Chloride channel blockers inhibit Ca2+ uptake by the smooth muscle sarcoplasmic reticulum

VSI: The anoctamins: Structure and function: "Intracellular" anoctamins

Plasma membrane localized anoctamin 1, 2 and 6 (TMEM16A, B, F) have been examined in great detail with respect to structure and function, but much less is known about the other seven intracellular members of this exciting family of proteins. This is probably due to their limited accessibility in intracellular membranous compartments, such as the endoplasmic reticulum (ER) or endosomes. However, these so-called intracellular anoctamins are also found in the plasma membrane (PM) which adds to the confusion regarding their cellular role. Probably all intracellular anoctamins except of ANO8 operate as intracellular phospholipid (PL) scramblases, allowing for Ca2+-activated, passive transport of phospholipids like phosphatidylserine between both membrane leaflets. Probably all of them also conduct ions, which is probably part of their physiological function. In this brief overview, we summarize key findings on the biological functions of ANO3, 4, 5, 7, 8, 9 and 10 (TMEM16C, D, E, G, H, J, K) that are gradually coming to light. Compartmentalized regulation of intracellular Ca2+ signals, tethering of the ER to specific PM contact sites, and control of intracellular vesicular trafficking appear to be some of the functions of intracellular anoctamins, while loss of function and abnormal expression are the cause for various diseases.

Chloride ions in health and disease

Chloride is a key anion involved in cellular physiology by regulating its homeostasis and rheostatic processes. Changes in cellular Cl- concentration result in differential regulation of cellular functions such as transcription and translation, post-translation modifications, cell cycle and proliferation, cell volume, and pH levels. In intracellular compartments, Cl- modulates the function of lysosomes, mitochondria, endosomes, phagosomes, the nucleus, and the endoplasmic reticulum. In extracellular fluid (ECF), Cl- is present in blood/plasma and interstitial fluid compartments. A reduction in Cl- levels in ECF can result in cell volume contraction. Cl- is the key physiological anion and is a principal compensatory ion for the movement of the major cations such as Na+, K+, and Ca2+. Over the past 25 years, we have increased our understanding of cellular signaling mediated by Cl-, which has helped in understanding the molecular and metabolic changes observed in pathologies with altered Cl- levels. Here, we review the concentration of Cl- in various organs and cellular compartments, ion channels responsible for its transportation, and recent information on its physiological roles.

A Superfolder Green Fluorescent Protein-Based Biosensor Allows Monitoring of Chloride in the Endoplasmic Reticulum

Though the concentration of chloride has been measured in the cytoplasm and in secretory granules of live cells, it cannot be measured within the endoplasmic reticulum (ER) due to poor fluorescence of existing biosensors. We developed a fluorescent biosensor composed of a chloride-sensitive superfolder GFP and long Stokes-shifted mKate2 for simultaneous chloride and pH measurements that retained fluorescence in the ER lumen. Using this sensor, we showed that the chloride concentration in the ER is significantly lower than that in the cytosol. This improved biosensor enables dynamic measurement of chloride in the ER and may be useful in other environments where protein folding is challenging.

Endoplasmic reticulum maintains ion homeostasis required for plasma membrane repair

Of the many crucial functions of the ER, homeostasis of physiological calcium increase is critical for signaling. Plasma membrane (PM) injury causes a pathological calcium influx. Here, we show that the ER helps clear this surge in cytoplasmic calcium through an ER-resident calcium pump, SERCA, and a calcium-activated ion channel, Anoctamin 5 (ANO5). SERCA imports calcium into the ER, and ANO5 supports this by maintaining electroneutrality of the ER lumen through anion import. Preventing either of these transporter activities causes cytosolic calcium overload and disrupts PM repair (PMR). ANO5 deficit in limb girdle muscular dystrophy 2L (LGMD2L) patient cells compromises their cytosolic and ER calcium homeostasis. By generating a mouse model of LGMD2L, we find that PM injury causes cytosolic calcium overload and compromises the ability of ANO5-deficient myofibers to repair. Addressing calcium overload in ANO5-deficient myofibers enables them to repair, supporting the requirement of the ER in calcium homeostasis in injured cells and facilitating PMR.

Multiscale Simulation Reveals Passive Proton Transport Through SERCA on the Microsecond Timescale

The sarcoplasmic reticulum Ca²⁺-ATPase (SERCA) transports two Ca²⁺ ions from the cytoplasm to the reticulum lumen at the expense of ATP hydrolysis. In addition to transporting Ca²⁺, SERCA facilitates bidirectional proton transport across the sarcoplasmic reticulum to maintain the charge balance of the transport sites and to balance the charge deficit generated by the exchange of Ca²⁺. Previous studies have shown the existence of a transient water-filled pore in SERCA that connects the Ca²⁺ binding sites with the lumen, but the capacity of this pathway to sustain passive proton transport has remained unknown. In this study, we used the multiscale reactive molecular dynamics method and free energy sampling to quantify the free energy profile and timescale of the proton transport across this pathway while also explicitly accounting for the dynamically coupled hydration changes of the pore. We find that proton transport from the central binding site to the lumen has a microsecond timescale, revealing a novel passive cytoplasm-to-lumen proton flow beside the well-known inverse proton countertransport occurring in active Ca²⁺ transport. We propose that this proton transport mechanism is operational and serves as a functional conduit for passive proton transport across the sarcoplasmic reticulum.

What biologists want from their chloride reporters – a conversation between chemists and biologists

Impaired chloride transport affects diverse processes ranging from neuron excitability to water secretion, which underlie epilepsy and cystic fibrosis, respectively. The ability to image chloride fluxes with fluorescent probes has been essential for the investigation of the roles of chloride channels and transporters in health and disease. Therefore, developing effective fluorescent chloride reporters is critical to characterizing chloride transporters and discovering new ones. However, each chloride channel or transporter has a unique functional context that demands a suite of chloride probes with appropriate sensing characteristics. This Review seeks to juxtapose the biology of chloride transport with the chemistries underlying chloride sensors by exploring the various biological roles of chloride and highlighting the insights delivered by studies using chloride reporters. We then delineate the evolution of small-molecule sensors and genetically encoded chloride reporters. Finally, we analyze discussions with chloride biologists to identify the advantages and limitations of sensors in each biological context, as well as to recognize the key design challenges that must be overcome for developing the next generation of chloride sensors.

Dysregulated calcium homeostasis prevents plasma membrane repair in Anoctamin 5/TMEM16E-deficient patient muscle cells

Autosomal recessive mutations in Anoctamin 5 (ANO5/TMEM16E), a member of the transmembrane 16 (TMEM16) family of Ca²⁺-activated ion channels and phospholipid scramblases, cause adult-onset muscular dystrophies (limb girdle muscular dystrophy 2L (LGMD2L) and Miyoshi Muscular Dystrophy (MMD3). However, the molecular role of ANO5 is unclear and ANO5 knockout mouse models show conflicting requirements of ANO5 in muscle. To study the role of ANO5 in human muscle cells we generated a myoblast line from a MMD3-patient carrying the c.2272C>T mutation, which we find causes the mutant protein to be degraded. The patient myoblasts exhibit normal myogenesis, but are compromised in their plasma membrane repair (PMR) ability. The repair deficit is linked to the poor ability of the endoplasmic reticulum (ER) to clear cytosolic Ca²⁺ increase caused by focal plasma membrane injury. Expression of wild-type ANO5 or pharmacological prevention of injury-triggered cytosolic Ca²⁺ overload enable injured patient muscle cells to repair. A homology model of ANO5 shows that several of the known LGMD2L/MMD3 patient mutations line the transmembrane region of the protein implicated in its channel activity. These results point to a role of cytosolic Ca²⁺ homeostasis in PMR, indicate a role for ANO5 in ER-mediated cytosolic Ca²⁺ uptake and identify normalization of cytosolic Ca²⁺ homeostasis as a potential therapeutic approach to treat muscular dystrophies caused by ANO5 deficit.

Cyclin-dependent kinase 5-mediated phosphorylation of chloride intracellular channel 4 promotes oxidative stress-induced neuronal death

Oxidative stress can cause apoptosis in neurons and may result in neurodegenerative diseases. However, the signaling mechanisms leading to oxidative stress–induced neuronal apoptosis are not fully understood. Oxidative stress stimulates aberrant activation of cyclin-dependent kinase 5 (CDK5), thought to promote neuronal apoptosis by phosphorylating many cell death-related substrates. Here, using protein pulldown methods, immunofluorescence experiments and in vitro kinase assays, we identified chloride intracellular channel 4 (CLIC4), the expression of which increases during neuronal apoptosis, as a CDK5 substrate. We found that activated CDK5 phosphorylated serine 108 in CLIC4, increasing CLIC4 protein stability, and accumulation. Pharmacological inhibition or shRNA-mediated silencing of CDK5 decreased CLIC4 levels in neurons. Moreover, CLIC4 overexpression led to neuronal apoptosis, whereas knockdown or pharmacological inhibition of CLIC4 attenuated H₂O₂-induced neuronal apoptosis. These results implied that CLIC4, by acting as a substrate of CDK5, mediated neuronal apoptosis induced by aberrant CDK5 activation. Targeting CLIC4 in neurons may therefore provide a therapeutic approach for managing progressive neurodegenerative diseases that arise from neuronal apoptosis.

Glycine Receptor Activation Impairs ATP-Induced Calcium Transients in Cultured Cortical Astrocytes

In central nervous system, glycine receptor (GlyR) is mostly expressed in the spinal cord and brainstem, but glycinergic transmission related elements have also been identified in the brain. Astrocytes are active elements at the tripartite synapse, being responsible for the maintenance of brain homeostasis and for the fine-tuning of synaptic activity. These cells communicate, spontaneously or in response to a stimulus, by elevations in their cytosolic calcium (calcium transients, Ca2+T) that can be propagated to other cells. How these Ca2+T are negatively modulated is yet poorly understood. In this work, we evaluated GlyR expression and its role on calcium signaling modulation in rat brain astrocytes. We first proved that GlyR, predominantly subunits α2 and β, was expressed in brain astrocytes and its localization was confirmed in the cytoplasm and astrocytic processes by immunohistochemistry assays. Calcium imaging experiments in cultured astrocytes showed that glycine (500 μM), a GlyR agonist, caused a concentration-dependent reduction in ATP-induced Ca2+T, an effect abolished by the GlyR antagonist, strychnine (0.8 μM), as well as by nocodazole (1 μM), known to impair GlyR anchorage to the plasma membrane. This effect was mimicked by activation of GABAAR, another Cl--permeable channel. In summary, we demonstrated that GlyR activation in astrocytes mediates an inhibitory effect upon ATP induced Ca2+T, which most probably involves changes in membrane permeability to Cl- and requires GlyR anchorage at the plasma membrane. GlyR in astrocytes may thus be part of a mechanism to modulate astrocyte-to-neuron communication.

CF airway smooth muscle transcriptome reveals a role for PYK2

Abnormal airway smooth muscle function can contribute to cystic fibrosis (CF) airway disease. We previously found that airway smooth muscle from newborn CF pigs had increased basal tone, an increased bronchodilator response, and abnormal calcium handling. Since CF pigs lack airway infection and inflammation at birth, these findings suggest intrinsic airway smooth muscle dysfunction in CF. In this study, we tested the hypothesis that CFTR loss in airway smooth muscle would produce a distinct set of changes in the airway smooth muscle transcriptome that we could use to develop novel therapeutic targets. Total RNA sequencing of newborn wild-type and CF airway smooth muscle revealed changes in muscle contraction-related genes, ontologies, and pathways. Using connectivity mapping, we identified several small molecules that elicit transcriptional signatures opposite of CF airway smooth muscle, including NVP-TAE684, an inhibitor of proline-rich tyrosine kinase 2 (PYK2). In CF airway smooth muscle tissue, PYK2 phosphorylation was increased and PYK2 inhibition decreased smooth muscle contraction. In vivo NVP-TAE684 treatment of wild-type mice reduced methacholine-induced airway smooth muscle contraction. These findings suggest that studies in the newborn CF pig may provide an important approach to enhance our understanding of airway smooth muscle biology and for discovery of novel airway smooth muscle therapeutics for CF and other diseases of airway hyperreactivity.

Importance of Altered Levels of SERCA, IP3R, and RyR in Vascular Smooth Muscle Cell

Calcium cycling between the sarcoplasmic reticulum (SR) and the cytosol via the sarco-/endoplasmic reticulum Ca-ATPase (SERCA) pump, inositol-1,4,5-triphosphate receptor (IP₃R), and Ryanodine receptor (RyR), plays a major role in agonist-induced intracellular calcium ([Ca²⁺]_cyt) dynamics in vascular smooth muscle cells (VSMC). Levels of these calcium handling proteins in SR get altered under disease conditions. We have developed a mathematical model to understand the significance of altered levels of SERCA, IP₃R, and RyR on the intracellular calcium dynamics of VSMC and to understand how variation in protein levels that arise due to diabetes contribute to different VSMC behavior and thus vascular disease. SR is modeled as a single continuous entity with homogeneous intra-SR calcium. Model results show that agonist-induced intracellular calcium dynamics can be modified by changing the levels of SERCA, IP₃R, and/or RyR. Lowering SERCA level will enable intracellular calcium oscillations at low agonist concentrations whereas lowered levels of IP₃R and RyR need higher agonist concentration for intracellular calcium oscillations. This research suggests that reduced SERCA level is the main factor responsible for the reduced intracellular calcium transients and contractility in VSMCs.

Cystic Fibrosis Transmembrane Conductance Regulator in Sarcoplasmic Reticulum of Airway Smooth Muscle. Implications for Airway Contractility

RATIONALE

An asthma-like airway phenotype has been described in people with cystic fibrosis (CF). Whether these findings are directly caused by loss of CF transmembrane conductance regulator (CFTR) function or secondary to chronic airway infection and/or inflammation has been difficult to determine.

OBJECTIVES

Airway contractility is primarily determined by airway smooth muscle. We tested the hypothesis that CFTR is expressed in airway smooth muscle and directly affects airway smooth muscle contractility.

METHODS

Newborn pigs, both wild type and with CF (before the onset of airway infection and inflammation), were used in this study. High-resolution immunofluorescence was used to identify the subcellular localization of CFTR in airway smooth muscle. Airway smooth muscle function was determined with tissue myography, intracellular calcium measurements, and regulatory myosin light chain phosphorylation status. Precision-cut lung slices were used to investigate the therapeutic potential of CFTR modulation on airway reactivity.

MEASUREMENTS AND MAIN RESULTS

We found that CFTR localizes to the sarcoplasmic reticulum compartment of airway smooth muscle and regulates airway smooth muscle tone. Loss of CFTR function led to delayed calcium reuptake following cholinergic stimulation and increased myosin light chain phosphorylation. CFTR potentiation with ivacaftor decreased airway reactivity in precision-cut lung slices following cholinergic stimulation.

CONCLUSIONS

Loss of CFTR alters porcine airway smooth muscle function and may contribute to the airflow obstruction phenotype observed in human CF. Airway smooth muscle CFTR may represent a therapeutic target in CF and other diseases of airway narrowing.

Calcium-activated chloride channels anoctamin 1 and 2 promote murine uterine smooth muscle contractility

OBJECTIVE

To determine the presence of calcium activated chloride channels anoctamin 1 (ANO1) and 2 (ANO2) in human and murine uterine smooth muscle (MUSM) and evaluate the physiologic role for these ion channels in murine myometrial contractility.

STUDY DESIGN

We performed reverse transcription polymerase chain reaction to determine whether ANO1 and 2 are expressed in human and murine uterine tissue to validate the study of this protein in mouse models. Immunohistochemical staining of ANO1 and 2 was then performed to determine protein expression in murine myometrial tissue. The function of ANO1 and 2 in murine uterine tissue was evaluated using electrophysiologic studies, organ bath, and calcium flux experiments.

RESULTS

ANO1 and 2 are expressed in human and MUSM cells. Functional studies show that selective antagonism of these channels promotes relaxation of spontaneous MUSM contractions. Blockade of ANO1 and 2 inhibits both agonist-induced and spontaneous transient inward currents and abolishes G-protein coupled receptor (oxytocin) mediated elevations in intracellular calcium.

CONCLUSION

The calcium activated chloride channels ANO1 and 2 are present in human and murine myometrial tissue and may provide novel potential therapeutic targets to achieve effective tocolysis.

Potassium fluxes across the endoplasmic reticulum and their role in endoplasmic reticulum calcium homeostasis

There are a number of known and suspected channels and exchangers in the endoplasmic reticulum that may participate in potassium flux across its membrane. They include trimeric intracellular cation channels permeable for potassium, ATP-sensitive potassium channels, calcium-activated potassium channels and the potassium-hydrogen exchanger. Apart from trimeric intracellular cation channels, which are specific to the endoplasmic reticulum, other potassium channels are also expressed in the plasma membrane and/or mitochondria, and their specific role in the endoplasmic reticulum has not yet been fully established. In addition to these potassium-selective channels, the ryanodine receptor and, potentially, the inositol 1,4,5-trisphosphate receptor are permeable to potassium ions. Also, the role of potassium fluxes across the endoplasmic reticulum membrane has remained elusive. It has been proposed that their main role is to balance the charge movement that occurs during calcium release and uptake from or to the endoplasmic reticulum. This review aims to summarize current knowledge on endoplasmic reticulum potassium channels and fluxes and their potential role in endoplasmic reticulum calcium uptake and release.

Chloride channel blockade relaxes airway smooth muscle and potentiates relaxation by β-agonists

Severe bronchospasm refractory to β-agonists continues to cause significant morbidity and mortality in asthmatic patients. We questioned whether chloride channels/transporters are novel targets for the relaxation of airway smooth muscle (ASM). We have screened a library of compounds, derivatives of anthranilic and indanyloxyacetic acid, that were originally developed to antagonize chloride channels in the kidney. We hypothesized that members of this library would be novel calcium-activated chloride channel blockers for the airway. The initial screen of this compound library identified 4 of 20 compounds that relaxed a tetraethylammonium chloride-induced contraction in guinea pig tracheal rings. The two most effective compounds, compounds 1 and 13, were further studied for their potential to either prevent the initiation of or relax the maintenance phase of an acetylcholine (ACh)-induced contraction or to potentiate β-agonist-mediated relaxation. Both relaxed an established ACh-induced contraction in human and guinea pig ex vivo ASM. In contrast, the prevention of an ACh-induced contraction required copretreatment with the sodium-potassium-chloride cotransporter blocker bumetanide. The combination of compound 13 and bumetanide also potentiated relaxation by the β-agonist isoproterenol in guinea pig tracheal rings. Compounds 1 and 13 hyperpolarized the plasma cell membrane of human ASM cells and blocked spontaneous transient inward currents, a measure of chloride currents in these cells. These functional and electrophysiological data suggest that modulating ASM chloride flux is a novel therapeutic target in asthma and other bronchoconstrictive diseases.

Contribution of Ca²⁺-dependent Cl⁻ channels to norepinephrine-induced contraction of femoral artery is replaced by increasing EDCF contribution during ageing

The activation of Ca²⁺-dependent Cl⁻ channels during norepinephrine-induced contraction of vascular smooth muscle was suggested to depolarize cell membrane and to increase Ca²⁺ entry. Hypertension and ageing are associated with altered Ca²⁺ handling including possible activation of Ca²⁺-dependent Cl⁻ channels. Our study was aimed to determine Ca²⁺-dependent Cl⁻ channels contribution to norepinephrine-induced contraction during hypertension and ageing. Norepinephrine-induced concentration-response curves of femoral arteries from 6- and 12-month-old spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were recorded using wire myograph. Pretreatment with Ca²⁺-dependent Cl- channel inhibitor indanyloxyacetic acid 94 [R(+)-IAA-94](IAA) attenuated norepinephrine-induced contraction in all groups, but relatively more in WKY than SHR arteries. The attenuation of norepinephrine-induced contraction after Ca²⁺-dependent Cl⁻ channels blockade was partially reduced in 12-month-old WKY rats, but substantially diminished in 12-month-old SHR. IAA effect was enhanced after NO synthase inhibition but decreased by ageing. In 20-month-old WKY rats norepinephrine-induced contraction was not affected by IAA but was almost abolished after cyclooxygenase inhibition by indomethacin or niflumic acid. In conclusion, contribution of Ca²⁺-dependent Cl⁻ channels to norepinephrine-induced contraction diminished with age, hypertension development, and/or NO synthesis inhibition. Ca²⁺-dependent Cl⁻ channels are important for maintenance of normal vascular tone while their inactivation/closing might be a pathological mechanism.

How the airway smooth muscle in cystic fibrosis reacts in proinflammatory conditions: implications for airway hyper-responsiveness and asthma in cystic fibrosis

Among patients with cystic fibrosis there is a high prevalence (40-70%) of asthma signs and symptoms such as cough and wheezing and airway hyper-responsiveness to inhaled histamine or methacholine. Whether these abnormal airway responses are due to a primary deficiency in the cystic fibrosis transmembrane conductance regulator (CFTR) or are secondary to the inflammatory environment in the cystic fibrosis lungs is not clear. A role for the CFTR in smooth muscle function is emerging, and alterations in contractile signalling have been reported in CFTR-deficient airway smooth muscle. Persistent bacterial infection, especially with Pseudomonas aeruginosa, stimulates interleukin-8 release from the airway epithelium, resulting in neutrophilic inflammation. Increased neutrophilia and skewing of CFTR-deficient T-helper cells to type 2 helper T cells creates an inflammatory environment characterised by high concentrations of tumour necrosis factor α, interleukin-8, and interleukin-13, which might all contribute to increased contractility of airway smooth muscle in cystic fibrosis. An emerging role of interleukin-17, which is raised in patients with cystic fibrosis, in airway smooth muscle proliferation and hyper-responsiveness is apparent. Increased understanding of the molecular mechanisms responsible for the altered smooth muscle physiology in patients with cystic fibrosis might provide insight into airway dysfunction in this disease.

Airway smooth muscle electrophysiology in a state of flux?

Activation of chloride currents and release of internally sequestered Ca2+ in airway smooth muscle have long been associated with excitation and contraction. Surprisingly, however, two recent publications (Deshpande DA, Wang WC, McIlmoyle EL, Robinett KS, Schillinger RM, An SS, Sham JS, Liggett SB. Nat Med 16: 1299–1304, 2010; Gallos G, Yim P, Chang S, Zhang Y, Xu D, Cook JM, Gerthoffer WT, Emala CW Sr. Am J Physiol Lung Cell Mol Physiol 302: L248–L256, 2012) have linked both events to relaxation. This begs a closer look at our understanding of airway smooth muscle electrophysiology and its contribution to excitation-contraction coupling. This Editorial Focus highlights those two aforementioned studies and several other equally paradoxical findings and proposes some possible reinterpretations of the data and/or new directions of research in which the answers might be found.

Endoplasmic reticulum potassium–hydrogen exchanger and small conductance calcium-activated potassium channel activities are essential for ER calcium uptake in neurons and cardiomyocytes

Calcium pumping into the endoplasmic reticulum (ER) lumen is thought to be coupled to a countertransport of protons through sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) and the members of the ClC family of chloride channels. However, pH in the ER lumen remains neutral, which suggests a mechanism responsible for proton re-entry. We studied whether cation–proton exchangers could act as routes for such a re-entry. ER Ca2+ uptake was measured in permeabilized immortalized hypothalamic neurons, primary rat cortical neurons and mouse cardiac fibers. Replacement of K+ in the uptake solution with Na+ or tetraethylammonium led to a strong inhibition of Ca2+ uptake in neurons and cardiomyocytes. Furthermore, inhibitors of the potassium–proton exchanger (quinine or propranolol) but not of the sodium–proton exchanger reduced ER Ca2+ uptake by 56–82%. Externally added nigericin, a potassium–proton exchanger, attenuated the inhibitory effect of propranolol. Inhibitors of small conductance calcium-sensitive K+ (SKCa) channels (UCL 1684, dequalinium) blocked the uptake of Ca2+ by the ER in all preparations by 48–94%, whereas inhibitors of other K+ channels (IKCa, BKCa and KATP) had no effect. Fluorescence microscopy and western blot analysis revealed the presence of both SKCa channels and the potassium–proton exchanger leucine zipper-EF-hand-containing transmembrane protein 1 (LETM1) in ER in situ and in the purified ER fraction. The data obtained demonstrate that SKCa channels and LETM1 reside in the ER membrane and that their activity is essential for ER Ca2+ uptake.

Mechanisms of copper accumulation in isolated mantle cells of the marine clam Mesodesma mactroides

In vivo copper accumulation was determined in tissues (mantle, gills, digestive gland, and hemolymph) following exposure to Cu (5 µM) for up to 96 h. Mantle was the tissue that accumulated the most Cu, followed by gill, digestive gland, and hemolymph. Therefore, in vitro Cu accumulation was evaluated in isolated mantle cells exposed to 0.5, 1.0, 2.5, and 5.0 µM Cu for 1 and 3 h. After both exposure times, no change in cell viability was observed. However, a significant Cu accumulation was observed in cells exposed to 2.5 and 5.0 µM Cu. Cell exposure to 2.5 µM Cu for 1 h did not affect the ionic (Na(+), K(+), Ca(2+), and Cl(-)) content of isolated mantle cells, characterizing an "ideal" noneffect concentration for the study of the involvement of different ion-transporting proteins (Na(+), K(+), and Cl(-) channels; Na(+)/K(+) 2Cl(-) and Na(+)/Cl(-) cotransporters; Na(+)/Ca(2+), Cl(-)/HCO3-, and Na(+)/H(+) exchangers; Na(+)/K(+) -ATPase; V-ATPase; and carbonic anhydrase) in Cu accumulation. Isolated cells were pre-exposed (30 min) to specific blockers or inhibitors of the ion-transporting proteins and then exposed (1 h) to Cu (2.5 µM) in the presence of the drug. A significant increase of 29.1 and 24.3% in Cu accumulation was observed after cell incubation with acetozalamide (carbonic anhydrase inhibitor) and NPPB (Cl(-) channels blocker), respectively. On the other hand, a significant decrease (48.2%) in Cu accumulation was observed after incubation with furosemide (Na(+) /K(+)/2Cl(-) blocker). Taken together, these findings indicate the mantle as an important route of Cu entry in M. mactroides, pointing to the cotransporter Na(+)/K(+)/2Cl(-) as a major mechanism of Cu accumulation in mantle cells of the clam.

Chloride channels of intracellular membranes

Proteins implicated as intracellular chloride channels include the intracellular ClC proteins, the bestrophins, the cystic fibrosis transmembrane conductance regulator, the CLICs, and the recently described Golgi pH regulator. This paper examines current hypotheses regarding roles of intracellular chloride channels and reviews the evidence supporting a role in intracellular chloride transport for each of these proteins.

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.

Spontaneous transient depolarizations in lymphatic vessels of the guinea pig mesentery: pharmacology and implication for spontaneous contractility

Guinea pig mesenteric lymphatic vessels exhibit rhythmic constrictions induced by action potential (AP)-like spikes and initiated by entrainment of spontaneous transient depolarizations (STDs). To characterize STDs and the signaling mechanisms responsible for their occurrence, we used intracellular microelectrodes, Ca2+ imaging, and pharmacological agents. In our investigation of the role of intracellular Ca2+ released from Ca2+ stores, we observed that intracellular Ca2+ transients accompanied some STDs, although there were many exceptions where Ca2+ transients occurred without accompanying STDs. STD frequency and amplitude were markedly affected by activators/inhibitors of inositol 1,4,5-trisphosphate receptors (IP3Rs) but not by treatments known to alter Ca2+ release via ryanodine receptors. A role for Ca2+-activated Cl(-) (Cl(Ca)) channels was indicated, as STDs were dependent on the Cl(-) but not Na+ concentration of the superfusing solution and were inhibited by the Cl(Ca) channel blockers niflumic acid (NFA), anthracene 9-carboxylic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid but not by the volume-regulated Cl(-) blocker DIDS. Increases in STD frequency and amplitude induced by agonist stimulation were also inhibited by NFA. Nifedipine, the hyperpolarization-activated inward current blocker ZD-7288, and the nonselective cation/store-operated channel blockers SKF-96365, Gd3+, and Ni2+ had no or marginal effects on STD activity. However, nifedipine, 2-aminoethoxydiphenyl borate, NFA, SKF-96365, Gd3+, and Ni2+ altered the occurrence of spontaneous APs. Our findings support a role for Ca2+ release through IP3Rs and a resultant opening of Cl(Ca) channels in STD generation and confirm the importance of these events in the initiation of lymphatic spontaneous APs and subsequent contractions. The abolition of spontaneous APs by blockers of other excitatory ion channels suggests a contribution of these conductances to lymphatic pacemaking.

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.

Evidence for a large conductance voltage gated cationic channel in rough endoplasmic reticulum of rat hepatocytes

In this work, we report the single channel characterization of a voltage gated cationic channel from rough endoplasmic reticulum (RER) membranes of rat hepatocytes incorporated into a planar lipid bilayer. The channel was found to be cation selective with a main conductance of 598+/-20 pS in 200 mM KCl cis/50 mM KCl trans. The channel open probability appeared voltage dependent with a voltage for half activation (V(1/2)) of 38 mV and an effective gating charge z of -6.66. Adding either 4-AP (5 mM) or ATP (2.5 mM) to the side corresponding to the cell internal medium caused a strong inhibition of the channel activity. This channel is likely to be involved in maintaining proper cation homeostasis in the RER of hepatocytes.

Intracellular Cl- fluxes play a novel role in Ca2+ handling in airway smooth muscle

Intracellular Ca(2+) is actively sequestered into the sarcoplasmic reticulum (SR), whereas the release of Ca(2+) from the SR can be triggered by activation of the inositol 1,4,5-trisphosphate and ryanodine receptors. Uptake and release of Ca(2+) across the SR membrane are electrogenic processes; accumulation of positive or negative charge across the SR membrane could electrostatically hinder the movement of Ca(2+) into or out of the SR, respectively. We hypothesized that the movement of intracellular Cl(-) (Cl(i)(-)) across the SR membrane neutralizes the accumulation of charge that accompanies uptake and release of Ca(2+). Thus inhibition of SR Cl(-) fluxes will reduce Ca(2+) sequestration and agonist-induced release. The Cl(-) channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 10(-4) M), previously shown to inhibit SR Cl(-) channels, significantly reduced the magnitude of successive acetylcholine-induced contractions of airway smooth muscle (ASM), suggesting a "run down" of sequestered Ca(2+) within the SR. Niflumic acid (10(-4) M), a structurally different Cl(-) channel blocker, had no such effect. Furthermore, NPPB significantly reduced caffeine-induced contraction and increases in intracellular Ca(2+) concentration ([Ca(2+)](i)). Depletion of Cl(i)(-), accomplished by bathing ASM strips in Cl(-)-free buffer, significantly reduced the magnitude of successive acetylcholine-induced contractions. In addition, Cl(-) depletion significantly reduced caffeine-induced increases in [Ca(2+)](i). Together these data suggest a novel role for Cl(i)(-) fluxes in Ca(2+) handling in smooth muscle. Because the release of sequestered Ca(2+) is the predominate source of Ca(2+) for contraction of ASM, targeting Cl(i)(-) fluxes may prove useful in the control of ASM hyperresponsiveness associated with asthma.

Looking chloride channels straight in the eye: bestrophins, lipofuscinosis, and retinal degeneration

Recent evidence suggests that Cl(-) ion channels are important for retinal integrity. Bestrophin Cl(-) channel mutations in humans are genetically linked to a juvenile form of macular degeneration, and disruption of some ClC Cl(-) channels in mice leads to retinal degeneration. In both cases, accumulation of lipofuscin pigment is a key feature of the cellular degeneration. Because Cl(-) channels regulate the ionic environment inside organelles in the endosomal-lysosomal pathway, retinal degeneration may result from defects in lysosomal trafficking or function.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Inhibition of SERCA2 Ca(2+)-ATPases by Cs(+)

Replacement of K(+) with Cs(+) on the cytoplasmic side of the sarcoplasmic reticulum (SR) membrane reduces the maximum velocity (V(max)) of Ca(2+) uptake into the SR of saponin-permeabilized rat ventricular myocytes. To compare the sensitivity of the cardiac and smooth muscle/non-muscle forms of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA2a and -2b respectively) to replacement of K(+) with Cs(+), SERCA2a and SERCA2b were expressed in HEK-293 cells. Ca(2+) uptake into HEK cell microsomes was inhibited by replacement of extravesicular K(+) with Cs(+) (V(max) of SERCA2a-mediated Ca(2+) uptake in CsCl was 80% of that in KCl; V(max) of SERCA2b-mediated uptake was 70% of that in KCl). The Ca(2+) sensitivity of uptake was decreased for both SERCA2a- and SERCA2b-mediated uptake and the Hill coefficients were increased in the presence of CsCl. The effects of Cs(+) on uptake were associated with direct inhibition of the ATPase activity of SERCA2a and SERCA2b. Our results indicate that cation binding sites are present in both SERCA2 isoforms, although the extent to which SERCA2b is inhibited by K(+) replacement is greater than that of SERCA2a or SERCA1. Consideration of these results and the recent molecular modeling work of others suggests that monovalent cations could interact with the Ca(2+) binding region of SERCA.

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.

Wolframin expression induces novel ion channel activity in endoplasmic reticulum membranes and increases intracellular calcium

Wolfram syndrome is an autosomal recessive neuro-degenerative disorder associated with juvenile onset non-autoimmune diabetes mellitus and progressive optic atrophy. The disease has been attributed to mutations in the WFS1 gene, which codes for a protein predicted to possess 9-10 transmembrane segments. Little is known concerning the function of the WFS1 protein (wolframin). Endoglycosidase H digestion, immunocytochemistry, and subcellular fractionation studies all indicated that wolframin is localized to the endoplasmic reticulum in rat brain hippocampus and rat pancreatic islet beta-cells, and after ectopic expression in Xenopus oocytes. Reconstitution of wolframin from oocyte membranes into planar lipid bilayers demonstrated that the protein induced a large cation-selective ion channel that was blocked by Mg2+ or Ca2+. Inositol triphosphate was capable of activating channels in the fused bilayers that were similar to channel components induced by wolframin expression. Expression of wolframin also increased cytosolic calcium levels in oocytes. Wolframin thus appears to be important in the regulation of intracellular Ca2+ homeostasis. Disruption of this function may place cells at risk to suffer inappropriate death decisions, thus accounting for the progressive beta-cell loss and neuronal degeneration associated with the disease.

Ca2+ regulation in the near-membrane microenvironment in smooth muscle cells

The microenvironment between the plasma membrane and the near-membrane sarcoplasmic reticulum (SR) may play an important role in Ca(2+) regulation in smooth muscle cells. We used a three-dimensional mathematical model of Ca(2+) diffusion and regulation and experimental measurements of SR Ca(2+) uptake and the distribution of the SR in isolated smooth muscle cells to predict the extent that the near-membrane SR could load Ca(2+) after the opening of single plasma membrane Ca(2+) channels. We also modeled the effect of SR uptake on 1), single-channel Ca(2+) transients in the near-membrane space; 2), the association of Ca(2+) with Ca(2+) buffers in this space; and 3), the amount of Ca(2+) reaching the central cytoplasm of the cell. Our results indicate that, although single-channel Ca(2+) transients could increase SR Ca(2+) to a certain extent, SR Ca(2+) uptake is not rapid enough to greatly affect the magnitude of these transients or their spread to the central cytoplasm unless the Ca(2+) uptake rate of the peripheral SR is an order-of-magnitude higher than the mean rate derived from our experiments. Immunofluorescence imaging, however, did not reveal obvious differences in the density of SR Ca(2+) pumps or phospholamban between the peripheral and central SR in smooth muscle cells.

Effects of 5-nitro-2-(3-phenylpropylamino) benzoic acid, anthracene-9-carboxylate, and zaprinast on endothelin-1-induced contractions of pregnant rat myometrium

OBJECTIVE

The effects of 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), anthracene-9-carboxylate (9-AC) (chloride channel blockers) and zaprinast (an inhibitor of phosphodiesterase-5) on endothelin-1 (ET-1) induced contractions of pregnant rat myometrium were investigated in vitro.

METHODS

Isolated myometrial strips were obtained from pregnant rats, and the strips were mounted in organ baths for recording of isometric tension (n=8). The effects of 10(-7) to 10(-4)M NPPB, 10(-7) to 10(-4)M 9-AC, and 10(-7) to 10(-4)M zaprinast on 10(-8)M ET-1-induced contractions of pregnant rat myometrial smooth muscle were recorded.

RESULTS

NPPB and 9-AC increased the amplitude of ET-1-induced myometrial contractions, while decreasing the frequency, in a concentration-dependent manner. The amplitude of myometrial contractions were significantly increased by NPPB and 9-AC beginning from the concentration of 10(-6)M. The frequency of myometrial contractions were significantly decreased by NPPB and 9-AC beginning from the concentration of 10(-6)M. Zaprinast inhibited the amplitude and frequency of ET-1-induced myometrial contractions in a concentration-dependent manner. Zaprinast-induced decreases in amplitude and frequency of myometrial contractions reached statistical significance beginning from the concentrations of 10(-7)M and 10(-5)M, respectively.

CONCLUSION

These data provide evidence that the ET-1-induced contractions of pregnant rat myometrial strips may be modulated by chloride channels. In addition, zaprinast effectively inhibited ET-1-induced contractions in pregnant rat myometrium.

Stimulation-dependent regulation of the pH, volume and quantal size of bovine and rodent secretory vesicles

Trapping of weak bases was utilized to evaluate stimulus-induced changes in the internal pH of the secretory vesicles of chromaffin cells and enteric neurons. The internal acidity of chromaffin vesicles was increased by the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; in vivo and in vitro) and by high K+ (in vitro); and in enteric nerve terminals by exposure to veratridine or a plasmalemmal [Ca2+]o receptor agonist (Gd3+). Stimulation-induced acidification of chromaffin vesicles was [Ca2+]o-dependent and blocked by agents that inhibit the vacuolar proton pump (vH+-ATPase) or flux through Cl- channels. Stimulation also increased the average volume of chromaffin vesicles and the proportion that displayed a clear halo around their dense cores (called active vesicles). Stimulation-induced increases in internal acidity and size were greatest in active vesicles. Stimulation of chromaffin cells in the presence of a plasma membrane marker revealed that membrane was internalized in endosomes but not in chromaffin vesicles. The stable expression of botulinum toxin E to prevent exocytosis did not affect the stimulation-induced acidification of the secretory vesicles of mouse neuroblastoma Neuro2A cells. Stimulation-induced acidification thus occurs independently of exocytosis. The quantal size of secreted catecholamines, measured by amperometry in cultured chromaffin cells, was found to be increased either by prior exposure to L-DOPA or stimulation by high K+, and decreased by inhibition of vH+-ATPase or flux through Cl- channels. These observations are consistent with the hypothesis that the content of releasable small molecules in secretory vesicles is increased when the driving force for their uptake is enhanced, either by increasing the transmembrane concentration or pH gradients.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Chloride channels and hepatocellular function: prospects for molecular identification

Hepatocytes possess chloride channels at the plasma membrane and in multiple intracellular compartments. These channels are required for cell volume regulation and acidification of intracellular organelles. Evidence also supports a role of chloride channels in modulation of apoptosis and cell growth. Swelling- and Ca(2+)-activated chloride channels have been identified in hepatocyte plasma membranes, and chloride channels have been observed in the membranes of lysosomes, endosomes, Golgi, endoplasmic reticulum, mitochondria, and the nucleus. This review summarizes the functions of these channels and discusses the specific channel molecules they may represent. Chloride channel molecules shown to be expressed in hepatocytes include members of the ClC channel family (ClC-2, ClC-3, ClC-5, and ClC-7), members of the newly identified CLIC family of intracellular chloride channels (CLIC-1 and CLIC-4), the mitochondrial voltage-dependent anion channel, and a newly identified intracellular channel, MCLC (Mid-1 related chloride channel). Current understanding does not include a molecular identification of most of the observed channel functions, but details of the molecular properties of these channel molecules should allow future identification and further understanding of chloride channel function in hepatocytes.

Actions of putative chloride channel blocking agents on canine lower esophageal sphincter (LES)

Niflumic acid (NA), a putative Cl–-channel blocker, has provided pharmacological evidence that Cl–-channel closures mediate hyperpolarization caused by NO in gastrointestinal smooth muscle. However, NA caused concentration- dependent relaxation of canine lower esophageal sphincter (LES) and failed to inhibit NO-mediated relaxations. DIDS also did not inhibit NO-mediated relaxations, but did abolish them when present with 20 mM TEA (tetraethyl ammonium ion), which was also ineffective alone. TEA reversed NA-induced relaxations, but with NA it did not inhibit NO-mediated relaxations. We investigated the modes of action of these agents further. Neither nerve-function block nor block of NOS activity affected the inhibition of LES tone by NA. In patch-clamp studies, NA increased outward currents from –30 to + 90 mV when [Ca2+]pipette was 50 nM. This was prevented by 20 mM TEA, but not by prior inhibition of NOS. At 200 nM [Ca2+]pipette, TEA markedly reduced outward currents, but did not prevent the increase from subsequent NA. In contrast, under similar conditions, application of DIDS after 20 mM TEA further reduced outward currents. When the patch pipette contained CsCl and TEA to block K+ currents, NA had no significant effect on currents between –50 and +90 mV. Thus, NA acted by opening K+ channels: some TEA-sensitive and some not. It had no detectable effect on currents when K+ channels were blocked. We conclude that NA is an unreliable pharmacological tool to evaluate Cl–-channel contributions to smooth muscle function. DIDS did not open K+ channels. Decreases in outward currents from DIDS may result from inhibition of K+ currents or currents carried by Cl– at depolarized membrane potentials.Key words: DIDS, niflumic acid, NO actions, smooth muscle.

Chloride in smooth muscle

Interest in the functions of intracellular chloride expanded about twenty years ago but mostly this referred to tissues other than smooth muscle. On the other hand, accumulation of chloride above equilibrium seems to have been recognised more readily in smooth muscle. Experimental data is used to show by calculation that the Donnan equilibrium cannot account for the chloride distribution in smooth muscle but it can in skeletal muscle. The evidence that chloride is normally above equilibrium in smooth muscle is discussed and comparisons are made with skeletal and cardiac muscle. The accent is on vascular smooth muscle and the mechanisms of accumulation and dissipation. The three mechanisms by which chloride can be accumulated are described with some emphasis on calculating the driving forces, where this is possible. The mechanisms are chloride/bicarbonate exchange, (Na+K+Cl) cotransport and a novel entity, "pump III", known only from own work. Their contributions to chloride accumulation vary and appear to be characteristic of individual smooth muscles. Thus, (Na+K+Cl) always drives chloride inwards, chloride/bicarbonate exchange is always present but does not always do it and "pump III" is not universal. Three quite different biophysical approaches to assessing chloride permeability are considered and the calculations underlying them are worked out fully. Comparisons with other tissues are made to illustrate that low chloride permeability is a feature of smooth muscle. Some of the functions of the high intracellular chloride concentrations are considered. This includes calculations to illustrate its depolarising influence on the membrane potential, a concept which, experience tells us, some people find confusing. The major topic is the role of chloride in the regulation of smooth muscle contractility. Whilst there is strong evidence that the opening of the calcium-dependent chloride channel leads to depolarisation, calcium entry and contraction in some smooth muscles, it appears that chloride serves a different function in others. Thus, although activation and inhibition of (Na+K+Cl) cotransport is associated with contraction and relaxation respectively, the converse association of inhibition and contraction has been seen. Nevertheless, inhibition of chloride/bicarbonate exchange and "pump III" and stimulation of (K+Cl) cotransport can all cause relaxation and this suggests that chloride is always involved in the contraction of smooth muscle. The evidence that (Na+K+Cl) cotransport more active in experimental hypertension is discussed. This is a common but not universal observation. The information comes almost exclusively from work on cultured cells, usually from rat aorta. Nevertheless, work on smooth muscle freshly isolated from hypertensive rats confirms that (Na+K+Cl) cotransport is activated in hypertension but there are several other differences, of which the depolarisation of the membrane potential may be the most important.Finally, a simple calculation is made which indicates as much as 40% of the energy put into the smooth muscle cell membrane by the sodium pump is necessary to drive (Na+K+Cl) cotransport. Notwithstanding the approximations in this calculation, this suggests that chloride accumulation is energetically expensive. Presumably, this is related to the apparently universal role of chloride in contraction.

Iodide and bromide inhibit Ca(2+) uptake by cardiac sarcoplasmic reticulum

Recent studies indicate that the Ca(2+) permeability of the sarcoplasmic reticulum (SR) can be affected by its anionic environment. Additionally, anions could directly modulate the SR Ca(2+) pump or the movement of compensatory charge across the SR membrane during Ca(2+) uptake or release. To examine the effect of anion substitution on cardiac SR Ca(2+) uptake, fluorometric Ca(2+) measurements and spectrophotometric ATPase assays were used. Ca(2+) uptake into SR vesicles was inhibited in a concentration-dependent manner when Br(-) or I(-) replaced extravesicular Cl(-) (when Br(-) completely replaced Cl(-), uptake velocity was approximately 70% of control; when I(-) completely replaced Cl(-), uptake velocity was approximately 39% of control). Replacement of Cl(-) with SO(2)(-4) had no effect on SR uptake. Although both I(-) and Br(-) inhibited net Ca(2+) uptake, neither anion directly inhibited the SR Ca(2+) pump nor did they increase the permeability of the SR membrane to Ca(2+). Our results support the hypothesis that an anionic current that occurs during SR Ca(2+) uptake is reduced by the substitution of Br(-) or I(-) for Cl(-).

Chloride channels and their functional roles in smooth muscle tone in the vasculature

Although evidence of important contributions by Cl- channels to agonist-induced currents have been reported in vascular smooth muscle cells, the functional roles played by Cl- channels in the smooth muscle contraction and in setting the membrane potential remain essentially obscure. All of the admittedly few papers published have focused on the physiological roles of Cl- channels in the contraction and membrane depolarization elicited by agonists. At present, it seems likely that in vascular cells: a) Cl- conductance contributes to membrane depolarization, with the subsequent contraction being due to Ca2+ release from the intracellular store sites, and b) Cl- movements through the membrane of the Ca2+ store sites also regulate Ca2+ release and Ca2+ uptake from/into the store sites. As a Ca2+-dependent Cl- current is most easily demonstrated under quasi-physiological conditions (by the perforated patch-clamp method), the contribution made by Cl- channels to smooth muscle function may be more important than previously thought. The development of the new, selective Cl--channel blockers as well as the identification and gene engineering of the channel molecules are essential if we are to advance our knowledge of the physiology and pharmacology of the Cl- channels residing in vascular smooth muscle cells.

Patch-clamp study of liver nuclear ionic channels reconstituted into giant proteoliposomes

Nuclear ionic channels (NICs) represent ubiquitous structures of living cells, although little is known about their functional properties and encoding genes. To characterize NICs, liver nuclear membrane vesicles were reconstituted into either planar lipid bilayers or proteoliposomes. Reconstitution of nuclear envelope (NE) vesicles into planar lipid bilayer proceeded with low efficiency. NE vesicle reconstitution into proteoliposomes led to NIC observations by the patch-clamp technique. Large conductance, voltage-gated, K(+)-permeant and Cl(-)-permeant NICs were characterized. An 80-105-pS K(+)-permeant NIC with conducting sub-state was also recorded. Our data establish that NICs can be characterized upon reconstitution into giant proteoliposomes and retain biophysical properties consistent with those described for native NICs.

Distinct ion channel classes are expressed on the outer nuclear envelope of T- and B-lymphocyte cell lines

The outer nuclear membrane, endoplasmic reticulum, and mitochondrial membrane ion channels are poorly understood, although they are important in the control of compartmental calcium levels, cell division, and apoptosis. Few direct recordings of these ion channels have been made because of the difficulty of accessing these intracellular membranes. Using patch-clamp techniques on isolated nuclei, we measured distinct ion channel classes on the outer nuclear envelope of T-cell (human Jurkat) and BFL5 cell (murine promyelocyte) lines. We first imaged the nuclear envelopes of both Jurkat and FL5 cells with atomic force microscopy to determine the density of pore proteins. The nuclear pore complex was intact at roughly similar densities in both cell types. In patch-clamp recordings of Jurkat nuclear membranes, Cl channels (105 +/- 5 pS) predominated and inactivated with negative pipette potentials. Nucleotides transiently inhibited the anion channel. In contrast, FL5 nuclear channels were cation selective (52 +/- 2 pS), were inactivated with positive membrane potentials, and were insensitive to GTPgammaS applied to the bath. We hypothesize that T- and B-cell nuclear membrane channels are distinct, and that this is perhaps related to their unique roles in the immune system.

Neuroeffector transmission to different layers of smooth muscle in the rat penile bulb

1. Using intracellular recording techniques, two distinct layers of smooth muscle were identified in the rat penile bulb. The inner muscle layer (parenchyma) exhibited spontaneous action potentials, while the outer sheet (sac) was electrically quiescent. 2. In the parenchyma, transmural stimulation initiated non-adrenergic, non-cholinergic (NANC) inhibitory junction potentials (IJPs) which were abolished by Nomeganitro-L-arginine (LNA) or 1H-[1,2, 4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The amplitude of IJPs was reduced by ouabain, dinitrophenol or decreasing the extracellular potassium concentration ([K+]o) but not by several K+ channel blockers. 3. The parenchyma also received an excitatory innervation mediated by alpha-adrenoceptors which caused a contraction that was not associated with a membrane potential change. 4. In the sac, transmural stimulation initiated two component excitatory junction potentials (EJPs) mediated by alpha-adrenoceptors and associated action potentials. The initial component was more dramatically suppressed than the secondary component by caffeine, ryanodine or cyclopiazonic acid (CPA). Lowering of the extracellular chloride concentration ([Cl-]o) selectively inhibited the rapid component of EJPs, while niflumic acid was less potent. 5. These results suggest that IJPs in the parenchyma result from the release of NO which stimulates sodium pump activity following the activation of guanylate cyclase. In the sac, the activation of alpha-adrenoceptors initiates EJPs by releasing Ca2+ from intracellular stores which activates Ca2+-activated channels.

Interactions between Ca(2+) and H(+) and functional consequences in vascular smooth muscle

Ca(2+) and H(+) ions can profoundly alter vascular tone. In many physiological and pathological processes, changes in the concentration of both ions occur. Thus, to understand the processes and mechanisms that modify force, it is necessary to understand what changes occur in these ions and, importantly, how they interact with each other. In this minireview, we highlight the quantitatively important mechanisms involved in the contractile responses of vascular tissues to pH change and discuss the cellular and molecular reasons underlying these responses.