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

Niflumic Acid Reverses Airway Mucus Excess and Improves Survival in the Rat Model of Steroid-Induced Pneumocystis Pneumonia

Although the role of adaptive immunity in fighting Pneumocystis infection is well known, the role of the innate, airway epithelium, responses remains largely unexplored. The concerted interaction of innate and adaptive responses is essential to successfully eradicate infection. Increased expression of goblet-cell-derived CLCA1 protein plus excess mucus in infant autopsy lungs and in murine models of primary Pneumocystis infection alert of innate immune system immunopathology associated to Pneumocystis infection. Nonetheless, whether blocking mucus-associated innate immune pathways decreases Pneumocystis-related immunopathology is unknown. Furthermore, current treatment of Pneumocystis pneumonia (PcP) relying on anti-Pneumocystis drugs plus steroids is not ideal because removes cellular immune responses against the fungal pathogen. In this study, we used the steroid-induced rat model of PcP to evaluate inflammation and mucus progression, and tested the effect of niflumic acid (NFA), a fenamate-type drug with potent CLCA1 blocker activity, in decreasing Pneumocystis-associated immunopathology. In this model, animals acquire Pneumocystis spontaneously and pneumonia develops owing to the steroids-induced immunodeficiency. Steroids led to decreased animal weight evidencing severe immunosuppression and to significant Pneumocystis-associated pulmonary edema as evidenced by wet-to-dry lung ratios that doubled those of uninfected animals. Inflammatory cuffing infiltrates were noticed first around lung blood vessels followed by bronchi, and both increased progressively. Similarly, airway epithelial and lumen mucus progressively increased. This occurred in parallel to increasing levels of MUC5AC and mCLCA3, the murine homolog of hCLCA1. Administration of NFA caused a significant decrease in total mucus, MUC5AC and mCLCA3 and also, in Pneumocystis-associated inflammation. Most relevant, NFA treatment improved survival at 8 weeks of steroids. Results suggest an important role of innate immune responses in immunopathology of steroid-induced PcP. They warrant evaluation of CLCA1 blockers as adjunctive therapy in this condition and describe a simple model to evaluate therapeutic interventions for steroid resistant mucus, a common condition in patients with chronic lung disease like asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Comparative Effects of Chloride Channel Inhibitors on LRRC8/VRAC-Mediated Chloride Conductance

Chloride channels play an essential role in a variety of physiological functions and in human diseases. Historically, the field of chloride channels has long been neglected owing to the lack of powerful selective pharmacological agents that are needed to overcome the technical challenge of characterizing the molecular identities of these channels. Recently, members of the LRRC8 family have been shown to be essential for generating the volume-regulated anion channel (VRAC) current, a chloride conductance that governs the regulatory volume decrease (RVD) process. The inhibitory effects of six commonly used chloride channel inhibitors on VRAC/LRRC8-mediated chloride transport were tested in wild-type HEK-293 cells expressing LRRC8 proteins and devoid of other types of chloride channels (CFTR and ANO1/2). We explored the effectiveness of the inhibitors using the patch-clamp whole-cell approach and fluorescence-based quantification of cellular volume changes during hypotonic challenge. Both DCPIB and NFA inhibited VRAC current in a whole-cell configuration, with IC₅₀ values of 5 ± 1 μM and 55 ± 2 μM, respectively. Surprisingly, GlyH-101 and PPQ-102, two CFTR inhibitors, also inhibited VRAC conductance at concentrations in the range of their current use, with IC₅₀ values of 10 ± 1 μM and 20 ± 1 μM, respectively. T16A_inh-A01, a so-called specific inhibitor of calcium-activated Cl^- conductance, blocked the chloride current triggered by hypo-osmotic challenge, with an IC₅₀ of 6 ± 1 μM. Moreover, RVD following hypotonic challenge was dramatically reduced by these inhibitors. CFTR_inh-172 was the only inhibitor that had almost no effect on VRAC/LRRC8-mediated chloride conductance. All inhibitors tested except CFTR_inh-172 inhibited VRAC/LRRC8-mediated chloride conductance and cellular volume changes during hypotonic challenge. These results shed light on the apparent lack of chloride channel inhibitors specificity and raise the question of how these inhibitors actually block chloride conductances.

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.

Influence of intracellular Ca2+ and alternative splicing on the pharmacological profile of ANO1 channels

Anoctamin-1 (ANO1) is a Ca(2+)-activated Cl(-) channel expressed in many types of cells. Splice variants of ANO1 have been shown to influence the biophysical properties of conductance. It has been suggested that several new antagonists of ANO1 with relatively high affinity and selectivity might be useful for experimental and, potentially, therapeutic purposes. We investigated the effects of intracellular Ca(2+) concentration ([Ca(2+)]i) at 100-1,000 nM, a concentration range that might be achieved in cells during physiological activation of ANO1 channels, on blockade of ANO1 channels expressed in HEK-293 cells. Whole cell and excised patch configurations of the patch-clamp technique were used to perform tests on a variety of naturally occurring splice variants of ANO1. Blockade of ANO1 currents with aminophenylthiazole (T16Ainh-A01) was highly dependent on [Ca(2+)]i Increasing [Ca(2+)]i reduced the potency of this blocker. Similar Ca(2+)-dependent effects were also observed with benzbromarone. Experiments on excised, inside-out patches showed that the diminished potency of the blockers caused by intracellular Ca(2+) might involve a competitive interaction for a common binding site or repulsion of the blocking drugs by electrostatic forces at the cytoplasmic surface of the channels. The degree of interaction between the channel blockers and [Ca(2+)]i depends on the splice variant expressed. These experiments demonstrate that the efficacy of ANO1 antagonists depends on [Ca(2+)]i, suggesting a need for caution when ANO1 blockers are used to determine the role of ANO1 in physiological functions and in their use as therapeutic agents.

Pharmacological characterization of TMEM16A currents

Recent studies have shown that transmembrane protein 16 A (TMEM16A) is a subunit of calcium-activated chloride channels (CACCs). Pharmacological agents have been used to probe the functional role of CACCs, however their effect on TMEM16A currents has not been systematically investigated. In the present study, we characterized the voltage and concentration-dependent effects of 2 traditional CACC inhibitors (niflumic acid and anthracene-9-carboxcylic acid) and 2 novel CACC / TMEM16A inhibitors (CACC(inh)A01 and T16A(inh)A01) on TMEM16A currents. The whole cell patch clamp technique was used to record TMEM16A currents from HE K 293 cells that stably expressed human TMEM16A. Niflumic acid, A-9-C, CACC(inh)A01 and T16A(inh)A01 inhibited TMEM16A currents with IC50 values of 12, 58, 1.7 and 1.5 μM, respectively, however, A-9-C and niflumic acid were less efficacious at negative membrane potentials. A-9-C and niflumic acid reduced the rate of TMEM16A tail current deactivation at negative membrane potentials and A-9-C (1 mM) enhanced peak TMEM16A tail current amplitude. In contrast, the inhibitory effects of CACC(inh)A01 and T16A(inh)A01 were independent of voltage and they did not prolong the rate of TMEM16A tail current deactivation. The effects of niflumic acid and A-9-C on TMEM16A currents were similar to previous observations on CACCs in vascular smooth muscle, strengthening the hypothesis that they are encoded by TMEM16A. However, CACC(inh)A01 and T16A(inh)A01 were more potent inhibitors of TMEM16A channels and their effects were not diminished at negative membrane potentials making them attractive candidates to interrogate the functional role of TMEM16A channels in future studies.

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.

Characterization of the effects of Cl⁻ channel modulators on TMEM16A and bestrophin-1 Ca²⁺ activated Cl⁻ channels

The Ca(2+) activated Cl(-) channels (CaCCs) play a multitude of important physiological functions. A number of candidate proteins have been proposed to form CaCC, but only two families, the bestrophins and the TMEM16 proteins, recapitulate the properties of native CaCC in expression systems. Studies of endogenous CaCCs are hindered by the lack of specific pharmacology as most Cl(-) channel modulators lack selectivity and a systematic comparison of the effects of these modulators on TMEM16A and bestrophin is missing. In the present study, we studied seven Cl(-) channel inhibitors: niflumic acid (NFA), NPPB, flufenamic acid (FFA), DIDS, tannic acid, CaCCinh-A01 and T16Ainh-A01 for their effects on TMEM16A and bestrophin-1 (Best1) stably expressed in CHO (Chinese hamster ovary) cells using patch clamp technique. Among seven inhibitors studied, NFA showed highest selectivity for TMEM16A (IC50 of 7.40 ± 0.95 μM) over Best1 (IC50 of 102.19 ± 15.05 μM). In contrast, DIDS displayed a reverse selectivity inhibiting Best1 with IC50 of 3.93 ± 0.73 μM and TMEM16A with IC50 of 548.86 ± 25.57 μM. CaCCinh-A01 was the most efficacious blocker for both TMEM16A and Best1 channels. T16Ainh-A01 partially inhibited TMEM16A currents but had no effect on Best1 currents. Tannic acid, NPPB and FFA had variable intermediate effects. Potentiation of channel activity by some of these modulators and the effects on TMEM16A deactivation kinetics were also described. Characterization of Cl(-) channel modulators for their effects on TMEM16A and Best1 will facilitate future studies of native CaCCs.

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.

Calcium dynamics underlying the myogenic response of the renal afferent arteriole

The renal afferent arteriole reacts to an elevation in blood pressure with an increase in muscle tone and a decrease in luminal diameter. This effect, known as the myogenic response, is believed to stabilize glomerular filtration and to protect the glomerulus from systolic blood pressure increases, especially in hypertension. To study the mechanisms underlying the myogenic response, we developed a mathematical model of intracellular Ca(2+) signaling in an afferent arteriole smooth muscle cell. The model represents detailed transmembrane ionic transport, intracellular Ca(2+) dynamics, the kinetics of myosin light chain phosphorylation, and the mechanical behavior of the cell. It assumes that the myogenic response is initiated by pressure-induced changes in the activity of nonselective cation channels. Our model predicts spontaneous vasomotion at physiological luminal pressures and KCl- and diltiazem-induced diameter changes comparable to experimental findings. The time-periodic oscillations stem from the dynamic exchange of Ca(2+) between the cytosol and the sarcoplasmic reticulum, coupled to the stimulation of Ca(2+)-activated potassium (KCa) and chloride (ClCa) channels, and the modulation of voltage-activated L-type channels; blocking sarco/endoplasmic reticulum Ca(2+) pumps, ryanodine receptors (RyR), KCa, ClCa, or L-type channels abolishes these oscillations. Our results indicate that the profile of the myogenic response is also strongly dependent on the conductance of ClCa and L-type channels, as well as the activity of plasmalemmal Ca(2+) pumps. Furthermore, inhibition of KCa is not necessary to induce myogenic contraction. Lastly, our model suggests that the kinetic behavior of L-type channels results in myogenic kinetics that are substantially faster during constriction than during dilation, consistent with in vitro observations (Loutzenhiser R, Bidani A, Chilton L. Circ. Res. 90: 1316-1324, 2002).

Low concentrations of niflumic acid enhance basal spontaneous and carbachol-induced contractions of the detrusor

PURPOSE

The urinary bladder expresses Ca(2+)-activated Cl(-) channels (CACC), but its physiological role in governing contractility remains to be defined. The CACC modulator niflumic acid (NFA) is widely used despite the variable results arisen from different drug concentrations used. This study was designed to examine the effects of NFA at low concentrations on detrusor strip contractility.

METHODS

Rat detrusor strips with mucosa-intact (+MU) and mucosa-denuded (-MU) were prepared in transverse (Tr) and longitudinal (Lg) with respect to the bladder orientation. Isometric force measurements were made at baseline (for spontaneous phasic contractile activity) and during drug stimulation (by carbachol, CCh) with and without NFA.

RESULTS

NFA (1 and 10 μmol/L) pretreatment enhanced CCh-induced contractions more in +MU than -MU strips with no selectivity on contractile direction. For spontaneous phasic contractions, NFA-treated strips in the Tr direction showed increased phasic amplitude, while phasic frequency was unchanged.

CONCLUSIONS

The findings suggest low concentrations of NFA having a potentiating effect on detrusor contractions that was sensitive to the MU and contractile direction.

Activation of Ca2+-activated Cl- channels by store-operated Ca2+ entry in arterial smooth muscle cells does not require reverse-mode Na+/Ca2+ exchange

The main purpose of this study was to characterize the stimulation of Ca(2+)-activated Cl(-) (Cl(Ca)) by store-operated Ca(2+) entry (SOCE) channels in rabbit pulmonary arterial smooth muscle cells (PASMCs) and determine if this process requires reverse-mode Na(+)/Ca(2+) exchange (NCX). In whole-cell voltage clamped PASMCs incubated with 1 μmol/L nifedipine (Nif) to inhibit Ca(2+) channels, 30 μmol/L cyclopiazonic acid (CPA), a SERCA pump inhibitor, activated a nonselective cation conductance permeable to Na(+) (I(SOC)) during an initial 1-3 s step, ranging from-120 to +60 mV, and Ca(2+)-activated Cl(-) current (I(Cl(Ca))) during a second step to +90 mV that increased with the level of the preceding hyperpolarizing step. Niflumic acid (100 μmol/L), a Cl(Ca) channel blocker, abolished I(Cl(Ca)) but had no effect on I(SOC), whereas the I(SOC) blocker SKF-96365 (50 μmol/L) suppressed both currents. Dual patch clamp and Fluo-4 fluorescence measurements revealed the appearance of CPA-induced Ca(2+) transients of increasing magnitude with increasing hyperpolarizing steps, which correlated with I(Cl(Ca)) amplitude. The absence of Ca(2+) transients at positive potentials following a hyperpolarizing step combined with the observation that SOCE-stimulated I(Cl(Ca)) was unaffected by the NCX blocker KB-R7943 (1 μmol/L) suggest that the SOCE/Cl(Ca) interaction does not require reverse-mode NCX in our conditions.

TMEM16A channels generate Ca²⁺-activated Cl⁻ currents in cerebral artery smooth muscle cells

Transmembrane protein (TMEM)16A channels are recently discovered membrane proteins that display electrophysiological properties similar to classic Ca(2+)-activated Cl(-) (Cl(Ca)) channels in native cells. The molecular identity of proteins that generate Cl(Ca) currents in smooth muscle cells (SMCs) of resistance-size arteries is unclear. Similarly, whether cerebral artery SMCs generate Cl(Ca) currents is controversial. Here, using molecular biology and patch-clamp electrophysiology, we examined TMEM16A channel expression and characterized Cl(-) currents in arterial SMCs of resistance-size rat cerebral arteries. RT-PCR amplified transcripts for TMEM16A but not TMEM16B-TMEM16H, TMEM16J, or TMEM16K family members in isolated pure cerebral artery SMCs. Western blot analysis using an antibody that recognized recombinant (r)TMEM16A channels detected TMEM16A protein in cerebral artery lysates. Arterial surface biotinylation and immunofluorescence indicated that TMEM16A channels are located primarily within the arterial SMC plasma membrane. Whole cell Cl(Ca) currents in arterial SMCs displayed properties similar to those generated by rTMEM16A channels, including Ca(2+) dependence, current-voltage relationship linearization by an elevation in intracellular Ca(2+) concentration, a Nerstian shift in reversal potential induced by reducing the extracellular Cl(-) concentration, and a negative reversal potential shift when substituting extracellular I(-) for Cl(-). A pore-targeting TMEM16A antibody similarly inhibited both arterial SMC Cl(Ca) and rTMEM16A currents. TMEM16A knockdown using small interfering RNA also inhibited arterial SMC Cl(Ca) currents. In summary, these data indicate that TMEM16A channels are expressed, insert into the plasma membrane, and generate Cl(Ca) currents in cerebral artery SMCs.

Characterizing behavior of corpus cavernosum in chloride-free condition

OBJECTIVE

To elucidate the role of chloride currents in erectile function through characterizing the behavior of corpus cavernosum (CC) in chloride-free (Cf) medium, which has not been evaluated before.

METHODS

Isolated rabbit CC strips were suspended in thermo-regulated organ baths containing oxygenated Tyrode for isometric tension recording. Cf Tyrode was prepared by substituting sodium chloride, calcium chloride, and potassium chloride (KCl) with equivalent molar concentrations of sodium acetate, calcium acetate, and potassium acetate salts. Resting cavernosal tone and contractions by noradrenaline, histamine, and KCl were assessed in Cf Tyrode with or without chloride channel blockers, niflumic acid (NFA), and anthracene-9-carboxylic acid (A9C).

RESULTS

Withdrawal of extracellular chloride caused myogenic contractions in the unstimulated CC strips (n = 18). In addition, peak contractions by noradrenaline (n = 14) and histamine (n = 13) were augmented in Cf buffer by 47.2 ± 5.9% and 85.4 ± 13.2%, respectively (P <.05), whereas KCl contractions were not significantly altered (17.6 ± 4.6%; n = 7). Interestingly, Cf buffer exerted opposing effects, potentiation and reduction, respectively, on the plateau phase of contractions mediated by noradrenaline and histamine. The stimulatory effect of Cf buffer on the intrinsic myogenic tone was diminished by NFA (30 μM), and A9C (300 μM-1 mM). NFA (30-100 μM), however, specifically reduced the plateau phase without significantly modifying the peak contraction of noradrenaline in Cf buffer.

CONCLUSIONS

These results reiterate the importance of chloride currents as a mechanism underlying the maintenance of penile cavernosal tone. Thus, chloride channel could be an effective alternative target to regulate penile erection.

Cholesterol depletion alters amplitude and pharmacology of vascular calcium-activated chloride channels

AIMS

Calcium-activated chloride channels (CACCs) share common pharmacological properties with Kcnma1-encoded large conductance K(+) channels (BK(Ca) or K(Ca)1.1) and it has been suggested that they may co-exist in a macromolecular complex. As K(Ca)1.1 channels are known to localize to cholesterol and caveolin-rich lipid rafts (caveolae), the present study investigated whether Ca(2+)-sensitive Cl(-) currents in vascular myocytes were affected by the cholesterol depleting agent methyl-beta-cyclodextrin (M-betaCD).

METHODS AND RESULTS

Calcium-activated chloride and potassium currents were recorded from single murine portal vein myocytes in whole cell voltage clamp. Western blot was undertaken following sucrose gradient ultracentrifugation using protein lysates from whole portal veins. Ca(2+)-activated Cl(-) currents were augmented by 3 mg mL(-1) M-betaCD with a rapid time course (t(0.5) = 1.8 min). M-betaCD had no effect on the bi-modal response to niflumic acid or anthracene-9-carboxylate but completely removed the inhibitory effects of the K(Ca)1.1 blockers, paxilline and tamoxifen, as well as the stimulatory effect of the K(Ca)1.1 activator NS1619. Discontinuous sucrose density gradients followed by western blot analysis revealed that the position of lipid raft markers caveolin and flotillin-2 was altered by 15 min application of 3 mg mL(-1) M-betaCD. The position of K(Ca)1.1 and the newly identified candidate for CACCs, TMEM16A, was also affected by M-betaCD.

CONCLUSION

These data reveal that CACC properties are influenced by lipid raft integrity.

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.

The role of CLCA proteins in inflammatory airway disease

Inflammatory airway diseases such as asthma and chronic obstructive pulmonary disease (COPD) exhibit stereotyped traits that are variably expressed in each person. In experimental mouse models of chronic lung disease, these individual disease traits can be genetically segregated and thereby linked to distinct determinants. Functional genomic analysis indicates that at least one of these traits, mucous cell metaplasia, depends on members of the calcium-activated chloride channel (CLCA) gene family. Here we review advances in the biochemistry of the CLCA family and the evidence of a role for CLCA family members in the development of mucous cell metaplasia and possibly airway hyperreactivity in experimental models and in humans. On the basis of this information, we develop the model that CLCA proteins are not integral membrane proteins with ion channel function but instead are secreted signaling molecules that specifically regulate airway target cells in healthy and disease conditions.

Inhibition of vascular calcium-gated chloride currents by blockers of KCa1.1, but not by modulators of KCa2.1 or KCa2.3 channels

BACKGROUND AND PURPOSE

Recent pharmacological studies have proposed there is a high degree of similarity between calcium-activated Cl(-) channels (CaCCs) and large conductance, calcium-gated K(+) channels (K(Ca)1.1). The goal of the present study was to ascertain whether blockers of K(Ca)1.1 inhibited calcium-activated Cl(-) currents (I(ClCa)) and if the pharmacological overlap between K(Ca)1.1 and CaCCs extends to intermediate and small conductance, calcium-activated K(+) channels.

EXPERIMENTAL APPROACHES

Whole-cell Cl(-) and K(+) currents were recorded from murine portal vein myocytes using the whole-cell variant of the patch clamp technique. CaCC currents were evoked by pipette solutions containing 500 nM free [Ca(2+)].

KEY RESULTS

The selective K(Ca)1.1 blocker paxilline (1 microM) inhibited I(ClCa) by approximately 90%, whereas penitrem A (1 microM) and iberiotoxin (100 and 300 nM) reduced the amplitude of I(ClCa) by approximately 20%, as well as slowing channel deactivation. Paxilline also abolished the stimulatory effect of niflumic acid on the CaCC. In contrast, an antibody against the Ca(2+)-binding domain of murine K(Ca)1.1 had no effect on I(ClCa) while inhibiting spontaneous K(Ca)1.1 currents. Structurally different modulators of small and intermediate conductance calcium-activated K(+) channels (K(Ca)2.1 and K(Ca)2.3), namely 1-EBIO, (100 microM); NS309, (1 microM); TRAM-34, (10 microM); UCL 1684, (1 microM) had no effect on I(ClCa).

CONCLUSIONS AND IMPLICATIONS

These data show that the selective K(Ca)1.1 blockers also reduce I(ClCa) considerably. However, the pharmacological overlap that exists between CaCCs and K(Ca)1.1 does not extend to the calcium-binding domain or to other calcium-gated K(+) channels.

Direct regulation of BK channels by phosphatidylinositol 4,5-bisphosphate as a novel signaling pathway

Large conductance, calcium- and voltage-gated potassium (BK) channels are ubiquitous and critical for neuronal function, immunity, and smooth muscle contractility. BK channels are thought to be regulated by phosphatidylinositol 4,5-bisphosphate (PIP(2)) only through phospholipase C (PLC)-generated PIP(2) metabolites that target Ca(2+) stores and protein kinase C and, eventually, the BK channel. Here, we report that PIP(2) activates BK channels independently of PIP(2) metabolites. PIP(2) enhances Ca(2+)-driven gating and alters both open and closed channel distributions without affecting voltage gating and unitary conductance. Recovery from activation was strongly dependent on PIP(2) acyl chain length, with channels exposed to water-soluble diC4 and diC8 showing much faster recovery than those exposed to PIP(2) (diC16). The PIP(2)-channel interaction requires negative charge and the inositol moiety in the phospholipid headgroup, and the sequence RKK in the S6-S7 cytosolic linker of the BK channel-forming (cbv1) subunit. PIP(2)-induced activation is drastically potentiated by accessory beta(1) (but not beta(4)) channel subunits. Moreover, PIP(2) robustly activates BK channels in vascular myocytes, where beta(1) subunits are abundantly expressed, but not in skeletal myocytes, where these subunits are barely detectable. These data demonstrate that the final PIP(2) effect is determined by channel accessory subunits, and such mechanism is subunit specific. In HEK293 cells, cotransfection of cbv1+beta(1) and PI4-kinaseIIalpha robustly activates BK channels, suggesting a role for endogenous PIP(2) in modulating channel activity. Indeed, in membrane patches excised from vascular myocytes, BK channel activity runs down and Mg-ATP recovers it, this recovery being abolished by PIP(2) antibodies applied to the cytosolic membrane surface. Moreover, in intact arterial myocytes under physiological conditions, PLC inhibition on top of blockade of downstream signaling leads to drastic BK channel activation. Finally, pharmacological treatment that raises PIP(2) levels and activates BK channels dilates de-endothelized arteries that regulate cerebral blood flow. These data indicate that endogenous PIP(2) directly activates vascular myocyte BK channels to control vascular tone.

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

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

Stimulation of Ca2+-gated Cl- currents by the calcium-dependent K+ channel modulators NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] and isopimaric acid

Because chloride (Cl(-)) channel blockers such as niflumic acid enhance large-conductance Ca(2+)-activated potassium channels (BK(Ca)), the aim of this study was to determine whether there is a reciprocal modification of Ca(2+)-activated chloride Cl(-) currents (I(ClCa)) by two selective activators of BK(Ca). Single smooth muscle cells were isolated by enzymatic digestion from murine portal vein and rabbit pulmonary artery. The BK(Ca) activators NS1619 [1,3-dihydro-1-[2-hydroxy-5-(trifluoromethyl-)phenyl]-5-(trifluoromethyl)-2H-benzimidazol-2-one] and isopimaric acid (IpA) augmented macroscopic I(ClCa) elicited by pipette solutions containing [Ca(2+)](i) > 100 nM without any alteration in current kinetics. Enhanced currents recorded in the presence of NS1619 or IpA reversed at the theoretical Cl(-) equilibrium potential, which was shifted by approximately -40 mV upon replacement of the external anion with the more permeable thiocyanate anion. NS1619 increased the sensitivity of calcium-activated chloride channel (Cl(Ca)) to Ca(2+) (approximately 100 nM at +60 mV) and induced a leftward shift in their voltage dependence (approximately 80 mV with 1 micro Ca(2+)). Single-channel experiments revealed that NS1619 increased the number of open channels times the open probability of small-conductance (1.8-3.1 pS) Cl(Ca) without any alteration in their unitary amplitude or number of observable unitary levels of activity. These data, in addition to the established stimulatory effects of niflumic acid on BK(Ca), show that there is similarity in the pharmacology of calcium-activated chloride and potassium channels. Although nonspecific interactions are possible, one alternative hypothesis is that the channel underlying vascular I(ClCa) shares some structural similarity to the BK(Ca) or that the latter K(+) channel physically interacts with Cl(Ca).

Overlapping pharmacology of Ca2+-activated Cl- and K+ channels

Research into Ca2+-activated Cl- channels is hampered by the inability to decipher their molecular identity and the fact that all extant Cl- channel blockers have effects on other ion channels. Most notably, Cl- channel blockers such as the fenamates (e.g. niflumic acid and flufenamic acid) activate Ca2+-dependent K+ channels, although other pharmacological overlaps have been discovered. In this article, we highlight the complex pharmacology of Ca2+-activated Cl- channels and the caveats associated with using these blockers--a necessary requirement because many researchers use Cl- channel blockers as probes for Cl- channel activity. Moreover, we discuss the argument for a common structural motif between Ca2+-activated Cl- channels and Ca2+-dependent K+ channels, which has led to the possibility that the molecular identity of Cl- channels will be revealed by research in this new direction, in addition to the use of existing candidates such as the CLCA, Bestrophin and tweety genes.

Mechanism of the inhibition of Ca2+-activated Cl- currents by phosphorylation in pulmonary arterial smooth muscle cells

The aim of the present study was to provide a mechanistic insight into how phosphatase activity influences calcium-activated chloride channels in rabbit pulmonary artery myocytes. Calcium-dependent Cl⁻ currents (I_ClCa) were evoked by pipette solutions containing concentrations between 20 and 1000 nM Ca²⁺ and the calcium and voltage dependence was determined. Under control conditions with pipette solutions containing ATP and 500 nM Ca²⁺, I_ClCa was evoked immediately upon membrane rupture but then exhibited marked rundown to ∼20% of initial values. In contrast, when phosphorylation was prohibited by using pipette solutions containing adenosine 5′-(β,γ-imido)-triphosphate (AMP-PNP) or with ATP omitted, the rundown was severely impaired, and after 20 min dialysis, I_ClCa was ∼100% of initial levels. I_ClCa recorded with AMP-PNP–containing pipette solutions were significantly larger than control currents and had faster kinetics at positive potentials and slower deactivation kinetics at negative potentials. The marked increase in I_ClCa was due to a negative shift in the voltage dependence of activation and not due to an increase in the apparent binding affinity for Ca²⁺. Mathematical simulations were carried out based on gating schemes involving voltage-independent binding of three Ca²⁺, each binding step resulting in channel opening at fixed calcium but progressively greater “on” rates, and voltage-dependent closing steps (“off” rates). Our model reproduced well the Ca²⁺ and voltage dependence of I_ClCa as well as its kinetic properties. The impact of global phosphorylation could be well mimicked by alterations in the magnitude, voltage dependence, and state of the gating variable of the channel closure rates. These data reveal that the phosphorylation status of the Ca²⁺-activated Cl⁻ channel complex influences current generation dramatically through one or more critical voltage-dependent steps.

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.