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.
- cation channel
- smooth muscle
- transient receptor potential (TRP) channel
- vascular function
The TRP (transient receptor potential) channel superfamily is composed of cation channels gated by temperature, light, pressure and chemical stimuli. TRP channels often act as cellular sensors and mediate responses to changes in the extracellular environment. All cells express multiple TRP channel proteins and these channels are involved in numerous critical functions. The present review attempts to summarize our current knowledge of TRP channels in the vascular endothelium and arterial myocytes and their roles in normal vascular function and pathophysiology.
OVERVIEW OF TRP CHANNELS
The TRP channel superfamily is diverse. There are 28 TRP-channel-encoding genes present in the mammalian genome and these are grouped into six channel subfamilies on the basis of DNA and protein sequence homology: TRPC (TRP canonical), TRPV (TRP vanilloid), TRPM (TRP melastatin), TRPA (TRP ankyrin), TRPP (TRP polycystin) and TRPML (TRP mucoliptin). TRP channel proteins are expressed as six-transmembrane-domain polypeptide subunits, and four subunits assemble in the plasma membrane to form functional ion channels. Channels can comprise identical subunits, forming homomeric channels, or can result from the assembly of two or three different TRP channel proteins to form heteromultimeric channels. These heteromultimeric channels often have properties that are distinct from those of homomeric channels. As most cells express multiple TRP subunits, it is likely that heteromultimeric channels exist in vivo. In addition, mRNA splice variants have been reported for up to 15 of the 28 TRP channel subunits, increasing further the number of individual subunits available for co-assembly. Much of what we understand about TRP channel function has been derived from studies using cell culture systems that overexpress cloned channel proteins, conditions that favour the formation of homomeric channels. These experimental conditions may not accurately reflect the in vivo setting, where heteromultimeric channels may be common. A brief introduction to the biophysical properties and unique characteristics of each channel follows. For more detailed information concerning the biophysical properties, channel domains and membrane topology of TRP channels, see the excellent previous reviews covering these topics [1–3].
The TRPC subfamily consists of six members in humans and seven in other mammalian species; TRPC1–7 are present in rats and mice, whereas TRPC2 is a non-functional pseudogene in humans. Certain members of the TRPC subfamily have been characterized as participants in SOC (store-operated Ca2+) influx, whereas others appear to mediate ROC (receptor-operated Ca2+) influx activity; however, a role for TRPC channels in SOC influx has been challenged. All TRPC channels are Ca2+ permeable, but they do not display significant selectivity for Ca2+ compared with Na+ ions. There are no reports of voltage-gated activation of TRPC channels.
Homomeric TRPC1 channels display mechanosensitivity and can form heteromultimeric channels with TRPC4 and/or TRPC5 that are sensitive to G-protein-coupled signalling pathways. Estimates of the single channel conductance of homomeric TRPC1 range from 2.5 to 30 pS. Functional TRPC2 is not present in humans but the channel is expressed in rats and mice where it may play a role in pheromone sensing. The channel has a unitary conductance of 42 pS in symmetrical cation solutions. TRPC3 channels are sensitive to the PLC (phospholipase C) metabolite DAG (diacylglycerol) and they appear to be an important receptor-operated channel in many types of cells. The unitary conductance of the homomeric TRPC3 channels has been estimated to be in the range of 20 to 65 pS. TRPC3 can form heteromultimeric channels with TRPC6 and TRPC7. TRPC4 can form G-protein-activated receptor-operated channels with a unitary conductance between 18 and 40 pS. The channel can exist as a heteromultimeric channel with TRPC1 and/or TRPC5. The unitary conductance of homomeric TRPC5 channels has been estimated to be 38 pS. TRPC5 can form heteromultimeric channels with TRPC1 and/or TRPC4, which are sensitive to G-protein-activated signalling pathways. There is strong evidence that TRPC6, like TRPC3, is a DAG-sensitive receptor-operated channel in many types of cells. The single channel conductance has been reported as 28 pS and 35 pS. TRPC6 can form heteromultimeric channels with TRPC3 and TRPC7. TRPC7 is outwardly rectifying with a unitary conductance of 50 pS at positive membrane potentials and 25 pS at negative potentials. TRPC7 can form heteromultimeric channels with TRPC3 and TRPC6. Homomeric TRPC7 is sensitive to DAG.
There are six members of the TRPV subfamily. TRPV1 is the receptor for capsaicin, a substance found in hot chilli peppers. The channel is modestly selective for Ca2+ compared with Na+ (by approx. 10-fold) and has a unitary conductance of approx. 80 pS in symmetrical cation solutions. TRPV2 channels appear to be involved in sensing thermal pain. TRPV2 channels have little selectivity for Ca2+ compared with Na+ (approx. 3-fold preference for Ca2+). The single-channel conductance of TRPV2 has not been reported. TRPV3 was initially identified as a warm-temperature-sensing channel. The channel shows modest selectivity for Ca2+ compared with Na+ (approx. 10-fold) and has a unitary conductance of approx. 170 pS, the largest of all of the TRPV channels. Heat- and hypotonicity-induced cell swelling activates TRPV4 channels. The channel is also activated by the phorbol compound 4α-PDD (4α-phorbol 12,13-didecanoate) and by EETs (epoxyeicosatrienoic acids). TRPV4 is slightly selective for Ca2+ compared with Na+ (approx. 6-fold) and has a single-channel conductance of 90 pS in symmetrical cation solutions. TRPV5 and TRPV6 channels are constitutively active and are highly selective for Ca2+ compared with Na+ (approx. 100-fold). TRPV5 and TRPV6 can reportedly form heteromultimeric channels.
There are eight members of the TRPM subfamily. TRPM1 is a non-selective cation channel that exhibits outward rectification under whole-cell conditions . The channel was originally identified as a protein that is down-regulated in melanomas and was initially described as a tumour suppressor gene, hence the name melastatin. TRPM1 is blocked by the trivalent ion La3+ . TRPM2 is a non-selective cation channel with a unitary conductance of 60 pS in symmetrical cation solutions. The channel is gated by the second messengers ADP-ribose, H2O2 and cADP-ribose. TRPM3 channels are activated by sphingolipids. The channel is somewhat selective for monovalent ions and has a unitary conductance of 73 pS under physiological conditions. TRPM3 may be an ionotropic steroid receptor in pancreatic islet cells . TRPM4 and TRPM5 are related channels with unusual biophysical properties. Both channels are highly selective for monovalent cations and require high levels of intracellular Ca2+ for activation. The single-channel conductance for monovalent cations for TRPM4 is 25 pS and for TRPM5 is 23 pS. TRPM6 and TRPM7 appear to be critical for Mg2+ homoeostasis in many types of cells. TRPM8 is a cool-temperature-activated channel and is also the receptor for menthol. The channel is not selective for monovalent or divalent cations and has a unitary conductance of 83 pS.
The TRPA subfamily consists of a single member, TRPA1. Electrophilic compounds activate TRPA1, including substances found in pungent foods, such as garlic and mustard oil. The channel is equally permeable to Ca2+ and Na+ ions and has a unitary conductance of ~90 pS.
TRPP and TRPML
The TRPP and TRPML subfamilies each consist of three members. These channels have a high degree of sequence homology compared with the other TRP subfamilies . Mutations in TRPP1 [also known as PKD1 (polycystic kidney disease 1] are associated with polycystic kidney disease. Little is known about regulation of the TRPP channels. Estimates of the single-channel conductances are 40–115 pS for TRPP1, 130 pS for TRPP2 and 300 pS for TRPP3. TRPML channels appear to be important for lysosome function and mutations in TRPML1 are associated with mucolipidosis type IV. Very little is known about regulation or biophysics of these channels.
TRP CHANNELS AND VASCULAR CELL FUNCTION
ECs (endothelial cells) regulate the permeability of the vascular wall and secrete potent factors that influence SMC (smooth muscle cell) contractility and oppose thrombus formation. In addition, in some segments of the vasculature, ECs can directly influence SMC membrane potential and contractility though electrical communication mediated by myoendothelial gap junctions. All of these processes are regulated by changes in endothelial intracellular Ca2+ level. It is clear that TRP channels have a significant role in the regulation of intracellular Ca2+ levels and EC function, but much remains to be discovered. Many studies rely on primary EC cultures and it is not known how culture conditions affect TRP channel expression. Identifying the TRP channel expression profile in native cells is critical to understanding how these channels influence EC function.
TRP channels also play key roles in vascular SMCs, mediating both vasoconstriction and vasodilation. TRP channels are involved in vasoconstriction associated with various forms of stimulation, including receptor activation, myogenic activation (stretch- or pressure-activated Ca2+ entry), and possibly via activation of SOC channels .
Vasoconstriction, particularly in small-diameter arteries and arterioles, typically results from vascular SMC membrane depolarization. Depolarization increases the activity of Cav (voltage-dependent Ca2+) channels, causing myocyte contraction. Evidence indicates that at least part of the depolarization is due to TRP channel activity. Direct Ca2+ entry through TRP channels may also contribute to increases in global Ca2+ levels leading to activation of the contractile apparatus. TRP channel-related vasodilation is probably due to TRP-mediated Ca2+-dependent activation of hyperpolarizing mechanisms or it may result from inhibition of TRP channels and reduced entry of Ca2+ via Cav or TRP channels. Below we review the evidence supporting the diverse roles for TRP channels in ECs and vascular SMCs.
Various reports suggest that all members of the TRPC subfamily may be present in ECs. However, there are currently no studies that demonstrate the function of TRPC2 or TRPC7 channels in the endothelium. TRPC channels are well represented in vascular smooth muscle and appear to play important roles in both vasoconstrictor and vasodilator mechanisms. TRPC channels probably contribute to Ca2+ entry and contraction of vascular smooth muscle stimulated by receptor-activation myogenic mechanisms or store depletion. Inhibition of TRPC channels may be involved in certain mechanisms of vasodilation.
Groundbreaking work demonstrates that STIM1 (stromal interaction molecule 1) and Orai1 (ORAI Ca2+ release-activated Ca2+ modulator 1) are required for SOC activity in many types of cells [8,9]. In addition, a number of reports suggest that TRPC1 is involved in SOC entry in ECs, perhaps as part of a heteromultimeric channel with TRPC4 [10–14]. For example, a cation current with biophysical properties consistent with TRPC1 is present in rat pulmonary artery ECs . In primary cultures of human pulmonary artery ECs, antisense-mediated down-regulation of TRPC1 expression diminished thapsigargin-induced increases in intracellular Ca2+ levels, suggesting a role for the channel in SOC influx . These findings and other reports have led some investigators to propose that TRPC1/TRPC4 channels are part of a SOC complex that includes STIM1 and Oria1 [17–19]. However, this model has been recently challenged by a study demonstrating that knockdown of TRPC1 and TRPC4 channels has no effect on SOC entry or the ICRAC (Ca2+ release-activated Ca2+ current) in HUVECs (human umbilical vein ECs) . These findings are supported by recent work from Putney and co-workers showing that agonist-dependent Ca2+ influx through TRPC1, TRPC3, TRPC5, TRPC6 and TRPC7 channels does not involve STIM1 . Thus the relationships between TRPC channels, STIM1 and Orai remain unresolved (for a recent review see ).
TRPC1 channels may also be involved in receptor-mediated responses in ECs. Mehta et al.  showed that the GTP-binding protein RhoA and IP3Rs [IP3 (inositol 1,4,5-trisphosphate) receptors] associate with TRPC1 in human pulmonary artery ECs following thrombin administration. In HUVECs coupling between TRPC1 and IP3R, induced by VEGF (vascular endothelial growth factor), is opposed by angiopoietin-1, a factor that blocks VEGF-induced increases in the permeability of the endothelium . Thus it appears that regulation of the TRPC1–IP3R interaction is critical for regulation of the permeability barrier . TRPC1 channels are present in bovine aortic ECs and have been implicated in Ca2+ influx associated with bFGF (basic fibroblast growth factor) . Exposure to TNFα (tumour necrosis factor α)  and activation of the NF-κB (nuclear factor κB) pathway with PAR-1 (protease-activated receptor-1)  cause increased expression of TRPC1 and enhanced thapsigargin- and thrombin-induced Ca2+ influx in HUVECs. Inhibition of PKCα (protein kinase Cα) diminishes thapsigargin-induced Ca2+ influx and store-operated cation currents in HUVECs . That study also reported that an anti-TRPC1 antibody caused similar inhibition of store-operated currents. In addition, thrombin and thapsigargin promote PKCα-mediated phosphorylation of TRPC1 .
TRPC1 channels also are known to interact with caveolin-1 in human pulmonary artery ECs via a CSD (caveolin-scaffolding domain) in the C-terminus of the channel. A CSD-conjugated peptide attenuates thrombin- and thapsigargin-induced Ca2+ influx and TRPC1 can be directly regulated by caveolin-1 .
TRPC1 has long been known to mediate receptor-activated Ca2+ entry in a variety of cell types and expression systems; however, only a few studies have implicated TRPC1 as mediating ROC influx in vascular smooth muscle. For instance, endothelin activates Ca2+ entry that is independent of SOC influx in primary cultured aortic SMCs  and in rabbit cerebral cortical arteries , although the entry does appear to involve TRPC1 channels. AngII (angiotensin II) activates TRPC1 channels in excised patches from rabbit mesenteric artery myocytes, implying direct channel activation (Figure 1) .
There is evidence both for and against the involvement of TRPC1 channels in SOC entry in SMCs. SOC entry is intact in aortic and cerebral artery SMCs from TRPC1-knockout mice, indicating that the channel is not involved in this response in these cells . However, overexpression of the human TRPC1 gene in rat pulmonary arteries enhanced Ca2+ entry and pulmonary vasoconstriction in response to depletion of Ca2+ stores . In addition, TRPC1 expression is up-regulated in cerebral arteries following organ culture and in these vessels store-depletion activates Ca2+ entry and constriction, which is inhibited by an anti-TRPC1 antibody . Activation of TRPC1 channels by store depletion in rabbit portal vein myocytes has been demonstrated, and it is facilitated by PIP2 (phosphatidlyinositol 4,5-bisphosphate) . TRPC1 cation currents and Ca2+ influx are activated following store depletion in cultured human saphenous vein SMCs . SOC entry in these cells involves molecular interactions between TRPC1 and STIM1. That study also suggested that TRPC1 activity may be associated with vascular SMC proliferation .
Changes in the expression or activity of TRPC1 channels have been observed for several vascular diseases. TRPC1 expression is increased during AngII-induced vascular hypertrophy  and during vascular hyperplasia associated with vascular occlusive disease . TRPC1 activity is also associated with the pulmonary artery SMC proliferation that occurs following activation of SOC entry . TRPC1, along with TRPC4 channels, are probably mediators of the Ca2+ entry and cerebrovascular constriction that occurs in response to activation of ET-1 (endothelin-1) receptors in a canine model of subarachnoid haemorrhage .
Expression of TRPC3 has been demonstrated by immunostaining in the rat mesenteric artery wall , primarily in ECs, and is detected in primary cultures of porcine aortic ECs using RT (reverse transcription)–PCR  and cell-surface biotinylation . Cation currents recorded from porcine aortic ECs have properties that are consistent with TRPC3 . This TRPC3-like current is activated by the oxidizing compound tBHP (t-butylhydroperoxide), suggesting that TRPC3 is an oxidant-activated channel in ECs and could play a role in redox sensitivity .
TRPC3 co-immunoprecipitates with TRPC4 in porcine aortic ECs and association of TRPC3 and TRPC4 expressed in HEK (human embryonic kidney)-293 cells has been demonstrated by FRET (fluorescence resonance energy transfer) analysis, suggesting that TRPC3 and TRPC4 can form heteromultimeric channels in ECs .
Recent evidence suggests that endothelial TRPC3 channels may influence the development of cardiovascular disease. In primary cultures of coronary artery ECs, down-regulation of TRPC3 using siRNA (small interfering RNA) results in a reduction in constitutive and ATP-induced Ca2+ influx . ATP-induced VCAM-1 (vascular cell adhesion molecule-1) activation and monocyte adhesion is also reduced in these cells, suggesting that dysregulation of TRPC3 channels may be involved in atherogenesis . Furthermore, TRPC3, but not TRPC6, expression is reportedly elevated in the vascular endothelium of preglomerular arteries isolated from human patients with malignant hypertension . It is not clear from that report how TRPC3 channels in the endothelium influence the development of the disease.
Abundant evidence supports the proposal that TRPC3 is a ROC channel in a host of different tissues and cell types. In vascular smooth muscle UTP  and ET-1  activate TRPC3 channels. In cerebral artery smooth muscle activation of TRPC3 currents by UTP causes depolarization, Ca2+ entry and vasoconstriction . Receptor activation of TRPC3 probably results from G-protein-mediated activation of PLC and subsequent generation of DAG and IP3; the action of DAG appears to be direct rather than via DAG-mediated activation of PKC , which in fact inhibits TRPC3 [48,49]. IP3 also appears to play a role in receptor-mediated activation of TRPC3 in expression systems  and in vascular smooth muscle , probably through a conformational coupling mechanism involving the TRPC3 channel and IP3Rs. Interestingly, TRPC3 channels in rabbit ear artery myocytes demonstrate constitutive activity, which may contribute to control of the resting membrane potential and Ca2+ entry in these cells ; the constitutive TRPC3 activity in this case is apparently due to tonic production of DAG . Evidence from expression systems suggests that constitutive TRPC3 activity is regulated by the channel glycosylation state , but this has not been shown for native TRPC3 channels.
Although TRPC3 may contribute to SOC entry in some cell types and expression systems there is no evidence to suggest a similar role for TRPC3 channels in vascular smooth muscle.
Recent evidence by Chen et al.  suggests a novel mechanism of vasodilation induced by NO. These investigators reported that NO-mediated activation of PKG (protein kinase G) is linked to inhibition of TRPC3 and TRPC1 channels and a related relaxation of rat carotid artery myocytes (Figure 2). This finding is consistent with a previous demonstration that PKG activation inhibits human TRPC3 channels expressed in HEK-293 cells .
A correlation between higher TRPC3 protein expression and hypertension, AngII activity and enhanced vasoconstrictor responses in vascular smooth muscle from SHRs (spontaneously hypertensive rats) compared with WKRs (Wistar–Kyoto rats) has recently been reported ; blood pressure reduction by ACE (angiotensin-converting enzyme) inhibition, but not by dihydropyridine treatment, in SHRs reduced TRPC3 expression and aortic vasoconstrictor responses. The results support the possibility that up-regulation of TRPC3 channels contributes to the development of hypertension in the SHR model.
Strong evidence suggests that TRPC4 is an important Ca2+ influx channel in the endothelium. Using primary cultures of mouse aortic ECs, Nilius and co-workers demonstrated that store-operated cation currents and Ca2+ entry are not present in ECs isolated from TRPC4-knockout mice . Furthermore, expression of TRPC4 was required for ACh (acetylcholine) and ATP-induced Ca2+ influx and hyperpolarization of the plasma membrane in these cells . Aortic rings from the TRPC4-knockout mice displayed impaired endothelium-dependent vasodilation in response to ACh . These findings suggest that TRPC4 is responsible for SOC influx in ECs following agonist stimulation and this function is necessary for endothelium-dependent relaxation of aortic rings.
TRPC4 channels are also involved in the regulation of the permeability of the pulmonary endothelium. Ca2+ influx in response to thrombin is impaired in primary cultures of pulmonary artery ECs isolated from the TRPC4-knockout mice . This defect in Ca2+ influx is associated with a lack of thrombin-induced actin stress fibre formation and a reduced retraction response in pulmonary ECs from the TRPC4-knockout mice. In isolated perfused mouse lungs, the PAR-1-agonist-peptide-induced increases in the microvessel filtration coefficient, a measure of vascular permeability, were significantly blunted in lungs from the TRPC4-knockout mice compared with controls . These results show that agonist-induced TRPC4-dependent Ca2+ entry in mouse pulmonary artery ECs is a key determinant of increased microvascular permeability.
The evidence for functional roles of TRPC4 in vascular SMCs is limited. TRPC4, probably as a TRPC1–TRPC4 heteromultimeric channel, participates in the heightened cation channel and vasoconstrictor responses to ET-1 stimulation of cerebral artery myocytes in a canine model of subarachnoid haemorrhage . It has also been reported that TRPC4 expression and SOC entry are enhanced in rat mesenteric and aortic vascular SMCs subjected to cyclic stretch . The effects of this up-regulation on myocyte contraction or relaxation responses have not been evaluated to date.
TRPC5 is present in primary cultures of bovine and mouse aortic ECs . In these cells, the channel translocates to the plasma membrane in response to lysophosphatidylcholine. The translocation response is dependent on TRPC6 expression . In addition, siRNA-mediated down-regulation of TRPC5 impairs lysophosphatidylcholine-induced Ca2+ influx and enhances migration of ECs . Lysophosphatidylcholine levels are elevated in the vascular wall during atherosclerosis, suggesting that that lysophosphatidylcholine-induced activation of TRPC5 channels could have an impact on the disease by inhibiting migration of ECs in atherosclerotic arteries.
NO-induced nitrosylation of TRPC5 channels following stimulation of G-protein-coupled receptors induces Ca2+ influx in ECs, suggesting the possibility that the channel acts as a NO sensor in these cells .
TRPC5 is involved in the responses of cerebral arteriolar SMCs to a variety of stimuli including Ca2+ store depletion where Ca2+ entry following store depletion was suppressed by an anti-TRPC5-blocking antibody . In light of a previous report indicating a similar role for TRPC1 in pial arterioles , the authors suggest that the SOC entry channel in this case consists of a TRPC1–TRPC5 heteromultimeric structure. TRPC5 is also activated by lysophospholipids . Activation of the TRPC5 channel does not constrict cerebral arteries, but rather may participate in the process of SMC migration and proliferation [63,64] A TRPC5–TRPC6 heteromultimeric channel contributes to SOC entry in rabbit coronary artery myocytes .
TRPC6 appears to be an important ROC influx channel in ECs. For example, down-regulation of TRPC6 using siRNA diminishes bradykinin-induced Ca2+ influx in H5V cells (transformed murine embryonic heart microvessel ECs) . In primary cultures of human pulmonary artery ECs, siRNA-mediated down-regulation of TRPC6 expression impairs Ca2+ influx and RhoA activation in response to the DAG analogue OAG (2-acetyl-sn-glycerol) and thrombin .
TRPC6 channels may be important in growth-factor-induced angiogenesis. Cheng et al.  recorded cation currents in primary cultures of human microvascular ECs in response to VEGF. A similar current was activated in these cells by OAG . The biophysical properties of this current were reconstituted in CHO (Chinese-hamster ovary) cells expressing VEGFR2 (VEGF receptor 2), TRPC6 and TRPC3, but not in CHO cells expressing VEGFR2 and TRPC6 or VEGFR2 and TRPC3 . These findings suggest that VEGF activates a heteromultimeric TRPC3–TRPC6 channel in human microvascular ECs. Consistent with these findings, expression of a dominant-negative form of TRPC6 decreased VEGF-induced Ca2+ influx, migration, sprouting and proliferation of primary cultures of human microvascular ECs, suggesting that TRPC6-dependent Ca2+ influx is necessary for VEGF-induced angiogenesis . Further support for TRPC6 in angiogenesis was reported by Ge et al. , who showed that VEGF induces a TRPC6-like cation current in HUVECs and that this current is absent in cells expressing a dominant-negative form of TRPC6. Expression of the dominant-negative form of TRPC6 or siRNA-mediated knockdown of TRPC6 expression suppressed VEGF- but not FGF-induced proliferation (Figure 3) and capillary tube formation in HUVEC cultures .
A variety of receptor ligands activate TRPC6 channels in vascular myocytes. The first observations of this role for TRPC6 in the vasculature were made by Inoue and co-workers in 2001 . These investigators determined that exposure to PE [phenyladrenaline (phenylephrine)], a vasoconstrictor agonist that activates α-adrenoreceptors coupled to PLC, produced a current in vascular SMCs isolated from the rabbit portal vein that had biophysical properties similar to cloned TRPC6 channels (e.g. dual rectification, divalent cation permeant and unitary conductance of 25–30 pS). The channel was activated by DAG and potentiated by flufenamte (a non-specific cation channel inhibitor that ‘uniquely’ enhances α-adrenoreceptor non-selective cation currents in the rabbit portal vein ) and extracellular Ca2+, and was inhibited by non-selective cation channel blockers including Cd2+, La3+, Gd3+, SK&F 96365 and amiloride. PE-induced currents, with virtually identical properties with those found in the native cells, were recorded from HEK cells overexpressing cloned TRPC6. Down-regulation of TRPC6 expression in primary cultured portal vein myocytes using antisense oligodeoxynucleotides suppressed the α-adrenoceptor-activated cation currents. These results showed that TRPC6 channels are critical mediators of PE-induced responses in portal vein SMCs. More recent evidence supports a role for TRPC6 in non-selective cation channel activity and Ca2+ influx induced by vasopressin [72,73], AngII [30,74] and ATP , and possibly by activation of 5-HT (5-hydroxytryptamine; serotonim) and PDGF (platelet-derived growth factor) receptors  in vascular myocytes. Na+ entry through TRPC6 channels, following activation of purinergic receptors, enhances Ca2+ influx via Na+/Ca2+ exchange in aortic myocytes . Interestingly, α-adrenoceptor-mediated vasoconstrictor responses are enhanced in TRPC6-knockout compared with wild-type mice . However, this unexpected result is probably due to the increased expression and possible compensatory role of vascular TRPC3 channels in the TRPC6-knockout mice.
TRPC6 channels have been linked to the signalling mechanism involved in mechano-activation (stretch or pressure) of vascular contraction. For instance, Welsh et al.  observed a non-selective cation current in cerebral artery myocytes that is activated by cell swelling. Down-regulation of TRPC6 expression using antisense oligodeoxynucleotides attenuates these currents, as well as smooth muscle membrane depolarization and vasoconstriction induced by elevation of intraluminal pressure. These results indicate that membrane stretch activates a TRPC6-dependent depolarizing cation current that contributes to myogenic constriction of cerebral arteries. Indeed, Spassova et al.  observed that osmotic (hypotonicity) and pressure (negative pipette pressure) stimuli activate TRPC6 channels expressed in HEK-293 or CHO cells. Such channel activation is blocked by GsMTx-4, a toxin isolated from the tarantula Grammostola spatulata, and a reported inhibitor of mechanosensitive ion channels , supporting the proposal that TRPC6 channels are direct sensors of mechanical stimuli. Alternatively, a recent study by Mederos y Schnitzler and co-workers indicates that membrane stretch does not directly gate mechanosensitive TRP channels, but rather causes an agonist-independent activation of Gq/11-coupled receptors, i.e. the AT1 (AngII receptor type 1), which subsequently activates TRPC channels via a G-protein- and PLC-coupled mechanism . This finding is consistent with other studies indicating the existence of common mechanisms  or synergistic interactions  between receptor and mechanical stimulation, which converge on TRPC6 channels.
Increased expression of TRPC6 enhances, whereas TRPC6 suppression decreases, SOC entry in pulmonary artery myocytes . In addition, as mentioned above, a TRPC5–TRPC6 heteromultimeric channel contributes to SOC entry in rabbit coronary artery myocytes .
TRPC6 expression is up-regulated in pulmonary artery myocytes from patients with idiopathic pulmonary hypertension and is correlated with the enhanced proliferation of pulmonary artery SMCs that occurs in this disease state . Expression levels of TRPC6 are also increased in mesenteric arterial SMCs from DOCA (deoxycorticosterone acetate)-salt-induced hypertensive rats and this is correlated with up-regulation of a serotonin-activated non-selective cation current . TRPC6 channels also appear to play a key role in the acute pulmonary vasoconstrictor response to hypoxia . This conclusion is based on the observations that hypoxia-induced pulmonary artery constriction, myocyte Ca2+ entry (Figure 4) and cation currents are absent in the TRPC6-knockout mice, and that regional hypoventilation causes severe hypoxaemia in the TRPC6-knockout mice. A recent report indicates that EETs contribute to hypoxic pulmonary artery constriction by enhancing the translocation of TRPC6 channels to the plasma membrane in pulmonary artery myocytes .
TRPC7 message has been detected in several types of arteries, including canine pulmonary and renal arteries  and murine aorta and cerebral arteries . TRPC7 contributes to ROC channel activity in vascular myocytes, probably as heteromeric TRPC6–TRPC7  or TRPC3–TRPC7 channels . ET-1 activates a Ca2+-permeable cation channel with TRPC7 properties, possibly as a heteromultimeric channel with TRPC3, in rabbit coronary artery myocytes .
Several studies demonstrate functional roles for TRPV1, TRPV3 and TRPV4 channels in the endothelium. TRPV2 channels may be present in human pulmonary artery ECs, but the function of the channel in these cells has not been reported. There are no reports of the expression of TRPV5 or TRPV6 channels in cultured or native ECs. Although TRPV1, TRPV2, TRPV3 and TRPV4 have been reported to be present in the vascular wall [89,90], only TRPV2 and TRPV4 have been shown to be physically present and functionally active in vascular SMCs.
Capsaicin-induced Ca2+ influx has been reported in primary culture of cerebral artery ECs, suggesting the presence of functional TRPV1 channels in these cells . Although TRPV1 mRNA was detected in rat intralobar arteries and aorta, two studies failed to detect TRPV1 mRNA from cerebral arteries. Thus it is unclear whether functional TRPV1 channels are present in the endothelium of cerebral vessels.
TRPV1 channels are present in the endothelium of pig coronary artery and appear to mediate endothelium-dependent dilation in response to capsaicin . Capsaicin-induced arterial relaxation is impaired in tissue isolated from obese pigs, indicating a possible role for TRPV1 channels in the development of coronary artery disease .
TRPV2 channel mRNA is present in rat  and mouse  aortic, mesenteric and cerebral artery myocytes. Cell swelling during exposure to hypo-osmotic bathing solution induces cation currents and Ca2+ influx in murine aortic myocytes, and these responses are suppressed following treatment of the cells with TRPV2 antisense oligonucleotides . This is consistent with the observation that TRPV2 channels in a variety of tissues are mechanosensitive . However, a functional role for TRPV2 channels in intact blood vessels remains to be determined.
A very recent report demonstrates that carvacrol, derived from oregano, elicits SMC hyperpolarization and vasodilation by activating TRPV3 channels in the endothelium of cerebral arteries . These findings suggest that dietary molecules can exert vasodilator and cardioprotective effects by activating chemosensitive TRP channels present in ECs.
Strong evidence has been reported that demonstrates an important role for TRPV4 channels in EC function. The presence of functional TRPV4 channels in the endothelium was first reported by Watanabe et al. [96–98], who showed that administration of 4α-PDD, heating to 43 °C and 5′,6′-EET activated cation currents with biophysical properties consistent with TRPV4 in mouse aortic SMCs. These stimuli also induced Ca2+ influx in these cells.
The EETs are CYP (cytochrome P450) epoxygenase metabolites of AA (arachidonic acid) produced by ECs [99,100]. These compounds are potent vasodilator and SMC-hyperpolarizing factors . Owing to their powerful effects, the consequences of EET-induced activation of TRPV4 channels in ECs are of considerable interest. Expression of CYP2C, the epoxygenase isoform responsible for EET synthesis in ECs , is increased in cells treated with the dihydropyridine compound nifedipine [102,103]. AA and hypotonicity-induced Ca2+ influx in nifedipine-treated aortic SMCs obtained from wild-type mice was elevated compared with non-treated controls, suggesting that CYP2C activity is involved in these responses . In contrast, treatment with sulfaphenazole, a blocker of CYP2C activty, diminishs Ca2+ influx in response to AA and hypotonicity-induced cell swelling . Nifedipine-induced increases in CYP2C expression did not alter EC Ca2+ increases in response to 4α-PDD, 5′,6′-EET and 8′,9′-EET, or heat, suggesting that these stimuli act downstream of expoxygenase activity. That study also demonstrates that 4α-PDD, 5′,6′-EET and 8′,9′-EET fail to evoke Ca2+ influx in mouse aortic SMCs from TRPV4-knockout mice, and changes in Ca2+ in response to AA, hypotonicity and heat were attenuated in these cells compared with wild-type. Furthermore, inhibition of soluble epoxide hydrolase and COX (cyclo-oxygenase) activity, the major pathways for EET hydrolysis, enhanced the Ca2+ responses to AA, hypotonicity and EETs, but responses to 4α-PDD and heat were not altered . These results suggest that CYP2C-derived EETs can act in an autocrine manner to provoke changes in EC Ca2+ level, in response to AA, and swelling by a mechanism that requires TRPV4. These findings were extended by Marrelli et al. , who detected expression of TRPV4 channels in the endothelium of rat middle cerebral arteries. This reports also shows that inhibition of PLA2 (phospholipase A2) activity blocked increases in EC Ca2+ level and cerebral artery dilation in response to the purinergic receptor agonist UTP. Vasodilation and EC Ca2+ increases in response to AA were not altered by PLA2 inhibition in that study. In addition, increases in EC Ca2+ in response to UTP and 4α-PDD were blocked with Ruthenium Red . These findings suggest that PLA2 activity is required for TRPV4-dependent endothelial Ca2+ influx and dilation of cerebral arteries. PLA2 probably contributes to this pathway by liberating AA from the plasma membrane, which is then converted into EETs through COX activity.
Köhler et al.  recorded TRPV4-like currents from rat carotid artery ECs using an in situ patch clamp technique and detected expression of TRPV4 in these cells by RT–PCR. The TRPV4 agonist 4α-PDD caused Ca2+ influx in freshly isolated rat aortic ECs. Administration of 4α-PDD resulted in dilation of pre-constricted carotid and gracilis arteries that was blocked by Ruthenium Red. 4α-PDD-induced dilation of carotid arteries was largely blocked by combined inhibition of the NOS (NO synthase) and COX pathway . In gracilis arteries, 4α-PDD-induced dilation was insensitive to NOS and COX inhibition, suggesting that TRPV4 activation causes vasodilation by different pathways in resistance compared with conduit vessels. Shear-stress-induced vasodilation was diminished by Ruthenium Red, suggesting a role for a TRPV channel in this response . These findings were confirmed by reports demonstrating an absence of shear-stress-induced vasodilation in carotid arteries isolated from TRPV4-knockout mice (Figure 5) [106,107].
Mesenteric arteries isolated from TRPV4-knockout mice display impaired endothelium-dependent vasodilation that is independent of NOS and COX activity , further demonstrating an important role for the channel in vascular regulation. Systolic blood pressure under resting conditions does not differ between control and TRPV4-knockout mice ; however, NOS-inhibition-induced hypertension is exacerbated in TRPV4-knockout mice. TRPV4 channels are also involved in the regulation of the permeability of the pulmonary vascular endothelium. Alvarez et al.  reported that 4α-PDD or EETs significantly increased the filtration coefficient (Kf), a measure of lung endothelial permeability, in perfused rat lungs in a Ca2+-dependent manner. These responses were blocked by Ruthenium Red, indicating involvement of a TRPV channel. Furthermore, 4α-PDD increased the Kf in lungs from wild-type mice, but was without effect in lungs from TRPV4-knockout mice . These findings provide strong evidence that activation of TRPV4 contributes to changes in endothelial vasodilator activity and barrier function. Interestingly, the 4α-PDD-invoked Ca2+ responses are diminished in ECs following siRNA-mediated down-regulation of caveolin-1. In addition a TRPV4 and caveolin-1 interaction can be demonstrated by co-immunoprecipition, suggesting that there might be a requirement for caveolar localization for TRPV4 function in these cells .
TRPV4 is present in vascular myocytes in the cerebral and mesenteric circulations [104,110] and in the aorta . Evidence for a functional role of TRPV4 in vascular smooth muscle comes from studies of cerebral  and mesenteric arteries . EETs, which are produced by the vascular endothelium and have been proposed to be one form of EDHF (endothelium-derived hyperpolarizing factor) , activate TRPV4 channels in isolated cerebral artery myocytes (Figure 6) . Ca2+ influx through the activated TRPV4 channels then induces the release of Ca2+ from ryanodine-sensitive receptors located on the sarcoplasmic reticulum. These Ca2+ release events, termed Ca2+ sparks , then activate nearby sarcolemmal voltage-activated BK channels (Ca2+-sensitive K+ channels) and increase the frequency of macroscopic outward K+ currents. The increase in BK currents results in smooth muscle hyperpolarization and vasodilation. These experiments support the proposal that TRPV4 forms a Ca2+ signalling complex that includes ryanodine receptors and BK channels, and also suggest that the TRPV4 channel serves as a receptor molecule for EETs in vascular smooth muscle. These findings have recently been verified in studies of mesenteric arterioles, suggesting the possibility that TRPV4 activity in vascular myocytes contributes to regulation of peripheral vascular resistance .
In rat pulmonary artery SMCs, serotonin-induced Ca2+ influx and cation currents may be mediated by TRPV4 channels . These responses were mimicked by the TRPV4 activator 4α-PDD and blocked by Ruthenium Red. TRPV4-mediated Ca2+ entry was linked to a mitogenic response of the pulmonary artery myocytes, suggesting a role of TRPV4 in proliferation of pulmonary vascular smooth muscle.
Expression of all the TRPM channel proteins except TRPM5 has been reported in ECs. However, there are currently no studies describing the function of TRPM1 and TRPM3–6 in ECs. All eight members of the TRPM subfamily are present in vascular SMCs. Possible functional roles in vascular smooth muscle have been reported for TRPM4, TRPM7 and TRPM8.
Functional TRPM2 channels are present in primary cultures of human pulmonary artery ECs, where the channel mediates Ca2+ influx in response to H2O2 . In addition, siRNA-mediated down-regulation of TRPM2 expression reduces H2O2-induced increases in the permeability of the endothelium, suggesting that TRPM2 is important for regulation of the pulmonary endothelial barrier during oxidative stress .
Several reports indicate that TRPM4 channels contribute to stretch-induced myogenic depolarization of cerebral artery myocytes [116–120]. TRPM4 channels, but not the closely related TRPM5 channel, are present in rat cerebral artery SMCs. TRPM4-like monovalent-cation-selective channels, activated by intracellular Ca2+, PKC and membrane stretch, were identified in isolated cerebral artery myocytes in these studies. The cation currents, as well as the smooth muscle depolarization and constriction of isolated cerebral arteries in response to elevated intravascular pressure or PKC activation, were greatly attenuated when expression of TRPM4 was reduced using TRPM4 antisense oligonucleotides in vitro or in vivo. Furthermore, cerebral arterial myogenic tone and cerebral blood flow autoregulation were reduced after exposing cerebral arteries to TRPM4 antisense oligonucleotides in the intact rat (Figure 7) . Collectively, these studies demonstrate a prominent role for vascular TRPM4 channels in the control of cerebral myogenic tone and autoregulation.
Recent evidence has been presented supporting the hypothesis that TRPM4 channels contribute to a secondary spinal haemorrhage response following spinal cord injury in a rat model . A day after spinal cord injury, TRPM4 expression in capillary endothelium distant from the point of injury is substantially increased. The increased TRPM4 expression is closely paralleled in time and location by capillary fragmentation and local haemorrhage events. Suppression of TRPM4 expression, or blocking of channel activity, greatly attenuated the secondary capillary pathology associated with spinal cord injury. It is hypothesized that a large Na+ influx into the ECs, due to the increased TRPM4 channel density and lower cellular ATP levels, results in osmotically driven capillary damage .
An important role for TRPM7 channels in the regulation of endothelial function is suggested by a recent study using a mouse model of hypomagnesaemia . Aortas from inbred mice exhibiting low intracellular erythrocyte magnesium concentration (MgL) demonstrate elevated TRPM7 expression and decreased expression of annexin-1, a substrate of the TRPM7 kinase domain, compared with mice with high intracellular erythrocyte magnesium (MgH). Interestingly, systolic blood pressure is elevated in MgL mice. Cultured aortic ECs and aortic tissue from MgL mice exhibited diminished eNOS (endothelial NOS) expression compared with MgH mice, and ACh-induced vasodilation of isolated mesenteric arteries from MgL was blunted compared with MgH mice. Functional TRPM7 channels are present in HUVECs. Knockdown of TRPM7 with siRNA, or inhibition of TRPM7 function with 2-aminoethoxydiphenyl borate, increased the phosphorylation of ERK (extracellular signal-regulated kinase) and enhanced the proliferation these cells . Silencing of TRPM7 using siRNA also increased the expression of NOS and NO production .
Recent studies have revealed an essential role of TRPM7 channels in control of Mg2+ homoeostasis in vascular SMCs. Touyz and co-workers found that TRPM7 channels are expressed in cultured vascular SMCs from mice, rats and humans . TRPM7 expression in these cells can be suppressed by TRPM7 siRNA. Basal intracellular Mg2+ levels are reduced when TRPM7 channels are suppressed. Chronic exposure to AngII increases intracellular Mg2+ and induces proliferation of the vascular SMCs, and these responses are inhibited by TRPM7 siRNA (Figure 8). These findings support the proposal that TRPM7 is a functionally important regulator of Mg2+ homoeostasis and growth in vascular myocytes. A recent study from this group demonstrates that bradykinin regulates Mg2+ levels and cell migration by modulating TRPM7 activity in vascular myocytes . This effect occurs through PLC-, PKC- and c-Src-dependent pathways.
Two studies suggest that TRPM7 channels may be mechanosensitive. Exposure of cultured aortic SMCs (A7r5 cells) to normal levels of shear stress causes rapid accumulation of TRPM7 channels at the plasma membrane . One implication of that work is that shear stress may increase surface expression of TRPM7 in vascular myocytes in areas where ECs have been damaged. Enhanced TRPM7-mediated Mg2+ influx and altered function of the vascular myocytes might then occur. Another study shows that currents activated by mechanical stretch are diminished in HeLa cells treated with siRNA against TRPM7 . Similar currents are present in HEK-293 cells overexpressing TRPM7, suggesting that the channel is inherently mechanosensitive . These findings have been disputed by Bessac and Fleig , who point out that the single channel conductance of the mechanosensitive channel in the study by Numata et al. , at 23 and 26 pS, does not match the single channel conductance of TRPM7 (40 pS). Bessac and Fleig  also express concerns about possible non-specificity of the siRNA procedures used to silence TRPM7 expression.
Basal TRPM7 expression levels are lower in cultured mesenteric artery SMCs from SHRs compared with WKRs . Low intracellular Mg2+ levels are associated with increased vascular tone, reduced vasodilator function, vascular remodelling and elevated blood pressure [129,130], and therefore this change in TRPM7 expression could be a key factor involved in some forms of hypertension.
Two studies suggest that TRPM8 channels may be present and functional in the vascular endothelium. The TRPM8 agonist icillin causes Ca2+ influx in immortalized human corneal ECs, suggesting that the channel is functional in these cells . Another study demonstrates that menthol, an activator of TRPM8 channels, dilates the rat tail artery and this response is partially dependent on the endothelium . That study also shows that menthol increases vascular conductance in human forearm and this response is blocked by NOS inhibition and atropine.
TRPM8 channels are expressed in rat pulmonary and aortic myocytes . Menthol stimulates Ca2+ influx in primary cultures of pulmonary and aortic SMCs. However, functional roles for TRPM8 channels in the vasculature remain to be established.
TRPA1 channels are present in the endothelium of intact cerebral arteries . Interestingly, TRPA1 channels appear to be concentrated in regions of the EC plasma membrane that span the internal elastic lamina and are in close proximity with SMCs . Stimulation of TRPA1 channel activity with AITC (allyl isothiocyanate), a compound found in mustard oil, causes endothelium-dependent SMC hyperpolarization and vasodilation of cerebral arteries by a mechanism that requires small- and intermediate-conductance Ca2+-activated K+ channels (Figure 9) . These findings suggest that the cardioprotective effects of foods such as garlic (a source of the TRPA1 agonist allicin) or mustard oil could result from endothelium-dependent vasodilation mediated by TRPA1.
TRPP and TRPML
There is current no evidence for expression of these channels in ECs or vascular SMCs.
The available evidence strongly supports the proposal that TRP channels play important roles in the normal function of vascular ECs and SMCs (Figure 10). TRP channels are implicated in ROC and SOC influx, endoplasmic and sarcoplasmic reticulum Ca2+ release mechanisms, mechanically activated (shear, pressure and stretch) cellular signalling, membrane potential regulation and Mg2+ handling, Additional reports imply that TRP channels are important contributors to the onset, maintenance, progression of or response to vascular pathologies or insults such as hypertension, subarachnoid haemorrhage, vascular hypertrophy, proliferation, ischeamia and oxidative stress. Further studies are needed to elucidate the tissue-specific mechanisms and protein interactions through which TRP channels regulate vascular function in health and disease.
The work in our laboratories was supported by the National Institutes of Health [grant numbers HL091905 (to S. E.), HL58231 (to J. E. B.)].
Abbreviations: AA, arachidonic acid; ACh, acetylcholine; AITC, allyl isothiocyanate; AngII, angiotensin II; BK channel, Ca2+-sensitive K+ channel; Cav channel, voltage-dependent Ca2+ channel; CHO, Chinese-hamster ovary; COX, cyclo-oxygenase; CSD, caveolin-scaffolding domain; CYP, cytochrome P450; DAG, diacylglycerol; EC, endothelial cell; EDHF, endothelium-derived hyperpolarizing factor; EET, epoxyeicosatrienoic acid; ET-1, endothelin isoform 1; FGF, fibroblast growth factor; HEK, human embryonic kidney; HUVEC, human umbilical vein EC; IP3, inositol 1,4,5-trisphosphate; IP3R, IP3 receptor; NOS, NO synthase; OAG, 2-acetyl-sn-glycerol; Orai, Orai Ca2+ release-activated Ca2+ modulator; PAR, protease-activated receptor; 4α-PDD, 4α-phorbol 12,13-didecanoate; PE, phenyladrenaline (phenylephrine); PIP2, phosphatidlyinositol 4,5-bisphosphate; PKC, protein kinase C; PKG, protein kinase G; PLA2, phospholipase A2; PLC, phospholipase C; ROC, receptor-operated Ca2+; RT, reverse transcription; SHR, spontaneously hypertensive rat; siRNA, small interfering RNA; SMC, smooth muscle cell; SOC, store-operated Ca2+; STIM1, stromal interaction molecule 1; TNFα, tumour necrosis factor α; TRP, transient receptor potential; TRPA, TRP ankyrin; TRPC, TRP canonical; TRPM, TRP melastatin; TRPML, TRP mucoliptin; TRPP, TRP polycystin; TRPV, TRP vanilloid; VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor; WKR, Wistar–Kyoto rat
- © The Authors Journal compilation © 2010 Biochemical Society