Time course of Ca(2+)-dependent K+ and Cl- currents in single smooth muscle cells of guinea-pig trachea

Reciprocal Relationship between Ca2+ Signaling and Ca2+-Gated Ion Channels as a Potential Target for Drug Discovery

Cellular Ca²⁺ signaling functions as one of the most common second messengers of various signal transduction pathways in cells and mediates a number of physiological roles in a cell-type dependent manner. Ca²⁺ signaling also regulates more general and fundamental cellular activities, including cell proliferation and apoptosis. Among ion channels, Ca²⁺-permeable channels in the plasma membrane as well as endo- and sarcoplasmic reticulum membranes play important roles in Ca²⁺ signaling by directly contributing to the influx of Ca²⁺ from extracellular spaces or its release from storage sites, respectively. Furthermore, Ca²⁺-gated ion channels in the plasma membrane often crosstalk reciprocally with Ca²⁺ signals and are central to the regulation of cellular functions. This review focuses on the physiological and pharmacological impact of i) Ca²⁺-gated ion channels as an apparatus for the conversion of cellular Ca²⁺ signals to intercellularly propagative electrical signals and ii) the opposite feedback regulation of Ca²⁺ signaling by Ca²⁺-gated ion channel activities in excitable and non-excitable cells.

Roles of LRRC26 as an auxiliary γ1-subunit of large-conductance Ca2+-activated K+ channels in bronchial smooth muscle cells

In visceral smooth muscle cells (SMCs), the large-conductance Ca²⁺-activated K⁺ (BK) channel is one of the key elements underlying a negative feedback mechanism that is essential for the regulation of intracellular Ca²⁺ concentration. Although leucine-rich repeat-containing (LRRC) proteins have been identified as novel auxiliary γ-subunits of the BK channel (BKγ) in several cell types, its physiological roles in SMCs are unclear. The BKγ expression patterns in selected SM tissues were examined using real-time PCR analyses and Western blotting. The functional contribution of BKγ1 to BK channel activity was examined by whole cell patch-clamp in SMCs and heterologous expression systems. BKγ1 expression in mouse bronchial SMCs (mBSMCs) was higher than in other several SMC types. Coimmunoprecipitation and total internal reflection fluorescence imaging analyses revealed molecular interaction between BKα and BKγ1 in mBSMCs. Under voltage-clamp, steady-state activation of BK channel currents at pCa 8.0 in mBSMCs occurred in a voltage range comparable to that of reconstituted BKα/BKγ1 complex. However, this range was much more negative than in mouse aortic SMCs (mASMCs) or in HEK293 cells expressing BKα alone and β-subunit (BKβ1). Mallotoxin, a selective activator of BK channel that lacks BKγ1, dose-dependently activated BK currents in mASMCs but not in mBSMCs. The abundant expression of BKγ1 in mBSMCs extensively facilitates BK channel activity to keep the resting membrane potential at negative values and prevents contraction under physiological conditions.

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.

Calcium signalling in smooth muscle

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

Calcium events in smooth muscles and their interstitial cells; physiological roles of sparks

The observation of spontaneous sporadic releases of packets of stored calcium made 20 years ago has opened up a number of new concepts in smooth muscle physiology: (1) the calcium release sites are ryanodine and inositol 1,4,5-trisphosphate (IP3) receptor channels which contribute to cell-wide increases in [Ca2+]i in response to cell depolarization, activation of IP3-generating receptors, or other stimuli; (2) changes in [Ca2+]i act back on the cell membrane to activate or modulate K+, Cl- and cation channel activity so affecting contraction, in arterial smooth muscle for example affecting blood pressure; (3) IP3 production is voltage dependent and is believed to contribute to pacemaker potentials and to refractory periods which control the rhythmical motility of many hollow organs. Most smooth muscle tissues contain interstitial cells (ICs) in addition to contractile smooth muscle cells (SMCs). The interactions of these internal mechanisms, and in turn the interactions of SMCs and ICs in various smooth muscle tissues, are major factors in determining the unique physiological profiles of individual smooth muscles.

Calcium-activated chloride channels

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

Signaling between SR and plasmalemma in smooth muscle: sparks and the activation of Ca2+-sensitive ion channels

Intracellular calcium ions are involved in the regulation of nearly every aspect of cell function. In smooth muscle, Ca2+ can be delivered to Ca2+-sensitive effector molecules either by influx through plasma membrane ion channels or by intracellular Ca2+ release events. Ca2+ sparks are transient local increases in intracellular Ca2+ that arise from the opening of ryanodine-sensitive Ca2+ release channels (ryanodine receptors) located in the sarcoplasmic reticulum. In arterial myocytes, Ca2+ sparks occur near the plasma membrane and act to deliver high (microM) local Ca2+ to plasmalemmal Ca2+-sensitive ion channels, without directly altering global cytosolic Ca2+ concentrations. The two major ion channel targets of Ca2+ sparks are Ca2+-activated chloride (Cl(Ca)) channels and large-conductance Ca2+-activated potassium (BK) channels. The activation of BK channels by Ca2+ sparks play an important role in the regulation of arterial diameter and appear to be involved in the action of a variety of vasodilators. The coupling of Ca2+ sparks to BK channels can be influenced by a number of factors including membrane potential and modulatory beta subunits of BK channels. Cl(Ca) channels, while not present in all smooth muscle, can also be activated by Ca2+ sparks in some types of smooth muscle. Ca2+ sparks can also influence the activity of Ca2+-dependent transcription factors and expression of immediate early response genes such as c-fos. In summary, Ca2+ sparks are local Ca2+ signaling events that in smooth muscle can act on plasma membrane ion channels to influence excitation-contraction coupling as well as gene expression.

Activation of Ca2+-activated Cl- current by depolarizing steps in rabbit urethral interstitial cells

Interstitial cells were isolated from strips of rabbit urethra for study using the amphotericin B perforated-patch technique. Depolarizing steps to -30 mV or greater activated a Ca2+ current (ICa), followed by a Ca2+-activated Cl- current, and, on stepping back to -80 mV, large Cl- tail currents were observed. Both currents were abolished when the cells were superfused with Ca2+-free bath solution, suggesting that Ca2+ influx was necessary for activation of the Cl- current. The Cl- current was also abolished when Ba2+ was substituted for Ca2+ in the bath or the cell was dialyzed with EGTA (2 mM). The Cl- current was also reduced by cyclopiazonic acid, ryanodine, 2-aminoethoxydiphenyl borate (2-APB), and xestospongin C, suggesting that Ca2+-induced Ca2+ release (CICR) involving both ryanodine and inositol 1,4,5-trisphosphate receptors contributes to its activation.

Mechanism of generation of spontaneous miniature outward currents (SMOCs) in retinal amacrine cells

A subtype of retinal amacrine cells displayed a distinctive array of K(+) currents. Spontaneous miniature outward currents (SMOCs) were observed in the narrow voltage range of -60 to -40 mV. Depolarizations above approximately -40 mV were associated with the disappearance of SMOCs and the appearance of transient (I(to)) and sustained (I(so)) outward K(+) currents. I(to) appeared at about -40 mV and its apparent magnitude was biphasic with voltage, whereas I(so) appeared near -30 mV and increased linearly. SMOCs, I(to), and a component of I(so) were Ca(2+) dependent. SMOCs were spike shaped, occurred randomly, and had decay times appreciably longer than the time to peak. In the presence of cadmium or cobalt, SMOCs with pharmacologic properties identical to those seen in normal Ringer's could be generated at voltages of -20 mV and above. Their mean amplitude was Nernstian with respect to [K(+)](ext) and they were blocked by tetraethylammonium. SMOCs were inhibited by iberiotoxin, were insensitive to apamin, and eliminated by nominally Ca(2+)-free solutions, indicative of BK-type Ca(2+)-activated K(+) currents. Dihydropyridine Ca(2+) channel antagonists and agonists decreased and increased SMOC frequencies, respectively. Ca(2+) permeation through the kainic acid receptor had no effect. Blockade of organelle Ca(2+) channels by ryanodine, or intracellular Ca(2+) store depletion with caffeine, eradicated SMOCs. Internal Ca(2+) chelation with 10 mM BAPTA eliminated SMOCs, whereas 10 mM EGTA had no effect. These results suggest a mechanism whereby Ca(2+) influx through L-type Ca(2+) channels and its subsequent amplification by Ca(2+)-induced Ca(2+) release via the ryanodine receptor leads to a localized elevation of internal Ca(2+). This amplified Ca(2+) signal in turn activates BK channels in a discontinuous fashion, resulting in randomly occurring SMOCs.

Neuronal Ca2+ -activated Cl- channels--homing in on an elusive channel species

Ca2+ -activated Cl- channels control electrical excitability in various peripheral and central populations of neurons. Ca2+ influx through voltage-gated or ligand-operated channels, as well as Ca2+ release from intracellular stores, have been shown to induce substantial Cl- conductances that determine the response to synaptic input, spike rate, and the receptor current of various kinds of neurons. In some neurons, Ca2+ -activated Cl- channels are localized in the dendritic membrane, and their contribution to signal processing depends on the local Cl- equilibrium potential which may differ considerably from those at the membranes of somata and axons. In olfactory sensory neurons, the channels are expressed in ciliary processes of dendritic endings where they serve to amplify the odor-induced receptor current. Recent biophysical studies of signal transduction in olfactory sensory neurons have yielded some insight into the functional properties of Ca2+ -activated Cl- channels expressed in the chemosensory membrane of these cells. Ion selectivity, channel conductance, and Ca2+ sensitivity have been investigated, and the role of the channels in the generation of receptor currents is well understood. However, further investigation of neuronal Ca2+ -activated Cl- channels will require information about the molecular structure of the channel protein, the regulation of channel activity by cellular signaling pathways, as well as the distribution of channels in different compartments of the neuron. To understand the physiological role of these channels it is also important to know the Cl- equilibrium potential in cells or in distinct cell compartments that express Ca2+ -activated Cl- channels. The state of knowledge about most of these aspects is considerably more advanced in non-neuronal cells, in particular in epithelia and smooth muscle. This review, therefore, collects results both from neuronal and from non-neuronal cells with the intent of facilitating research into Ca2+ -activated Cl- channels and their physiological functions in neurons.

Effect of exhaustive exercise on membrane estradiol concentration, intracellular calcium, and oxidative damage in mouse thymic lymphocytes

Early Ca2+ signaling events in cells of the immune system after exhaustive exercise challenge (8% slope, 32 m/min(-1) speed) of female C57BL/6 mice, and their effects on oxidative reactions in thymus were studied. Intracellular Ca2+ and the oscillation of free extracellular Ca2+ were imaged with cell permeant cell and cell impermeant Fluo 3 calcium indicator in thymocytes. The role of estradiol was assessed by RIA for levels of membrane bound estradiol. Oxidative product release and membrane lipid peroxide were also evaluated. Intracellular Ca2+ levels were significantly higher in thymocytes of exercised compared with control mice (p < .001). There was a continuous flux of Ca2+ after exercise when cells were monitored in Ca2+ rich medium, with a significant influx between 160 and 200 sec (p < .001). Membrane bound estradiol was elevated in thymocytes of exercised compared to control mice (p < .05). Immediately after exercise there was a greater release of oxidative products by thymocytes in exhaustively exercised compared with control animals. There was also significant generation of lipid peroxide in thymus of exercised mice (p < .001). The findings suggest that exhaustive exercise may stimulate estradiol uptake by receptors on thymocytes, with a possible opening up of estradiol-receptor operated channels for Ca2+ entry into cells. This may have damaging effects on thymic lymphocytes by the triggering of oxidative reactions as determined by higher oxidative product release and greater generation of lipid peroxide.

Ca2+ sparks activate K+ and Cl- channels, resulting in spontaneous transient currents in guinea-pig tracheal myocytes

1. Local changes in cytosolic [Ca2+] were imaged with a wide-field, high-speed, digital imaging system while membrane currents were simultaneously recorded using whole-cell, perforated patch recording in freshly dissociated guinea-pig tracheal myocytes. 2. Depending on membrane potential, Ca2+ sparks triggered 'spontaneous' transient inward currents (STICs), 'spontaneous' transient outward currents (STOCs) and biphasic currents in which the outward phase always preceded the inward (STOICs). The outward currents resulted from the opening of large-conductance Ca2+-activated K+ (BK) channels and the inward currents from Ca2+-activated Cl- (ClCa) channels. 3. A single Ca2+ spark elicited both phases of a STOIC, and sparks originating from the same site triggered STOCs, STICs and STOICs, depending on membrane potential. 4. STOCs had a shorter time to peak (TTP) than Ca2+ sparks and a much shorter half-time of decay. In contrast, STICs had a somewhat longer TTP than sparks but the same half-time of decay. Thus, the STIC, not the STOC, more closely reflected the time course of cytosolic Ca2+ elevation during a Ca2+ spark. 5. These findings suggest that ClCa channels and BK channels may be organized spatially in quite different ways in relation to points of Ca2+ release from intracellular Ca2+ stores. The results also suggest that Ca2+ sparks may have functions in smooth muscle not previously suggested, such as a stabilizing effect on membrane potential and hence on the contractile state of the cell, or as activators of voltage-gated Ca2+ channels due to depolarization mediated by STICs.

Cyclic GMP-dependent but G-kinase-independent inhibition of Ca2+-dependent Cl- currents by NO donors in cat tracheal smooth muscle

1. The effects of NO donors on Ca2+-dependent Cl- currents (ICl(Ca)) were investigated in freshly isolated cat tracheal myocytes using the whole-cell patch clamp technique. 2. With nystatin-perforated whole-cell recording, carbachol (CCh, >/= 1 microM) induced a transient inward current (ICCh) with a reversal potential of about -20 mV. Activation of ICCh probably occurred through the M3 muscarinic receptor, since nanomolar concentrations of 4-diphenylacetoxy-N-methylpiperidine methobromide (4-DAMP) greatly inhibited this current, while 11-(2-(diethylamino)methyl)-1-piperidinylacetyl)-5, 11-dihydro-6H-pyrido (2,3beta) (1,4)benzodiazepine-6-one (AF-DX 116) or pirenzepine at concentrations of up to 1 microM were almost ineffective. 3. Chloride channel/transporter blockers such as DIDS (100 microM), anthracene-9-carboxylic acid (9-AC, 100 microM) and niflumic acid (100 microM) greatly inhibited ICCh, but cation channel blockers, such as nifedipine (10 microM), Zn2+ (500 microM) or Gd3+ (500 microM), were without effect. 4. Activation of ICCh was strongly attenuated by pretreatment with ryanodine (4 microM) plus caffeine (10 mM). Addition of neomycin (1 mM) into the bath or inclusion of heparin (3 mg ml-1) in the pipette abolished a substantial part of ICCh. These results suggest that ICCh is ICl(Ca), which is activated by inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release. 5. The nitric oxide donor S-nitroso-N-acetyl penicillamine (SNAP) reduced the amplitude of ICCh dose dependently (IC50, approximately 10 microM). Similar inhibition was also exerted by other types of NO donor such as glyceryl trinitrate (GTN) and (+/-)-E-methyl-2-(E-hydroxyimitol)-5-nitro-6-methoxy-3- hexeneamide (NO-R). 6. SNAP-induced ICCh inhibition was effectively antagonized by Methylene Blue (1-100 nM), and mimicked by dibutyryl cGMP (db-cGMP) (0.5-1 mM), whereas two structurally distinct types of cGMP-dependent (G)-kinase inhibitor, N-(2-aminoethyl)-5-isoquinilinesulphonamide (H-8, 2.5 microM) and KT5823 (1 microM), failed to counteract the inhibitory effects of SNAP or db-cGMP. Another G-kinase-specific inhibitor Rp-8-(para-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate (Rp-8-pCPT-cGMPS; 1 microM) itself caused a marked reduction in ICCh. 7. SNAP (100 microM) or db-cGMP (100 microM) exhibited no inhibitory actions, when caffeine (10 mM) or photolytically released IP3 were used instead of CCh to activate the inward current. 8. These results suggest that inhibition of ICCh by NO donors involves a cGMP-dependent but G-kinase-independent mechanism, which may operate at a site(s) between the muscarinic (M3) and IP3 receptors.

Direct modulation of tracheal Cl--channel activity by 5,6- and 11,12-EET

Using microelectrode potential measurements, we tested the involvement of Cl- conductances in the hyperpolarization induced by 5,6- and 11,12-epoxyeicosatrienoic acid (EET) in airway smooth muscle (ASM) cells. 5,6-EET and 11,12-EET (0.75 microM) caused -5.4 +/- 1.1- and -3.34 +/- 0.95-mV hyperpolarizations, respectively, of rabbit tracheal cells (from a resting membrane potential of -53.25 +/- 0.44 mV), with significant residual repolarizations remaining after the Ca2+-activated K+ channels had been blocked by 10 nM iberiotoxin. In bilayer reconstitution experiments, we demonstrated that the EETs directly inhibit a Ca2+-insensitive Cl- channel from bovine ASM; 1 microM 5,6-EET and 1.5 microM 11,12-EET lowered the unitary current amplitude by 40 (n = 6 experiments) and 44.7% (n = 4 experiments), respectively. Concentration-dependent decreases in channel open probability were observed, with estimated IC50 values of 0.26 microM for 5,6- and 1.15 microM for 11,12-EET. Furthermore, pharmacomechanical tension measurements showed that both regioisomers induced significant bronchorelaxations in epithelium-denuded ASM strips. These results suggest that 5,6- and 11,12-EET can act in ASM as epithelium-derived hyperpolarizing factors.

Variability in spontaneous subcellular calcium release in guinea-pig ileum smooth muscle cells

1. Spontaneous, localized transient increases in [Ca2+]i ('Ca2+ sparks') were observed in about 40 % of fluo-3-loaded myocytes examined using laser scanning confocal microscopy. Ca2+ sparks persisted after application of Cd2+ (200 microM), but were abolished by ryanodine (30 microM) or thapsigargin (0.1 microM), suggesting that they arise from the spontaneous activation of ryanodine receptors (RyR) in the sarcoplasmic reticulum (SR). 2. Ca2+ sparks occurred much more frequently at certain sites (or 'frequent discharge sites', FDSs) within any confocal plane of the cell and line-scan imaging revealed a wide variation in their spatial size, amplitude and time course. Some spontaneous local transients were very similar to 'Ca2+ sparks' observed in heart, i.e. lasting approximately 200 ms with a peak fluorescence ratio of 1.75 +/- 0.23 (mean +/- s.d., n = 33). Other events were faster and smaller, lasting only approximately 40 ms with a peak normalized fluorescence of 1.36 +/- 0.09 (mean +/- s.d., n = 28). 3. Spontaneous Ca2+ waves with a wide range of propagation velocities (between 30 and 260 micron s-1) were also observed. In about 60 % of records (n = 33), Ca2+ sparks could be detected at the sites of wave initiation. Waves of elevated [Ca2+]i propagated with non-constant velocity and in some cases terminated. These observations could be explained by heterogeneity in the distribution of subcellular release sites as well as variability in the contribution of each release site to the wave. 4. Spontaneous [Ca2+]i transients in single dispersed visceral smooth muscle cells have a wide spectrum of behaviour that is likely to be the result of spatio-temporal recruitment of smaller local events, probably via a calcium-induced calcium release (CICR) mechanism. The spatial non-uniformity of SR and RyR distribution within the cell may account for the existence of 'frequent discharge sites' firing the majority of the smooth muscle Ca2+ sparks and the wide variation in the Ca2+ wave propagation velocities observed.

Spontaneous transient outward currents in smooth muscle cells

Spontaneous transient outward currents (STOCs) lasting about 100 ms occur in single smooth muscle cells and represent the simultaneous opening of up to a hundred calcium-activated potassium (BK) channels. The recent observation of brief focal releases of sarcoplasmic reticulum (SR) calcium ('sparks') in smooth muscle cells has provided support for the original suggestion that STOCs arise due to the spontaneous releases of calcium from the SR close to the sarcolemma. However, it is possible that such releases occur in a region of close apposition of SR membrane and sarcolemma about 0.1 microns wide ('junctional space') in which case they would be detectable by endogenous calcium-sensitive molecules such as BK channels but, using present confocal microscopy technique, not by calcium-indicator dyes introduced into the cell; should calcium escape from the junctional space then it may be visualised as 'sparks' by the fluorescent emission from calcium-indicator dyes using confocal microscopy. Some STOCs seem too large to represent the effect of a single 'spark' and some form of calcium-induced calcium release or 'macrospark' may be involved in their generation. Depletion of calcium stores by caffeine, ryanodine, or by activation of receptors linked to the phospholipase C/inositol trisphosphate system abolishes STOCs. However, low concentrations of caffeine or inositol trisphosphate accelerate STOC discharge by an unknown mechanism and often decrease STOC size presumably by depleting store calcium; similar effects are produced by agents such as cyclopiazonic acid and thapsigargin which inhibit calcium storage mechanisms (largely the SR calcium pump).