Ca(2+)-activated Cl- currents are activated by metabolic inhibition in rat pulmonary artery smooth muscle cells

Inhibitors of connexin and pannexin channels as potential therapeutics

While gap junctions support the exchange of a number of molecules between neighboring cells, connexin hemichannels provide communication between the cytosol and the extracellular environment of an individual cell. The latter equally holds true for channels composed of pannexin proteins, which display an architecture reminiscent of connexin hemichannels. In physiological conditions, gap junctions are usually open, while connexin hemichannels and, to a lesser extent, pannexin channels are typically closed, yet they can be activated by a number of pathological triggers. Several agents are available to inhibit channels built up by connexin and pannexin proteins, including alcoholic substances, glycyrrhetinic acid, anesthetics and fatty acids. These compounds not always strictly distinguish between gap junctions, connexin hemichannels and pannexin channels, and may have effects on other targets as well. An exception lies with mimetic peptides, which reproduce specific amino acid sequences in connexin or pannexin primary protein structure. In this paper, a state-of-the-art overview is provided on inhibitors of cellular channels consisting of connexins and pannexins with specific focus on their mode-of-action and therapeutic potential.

Regulation of ion transport by oxidants

Ion channels perform a variety of cellular functions in lung epithelia. Oxidant- and antioxidant-mediated mechanisms (that is, redox regulation) of ion channels are areas of intense research. Significant progress has been made in our understanding of redox regulation of ion channels since the last Experimental Biology report in 2003. Advancements include: 1) identification of nonphagocytic NADPH oxidases as sources of regulated reactive species (RS) production in epithelia, 2) an understanding that excessive treatment with antioxidants can result in greater oxidative stress, and 3) characterization of novel RS signaling pathways that converge upon ion channel regulation. These advancements, as discussed at the 2013 Experimental Biology Meeting in Boston, MA, impact our understanding of oxidative stress in the lung, and, in particular, illustrate that the redox state has profound effects on ion channel and cellular function.

Chronic hypoxia-induced upregulation of Ca2+-activated Cl- channel in pulmonary arterial myocytes: a mechanism contributing to enhanced vasoreactivity

Chronic hypoxic pulmonary hypertension (CHPH) is associated with altered expression and function of cation channels in pulmonary arterial smooth muscle cells (PASMCs), but little is known for anion channels. The Ca(2+)-activated Cl(-) channel (CaCC), recently identified as TMEM16A, plays important roles in pulmonary vascular function. The present study sought to determine the effects of chronic hypoxia (CH) on the expression and function of CaCCs in PASMCs, and their contributions to the vascular hyperreactivity in CHPH. Male Wistar rats were exposed to room air or 10% O(2) for 3–4 weeks to generate CHPH. CaCC current (I(CI.Ca)) elicited by caffeine-induced Ca(2+) release or by depolarization at a constant high [Ca(2+)](i) (500 or 750 nm) was significantly larger in PASMCs of CH rats compared to controls. The enhanced I(CI.Ca)) density in CH PASMCs was unrelated to changes in amplitude of Ca(2+) release, Ca(2+)-dependent activation, voltage-dependent properties or calcineurin-dependent modulation of CaCCs, but was associated with increased TMEM16A mRNA and protein expression. Maximal contraction induced by serotonin, an important mediator of CHPH, was potentiated in endothelium-denuded pulmonary arteries of CH rats. The enhanced contractile response was prevented by the CaCC blockers niflumic acid and T16A(inh)-A01, or by the L-type Ca(2+) channel antagonist nifedipine. The effects of niflumic acid and nifedipine were non-additive. Our results demonstrate for the first time that CH increases I(CI.Ca) density, which is attributable to an upregulation of TMEM16A expression in PASMCs. The augmented CaCC activity in PASMCs may potentiate membrane depolarization and L-type channel activation in response to vasoconstrictors and enhance pulmonary vasoreactivity in CHPH.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Role of ROS signaling in differential hypoxic Ca2+ and contractile responses in pulmonary and systemic vascular smooth muscle cells

Hypoxia causes a large increase in [Ca2+]i and attendant contraction in pulmonary artery smooth muscle cells (PASMCs), but not in systemic artery SMCs. The different responses meet the respective functional needs in these two distinct vascular myocytes; however, the underlying molecular mechanisms are not well known. We and other investigators have provided extensive evidence to reveal that voltage-dependent K+ (KV) channels, canonical transient receptor potential (TRPC) channels, ryanodine receptor Ca2+ release channels (RyRs), cyclic adenosine diphosphate-ribose, FK506 binding protein 12.6, protein kinase C, NADPH oxidase and reactive oxygen species (ROS) are the essential effectors and signaling intermediates in the hypoxic increase in [Ca2+]i in PASMCs and HPV, but they may not primarily underlie the diverse cellular responses in pulmonary and systemic vascular myocytes. Hypoxia significantly increases mitochondrial ROS generation in PASMCs, which can induce intracellular Ca2+ release by opening RyRs, and may also cause extracellular Ca2+ influx by inhibiting KV channels and activating TRPC channels, leading to a large increase in [Ca2+]i in PASMCs and HPV. In contrast, hypoxia has no or a minor effect on mitochondrial ROS generation in systemic SMCs, thereby causing no change or a negligible increase in [Ca2+]i and contraction. Further preliminary work indicates that Rieske iron-sulfur protein in the mitochondrial complex III may perhaps serve as a key initial molecular determinant for the hypoxic increase in [Ca2+]i in PASMCs and HPV, suggesting its potential important role in different cellular changes to respond to hypoxic stimulation in pulmonary and systemic artery myocytes. All these findings have greatly improved our understanding of the molecular processes for the differential hypoxic Ca2+ and contractile responses in vascular SMCs from distinct pulmonary and systemic circulation systems.

Mechanisms involved in the regulation of bovine pulmonary vascular tone by the 5-HT1B receptor

BACKGROUND AND PURPOSE

5-HT(1B) receptors may have a role in pulmonary hypertension. Their relationship with the activity of BK(Ca,) a T-type voltage-operated calcium channel (VOCC) and cyclic nucleotide-mediated relaxation was examined.

EXPERIMENTAL APPROACH

Ring segments of bovine pulmonary arteries were mounted in organ baths in modified Krebs-Henseleit buffer (37 degrees C) under a tension of 20 mN and gassed with 95% O(2)/5% CO(2). Isometric recordings were made using Chart 5 software.

KEY RESULTS

Contractile responses to 5-HT (10 nM-300 microM) were inhibited similarly by the 5-HT(1B) receptor antagonist SB216641 (100 nM) and the T-type VOCC blockers mibefradil (10 microM) and NNC550396 (10 microM) with no additive effect between SB216641 and mibefradil. Inhibition by SB216641 was prevented by the potassium channel blocker, charybdotoxin (100 nM). 5-HT(1B) receptor activation and charybdotoxin produced a mibefradil-sensitive potentiation of responses to U46619. Bradykinin (0.1 nM-30 microM), sodium nitroprusside (0.01 nM-3 microM), zaprinast (1 nM-3 microM), isoprenaline (0.1 nM-10 microM) and rolipram (1 nM-3 microM) produced 50% relaxation of arteries constricted with 5-HT (1-3 microM) or U46619 (30-50 nM) in the presence of 5-HT(1B) receptor activation, but full relaxation of arteries constricted with U46619, the 5-HT(2A) receptor agonist 2,5 dimethoxy-4 iodoamphetamine (1 microM) or 5-HT in the presence of 5-HT(1B) receptor antagonism. Enhanced relaxation of 5-HT-constricted arteries by cGMP-dependent pathways, seen in the presence of the 5-HT(1B) receptor antagonist, was reversed by charybdotoxin whereas cAMP-dependent relaxation was only partly reversed by charybdotoxin.

CONCLUSIONS AND IMPLICATIONS

5-HT(1B) receptors couple to inhibition of BK(Ca), thus increasing tissue sensitivity to contractile agonists by activating a T-type VOCC and impairing cGMP-mediated relaxation. Impaired cAMP-mediated relaxation was only partly mediated by inhibition of BK(Ca).

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.

Human ClCa1 modulates anionic conduction of calcium-dependent chloride currents

Proteins of the CLCA gene family including the human ClCa1 (hClCa1) have been suggested to constitute a new family of chloride channels mediating Ca(2+)-dependent Cl- currents. The present study examines the relationship between the hClCa1 protein and Ca(2+)-dependent Cl- currents using heterologous expression of hClCa1 in HEK293 and NCIH522 cell lines and whole cell recordings. By contrast to previous reports claiming the absence of Cl- currents in HEK293 cells, we find that HEK293 and NCIH522 cell lines express constitutive Ca(2+)-dependent Cl- currents and show that hClCa1 increases the amplitude of Ca(2+)-dependent Cl- currents in those cells. We further show that hClCa1 does not modify the permeability sequence but increases the Cl- conductance while decreasing the G(SCN-)/G(Cl-) conductance ratio from approximately 2-3 to approximately 1. We use an Eyring rate theory (two barriers, one site channel) model and show that the effect of hClCa1 on the anionic channel can be simulated by its action on lowering the first and the second energy barriers. We conclude that hClCa1 does not form Ca(2+)-dependent Cl- channels per se or enhance the trafficking/insertion of constitutive channels in the HEK293 and NCIH522 expression systems. Rather, hClCa1 elevates the single channel conductance of endogenous Ca(2+)-dependent Cl- channels by lowering the energy barriers for ion translocation through the pore.

Properties of gap junction blockers and their behavioural, cognitive and electrophysiological effects: animal and human studies

Gap junctions play an important role in brain physiology. They synchronize neuronal activity and connect glial cells participating in the regulation of brain metabolism and homeostasis. Gap junction blockers (GJBs) include various chemicals that impair gap junction communication, disrupt oscillatory neuronal activity over a wide range of frequencies, and decrease epileptic discharges. The behavioural and clinical effects of GJBs suggest that gap junctions can be involved in the regulation of locomotor activity, arousal, memory, and breathing. Severe neuropsychiatric side effects suggest the involvement of gap junctions in mechanisms of consciousness. Unfortunately, the available GJBs are not selective and can bind to targets other than gap junctions. Other problems in behavioural studies include the possible adverse effects of GJBs, for example, retinal toxicity and hearing disturbances, changes in blood-brain transport, and the metabolism of other drugs. Therefore, it is necessary to design experiments properly to avoid false, misleading or uninterpretable results. We review the pharmacological properties and electrophysiological, behavioural and cognitive effects of the available gap junction blockers, such as carbenoxolone, glycyrrhetinic acid, quinine, quinidine, mefloquine, heptanol, octanol, anandamide, fenamates, 2-APB, several anaesthetics, retinoic acid, oleamide, spermine, aminosulfonates, and sodium propionate. It is concluded that despite a number of different problems, the currently used gap junction blockers could be useful tools in pharmacology and neuroscience.

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.

Sodium-Calcium Exchanger in Pulmonary Artery Smooth Muscle Cells

The expression and function of the Na+/Ca2+ exchanger (NCX) in the regulation of intracellular Ca2+ homeostasis have been well studied in cardiac, skeletal, and systemic vascular myocytes, but not in pulmonary artery smooth muscle cells (SMCs). We have recently demonstrated that the NCX current is present in freshly isolated pulmonary artery SMCs using the patch-clamp technique. The current has a mean amplitude of 13 pA under near physiological resting conditions. The NCX may function in the forward mode to make a significant contribution to the decay of intracellular Ca2+ following Ca2+ release and/or depolarization. Hypoxic stimulation inhibits the NCX current, reduces the removal of intracellular Ca2+, and enhances Ca2+ release from the sarcoplasmic reticulum. Using RT-PCR, subcloning and sequence analysis, we have shown that three NCX1 splice variants: NCX1.2 (containing exons B, C, and D), NCX1.3 (exons B and D), and NCX1.7 (exons B, D, and F) are expressed in pulmonary artery smooth muscle. Each of these splice variants expressed in HEK293 cells it likely to show a distinct activity in the removal of intracellular Ca2+. Taken together, we provide clear evidence that NCX1 is functionally and molecularly expressed and plays a physiological role in pulmonary artery SMCs.

Type-3 ryanodine receptors mediate hypoxia-, but not neurotransmitter-induced calcium release and contraction in pulmonary artery smooth muscle cells

In this study we examined the expression of RyR subtypes and the role of RyRs in neurotransmitter- and hypoxia-induced Ca²⁺ release and contraction in pulmonary artery smooth muscle cells (PASMCs). Under perforated patch clamp conditions, maximal activation of RyRs with caffeine or inositol triphosphate receptors (IP₃Rs) with noradrenaline induced equivalent increases in [Ca²⁺]_i and Ca²⁺-activated Cl⁻ currents in freshly isolated rat PASMCs. Following maximal IP₃-induced Ca²⁺ release, neither caffeine nor chloro-m-cresol induced a response, whereas prior application of caffeine or chloro-m-cresol blocked IP₃-induced Ca²⁺ release. In cultured human PASMCs, which lack functional expression of RyRs, caffeine failed to affect ATP-induced increases in [Ca²⁺]_i in the presence and absence of extracellular Ca²⁺. The RyR antagonists ruthenium red, ryanodine, tetracaine, and dantrolene greatly inhibited submaximal noradrenaline– and hypoxia-induced Ca²⁺ release and contraction in freshly isolated rat PASMCs, but did not affect ATP-induced Ca²⁺ release in cultured human PASMCs. Real-time quantitative RT-PCR and immunofluorescence staining indicated similar expression of all three RyR subtypes (RyR1, RyR2, and RyR3) in freshly isolated rat PASMCs. In freshly isolated PASMCs from RyR3 knockout (RyR3^−/−) mice, hypoxia-induced, but not submaximal noradrenaline–induced, Ca²⁺ release and contraction were significantly reduced. Ruthenium red and tetracaine can further inhibit hypoxic increase in [Ca²⁺]_i in RyR3^−/− mouse PASMCs. Collectively, our data suggest that (a) RyRs play an important role in submaximal noradrenaline– and hypoxia-induced Ca²⁺ release and contraction; (b) all three subtype RyRs are expressed; and (c) RyR3 gene knockout significantly inhibits hypoxia-, but not submaximal noradrenaline–induced Ca²⁺ and contractile responses in PASMCs.

Hypoxic pulmonary vasoconstriction: role of ion channels

Acute hypoxia induces pulmonary vasoconstriction and chronic hypoxia causes structural changes of the pulmonary vasculature including arterial medial hypertrophy. Electro- and pharmacomechanical mechanisms are involved in regulating pulmonary vasomotor tone, whereas intracellular Ca(2+) serves as an important signal in regulating contraction and proliferation of pulmonary artery smooth muscle cells. Herein, we provide a basic overview of the cellular mechanisms involved in the development of hypoxic pulmonary vasoconstriction. Our discussion focuses on the roles of ion channels permeable to K(+) and Ca(2+), membrane potential, and cytoplasmic Ca(2+) in the development of acute hypoxic pulmonary vasoconstriction and chronic hypoxia-mediated pulmonary vascular remodeling.

Role of FKBP12.6 in hypoxia- and norepinephrine-induced Ca2+ release and contraction in pulmonary artery myocytes

The cellular and molecular processes underlying the regulation of ryanodine receptor (RyR) Ca(2+) release in smooth muscle cells (SMCs) are incompletely understood. Here we show that FKBP12.6 proteins are expressed in pulmonary artery (PA) smooth muscle and associated with type-2 RyRs (RyR2), but not RyR1, RyR3, or IP(3) receptors (IP(3)Rs) in PA sarcoplasmic reticulum. Application of FK506, which binds to FKBPs and dissociates these proteins from RyRs, induced an increase in [Ca(2+)](i) and Ca(2+)-activated Cl(-) and K(+) currents in freshly isolated PASMCs, whereas cyclosporin, an agent known to inhibit calcineurin but not to interact with FKBPs, failed to induce an increase in [Ca(2+)](i). FK506-induced [Ca(2+)](i) increase was completely blocked by the RyR antagonist ruthenium red and ryanodine, but not the IP(3)R antagonist heparin. Hypoxic Ca(2+) response and hypoxic vasoconstriction were significantly enhanced in FKBP12.6 knockout mouse PASMCs. FK506 or rapamycin pretreatment also enhanced hypoxic increase [Ca(2+)](i), but did not alter caffeine-induced Ca(2+) release (SR Ca(2+) content) in PASMCs. Norepinephrine-induced Ca(2+) release and force generation were also markedly enhanced in PASMCs from FKBP12.6 null mice. These findings suggest that FKBP12.6 plays an important role in hypoxia- and neurotransmitter-induced Ca(2+) and contractile responses by regulating the activity of RyRs in PASMCs.

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.

Role of sarcoplasmic reticulum in control of membrane potential and nitrergic response in opossum lower esophageal sphincter

1. We previously demonstrated that a balance of Ca2+-activated Cl- current (ICl(Ca)) and K+ current activity sets the resting membrane potential of opossum lower esophageal sphincter (LES) circular smooth muscle at approximately -41 mV, which leads to continuous spike-like action potentials and the generation of basal tone. Ionic mechanisms underlying this basal ICl(Ca) activity and its nitrergic regulation remain unclear. Recent studies suggest that spontaneous Ca2+ release from sarcoplasmic reticulum (SR) and myosin light chain kinase (MLCK) play important roles. The current study investigated this possibility. Conventional intracellular recordings were performed on circular smooth muscle of opossum LES. Nerve responses were evoked by electrical square wave pulses of 0.5 ms duration at 20 Hz. 2. In the presence of nifedipine (1 microm), substance P (1 microm), atropine (3 microm) and guanethidine (3 microm), intracellular recordings demonstrated a resting membrane potential (MP) of -38.1+/-0.7 mV (n=25) with spontaneous membrane potential fluctuations (MPfs) of 1-3 mV. Four pulses of nerve stimulation induced slow inhibitory junction potentials (sIJPs) with an amplitude of 6.1+/-0.3 mV and a half-amplitude duration of 1926+/-147 ms (n=25). 3. 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific guanylyl cyclase inhibitor, abolished sIJPs, but had no effects on MPfs. Caffeine, a ryanodine receptor agonist, hyperpolarized MP and abolished sIJPs and MPfs. Ryanodine (20 microm) inhibited the sIJP and induced biphasic effects on MP, an initial small hyperpolarization followed by a large depolarization. sIJPs and MPfs were also inhibited by cyclopiazonic acid, an SR Ca2+ ATPase inhibitor. Specific ICl(Ca) and MLCK inhibitors hyperpolarized the MP and inhibited MPfs and sIJPs. 4. These data suggest that (1). spontaneous release of Ca2+ from the SR activates ICl(Ca), which in turn contributes to resting membrane potential; (2). MLCK is involved in activation of ICl(Ca); (3). inhibition of ICl(Ca) is likely to underlie sIJPs induced by nitrergic innervation.

Multiple conductance states of single Ca2+-activated Cl- channels in rabbit pulmonary artery smooth muscle cells

Ca2+-activated Cl- channels contribute to agonist-evoked contraction and spontaneous activity in some smooth muscle preparations. Patch pipette techniques were used to study the properties of single Ca2+-activated Cl- channels in freshly dispersed rabbit pulmonary artery myocytes. In the cell-attached recording mode, two conductance states of 3.5 and 1.8 pS were recorded either spontaneously or in response to increasing [Ca2+]i. With inside-out patches, the 3.5 pS channel current predominated at 50 nM [Ca2+]i, but at 500 nM [Ca2+]i most channels opened to the 1.8 pS level and an additional 1.2 pS channel conductance was resolved. At 1 microM [Ca2+]i all of the Cl- channels opened either to the 1.8 pS or 1.2 pS level. In 0 [Ca2+]i, no channel activity was observed at -100 mV to +100 mV, but with 10-250 nM [Ca2+]i the total single channel open probability (NP(o)) increased with depolarisation. This voltage dependence was not seen at higher values of [Ca2+]i. The plot of NPo vs. [Ca2+]i yielded Ca2+ affinity constants of 8 and 250 nM and Hill slopes of 1.3 and 2.3 at +100 and -100 mV, respectively. The distribution of open times was fitted by two exponentials of about 5 and 30 ms, which were neither voltage nor Ca2+ dependent. Replacement of external Cl- by I- shifted the reversal potential by about -30 mV and lengthened the longer of the two mean open times without significant effects on other kinetic parameters. Based on these data, a model for the activation of Ca2+-activated Cl- channels is proposed.

Metabolic inhibition with cyanide induces calcium release in pulmonary artery myocytes and Xenopus oocytes

We examined the effects of metabolic inhibition on intracellular Ca(2+) release in single pulmonary arterial smooth muscle cells (PASMCs). Severe metabolic inhibition with cyanide (CN, 10 mM) increased intracellular calcium concentration ([Ca(2+)](i)) and activated Ca(2+)-activated Cl(-) currents [I(Cl(Ca))] in PASMCs, responses that were greatly inhibited by BAPTA-AM or caffeine. Mild metabolic inhibition with CN (1 mM) increased spontaneous transient inward currents and Ca(2+) sparks in PASMCs. In Xenopus oocytes, CN also induced Ca(2+) release and activated I(Cl(Ca)), and these responses were inhibited by thapsigargin and cyclopiazonic acid to deplete sarcoplasmic reticulum (SR) Ca(2+), whereas neither heparin nor anti-inositol 1,4,5-trisphosphate receptor (IP(3)R) antibodies affected CN responses. In both PASMCs and oocytes, CN-evoked Ca(2+) release was inhibited by carbonyl cyanide m-chlorophenylhydrazone (CCCP) and oligomycin or CCCP and thapsigargin. Whereas hypoxic stimuli resulted in Ca(2+) release in pulmonary but not mesenteric artery myocytes, CN induced release in both cell types. We conclude that metabolic inhibition with CN increases [Ca(2+)](i) in both pulmonary and systemic artery myocytes by stimulating Ca(2+) release from the SR and mitochondria.

The large conductance potassium channel beta-subunit can interact with and modulate the functional properties of a calcium-activated chloride channel, CLCA1

We have recently compared the biophysical and pharmacological properties of native Ca(2+)-activated Cl(-) currents in murine portal vein with mCLCA1 channels cloned from murine portal vein myocytes (Britton, F. C., Ohya, S., Horowitz, B., and Greenwood, I. A. (2002) J. Physiol. (Lond.) 539, 107-117). These channels shared a similar relative permeability to various anions, but the expressed channel current lacked the marked time dependence of the native current. In addition, the expressed channel showed a lower Ca(2+) sensitivity than the native channel. As non-pore-forming regulatory beta-subunits alter the kinetics and increase the Ca(2+) sensitivity of Ca(2+)-dependent K(+) channels (BK channels) we investigated whether co-expression of beta-subunits with CLCA1 would alter the kinetics/Ca(2+) sensitivity of mCLCA1. Internal dialysis of human embryonic kidney cells stably expressing CLCA1 with 500 nM Ca(2+) evoked a significantly larger current when the beta-subunit KCNMB1 was co-expressed. In a small number of co-transfected cells marked time dependence to the activation kinetics was observed. Interaction studies using the mammalian two-hybrid technique demonstrated a physical association between CLCA1 and KCNMB1 when co-expressed in human embryonic kidney cells. These data suggest that activation of CLCA1 can be modified by accessory subunits.

Cell-to-cell communication via nitric oxide modulation of oscillatory Cl(-) currents in rat intact cerebral arterioles

1. Diffusion-mediated changes in ion channel function within blood vessels have not been demonstrated directly in a patch-clamp study. Here, we examined the hypothesis that endothelium-derived diffusible bioactive substances would modify endothelin-1 (ET-1)-evoked membrane currents in smooth muscle cells situated within intact arterioles. 2. In pieces of arterioles dissected from the rat cerebral pial membrane, patch electrodes were placed on single smooth muscle cells identified under the microscope. Under perforated patch-clamp conditions, ET-1 evoked an oscillatory inward current at negative potentials in such cells in the presence of the gap junction disrupter 18alpha-glycyrrhetinic acid. ET-1 also elicited an oscillation superimposed on a membrane depolarization in current-clamp mode. 3. The oscillatory current exhibited an outwardly rectifying current-voltage relationship, a sensitivity to niflumic acid, a requirement for inositol 1,4,5-trisphosphate (IP(3))- and caffeine-sensitive Ca(2+) stores and for external Ca(2+) and a rank order of anion permeabilities characteristic of Ca(2+)-activated Cl(-) currents (I(Ca(Cl))). 4. This oscillatory response was inhibited by bradykinin (an effect distinct from the electrical propagation of hyperpolarization) and this effect was attenuated by the NO-synthase inhibitor N(omega)-nitro-L-arginine and by the NO scavenger oxyhaemoglobin but not by the cyclo-oxygenease inhibitor indomethacin. 8-Bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) and nitroprusside closely mimicked the effect of bradykinin. 5. The present patch-clamp study has revealed diffusion-mediated cell-to-cell interaction in an intact blood vessel: bradykinin appears to cause NO to move from endothelium to smooth muscle, there to inhibit an ET-1-evoked oscillatory I(Ca(Cl)) via the NO-cGMP pathway.

Invited review: significance of spatial and temporal heterogeneity of calcium transients in smooth muscle

The multiplicity of mechanisms involved in regulation of intracellular Ca(2+) concentration ([Ca(2+)](i)) in smooth muscle results in both intra- and intercellular heterogeneities in [Ca(2+)](i). Heterogeneity in [Ca(2+)](i) regulation is reflected by the presence of spontaneous, localized [Ca(2+)](i) transients (Ca(2+) sparks) representing Ca(2+) release through ryanodine receptor (RyR) channels. Ca(2+) sparks display variable spatial Ca(2+) distributions with every occurrence within and across cellular regions. Individual sparks are often grouped, and fusion of sparks produces large local elevations in [Ca(2+)](i) that occasionally trigger propagating [Ca(2+)](i) waves. Ca(2+) sparks may modulate membrane potential and thus smooth muscle contractility. Sparks may also be the target of other regulatory factors in smooth muscle. Agonists induce propagating [Ca(2+)](i) oscillations that originate from foci with high spark incidence and also represent Ca(2+) release through RyR channels. With increasing agonist concentration, the peak of regional [Ca(2+)](i) oscillations remains relatively constant, whereas both frequency and propagation velocity increase. In contrast, the global cellular response appears as a concentration-dependent increase in peak as well as mean cellular [Ca(2+)](i), representing a spatial and temporal integration of the oscillations. The significance of agonist-induced [Ca(2+)](i) oscillations lies in the establishment of a global [Ca(2+)](i) level for slower Ca(2+)-dependent physiological processes.

Hypoxia inhibits the Na(+)/Ca(2+) exchanger in pulmonary artery smooth muscle cells

The cellular mechanisms underlying hypoxic pulmonary vasoconstriction are not fully understood. We examined the effect of hypoxia on Ca(2+) efflux from the cytosol in single Fura-2-loaded pulmonary artery myocytes. During mild hypoxia (pO(2)=50-60 Torr), peak [Ca(2+)](i) was increased and the rate of Ca(2+) removal from the cytosol was markedly slowed after stimuli that elevated [Ca(2+)](i). Removal of extracellular Na(+) potentiated the peak [Ca(2+)](i) rise and slowed the Ca(2+) decay rate in cells recorded under normoxic conditions; it did not further slow the Ca(2+) decay rate or potentiate the [Ca(2+)](i) increase in hypoxic cells. An Na(+)/Ca(2+) exchange current was recorded in isolated pulmonary artery myocytes. Switching from Li(+) to Na(+) (130 mM) revealed an inward current with reversal potential consistent with the Na(+)/Ca(2+) exchange current in cells in which [Ca(2+)](i) was clamped at 1 microM similar currents, although smaller, were observed with normal resting [Ca(2+)](i) using the perforated patch clamp technique. The Na(+)/Ca(2+) exchange current was markedly inhibited in myocytes exposed to mild hypoxia. RT-PCR revealed the expression of specific alternatively spliced RNAs of NCX1 in rat pulmonary arteries. These findings provide an enhanced understanding of the molecular mechanisms underlying hypoxic sensing in pulmonary arteries.

Calcium sparks in smooth muscle

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

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.

Inhibition of hKv2.1, a major human neuronal voltage-gated K+ channel, by meclofenamic acid

Using the standard patch clamp whole cell recording method, we assessed the pharmacological activity of four fenamate nonsteroidal anti-inflammatory drugs, meclofenamic acid, flufenamic acid, mefenamic acid and niflumic acid, on hKv2.1, a major human neuronal voltage-gated potassium channel stably expressed heterologously in Chinese hamster ovary cells. Meclofenamic acid inhibited hKv2.1 in a concentration-dependent manner whereas the other three fenamates had weaker or no effect on these channels at a concentration of 100 microM. The estimated IC50 of meclofenamic acid was 56.0 microM for hKv2.1 compared an IC50 of 155.9 microM for another human neuronal K channel (hKv1.1). Meclofenamic acid reached its maximum inhibition within 5 min of bath application and its effect was readily reversed upon wash. Kinetic analysis revealed that this drug did not alter the channel activation or deactivation time courses. Moreover, the effect of meclofenamic acid on hKv2.1 channels was not voltage-dependent. Indomethacin, another inhibitor of the cyclooxygenase that catalyses the synthesis of prostaglandin from arachidonic acid, had no effect on either hKv2.1 or hKv1.1. These results indicate that meclofenamic acid inhibits hKv2.1 more potently than hKv1.1 and it is likely that this compound acts directly on the channel proteins.

Contribution of Ca2+-activated K+ channels and non-selective cation channels to membrane potential of pulmonary arterial smooth muscle cells of the rabbit

1. Using the perforated patch-clamp or whole-cell clamp technique, we investigated the contribution of Ca2+-activated K+ current (IK(Ca)) and non-selective cation currents (INSC) to the membrane potential in small pulmonary arterial smooth muscle cells of the rabbit. 2. The resting membrane potential (Vm) was -39.2 +/- 0.9 mV (n = 72). It did not stay at a constant level, but hyperpolarized irregularly, showing spontaneous transient hyperpolarizations (STHPs). The mean frequency and amplitude of the STHPs was 5.6 +/- 1. 1 Hz and -7.7 +/- 0.7 mV (n = 12), respectively. In the voltage-clamp mode, spontaneous transient outward currents (STOCs) were recorded with similar frequency and irregularity. 3. Intracellular application of BAPTA or extracellular application of TEA or charybdotoxin suppressed both the STHPs and STOCs. The depletion of intracellular Ca2+ stores by caffeine or ryanodine, and the removal of extracellular Ca2+ also abolished STHPs and STOCs. 4. Replacement of extracellular Na+ with NMDG+ caused hyperpolarization Vm of without affecting STHPs. Removal of extracellular Ca2+ induced a marked depolarization of Vm along with the disappearance of STHPs. 5. The ionic nature of the background inward current was identified. The permeability ratio of K+ : Cs+ : Na+ : Li+ was 1.7 : 1.3 : 1 : 0. 9, indicating that it is a non-selective cation current (INSC). The reversal potential of this current in control conditions was calculated to be -13.9 mV. The current was blocked by millimolar concentrations of extracellular Ca2+ and Mg2+. 6. From these results, it was concluded that (i) hyperpolarizing currents are mainly contributed by Ca2+-activated K+ (KCa) channels, and thus STOCs result in transient membrane hyperpolarization, and (ii) depolarizing currents are carried through NSC channels.

Calcium release and calcium-activated chloride channels in airway smooth muscle cells

Rapid progress has been made in the determination of specific ion channels expressed in airway smooth muscle cells and their role in excitation-contraction coupling. The combination of molecular biology and molecular physiology has provided insight into the properties of voltage-dependent cation (calcium and potassium) channels and their regulation by excitatory and inhibitory signaling processes. In this brief review, we will focus on calcium release and calcium-activated chloride channels. The former channels mediate receptor-activated calcium release, and the latter channels are opened following this release event. Moreover, the discovery of spontaneous calcium release events, or "calcium sparks," in smooth muscle, suggests an unanticipated level of regulation. Intracellular calcium release can drive electrical activity by the activation of calcium-dependent sarcolemmal ion channels, including calcium-activated chloride channels. These channels activate briefly but undergo a rapid phosphorylation by calcium/calmodulin-dependent protein kinase, which uncouples channel activity from cytosolic calcium. The coupling between intracellular calcium release and depolarizing chloride currents represents a potentially important signaling system in airway smooth muscle.