expression and function of different families of ion channels. We will focus on L-type voltage-dependent Ca2+ channels (Cav1.2), canonical transient receptor potential channels and four different classes of K+ channels: voltage-activated (Kv), large conductance Ca2+-activated (BK), inward rectifiers (Kir) and ATP-sensitive (KATP) K+ channels. We will describe the role of these channels in hypertension and we will discuss the importance of integrating individual changes in a global context to understand the complex interplay of ion channels in hypertension.

Coupling of store-operated calcium entry to vasoconstriction is acid-sensing ion channel 1a dependent in pulmonary but not mesenteric arteries

Although voltage-gated Ca²⁺ channels (VGCC) are a major Ca²⁺ entry pathway in vascular smooth muscle cells (VSMCs), several other Ca²⁺-influx mechanisms exist and play important roles in vasoreactivity. One of these is store-operated Ca²⁺ entry (SOCE), mediated by an interaction between STIM1 and Orai1. Although SOCE is an important mechanism of Ca²⁺ influx in non-excitable cells (cells that lack VGCC); there is debate regarding the contribution of SOCE to regulate VSMC contractility and the molecular components involved. Our previous data suggest acid-sensing ion channel 1a (ASIC1a) is a necessary component of SOCE and vasoconstriction in small pulmonary arteries. However, it is unclear if ASIC1a similarly contributes to SOCE and vascular reactivity in systemic arteries. Considering the established role of Orai1 in mediating SOCE in the systemic circulation, we hypothesize the involvement of ASIC1a in SOCE and resultant vasoconstriction is unique to the pulmonary circulation. To test this hypothesis, we examined the roles of Orai1 and ASIC1a in SOCE- and endothelin-1 (ET-1)-induced vasoconstriction in small pulmonary and mesenteric arteries. We found SOCE is coupled to vasoconstriction in pulmonary arteries but not mesenteric arteries. In pulmonary arteries, inhibition of ASIC1a but not Orai1 attenuated SOCE- and ET-1-induced vasoconstriction. However, neither inhibition of ASIC1a nor Orai1 altered ET-1-induced vasoconstriction in mesenteric arteries. We conclude that SOCE plays an important role in pulmonary, but not mesenteric, vascular reactivity. Furthermore, in contrast to the established role of Orai1 in SOCE in non-excitable cells, the SOCE response in pulmonary VSMCs is largely mediated by ASIC1a.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Differences in TRPC3 and TRPC6 channels assembly in mesenteric vascular smooth muscle cells in essential hypertension

KEY POINTS

Canonical transient receptor potential (TRPC)3 and TRPC6 channels of vascular smooth muscle cells (VSMCs) mediate stretch- or agonist-induced cationic fluxes, contributing to membrane potential and vascular tone. Native TRPC3/C6 channels can form homo- or heterotetrameric complexes, which can hinder individual TRPC channel properties. The possibility that the differences in their association pattern may change their contribution to vascular tone in hypertension is unexplored. Functional characterization of heterologously expressed channels showed that TRPC6-containing complexes exhibited Pyr3/Pyr10-sensitive currents, whereas TRPC3-mediated currents were blocked by anti-TRPC3 antibodies. VSMCs from hypertensive (blood pressure high; BPH) mice have larger cationic basal currents insensitive to Pyr10 and sensitive to anti-TRPC3 antibodies. Consistently, myography studies showed a larger Pyr3/10-induced vasodilatation in BPN (blood pressure normal) mesenteric arteries. We conclude that the increased TRPC3 channel expression in BPH VSMCs leads to changes in TRPC3/C6 heteromultimeric assembly, with a higher TRPC3 channel contribution favouring depolarization of hypertensive VSMCs.

ABSTRACT

Increased vascular tone in essential hypertension involves a sustained rise in total peripheral resistance. A model has been proposed in which the combination of membrane depolarization and higher L-type Ca²⁺ channel activity generates augmented Ca²⁺ influx into vascular smooth muscle cells (VSMCs), contraction and vasoconstriction. The search for culprit ion channels responsible for membrane depolarization has provided several candidates, including members of the canonical transient receptor potential (TRPC) family. TRPC3 and TRPC6 are diacylglycerol-activated, non-selective cationic channels contributing to stretch- or agonist-induced depolarization. Conflicting information exists regarding changes in TRPC3/TRPC6 functional expression in hypertension. However, although TRPC3-TRPC6 channels can heteromultimerize, the possibility that differences in their association pattern may change their functional contribution to vascular tone is largely unexplored. We probe this hypothesis using a model of essential hypertension (BPH mice; blood pressure high) and its normotensive control (BPN mice; blood pressure normal). First, non-selective cationic currents through homo- and heterotetramers recorded from transfected Chinese hamster ovary cells indicated that TRPC currents were sensitive to the selective antagonist Pyr10 only when TRPC6 was present, whereas intracellular anti-TRPC3 antibody selectively blocked TRPC3-mediated currents. In mesenteric VSMCs, basal and agonist-induced currents were more sensitive to Pyr3 and Pyr10 in BPN cells. Consistently, myography studies showed a larger Pyr3/10-induced vasodilatation in BPN mesenteric arteries. mRNA and protein expression data supported changes in TRPC3 and TRPC6 proportions and assembly, with a higher TRPC3 channel contribution in BPH VSMCs that could favour cell depolarization. These differences in functional and pharmacological properties of TRPC3 and TRPC6 channels, depending on their assembly, could represent novel therapeutical opportunities.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

TRP channels: a missing bond in the entrainment mechanism of peripheral clocks throughout evolution

Circadian rhythm may be understood as a temporal organization that works to orchestrate physiological processes and behavior in a period of approximately 24 h. Because such temporal organization has evolved in the presence of predictable environmental clues, such as day length, tides, seasons, and temperature, the organism has confronted the natural selection in highly precise intervals of opportunities and risks, generating temporal programs and resetting mechanisms, which are well conserved among different taxa of animals. The present review brings some evidence of how these programs may have co-evolved in systems able to deal with 2 or more environmental clues, and how they similarly function in different group of animals, stressing how important temperature and light were to establish the temporal organizations. For example, melanopsin and rhodopsin, photopigments present respectively in circadian and visual photoreceptors, are required for temperature discrimination in Drosophila melanogaster. These pigments may signal light and temperature via activation of cationic membrane channel, named transient-receptor potential channel (TRP). In fact, TRPs have been suggested to function as thermal sensor for various groups of animals. Another example is the clock machinery at the molecular level. A set of very-well conserved proteins, known as clock proteins, function as transcription factors in positive and negative auto-regulatory loops generating circadian changes of their expression, and of clock-controlled genes. Similar molecular machinery is present in organisms as diverse as cyanobacteria (Synechococcus), fungi (Neurospora), insects (Drosophila), and vertebrates including humans.

Diverse Kir expression contributes to distinct bimodal distribution of resting potentials and vasotone responses of arterioles

The resting membrane potential (RP) of vascular smooth muscle cells (VSMCs) is a major determinant of cytosolic calcium concentration and vascular tone. The heterogeneity of RPs and its underlying mechanism among different vascular beds remain poorly understood. We compared the RPs and vasomotion properties between the guinea pig spiral modiolar artery (SMA), brain arterioles (BA) and mesenteric arteries (MA). We found: 1) RPs showed a robust bimodal distribution peaked at -76 and -40 mV evenly in the SMA, unevenly at -77 and -51 mV in the BA and ~-71 and -52 mV in the MA. Ba(2+) 0.1 mM eliminated their high RP peaks ~-75 mV. 2) Cells with low RP (~-45 mV) hyperpolarized in response to 10 mM extracellular K(+), while cells with a high RP depolarized, and cells with intermediate RP (~-58 mV) displayed an initial hyperpolarization followed by prolonged depolarization. Moderate high K(+) typically induced dilation, constriction and a dilation followed by constriction in the SMA, MA and BA, respectively. 3) Boltzmann-fit analysis of the Ba(2+)-sensitive inward rectifier K(+) (Kir) whole-cell current showed that the maximum Kir conductance density significantly differed among the vessels, and the half-activation voltage was significantly more negative in the MA. 4) Corresponding to the whole-cell data, computational modeling simulated the three RP distribution patterns and the dynamics of RP changes obtained experimentally, including the regenerative swift shifts between the two RP levels after reaching a threshold. 5) Molecular works revealed strong Kir2.1 and Kir2.2 transcripts and Kir2.1 immunolabeling in all 3 vessels, while Kir2.3 and Kir2.4 transcript levels varied. We conclude that a dense expression of functional Kir2.X channels underlies the more negative RPs in endothelial cells and a subset of VSMC in these arterioles, and the heterogeneous Kir function is primarily responsible for the distinct bimodal RPs among these arterioles. The fast Kir-based regenerative shifts between two RP states could form a critical mechanism for conduction/spread of vasomotion along the arteriole axis.

Photosensitive TRPs

The Drosophila "transient receptor potential" channel is the prototypical TRP channel, belonging to and defining the TRPC subfamily. Together with a second TRPC channel, trp-like (TRPL), TRP mediates the transducer current in the fly's photoreceptors. TRP and TRPL are also implicated in olfaction and Malpighian tubule function. In photoreceptors, TRP and TRPL are localised in the ~30,000 packed microvilli that form the photosensitive "rhabdomere"-a light-guiding rod, housing rhodopsin and the rest of the phototransduction machinery. TRP (but not TRPL) is assembled into multimolecular signalling complexes by a PDZ-domain scaffolding protein (INAD). TRPL (but not TRP) undergoes light-regulated translocation between cell body and rhabdomere. TRP and TRPL are also found in photoreceptor synapses where they may play a role in synaptic transmission. Like other TRPC channels, TRP and TRPL are activated by a G protein-coupled phospholipase C (PLCβ4) cascade. Although still debated, recent evidence indicates the channels can be activated by a combination of PIP2 depletion and protons released by the PLC reaction. PIP2 depletion may act mechanically as membrane area is reduced by cleavage of PIP2's bulky inositol headgroup. TRP, which dominates the light-sensitive current, is Ca(2+) selective (P Ca:P Cs >50:1), whilst TRPL has a modest Ca(2+) permeability (P Ca:P Cs ~5:1). Ca(2+) influx via the channels has profound positive and negative feedback roles, required for the rapid response kinetics, with Ca(2+) rapidly facilitating TRP (but not TRPL) and also inhibiting both channels. In trp mutants, stimulation by light results in rapid depletion of microvillar PIP2 due to lack of Ca(2+) influx required to inhibit PLC. This accounts for the "transient receptor potential" phenotype that gives the family its name and, over a period of days, leads to light-dependent retinal degeneration. Gain-of-function trp mutants with uncontrolled Ca(2+) influx also undergo retinal degeneration due to Ca(2+) cytotoxicity. In vertebrate retina, mice knockout studies suggest that TRPC6 and TRPC7 mediate a PLCβ4-activated transducer current in intrinsically photosensitive retinal ganglion cells, expressing melanopsin. TRPA1 has been implicated as a "photo-sensing" TRP channel in human melanocytes and light-sensitive neurons in the body wall of Drosophila.

TRPC5

Human canonical transient receptor potential channel 5 (TRPC5) has been cloned from the Xq23 region on chromosome X as a suspect in nonsyndromic mental retardation. TRPC5 is a Ca(2+)-permeable cation channel predominantly expressed in the CNS, including the hippocampus, cerebellum, amygdala, sensory neurons, and retina. It also shows more restricted expression in the periphery, notably in the kidney and cardiovascular system. Homotetrameric TRPC5 channels are primarily activated by receptors coupled to Gq and phospholipase C and/or Gi proteins, but TRPC5 channels may also gate in a store-dependent manner, which requires other partner proteins such TRPC1, STIM1, and Orai1. There is an impressive array of other activators of TRPC5 channels, such as nitric oxide, lysophospholipids, sphingosine-1-phosphate, reduced thioredoxin, protons, lanthanides, and calcium, and many can cause its direct activation. Moreover, TRPC5 shows constitutive activity, and it is responsive to membrane stretch and cold. Thus, TRPC5 channels have significant potential for synergistic activation and may serve as an important focal point in Ca(2+) signalling and electrogenesis. Moreover, TRPC5 functions in partnership with about 60 proteins, including TRPC1, TRPC4, calmodulin, IP3 receptors, NHERF, NCS-1, junctate, stathmin 2, Ca(2+)-binding protein 1, caveolin, and SESTD1, while its desensitisation is mediated by both protein kinases A and C. TRPC5 has a distinct voltage dependence shared only with its closest relative, TRPC4. Its unique N-shaped activation curve underlined by intracellular Mg(2+) block seems to be perfectly "shaped" to trigger action potential discharge, but not to grossly interfere with the action potential shape. The range of biological functions of TRPC5 channels is also impressive, from neurotransmission to control of axon guidance and vascular smooth muscle cell migration and contractility. Recent studies of Trpc5 gene knockouts begin to uncover its roles in fear, anxiety, seizures, and cold sensing.

Vascular KCNQ (Kv7) potassium channels as common signaling intermediates and therapeutic targets in cerebral vasospasm

Cerebral vasospasm after subarachnoid hemorrhage (SAH) is characterized by prolonged severe constriction of the basilar artery, which often leads to ischemic brain damage. Locally elevated concentrations of spasmogenic substances induce persistent depolarization of myocytes in the basilar artery, leading to continuous influx of calcium (Ca) through voltage-sensitive Ca channels and myocyte contraction. Potassium (K) channel openers may have therapeutic utility to oppose membrane depolarization, dilate the arteries, and reduce ischemia. Here, we examined the involvement of vascular Kv7 K channels in the pathogenesis of cerebral vasospasm and tested whether Kv7 channel openers are effective therapeutic agents in a rat model of SAH. Patch-clamp experiments revealed that 3 different spasmogens (serotonin, endothelin, and vasopressin) suppressed Kv7 currents and depolarized freshly isolated rat basilar artery myocytes. These effects were significantly reduced in the presence of a Kv7 channel opener, retigabine. Retigabine (10 μM) also significantly blocked L-type Ca channels, reducing peak inward currents by >50%. In the presence of a selective Kv7 channel blocker, XE991, the spasmogens did not produce additive constriction responses measured using pressure myography. Kv7 channel openers (retigabine or celecoxib) significantly attenuated basilar artery spasm in rats with experimentally induced SAH. In conclusion, we identify Kv7 channels as common targets of vasoconstrictor spasmogens and as candidates for therapeutic intervention for cerebral vasospasm.

Involvement of phosphatidylcholine-specific phospholipase C in thromboxane A₂ receptor-mediated extracellular Ca²⁺ influx in rat aorta

An involvement of signal transduction other than phosphatidylinositol turnover in thromboxane A(2) receptor (TP receptor)-mediated vascular contraction was investigated in rat aorta. The contraction induced by U46619, a TP receptor agonist, at low concentrations (≤ 30 nM) was partially inhibited by verapamil, an inhibitor of voltage-dependent Ca(2+) channels (VDCC), and was further diminished in Ca(2+)-free solution. Twenty nanomolar of U46619 induced contraction and elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)), which were consisted of two phases; slowly developing first phase followed by quickly rising second phase. The second phase was inhibited by verapamil, and all the [Ca(2+)](i) response was abolished in Ca(2+)-free solution. The contraction and [Ca(2+)](i) elevation induced by 20 nM U46619 were not inhibited by U73122, an inhibitor of phosphatidylinositol-specific phospholipase C, or GF109203X, a protein kinase C inhibitor, but were abolished by D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC). However, D609 had no effect on those induced by 1 μM phenylephrine. The U46619-induced responses were also partially inhibited by cation channel blockers, 2-APB and LOE908. The inhibition by LOE908 was abolished in the presence of verapamil, suggesting that LOE908-sensitive cation channels lead to the activation of VDCC by depolarizing plasma membrane. In contrast, 2-APB further diminished the U46619-induced [Ca(2+)](i) elevation in the presence of verapamil. In conclusion, TP receptor stimulation is suggested to be coupled with PC-PLC. Diacylglycerol produced by PC-PLC seems to activate two types of cation channels independently of PKC, which in turn leads to VDCC-dependent and independent Ca(2+) influx, thereby eliciting contraction.

Intrinsic and integrative properties of substantia nigra pars reticulata neurons

The GABA projection neurons of the substantia nigra pars reticulata (SNr) are output neurons for the basal ganglia and thus critical for movement control. Their most striking neurophysiological feature is sustained, spontaneous high frequency spike firing. A fundamental question is: what are the key ion channels supporting the remarkable firing capability in these neurons? Recent studies indicate that these neurons express tonically active type 3 transient receptor potential (TRPC3) channels that conduct a Na-dependent inward current even at hyperpolarized membrane potentials. When the membrane potential reaches -60 mV, a voltage-gated persistent sodium current (I(NaP)) starts to activate, further depolarizing the membrane potential. At or slightly below -50 mV, the large transient voltage-activated sodium current (I(NaT)) starts to activate and eventually triggers the rapid rising phase of action potentials. SNr GABA neurons have a higher density of I(NaT), contributing to the faster rise and larger amplitude of action potentials, compared with the slow-spiking dopamine neurons. I(NaT) also recovers from inactivation more quickly in SNr GABA neurons than in nigral dopamine neurons. In SNr GABA neurons, the rising phase of the action potential triggers the activation of high-threshold, inactivation-resistant Kv3-like channels that can rapidly repolarize the membrane. These intrinsic ion channels provide SNr GABA neurons with the ability to fire spontaneous and sustained high frequency spikes. Additionally, robust GABA inputs from direct pathway medium spiny neurons in the striatum and GABA neurons in the globus pallidus may inhibit and silence SNr GABA neurons, whereas glutamate synaptic input from the subthalamic nucleus may induce burst firing in SNr GABA neurons. Thus, afferent GABA and glutamate synaptic inputs sculpt the tonic high frequency firing of SNr GABA neurons and the consequent inhibition of their targets into an integrated motor control signal that is further fine-tuned by neuromodulators including dopamine, serotonin, endocannabinoids, and H₂O₂.

TRPM channels in the vasculature

Recent studies show that mammalian melastatin TRPM nonselective cation channels (TRPM1-8), members of the largest and most diverse TRP subfamily, are widely expressed in the endothelium and vascular smooth muscles. When activated, these channels similarly to other TRPs permit the entry of sodium, calcium and magnesium, thus causing membrane depolarisation. Although membrane depolarisation reduces the driving force for calcium entry via TRPMs as well as other pathways for calcium entry, in smooth muscle myocytes expressing voltage-gated Ca(2+) channels the predominant functional effect is an increase in intracellular Ca(2+) concentration and myocyte contraction. This review focuses on several best documented aspects of vascular functions of TRPMs, including the role of TRPM2 in oxidant stress, regulation of endothelial permeability and cell death, the connection between TRPM4 and myogenic response, significance of TRPM7 for magnesium homeostasis, vessel injury and hypertension, and emerging evidence that the cold and menthol receptor TRPM8 is involved in the regulation of vascular tone.

A transient receptor potential channel regulates basal ganglia output

The substantia nigra pars reticulata (SNr) is a key basal ganglia output nucleus critical for movement control. A hallmark of the SNr gamma-aminobutyric acid (GABA)-containing projection neurons is their depolarized membrane potential, accompanied by rapid spontaneous spikes. Parkinsonian movement disorders are often associated with abnormalities in SNr GABA neuron firing intensity and/or pattern. A fundamental question is the molecular identity of the ion channels that drive these neurons to a depolarized membrane potential. Recent data show that SNr GABA projection neurons selectively express type 3 canonical transient receptor potential (TRPC3) channels. Such channels are tonically active and mediate an inward, Na(+)-dependent current, leading to a substantial depolarization and ensuring appropriate firing intensity and pattern in SNr GABA projection neurons. Equally important, TRPC3 channels in SNr GABA neurons are up-regulated by dopamine (DA) released from neighboring nigral DA neuron dendrites. Co-activation of D1 and D5 DA receptors leads to a TRPC3 channel-mediated inward current and increased firing in SNr GABA neurons, whereas D1-like receptor blockade reduces SNr GABA neuron firing frequency and increases their firing irregularity. TRPC3 channels serve as the effector channels mediating an ultra-short SNc-->SNr DA pathway that regulates the firing intensity and pattern of the basal ganglia output neurons.

TRPC6 channels stimulated by angiotensin II are inhibited by TRPC1/C5 channel activity through a Ca2+- and PKC-dependent mechanism in native vascular myocytes

The present work investigated interactions between TRPC1/C5 and TRPC6 cation channel activities evoked by angiotensin II (Ang II) in native rabbit mesenteric artery vascular smooth muscle cells (VSMCs). In low intracellular Ca(2+) buffering conditions (0.1 mm BAPTA), 1 nm and 10 nm Ang II activated both 2 pS TRPC1/C5 channels and 15-45 pS TRPC6 channels in the same outside-out patches. However, increasing Ang II to 100 nm abolished TRPC6 activity but further increased TRPC1/C5 channel activity. Comparison of individual patches revealed an inverse relationship between TRPC1/C5 and TRPC6 channel activity suggesting that TRPC1/C5 inhibits TRPC6 channel activity. Inclusion of anti-TRPC1 and anti-TRPC5 antibodies, raised against intracellular epitopes, in the patch pipette solution blocked TRPC1/C5 channel currents but potentiated by about six-fold TRPC6 channel activity evoked by 1-100 nm Ang II in outside-out patches. Bath application of T1E3, an anti-TRPC1 antibody raised against an extracellular epitope, also increased Ang II-evoked TRPC6 channel activity. With high intracellular Ca(2+) buffering conditions (10 mm BAPTA), 10 nm Ang II-induced TRPC6 channel activity was increased by about five-fold compared to channel activity with low Ca(2+) buffering. In addition, increasing intracellular Ca(2+) levels ([Ca(2+)](i)) at the cytosolic surface inhibited 10 nm Ang II-evoked TRPC6 channel activity in inside-out patches. Moreover, in zero external Ca(2+) (0 [Ca(2+)](o)) 100 nm Ang II induced TRPC6 channel activity in outside-out patches. Pre-treatment with the PKC inhibitor, chelerythrine, markedly increased TRPC6 channel activity evoked by 1-100 nm Ang II and blocked the inhibitory action of [Ca(2+)](i) on TRPC6 channel activity. Co-immunoprecipitation studies shows that Ang II increased phosphorylation of TRPC6 proteins which was inhibited by chelerythrine, 0 [Ca(2+)](o) and the anti-TRPC1 antibody T1E3. These results show that TRPC6 channels evoked by Ang II are inhibited by TRPC1/C5-mediated Ca(2+) influx and stimulation of PKC, which phosphorylates TRPC6 subunits. These conclusions represent a novel interaction between two distinct vasoconstrictor-activated TRPC channels expressed in the same native VSMCs.

Upregulation of Na+/Ca2+ exchanger and TRPC6 contributes to abnormal Ca2+ homeostasis in arterial smooth muscle cells from Milan hypertensive rats

The Milan hypertensive strain (MHS) of rats is a model for hypertension in humans. Inherited defects in renal function have been well studied in MHS rats, but the mechanisms that underlie the elevated vascular resistance are unclear. Altered Ca(2+) signaling plays a key role in the vascular dysfunction associated with arterial hypertension. Here we compared Ca(2+) signaling in mesenteric artery smooth muscle cells from MHS rats and its normotensive counterpart (MNS). Systolic blood pressure was higher in MHS than in MNS rats (144 +/- 2 vs. 113 +/- 1 mmHg, P < 0.05). Resting cytosolic free Ca(2+) concentration (measured with fura-2) and ATP-induced Ca(2+) transients were augmented in freshly dissociated arterial myocytes from MHS rats. Ba(2+) entry activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol (a measure of receptor-operated channel activity) was much greater in MHS than MNS arterial myocytes. This correlated with a threefold upregulation of transient receptor potential canonical 6 (TRPC6) protein. TRPC3, the other component of receptor-operated channels, was marginally, but not significantly, upregulated. The expression of TRPC1/5, components of store-operated channels, was not altered in MHS mesenteric artery smooth muscle. Immunoblots also revealed that the Na(+)/Ca(2+) exchanger-1 (NCX1) was greatly upregulated in MHS mesenteric artery (by approximately 13-fold), whereas the expression of plasma membrane Ca(2+)-ATPase was not altered. Ca(2+) entry via the reverse mode of NCX1 evoked by the removal of extracellular Na(+) induced a rapid increase in cytosolic free Ca(2+) concentration that was significantly larger in MHS arterial myocytes. The expression of alpha(1)/alpha(2) Na(+) pumps in MHS mesenteric arteries was not changed. Immunocytochemical observations showed that NCX1 and TRPC6 are clustered in plasma membrane microdomains adjacent to the underlying sarcoplasmic reticulum. In summary, MHS arteries exhibit upregulated TRPC6 and NCX1 and augmented Ca(2+) signaling. We suggest that the increased Ca(2+) signaling contributes to the enhanced vasoconstriction and elevated blood pressure in MHS rats.

Calcium-sensing receptor activation contributed to apoptosis stimulates TRPC6 channel in rat neonatal ventricular myocytes

Capacitative calcium entry (CCE) refers to the influx of calcium through plasma membrane channels activated on depletion of endoplasmic sarcoplasmic/reticulum (ER/SR) Ca(2+) stores, which is performed mainly by the transient receptor potential (TRP) channels. TRP channels are expressed in cardiomyocytes. Calcium-sensing receptor (CaR) is also expressed in rat cardiac tissue and plays an important role in mediating cardiomyocyte apoptosis. However, there are no data regarding the link between CaR and TRP channels in rat heart. In this study, in rat neonatal myocytes, by Ca(2+) imaging, we found that the depletion of ER/SR Ca(2+) stores by thapsigargin (TG) elicited a transient rise in cytoplasmic Ca(2+) ([Ca(2+)](i)), followed by sustained increase depending on extracellular Ca(2+). But, TRP channels inhibitor (SKF96365), not L-type channels or the Na(+)/Ca(2+) exchanger inhibitors, inhibited [Ca(2+)](i) relatively high. Then, we found that the stimulation of CaR with its activator gadolinium chloride (GdCl(3)) or by an increased extracellular Ca(2+)([Ca(2+)](o)) increased the concentration of intracelluar Ca(2+), whereas, the sustained elevation of [Ca(2+)](i) was reduced in the presence of SKF96365. Similarly, the duration of [Ca(2+)](i) increase was also shortened in the absence of extracellular Ca(2+). Western blot analysis showed that GdCl(3) increased the expression of TRPC6, which was reversed by SKF96365. Additionally, SKF96365 reduced cardiomyocyte apoptosis induced by GdCl(3). Our results suggested that CCE exhibited in rat neonatal myocytes and CaR activation induced Ca(2+)-permeable cationic channels TRPCs to gate the CCE, for which TRPC6 was one of the most likely candidates. TRPC6 channel was functionally coupled with CaR to enhance the cardiomyocyte apoptosis.

Ins(1,4,5)P3 interacts with PIP2 to regulate activation of TRPC6/C7 channels by diacylglycerol in native vascular myocytes

We investigated synergism between inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) and diacylglycerol (DAG) on TRPC6-like channel activity in rabbit portal vein myocytes using single channel recording and immunoprecipitation techniques. Ins(1,4,5)P(3) at 10 microm increased 3-fold TRPC6-like activity induced by 10 microm 1-oleoyl-2-acetyl-sn-glycerol (OAG), a DAG analogue. Ins(1,4,5)P(3) had no effect on OAG-induced TRPC6 activity in mesenteric artery myocytes. Anti-TRPC6 and anti-TRPC7 antibodies blocked channel activity in portal vein but only anti-TRPC6 inhibited activity in mesenteric artery. TRPC6 and TRPC7 proteins strongly associated in portal vein but only weakly associated in mesenteric artery tissue lysates. Therefore in portal vein the conductance consists of TRPC6/C7 subunits, while OAG activates a homomeric TRPC6 channel in mesenteric artery myocytes. Wortmannin at 20 microm reduced phosphatidylinositol 4,5-bisphosphate (PIP(2)) association with TRPC6 and TRPC7, and produced a 40-fold increase in OAG-induced TRPC6/C7 activity. Anti-PIP(2) antibodies evoked TRPC6/C7 activity, which was blocked by U73122, a phospholipase C inhibitor. DiC8-PIP(2), a water-soluble PIP(2) analogue, inhibited OAG-induced TRPC6/C7 activity with an IC(50) of 0.74 microm. Ins(1,4,5)P(3) rescued OAG-induced TRPC6/C7 activity from inhibition by diC8-PIP(2) in portal vein myocytes, and this was not prevented by the Ins(1,4,5)P(3) receptor antagonist heparin. In contrast, Ins(1,4,5)P(3) did not overcome diC8-PIP(2)-induced inhibition of TRPC6 activity in mesenteric artery myocytes. 2,3,6-Tri-O-butyryl-Ins(1,4,5)P(3)/AM (6-Ins(1,4,5)P(3)), a cell-permeant analogue of Ins(1,4,5)P(3), at 10 microm increased TRPC6/C7 activity in portal vein and reduced association between TRPC7 and PIP(2), but not TRPC6 and PIP(2). In contrast, 10 microm OAG reduced association between TRPC6 and PIP(2), but not between TRPC7 and PIP(2). The present work provides the first evidence that Ins(1,4,5)P(3) modulates native TRPC channel activity through removal of the inhibitory action of PIP(2) from TRPC7 subunits.

Pharmacology of transient receptor potential melastatin channels in the vasculature

Mammalian transient receptor potential melastatin (TRPM) non-selective cation channels, the largest TRP subfamily, are widely expressed in excitable and non-excitable cells where they perform diverse functions ranging from detection of cold, taste, osmolarity, redox state and pH to control of Mg(2+) homeostasis and cell proliferation or death. Recently, TRPM gene expression has been identified in vascular smooth muscles with dominance of the TRPM8 channel. There has been in parallel considerable progress in decoding the functional roles of several TRPMs in the vasculature. This research on native cells is aided by the knowledge of the activation mechanisms and pharmacological properties of heterologously expressed TRPM subtypes. This paper summarizes the present state of knowledge of vascular TRPM channels and outlines several anticipated directions of future research in this area.

New tricks for old dogs: KCNQ expression and role in smooth muscle

Ion channels encoded by the KCNQ gene family (K(v)7.1-7.5) are major determinants of neuronal membrane potential and the cardiac action potential. This key physiological role is highlighted by the existence of a number of hereditary disorders caused by mutations to KCNQ genes. Recently, KCNQ gene expression has been identified in vascular and non-vascular smooth muscles. In addition, experiments with an array of pharmacological modulators of KCNQ channels have supported a crucial role for these channels in regulating smooth muscle contractility. This article will provide an overview of present understanding in this nascent area of KCNQ research and will offer guidance as to future directions.

Attenuation of canonical transient receptor potential-like channel 6 expression specifically reduces the diacylglycerol-mediated increase in intracellular calcium in human myometrial cells

An increase in intracellular Ca(2+) ([Ca(2+)](i)) as a result of release of Ca(2+) from intracellular stores or influx of extracellular Ca(2+) contributes to the regulation of smooth muscle contractile activity. Human uterine smooth muscle cells exhibit receptor-, store-, and diacylglycerol (OAG)-mediated extracellular Ca(2+)-dependent increases in [Ca(2+)](i) (SRCE) and express canonical transient receptor potential-like channels (TRPC) mRNAs (predominantly TRPC1, -4, and -6) that have been implicated in SRCE. To determine the role of TRPC6 in human myometrial SRCE, short hairpin RNA constructs were designed that effectively targeted a TRPC6 mRNA reporter for degradation. One sequence was used to produce an adenovirus construct (TC6sh1). TC6sh1 reduced TRPC6 mRNA but not TRPC1, -3, -4, -5, or -7 mRNAs in PHM1-41 myometrial cells. Compared with uninfected cells or cells infected with empty vector, the increase in [Ca(2+)](i) in response to OAG was specifically inhibited by TC6sh1, whereas SRCE responses elicited by either oxytocin or thapsigargin were not changed. Similar findings were observed in primary pregnant human myometrial cells. When PHM1-41 cells were activated by OAG in the absence of extracellular Na(+), the increase in [Ca(2+)](i) was partially reduced. Furthermore, pretreatment with nifedipine, an L-type calcium channel blocker, also partially reduced the OAG-induced [Ca(2+)](i) increase. Similar effects were observed in primary human myometrial cells. These findings suggest that OAG activates channels containing TRPC6 in myometrial cells and that these channels act via both enhanced Na(+) entry coupled to activation of voltage-dependent Ca(2+) entry channels and a nifedipine-independent Ca(2+) entry mechanism to promote elevation of intracellular Ca(2+).

Alpha(1D)-adrenergic receptor insensitivity is associated with alterations in its expression and distribution in cultured vascular myocytes

AIM

It is unclear why alpha(1D)-adrenergic receptors (alpha(1D)-ARs) play a critical role in the mediation of peripheral vascular resistance and blood pressure in situ but function inefficiently when studied in vitro. The present study examined the causes for these inconsistencies in native alpha(1)-adrenergic functional performance between the vascular smooth muscle and myocytes.

METHODS

The alpha(1)-adrenergic mediated contraction, Ca(2+) signaling and the subcellular receptor distribution were evaluated using the Fluo-4, BODIPY-FL prazosin and subtype-specific antibodies.

RESULTS

Rat aortic rings and freshly dissociated myocytes displayed contractile and increased intracellular Ca(2+) responses to stimulation with phenylephrine (PE, 10 micromol), respectively. However, the PE-induced responses disappeared completely in cultured aortic myocytes, whereas PE-enhanced Ca(2+) transients were seen in cultured rat cardiac myocytes. Further studies indicated that alpha(1D)-ARs, the major receptor subtype responsible for the alpha(1)-adrenergic regulation of aortic contraction, were distributed both intracellularly and at the cell membrane in freshly dispersed aortic myocytes, similar to the alpha(1A)-AR subcellular localization in the cultured cardiomyocytes. In the cultured aortic myocytes, however, in addition to a marked decrease in their protein expression relative to the aorta, most labeling signals for alpha(1D)-ARs were found in the cytoplasm. Importantly, treating the culture medium with charcoal/dextran caused the reappearance of alpha(1D)-ARs at the cell surface and a partial restoration of the Ca(2+) signal response to PE in approximately 30% of the cultured cells.

CONCLUSION

Reduction in alpha(1D)-AR total protein expression and disappearance from the cell surface contribute to the insensitivity of cultured vascular smooth muscle cells to alpha(1)-adrenergic receptor activation.

Upregulation of Na+ and Ca2+ transporters in arterial smooth muscle from ouabain-induced hypertensive rats

Prolonged ouabain administration (25 microg kg(-1) day(-1) for 5 wk) induces "ouabain hypertension" (OH) in rats, but the molecular mechanisms by which ouabain elevates blood pressure are unknown. Here, we compared Ca(2+) signaling in mesenteric artery smooth muscle cells (ASMCs) from normotensive (NT) and OH rats. Resting cytosolic free Ca(2+) concentration ([Ca(2+)](cyt); measured with fura-2) and phenylephrine-induced Ca(2+) transients were augmented in freshly dissociated OH ASMCs. Immunoblots revealed that the expression of the ouabain-sensitive alpha(2)-subunit of Na(+) pumps, but not the predominant, ouabain-resistant alpha(1)-subunit, was increased (2.5-fold vs. NT ASMCs) as was Na(+)/Ca(2+) exchanger-1 (NCX1; 6-fold vs. NT) in OH arteries. Ca(2+) entry, activated by sarcoplasmic reticulum (SR) Ca(2+) store depletion with cyclopiazonic acid (SR Ca(2+)-ATPase inhibitor) or caffeine, was augmented in OH ASMCs. This reflected an augmented expression of 2.5-fold in OH ASMCs of C-type transient receptor potential TRPC1, an essential component of store-operated channels (SOCs); two other components of some SOCs were not expressed (TRPC4) or were not upregulated (TRPC5). Ba(2+) entry activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol [a measure of receptor-operated channel (ROC) activity] was much greater in OH than NT ASMCs. This correlated with a sixfold upregulation of TRPC6 protein, a ROC family member. Importantly, in primary cultured mesenteric ASMCs from normal rats, 72-h treatment with 100 nM ouabain significantly augmented NCX1 and TRPC6 protein expression and increased resting [Ca(2+)](cyt) and ROC activity. SOC activity was also increased. Silencer RNA knockdown of NCX1 markedly downregulated TRPC6 and eliminated the ouabain-induced augmentation; silencer RNA knockdown of TRPC6 did not affect NCX1 expression but greatly attenuated its upregulation by ouabain. Clearly, NCX1 and TRPC6 expression are interrelated. Thus, prolonged ouabain treatment upregulates the Na(+) pump alpha(2)-subunit-NCX1-TRPC6 (ROC) Ca(2+) signaling pathway in arterial myocytes in vitro as well as in vivo. This may explain the augmented myogenic responses and enhanced phenylephrine-induced vasoconstriction in OH arteries (83) as well as the high blood pressure in OH rats.

An ultra-short dopamine pathway regulates basal ganglia output

Substantia nigra pars reticulata (SNr) is a key basal ganglia output nucleus critical for movement control. Its GABA-containing projection neurons intermingle with nigral dopamine (DA) neuron dendrites. Here we show that SNr GABA neurons coexpress dopamine D(1) and D(5) receptor mRNAs and also mRNA for TRPC3 channels. Dopamine induced an inward current in these neurons and increased their firing frequency. These effects were mimicked by D(1)-like agonists, blocked by a D(1)-like antagonist. D(1)-like receptor blockade reduced SNr GABA neuron firing frequency and increased their firing irregularity. These D(1)-like effects were absent in D(1) or D(5) receptor knock-out mice and inhibited by intracellularly applied D(1) or D(5) receptor antibody. These D(1)-like effects were also inhibited when the tonically active TRPC3 channels were inhibited by intracellularly applied TRPC3 channel antibody. Furthermore, stimulation of DA neurons induced a direct inward current in SNr GABA neurons that was sensitive to D(1)-like blockade. Manipulation of DA neuron activity and DA release and inhibition of dopamine reuptake affected SNr GABA neuron activity in a D(1)-like receptor-dependent manner. Together, our findings indicate that dendritically released dopamine tonically excites SNr GABA neurons via D(1)-D(5) receptor coactivation that enhances constitutively active TRPC3 channels, forming an ultra-short substantia nigra pars compacta --> SNr dopamine pathway that regulates the firing intensity and pattern of these basal ganglia output neurons.

Transient receptor potential melastatin 8 channel involvement in the regulation of vascular tone

The transient receptor potential melastatin 8 (TRPM8) channel has been characterized as a cold and menthol receptor expressed in a subpopulation of sensory neurons but was recently identified in other tissues, including the respiratory tract, urinary system, and vasculature. Thus TRPM8 may play multiple functional roles, likely to be in a tissue- and activation state-dependent manner. We examined the TRPM8 channel presence in large arteries from rats and the functional consequences of their activation. We also aimed to examine whether these channels contribute to control of conscious human skin blood flow. TRPM8 mRNA and protein were detected in rat tail, femoral and mesenteric arteries, and thoracic aorta. This was confirmed in single isolated vascular myocytes by immunocytochemistry. Isometric contraction studies on endothelium-denuded relaxed rat vessels found small contractions on application of the TRPM8-specific agonist menthol (300 microM). However, both menthol and another agonist icilin (50 microM) caused relaxation of vessels precontracted with KCl (60 mM) or the alpha-adrenoceptor agonist phenylephrine (2 microM) and a reduction in sympathetic nerve-mediated contraction. These effects were antagonized by bromoenol lactone treatment, suggesting the involvement of Ca(2+)-independent phospholipase A(2) activation in TRPM8-mediated vasodilatation. In thoracic aorta with intact endothelium, menthol-induced inhibition of KCl-induced contraction was enhanced. This was unaltered by preincubation with either N(omega)-nitro-l-arginine methyl ester (l-NAME; 100 nM), a nitric oxide synthase inhibitor, or the ACh receptor antagonist atropine (1 microM). Application of menthol (3% solution, topical application) to skin caused increased blood flow in conscious humans, as measured by laser Doppler fluximetry. Vasodilatation was markedly reduced or abolished by prior application of l-NAME (passive application, 10 mM) or atropine (iontophoretic application, 100 nM, 30 s at 70 microA). We conclude that TRPM8 channels are present in rat artery vascular smooth muscle and on activation cause vasoconstriction or vasodilatation, dependent on previous vasomotor tone. TRPM8 channels may also contribute to human cutaneous vasculature control, likely with the involvement of additional neuronal mechanisms.

Obligatory role for phosphatidylinositol 4,5-bisphosphate in activation of native TRPC1 store-operated channels in vascular myocytes

In the present study the effect of phosphatidylinositol 4,5-bisphosphate (PIP₂) was studied on a native TRPC1 store-operated channel (SOC) in freshly dispersed rabbit portal vein myocytes. Application of diC8-PIP₂, a water soluble form of PIP₂, to quiescent inside-out patches evoked single channel currents with a unitary conductance of 1.9 pS. DiC8-PIP₂-evoked channel currents were inhibited by anti-TRPC1 antibodies and these characteristics are identical to SOCs evoked by cyclopiazonic acid (CPA) and BAPTA-AM. SOCs stimulated by CPA, BAPTA-AM and the phorbol ester phorbol 12,13-dibutyrate (PDBu) were reduced by anti-PIP₂ antibodies and by depletion of tissue PIP₂ levels by pre-treatment of preparations with wortmannin and LY294002. However, these reagents did not alter the ability of PIP₂ to activate SOCs in inside-out patches. Co-immunoprecipitation techniques demonstrated association between TRPC1 and PIP₂ at rest, which was greatly decreased by wortmannin and LY294002. Pre-treatment of cells with PDBu, which activates protein kinase C (PKC), augmented SOC activation by PIP₂ whereas the PKC inhibitor chelerythrine decreased SOC stimulation by PIP₂. Co-immunoprecipitation experiments provide evidence that PKC-dependent phosphorylation of TRPC1 occurs constitutively and was increased by CPA and PDBu but decreased by chelerythrine. These novel results show that PIP₂ can activate TRPC1 SOCs in native vascular myocytes and plays an important role in SOC activation by CPA, BAPTA-AM and PDBu. Moreover, the permissive role of PIP₂ in SOC activation requires PKC-dependent phosphorylation of TRPC1.

A non-selective cationic channel activated by diacylglycerol in mouse intestinal myocytes

Application of 1-oleoyl-2-acetyl-sn-glycerol (OAG), an analogue of diacylglycerol (DAG) formed via M(3) muscarinic receptors, induced inward cationic currents via a protein kinase C-independent mechanism and produced membrane depolarization with increased action potential discharges in mouse intestinal myocytes. Outside-out patches from the myocytes responded to OAG with openings of 115-pS channels characterized by a mean open time (O(tau)) of 0.15 ms. M(3) receptor stimulation is reportedly capable of causing brief openings (O(tau)=0.23 ms) of 120-pS cationic channels in intestinal myocytes, thus the present results strongly support the idea that the M(3)-mediated 120-pS channel opening is brought about via DAG-dependent mechanisms.

Mechanical deformation of ventricular myocytes modulates both TRPC6 and Kir2.3 channels

Cardiomyocytes respond to mechanical stretch with an increase [Ca2+]i. Here, we analyzed which ion channels could mediate this effect. Murine ventricular myocytes were attached to a glass coverslip and a cell-attached glass stylus sheared the upper cell part versus the attached cell bottom. At negative clamp potentials, stretch induced inward currents that increased with the extent of stretch and reversed within 2 min after relaxation from stretch. Stretch activated a nearly voltage-independent GsMTx-4-sensitive non-selective cation conductance Gns, antibodies against TRPC6 prevented Gns activation. In addition, stretch deactivated a Cs+-sensitive inwardly rectifying potassium conductance GK1, antibodies against Kir2.3 inhibited this effect. Immunolabeling localized TRPC6 and Kir2.3 in T-tubular membranes, and stretch-induced changes in membrane currents were absent in cells whose T-tubules had been removed. In absence of stretch, we could activate Gns and deactivate GK1 by 1-oleoyl-2-acetyl-sn-glycerol (OAG) and other amphipaths. We interpret that the function of TRPC6 and Kir2.3 channels is controlled by both tension and curvature of the surrounding lipid bilayer that are changed by incorporation of amphipaths. Stretch-activation of TRPC6 channels may increase Ca2+ influx directly and indirectly, by membrane depolarization (activation of voltage-gated Ca2+ channels) and by elevated [Na+]i (augmented Na+,Ca2+-exchange).

Inhibition of native TRPC6 channel activity by phosphatidylinositol 4,5-bisphosphate in mesenteric artery myocytes

The present work investigates the effect of phosphatidylinositol-4,5-bisphosphate (PIP(2)) on native TRPC6 channel activity in freshly dispersed rabbit mesenteric artery myocytes using patch clamp recording and co-immunoprecipitation methods. Inclusion of 100 microM diC8-PIP(2) in the patch pipette and bathing solutions, respectively, inhibited angiotensin II (Ang II)-evoked whole-cell cation currents and TRPC6 channel activity by over 90%. In inside-out patches diC8-PIP(2) also inhibited TRPC6 activity induced by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) with an IC(50) of 7.6 microM. Anti-PIP(2) antibodies potentiated Ang II- and OAG-evoked TRPC6 activity by about 2-fold. Depleters of tissue PIP(2) wortmannin and LY294002 stimulated TRPC6 activity, as did the polycation PIP(2) scavenger poly-L-lysine. Wortmannin reduced Ang II-evoked TRPC6 activity by over 75% but increased OAG-induced TRPC6 activity by over 50-fold. Co-immunoprecipitation studies demonstrated association between PIP(2) and TRPC6 proteins in tissue lysates. Pre-treatment with Ang II, OAG and wortmannin reduced TRPC6 association with PIP(2). These results provide for the first time compelling evidence that constitutively produced PIP(2) exerts a powerful inhibitory action on native TRPC6 channels.

Examination of signalling pathways involved in muscarinic responses in bovine ciliary muscle using YM-254890, an inhibitor of the Gq/11 protein

BACKGROUND AND PURPOSE

In the ciliary muscle, the tonic component of the contraction produced by cholinergic agonists is highly dependent on Ca2+ provided by influx through non-selective cation channels (NSCCs) opened by stimulation of M3 muscarinic receptors. We examined effects of YM-254890 (YM), a Gq/11-specific inhibitor, on contraction, NSCC currents and [Ca2+]i elevation induced by carbachol (CCh).

EXPERIMENTAL APPROACH

Isometric tension was recorded from ciliary muscle bundles excised from bovine eyes. In ciliary myocytes dispersed with collagenase and cultured for 1-5 days, whole-cell currents were recorded by voltage clamp and the intracellular free Ca2+ concentration [Ca2+]i was monitored using the Fluo-4 fluorophore. Existence and localization of M3 receptors and the alpha subunit of Gq/11 (Galpha(q/11)) were examined by immunofluorescence microscopy using AlexaFluor-conjugated antibodies.

KEY RESULTS

Both phasic and tonic components of contractions evoked by 2 microM CCh were inhibited by YM (3-10 microM) in a dose-dependent manner. In the cultured cells, CCh (0.05-10 microM) evoked an NSCC current as well as an elevation of the [Ca2+]i. Both initial and sustained phases of these CCh-evoked responses were abolished by YM (3-10 microM). Immunostaining of the cytoplasmic side of the plasma membrane of ciliary myocytes revealed a dense distribution of M3 receptors and Galpha(q/11).

CONCLUSIONS AND IMPLICATIONS

The tonic as well as phasic component of the ciliary muscle contraction appears to be under control of signals conveyed by a G(q/11)-coupled pathway. YM is a useful tool to assess whether Gq/11 is involved in a signal transduction system.

Diverse properties of store-operated TRPC channels activated by protein kinase C in vascular myocytes

In vascular smooth muscle, store-operated channels (SOCs) contribute to many physiological functions including vasoconstriction and cell growth and proliferation. In the present work we compared the properties of SOCs in freshly dispersed myocytes from rabbit coronary and mesenteric arteries and portal vein. Cyclopiazonic acid (CPA)-induced whole-cell SOC currents were sixfold greater at negative membrane potentials and displayed markedly different rectification properties and reversal potentials in coronary compared to mesenteric artery myocytes. Single channel studies showed that endothelin-1, CPA and the cell-permeant Ca(2+) chelator BAPTA-AM activated the same 2.6 pS SOC in coronary artery. In 1.5 mM [Ca(2+)](o) the unitary conductance of SOCs was significantly greater in coronary than in mesenteric artery. Moreover in 0 mM [Ca(2+)](o) the conductance of SOCs in coronary artery was unaltered whereas the conductance of SOCs in mesenteric artery was increased fourfold. In coronary artery SOCs were inhibited by the protein kinase C (PKC) inhibitor chelerythrine and activated by the phorbol ester phorbol 12,13-dibutyrate (PDBu), the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) and a catalytic subunit of PKC. These data infer an important role for PKC in activation of SOCs in coronary artery similar to mesenteric artery and portal vein. Anti-TRPC1 and -TRPC5 antibodies inhibited SOCs in coronary and mesenteric arteries and portal vein but anti-TRPC6 blocked SOCs only in coronary artery and anti-TRPC7 blocked SOCs only in portal vein. Immunoprecipitation showed associations between TRPC1 and TRPC5 in all preparations but between TRPC5 and TRPC6 only in coronary artery and between TRPC5 and TRPC7 only in portal vein. Finally, flufenamic acid increased SOC activity in coronary artery but inhibited SOCs in mesenteric artery and portal vein myocytes. These data provide strong evidence that vascular myocytes express diverse SOC isoforms, which are likely to be composed of different TRPC proteins and have different physiological functions.

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.

ACh-induced depolarization in inner ear artery is generated by activation of a TRP-like non-selective cation conductance and inactivation of a potassium conductance

Adequate cochlear blood supply by the spiral modiolar artery (SMA) is critical for normal hearing. ACh may play a role in neuroregulation of the SMA but several key issues including its membrane action mechanisms remain poorly understood. Besides its well-known endothelium-dependent hyperpolarizing action, ACh can induce a depolarization in vascular cells. Using intracellular and whole-cell recording techniques on cells in guinea pig in vitro SMA, we studied the ionic mechanism underlying the ACh-depolarization and found that: (1) ACh induced a DAMP-sensitive depolarization when intermediate conductance KCa channels were blocked by charybdotoxin or nitrendipine. The ACh-depolarization was associated with a decrease in input resistance (R(input)) in high membrane potential (V(m)) ( approximately -40 mV) cells but with no change or an increase in R input in low Vm ( approximately -75 mV) cells. ACh-depolarization was attenuated by background membrane depolarization from approximately -70 mV in the majority of cells; (2) ACh-induced inward current in smooth muscle cells embedded in a SMA segment often showed a U-shaped I/V curve, the reversal potential of its two arms being near EK and 0 mV, respectively; (3) ACh-depolarization was reduced by low Na+, zero K+ or 20mM K+ bath solutions; (4) ACh-depolarization was inhibited by La3+ in all cells tested, by 4-AP and flufenamic acid in low Vm cells, but was not sensitive to Cd2+, Ni2+, nifedipine, niflumic acid, DIDS, IAA94, linopirdine or amiloride. We conclude that ACh-induced vascular depolarization was generated mainly by activation of a TRP-like non-selective cation channel and by inactivation of an inward rectifier K+ channel.

Synergistic actions of diacylglycerol and inositol 1,4,5 trisphosphate for Ca2+-dependent inactivation of TRPC7 channel

AIM

The aim of the present study was to explore the mechanism for the Ca2+- dependent inactivation of the canonical transient receptor potential (TRPC) 7 channel expressed in human embryonic kidney 293 cells.

METHOD

The whole-cell patch-clamp technique was used in the study.

RESULTS

With Ca2+-free external solution, the perfusion of 100 micromol/L carbachol to, or dialysis of the cell with 100 micromol/L guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), induced large inward currents, respectively. These currents were rapidly inhibited by the addition of 1 mmol/L Ca2+ into the bath, and recovery from this inhibition was only partial after the Ca2+ removal, unless vigorous intracellular Ca2+ buffering with 10 mmol/L 1,2 bis(2- aminophenoxy)ethane-N,N,No,No-tetraacetic acid (BAPTA) (plus 4 mmol/L Ca2+) was employed. In contrast, the current induced by a membrane-permeable analog of diacylglycerol (DAG), 1-oleoyl-2-acetyl-sn-glycerol (OAG; 100 micromol/L) did not undergo the inhibition persisting after Ca2+ removal. Interestingly, the inclusion of inositol 1,4,5 trisphosphate (IP3; 100 micromol/L) in the patch pipette rendered the OAG-induced current susceptible to the persistent Ca2+-mediated inhibition independent of the IP3 receptor in the majority of the tested cells, as evidenced by the inability of heparin and thapsigargin in reversing the effect of IP3.

CONCLUSION

The present results suggest that Ca2+ entry via the activated TRPC7 channel plays a critical role in inactivating the channel where the cooperative actions of DAG and IP3 are essentially involved.

Angiotensin II-stimulated Ca2+ entry mechanisms in afferent arterioles: role of transient receptor potential canonical channels and reverse Na+/Ca2+ exchange

In afferent arterioles, the signaling events that lead to an increase in cytosolic Ca2+ concentration ([Ca2+]i) and initiation of vascular contraction are increasingly being delineated. We have recently studied angiotensin II (ANG II)-mediated effects on sarcoplasmic reticulum (SR) mobilization of Ca2+ and the role of superoxide and cyclic adenosine diphosphoribose in these processes. In the current study we investigated the participation of transient receptor potential canonical channels (TRPC) and a Na+/Ca2+ exchanger (NCX) in Ca2+ entry mechanisms. Afferent arterioles, isolated with the magnetized polystyrene bead method, were loaded with fura-2 to measure [Ca2+]i ratiometrically. We observed that the Ca2+-dependent chloride channel blocker niflumic acid (10 and 50 μ M) affects neither the peak nor plateau [Ca2+]i response to ANG II. Arterioles were pretreated with ryanodine (100 μM) and TMB-8 to block SR mobilization via the ryanodine receptor and inositol trisphosphate receptor, respectively. The peak [Ca2+]i response to ANG II was reduced by 40%. Addition of 2-aminoethoxydiphenyl borane to block TRPC-mediated Ca2+ entry inhibited the peak [Ca2+]i ANG II response by 80% and the plateau by 74%. Flufenamic acid (FFA; 50 μM), which stimulates TRPC6, caused a sustained increase of [Ca2+]i of 146 nM. This response was unaffected by diltiazem or nifedipine. KB-R7943 (at the low concentration of 10 μM) inhibits reverse (but not forward) mode NCX. KB-R7943 decreased the peak [Ca2+]i response to ANG II by 48% and to FFA by 38%. We conclude that TRPC6 and reverse-mode NCX may be important Ca2+ entry pathways in afferent arterioles.

Effects of the muscarinic agonist, 5-methylfurmethiodide, on contraction and electrophysiology of Ascaris suum muscle

Contraction and electrophysiological effects of 5-methylfurmethiodide (MFI), a selective muscarinic agonist in mammals, were tested on Ascaris suum muscle strips. In a contraction assay, MFI produced weak contraction and was less potent than levamisole and acetylcholine. Atropine (3microM) a non-selective muscarinic antagonist in mammalian preparations, did not affect contractions produced by MFI. Mecamylamine (3microM) a nicotinic antagonist in A. suum preparations, blocked the MFI contractions indicating that MFI had weak nicotinic agonist actions. In two-micropipette current-clamp experiments MFI, at concentrations greater than 10microM, produced concentration-dependent depolarizations and small increases in membrane conductance. The depolarizing effects were not abolished by perfusing the preparation in a calcium-free Ascaris Ringer solution to block synaptic transmission, suggesting that MFI effects were mediated by receptors on the muscle and were calcium-independent. A high concentration of mecamylamine, 30microM, only reduced the depolarizing responses by 42%, indicating that MFI also had effects on non-nicotinic receptors. Three non-nicotinic effects in the presence of 30microM mecamylamine were identified using voltage-clamp techniques: (i) MFI produced opening of mecamylamine-resistant non-selective-cation channel currents; (ii) MFI inhibited opening of voltage-activated potassium currents; and (iii) MFI increased the threshold of voltage-activated calcium currents. We suggest that a drug that is more selective for voltage-activated potassium currents, without effects on other channels like MFI, may be exploited pharmacologically as a novel anthelmintic or as an agent to potentiate the action of levamisole. In a larval migration assay we demonstrated that 4-aminopyridine (4-AP: a potassium channel blocker) potentiated the effects of levamisole but MFI did not.

Indirect coupling between Cav1.2 channels and ryanodine receptors to generate Ca2+ sparks in murine arterial smooth muscle cells

In arterial vascular smooth muscle cells (VSMCs), Ca(2+) sparks stimulate nearby Ca(2+)-activated K(+) (BK) channels that hyperpolarize the membrane and close L-type Ca(2+) channels. We tested the contribution of L-type Ca(v)1.2 channels to Ca(2+) spark regulation in tibial and cerebral artery VSMCs using VSMC-specific Ca(v)1.2 channel gene disruption in (SMAKO) mice and an approach based on Poisson statistical analysis of activation frequency and first latency of elementary events. Ca(v)1.2 channel gene inactivation reduced Ca(2+) spark frequency and amplitude by approximately 50% and approximately 80%, respectively. These effects were associated with lower global cytosolic Ca(2+) levels and reduced sarcoplasmic reticulum (SR) Ca(2+) load. Elevating cytosolic Ca(2+) levels reversed the effects completely. The activation frequency and first latency of elementary events in both wild-type and SMAKO VSMCs weakly reflected the voltage dependency of L-type channels. This study provides evidence that local and tight coupling between the Ca(v)1.2 channels and ryanodine receptors (RyRs) is not required to initiate Ca(2+) sparks. Instead, Ca(v)1.2 channels contribute to global cytosolic [Ca(2+)], which in turn influences luminal SR calcium and thus Ca(2+) sparks.

Dynamics of a three-variable nonlinear model of vasomotion: comparison of theory and experiment

The effects of pharmacological interventions that modulate Ca(2+) homeodynamics and membrane potential in rat isolated cerebral vessels during vasomotion (i.e., rhythmic fluctuations in arterial diameter) were simulated by a third-order system of nonlinear differential equations. Independent control variables employed in the model were [Ca(2+)] in the cytosol, [Ca(2+)] in intracellular stores, and smooth muscle membrane potential. Interactions between ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores and transmembrane ion fluxes via K(+) channels, Cl(-) channels, and voltage-operated Ca(2+) channels were studied by comparing simulations of oscillatory behavior with experimental measurements of membrane potential, intracellular free [Ca(2+)] and vessel diameter during a range of pharmacological interventions. The main conclusion of the study is that a general model of vasomotion that predicts experimental data can be constructed by a low-order system that incorporates nonlinear interactions between dynamical control variables.

Three distinct muscarinic signalling pathways for cationic channel activation in mouse gut smooth muscle cells

Using mutant mice genetically lacking certain subtypes of muscarinic receptor, we have studied muscarinic signal pathways mediating cationic channel activation in intestinal smooth muscle cells. In cells from M2 subtype-knockout (M2-KO) or M3-KO mice, carbachol (100 microM) evoked a muscarinic cationic current (mI(Cat)) as small as approximately 10% of mI(Cat) in wild-type (WT) cells. No appreciable current was evoked in M2/M3 double-KO cells. All mutant type cells preserved normal G-protein-cationic channel coupling. The M3-KO and WT mI(Cat) each showed a U-shaped current-voltage (I-V) relationship, whereas the M2-KO mI(Cat) displayed a linear I-V relationship. Channel analysis in outside-out patches recognized 70-pS and 120-pS channels as the major muscarinic cationic channels. Active patches of M2-KO cells exhibited both 70-pS and 120-pS channel activity usually together, either of which consisted of brief openings (the respective mean open times O(tau) = 0.55 and 0.23 ms). In contrast, active M3-KO patches showed only 70-pS channel activity, which had three open states (O(tau) = 0.55, 3.1 and 17.4 ms). In WT patches, besides the M2-KO and M3-KO types, another type of channel activity was also observed that consisted of 70-pS channel openings with four open states (O(tau) = 0.62, 2.7, 16.9 and 121.1 ms), and patch current of this channel activity showed a U-shaped I-V curve similar to the WT mI(Cat). The present results demonstrate that intestinal myocytes are endowed with three distinct muscarinic pathways mediating cationic channel activation and that the M2/M3 pathway targeting 70-pS channels, serves as the major contributor to mI(Cat) generation. The delineation of this pathway is consistent with the formation of a functional unit by the M2-Go protein and the M3-PLC systems predicted to control cationic channels.

Inhibition of KATP channels in the rat tail artery by neurally released noradrenaline acting on postjunctional alpha2-adrenoceptors

In rat tail artery, activation of postjunctional alpha(2)-adrenoceptors by noradrenaline (NA) released from sympathetic axons produces a slow depolarization (NAD) of the smooth muscle through a decrease in K(+) conductance. In this study we used intracellular recording to investigate whether the K(+) channel involved is the ATP-sensitive K(+) (K(ATP)) channel. Changes in membrane resistance were monitored by measuring the time constant of decay of excitatory junction potentials. The K(ATP) channel blockers, glibenclamide (10 microm) and PNU 37883A (5 microm), depolarized the smooth muscle and increased membrane resistance. Conversely, the K(ATP) channel openers, pinacidil (0.1 and 0.5 microm) and levcromakalim (0.1 microm), hyperpolarized the smooth muscle and decreased membrane resistance. Activation of K(ATP) channels with calcitonin gene-related peptide (CGRP; 10 nM) also hyperpolarized the smooth muscle and decreased membrane resistance. The NAD was abolished by both glibenclamide and PNU 37883A but was potentiated by CGRP. However, unlike CGRP, the directly acting K(ATP) channel openers, pinacidil and levcromakalim, inhibited the NAD. The effects of other K(+) channel blockers were also determined. A high concentration of Ba(2+)(1 mM), which would be expected to block K(ATP) channels, abolished the NAD, whereas teteraethylammonium (1 mM) and 4-aminopyridine (1 mM) increased its amplitude. Apamin (0.5 microm) and a lower concentration of Ba(2+) (0.1 mM) did not affect the NAD. These findings indicate that activation of alpha(2)-adrenoceptors by neurally released NA depolarizes the membrane of vascular smooth muscle by inhibiting K(ATP) channels open in the resting membrane.

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.

Upregulation of Na+/Ca2+ exchanger contributes to the enhanced Ca2+ entry in pulmonary artery smooth muscle cells from patients with idiopathic pulmonary arterial hypertension

A rise in cytosolic Ca(2+) concentration ([Ca(2+)](cyt)) in pulmonary artery smooth muscle cells (PASMC) is a trigger for pulmonary vasoconstriction and a stimulus for PASMC proliferation and migration. Multiple mechanisms are involved in regulating [Ca(2+)](cyt) in human PASMC. The resting [Ca(2+)](cyt) and Ca(2+) entry are both increased in PASMC from patients with idiopathic pulmonary arterial hypertension (IPAH), which is believed to be a critical mechanism for sustained pulmonary vasoconstriction and excessive pulmonary vascular remodeling in these patients. Here we report that protein expression of NCX1, an NCX family member of Na(+)/Ca(2+) exchanger proteins is upregulated in PASMC from IPAH patients compared with PASMC from normal subjects and patients with other cardiopulmonary diseases. The Na(+)/Ca(2+) exchanger operates in a forward (Ca(2+) exit) and reverse (Ca(2+) entry) mode. By activating the reverse mode of Na(+)/Ca(2+) exchange, removal of extracellular Na(+) caused a rapid increase in [Ca(2+)](cyt), which was significantly enhanced in IPAH PASMC compared with normal PASMC. Furthermore, passive depletion of intracellular Ca(2+) stores using cyclopiazonic acid (10 microM) not only caused a rise in [Ca(2+)](cyt) due to Ca(2+) influx through store-operated Ca(2+) channels but also mediated a rise in [Ca(2+)](cyt) via the reverse mode of Na(+)/Ca(2+) exchange. The upregulated NCX1 in IPAH PASMC led to an enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange, but did not accelerate Ca(2+) extrusion via the forward mode of Na(+)/Ca(2+) exchange. These observations indicate that the upregulated NCX1 and enhanced Ca(2+) entry via the reverse mode of Na(+)/Ca(2+) exchange are an additional mechanism responsible for the elevated [Ca(2+)](cyt) in PASMC from IPAH patients.