A TRPC-like non-selective cation current activated by α1-adrenoceptors in rat mesenteric artery smooth muscle cells

α1-Adrenergic Stimulation Increases Platelet Adhesion to Endothelial Cells Mediated by TRPC6

Stimulation of a1-adrenergic nervous system is increased during systemic inflammation and other pathological conditions with the consequent adrenergic receptors (ARs) activation. It has been reported that a1-stimulation contributes to coagulation since a1-AR blockers inhibit coagulation and its organic consequences. Also, coagulation induced by a1-AR stimulation can be greatly decreased using a1-AR blockers. In health, endothelial cells (ECs) perform anticoagulant actions at cellular and molecular level. However, during inflammation, ECs turn dysfunctional promoting a procoagulant state. Endothelium-dependent coagulation progresses at cellular and molecular levels, promoting endothelial acquisition of procoagulant properties to potentiate coagulation by means of prothrombotic and antifibrinolytic proteins expression increase in ECs releasing them to circulation, the thrombus formation is strengthened. Calcium signaling is a main feature of coagulation. Inhibition of ion channels involved in Ca²⁺ entry severely decreases coagulation. The transient receptor potential canonical 6 (TRPC6) is a non-selective Ca²⁺-permeable ion channel. TRPC6 activity is induced by diacylglycerol, suggesting that is regulated by a1-ARs. Furthermore, a1-ARs stimulation elicits a TRPC-like current in rat mesenteric artery smooth muscle and mesangial cells. However, whether TRPC6 could promote an ECs-mediated platelet adhesion induced by a1-adrenergic stimulation is currently not known. Therefore, the aim of this study was to examine if the TRPC6 calcium channel mediates platelet adhesion induced by a1-adrenergic stimulation. Our results suggest that platelet adhesion to ECs is enhanced by the a1-adrenergic stimulation evoked by phenylephrine mediated by TRPC6 activity. We conclude that TRPC6 is a molecular determinant in platelet adhesion to ECs with implications in systemic inflammatory diseases treatment.

Magnolol and honokiol target TRPC4 to regulate extracellular calcium influx and relax intestinal smooth muscle

ETHNOPHARMACOLOGICAL RELEVANCE

Magnolia officinalis Cortex (M. officinalis) is a classical traditional Chinese medicine (TCM) widely used to treat digestive system diseases. It effectively regulates gastrointestinal motility to improve abdominal pain, abdominal distension and other symptoms. Magnolol (MAG) and honokiol (HON) are the main pharmacodynamic components responsible for the gastrointestinal activity of M. officinalis.

AIM OF THE STUDY

The transient receptor potential (TRP) family is highly expressed in the gastrointestinal tract and participates in the regulation of gastrointestinal motility, visceral hypersensitivity, visceral secretion and other physiological activities. In this study, the calcium-lowering mechanisms of MAG and HON contributing to the smooth muscle relaxation associated with TRP are discussed.

MATERIALS AND METHODS

The relaxation smooth muscle effects of MAG and HON were tested by the isolated intestine tone tests. A synthetic MAG probe (MAG-P) was used to target fishing for their possible target. The distribution of MAG on the smooth muscle was identified by a molecular tracer based on chemical biology. Ca²⁺ imaging and dual-luciferase reporter assays were used to determine the effects on the target proteins. Finally, the calcium-mediating effects of MAG and HON on smooth muscle cells and TRPC4-knockdown cells were compared to verify the potential mechanism.

RESULTS

After confirming the smooth muscle relaxation in the small intestine induced by MAG and HON, the relaxation effect was identified mainly due to the downregulation of intracellular calcium by controlling external calcium influx. Although MAG and HON inhibited both TRPV4 and TRPC4 channels to reduce calcium levels, the inhibitory effect on TRPC4 channels is an important mechanism of their smooth muscle relaxation effect, since TRPC4 is widely expressed in the small intestinal smooth muscle cells.

CONCLUSIONS

The inhibition of MAG and HON on TRPC4 channels contributes to the relaxation of intestinal smooth muscle.

Vascular smooth muscle TRPC3 channels facilitate the inverse hemodynamic response during status epilepticus

Human status epilepticus (SE) is associated with a pathological reduction in cerebral blood flow termed the inverse hemodynamic response (IHR). Canonical transient receptor potential 3 (TRPC3) channels are integral to the propagation of seizures in SE, and vascular smooth muscle cell (VSMC) TRPC3 channels participate in vasoconstriction. Therefore, we hypothesize that cerebrovascular TRPC3 channels may contribute to seizure-induced IHR. To examine this possibility, we developed a smooth muscle-specific TRPC3 knockout (TRPC3smcKO) mouse. To quantify changes in neurovascular coupling, we combined laser speckle contrast imaging with simultaneous electroencephalogram recordings. Control mice exhibited multiple IHRs, and a limited increase in cerebral blood flow during SE with a high degree of moment-to-moment variability in which blood flow was not correlated with neuronal activity. In contrast, TRPC3smcKO mice showed a greater increase in blood flow that was less variable and was positively correlated with neuronal activity. Genetic ablation of smooth muscle TRPC3 channels shortened the duration of SE by eliminating a secondary phase of intense seizures, which was evident in littermate controls. Our results are consistent with the idea that TRPC3 channels expressed by cerebral VSMCs contribute to the IHR during SE, which is a critical factor in the progression of SE.

Membrane potential and Ca2+ concentration dependence on pressure and vasoactive agents in arterial smooth muscle: A model

A mathematical model incorporating junctional and stretch-activated microdomains and 37 protein components describes the myogenic response in arterial smooth muscle cells.

Arterial smooth muscle (SM) cells respond autonomously to changes in intravascular pressure, adjusting tension to maintain vessel diameter. The values of membrane potential (V_m) and sarcoplasmic Ca²⁺ concentration (Ca_in) within minutes of a change in pressure are the results of two opposing pathways, both of which use Ca²⁺ as a signal. This works because the two Ca²⁺-signaling pathways are confined to distinct microdomains in which the Ca²⁺ concentrations needed to activate key channels are transiently higher than Ca_in. A mathematical model of an isolated arterial SM cell is presented that incorporates the two types of microdomains. The first type consists of junctions between cisternae of the peripheral sarcoplasmic reticulum (SR), containing ryanodine receptors (RyRs), and the sarcolemma, containing voltage- and Ca²⁺-activated K⁺ (BK) channels. These junctional microdomains promote hyperpolarization, reduced Ca_in, and relaxation. The second type is postulated to form around stretch-activated nonspecific cation channels and neighboring Ca²⁺-activated Cl⁻ channels, and promotes the opposite (depolarization, increased Ca_in, and contraction). The model includes three additional compartments: the sarcoplasm, the central SR lumen, and the peripheral SR lumen. It incorporates 37 protein components. In addition to pressure, the model accommodates inputs of α- and β-adrenergic agonists, ATP, 11,12-epoxyeicosatrienoic acid, and nitric oxide (NO). The parameters of the equations were adjusted to obtain a close fit to reported V_m and Ca_in as functions of pressure, which have been determined in cerebral arteries. The simulations were insensitive to ±10% changes in most of the parameters. The model also simulated the effects of inhibiting RyR, BK, or voltage-activated Ca²⁺ channels on V_m and Ca_in. Deletion of BK β1 subunits is known to increase arterial–SM tension. In the model, deletion of β1 raised Ca_in at all pressures, and these increases were reversed by NO.

Regulation of calcium channels in smooth muscle: new insights into the role of myosin light chain kinase

Smooth muscle myosin light chain kinase (MLCK) plays a crucial role in artery contraction, which regulates blood pressure and blood flow distribution. In addition to this role, MLCK contributes to Ca(2+) flux regulation in vascular smooth muscle (VSM) and in non-muscle cells, where cytoskeleton has been suggested to help Ca(2+) channels trafficking. This conclusion is based on the use of pharmacological inhibitors of MLCK and molecular and cellular techniques developed to down-regulate the enzyme. Dissimilarities have been observed between cells and whole tissues, as well as between large conductance and small resistance arteries. A differential expression in MLCK and ion channels (either voltage-dependent Ca(2+) channels or non-selective cationic channels) could account for these observations, and is in line with the functional properties of the arteries. A potential involvement of MLCK in the pathways modulating Ca(2+) entry in VSM is described in the present review.

Gap junctions suppress electrical but not [Ca(2+)] heterogeneity in resistance arteries

Despite stochastic variation in the molecular composition and morphology of individual smooth muscle and endothelial cells, the membrane potential along intact microvessels is remarkably uniform. This is crucial for coordinated vasomotor responses. To investigate how this electrical homogeneity arises, a virtual arteriole was developed that introduces variation in the activities of ion-transport proteins between cells. By varying the level of heterogeneity and subpopulations of gap junctions (GJs), the resulting simulations shows that GJs suppress electrical variation but can only reduce cytosolic [Ca(2+)] variation. The process of electrical smoothing, however, introduces an energetic cost due to permanent currents, one which is proportional to the level of heterogeneity. This cost is particularly large when electrochemically different endothelial-cell and smooth-muscle-cell layers are coupled. Collectively, we show that homocellular GJs in a passively open state are crucial for electrical uniformity within the given cell layer, but homogenization may be limited by biophysical or energetic constraints. Owing to the ubiquitous presence of ion transport-proteins and cell-cell heterogeneity in biological tissues, these findings generalize across most biological fields.

Participation of the TRP channel in the cardiovascular effects induced by carvacrol in normotensive rat

Carvacrol has been described as an agonist/antagonist of different transient receptor potential (TRP) channels and voltage-dependent calcium channels (Cavs). The aim of this study was to evaluate the role of Cav and TRP channels following carvacrol stimulation. Initially, in mesenteric artery rings carvacrol relaxed phenylephrine-induced contractions. Furthermore, carvacrol inhibited contraction elicited by CaCl2 in depolarizing nominally without Ca2+ medium and antagonized the contractions induced by S(-)-Bay K 8644 and inhibited Ca2+ currents indicating the inhibition of Ca2+ influx through L-type Cav. Additionally, carvacrol antagonized the contractions induced by CaCl2 in the presence of nifedipine/Cyclopiazonic acid/phenylephrine or nifedipine/Cyclopiazonic acid/KCl 60, suggesting a possible inhibition of calcium influx by store operated channels (SOCs), receptor operated channels (ROCs) and/or TRP channels. Interestingly, among the TRP channel blockers used, the effect induced by carvacrol was attenuated by Mg2+ and potentiated by La3+ and Gd3+, suggesting that TRP channels are involved in relaxation induced by carvacrol. Monoterpene also induced hypotension and bradycardia in non-anesthetized normotensive rats and negative inotropic and chronotropic effects. In conclusion, these results suggest that the hypotensive effect of carvacrol is probably due to bradycardia and a peripheral vasodilatation that involves, at least, the inhibition of the Ca2+ influx through Cav and TRP channels.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Store-operated calcium entry induced by activation of Gq-coupled alpha1B adrenergic receptor in human osteoblast

Recent studies have revealed that the sympathetic nervous system is involved in bone metabolism. We previously reported that noradrenaline (NA) suppressed K(+) currents via Gi/o protein-coupled alpha1B-adrenergic receptor (α1B-AR) in human osteoblast SaM-1 cells. Additionally, it has been demonstrated that the intracellular Ca(2+) level ([Ca(2+)]i) was increased by NA via α1B-AR. In this study, we investigated the signal pathway of NA-induced [Ca(2+)]i elevation by using Ca(2+) fluorescence imaging in SaM-1 cells. NA-induced [Ca(2+)]i elevation was suppressed by pretreatment with a PLC inhibitor, U73122. This suggested that the [Ca(2+)]i elevation was mediated by Gq protein-coupled α1B-AR. On the other hand, NA-induced [Ca(2+)]i elevation was completely abolished in Ca(2+)-free solution, which suggested that Ca(2+) influx is the predominant pathway of NA-induced [Ca(2+)]i elevation. Although the inhibition of K(+) channel by NA caused membrane depolarization, the [Ca(2+)]i elevation was not affected by voltage-dependent Ca(2+) channel blockers, nifedipine and mibefradil. Meanwhile, NA-induced [Ca(2+)]i elevation was abolished following activation of store-operated Ca(2+) channel by thapsigargin. Additionally, the [Ca(2+)]i elevation was suppressed by store-operated channel inhibitors, 2-APB, flufenamate, GdCl3 and LaCl3. These results suggest that Ca(2+) influx through store-operated Ca(2+) channels plays a critical role in the signal transduction pathway of Gq protein-coupled α1B-AR in human osteoblasts.

Fenamates block gap junction coupling and potentiate BKCa channels in guinea pig arteriolar cells

We determined the actions of the fenamates, flufenamic acid (FFA) and niflumic acid (NFA), on gap junction-mediated intercellular coupling between vascular smooth muscle cells (VSMC) in situ of acutely isolated arteriole segments from the three vascular beds: the spiral modiolar artery (SMA), anterior inferior cerebellar artery (AICA) and mesenteric artery (MA), and on non-junctional membrane channels in dispersed VSMCs. Conventional whole-cell recording methods were used. FFA reversibly suppressed the input conductance (Ginput) or increased the input resistance (Rinput) in a concentration dependent manner, with slightly different IC50s for the SMA, AICA and MA segments (26, 33 and 56 μM respectively, P>0.05). Complete electrical isolation of the recorded VSMC was normally reached at ≥ 300 μM. NFA had a similar effect on gap junction among VSMCs with an IC50 of 40, 48 and 62 μM in SMA, AICA and MA segments, respectively. In dispersed VSMCs, FFA and NFA increased outward rectifier K(+)-current mediated by the big conductance calcium-activated potassium channel (BKCa) in a concentration-dependent manner, with a similar EC50 of ∼300 μM for both FFA and NFA in the three vessels. Iberiotoxin, a selective blocker of the BKCa, suppressed the enhancement of the BKCa by FFA and NFA. The KV blocker 4-AP had no effect on the fenamates-induced K(+)-current enhancement. We conclude that FFA and NFA blocked the vascular gap junction mediated electrical couplings uniformly in arterioles of the three vascular beds, and complete electrical isolation of the recorded VSMC is obtained at ≧300μM; FFA and NFA also activate BKCa channels in the arteriolar smooth muscle cells in addition to their known inhibitory effects on chloride channels.

Nuclear factor of activated T cells mediates RhoA-induced fibronectin upregulation in glomerular podocytes

Glomerulosclerosis is featured by accumulation of the extracellular matrixes in the glomerulus. We showed previously that activation of the small GTPase RhoA in podocytes induces heavy proteinuria and glomerulosclerosis in the mouse. In the current study, we investigated the mechanism by which RhoA stimulates the production of one of the extracellular matrixes, fibronectin, by podocytes, specifically testing the role of nuclear factor of activated T cells (NFAT). Expression of constitutively active RhoA in cultured podocytes activated the fibronectin promoter, upregulated fibronectin protein, and activated NFAT. Expression of constitutively active NFAT in podocytes also activated the fibronectin promoter and upregulated fibronectin protein. RhoA-induced NFAT activation and fibronectin upregulation were both dependent on the calcium/calmodulin pathway and Rho kinase. NFAT activation was also observed in vivo in the rat and mouse models of podocyte injury and proteinuria, and NFAT inhibition ameliorated fibronectin upregulation in the latter. RhoA activation induced a rise of intracellular calcium ion concentration ([Ca(2+)]i), which was at least in part dependent on the transient receptor potential canonical 6 (TRPC6) cation channel. The results indicate that RhoA activates NFAT by inducing a rise of [Ca(2+)]i in podocytes, which in turn contributes to fibronectin upregulation. This pathway may be responsible for the pathogenesis of certain glomerular diseases such as hypertension-mediated glomerulosclerosis.

Rho kinase regulation of vasopressin-induced calcium entry in vascular smooth muscle cell: comparison between rat isolated aorta and cultured aortic cells

In addition to its role in artery contraction, Rho kinase (ROCK) is reported to be involved in the Ca(2+) response to vasoconstrictor agonist in rat aorta. However the signaling pathway mediated by ROCK had not been investigated so far and it was not known whether ROCK also contributed to Ca(2+) signaling in cultured vascular smooth muscle cells (VSMC), which undergo profound phenotypic changes. Our results showed that in VSMC, ROCK inhibition by Y-27632 or H-1152 had no effect on the Ca(2+) response to vasopressin, while in aorta the vasopressin-induced Ca(2+) entry was significantly decreased. The inhibition of myosin light chain kinase (MLCK) by ML-7 depressed the vasopressin-induced Ca(2+) signal in aorta but not in VSMC. The difference in ROCK sensitivity of vasopressin-induced Ca(2+) entry between aorta and VSMC was not related to an alteration of the RhoA/ROCK pathway. However, MLCK expression and activity were depressed in cultured cells compared to aorta. We concluded that the regulation of vasopressin-induced Ca(2+) entry by ROCK in aorta could involve the myosin cytoskeleton and could be prevented by the downregulation of MLCK in VSMC. These results underline the important differences in Ca(2+) regulation between whole tissue and cultured cells.

Transient receptor potential canonical type 3 channels facilitate endothelium-derived hyperpolarization-mediated resistance artery vasodilator activity

AIMS

Microdomain signalling mechanisms underlie key aspects of artery function and the modulation of intracellular calcium, with transient receptor potential (TRP) channels playing an integral role. This study determines the distribution and role of TRP canonical type 3 (C3) channels in the control of endothelium-derived hyperpolarization (EDH)-mediated vasodilator tone in rat mesenteric artery.

METHODS AND RESULTS

TRPC3 antibody specificity was verified using rat tissue, human embryonic kidney (HEK)-293 cells stably transfected with mouse TRPC3 cDNA, and TRPC3 knock-out (KO) mouse tissue using western blotting and confocal and ultrastructural immunohistochemistry. TRPC3-Pyr3 (ethyl-1-(4-(2,3,3-trichloroacrylamide)phenyl)-5-(trifluoromethyl)-1H-pyrazole-4-carboxylate) specificity was verified using patch clamp of mouse mesenteric artery endothelial and TRPC3-transfected HEK cells, and TRPC3 KO and wild-type mouse aortic endothelial cell calcium imaging and mesenteric artery pressure myography. TRPC3 distribution, expression, and role in EDH-mediated function were examined in rat mesenteric artery using immunohistochemistry and western blotting, and pressure myography and endothelial cell membrane potential recordings. In rat mesenteric artery, TRPC3 was diffusely distributed in the endothelium, with approximately five-fold higher expression at potential myoendothelial microdomain contact sites, and immunoelectron microscopy confirmed TRPC3 at these sites. Western blotting and endothelial damage confirmed primary endothelial TRPC3 expression. In rat mesenteric artery endothelial cells, Pyr3 inhibited hyperpolarization generation, and with individual SK(Ca) (apamin) or IK(Ca) (TRAM-34) block, Pyr3 abolished the residual respective IK(Ca)- and SK(Ca)-dependent EDH-mediated vasodilation.

CONCLUSION

The spatial localization of TRPC3 and associated channels, receptors, and calcium stores are integral for myoendothelial microdomain function. TRPC3 facilitates endothelial SK(Ca) and IK(Ca) activation, as key components of EDH-mediated vasodilator activity and for regulating mesenteric artery tone.

Multiple factors influence calcium synchronization in arterial vasomotion

The intercellular synchronization of spontaneous calcium (Ca(2+)) oscillations in individual smooth muscle cells is a prerequisite for vasomotion. A detailed mathematical model of Ca(2+) dynamics in rat mesenteric arteries shows that a number of synchronizing and desynchronizing pathways may be involved. In particular, Ca(2+)-dependent phospholipase C, the intercellular diffusion of inositol trisphosphate (IP(3), and to a lesser extent Ca(2+)), IP(3) receptors, diacylglycerol-activated nonselective cation channels, and Ca(2+)-activated chloride channels can contribute to synchronization, whereas large-conductance Ca(2+)-activated potassium channels have a desynchronizing effect. Depending on the contractile state and agonist concentrations, different pathways become predominant, and can be revealed by carefully inhibiting the oscillatory component of their total activity. The phase shift between the Ca(2+) and membrane potential oscillations can change, and thus electrical coupling through gap junctions can mediate either synchronization or desynchronization. The effect of the endothelium is highly variable because it can simultaneously enhance the intercellular coupling and affect multiple smooth muscle cell components. Here, we outline a system of increased complexity and propose potential synchronization mechanisms that need to be experimentally tested.

Modeling Ca2+ signaling in the microcirculation: intercellular communication and vasoreactivity

A network of intracellular signaling pathways and complex intercellular interactions regulate calcium mobilization in vascular cells, arteriolar tone, and blood flow. Different endothelium-derived vasoreactive factors have been identified and the importance of myoendothelial communication in vasoreactivity is now well appreciated. The ability of many vascular networks to conduct signals upstream also is established. This phenomenon is critical for both short-term changes in blood perfusion as well as long-term adaptations of a vascular network. In addition, in a phenomenon termed vasomotion, arterioles often exhibit spontaneous oscillations in diameter. This is thought to improve tissue oxygenation and enhance blood flow. Experimentation has begun to reveal important aspects of the regulatory machinery and the significance of these phenomena for the regulation of local perfusion and oxygenation. Mathematical modeling can assist in elucidating the complex signaling mechanisms that participate in these phenomena. This review highlights some of the important experimental studies and relevant mathematical models that provide the current understanding of these mechanisms in vasoreactivity.

Regulation of cardiovascular TRP channel functions along the NO-cGMP-PKG axis

There is growing body of evidence that nitric oxide (NO)-cGMP-PKG signaling plays a central role in negative regulation of cardiovascular (CV) responses and its disorders through suppressed Ca(2+) dynamics. Other lines of evidence also reveal the stimulatory effects of this signaling on some CV functions. Recently, transient receptor potential (TRP) channels have received much attention as non-voltage-gated Ca(2+) channels involved in CV physiology and pathophysiology. Available information suggests that these channels undergo both inhibition and activation by NO via PKG-mediated phosphorylation and S-nitrosylation, respectively, and also act as upstream regulators to promote endothelial NO production. This review summarizes the roles of NO-cGMP-PKG signaling pathway, particularly in regulating TRP channel functions with their associated physiology and pathophysiology.

Calcium dynamics and signaling in vascular regulation: computational models

Calcium is a universal signaling molecule with a central role in a number of vascular functions including in the regulation of tone and blood flow. Experimentation has provided insights into signaling pathways that lead to or affected by Ca(2+) mobilization in the vasculature. Mathematical modeling offers a systematic approach to the analysis of these mechanisms and can serve as a tool for data interpretation and for guiding new experimental studies. Comprehensive models of calcium dynamics are well advanced for some systems such as the heart. This review summarizes the progress that has been made in modeling Ca(2+) dynamics and signaling in vascular cells. Model simulations show how Ca(2+) signaling emerges as a result of complex, nonlinear interactions that cannot be properly analyzed using only a reductionist's approach. A strategy of integrative modeling in the vasculature is outlined that will allow linking macroscale pathophysiological responses to the underlying cellular mechanisms.

Dual signaling pathways of arterial constriction by extracellular uridine 5'-triphosphate in the rat

We investigated actions of uridine 5'-triphosphate (UTP) in rat aorta, cerebral and mesenteric arteries, and their single myocytes. UTP (≥10 µM) elicited an inward-rectifying current strongly reminiscent of activation of P2X(1) receptor, and a similar current was also induced by α,β-methylene adenosine 5'-triphosphate (ATP) (≥100 nM). UTP desensitized α,β-methylene ATP-evoked current, and vice versa. The UTP-activated current was insensitive to G-protein modulators, TRPC3 inhibitors, or TRPC3 antibody, but was sensitive to P2-receptor inhibitors or P2X(1)-receptor antibody. Both UTP (1 mM) and α,β-methylene ATP (10 µM) elicited similar conductance single channel activities. UTP (≥10 µM) provoked a dose-dependent contraction of de-endothelialized aortic ring preparation consisting of phasic and tonic components. Removal of extracellular Ca(2+) or bath-applied 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) (30 µM) or nifedipine (10 µM) completely inhibited the phasic contraction while only partially reducing the tonic one. The tonic contraction was almost completely abolished by additional application of thapsigargin (2 µM). Similar biphasic rises in [Ca(2+)](i) were also evoked by UTP in rat aortic myocytes. In contrast to the low expression of TRPC3, significant expression of P2X(1) receptor was detected in all arteries by RT-PCR and immunoblotting, and its localization was limited to plasma membrane of myocytes as indicated by immunohistochemistry. These results suggest that UTP dually activates P2X(1)-like and P2Y receptors, but not TRPC3.

Na+-independent, nifedipine-resistant rat afferent arteriolar Ca2+ responses to noradrenaline: possible role of TRPC channels

AIM

In rat afferent arterioles we investigated the role of Na(+) entry in noradrenaline (NA)-induced depolarization and voltage-dependent Ca(2+) entry together with the importance of the transient receptor potential channel (TRPC) subfamily for non-voltage-dependent Ca(2+) entry.

METHODS

R (340/380) Fura-2 fluorescence was used as an index for intracellular free Ca(2+) concentration ([Ca(2+)](i)). Immunofluorescence detected the expression of TRPC channels.

RESULTS

TRPC 1, 3 and 6 were expressed in afferent arteriolar vascular smooth muscle cells. Under extracellular Na(+)-free (0 Na) conditions, the plateau response to NA was 115% of the baseline R(340/380) (control response 123%). However, as the R(340/380) baseline increased (7%) after 0 Na the plateau reached the same level as during control conditions. Similar responses were obtained after blockade of the Na(+)/Ca(2+) exchanger. The L-type blocker nifedipine reduced the plateau response to NA both under control (from 134% to 116% of baseline) and 0 Na conditions (from 112% to 103% of baseline). In the presence of nifedipine, the putative TRPC channel blockers SKF 96365 (30 μm) and Gd(3+) (100 μm) further reduced the plateau Ca(2+) responses to NA (from 117% to 102% and from 117% to 110% respectively).

CONCLUSION

We found that Na(+) is not crucial for the NA-induced depolarization that mediates Ca(2+) entry via L-type channels. In addition, the results are consistent with the idea that TRPC1/3/6 Ca(2+) -permeable cation channels expressed in afferent arteriolar smooth muscle cells mediate Ca(2+) entry during NA stimulation.

Two types of independent bursting mechanisms in inspiratory neurons: an integrative model

The network of coupled neurons in the pre-Bötzinger complex (pBC) of the medulla generates a bursting rhythm, which underlies the inspiratory phase of respiration. In some of these neurons, bursting persists even when synaptic coupling in the network is blocked and respiratory rhythmic discharge stops. Bursting in inspiratory neurons has been extensively studied, and two classes of bursting neurons have been identified, with bursting mechanism depends on either persistent sodium current or changes in intracellular Ca(2+), respectively. Motivated by experimental evidence from these intrinsically bursting neurons, we present a two-compartment mathematical model of an isolated pBC neuron with two independent bursting mechanisms. Bursting in the somatic compartment is modeled via inactivation of a persistent sodium current, whereas bursting in the dendritic compartment relies on Ca(2+) oscillations, which are determined by the neuromodulatory tone. The model explains a number of conflicting experimental results and is able to generate a robust bursting rhythm, over a large range of parameters, with a frequency adjusted by neuromodulators.

Functional distinctions in cytosolic calcium regulation between cells of the glomerular filtration barrier

The importance of intracellular calcium ([Ca(2+)]i) regulation in the glomerular filtration barrier (GFB) has recently been highlighted by mutations in the cation channel TRPC6, resulting in a renal-specific phenotype. We examined the effects of FFA, a tool that can activate TRPC6, on [Ca(2+)]i in human conditionally immortalised glomerular endothelial cells (ciGEnC) and human podocytes (ciPod) that form the GFB. Changes in [Ca(2+)]i stimulated by FFA were measured in Fura 2-AM loaded cells. In GEnC, cell activation by FFA was dependent on external Ca(2+), yet in ciPod it was not. Depletion of internal Ca(2+) stores with thapsigargin did not affect cell activation by FFA in ciGEnC, but inhibited it in ciPod in a nephrin-dependent manner, demonstrated using nephrin deficient (ND) ciPod in conjunction with nephrin rescue experiments. FFA induced [Ca(2+)]i store release in ciPod, but not in ciGEnC or ND ciPod. In parallel, there were differences in the localisation of overexpressed TRPC6 between ciGEnC and ciPod. Furthermore, co-transfection of nephrin with TRPC6 in HEK293 cells reduced the FFA-induced increase in [Ca(2+)]i and nephrin clustering altered TRPC6 distribution. In conclusion, cell activation by FFA in podocytes stimulates the opening of a Ca(2+) channel, probably TRPC6, in a nephrin-dependent manner with a different activation profile to GEnC.

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.

TRPC channels in smooth muscle cells

Transient receptor potential canonical (TRPC) proteins constitute a family of seven (TRPC1-7) nonselective cation channels within the wider TRP superfamily. TRPC1, TRPC3, TRPC4, TRPC5 and TRPC6 channels are expressed in vascular smooth muscle cells from human vessels of all calibers and in smooth muscle from organs such as the uterus and the gastrointestinal tract. TRPC channels have recently emerged as important players in the control of smooth muscle function. This review will focus on the retrospective analysis of studies proposing contributions of TRPC channels to native calcium entry pathways in smooth muscle and to physiological and pathophysiological responses with emphasis on the vascular system.

Transient receptor potential channels and vascular function

TRP (transient receptor potential) channels play important roles in the regulation of normal and pathological cellular function. In the vasculature, TRP channels are present both in ECs (endothelial cells) and vascular SMCs (smooth muscle cells) and contribute to vasomotor control mechanisms in most vascular beds. Vascular TRP channels are activated by various stimuli, such as mechanical perturbation, receptor activation and dietary molecules. Some of the specific roles of these channels in normal and impaired vascular function have emerged in recent years and include participation in vascular signalling processes, such as neurotransmission, hormonal signalling, NO production, myogenic tone and autoregulation of blood flow, thermoregulation, responses to oxidative stress and cellular proliferative activity. Current research is aimed at understanding the interactions of TRP channels with other vascular proteins and signalling mechanisms. These studies should reveal new targets for pharmacological therapy of vascular diseases, such as hypertension, ischaemia and vasospasm, and vascular proliferative states.

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.

Mechanosensitive TRP channels in cardiovascular pathophysiology

Transient receptor potential (TRP) proteins constitute a large non-voltage-gated cation channel superfamily, activated polymodally by various physicochemical stimuli, and are implicated in a variety of cellular functions. Known activators for TRP include not only chemical stimuli such as receptor stimulation, increased acidity and pungent/cooling agents, but temperature change and various forms of mechanical stimuli such as osmotic stress, membrane stretch, and shear force. Recent investigations have revealed that at least ten mammalian TRPs exhibit mechanosensitivity (TRPC1, 5, 6; TRPV1, 2, 4; TRPM3, 7; TRPA1; TRPP2), but the mechanisms underlying it appear considerably divergent and complex. The proposed mechanisms are associated with lipid bilayer mechanics, specialized force-transducing structures, biochemical reactions, membrane trafficking and transcriptional regulation. Many of mechanosensitive (MS)-TRP channel likely undergo multiple regulations via these mechanisms. In the cardiovascular system in which hemodynamic forces constantly operate, the impact of mechanical stress may be particularly significant. Extensive morphological and functional studies have indicated that several MS-TRP channels are expressed in cardiac muscle, vascular smooth muscle, endothelium and vasosensory neurons, each differentially contributing to cardiovascular (CV) functions. To further complexity, the recent evidence suggests that mechanical stress may synergize with neurohormonal mechanisms thereby amplifying otherwise marginal responses. Furthermore, the currently available data suggest that MS-TRP channels may be involved in CV pathophysiology such as cardiac arrhythmia, cardiac hypertrophy/myopathy, hypertension and aneurysms. This review will overview currently known mechanisms for mechanical activation/modulation of TRPs and possible connections of MS-TRP channels to CV disorders.

Diacylglycerol analogues activate second messenger-operated calcium channels exhibiting TRPC-like properties in cortical neurons

The lipid diacylglycerol (DAG) analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG) was used to verify the existence of DAG-sensitive channels in cortical neurons dissociated from E13 mouse embryos. Calcium imaging experiments showed that OAG increased the cytosolic concentration of Ca(2+) ([Ca(2+)]i) in nearly 35% of the KCl-responsive cells. These Ca(2+) responses disappeared in a Ca(2+)-free medium supplemented with EGTA. Mn(2+) quench experiments showed that OAG activated Ca(2+)-conducting channels that were also permeant to Ba(2+). The OAG-induced Ca(2+) responses were unaffected by nifedipine or omega-conotoxin GVIA (Sigma-Aldrich, Saint-Quentin Fallavier, France) but blocked by 1-[beta-(3-(4-Methoxyphenyl)propoxy)-4-methoxyphenethyl]-1H-imidazole hydrochloride (SKF)-96365 and Gd(3+). Replacing Na(+) ions with N-methyl-D-glucamine diminished the amplitude of the OAG-induced Ca(2+) responses showing that the Ca(2+) entry was mediated via Na(+)-dependent and Na(+)-independent mechanisms. Experiments carried out with the fluorescent Na(+) indicator CoroNa Green showed that OAG elevated [Na(+)]i. Like OAG, the DAG lipase inhibitor RHC80267 increased [Ca(2+)]i but not the protein kinase C activator phorbol 12-myristate 13-acetate. Moreover, the OAG-induced Ca(2+) responses were not regulated by protein kinase C activation or inhibition but they were augmented by flufenamic acid which increases currents through C-type transient receptor potential protein family (TRPC) 6 channels. In addition, application of hyperforin, a specific activator of TRPC6 channels, elevated [Ca(2+)]i. Whole-cell patch-clamp recordings showed that hyperforin activated non-selective cation channels. They were blocked by SKF-96365 but potentiated by flufenamic acid. Altogether, our data show the presence of hyperforin- and OAG-sensitive Ca(2+)-permeable channels displaying TRPC6-like properties. This is the first report revealing the existence of second messenger-operated channels in cortical neurons.

VEGF-mediated elevated intracellular calcium and angiogenesis in human microvascular endothelial cells in vitro are inhibited by dominant negative TRPC6

OBJECTIVE

Vascular endothelial growth factor (VEGF)-induced vascular permeability has been shown to be dependent on calcium influx, possibly through a transient receptor potential cation channel (TRPC)-mediated cation channel with properties of the TRPC3/6/7 subfamily. To investigate further the involvement of this subfamily, we determined the effects of dominant negative TRPC6 overexpression on VEGF-mediated changes of human microvascular endothelial cell (HMVEC) calcium, proliferation, migration, and sprouting.

METHODS

Cytoplasmic calcium concentration was estimated by fura-2 fluorescence spectrophotometry, migration by Boyden chamber assay, sprouting by immunofluorescence imaging of stimulated endothelial cells, and proliferation by flow cytometry.

RESULTS

Overexpression of a dominant negative TRPC6 construct in HMVECs inhibited the VEGF-mediated increases in cytosolic calcium, migration, sprouting, and proliferation. In contrast, overexpression of a wild-type TRPC6 construct increased the proliferation and migration of HMVECs.

CONCLUSIONS

TRPC6 is an obligatory component of cation channels required for the VEGF-mediated increase in cytosolic calcium and subsequent downstream signaling that leads to processes associated with angiogenesis.

Flufenamic acid is a tool for investigating TRPC6-mediated calcium signalling in human conditionally immortalised podocytes and HEK293 cells

Mutations in the cation channel TRPC6 result in a renal-specific phenotype of familial nephrotic syndrome, affecting intracellular calcium ([Ca(2+)](i)) signalling in the glomerular podocyte. Tools to study native TRPC6 activity are scarce, although there has been recent success with flufenamic acid (FFA). We confirm the specificity of FFA for TRPC6 both in an artificial expression system and in a human conditionally immortalised podocyte cell line (ciPod). Cells were loaded with fura-2AM and changes in intracellular calcium ([Ca(2+)](i)) were calculated. 200microM FFA induced an increase in [Ca(2+)](i) in HEK293 cells with native TRPC6 expression, which was enhanced by overexpression of TRPC6 and completely blocked in the absence of extracellular calcium. Expressed TRPC7 did not significantly affect the response to FFA whereas expressed TRPC3 reduced it. FFA also induced an increase ciPod in [Ca(2+)](i), which was inhibited using SKF96365 and 2-APB, but not indomethacin. In ciPod, adenovirus (Ad-v) wild type (WT) TRPC6 increased [Ca(2+)](i) activity to FFA compared to native TRPC6, whereas activity was significantly reduced with Ad-v dominant negative (DN) TRPC6. The niflumic acid (NFA) induced increase in [Ca(2+)](i) in ciPod was not affected by Ad-v TRPC6 DN, and in HEK293 cells was not affected by WT TRPC6. In conclusion, FFA activates TRPC6 [Ca(2+)](i) signalling in both ciPod and HEK293 cells independently of TRPC3 and TRPC7, and independently of properties of the fenamate family.

A mathematical model of Ca2+ dynamics in rat mesenteric smooth muscle cell: agonist and NO stimulation

A mathematical model of calcium dynamics in vascular smooth muscle cell (SMC) was developed based on data mostly from rat mesenteric arterioles. The model focuses on (a) the plasma membrane electrophysiology; (b) Ca2+ uptake and release from the sarcoplasmic reticulum (SR); (c) cytosolic balance of Ca2+, Na+, K+, and Cl ions; and (d) IP3 and cGMP formation in response to norepinephrine(NE) and nitric oxide (NO) stimulation. Stimulation with NE induced membrane depolarization and an intracellular Ca2+ ([Ca2+]i) transient followed by a plateau. The plateau concentrations were mostly determined by the activation of voltage-operated Ca2+ channels. NE causes a greater increase in [Ca2+]i than stimulation with KCl to equivalent depolarization. Model simulations suggest that the effect of[Na+]i accumulation on the Na+/Ca2+ exchanger (NCX) can potentially account for this difference.Elevation of [Ca2+]i within a concentration window (150-300 nM) by NE or KCl initiated [Ca2+]i oscillations with a concentration-dependent period. The oscillations were generated by the nonlinear dynamics of Ca2+ release and refilling in the SR. NO repolarized the NE-stimulated SMC and restored low [Ca2+]i mainly through its effect on Ca2+-activated K+ channels. Under certain conditions, Na+-K+-ATPase inhibition can result in the elevation of [Na+]i and the reversal of NCX, increasing resting cytosolic and SR Ca2+ content, as well as reactivity to NE. Blockade of the NCX's reverse mode could eliminate these effects. We conclude that the integration of the selected cellular components yields a mathematical model that reproduces, satisfactorily, some of the established features of SMC physiology. Simulations suggest a potential role of intracellular Na+ in modulating Ca2+ dynamics and provide insights into the mechanisms of SMC constriction, relaxation, and the phenomenon of vasomotion. The model will provide the basis for the development of multi-cellular mathematical models that will investigate microcirculatory function in health and disease.

Nitric oxide-cGMP-protein kinase G pathway negatively regulates vascular transient receptor potential channel TRPC6

We investigated the inhibitory role of the nitric oxide (NO)-cGMP-protein kinase G (PKG) pathway on receptor-activated TRPC6 channels in both a heterologous expression system (HEK293 cells) and A7r5 vascular myocytes. Cationic currents due to TRPC6 expression were strongly suppressed (by approximately 70%) by a NO donor SNAP (100 microm) whether it was applied prior to muscarinic receptor stimulation with carbachol (CCh; 100 microm) or after G-protein activation with intracellular perfusion of GTPgammaS (100 microm). A similar extent of suppression was also observed with a membrane-permeable analogue of cGMP, 8Br-cGMP (100 microm). The inhibitory effects of SNAP and 8Br-cGMP on TRPC6 channel currents were strongly attenuated by the presence of inhibitors for guanylyl cyclase and PKG such as ODQ, KT5823 and DT3. Alanine substitution for the PKG phosphorylation candidate site at T69 but not at other sites (T14A, S28A, T193A, S321A) of TRPC6 similarly attenuated the inhibitory effects of SNAP and 8Br-cGMP. SNAP also significantly reduced single TRPC6 channel activity recorded in the inside-out configuration in a PKG-dependent manner. SNAP-induced PKG activation stimulated the incorporation of (32)P into wild-type and S321A-mutant TRPC6 proteins immunoprecipitated by TRPC6-specific antibody, but this was greatly attenuated in the T69A mutant. SNAP or 8Br-cGMP strongly suppressed TRPC6-like cation currents and membrane depolarization evoked by Arg(8)-vasopressin in A7r5 myocytes. These results strongly suggest that TRPC6 channels can be negatively regulated by the NO-cGMP-PKG pathway, probably via T69 phosphorylation of the N-terminal. This mechanism may be physiologically important in vascular tissues where NO is constantly released from vascular endothelial cells or nitrergic nerves.

Ca2+ handling is altered when arterial myocytes progress from a contractile to a proliferative phenotype in culture

Phenotypic modulation of vascular myocytes is important for vascular development and adaptation. A characteristic feature of this process is alteration in intracellular Ca(2+) handling, which is not completely understood. We studied mechanisms involved in functional changes of inositol 1,4,5-trisphosphate (IP(3))- and ryanodine (Ry)-sensitive Ca(2+) stores, store-operated Ca(2+) entry (SOCE), and receptor-operated Ca(2+) entry (ROCE) associated with arterial myocyte modulation from a contractile to a proliferative phenotype in culture. Proliferating, cultured myocytes from rat mesenteric artery have elevated resting cytosolic Ca(2+) levels and increased IP(3)-sensitive Ca(2+) store content. ATP- and cyclopiazonic acid [CPA; a sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor]-induced Ca(2+) transients in Ca(2+)-free medium are significantly larger in proliferating arterial smooth muscle cells (ASMCs) than in freshly dissociated myocytes, whereas caffeine (Caf)-induced Ca(2+) release is much smaller. Moreover, the Caf/Ry-sensitive store gradually loses sensitivity to Caf activation during cell culture. These changes can be explained by increased expression of all three IP(3) receptors and a switch from Ry receptor type II to type III expression during proliferation. SOCE, activated by depletion of the IP(3)/CPA-sensitive store, is greatly increased in proliferating ASMCs. Augmented SOCE and ROCE (activated by the diacylglycerol analog 1-oleoyl-2-acetyl-sn-glycerol) in proliferating myocytes can be attributed to upregulated expression of, respectively, transient receptor potential proteins TRPC1/4/5 and TRPC3/6. Moreover, stromal interacting molecule 1 (STIM1) and Orai proteins are upregulated in proliferating cells. Increased expression of IP(3) receptors, SERCA2b, TRPCs, Orai(s), and STIM1 in proliferating ASMCs suggests that these proteins play a critical role in an altered Ca(2+) handling that occurs during vascular growth and remodeling.

Expression of TRPC6 channels in human epithelial breast cancer cells

Background

TRP channels have been shown to be involved in tumour generation and malignant growth. However, the expression of these channels in breast cancer remains unclear. Here we studied the expression and function of endogenous TRPC6 channels in a breast cancer cell line (MCF-7), a human breast cancer epithelial primary culture (hBCE) and in normal and tumour breast tissues.

Methods

Molecular (Western blot and RT-PCR), and immunohistochemical techniques were used to investigate TRPC6 expression. To investigate the channel activity in both MCF-7 cells and hBCE we used electrophysiological technique (whole cell patch clamp configuration).

Results

A non selective cationic current was activated by the oleoyl-2-acetyl-sn-glycerol (OAG) in both hBCE and MCF-7 cells. OAG-inward current was inhibited by 2-APB, SK&F 96365 and La³⁺. TRPC6, but not TRPM7, was expressed both in hBCE and in MCF-7 cells. TRPC3 was only expressed in hBCE. Clinically, TRPC6 mRNA and protein were elevated in breast carcinoma specimens in comparison to normal breast tissue. Furthermore, we found that the overexpression of TRPC6 protein levels were not correlated with tumour grades, estrogen receptor expression or lymph node positive tumours.

Conclusion

Our results indicate that TRPC6 channels are strongly expressed and functional in breast cancer epithelial cells. Moreover, the overexpression of these channels appears without any correlation with tumour grade, ER expression and lymph node metastasis. Our findings support the idea that TRPC6 may have a role in breast carcinogenesis.

Flufenamic acid affects multiple currents and causes intracellular Ca2+ release in Aplysia bag cell neurons

Flufenamic acid (FFA) is a nonsteroidal antiinflammatory agent, commonly used to block nonselective cation channels. We previously reported that FFA potentiated, rather than inhibited, a cation current in Aplysia bag cell neurons. Prompted by this paradoxical result, the present study examined the effects of FFA on membrane currents and intracellular Ca2+ in cultured bag cell neurons. Under whole cell voltage clamp, FFA evoked either outward (I out) or inward (I in) currents. I out had a rapid onset, was inhibited by the K+ channel blocker, tetraethylammonium, and was associated with both an increase in membrane conductance and a negative shift in the whole cell current reversal potential. I in developed more slowly, was inhibited by the cation channel blocker, Gd3+, and was concomitant with both an increased conductance and positive shift in reversal potential. FFA also enhanced the use-dependent inactivation and caused a positive-shift in the activation curve of the voltage-dependent Ca2+ current. Furthermore, as measured by ratiometric imaging, FFA produced a rise in intracellular Ca2+ that persisted in the absence of extracellular Ca2+ and was reduced by depleting either the endoplasmic reticulum and/or mitochondrial stores. Ca2+ appeared to be involved in the activation of I in, as strong intracellular Ca2+ buffering effectively eliminated I in but did not alter I out. Finally, the effects of FFA were likely not due to block of cyclooxygenase given that the general cyclooxygenase inhibitor, indomethacin, failed to evoke either current. That FFA influences a number of neuronal properties needs to be taken into consideration when employing it as a cation channel antagonist.

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.

Rho kinase is involved in Ca2+ entry of rat penile small arteries

Tonic physiological activity of RhoA/Rho kinase contributes to the maintenance of penile flaccidity through its involvement in the Ca(2+) sensitization of erectile tissue smooth muscle. The present study hypothesized that Rho kinase is also involved in the modulation of Ca(2+) entry induced by alpha(1)-adrenoceptor stimulation of penile arteries. Rat penile arteries were mounted in microvascular myographs for simultaneous measurements of intracellular Ca(2+) ([Ca(2+)](i)) and force. The Rho-kinase inhibitor Y-27632 markedly reduced norepinephrine-mediated electrically induced contractions and the increases in both [Ca(2+)](i) and tension elicited by the alpha(1)-adrenoceptor agonist phenylephrine (Phe). In contrast, the protein kinase C (PKC) inhibitor Ro-31-8220 reduced tension without altering the Phe-induced increase in [Ca(2+)](i). In the presence of nifedipine, Y-27632 still inhibited the non-L-type Ca(2+) signal and blunted Phe contraction. Y-27632 did not impair the capacitative Ca(2+) entry evoked by store depletion with cyclopiazonic acid but largely reduced the Ba(2+) influx stimulated by Phe in fura-2 AM-loaded arteries. The addition of Y-27632 to arteries depolarized with high KCl markedly reduced tension without changing [Ca(2+)](i). In alpha-toxin-permeabilized penile arteries stimulated with threshold Ca(2+) concentrations, Y-27632 inhibited the sensitization induced by either guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) or Phe in the presence of GTPgammaS. However, Y-27632 failed to alter contractions induced by a maximal concentration of free Ca(2+). These results suggest that Rho kinase, besides its contribution to the Ca(2+) sensitization of the contractile proteins, is also involved in the regulation of Ca(2+) entry through a nonselective cation channel activated by alpha(1)-adenoceptor stimulation in rat penile arteries.

Decreased activity of the smooth muscle Na+/Ca2+ exchanger impairs arteriolar myogenic reactivity

Arteriolar myogenic vasoconstriction occurs when stretch or increased membrane tension leads to smooth muscle cell (SMC) depolarization and opening of voltage-gated Ca(2+) channels. While the mechanism underlying the depolarization is uncertain a role for non-selective cation channels has been demonstrated. As such channels may be expected to pass Na(+), we hypothesized that reverse mode Na(+)/Ca(2+) exchange (NCX) may act to remove Na(+) and in addition play a role in myogenic signalling through coupled Ca(2+) entry. Further, reverse (Ca(2+) entry) mode function of the NCX is favoured by the membrane potential found in myogenically active arterioles. All experiments were performed on isolated rat cremaster muscle first order arterioles (passive diameter approximately 150 mum) which were pressurized in the absence of intraluminal flow. Reduction of extracellular Na(+) to promote reverse-mode NCX activity caused significant, concentration-dependent vasoconstriction and increased intracellular Ca(2+). This vasoconstriction was attenuated by the NCX inhibitors KB-R7943 and SEA 04000. Western blotting confirmed the existence of NCX protein while real-time PCR studies demonstrated that the major isoform expressed in the arteriolar wall was NCX1. Oligonucleotide knockdown (24 and 36 h) of NCX inhibited the vasoconstrictor response to reduced extracellular Na(+) while also impairing both steady-state myogenic responses (as shown by pressure-diameter relationships) and acute reactivity to a 50 to 120 mmHg pressure step. The data are consistent with reverse mode activity of the NCX in arterioles and a contribution of this exchanger to myogenic vasoconstriction.

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.

Analysis of TRPC3-interacting proteins by tandem mass spectrometry

Mammalian transient receptor potential canonical (TRPC) channels are a family of nonspecific cation channels that are activated in response to stimulation of phospholipase C (PLC)-dependent hydrolysis of the membrane lipid phosphatidylinositol 4,5-bisphosphate. Despite extensive studies, the mechanism(s) involved in regulation of mammalian TRPC channels remains unknown. Presence of various protein-interacting domains in TRPC channels have led to the suggestion that they associate with proteins that are involved in their function and regulation. This study was directed toward identifying the proteins associated with native TRPC3 using a shotgun proteomic approach. Anti-TRPC3 antibody was used to immunoprecipitate TRPC3 from solubilized rat brain crude membranes under conditions that allow retention of TRPC3 function. Proteins in the TRPC3 (using anti-TRPC3 antibody) and control (using rabbit IgG) immunoprecipitates were separated by SDS-PAGE, the gel was sectioned, and the resolved proteins were digested by trypsin in situ. After extraction of the peptides, the peptides were separated by HPLC and sequences derived by MS/MS. Analysis of the data revealed 64 specific TRPC3-associated proteins which can be grouped in terms of their cellular location and involvement in specific cellular function. Many of the proteins identified have been previously reported as TRPC3-regulatory proteins, such as IP3Rs and vesicle trafficking proteins. In addition, we report novel putative TRPC3-interacting proteins, including those involved in protein endocytosis and neuronal growth. To our knowledge, this is the first comprehensive proteomic analysis of a native TRPC channel. These data reveal potential TRPC3 regulatory proteins and provide novel insights of the mechanism(s) regulating TRPC3 channels as well as the possible cellular functions where the channel might be 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.

TRPCs as MS Channels

This chapter reviews recent evidence indicating that canonical or classical transient receptor potential (TRPC) channels are directly or indirectly mechanosensitive (MS) and can therefore be designated as mechano-operated channels (MOCs). The MS functions of TRPCs may be mechanistically related to their better known functions as store-operated and receptor-operated channels (SOCs and ROCs). Mechanical forces may be conveyed to TRPC channels through the "conformational coupling" mechanism that transmits information regarding the status of internal Ca(2+) stores. All TRPCs are regulated by receptors coupled to phospholipases that are themselves MS and can regulate channels via lipidic second messengers. Accordingly, there may be several nonexclusive mechanisms by which mechanical forces may regulate TRPC channels, including direct sensitivity to bilayer mechanics, physical coupling to internal membranes and/or cytoskeletal proteins, and sensitivity to lipidic second messengers generated by MS enzymes. Various strategies that can be used for separating out different MS-gating mechanisms and their possible role in specific TRPCs are discussed.

Activity-Dependent Initiation of a Prolonged Depolarization in Aplysia Bag Cell Neurons: Role for a Cation Channel

The translation of prior activity into changes in excitability is essential for memory and the initiation of behavior. After brief synaptic input, the bag cell neurons of Aplysia californica undergo a nearly 30-min afterdischarge to release egg-laying hormone. The present study examines a prolonged depolarization in cultured bag cell neurons. A 5-Hz, 10-s action potential train elicited a depolarization of about 10 mV, which lasted ≤30 min and was reduced by calmodulin kinase inhibition. Very broad action potentials (resulting from TEA application) decreased prolonged depolarization amplitude, indicating that strong Ca2+ influx did not necessarily promote the response. The prolonged depolarization current ( IPD) was recorded after 5-Hz, 10-s trains of square voltage pulses of varying duration (10–150 ms). Despite Ca2+ influx increasing steadily with pulse duration, IPD was most reliably initiated at 100 ms, suggesting a Ca2+ window or limit exists for triggering IPD. Consistent with this, modestly broader action potentials, evoked by lengthening the train current-pulse duration, resulted in smaller prolonged depolarizations. With respect to the properties of IPD, it displayed a linear current–voltage relationship with a reversal potential of about −45 mV that was shifted to approximately −25 mV by lowering internal K+ or about −56 mV by lowering external Na+ and Ca2+. IPD was blocked by Gd3+, but was not antagonized by MDL-123302A, SKF-96365, 2-APB, tetrodotoxin, or flufenamic acid. Optimal Ca2+ influx may activate calmodulin kinase and a voltage-independent, nonselective cation channel to initiate the prolonged depolarization, thereby contributing to the afterdischarge and reproduction.

Inhibition of eryptosis and intraerythrocytic growth of Plasmodium falciparum by flufenamic acid

Non-selective (NSC) cation channels participate in the Ca(2+) leak of human erythrocytes. Sustained activity of these channels triggers suicidal erythrocyte death (eryptosis), which is characterized by Ca(2+)-stimulated cell shrinkage and phosphatidylserine (PS) exposure. PS-exposing erythrocytes are rapidly cleared from circulating blood. PGE(2) activates the NSC channels, and erythrocyte PGE(2) formation is stimulated by a decrease in intra- or extracellular Cl(-) concentration. In addition, the intraerythrocytic malaria parasite Plasmodium falciparum activates the NSC channels, most probably to accomplish Na(+) and Ca(2+) entry into the erythrocyte cytosol required for parasite development. By Ca(2+) uptake the parasite maintains a low Ca(2+) concentration in the erythrocyte cytosol and thus delays the suicidal death of the host erythrocyte. Flufenamic acid has previously been shown to inhibit NSC channels. The present study thus explored the effect of flufenamic acid on erythrocyte Ca(2+) entry, on suicidal erythrocyte death and on intraerythrocytic growth of P. falciparum. Within 48 h, replacement of extracellular Cl(-) with gluconate or application of PGE(2) (50 microM) increased Fluo3 fluorescence reflecting cytosolic Ca(2+) activity, decreased forward scatter reflecting cell volume and increased annexin V binding reflecting PS exposure in FACS analysis. All those effects were significantly blunted in the presence of flufenamic acid (10 microM). Flufenamic acid (25 microM) further significantly delayed the intraerythrocytic growth of P. falciparum and the PS exposure of the infected erythrocytes. The present observations disclose a novel effect of flufenamic acid, which may allow the pharmacological manipulation of erythrocyte survival and the course of malaria.

Angiotensin II activates two cation conductances with distinct TRPC1 and TRPC6 channel properties in rabbit mesenteric artery myocytes

Angiotensin II (Ang II) is a potent vasoconstrictor with an important role in controlling blood pressure; however, there is little information on cellular mechanisms underlying Ang II-evoked vasoconstrictor responses. The aim of the present study is to investigate the effect of Ang II on cation conductances in freshly dispersed rabbit mesenteric artery myocytes at the single-channel level using patch-clamp techniques. In cell-attached patches, bath application of low concentrations of Ang II (1 nM) activated cation channel currents (Icat1) with conductances states of about 15, 30 and 45 pS. At relatively high concentrations, Ang II (100 nM) inhibited Icat1 but evoked another cation channel (Icat2) with a conductance of approximately 2 pS. Ang II-evoked Icat1 and Icat2 were inhibited by the AT1 receptor antagonist losartan and the phospholipase C (PLC) inhibitor U73122. The diacylglycerol (DAG) lipase inhibitor RHC80267 initially induced Icat1 which was subsequently inhibited to reveal Icat2. The DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (1 microM) activated Icat1 and Icat2 but inositol 1,4,5-trisphosphate did not evoke either conductance. The protein kinase C (PKC) inhibitor chelerythrine (3 microM) potentiated Ang II-evoked Icat1 and inhibited Icat2 whereas the PKC activator phorbol-12,13-dibutyrate (1 microM) reduced Ang II-induced Icat1 but activated Icat2. Moreover in cell-attached patches pretreated with chelerythrine, application of 100 nM Ang II activated Icat1. These data indicate that PKC inhibits Icat1 but stimulates Icat2. Agents that deplete intracellular Ca2+ stores also activated cation channel currents with similar properties to Icat2. Bath application of anti-TRPC6 and anti-TRPC1 antibodies to inside-out patches inhibited Icat1 and Icat2, respectively. Also flufenamic acid and zero external Ca2+ concentration, respectively, potentiated and reduced Ang II-evoked Icat1. Immunocytochemical studies showed TRPC6 and TRPC1 expression with TRPC6 preferentially distributed in the plasma membrane and TRPC1 expression located throughout the myocyte. These results indicate that Ang II activates two distinct cation conductances in mesenteric artery myocytes by stimulation of AT1 receptors linked to PLC. Icat1 is activated by DAG via a PKC-independent mechanism whereas Icat2 involves DAG acting via a PKC-dependent pathway. Higher concentrations of Ang II inhibit Icat1 by activating an inhibitory effect of PKC. It is proposed that TRPC6 and TRPC1 channel proteins are important components of Ang II-induced Icat1 and Icat2, respectively.

Transient receptor potential channels in cardiovascular function and disease

Sustained elevation in the intracellular Ca2+ concentration via Ca2+ influx, which is activated by a variety of mechanisms, plays a central regulatory role for cardiovascular functions. Recent molecular biological research has disclosed an unexpectedly diverse array of Ca(2+-entry channel molecules involved in this Ca2+ influx. These include more than ten transient receptor potential (TRP) superfamily members such as TRPC1, TRPC3-6, TRPV1, TRPV2, TRPV4, TRPM4, TRPM7, and polycystin (TRPP2). Most of them appear to be multimodally activated or modulated and show relevant features to both acute hemodynamic control and long-term remodeling of the cardiovascular system, and many of them have been found to respond not only to receptor stimulation but also to various forms of stimuli. There is good evidence to implicate TRPC1 in neointimal hyperplasia after vascular injury via store-depletion-operated Ca2+ entry. TRPC6 likely contributes to receptor-operated and mechanosensitive Ca2+ mobilizations, being involved in vasoconstrictor and myogenic responses and pulmonary arterial proliferation and its associated disease (idiopathic pulmonary arterial hypertension). Considerable evidence has also been accumulated for unique involvement of TRPV1 in blood flow/pressure regulation via sensory vasoactive neuropeptide release. New lines of evidence suggest that TRPV2 may act as a Ca2+-overloading pathway associated with dystrophic cardiomyopathy, TRPV4 as a mediator of endothelium-dependent hyperpolarization, TRPM7 as a proproliferative vascular Mg2+ entry channel, and TRPP2 as a Ca2+-entry channel requisite for vascular integrity. This review attempts to provide an overview of the current knowledge on TRP proteins and discuss their possible roles in cardiovascular functions and diseases.