Effect of niflumic acid on noradrenaline-induced contractions of the rat aorta

Calcium-Activated Chloride Channels in Myometrial and Vascular Smooth Muscle

In smooth muscle tissues, calcium-activated chloride channels (CaCC) provide the major anionic channel. Opening of these channels leads to chloride efflux and depolarization of the myocyte membrane. In this way, activation of the channels by a rise of intracellular [Ca²⁺], from a variety of sources, produces increased excitability and can initiate action potentials and contraction or increased tone. We now have a good mechanistic understanding of how the channels are activated and regulated, due to identification of TMEM16A (ANO1) as the molecular entity of the channel, but key questions remain. In reviewing these channels and comparing two distinct smooth muscles, myometrial and vascular, we expose the differences that occur in their activation mechanisms, properties, and control. We find that the myometrium only expresses “classical,” Ca²⁺-activated, and voltage sensitive channels, whereas both tonic and phasic blood vessels express classical, and non-classical, cGMP-regulated CaCC, which are voltage insensitive. This translates to more complex activation and regulation in vascular smooth muscles, irrespective of whether they are tonic or phasic. We therefore tentatively conclude that although these channels are expressed and functionally important in all smooth muscles, they are probably not part of the mechanisms governing phasic activity. Recent knockdown studies have produced unexpected functional results, e.g. no effects on labour and delivery, and tone increasing in some but decreasing in other vascular beds, strongly suggesting that there is still much to be explored concerning CaCC in smooth muscle.

A small molecule inhibitor of the chloride channel TMEM16A blocks vascular smooth muscle contraction and lowers blood pressure in spontaneously hypertensive rats

Hypertension is a major cause of cardiovascular morbidity and mortality, despite the availability of antihypertensive drugs with different targets and mechanisms of action. Here, we provide evidence that pharmacological inhibition of TMEM16A (ANO1), a calcium-activated chloride channel expressed in vascular smooth muscle cells, blocks calcium-activated chloride currents and contraction in vascular smooth muscle in vitro and decreases blood pressure in spontaneously hypertensive rats. The acylaminocycloalkylthiophene TMinh-23 fully inhibited calcium-activated TMEM16A chloride current with nanomolar potency in Fischer rat thyroid cells expressing TMEM16A, and in primary cultures of rat vascular smooth muscle cells. TMinh-23 reduced vasoconstriction caused by the thromboxane mimetic U46619 in mesenteric resistance arteries of wild-type and spontaneously hypertensive rats, with a greater inhibition in spontaneously hypertensive rats. Blood pressure measurements by tail-cuff and telemetry showed up to a 45-mmHg reduction in systolic blood pressure lasting for four-six hours in spontaneously hypertensive rats after a single dose of TMinh-23. A minimal effect on blood pressure was seen in wild-type rats or mice treated with TMinh-23. Five-day twice daily treatment of spontaneously hypertensive rats with TMinh-23 produced sustained reductions of 20-25 mmHg in daily mean systolic and diastolic blood pressure. TMinh-23 action was reversible, with blood pressure returning to baseline in spontaneously hypertensive rats by three days after treatment discontinuation. Thus, our studies provide validation for TMEM16A as a target for antihypertensive therapy and demonstrate the efficacy of TMinh-23 as an antihypertensive with a novel mechanism of action.

A paradoxical increase of force development in saphenous and tail arteries from heterozygous ANO1 knockout mice

A Ca²⁺‐activated Cl⁻ channel protein, ANO1, is expressed in vascular smooth muscle cells where Cl⁻ current is thought to potentiate contraction by contributing to membrane depolarization. However, there is an inconsistency between previous knockout and knockdown studies on ANO1’s role in small arteries. In this study, we assessed cardiovascular function of heterozygous mice with global deletion of exon 7 in the ANO1 gene. We found decreased expression of ANO1 in aorta, saphenous and tail arteries from heterozygous ANO1 knockout mice in comparison with wild type. Accordingly, ANO1 knockdown reduced the Ca²⁺‐activated Cl⁻ current in smooth muscle cells. Consistent with conventional hypothesis, the contractility of aorta from ANO1 heterozygous mice was reduced. Surprisingly, we found an enhanced contractility of tail and saphenous arteries from ANO1 heterozygous mice when stimulated with noradrenaline, vasopressin, and K⁺‐induced depolarization. This difference was endothelium‐independent. The increased contractility of ANO1 downregulated small arteries was due to increased Ca²⁺ influx. The expression of L‐type Ca²⁺ channels was not affected but expression of the plasma membrane Ca²⁺ ATPase 1 and the Piezo1 channel was increased. Expressional analysis of tail arteries further suggested changes of ANO1 knockdown smooth muscle cells toward a pro‐contractile phenotype. We did not find any difference between genotypes in blood pressure, heart rate, pressor response, and vasorelaxation in vivo. Our findings in tail and saphenous arteries contrast with the conventional hypothesis and suggest additional roles for ANO1 as a multifunctional protein in the vascular wall that regulates Ca²⁺ homeostasis and smooth muscle cell phenotype.

Regional Heterogeneity in the Regulation of Vasoconstriction in Arteries and Its Role in Vascular Mechanics

Vasoconstriction and vasodilation play important roles in the circulatory system and can be regulated through different pathways that depend on myriad biomolecules. These different pathways reflect the various functions of smooth muscle cell (SMC) contractility within the different regions of the arterial tree and how they contribute to both the mechanics and the mechanobiology. Here, we review the primary regulatory pathways involved in SMC contractility and highlight their regional differences in elastic, muscular, and resistance arteries. In this way, one can begin to assess how these properties affect important biomechanical and mechanobiological functions in the circulatory system in health and disease.

Negative News: Cl− and HCO3− in the Vascular Wall

Cl− and HCO3− are the most prevalent membrane-permeable anions in the intra- and extracellular spaces of the vascular wall. Outwardly directed electrochemical gradients for Cl− and HCO3− permit anion channel opening to depolarize vascular smooth muscle and endothelial cells. Transporters and channels for Cl− and HCO3− also modify vascular contractility and structure independently of membrane potential. Transport of HCO3− regulates intracellular pH and thereby modifies the activity of enzymes, ion channels, and receptors. There is also evidence that Cl− and HCO3− transport proteins affect gene expression and protein trafficking. Considering the extensive implications of Cl− and HCO3− in the vascular wall, it is critical to understand how these ions are transported under physiological conditions and how disturbances in their transport can contribute to disease development. Recently, sensing mechanisms for Cl− and HCO3− have been identified in the vascular wall where they modify ion transport and vasomotor function, for instance, during metabolic disturbances. This review discusses current evidence that transport (e.g., via NKCC1, NBCn1, Ca2+-activated Cl− channels, volume-regulated anion channels, and CFTR) and sensing (e.g., via WNK and RPTPγ) of Cl− and HCO3− influence cardiovascular health and disease.

Molecular and functional significance of Ca(2+)-activated Cl(-) channels in pulmonary arterial smooth muscle

Increased peripheral resistance of small distal pulmonary arteries is a hallmark signature of pulmonary hypertension (PH) and is believed to be the consequence of enhanced vasoconstriction to agonists, thickening of the arterial wall due to remodeling, and increased thrombosis. The elevation in arterial tone in PH is attributable, at least in part, to smooth muscle cells of PH patients being more depolarized and displaying higher intracellular Ca(2+) levels than cells from normal subjects. It is now clear that downregulation of voltage-dependent K(+) channels (e.g., Kv1.5) and increased expression and activity of voltage-dependent (Cav1.2) and voltage-independent (e.g., canonical and vanilloid transient receptor potential [TRPC and TRPV]) Ca(2+) channels play an important role in the functional remodeling of pulmonary arteries in PH. This review focuses on an anion-permeable channel that is now considered a novel excitatory mechanism in the systemic and pulmonary circulations. It is permeable to Cl(-) and is activated by a rise in intracellular Ca(2+) concentration (Ca(2+)-activated Cl(-) channel, or CaCC). The first section outlines the biophysical and pharmacological properties of the channel and ends with a description of the molecular candidate genes postulated to encode for CaCCs, with particular emphasis on the bestrophin and the newly discovered TMEM16 and anoctamin families of genes. The second section provides a review of the various sources of Ca(2+) activating CaCCs, which include stimulation by mobilization from intracellular Ca(2+) stores and Ca(2+) entry through voltage-dependent and voltage-independent Ca(2+) channels. The third and final section summarizes recent findings that suggest a potentially important role for CaCCs and the gene TMEM16A in PH.

Inhibitory role of phosphatidylinositol 4,5-bisphosphate on TMEM16A-encoded calcium-activated chloride channels in rat pulmonary artery

BACKGROUND AND PURPOSE

Calcium-activated chloride channels (CaCCs) are key depolarizing mechanisms that have an important role in vascular smooth muscle contraction. Here, we investigated whether these channels are regulated by phosphatidylinositol (4,5) bisphosphate [P(4,5)P2 ], a known regulator of various ion channels.

EXPERIMENTAL APPROACH

Calcium-activated Cl(-) currents (IClCa ) were recorded by patch clamp electrophysiology of rat isolated pulmonary artery smooth muscle cells. TMEM16A protein-phosphoinositide interaction was studied by co-immunoprecipitation and phosphoinositide binding arrays on protein lysates from whole pulmonary arteries and HEK293 cells overexpressing TMEM16A, the molecular correlate.

KEY RESULTS

PI(4,5)P2 and other phospholipids were shown to bind directly to TMEM16A isolated from whole pulmonary artery (PA) and TMEM16A-eGFP expressed in HEK293 cells. Agents that reduced PI(4,5)P2 levels through different routes [PLC activation, PI4K inhibition, PI(4,5)P2 scavenging and absorption] all increased IClCa evoked by solutions containing clamped-free [Ca(2+) ], whereas enrichment of activating solutions with PI(4,5)P2 inhibited IClca in PA smooth muscle cells with approximately 50% reduction at 1 μM.

CONCLUSIONS AND IMPLICATIONS

These data are the first to show a negative regulation of TMEM16A-encoded CaCCs by PI(4,5)P2 and propose that control of PI(4,5)P2 levels is a key determinant of arterial physiology.

Cl⁻ channels in smooth muscle cells

In smooth muscle cells (SMCs), the intracellular chloride ion (Cl−) concentration is high due to accumulation by Cl−/HCO3− exchange and Na+–K+–Cl− cotransportation. The equilibrium potential for Cl− (ECl) is more positive than physiological membrane potentials (Em), with Cl− efflux inducing membrane depolarization. Early studies used electrophysiology and nonspecific antagonists to study the physiological relevance of Cl− channels in SMCs. More recent reports have incorporated molecular biological approaches to identify and determine the functional significance of several different Cl− channels. Both "classic" and cGMP-dependent calcium (Ca2+)-activated (ClCa) channels and volume-sensitive Cl− channels are present, with TMEM16A/ANO1, bestrophins, and ClC-3, respectively, proposed as molecular candidates for these channels. The cystic fibrosis transmembrane conductance regulator (CFTR) has also been described in SMCs. This review will focus on discussing recent progress made in identifying each of these Cl− channels in SMCs, their physiological functions, and contribution to diseases that modify contraction, apoptosis, and cell proliferation.

Structure and function of TMEM16 proteins (anoctamins)

TMEM16 proteins, also known as anoctamins, are involved in a variety of functions that include ion transport, phospholipid scrambling, and regulation of other membrane proteins. The first two members of the family, TMEM16A (anoctamin-1, ANO1) and TMEM16B (anoctamin-2, ANO2), function as Ca2+-activated Cl- channels (CaCCs), a type of ion channel that plays important functions such as transepithelial ion transport, smooth muscle contraction, olfaction, phototransduction, nociception, and control of neuronal excitability. Genetic ablation of TMEM16A in mice causes impairment of epithelial Cl- secretion, tracheal abnormalities, and block of gastrointestinal peristalsis. TMEM16A is directly regulated by cytosolic Ca2+ as well as indirectly by its interaction with calmodulin. Other members of the anoctamin family, such as TMEM16C, TMEM16D, TMEM16F, TMEM16G, and TMEM16J, may work as phospholipid scramblases and/or ion channels. In particular, TMEM16F (ANO6) is a major contributor to the process of phosphatidylserine translocation from the inner to the outer leaflet of the plasma membrane. Intriguingly, TMEM16F is also associated with the appearance of anion/cation channels activated by very high Ca2+ concentrations. Furthermore, a TMEM16 protein expressed in Aspergillus fumigatus displays both ion channel and lipid scramblase activity. This finding suggests that dual function is an ancestral characteristic of TMEM16 proteins and that some members, such as TMEM16A and TMEM16B, have evolved to a pure channel function. Mutations in anoctamin genes (ANO3, ANO5, ANO6, and ANO10) cause various genetic diseases. These diseases suggest the involvement of anoctamins in a variety of cell functions whose link with ion transport and/or lipid scrambling needs to be clarified.

Ca2+-activated Cl- channels

Ca(2+)-activated Cl(-) channels (CaCCs) are plasma membrane proteins involved in various important physiological processes. In epithelial cells, CaCC activity mediates the secretion of Cl(-) and of other anions, such as bicarbonate and thiocyanate. In smooth muscle and excitable cells of the nervous system, CaCCs have an excitatory role coupling intracellular Ca(2+) elevation to membrane depolarization. Recent studies indicate that TMEM16A (transmembrane protein 16 A or anoctamin 1) and TMEM16B (transmembrane protein 16 B or anoctamin 2) are CaCC-forming proteins. Induced expression of TMEM16A and B in null cells by transfection causes the appearance of Ca(2+)-activated Cl(-) currents similar to those described in native tissues. Furthermore, silencing of TMEM16A by RNAi causes disappearance of CaCC activity in cells from airway epithelium, biliary ducts, salivary glands, and blood vessel smooth muscle. Mice devoid of TMEM16A expression have impaired Ca(2+)-dependent Cl(-) secretion in the epithelial cells of the airways, intestine, and salivary glands. These animals also show a loss of gastrointestinal motility, a finding consistent with an important function of TMEM16A in the electrical activity of gut pacemaker cells, that is, the interstitial cells of Cajal. Identification of TMEM16 proteins will help to elucidate the molecular basis of Cl(-) transport.

TMEM16A knockdown abrogates two different Ca(2+)-activated Cl (-) currents and contractility of smooth muscle in rat mesenteric small arteries

The presence of Ca²⁺-activated Cl⁻ channels (CaCCs) in vascular smooth muscle cells (SMCs) is well established. Their molecular identity is, however, elusive. Two distinct Ca²⁺-activated Cl⁻ currents (I_Cl(Ca)) were previously characterized in SMCs. We have shown that the cGMP-dependent I_Cl(Ca) depends on bestrophin expression, while the “classical” I_Cl(Ca) is not. Downregulation of bestrophins did not affect arterial contraction but inhibited the rhythmic contractions, vasomotion. In this study, we have used in vivo siRNA transfection of rat mesenteric small arteries to investigate the role of a putative CaCC, TMEM16A. Isometric force, [Ca²⁺]_i, and SMC membrane potential were measured in isolated arterial segments. I_Cl(Ca) and GTPγS-induced nonselective cation current were measured in isolated SMCs. Downregulation of TMEM16A resulted in inhibition of both the cGMP-dependent I_Cl(Ca) and the “classical” I_Cl(Ca) in SMCs. TMEM16A downregulation also reduced expression of bestrophins. TMEM16A downregulation suppressed vasomotion both in vivo and in vitro. Downregulation of TMEM16A reduced agonist (noradrenaline and vasopressin) and K⁺-induced contractions. In accordance with the depolarizing role of CaCCs, TMEM16A downregulation suppressed agonist-induced depolarization and elevation in [Ca²⁺]_i. Surprisingly, K⁺-induced depolarization was unchanged but Ca²⁺ entry was reduced. We suggested that this is due to reduced expression of the L-type Ca²⁺ channels, as observed at the mRNA level. Thus, the importance of TMEM16A for contraction is, at least in part, independent from membrane potential. This study demonstrates the significance of TMEM16A for two SMCs I_Cl(Ca) and vascular function and suggests an interaction between TMEM16A and L-type Ca²⁺ channels.

Potent vasorelaxant activity of the TMEM16A inhibitor T16A(inh) -A01

BACKGROUND AND PURPOSE

T16A(inh) -A01 is a recently identified inhibitor of the calcium-activated chloride channel TMEM16A. The aim of this study was to test the efficacy of T16A(inh) -A01 for inhibition of calcium-activated chloride channels in vascular smooth muscle and consequent effects on vascular tone.

EXPERIMENTAL APPROACH

Single channel and whole cell patch clamp was performed on single smooth muscle cells from rabbit pulmonary artery and mouse thoracic aorta. Isometric tension studies were performed on mouse thoracic aorta and mesenteric artery as well as human abdominal visceral adipose artery.

KEY RESULTS

In rabbit pulmonary artery myocytes T16A(inh) -A01 (1-30 μM) inhibited single calcium (Ca(2+) )-activated chloride (Cl(-) ) channels and whole cell currents activated by 500 nM free Ca(2+) . Similar effects were observed for single Ca(2+) -activated Cl(-) channels in mouse thoracic aorta, and in both cell types, channel activity was abolished by two antisera raised against TMEM16A but not by a bestrophin antibody. The TMEM16A potentiator, F(act) (10 μM), increased single channel and whole cell Ca(2+) -activated Cl(-) currents in rabbit pulmonary arteries. In isometric tension studies, T16A(inh) -A01 relaxed mouse thoracic aorta pre-contracted with methoxamine with an IC(50) of 1.6 μM and suppressed the methoxamine concentration-effect curve. T16A(inh) -A01 did not affect the maximal contraction produced by 60 mM KCl and the relaxant effect of 10 μM T16A(inh) -A01 was not altered by incubation of mouse thoracic aorta in a cocktail of potassium (K(+) ) channel blockers. T16A(inh) -A01 (10 μM) also relaxed human visceral adipose arteries by 88 ± 3%.

CONCLUSIONS AND IMPLICATIONS

T16A(inh) -A01 blocks calcium-activated chloride channels in vascular smooth muscle cells and relaxes murine and human blood vessels.

A Ca²⁺-dependent chloride current and Ca²⁺ influx via Ca(v)1.2 ion channels play major roles in P2Y receptor-mediated pulmonary vasoconstriction

BACKGROUND AND PURPOSE

ATP, UTP and UDP act at smooth muscle P2X and P2Y receptors to constrict rat intrapulmonary arteries, but the underlying signalling pathways are poorly understood. Here, we determined the roles of the Ca²⁺ -dependent chloride ion current (I(Cl,Ca)), Ca(v)1.2 ion channels and Ca²⁺ influx.

EXPERIMENTAL APPROACH

Isometric tension was recorded from endothelium-denuded rat intrapulmonary artery rings (i.d. 200-500 µm) mounted on a wire myograph.

KEY RESULTS

The I(Cl,Ca) blockers, niflumic acid and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid and the Ca(v)1.2 channel blocker, nifedipine, reduced peak amplitude of contractions evoked by UTP and UDP by ∼45-50% and in a non-additive manner. Ca²⁺-free buffer inhibited responses by ∼70%. Niflumic acid and nifedipine similarly depressed contractions to ATP, but Ca²⁺-free buffer almost abolished the response. After peaking, contractions to UTP and UDP decayed slowly by 50-70% to a sustained plateau, which was rapidly inhibited by niflumic acid and nifedipine. Contractions to ATP, however, reversed rapidly and fully. Tannic acid contracted tissues per se and potentiated nucleotide-evoked contractions.

CONCLUSIONS AND IMPLICATIONS

I (Cl,Ca) and Ca²⁺ influx via Ca(v)1.2 ion channels contribute substantially and equally to contractions of rat intrapulmonary arteries evoked by UTP and UDP, via P2Y receptors. ATP also activates these mechanisms via P2Y receptors, but the greater dependence on extracellular Ca²⁺ most likely reflects additional influx through the P2X1 receptor pore. The lack of a sustained response to ATP is probably due to it acting at P2 receptor subtypes that desensitize rapidly. Thus multiple signalling mechanisms contribute to pulmonary artery vasoconstriction mediated by P2 receptors.

Inhibition of Ca2+-activated Cl- channels by gallotannins as a possible molecular basis for health benefits of red wine and green tea

TMEM16A was found recently to be a calcium-activated Cl(-) channel (CaCC). CaCCs perform important functions in cell physiology, including regulation of epithelial secretion, cardiac and neuronal excitability, and smooth muscle contraction. CaCC modulators are of potential utility for treatment of hypertension, diarrhea, and cystic fibrosis. Screening of drug and natural product collections identified tannic acid as an inhibitor of TMEM16A, with IC(50) ∼ 6 μM and ∼100% inhibition at higher concentrations. Tannic acid inhibited CaCCs in multiple cell types but did not affect CFTR Cl(-) channels. Structure-activity analysis indicated the requirement of gallic or digallic acid substituents on a macromolecular scaffold (gallotannins), as are present in green tea and red wine. Other polyphenolic components of teas and wines, including epicatechin, catechin, and malvidin-3-glucoside, poorly inhibited CaCCs. Remarkably, a 1000-fold dilution of red wine and 100-fold dilution of green tea inhibited CaCCs by >50%. Tannic acid, red wine, and green tea inhibited arterial smooth muscle contraction and intestinal Cl(-) secretion. Gallotannins are thus potent CaCC inhibitors whose biological activity provides a potential molecular basis for the cardioprotective and antisecretory benefits of red wine and green tea.

Phosphorylation alters the pharmacology of Ca(2+)-activated Cl channels in rabbit pulmonary arterial smooth muscle cells

BACKGROUND AND PURPOSE

Ca(2+)-activated Cl(-) currents (I(Cl(Ca))) in arterial smooth muscle cells are inhibited by phosphorylation. The Ca(2+)-activated Cl(-) channel (Cl(Ca)) blocker niflumic acid (NFA) produces a paradoxical dual effect on I(Cl(Ca)), causing stimulation or inhibition at potentials below or above 0 mV respectively. We tested whether the effects of NFA on I(Cl(Ca)) were modulated by phosphorylation.

EXPERIMENTAL APPROACH

I(Cl(Ca)) was elicited with 500 nM free internal Ca(2+) in rabbit pulmonary artery myocytes. The state of global phosphorylation was altered by cell dialysis with either 5 mM ATP or 0 mM ATP with or without an inhibitor of calmodulin-dependent protein kinase type II, KN-93 (10 microM).

KEY RESULTS

Dephosphorylation enhanced the ability of 100 microM NFA to inhibit I(Cl(Ca)). This effect was attributed to a large negative shift in the voltage-dependence of block, which was converted to stimulation at potentials <-50 mV, approximately 70 mV more negative than cells dialysed with 5 mM ATP. NFA dose-dependently blocked I(Cl(Ca)) in the range of 0.1-250 microM in cells dialysed with 0 mM ATP and KN-93, which contrasted with the stimulation induced by 0.1 microM, which converted to block at concentrations >1 microM when cells were dialysed with 5 mM ATP.

CONCLUSIONS AND IMPLICATIONS

Our data indicate that the presumed state of phosphorylation of the pore-forming or regulatory subunit of Cl(Ca) channels influenced the interaction of NFA in a manner that obstructs interaction of the drug with an inhibitory binding site.

Mechanisms of U46619-induced contraction of rat pulmonary arteries in the presence and absence of the endothelium

BACKGROUND AND PURPOSE

Thromboxane A(2) and endothelial dysfunction are implicated in the development of pulmonary hypertension. The receptor-transduction pathway for U46619 (9,11-dideoxy-9 alpha, 11 alpha-methanoepoxy prostaglandin F(2 alpha))-induced contraction was examined in endothelium-intact (E+) and denuded (E-) rat pulmonary artery rings.

EXPERIMENTAL APPROACH

Artery rings were mounted on a wire myograph under a tension of 7-7.5 mN at 37 degrees C and gassed with 95% O(2)/5% CO(2). Isometric recording was made by using Powerlab data collection and Chart 5 software.

KEY RESULTS

Both E+ and E- contractile responses were sensitive to Rho-kinase inhibition and the chloride channel blocker NPPB [5-nitro-2-(3-phenylpropylamino)benzoic acid]. The E+ response was sensitive to the store-operated calcium channel blockers SKF-96365 {1-[B-[3-(4-methoxyphenyl)propoxy]-4-methoxy-phenethyl]-1H-imidazole hydrochloride} and 2-APB (2-amino ethoxy diphenylborate) (75-100 micromol x L(-1)). The E- response was sensitive to 2-APB (10-30 micromol x L(-1)), a putative IP(3) receptor antagonist, and the calcium and chloride channel blockers nifedipine, DIDS (4,4'-diisothiocyanostilbene-2,2'-disulphonic acid) and niflumic acid but was insensitive to SKF-96365. Inhibiting K(V) with 4-AP in E+ rings exposed a contraction sensitive to nifedipine, DIDS and niflumic acid, whereas inhibiting BK(Ca) exposed a contraction sensitive to mibefradil, DIDS and niflumic acid. This indicates that removal of the endothelium allows the TP receptor to inhibit K(V), which may involve coupling to phospholipase C, because inhibition of phospholipase C with U73122 (1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-y]amino]hexyl]- 1H-pyrrole-2,5-dione) switched the E- pathway to the E+ pathway.

CONCLUSIONS AND IMPLICATIONS

The results from this study indicate that distinct transduction pathways can be employed by the TP receptor to produce contraction and that the endothelium is able to influence the coupling of the TP receptor.

Ca2+-activated Cl- current in retinal arteriolar smooth muscle

PURPOSE

To characterize the biophysical, pharmacologic, and functional properties of the Ca(2+)-activated Cl(-) current in retinal arteriolar myocytes.

METHODS

Whole-cell perforated patch-clamp recordings were made from myocytes within intact isolated arteriolar segments. Arteriolar tone was assessed using pressure myography.

RESULTS

Depolarizing of voltage steps to -40 mV and greater activated an L-type Ca(2+) current (I(Ca(L))) that was followed by a sustained current. Large tail currents (I(tail)) were observed on stepping back to -80 mV. The sustained current and I(tail) reversed close to 0 mV in symmetrical Cl(-) concentrations. The ion selectivity sequence for I(tail) was I(-)> Cl(-)> glucuronate. Outward I(tail) was sensitive to the Cl(-) channel blockers 9-anthracene-carboxylic acid (9-AC; 1 mM), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS; 1 mM), and disodium 4,4'-diisothiocyanatostilbene-2,2'-disulfonate (DIDS; 1 mM), but only DIDS produced a substantial (78%) block of inward tail currents at -100 mV. I(tail) was decreased in magnitude when the normal bathing medium was substituted with Ca(2+)-free solution or if I(Ca(L)) was inhibited by 1 microM nimodipine. Caffeine (10 mM) produced large transient currents that reversed close to the Cl(-) equilibrium potential and were blocked by 1 mM DIDS or 100 microM tetracaine. DIDS had no effect on basal vascular tone in pressurized arterioles but dramatically reduced the level of vasoconstriction observed in the presence of 10 nM endothelin-1.

CONCLUSIONS

Retinal arteriolar myocytes have I(Cl(Ca)), which may be activated by Ca(2+) entry through L-type Ca(2+) channels or Ca(2+) release from intracellular stores. This current appears to contribute to agonist-induced retinal vasoconstriction.

Calcium signalling in smooth muscle

Calcium signalling in smooth muscles is complex, but our understanding of it has increased markedly in recent years. Thus, progress has been made in relating global Ca2+ signals to changes in force in smooth muscles and understanding the biochemical and molecular mechanisms involved in Ca2+ sensitization, i.e. altering the relation between Ca2+ and force. Attention is now focussed more on the role of the internal Ca2+ store, the sarcoplasmic reticulum (SR), global Ca2+ signals and control of excitability. Modern imaging techniques have shown the elaborate SR network in smooth muscles, along with the expression of IP3 and ryanodine receptors. The role and cross-talk between these two Ca(2+) release mechanisms, as well as possible compartmentalization of the SR Ca2+ store are discussed. The close proximity between SR and surface membrane has long been known but the details of this special region to Ca2+ signalling and the role of local sub-membrane Ca2+ concentrations and membrane microdomains are only now emerging. The activation of K+ and Cl- channels by local Ca2+ signals, can have profound effects on excitability and hence contraction. We examine the evidence for both Ca2+ sparks and puffs in controlling ion channel activity, as well as a fundamental role for Ca2+ sparks in governing the period of inexcitability in smooth muscle, i.e. the refractory period. Finally, the relation between different Ca2+ signals, e.g. sparks, waves and transients, to smooth muscle activity in health and disease is becoming clearer and will be discussed.

Effects of calcium channel blockers on antidepressant action of Alprazolam and Imipramine

Alprazolam is effective as an anxiolytic and in the adjunct treatment of depression. In this study, the effects of calcium channel antagonists on the antidepressant action of alprazolam and imipramine were investigated. A forced swimming maze was used to study behavioral despair in albino mice. Mice were divided into nine groups (n = 7 per group). One group received a single dose of 1% Tween 80; two groups each received a single dose of the antidepressant alone (alprazolam or imipramine); two groups each received a single dose of the calcium channel blocker (nifedipine or verapamil); four groups each received a single dose of the calcium channel blocker followed by a single dose of the antidepressant (with same doses used for either in the previous four groups). Drug administration was performed concurrently on the nine groups. Our data confirmed the antidepressant action of alprazolam and imipramine. Both nifedipine and verapamil produced a significant antidepressant effect (delay the onset of immobility) when administered separately. Verapamil augmented the antidepressant effects of alprazolam and imipramine (additive antidepressant effect). This may be due to the possibility that verapamil might have antidepressant-like effect through different mechanism. Nifedipine and imipramine combined led to a delay in the onset of immobility greater than their single use but less than the sum of their independent administration. This may be due to the fact that nifedipine on its own might act as an antidepressant but blocks one imipramine mechanism that depends on L-type calcium channel activation. Combining nifedipine with alprazolam produced additional antidepressant effects, which indicates that they exert antidepressant effects through different mechanisms.

Effects of anethole and structural analogues on the contractility of rat isolated aorta: Involvement of voltage-dependent Ca2+-channels

Anethole is a naturally occurring aromatic oxidant, present in a variety of medicinal plant extracts, which is commonly used by the food and beverage industry. Despite its widespread occurrence and commercial use, there is currently little information regarding effects of this compound on the vasculature. Therefore the actions of anethole on the contractility of rat isolated aorta were compared with those of eugenol, and their respective isomeric forms, estragole and isoeugenol. In aortic rings precontracted with phenylephrine (PE; 1 microM), anethole (10(-6) M-10(-4) M) induced contraction in preparations possessing an intact endothelium, but not in endothelium-denuded tissues. At higher concentrations (10(-3) M-10(-2) M), anethole-induced concentration-dependent and complete relaxation of all precontracted preparations, irrespective of whether the endothelium was intact or not, an action shared by eugenol, estragole and isoeugenol. The contractile and relaxant effects of anethole in PE-precontracted preparations were not altered by L-NAME (10 microM) or indomethacin (10 microM), indicating that neither nitric oxide nor prostaglandins were involved in these actions. The mixed profile of effects was not confined to PE-mediated contraction, since similar responses were obtained to anethole when tissues were precontracted with 25 mM KCl. Anethole and estragole (10(-6)-10(-4) M), but not eugenol or isoeugenol, increased the basal tonus of endothelium-denuded aortic rings, an action that was abolished by VDCC blockers nifedipine (1 microM) and diltiazem (1 microM), or by withdrawal of extracellular Ca(2+). Our data suggest complex effects of anethole on isolated blood vessels, inducing contraction at lower doses, mediated via opening of voltage-dependent Ca(2+)-channels, and relaxant effects at higher concentrations that are shared by structural analogues.

Mechanisms of U46619- and 5-HT-induced contraction of bovine pulmonary arteries: role of chloride ions

BACKGROUND AND PURPOSE

Thromboxane A(2) and 5-hydroxytryptamine (5-HT) are implicated in pulmonary hypertension. The involvement of chloride, voltage-operated calcium channels (VOCCs), store-operated calcium channels (SOCCs) and the Rho kinase in the contractile response of bovine pulmonary arteries (BPA) to the thromboxane A(2) mimetic U46619 and 5-HT was investigated.

EXPERIMENTAL APPROACH

Endothelium-intact ring segments of BPA were mounted in Krebs/Henseleit buffer (37 degrees C) under a tension of 2g and gassed with 95%O(2)/5%CO(2).

KEY RESULTS

Depletion or removal of extracellular chloride, inhibition of chloride and SOCC, Na:K:2Cl, Cl/HCO(3), Rho kinase inhibited contractions to U46619. Combining Rho kinase inhibition and chloride channel blockade (with NPPB) almost abolished the contractions to U46619. In contrast 5-HT-induced contraction was inhibited by verapamil and mibefradil. Depletion of stored calcium with caffeine almost abolished the response to U46619 but not 5-HT. The contraction by the sarco(endo)plasmic reticulum Ca(2+)-ATPase inhibitor CPA was abolished by SOCC and chloride channel blockade (with NPPB) and by chloride depletion.

CONCLUSIONS AND IMPLICATIONS

This study suggests that the contractile response of BPA to U46619 involves Rho kinase together with a chloride-sensitive mechanism, which does not involve VOCC but may have a role in calcium release and calcium entry via SOCC. In contrast contraction of the BPA by 5-HT appears to involve verapamil- and mibefradil-sensitive VOCC. This study may indicate that the use of calcium channel blockers in the management of pulmonary hypertension may not always be effective and that Rho kinase and chloride channels may be targets for the development of new therapies.

Na+ -K+ -2Cl- cotransporter is implicated in gender differences in the response of the rat aorta to phenylephrine

Inhibition of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) with bumetanide reduced contractile responses to phenylephrine (PE) in male rat aortas (129+/-4% of 60 mM KCl-induced contraction control vs 108+/-7% bumetanide; PE 10(-5) M; P<0.01) but did not change equivalent responses in female rat aortas. Removal of the endothelium blunted the effect of NKCC1 inhibition on the response to PE (10(-5) M) in males, whereas in denuded aorta from female rats, bumetanide reduced this response (162+/-5% control vs 146+/-3% bumetanide; P<0.05). NKCC1 basal activity did not show gender differences in intact aortic rings, but in the presence of PE, bumetanide-sensitive (86)Rb(+)/K(+) uptake increased more in male than female aortas (179+/-8 in males vs 158+/-5 nmol (86)Rb(+)/K(+) min(-1) (g aorta)(-1) in females; P<0.05). PE did not stimulate NKCC1 activity in denuded aorta from male rats. However, in female rats, PE increased NKCC1 activity similarly in both denuded (169+/-11 nmol (86)Rb(+)/K(+) min(-1) (g aorta)(-1)) and intact aortas. Ovariectomy increased the bumetanide-sensitive (86)Rb(+)/K(+) uptake increase elicited by PE (223+/-17 nmol (86)Rb(+)/K(+) min(-1) (g aorta)(-1)) and hormone replacement with 17beta-estradiol prevented this effect (159+/-29 nmol (86)Rb(+)/K(+) min(-1) (g aorta)(-1)). Na(+),K(+)-ATPase basal activity, measured as ouabain-sensitive (86)Rb(+)/K(+) uptake, was similar in male and female rats, but the effect of PE was significantly less in intact male aortas (232+/-16 in males vs 296+/-25 nmol (86)Rb(+)/K(+) min(-1) (g aorta)(-1) in females; P<0.05). Our results suggest that PE induced activation of NKCC1 and Na(+),K(+)-ATPase in the rat aorta in a gender-dependent way.

Effects of chloride substitution in isolated mesenteric blood vessels from Dahl normotensive and hypertensive rats

The purpose of this investigation was to examine the effect of Cl-free medium, nitric oxide synthase inhibitor (N nitro-L-arginine methyl ester; L-NAME), and Cl channel antagonist (niflumic acid), on alpha1-adrenoceptor (cirazoline) mediated responses in the isolated mesenteric blood vessels from Dahl salt-resistant normotensive (SRN) and salt-sensitive hypertensive (SSH) rats on a 4% salt diet for 7 weeks. Cirazoline produced dose-dependent vasoconstriction in blood vessels of SRN and SSH rats. Replacement of extracellular Cl with propionate ions significantly inhibited (P < 0.05) cirazoline-mediated vasoconstriction in SRN but not in SSH rats. Perfusion with L-NAME (10 microM) augmented responses to cirazoline in SRN but not in SSH rats. In Cl-free medium, addition of L-NAME had a biphasic effect on cirazoline responses; potentiation of responses at the lower doses and attenuation at the highest dose. Niflumic acid (10 microM) significantly inhibited cirazoline responses with the inhibition being more pronounced in SRN than SSH rats. The resting Em of smooth muscle cells was -68.0 +/- 4.2 mV (mean +/- SD; n = 87) and -67.2 +/- 4.8 mV (n = 88), in SRN and SSH rats, respectively. Perfusion with Cl-free medium produced a significant depolarization that was larger in smooth muscle cells of SSH (-57.4 +/- 4.8 mV, n = 38) than SRN (-61.3 +/- 5.4 mV, n = 35) rats, while L-NAME depolarized the smooth muscle cells of SRN (-62.1 +/- 6.5 mV, n = 36) but not SSH (-67.5 +/- 4.2 mV, n = 34) rats. The data supports the view that Cl handling and Ca-dependent Cl channels seem to undergo modification as a consequence of salt-induced hypertension. It is also possible that the modified role of nitric oxide on membrane potential may have a direct bearing on the changes observed in Cl handling in blood vessels of SRN versus SSH rats.

Airway smooth muscle excitation-contraction coupling and airway hyperresponsiveness

The primary complaints from patients with asthma pertain to function of airway smooth muscle (ASM) function including shortness of breath, wheezing, and coughing. Thus, it is imperative to better understand the mechanisms underlying excitation-contraction coupling in ASM. Here, we review the various signaling pathways underlying contraction in ASM, and then examine how these are altered in asthma and airway hyperresponsiveness (a hallmark feature of asthma). Throughout, we highlight how studies of vascular smooth muscle have helped or hindered progress in understanding ASM physiology and pathophysiology.

Regulation of calcium-activated chloride channels in smooth muscle cells: a complex picture is emerging

Calcium-activated chloride channels (ClCa) are ligand-gated anion channels as they have been shown to be activated by a rise in intracellular Ca2+ concentration in various cell types including cardiac, skeletal and vascular smooth muscle cells, endothelial and epithelial cells, as well as neurons. Because ClCa channels are normally closed at resting, free intracellular Ca2+ concentration (approximately 100 nmol/L) in most cell types, they have generally been considered excitatory in nature, providing a triggering mechanism during signal transduction for membrane excitability, osmotic balance, transepithelial chloride movements, or fluid secretion. Unfortunately, the genes responsible for encoding this class of ion channels is still unknown. This review centers primarily on recent findings on the properties of these channels in smooth muscle cells. The first section discusses the functional significance and biophysical and pharmacological properties of ClCa channels in smooth muscle cells, and ends with a description of 2 candidate gene families (i.e., CLCA and Bestrophin) that are postulated to encode for these channels in various cell types. The second section provides a summary of recent findings demonstrating the regulation of native ClCa channels in vascular smooth muscle cells by calmodulin-dependent protein kinase II and calcineurin and how their fine tuning by these enzymes may influence vascular tone.

Impact of Nitric Oxide Synthase Inhibitor and Chloride Channel Antagonist on Mesenteric Vascular Conductance in Anesthetized Dahl Normotensive and Hypertensive Rats

The effects of nitric oxide synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME) and chloride channel antagonist niflumic acid on vascular responsiveness to the effect of alpha1-adrenoceptor stimulation in the mesenteric bed of Dahl salt-resistant normotensive (SRN) and salt-sensitive hypertensive (SSH) rats were examined. Dahl salt-resistant and salt-sensitive rats were fed a high-salt diet (4% NaCl) for 7 weeks, and blood pressure, heart rate, and mesenteric blood flow were measured before and after treatment with L-NAME (0.3 mg/kg, IV) and/or niflumic acid (10 mg/kg, IV). Morphometry of the primary mesenteric blood vessel was also assessed. Administration of alpha1-adrenoceptor agonist cirazoline produced a dose-dependent increase in blood pressure, decrease in heart rate, mesenteric blood flow, and mesenteric vascular conductance in SRN and SSH rats. L-NAME significantly increased basal blood pressure and decreased basal mesenteric blood flow and vascular conductance in SRN but not in SSH rats. Niflumic acid attenuation of cirazoline-mediated decreases in mesenteric blood flow and vascular conductance was more pronounced in the SRN than SSH rats. This difference in the inhibitory actions of niflumic acid was absent following its concomitant administration with L-NAME. It seems that tonic release of nitric oxide modulates niflumic acid-sensitive chloride channels in vascular muscle. Blood vessels from SSH rats had significantly larger smooth muscle thickness and lumen diameter, but the ratio of the 2 were not different between the SRN and SSH. Our findings support the view that alterations in receptor-mediated signal transduction, rather than just changes in blood vessel architecture, are responsible for differences in behavior of blood vessels in salt-induced hypertensive rats.

Calcium-activated chloride channels

Calcium-activated chloride channels (CaCCs) play important roles in cellular physiology, including epithelial secretion of electrolytes and water, sensory transduction, regulation of neuronal and cardiac excitability, and regulation of vascular tone. This review discusses the physiological roles of these channels, their mechanisms of regulation and activation, and the mechanisms of anion selectivity and conduction. Despite the fact that CaCCs are so broadly expressed in cells and play such important functions, understanding these channels has been limited by the absence of specific blockers and the fact that the molecular identities of CaCCs remains in question. Recent status of the pharmacology and molecular identification of CaCCs is evaluated.

Activation of chloride currents in murine portal vein smooth muscle cells by membrane depolarization involves intracellular calcium release

The present study describes the first characterization of Ca2+-activated Cl− currents ( IClCa) in single smooth muscle cells from a murine vascular preparation (portal veins). IClCa was recorded using the perforated patch version of the whole cell voltage-clamp technique and was evoked using membrane depolarization. Generation of IClCa relied on Ca2+ entry through dihydropyridine-sensitive Ca2+ channels because IClCa was abolished by 1 μM nicardipine and enhanced by raising external Ca2+ concentration or by application of BAY K 8644. IClCa was characterized by the sensitivity to Cl− channel blockers and the effect of altering the external anion on reversal potential. Activation of IClCa after membrane depolarization was dependent on Ca2+ release from intracellular stores. Thus the amplitude of IClCa was diminished by the SR-ATPase inhibitor cyclopiazonic acid, the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenyl borate (2-APB), and the ryanodine receptor blocker tetracaine. The degree of inhibition produced by the application of 2-APB and tetracaine together was significantly greater than the effect of each agent applied alone. In current-clamp mode, injection of depolarizing current elicited a biphasic action potential, with the later depolarization being sensitive to niflumic acid (NFA; 10 μM). In isometric tension recordings, NFA inhibited spontaneous contractions. These data support a role for this conductance in portal vein excitability.

Dynamics of Ca2+-dependent Cl- channel modulation by niflumic acid in rabbit coronary arterial myocytes

Calcium-activated chloride channels (Cl(Ca)) are crucial regulators of vascular tone by promoting a depolarizing influence on the resting membrane potential of vascular smooth muscle cells. Niflumic acid (NFA), a potent blocker of Cl(Ca) in vascular myocytes, was shown recently to cause inhibition and paradoxical stimulation of sustained calcium-activated chloride currents [I(Cl(Ca))] in rabbit pulmonary artery myocytes. The aims of the present study were to investigate whether NFA produced a similar dual effect in coronary artery smooth muscle cells and to determine the concentration-dependence and dynamics of such a phenomenon. Sustained I(Cl(Ca)) evoked by intracellular Ca(2+) clamped at 500 nM were dose-dependently inhibited by NFA (IC(50) = 159 microM) and transiently augmented in a concentration-independent manner (10 microM to 1 mM) approximately 2-fold after NFA removal. However, the time to peak and duration of NFA-enhanced I(Cl(Ca)) increased in a concentration-dependent fashion. Moreover, the rate of recovery was reduced by membrane depolarization, suggesting the involvement of a voltage-dependent step in the interaction of NFA, leading to stimulation of I(Cl(Ca)). Computer simulations derived from a kinetic model involving low (K(i) = 1.25 mM) and high (K(i) < 30 microM) affinity sites could reproduce the properties of the NFA-modulated I(Cl(Ca)) fairly well.

Chloride channels and α1-adrenoceptor-mediated pulmonary artery smooth muscle contraction: effect of pulmonary hypertension

Noradrenaline induced concentration-dependent contractions of pulmonary artery segments from control and monocrotaline-treated rats. There was a significant decrease in the maximum response but not sensitivity in artery segments from monocrotaline-treated rats. At a concentration (10(-6) M) that abolished KCl-induced contraction, nifedipine attenuated but did not abolish, noradrenaline-induced contraction in both groups. However, noradrenaline-induced contraction in artery segments from pulmonary hypertensive rats was more susceptible to inhibition by nifedipine. Bumetanide (10(-4) M), a chloride transport inhibitor and niflumic acid, a chloride channel inhibitor, reduced noradrenaline-induced contraction of the pulmonary artery in control and pulmonary hypertensive groups. These compounds were more effective in ring segments from pulmonary hypertensive rats. It was concluded that activation of chloride channels was involved in noradrenaline-induced contraction and that the contribution of chloride channels was enhanced in pulmonary hypertensive rats.

Effects of chloride substitution on electromechanical responses in the pulmonary artery of Dahl normotensive and hypertensive rats

1. We have investigated the in vitro interaction between chloride ions and endothelium as revealed by alterations in vascular contractility and smooth muscle cell membrane potential in isolated pulmonary arteries from Dahl salt-resistant normotensive and salt-sensitive hypertensive rats. 2. Exposure to nitro-l-arginine methyl ester (l-NAME) of tissues from normotensive but not hypertensive rats augmented contractions to cirazoline. While chloride removal did not alter cirazoline-induced contractions, it completely abolished the augmentation by l-NAME in normotensive rats. However, in hypertensive rats, removal of chloride ions significantly attenuated contractions elicited by cirazoline, and l-NAME effectively reversed this inhibition. 3. Methacholine-induced endothelium-dependent relaxations of the same magnitude were evident in both normotensive and hypertensive rats. However, basal cyclic GMP levels were found to be significantly higher (7.8-fold) in blood vessels of normotensive rats compared to hypertensive rats. 4. The resting membrane potential in pulmonary arteries of hypertensive rats (-52.1+/-1.04 mV) revealed a significant hyperpolarisation when compared with that of normotensive rats (-46.4+/-1.58 mV). Cirazoline did not produce a significant depolarisation in blood vessels of either normotensive or hypertensive rats. Perfusion with chloride-free solution resulted in a modest but significant hyperpolarisation (-8.0 mV) in the blood vessels of hypertensive but not in normotensive rats. 5. We conclude that salt-dependent hypertension in Dahl rats is accompanied by functional and biochemical changes in low-pressure blood vessels. These changes can, in part, be attributed to impairment in the basal, but not methacholine-stimulated, release of nitric oxide, and to altered chloride ion handling.

Identification of an N-terminal amino acid of the CLC-3 chloride channel critical in phosphorylation-dependent activation of a CaMKII-activated chloride current

CLC-3, a member of the CLC family of chloride channels, mediates function in many cell types in the body. The multifunctional calcium-calmodulin-dependent protein kinase II (CaMKII) has been shown to activate recombinant CLC-3 stably expressed in tsA cells, a human embryonic kidney cell line derivative, and natively expressed channel protein in a human colonic tumour cell line T84. We examined the CaMKII-dependent regulation of CLC-3 in a smooth muscle cell model as well as in the human colonic tumour cell line, HT29, using whole-cell voltage clamp. In CLC-3-expressing cells, we observed the activation of a Cl(-) conductance following intracellular introduction of the isolated autonomous CaMKII into the voltage-clamped cell via the patch pipette. The CaMKII-dependent Cl(-) conductance was not observed following exposure of the cells to 1 microm autocamtide inhibitory peptide (AIP), a selective inhibitor of CaMKII. Arterial smooth muscle cells express a robust CaMKII-activated Cl(-) conductance; however, CLC-3(-/-) cells did not. The N-terminus of CLC-3, which contains a CaMKII consensus sequence, was phosphorylated by CaMKII in vitro, and mutation of the serine at position 109 (S109A) abolished the CaMKII-dependent Cl(-) conductance, indicating that this residue is important in the gating of CLC-3 at the plasma membrane.

Effects of chloride channel blockers on hypotonicity-induced contractions of the rat trachea

1. We have investigated the inhibitory effects of blockers of volume-activated (Cl(vol)) and calcium-activated (Cl(Ca)) chloride channels on hypotonic solution (HS)-induced contractions of rat trachea, comparing their effects with those of the voltage-dependent calcium channel (VDCC) blocker nifedpine. 2. HS elicited large, stable contractions that were partially dependent on the cellular chloride gradient; a reduction to 41.45+/-7.71% of the control response was obtained when extracellular chloride was removed. In addition, HS-induced responses were reduced to 26.8+/-5.6% of the control by 1 microm nifedipine, and abolished under calcium-free conditions, indicating a substantial requirement for extracellular calcium entry, principally via VDCCs. 3. The established Cl(vol) blockers tamoxifen (</=10 microm) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (1-100 microm), at concentrations previously reported to inhibit Cl(vol) in smooth muscle, did not significantly inhibit HS-induced contractions. 4. In contrast, the recognized Cl(Ca) blocker niflumic acid (NFA; 1-100 microm) produced a reversible, concentration-dependent inhibition of HS responses, with a reduction to 36.6+/-6.4% of control contractions at the highest concentration. The mixed Cl(vol) and Cl(Ca) blocker, 5-nitro 2-(3-phenylpropylamine) benzoic acid (NPPB; 10-100 microm) also elicited concentration-related inhibition of HS-induced contractions, producing a decrease to 35.9+/-11.3% of the control at 100 microm. 5. Our results show that HS induces reversible, chloride-dependent contractions of rat isolated trachea that were inhibited by NFA and NPPB, while exhibiting little sensitivity to recognized blockers of Cl(vol). The data support the possibility that opening of calcium-activated chloride channels under hypotonic conditions in respiratory smooth muscle may ultimately lead to VDCC-mediated calcium entry and contraction.

The Cl(-) channel blocker niflumic acid releases Ca(2+) from an intracellular store in rat pulmonary artery smooth muscle cells

The effect of the Cl- channel blockers niflumic acid (NFA), 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB), 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), and anthracene-9-carboxylic acid (A-9-C), on Ca2+ signalling in rat pulmonary artery smooth muscle cells was examined. Intracellular Ca2+ concentration ([Ca2+]i) was monitored with either fura-2 or fluo-4, and caffeine was used to activate the ryanodine receptor, thereby releasing Ca2+ from the sarcoplasmic reticulum (SR). NFA and NPPB significantly increased basal [Ca2+]i and attenuated the caffeine-induced increase in [Ca2+]i. These Cl- channel blockers also increased the half-time (t1/2) to peak for the caffeine-induced [Ca2+]i transient, and slowed the removal of Ca2+ from the cytosol following application of caffeine. Since DIDS and A-9-C were found to adversely affect fura-2 fluorescence, fluo-4 was used to monitor intracellular Ca2+ in studies involving these Cl- channel blockers. Both DIDS and A-9-C increased basal fluo-4 fluorescence, indicating an increase in intracellular Ca2+, and while DIDS had no significant effect on the t1/2 to peak for the caffeine-induced Ca2+ transient, it was significantly increased by A-9-C. In the absence of extracellular Ca2+, NFA significantly increased basal [Ca2+]i, suggesting that the release of Ca2+ from an intracellular store was responsible for the observed effect. Depleting the SR with the combination of caffeine and cyclopiazonic acid prevented the increase in basal [Ca2+]i induced by NFA. Additionally, incubating the cells with ryanodine also prevented the increase in basal [Ca2+]i induced by NFA. These data show that Cl- channel blockers have marked effects on Ca2+ signalling in pulmonary artery smooth muscle cells. Furthermore, examination of the NFA-induced increase in [Ca2+]i indicates that it is likely due to Ca2+ release from an intracellular store, most probably the SR.

Endothelium-independent relaxation to cannabinoids in rat-isolated mesenteric artery and role of Ca2+ influx

(1) Three cannabinoid receptor agonists, anandamide (CB(1) receptor-selective) and the aminoalkyl-indoles, JWH 015(2-methyl-1-propyl-1H-indol-3-yl)-1-napthalenylmethanone; (CB(2) receptor-selective), R-(+)-WIN 55,212-2 (R-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolol[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone; slightly CB(2) receptor-selective), as well as the enantiomer S-(-)-WIN 55,212-3(S-(-)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolol[1,2,3-de]-1,4-benzoxazin-6-yl]-1-napthalenylmethanone; inactive at cannabinoid receptors), induced endothelium-independent relaxation of methoxamine-precontracted isolated small mesenteric artery of rat. KCL (60 mM) precontraction did not affect relaxation to the aminoalkylindoles, but reduced that to anandamide. (2) SR14176A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; 3 micro M; CB(1) receptor antagonist) inhibited relaxation only to JWH 015 and anandamide. Neither AM 251 (N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide; CB(1) antagonist) nor SR 144528 (N-[(1S)-endo-1,3,3-trimethyl bicyclo[2.2.1] heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methylbenzyl)-pyrazole-3-carboxamide; CB(2) antagonist; both at 3 micro M) affected any of the relaxations. (3) Vanilloid receptor desensitisation with capsaicin reduced anandamide relaxation; addition of SR 141716A (3 micro M) then caused further inhibition. SR 141716A did not affect capsaicin-induced relaxation. (4) The aminoalkylindoles inhibited CaCl(2)-induced contractions in methoxamine-stimulated vessels previously depleted of intracellular Ca(2+). These inhibitory effects were greatly reduced or abolished in ionomycin-(a calcium ionophore) contracted vessels. Anandamide also caused vanilloid receptor-independent, SR 141716A- (3 micro M) insensitive, inhibition of CaCl(2) contractions. (5) In conclusion, the aminoalkylindoles JWH 015, R-(+)-WIN 55,212-2 and S-(-)-WIN 55,212-3 relax rat small mesenteric artery mainly by inhibiting Ca(2+) influx into vascular smooth muscle. Anandamide causes vasorelaxation by activating vanilloid receptors, but may also inhibit Ca(2+) entry. Relaxation to JWH 015 and anandamide was sensitive to SR 141716A, but there is no other evidence for the involvement of CB(1) or CB(2) receptors in responses to these compounds.

Phasic contractions of the rat portal vein depend on intracellular Ca2+ release stimulated by depolarization

The phasic contraction to phenylephrine of the rat isolated portal vein was investigated using functional studies. Phasic contractions to phenylephrine and caffeine could be produced after several minutes in Ca(2+)-free Krebs solution, which were inhibited by cyclopiazonic acid or ryanodine. The phenylephrine and caffeine contractions were abolished, however, within 10 min in Ca(2+)-free Krebs solution and by nifedipine. This indicated the Ca(2+) stores were depleted in the absence of Ca(2+) influx through voltage-gated channels. The phasic contraction to phenylephrine was also abolished by niflumic acid even in Ca(2+)-free Krebs solution. This showed that the response depended on intracellular Ca(2+) release stimulated directly by depolarization, resulting from opening of Ca(2+)-activated Cl(-) channels, but did not require Ca(2+) influx. In support of this, K(+)-induced phasic contractions were also produced in Ca(2+)-free Krebs solution. The phenylephrine but not K(+)-induced phasic contractions in Ca(2+)-free Krebs solution were inhibited by ryanodine or cyclopiazonic acid. This would be consistent with Ca(2+) release from more superficial intracellular stores (affected most by these agents), probably by inositol 1,4,5-trisphospate, being required to stimulate the phenylephrine depolarization.

Anion channels influence ECC by modulating L-type Ca(2+) channel in ventricular myocytes

Anion channels are extensively expressed in the heart, but their roles in cardiac excitation-contraction coupling (ECC) are poorly understood. We, therefore, investigated the effects of anion channels on cardiac ventricular ECC. Edge detection, fura 2 fluorescence measurements, and whole cell patch-clamp techniques were used to measure cell shortening, the intracellular Ca(2+) transient, and the L-type Ca(2+) current (I(Ca,L)) in single rat ventricular myocytes. The anion channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid reversibly inhibited the Ca(2+) transients and cell shortening in a dose-dependent manner. Comparable results were observed when the majority of the extracellular Cl(-) was replaced with the relatively impermeant anions glutamate (Glt(-)) and aspartate (Asp(-)). NPPB and niflumic acid or the Cl(-) substitutes did not affect the resting intracellular Ca(2+) concentration but significantly inhibited I(Ca,L). In contrast, replacement of extracellular Cl(-) with the permeant anions NO, SCN(-), and Br(-) supported the ECC and I(Ca,L), which were still sensitive to blockade by NPPB. Exposure of cardiac ventricular myocytes to a hypotonic bath solution enhanced the amplitude of cell shortening and supported I(Ca,L), whereas hypertonic stress depressed the contraction and I(Ca,L). Moreover, cardiac contraction was completely abolished by NPPB (50 microM) under hypotonic conditions. It is concluded that a swelling-activated anion channel may be involved in the regulation of cardiac ECC through modulating L-type Ca(2+) channel activity.

Functional role of Cl- channels in acidic pH-induced contraction of the aorta of spontaneously hypertensive and Wistar Kyoto rats

pH regulates various cellular functions. Previously, we have described that acidic pH produces depolarization and contraction in isolated aorta from spontaneously hypertensive (SHR) and Wistar Kyoto (WKY) rats [Br. J. Pharmacol. 118 (1996) 485]. The aim of the present study was to investigate the involvement of Cl- channels in acidic pH-induced contraction. Changing the pH of the bathing solution from 7.4 to 6.5 induced a contraction in both SHR and WKY aorta, which was 127.50+/-13.32% and 79.27+/-0.94% of the 64.8 mM KCl-induced contraction, respectively. The acidic pH-induced contraction was partially inhibited by the voltage-dependent Ca2+ channel (VDCC) blockers, verapamil (1 microM) and nifedipine (0.1 microM). The Cl- channel inhibitors, diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) (0.5 mM), 9-anthracene chloride (0.5 mM), indanyloxyacetic acid (30 microM) and niflumic acid (3 microM) also inhibited the acidic pH-induced contraction and the degree of attenuation was comparable to that of VDCC blockers. DIDS, 9-anthracene chloride and niflumic acid at concentrations used to inhibit the acidic pH-induced contraction also inhibited the 10 microM phenylephrine-induced contraction partially, without affecting the 64.8 mM KCl-induced contraction, whereas both the contractions were inhibited by indanyloxyacetic acid with equal efficacy. Indanyloxyacetic acid but not DIDS, 9-anthracene chloride or niflumic acid inhibited the 24.8 mM KCl-induced contraction. Simultaneous measurement of cytosolic Ca2+ and tension showed that niflumic acid reversed the increase in intracellular Ca2+ level and inhibited the contraction caused by acidic pH. Similarly, acidic pH depolarized the cultured vascular smooth muscle cells from SHR and the depolarization was completely reversible after the administration of niflumic acid. All these results suggest that the activation of Cl- channels is an important mechanism underlying the depolarization and contraction induced by acidic pH in SHR and WKY aortas.

Ca2+ entry channels involved in contractions of rat aorta induced by endothelin-1, noradrenaline, and vasopressin

Endothelin-1 (ET-1) has been shown to activate three types of Ca2+ channel, namely two Ca2+-permeable nonselective cation channels (designated NSCC-1 and NSCC-2) and a store-operated Ca2+ channel (SOCC), and that these channels can be discriminated by Ca2+ channel blockers such as LOE 908 (a blocker of NSCC-1 and NSCC-2) and SK&F 96365 (a blocker of NSCC-2 and SOCC). This study pharmacologically compared Ca2+ entry channels involved in contractions of rat thoracic aorta without endothelium induced by ET-1, noradrenaline (NA), or arginine-vasopressin (AVP). These agonists-induced contractions of aortic rings without endothelium and increases in the intracellular free Ca2+ concentration ([Ca2+]i) of cultured aortic smooth muscle cells were abolished by removal of extracellular Ca2+. A blocker of L-type voltage-operated Ca2+ channel (VOCC), nifedipine had no effect on the responses to ET-1, but it suppressed the responses to NA and AVP to 70% and 65% of control responses, respectively. LOE 908 partially suppressed the nifedipine-resistant responses to ET-1 and AVP, but not those to NA. SK&F 96365 also partially suppressed the nifedipine-resistant responses to ET-1 and AVP, whereas it abolished the responses to NA. LOE 908 in combination with SK&F 96365 abolished the nifedipine-resistant responses to either of the agonists. These results show that the contraction of rat aorta involves different Ca2+ entry channel depending on agonists: (a) NSCC-1, NSCC-2, and SOCC for ET-1; (b) VOCC and SOCC for NA; and (c) VOCC, NSCC-1, NSCC-2, and SOCC for AVP.

A comparative study of the effects of Cl(-) channel blockers on mesenteric vascular conductance in anaesthetized rat

There is evidence to suggest that niflumic acid is capable of selectively inhibiting Ca(2+)-dependent Cl(-) channels. Furthermore, it has been demonstrated that niflumic acid is capable of antagonizing contractile responses due to activation of alpha(1)-adrenoceptor in mesenteric vasculature. Here, we have examined the effects of three Cl(-) channel blockers, niflumic acid, indanyloxyacetic acid 94 (IAA-94) and diphenylamine-2-carboxylic acid (DPC) on cirazoline-mediated vasoconstriction in mesenteric blood vessel in vivo. Infusion of cirazoline produced a dose-dependent increase in blood pressure, decrease in superior mesenteric blood flow, mesenteric vascular conductance and heart rate. While niflumic acid and IAA-94 did not have any impact on cirazoline-induced changes in blood pressure, DPC accentuated the pressor effect of cirazoline. Neither agent affected cirazoline-mediated reflex reduction in the heart rate. Niflumic acid, IAA-94 and DPC attenuated alpha(1)-adrenoceptor mediated decrease in mesenteric blood flow and vascular conductance. Based on the profile of the actions of these compounds, it may be suggested that IAA-94 did not appear to act as selective inhibitor of Ca(2+)-activated Cl(-) channels when compared to niflumic acid in the mesenteric blood vessels. In addition, while DPC seems to be as effective as niflumic acid in its effects on mesenteric blood vessels, its actions may be attributed to other pharmacological effects.

Molecular and functional characterization of a murine calcium-activated chloride channel expressed in smooth muscle

To identify the gene products responsible for the calcium-activated chloride current in smooth muscle, reverse transcription-PCR with degenerate primers was performed on mouse intestine and other organs. A new member of the CLCA gene family was identified, mCLCA4, that is expressed preferentially in organs containing a high percentage of smooth muscle cells, including intestine, stomach, uterus, bladder, and aorta. Reverse transcription-PCR using template RNA prepared from mouse bladder and stomach smooth muscle layers dissected free of mucosa yielded mCLCA4-specific bands. In situ hybridization with an mCLCA4-specific probe confirmed prominent expression in smooth muscle of major vessels of the heart but not cardiac muscle. High expression was also detected in the gastrointestinal tract, in bronchioles, and in aortic and lung endothelial cells. Transient expression of mCLCA4 in 293T cells resulted in the appearance of a prominent calcium-activated chloride current. Whole-cell currents activated by ionomycin or methacholine were anion-selective and showed minimal rectification or voltage-dependent gating. Similar to endogenous currents in smooth muscle cells, methacholine-induced currents were transient, and spontaneous transient inward currents were occasionally observed at resting membrane potentials. These results link calcium-activated chloride channels in smooth muscle with a gene family whose members have been implicated in cystic fibrosis, cancer, and asthma.