Ca2+ and Ca(2+)-activated Cl- currents in rabbit oesophageal smooth muscle

Calcium-Activated Chloride Channels in Myometrial and Vascular Smooth Muscle

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

Network model predicts that CatSper is the main Ca2+ channel in the regulation of sea urchin sperm motility

Spermatozoa sea urchin swimming behaviour is regulated by small peptides from the egg outer envelope. Speract, such a peptide, after binding to its receptor in Strongylocentrotus purpuratus sperm flagella, triggers a signaling pathway that culminates with a train of intracellular calcium oscillations, correlated with changes in sperm swimming pattern. This pathway has been widely studied but not fully characterized. Recent work on Arbacia punctulata sea urchin spermatozoa has documented the presence of the Ca²⁺ CatSper channel in their flagella and its involvement in chemotaxis. However, if other calcium channels participate in chemotaxis remains unclear. Here, based on an experimentally-backed logical network model, we conclude that CatSper is fundamental in the S. purpuratus speract-activated sea urchin sperm signaling cascade, although other Ca²⁺ channels could still be relevant. We also present for the first time experimental corroboration of its active presence in S. purpuratus sperm flagella. We argue, prompted by in silico knock-out calculations, that CatSper is the main generator of calcium oscillations in the signaling pathway and that other calcium channels, if present, have a complementary role. The approach adopted here allows us to unveil processes, which are hard to detect exclusively by experimental procedures.

Pharmacological characterization of TMEM16A currents

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

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

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

Sensitization of enteric neurons to morphine by HIV-1 Tat protein

BACKGROUND

Gastrointestinal (GI) dysfunction is a major cause of morbidity in acquired immunodeficiency syndrome (AIDS). HIV-1-induced neuropathogenesis is significantly enhanced by opiate abuse, which increases proinflammatory chemokine/cytokine release, the production of reactive species, glial reactivity, and neuronal injury in the central nervous system. Despite marked interactions in the gut, little is known about the effects of HIV-1 in combination with opiate use on the enteric nervous system.

METHODS

To explore HIV-opiate interactions in myenteric neurons, the effects of Tat ± morphine (0.03, 0.3, and 3 μM) were examined in isolated neurons from doxycycline- (DOX-) inducible HIV-1 Tat(1-86) transgenic mice or following in vitro Tat 100 nM exposure (>6 h).

KEY RESULTS

Current clamp recordings demonstrated increased neuronal excitability in neurons of inducible Tat(+) mice (Tat+/DOX) compared to control Tat-/DOX mice. In neurons from Tat+/DOX, but not from Tat-/DOX mice, 0.03 μM morphine significantly reduced neuronal excitability, fast transient and late long-lasting sodium currents. There was a significant leftward shift in V(0.5) of inactivation following exposure to 0.03 μM morphine, with a 50% decrease in availability of sodium channels at -100 mV. Similar effects were noted with in vitro Tat exposure in the presence of 0.3 μM morphine. Additionally, GI motility was significantly more sensitive to morphine in Tat(+) mice than Tat(-) mice.

CONCLUSIONS & INFERENCES

Overall, these data suggest that the sensitivity of enteric neurons to morphine is enhanced in the presence of Tat. Opiates and HIV-1 may uniquely interact to exacerbate the deleterious effects of HIV-1-infection and opiate exposure on GI function.

Effects of HIV-1 Tat on enteric neuropathogenesis

The gastrointestinal (GI) tract presents a major site of immune modulation by HIV, resulting in significant morbidity. Most GI processes affected during HIV infection are regulated by the enteric nervous system. HIV has been identified in GI histologic specimens in up to 40% of patients, and the presence of viral proteins, including the trans-activator of transcription (Tat), has been reported in the gut indicating that HIV itself may be an indirect gut pathogen. Little is known of how Tat affects the enteric nervous system. Here we investigated the effects of the Tat protein on enteric neuronal excitability, proinflammatory cytokine release, and its overall effect on GI motility. Direct application of Tat (100 nm) increased the number of action potentials and reduced the threshold for action potential initiation in isolated myenteric neurons. This effect persisted in neurons pretreated with Tat for 3 d (19 of 20) and in neurons isolated from Tat(+) (Tat-expressing) transgenic mice. Tat increased sodium channel isoforms Nav1.7 and Nav1.8 levels. This increase was accompanied by an increase in sodium current density and a leftward shift in the sodium channel activation voltage. RANTES, IL-6, and IL-1β, but not TNF-α, were enhanced by Tat. Intestinal transit and cecal water content were also significantly higher in Tat(+) transgenic mice than Tat(-) littermates (controls). Together, these findings show that Tat has a direct and persistent effect on enteric neuronal excitability, and together with its effect on proinflammatory cytokines, regulates gut motility, thereby contributing to GI dysmotilities reported in HIV patients.

Multiple effects of anthracene-9-carboxylic acid on the TMEM16B/anoctamin2 calcium-activated chloride channel

Ca(2+)-activated Cl(-) currents (CaCCs) play important roles in many physiological processes. Recent studies have shown that TMEM16A/anoctamin1 and TMEM16B/anoctamin2 constitute CaCCs in several cell types. Here we have investigated for the first time the extracellular effects of the Cl(-) channel blocker anthracene-9-carboxylic acid (A9C) and of its non-charged analogue anthracene-9-methanol (A9M) on TMEM16B expressed in HEK 293T cells, using the whole-cell patch-clamp technique. A9C caused a voltage-dependent block of outward currents and inhibited a larger fraction of the current as depolarization increased, whereas the non-charged A9M produced a small, not voltage dependent block of outward currents. A similar voltage-dependent block by A9C was measured both when TMEM16B was activated by 1.5 and 13μM Ca(2+). However, in the presence of 1.5μM Ca(2+) (but not in 13μM Ca(2+)), A9C also induced a strong potentiation of tail currents measured at -100mV after depolarizing voltages, as well as a prolongation of the deactivation kinetics. On the contrary, A9M did not produce potentiation of tail currents, showing that the negative charge is required for potentiation. Our results provide the first evidence that A9C has multiple effects on TMEM16B and that the negative charge of A9C is necessary both for voltage-dependent block and for potentiation. Future studies are required to identify the molecular mechanisms underlying these complex effects of A9C on TMEM16B. Understanding these mechanisms will contribute to the elucidation of the structure and functional properties of TMEM16B channels.

Morphine dependence in single enteric neurons from the mouse colon requires deletion of β-arrestin2

Chronic administration of morphine results in the development of tolerance to the analgesic effects and to inhibition of upper gastrointestinal motility but not to colonic motility, resulting in persistent constipation. In this study we examined the effect of chronic morphine in myenteric neurons from the adult mouse colon. Similar to the ileum, distinct neuronal populations exhibiting afterhyperpolarization (AHP)-positive and AHP-negative neurons were identified in the colon. Acute morphine (3 μM) decreased the number of action potentials, and increased the threshold for action potential generation indicative of reduced excitability in AHP-positive neurons. In neurons from the ileum of mice that were rendered antinociceptive tolerant by morphine-pellet implantation for 5 days, the opioid antagonist naloxone precipitated withdrawal as evidenced by increased neuronal excitability. Overnight incubation of ileum neurons with morphine also resulted in enhanced excitability to naloxone. Colonic neurons exposed to long-term morphine, remained unresponsive to naloxone suggesting that precipitated withdrawal does not occur in colonic neurons. However, morphine-treated colonic neurons from β-arrestin2 knockout mice demonstrated increased excitability upon treatment with naloxone as assessed by change in rheobase, number of action potentials and input resistance. These data suggest that similar to the ileum, acute exposure to morphine in colonic neurons results in reduced excitability due to inhibition of sodium currents. However, unlike the ileum, dependence to chronic exposure of morphine develops in colonic neurons from the β-arrestin2 knockout mice. These studies corroborate the in-vivo findings of the differential role of neuronal β-arrestin2 in the development of morphine tolerance/dependence in the ileum and colon.

In silico determination of the effect of multi-target drugs on calcium dynamics signaling network underlying sea urchin spermatozoa motility

The motility of spermatozoa of both Lytechinus pictus and Strongylocentrotus purpuratus sea urchin species is modulated by the egg-derived decapeptide speract via an oscillatory [Ca2+]-dependent signaling pathway. Comprehension of this pathway is hence directly related to the understanding of regulated sperm swimming. Niflumic acid (NFA), a nonsteroidal anti-inflammatory drug alters several ion channels. Though unspecific, NFA profoundly affects how sea urchin sperm respond to speract, increasing the [Ca2+]i oscillation period, amplitude, peak and average level values of the responses in immobilized and swimming cells. A previous logical network model we developed for the [Ca2+] dynamics of speract signaling cascade in sea urchin sperm allows integrated dissection of individual and multiple actions of NFA. Among the channels affected by NFA are: hyperpolarization-activated and cyclic nucleotide gated Na+ channels (HCN), [Ca2+]-dependent Cl- channels (CaCC) and [Ca2+]-dependent K+ channels (CaKC), all present in the sea urchin genome. Here, using our model we investigated the effect of blocking in silico HCN and CaCC channels suggested by experiments. Regarding CaKC channels, arguments can be provided for either their blockage or activation by NFA. Our study yielded two scenarios compliant with experimental observations: i) under CaKC inhibition, this [Ca2+]-dependent K+ channel should be different from the Slo1 channel and ii) under activation of the CaKC channel, another [Ca2+] channel not considered previously in the network is required, such as the pH-dependent CatSper channel. Additionally, our findings predict cause-effect relations resulting from a selective inhibition of those channels. Knowledge of these relations may be of consequence for a variety of electrophysiological studies and have an impact on drug related investigations. Our study contributes to a better grasp of the network dynamics and suggests further experimental work.

Ca2+-activated Cl- channels

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

Electrophysiological characteristics of enteric neurons isolated from the immortomouse

BACKGROUND

Recently, two enteric neuronal cell lines, one fetal and the other post-natal (IM-PEN), have been developed from the H-2K(b)-tsA58 transgenic mouse (immortomouse). However, their electrophysiological properties are not known. The goal of this study was to determine the electrical excitability and ionic conductance of the immortalized postnatal enteric neuronal (IM-PEN) cell line.

METHODS

Whole cell patch clamp studies, immunohistochemistry and RT-PCR were performed on differentiated IM-PEN cells following propagation at 33 °C and differentiation at 37 °C.

RESULTS

Differentiated IM-PEN cells stained positively for the neuron specific markers βIII-tubulin and PGP9.5. The mRNA for several ion channels expressed in enteric neurons were detected by RT-PCR. In current clamp, the resting membrane potential was -24.6 ± 2.1 mV (n = 6) for IM-FEN and -29.8 ± 0.9 mV (n = 30) for IM-PEN. Current injections from Vh -80 mV resulted in passive responses but not action potentials. Depolarizing pulses in the whole cell voltage clamp configuration from Vh -80 mV elicited small nifedipine-sensitive inward currents. Additionally, outward currents with slow deactivating tail currents were blocked by niflumic acid and low chloride solution. A volume-regulated anion current was elicited by hypo-osmotic solution and inhibited by 10 μM DCPIB. Growth with rabbit gastrointestinal smooth muscle did not yield significant differences in the active properties of the IM-PEN cell line. Transient expression of L-type Ca(2+) channels produced large inward currents demonstrating a working mechanism for protein folding and transport.

CONCLUSION

The electrophysiological characteristics of IM-PEN cells suggest that chloride channels in IM-PEN cells play an important role in their resting state, and membrane trafficking of some of the ion channels may preclude their electrical excitability.

Niflumic acid disrupts marine spermatozoan chemotaxis without impairing the spatiotemporal detection of chemoattractant gradients

In many broadcast-spawning marine organisms, oocytes release chemicals that guide conspecific spermatozoa towards their source through chemotaxis. In the sea urchin Lytechinus pictus, the chemoattractant peptide speract triggers a train of fluctuations of intracellular Ca2+ concentration in the sperm flagella. Each transient Ca2+ elevation leads to a momentary increase in flagellar bending asymmetry, known as a chemotactic turn. Furthermore, chemotaxis requires a precise spatiotemporal coordination between the Ca2+-dependent turns and the form of chemoattractant gradient. Spermatozoa that display Ca2+-dependent turns while swimming down the chemoattractant gradient, and conversely suppress turning events while swimming up gradient, successfully approach the center of the gradient. Previous experiments in Strongylocentrotus purpuratus sea urchin spermatozoa showed that niflumic acid (NFA), an inhibitor of several ion channels, drastically altered the speract-induced Ca2+ fluctuations and swimming patterns. In this study, mathematical modeling of the speract-dependent Ca2+ signaling pathway suggests that NFA, by potentially affecting HCN, CaCC and CaKC channels, may alter the temporal organization of Ca2+ fluctuations, and therefore disrupt chemotaxis. Here we investigate our hypothesis using a novel automated method for analyzing sperm behavior. We show that NFA does indeed disrupt chemotactic responses of L. pictus spermatozoa, although the temporal coordination between the Ca2+-dependent turns and the form of chemoattractant gradient is unaltered. Instead, NFA disrupts sperm chemotaxis by altering the arc length traveled during each chemotactic turning event. This alteration in the chemotactic turn trajectory disorientates spermatozoa at the termination of the turning event. We conclude that NFA disrupts chemotaxis without affecting how the spermatozoa decode environmental cues.

Morphine decreases enteric neuron excitability via inhibition of sodium channels

Gastrointestinal peristalsis is significantly dependent on the enteric nervous system. Constipation due to reduced peristalsis is a major side-effect of morphine, which limits the chronic usefulness of this excellent pain reliever in man. The ionic basis for the inhibition of enteric neuron excitability by morphine is not well characterized as previous studies have mainly utilized microelectrode recordings from whole mount myenteric plexus preparations in guinea pigs. Here we have developed a Swiss-Webster mouse myenteric neuron culture and examined their electrophysiological properties by patch-clamp techniques and determined the mechanism for morphine-induced decrease in neuronal excitability. Isolated neurons in culture were confirmed by immunostaining with pan-neuronal marker, β-III tubulin and two populations were identified by calbindin and calretinin staining. Distinct neuronal populations were further identified based on the presence and absence of an afterhyperpolarization (AHP). Cells with AHP expressed greater density of sodium currents. Morphine (3 µM) significantly reduced the amplitude of the action potential, increased the threshold for spike generation but did not alter the resting membrane potential. The decrease in excitability resulted from inhibition of sodium currents. In the presence of morphine, the steady-state voltage dependence of Na channels was shifted to the left with almost 50% of channels unavailable for activation from hyperpolarized potentials. During prolonged exposure to morphine (two hours), action potentials recovered, indicative of the development of tolerance in single enteric neurons. These results demonstrate the feasibility of isolating mouse myenteric neurons and establish sodium channel inhibition as a mechanism for morphine-induced decrease in neuronal excitability.

Ionic conductances regulating the excitability of colonic smooth muscles

The tunica muscularis of the gastrointestinal (GI) tract contains two layers of smooth muscle cells (SMC) oriented perpendicular to each other. SMC express a variety of voltage-dependent and voltage-independent ionic conductance(s) that develop membrane potential and control excitability. Resting membrane potentials (RMP) vary through the GI tract but generally are within the range of -80 to -40 mV. RMP sets the 'gain' of smooth muscle and regulates openings of voltage-dependent Ca(2+) channels. A variety of K(+) channels contribute to setting RMP of SMC. In most regions, RMP is considerably less negative than the K(+) equilibrium potential, due to a finely tuned balance between background K(+) channels and non-selective cation channels (NSCC). Variations in expression patterns and openings of K(+) channels and NSCC account for differences of the RMP in different regions of the GI tract. Smooth muscle excitability is also regulated by interstitial cells (interstitial cells of Cajal (ICC) and PDGFRα(+) cells) that express additional conductances and are electrically coupled to SMC. Thus, 'myogenic' activity results from the integrated behavior of the SMC/ICC/PDGFRα(+) cell (SIP) syncytium. Inputs from excitatory and inhibitory motor neurons are required to produce the complex motor patterns of the gut. Motor neurons innervate three cell types in the SIP, and receptors, second messenger pathways, and ion channels in these cells mediate postjunctional responses. Studies of isolated SIP cells have begun to unravel the mechanisms responsible for neural responses. This review discusses ion channels that set and regulate RMP of SIP cells and how neurotransmitters regulate membrane potential.

CaMKII inhibition hyperpolarizes membrane and blocks nitrergic IJP by closing a Cl(-) conductance in intestinal smooth muscle

The ionic basis of nitrergic "slow'" inhibitory junction potential (sIJP) is not fully understood. The purpose of the present study was to determine the nature and the role of calmodulin-dependent protein kinase II (CaMKII)-dependent ion conductance in nitrergic neurotransmission at the intestinal smooth muscle neuromuscular junction. Studies were performed in guinea pig ileum. The modified Tomita bath technique was used to induce passive hyperpolarizing electrotonic potentials (ETP) and membrane potential change due to sIJP or drug treatment in the same cell. Changes in membrane potential and ETP were recorded in the same smooth muscle cell, using sharp microelectrode. Nitrergic IJP was elicited by electrical field stimulation in nonadrenergic, noncholinergic conditions and chemical block of purinergic IJP. Modification of ETP during hyperpolarization reflected active conductance change in the smooth muscle. Nitrergic IJP was associated with decreased membrane conductance. The CAMKII inhibitor KN93 but not KN92, the Cl(-) channel blocker niflumic acid (NFA), and the K(ATP)-channel opener cromakalim hyperpolarized the membrane. However, KN93 and NFA were associated with decreased and cromakalim was associated with increased membrane conductance. After maximal NFA-induced hyperpolarization, hyperpolarization associated with KN93 or sIJP was not seen, suggesting a saturation block of the Cl(-) channel signaling. These studies suggest that inhibition of CaMKII-dependent Cl(-) conductance mediates nitrergic sIJP by causing maximal closure of the Cl(-) conductance.

Sarcoplasmic reticulum function in smooth muscle

The sarcoplasmic reticulum (SR) of smooth muscles presents many intriguing facets and questions concerning its roles, especially as these change with development, disease, and modulation of physiological activity. The SR's function was originally perceived to be synthetic and then that of a Ca store for the contractile proteins, acting as a Ca amplification mechanism as it does in striated muscles. Gradually, as investigators have struggled to find a convincing role for Ca-induced Ca release in many smooth muscles, a role in controlling excitability has emerged. This is the Ca spark/spontaneous transient outward current coupling mechanism which reduces excitability and limits contraction. Release of SR Ca occurs in response to inositol 1,4,5-trisphosphate, Ca, and nicotinic acid adenine dinucleotide phosphate, and depletion of SR Ca can initiate Ca entry, the mechanism of which is being investigated but seems to involve Stim and Orai as found in nonexcitable cells. The contribution of the elemental Ca signals from the SR, sparks and puffs, to global Ca signals, i.e., Ca waves and oscillations, is becoming clearer but is far from established. The dynamics of SR Ca release and uptake mechanisms are reviewed along with the control of luminal Ca. We review the growing list of the SR's functions that still includes Ca storage, contraction, and relaxation but has been expanded to encompass Ca homeostasis, generating local and global Ca signals, and contributing to cellular microdomains and signaling in other organelles, including mitochondria, lysosomes, and the nucleus. For an integrated approach, a review of aspects of the SR in health and disease and during development and aging are also included. While the sheer versatility of smooth muscle makes it foolish to have a "one model fits all" approach to this subject, we have tried to synthesize conclusions wherever possible.

The effect of tyrosine nitration of L-type Ca2+ channels on excitation-transcription coupling in colonic inflammation

BACKGROUND AND PURPOSE

Excitation-transcriptional coupling involves communication between plasma membrane ion channels and gene expression in the nucleus. Calcium influx through L-type Ca(2+) channels induces phosphorylation of the transcription factor, cyclic-AMP response element binding protein (CREB) and downstream activation of the cyclic-AMP response element (CRE) promoter regions. Tyrosine nitration of Ca(2+) channels attenuates interactions with c-Src kinase, decreasing Ca(2+) channel currents and smooth muscle contraction during colonic inflammation. In this study we examined the effect of tyrosine nitration and colonic inflammation on Ca(2+) channel mediated phosphorylation of CREB and CRE activation.

EXPERIMENTAL APPROACH

CREB and phospho-CREB were detected by Western blots and CRE activation measured by dual luciferase assay. Chinese hamster ovary (CHO) cells were transfected with hCa(v)1.2b and hCa(v)1.2b c-terminal mutants. Colonic inflammation was induced by intracolonic instillation of 2,4,6 trinitrobenzene sulphonic acid in mouse colon.

KEY RESULTS

In hCa(v)1.2b transfected CHO cells and in native colonic smooth muscle, depolarization with 80 mM KCl induced CREB phosphorylation (pCREB). Treatment with peroxynitrite inhibited KCl-induced pCREB. Following experimental colitis, KCl-induced CREB phosphorylation was abolished in smooth muscle, concomitant with tyrosine nitration of Ca(2+) channels. Depolarization increased CRE activation in hCa(v)1.2b CHO cells by 2.35 fold which was blocked by nifedipine and by protein nitration of Ca(2+) channels with peroxynitrite. The Src-kinase inhibitor, PP2, blocked depolarization-induced CRE activation. Mutation of the C-terminus tyrosine residue, Y2134F, but not Y1861F, blocked CRE activation.

CONCLUSIONS AND IMPLICATIONS

Post-translational modification of Ca(2+) channels due to tyrosine nitration modified excitation-transcriptional coupling in colonic inflammation.

A quantitative model of gastric smooth muscle cellular activation

A physiologically realistic quantitative description of the electrical behavior of a gastric smooth muscle (SM) cell is presented. The model describes the response of a SM cell when activated by an electrical stimulus coming from the network of interstitial cells of Cajal (ICC) and is mediated by the activation of different ion channels species in the plasma membrane. The conductances (predominantly Ca2+ and K+) that are believed to substantially contribute to the membrane potential fluctuations during slow wave activity have been included in the model. A phenomenological description of intracellular Ca2+ dynamics has also been included because of its primary importance in regulating a number of cellular processes. In terms of shape, duration, and amplitude, the resulting simulated smooth muscle depolarizations (SMDs) are in good agreement with experimentally recordings from mammalian gastric SM in control and altered conditions. This model has also been designed to be suitable for incorporation into large scale multicellular simulations.

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

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

Regional differences in L-type Ca2+ channel expression in feline lower esophageal sphincter

In humans and cats, muscle from the lower esophageal sphincter (LES) circular region exhibits greater spontaneous tone than LES sling muscle, whereas the sling muscle is much more responsive to cholinergic stimulation. Despite physiological and pharmacological evidence for the presence of L-type Ca2+ channel current (ICa,L) activity in LES circular muscle, the identity of this channel has not been demonstrated biochemically or electrophysiologically fingerprinted. Furthermore, there is no information on the channel's presence and role in the sling region of the LES. We hypothesized that regional differences in the expression of ICa,L between LES circular and sling muscles, if present, could contribute to the functional asymmetry observed within the LES. ICa,L expression was compared between circular and sling regions of the LES by Western blot analysis. The patch-clamp technique was used to study ICa,L. Muscle strip studies assessed ICa,L contribution to contractile activity. We found both protein expression of ICa,L and ICa,L density to be greater in LES circular muscle than sling muscle. ICa,L voltage- and time-dependent activation and inactivation curves were similar in cells from both regions. ICa,L blockade with nifedipine inhibited spontaneous tone and ACh-induced contractions only in circular muscle but was able to abolish depolarization (KCl)-induced contractions in both sling and circular muscles. In contrast, La3+ inhibited tone and ACh-induced contractions in muscles from both regions. Therefore, regional myogenic differences in ICa,L expression within the LES circular and sling muscle exist and provide one explanation for the differential contribution of sling and circular muscle to LES contractility.

Calcineurin Aα but Not Aβ Augments ICl(Ca) in Rabbit Pulmonary Artery Smooth Muscle Cells

Activation of Ca(2+)-dependent Cl(-) currents (I(Cl(Ca))) increases membrane excitability in vascular smooth muscle cells. Previous studies showed that Ca(2+)-dependent phosphorylation suppresses I(Cl(Ca)) in pulmonary artery myocytes, and the aim of the present study was to determine the role of the Ca(2+)-dependent phosphatase calcineurin on chloride channel activity. Immunocytochemical and Western blot studies with isoform-specific antibodies revealed that the alpha and beta forms of the CaN catalytic subunit are expressed in PA cells but that only the alpha variant translocated to the cell periphery upon a rise in intracellular [Ca(2+)]. I(Cl(Ca)) evoked by pipette solutions containing a [Ca(2+)] set at 500 nm was considerably larger when the pipette solution included constitutively active CaN containing the alpha catalytic isoform. This stimulatory effect was lost by boiling the enzyme or by the inclusion of a specific CaN inhibitory peptide and was not shared by the inclusion of the beta form of the catalytic subunit. In the absence of constitutively active CaN, cyclosporin A, an inhibitor of CaN, suppressed I(Cl(Ca)) evoked by 500 nm Ca(2+) when the current amplitude was relatively large but was ineffective in cells with smaller currents. In perforated patch recordings, cyclosporin A consistently inhibited I(Cl(Ca)) evoked as a consequence of Ca(2+) influx through voltage-dependent calcium channels. These novel data show that in PA myocytes activation of I(Cl(Ca)) is enhanced by Ca(2+)-dependent dephosphorylation and that the regulation of this conductance is highly isoform-specific.

L-type Ca(2+) channel expression along feline smooth muscle oesophagus

Muscle from the proximal smooth muscle (SM) oesophagus of the cat demonstrates contractions of greater amplitude and greater sensitivity to cholinergic stimulation than muscle from the distal SM oesophagus. In the light of the central role of calcium influx in SM contractility, we hypothesized that regional differences in oesophageal contractility may be associated with differential expression of L-type calcium channels (L(Ca)) along the SM oesophagus. L(Ca) expression was compared between proximal and distal regions of the circular SM oesophagus by Western blots. Patch clamp technique was utilized to study L(Ca) currents. Muscle strip studies assessed L(Ca) contribution to contractile activity. The protein expression of L(Ca) and L(Ca) current density was greater in the proximal than distal region. L(Ca) voltage and time-dependent activation and inactivation curves were similar in cells from both regions. Stimulation of muscle strips with acetylcholine (ACh) in the presence of tetrodotoxin resulted in contractions of greater amplitude in the proximal region. The L(Ca) agonist Bay K 8644 caused a greater increase in ACh-induced contraction amplitude in muscle strips from the proximal region. Therefore, regional myogenic differences in L(Ca) expression along the circular SM oesophageal body exist and may contribute to the nature of oesophageal contractions.

Electrophysiological characterization and functional importance of calcium-activated chloride channel in rat uterine myocytes

In order to better understand the mechanisms underlying excitation of the uterus, we have elucidated the characteristics and functional importance of Ca(2+)-activated Cl(-) currents ( I(Cl-Ca)) in pregnant rat myometrium. In 101/320 freshly isolated myocytes, there was a slowly inactivating tail current (162+/-48 pA) upon repolarization following depolarising steps. This current has a reversal potential close to that for chloride, and was shifted when [Cl(-)] was altered. It was activated by Ca(2+) (but not Ba(2+)) entry through L-type Ca(2+) channels, enhanced by the Ca(2+) channel agonist Bay K8644 (2 microM), and inhibited by the Cl(-) channel blockers, niflumic acid (10 microM) and anthracene-9-carboxylic acid (9-AC, 100 microM). We therefore conclude that the pregnant rat myometrium contains Ca(2+)-activated Cl(-) channels producing inward current in ~30% of its cells. When these channels were inhibited by niflumic acid or 9-AC in intact tissues, the frequency of spontaneous contractions, was significantly reduced. Niflumic acid was also shown to inhibit oxytocin-induced contractions and Ca(2+) transients. Neither 9-AC nor niflumic acid had any effect on high-K-invoked contractions. Taken together these data suggest that Ca(2+)-activated Cl(-) channels are activated by Ca(2+) entry and play a functionally important role in myometrium, probably by contributing to membrane potential and firing frequency (pacemakers) in these cells.

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

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

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.

SNAP-25 inhibits L-type Ca2+ channels in feline esophagus smooth muscle cells

We recently reported that non-secretory gastrointestinal smooth muscle cells also possessed SNARE proteins, of which SNAP-25 regulated Ca(2+)-activated (K(Ca)) and delayed rectifier K(+) channels (K(V)). Voltage-gated, long lasting (L-type) calcium channels (L(Ca)) play an important role in excitation-contraction coupling of smooth muscle. Here, we show that SNAP-25 could also directly inhibit the L-type Ca(2+) channels in feline esophageal smooth muscle cells at the SNARE complex binding synprint site. SNARE proteins could therefore regulate additional cell actions other than membrane fusion and secretion, in particular, coordinated muscle membrane excitability and contraction, through their actions on membrane Ca(2+) and K(+) channels.

Anomalous effect of anthracene-9-carboxylic acid on calcium-activated chloride currents in rabbit pulmonary artery smooth muscle cells

1 Ca(2+)-activated Cl(-) currents (I(Cl(Ca))) evoked by K(+)-free pipette solutions containing 500 nM Ca(2+) were recorded in rabbit pulmonary artery smooth muscle cells. A voltage step protocol in which the cells were stepped to +70 mV and then to -80 mV produced outward and inward Cl(-) currents respectively that exhibited distinctive voltage- and time-dependent kinetics that remained consistent for the recording period. 2 Application of the Cl(-) channel inhibitor anthracene-9-carboxylic acid (A-9-C, 500 micro M), produced a small inhibition of the maximum outward Cl(-) current at +70 mV (21+/-10%) but augmented the amplitude of the instantaneous inward relaxation at -80 mV by 321+/-34% (n=12). 3 The current recorded in the absence and presence of A-9-C reversed at the theoretical Cl(-) equilibrium potential and the reversal potential was shifted by about -40 mV upon replacement of external chloride ion by the more permeant anion thiocyanate. Currents in the absence and presence of A-9-C were similarly affected by 100 micro M niflumic acid. 4 Augmentation of the inward current at -80 mV by A-9-C required prior depolarization, i.e. A-9-C did not simply activate a Cl(-) current at negative membrane potentials. Moreover the degree of augmentation was independent of the internal Ca(2+) for concentrations between 100 nM and 1 micro M Ca(2+). 5 The data from the present study confirm previous observations that the inhibitory effect of Cl(-) channel blockers is modified when [Ca(2+)](i) is maintained at higher than normal resting concentrations.

Control of descending vasa recta pericyte membrane potential by angiotensin II

Using nystatin perforated-patch whole cell recording, we investigated the role of Cl(-) conductance in the modulation of outer medullary descending vasa recta (OMDVR) pericyte membrane potential (Psi m) by ANG II. ANG II (10(-11) to 10(-7) M) consistently depolarized OMDVR and induced Psi m oscillations at lower concentrations. The Cl(-) channel blockers anthracene-9-decarboxylate (1 mM) and niflumic acid (10 microM) hyperpolarized resting pericytes and repolarized ANG II-treated pericytes. In voltage-clamp experiments, ANG II-treated pericytes exhibited slowly activating currents that were nearly eliminated by treatment with niflumic acid (10 microM) or removal of extracellular Ca(2+). Those currents reversed at -31 and -10 mV when extracellular Cl(-) concentration was 152 and 34 mM, respectively. In pericytes held at -70 mV, oscillating inward currents were sometimes observed; the reversal potential also shifted with extracellular Cl(-) concentration. We conclude that ANG II activates a Ca(2+)-dependent Cl(-) conductance in OMDVR pericytes to induce membrane depolarization and Psi m oscillations.

Comparative inhibitory effects of niflumic acid and novel synthetic derivatives on the rat isolated stomach fundus

Novel derivatives of 2-[3-(trifluoromethyl)-analino]nicotinic acid (niflumic acid) were synthesized. The compounds were compared for their inhibitory effects on 5-hydroxytryptamine (5-HT)- and KCI-induced contraction of the rat fundus. The aim was to assess structure-activity relationships regarding the selectivity and potency of these compounds. Niflumic acid (1-100 microM) concentration-dependently inhibited 5-HT-induced tonic contractions with an IC50 value (concentration reducing the control contractile response by 50%, calculated from semi-log graphs) of 0.24 x 10(4) M (n = 9). In contrast, it was significantly less potent at inhibiting KCl-induced responses (IC50 = 1.49 x 10(4) M, n = 9). The methyl ester (NFAme) and amido (NFAm) analogues showed no selectivity between 5-HT- and KCl-induced contractions with IC50 values of 1.64 x 10(-4) M (n = 8) and 1.87 x 10(-4) M (n = 9) for 5-HT responses, and 2.61 x 10(-4) M (n = 8) and 2.55 x 10(-4) M (n = 7) for KCl-induced responses, respectively. Our results suggest that alteration of the carboxylic acid moiety of niflumic acid reduces the selectivity and potency of its inhibitory action on 5-HT-induced contractile responses of the rat fundus, possibly via a reduced interaction with calcium-activated chloride channels.

Role of Cl- current in endothelin-1-induced contraction in rabbit basilar artery

Cl- efflux induces depolarization and contraction of smooth muscle cells. This study was undertaken to explore the role of Cl- channels in endothelin-1 (ET-1)-induced contraction in rabbit basilar artery. Male New Zealand White rabbits (n = 26), weighing 1.8-2.5 kg, were euthanized by an overdose of pentobarbital. The basilar arteries were removed for isometric tension recording. ET-1 produced a concentration-dependent contraction of the rabbit basilar artery in the normal Cl- Krebs-Henseleit bicarbonate buffer (123 mM Cl-). The ET-1-induced contraction was reduced by the following manipulations: 1) inhibition of Na+-K+-2Cl- cotransporter with bumetanide (3 x 10(-5) and 10(-4) M), 2) bicarbonate-free solution to disable Cl-/HCO exchanger, and 3) preincubation of rings with the Cl- channel blockers niflumic acid, 5-nitro-2-(3-phenylpropylamino)benzoic acid, and indanyloxyacetic acid 94. The ET-1-induced contraction was enhanced by substitution of extracellular Cl- (10 mM) with methanesulfonic acid (113 mM). Cl- channels are involved in ET-1-induced contraction in the rabbit basilar artery.

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.

Ca(2+)-activated Cl(-) current in sheep lymphatic smooth muscle

Freshly dispersed sheep mesenteric lymphatic smooth muscle cells were studied at 37 degrees C using the perforated patch-clamp technique with Cs(+)- and K(+)-filled pipettes. Depolarizing steps evoked currents that consisted of L-type Ca(2+) [I(Ca(L))] current and a slowly developing current. The slow current reversed at 1 +/- 1.5 mV with symmetrical Cl(-) concentrations compared with 23.2 +/- 1.2 mV (n = 5) and -34.3 +/- 3.5 mV (n = 4) when external Cl(-) was substituted with either glutamate (86 mM) or I(-) (125 mM). Nifedipine (1 microM) blocked and BAY K 8644 enhanced I(Ca(L)), the slow-developing sustained current, and the tail current. The Cl(-) channel blocker anthracene-9-carboxylic acid (9-AC) reduced only the slowly developing inward and tail currents. Application of caffeine (10 mM) to voltage-clamped cells evoked currents that reversed close to the Cl(-) equilibrium potential and were sensitive to 9-AC. Small spontaneous transient depolarizations and larger action potentials were observed in current clamp, and these were blocked by 9-AC. Evoked action potentials were triphasic and had a prominent plateau phase that was selectively blocked by 9-AC. Similarly, fluid output was reduced by 9-AC in doubly cannulated segments of spontaneously pumping sheep lymphatics, suggesting that the Ca(2+)-activated Cl(-) current plays an important role in the electrical activity underlying spontaneous activity in this tissue.

Selective inhibitory effects of niflumic acid on 5-HT-induced contraction of the rat isolated stomach fundus

The effects of niflumic acid (NFA), an inhibitor of calcium-activated chloride currents I(Cl(Ca)), were compared with the actions of the voltage-dependent calcium channel (VDCC) blocker nifedipine on 5-hydroxtryptamine (5-HT)- and acetylcholine (ACh)-induced contractions of the rat isolated fundus. NFA (1 - 30 microM) elicited a concentration-dependent inhibition of contractions induced by 5-HT (10 microM) with a reduction to 15. 5+/-6.0% of the control value at 30 microM. 1 microM nifedipine reduced 5-HT-induced contraction to 15.2+/-4.9% of the control, an effect not greater in the additional presence of 30 microM NFA. In contrast, the contractile response to ACh (10 microM) was not inhibited by NFA in concentrations </=100 microM, although this response was partly inhibited by nifedipine (1 microM) to 67.6+/-11. 8% of the control value. NFA (1 - 30 microM) did not affect contraction induced by either 20 mM or 60 mM KCl, suggesting that this drug was not acting via blockade of VDCCs or activation of potassium channels. In contrast, 3, 5-dichlorophenylamine-2-carboxylic acid and 4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid were less selective in their inhibitory effects, inducing reductions of 60 mM KCl-induced contraction at concentrations >/=10 microM. Our results show that NFA can exert selective inhibitory effects on the chloride-dependent 5-HT-induced contractions of the rat fundus. The data support the hypothesis that activation of Cl((Ca)) channels leading to calcium entry via VDCCs is a mechanism utilized by 5-HT, but not by ACh, to elicit contraction of the rat fundus.

Comparative effects of niflumic acid and nifedipine on 5-hydroxytryptamine- and acetylcholine-induced contraction of the rat trachea

The effects of niflumic acid, an inhibitor of Ca(2+)-activated Cl(-) (Cl((Ca))) channels, were compared with those of the voltage-dependent Ca(2+) channel (VDCC) blocker nifedipine on 5-hydroxytryptamine (5-HT)- and acetylcholine-induced contractions of the rat isolated trachea. Niflumic acid (3-100 microM) induced a concentration-dependent inhibition of 5-HT (10 microM)-induced contractions, with a reduction to 37.0+/-9.5% of the control at the highest concentration. One micromolar nifedipine, which completely blocked 60 mM KCl-induced contractions, reduced the response to 5-HT similarly to 39.2+/-11.5% of the control. The inhibition of the 5-HT response was not significantly different from that produced by the combined presence of nifedipine (1 microM) and niflumic acid (100 microM), suggesting that their effects were not additive. In contrast, neither niflumic acid (3-100 microM) nor nifedipine (1 microM) inhibited acetylcholine-induced contractions. The contraction to 5-HT (10 microM) in Cl(-)-free solution was decreased by more than approximately 85% of the control, whilst that of acetylcholine was reduced only by approximately 36%. Our data show that niflumic acid exerts selective inhibitory effects on 5-HT-induced contraction, and suggest that activation of Cl((Ca)) channels may be a mechanism whereby 5-HT (but not acetylcholine) induces Ca(2+) entry via VDCCs to elicit contraction.

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.

Effects of anion channel antagonists in canine colonic myocytes: comparative pharmacology of Cl-, Ca2+ and K+ currents

1. Volume-Sensitive, Outwardly Rectifying (VSOR) Cl- currents were measured in canine colonic myocytes by whole-cell patch clamp. Decreasing extracellular osmolarity 50 milliosmoles l-1 activated current that was carried by Cl- and 5 - 7 times greater in the outward direction. 2. Niflumic acid, an inhibitor of Ca2+-activated Cl- channels, did not inhibit VSOR Cl- current. Glibenclamide, an antagonist of CFTR, and anthracene-9-carboxylate (9-AC) inhibited current less than 25% at 100 microM. 3. DIDS (4, 4-diisothiocyanato-stilbene-2,2'disulphonate) inhibited VSOR Cl- current more potently than SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonate). IC50s were 0.84 and 226 microM, respectively. 4. VSOR Cl- current was strongly inhibited by tamoxifen ([Z]-1-[p-dimethylaminoethoxy-phenyl]-1,2-diphenyl-1-butene), an anti-oestrogen compound (IC50=0.57 microM). 5. Gd3+ antagonized VSOR Cl- current more potently than La3+. The IC50 for Gd3+ was 23 microM. In contrast, 100 microM La3+ inhibited current only 35+/-7%. 6. Antagonists of VSOR Cl- current had non-specific effects. These compounds blocked voltage-dependent K+ and Ca2+ currents in colonic myocytes. Tamoxifen (10 microM) and DIDS (10 microM) inhibited L-type Ca2+ current 87+/-7 and 31+/-5%, respectively. Additionally, in the presence of 300 nM charybdotoxin, tamoxifen (1 microM) and DIDS (10 microM) inhibited delayed rectifier K+ current 38+/-8 and 10+/-2%, respectively. 7. The pharmacology of VSOR Cl- channels overlaps with voltage-dependent cation channels. DIDS and tamoxifen inhibited VSOR Cl- equally. However, because DIDS had much less effect on L-type Ca2+ and delayed rectifier K+ channels than did tamoxifen, it might be useful in experiments to investigate the physiological and pathophysiological role of this conductance in whole tissues.

Excitation-contraction coupling in gastrointestinal and other smooth muscles

The main contributors to increases in [Ca2+]i and tension are the entry of Ca2+ through voltage-dependent channels opened by depolarization or during action potential (AP) or slow-wave discharge, and Ca2+ release from store sites in the cell by the action of IP3 or by Ca(2+)-induced Ca(2+)-release (CICR). The entry of Ca2+ during an AP triggers CICR from up to 20 or more subplasmalemmal store sites (seen as hot spots, using fluorescent indicators); Ca2+ waves then spread from these hot spots, which results in a rise in [Ca2+]i throughout the cell. Spontaneous transient releases of store Ca2+, previously detected as spontaneous transient outward currents (STOCs), are seen as sparks when fluorescent indicators are used. Sparks occur at certain preferred locations--frequent discharge sites (FDSs)--and these and hot spots may represent aggregations of sarcoplasmic reticulum scattered throughout the cytoplasm. Activation of receptors for excitatory signal molecules generally depolarizes the cell while it increases the production of IP3 (causing calcium store release) and diacylglycerols (which activate protein kinases). Activation of receptors for inhibitory signal molecules increases the activity of protein kinases through increases in cAMP or cGMP and often hyperpolarizes the cell. Other receptors link to tyrosine kinases, which trigger signal cascades interacting with trimeric G-protein systems.

Ionic conductances in gastrointestinal smooth muscles and interstitial cells of Cajal

Ion channels are the unitary elements that underlie electrical activity of gastrointestinal smooth muscle cells and of interstitial cells of Cajal. The result of ion channel activity in the gastrointestinal smooth muscle layers is a rhythmic change in membrane potential that in turn underlies events leading to organized motility patterns. Gastrointestinal smooth muscle cells and interstitial cells of Cajal express a wide variety of ion channels that are tightly regulated. This review summarizes 20 years of data obtained from patch-clamp studies on gastrointestinal smooth muscle cells and interstitial cells, with a focus on regulation.

Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels

Visceral smooth muscle cells (VSMC) play an essential role, through changes in their contraction-relaxation cycle, in the maintenance of homeostasis in biological systems. The features of these cells differ markedly by tissue and by species; moreover, there are often regional differences within a given tissue. The biophysical features used to investigate ion channels in VSMC have progressed from the original extracellular recording methods (large electrode, single or double sucrose gap methods), to the intracellular (microelectrode) recording method, and then to methods for recording from membrane fractions (patch-clamp, including cell-attached patch-clamp, methods). Remarkable advances are now being made thanks to the application of these more modern biophysical procedures and to the development of techniques in molecular biology. Even so, we still have much to learn about the physiological features of these channels and about their contribution to the activity of both cell and tissue. In this review, we take a detailed look at ion channels in VSMC and at receptor-operated ion channels in particular; we look at their interaction with the contraction-relaxation cycle in individual VSMC and especially at the way in which their activity is related to Ca2+ movements and Ca2+ homeostasis in the cell. In sections II and III, we discuss research findings mainly derived from the use of the microelectrode, although we also introduce work done using the patch-clamp procedure. These sections cover work on the electrical activity of VSMC membranes (sect. II) and on neuromuscular transmission (sect. III). In sections IV and V, we discuss work done, using the patch-clamp procedure, on individual ion channels (Na+, Ca2+, K+, and Cl-; sect. IV) and on various types of receptor-operated ion channels (with or without coupled GTP-binding proteins and voltage dependent and independent; sect. V). In sect. VI, we look at work done on the role of Ca2+ in VSMC using the patch-clamp procedure, biochemical procedures, measurements of Ca2+ transients, and Ca2+ sensitivity of contractile proteins of VSMC. We discuss the way in which Ca2+ mobilization occurs after membrane activation (Ca2+ influx and efflux through the surface membrane, Ca2+ release from and uptake into the sarcoplasmic reticulum, and dynamic changes in Ca2+ within the cytosol). In this article, we make only limited reference to vascular smooth muscle research, since we reviewed the features of ion channels in vascular tissues only recently.

Ca2+ current and Ca(2+)-activated chloride current in isolated smooth muscle cells of the sheep urethra

1. Isolated sheep urethral cells were studied using the perforated patch clamp technique (T = 37 degrees C). Depolarizing steps ranging from -40 to -10 mV evoked an inward current that peaked within 10 ms and a slower inward current. Stepping back to the holding potential of -80 mV evoked large inward tail currents. All three currents were abolished by nifedipine (1 microM). Substitution of external Ca2+ with Ba2+ resulted in potentiation of the fast inward current and blockade of the slow current and tails. 2. Changing the chloride equilibrium potential (ECl) from 0 to +27 mV shifted the reversal potential of the tail currents from 1 +/- 1 to 27 +/- 1 mV (number of cells, n = 5). Chloride channel blockers, niflumic acid (10 microM) and anthracene-9-carboxylic acid (9AC, 1 mM), reduced the slow current and tails suggesting that these were Ca(2+)-activated Cl- currents, ICl(Ca). 4. Caffeine (10 mM) induced currents that reversed at ECl and were blocked by niflumic acid (10 microM). 5. In current clamp mode, some cells developed spontaneous transient depolarizations (STDs) and action potentials. Short exposure to nifedipine blocked the action potentials and unmasked STDs. In contrast, 9AC and niflumic acid reduced the amplitude of the STDs and blocked the action potentials. 6. In conclusion, these cells have both L-type ICa and ICl(Ca). The former appears to be responsible for the upstroke of the action potential, while the latter may act as a pacemaker current.

Inhibitory action of niflumic acid on noradrenaline- and 5-hydroxytryptamine-induced pressor responses in the isolated mesenteric vascular bed of the rat

1. The effects of niflumic acid, an inhibitor of calcium-activated chloride currents, were compared with the actions of the calcium channel blocker nifedipine on noradrenaline- and 5-hydroxytryptamine (5-HT)-induced pressor responses of the rat perfused isolated mesenteric vascular bed. 2. Bolus injections of noradrenaline (1 and 10 nmol) increased the perfusion pressure in a dose-dependent manner. Nifedipine (1 microM) inhibited the increase in pressure produced by 1 nmol noradrenaline by 31 +/- 5%. Niflumic acid (10 and 30 microM) also inhibited the noradrenaline-induced increase in perfusion pressure and 30 microM niflumic acid reduced the pressor response to 1 nmol noradrenaline by 34 +/- 6%. 3. The increases in perfusion elicited by 5-HT (0.3 and 3 nmol) were reduced by niflumic acid (10 and 30 microM) in a concentration-dependent manner and 30 microM niflumic acid inhibited responses to 0.3 and 3 nmol 5-HT by, respectively, 49 +/- 8% and 50 +/- 7%. Nifedipine (1 microM) decreased the pressor response to 3 nmol 5-HT by 44 +/- 9%. 4. In the presence of a combination of 30 microM niflumic acid and 1 microM nifedipine the inhibition of the pressor effects of noradrenaline (10 nmol) and 5-HT (3 nmol) was not significantly greater than with niflumic acid (30 microM) alone. Thus the effects of niflumic acid and nifedipine were not additive. 5. In Ca-free conditions the transient contractions induced by 5-HT (3 nmol) were not reduced by 30 microM niflumic acid, suggesting that this agent does not inhibit calcium release from the intracellular store or the binding of 5-HT to its receptor. 6. Niflumic acid 30 microM did not inhibit the pressor responses induced by KCl (20 and 60 mumol) which were markedly reduced by 1 microM nifedipine. In addition, 1 microM levcromakalim decreased pressor responses produced by 20 mumol KCl. These data suggest that niflumic acid does not block directly calcium channels or activate potassium channels. 7. It is concluded that niflumic acid selectively reduces a component of noradrenaline- and 5-HT-induced pressor responses by inhibiting a mechanism which leads to the opening of voltage-gated calcium channels. Our data suggest that the Ca(2+)-activated chloride conductance may play a pivotal role in the activation of voltage-gated calcium channels in agonist-induced constriction of resistance blood vessels.

Analysis of the time course of calcium-activated chloride "tail" currents in rabbit portal vein smooth muscle cells

The time course of calcium-activated chloride "tail" currents (Itail) in single cells of the rabbit portal vein was studied. These currents were activated by the influx of calcium through voltage-dependent calcium channels (VDCCs). At -50 mV, Itail decayed exponentially with a time constant (tau) of 80-100 ms that was independent of amplitude and was similar to the tau of the decay of spontaneous transient inward currents (STICs; calcium-activated chloride currents). The decays of the STIC and Itail had a similar voltage dependence between -50 and -110 mV and were similarly affected by the chloride channel blocker, niflumic acid. However, at more positive potentials (-20 to +40 mV), Itail was sustained for the duration of the test pulse in most cells, in contrast to STICs which decayed exponentially. At very positive potentials (e.g. +100 mV), when little calcium enters the cell through VDCCs, Itail decayed exponentially. Measurement of calcium current (ICa) at various potentials showed that the VDCCs did not inactivate fully at potentials between -20 and +30 mV. We propose that at negative potentials the decay of Itail is determined by slow gating of the chloride channel, but at positive potentials a sustained Itail is produced by persistent influx of calcium through non-inactivating VDCCs.

Characteristics and physiological role of the Ca(2+)-activated Cl- conductance in smooth muscle

In this review we discuss the properties of the Ca(2+)-activated Cl- current [ICl(Ca)] recorded in isolated smooth muscle cells with electrophysiological techniques and speculate on the possible physiological role(s) of ICl(Ca) in smooth muscle function. In particular, we concentrate on 1) the Ca2+ dependence of ICl(Ca), 2) the mechanisms that link pharmacological receptor stimulation on the cell surface membrane to activation of ICl(Ca), 3) the biophysical properties of ICl(Ca), and 4) the pharmacology of ICl(Ca). It is evident that a diverse array of pharmacological agonists can evoke ICl(Ca) in many types of smooth muscle, and it seems that the well-established G protein-phosphoinositide metabolism (inositol 1,4,5-trisphosphate)-intracellular Ca2+ store pathway couples the receptor to the membrane channels. Also, the results indicate that the biophysical and pharmacological properties of ICl(Ca) are not only similar in the various smooth muscle types studied so far but, possibly, are also similar to ICl(Ca) in non-smooth muscle tissue. Evidence is presented that the Ca(2+)-activated Cl- channel exists in two states, open and closed, with a relatively long mean open time and that some of the agents that inhibit ICl(Ca) interact directly with the open channel. It is suggested that the most likely role of ICl(Ca) in smooth muscle is to produce membrane depolarization and contraction to neurotransmitters and local mediators.

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

The time course of two types of Ca(2+)-dependent currents were compared in single smooth muscle cells freshly isolated from guinea-pig trachea. When the pipette solution contained mainly 140 mM KCl, depolarization from -60 mV to 0 mV evoked an initial inward current followed by an outward current which consisted of transient (I(to)) and sustained components. In addition, a long-lasting inward tail current (Itail) was occasionally observed after the repolarization to -60 mV. Although I(to) often occurred repetitively during depolarization, the first I(to) reached the peak of approximately 50 ms after the start of depolarization and had the largest amplitude in most cells examined. The amplitude of Itail increased with the increase in depolarization period up to about 500 ms. Pharmacological analyses indicate that I(to) and Itail are Ca(2+)-dependent K+ and Cl- currents (IK-Ca and ICl-Ca), respectively, and suggest that not only Ca(2+)-influx through Ca2+ channels but also subsequent Ca2+ release from stores contributes to activate these currents. Spontaneous transient outward and inward currents, IK-Ca and ICl-Ca, respectively, were simultaneously recorded at -40 mV. In over 80% of the spontaneous current events, outward and inward currents coupled one to one and always occurred in this order. Puff-application of 10 mM caffeine also induced IK-Ca and ICl-Ca in this order at -40 mV. When caffeine was applied twice with various intervals, the current amplitude in the second application depended upon the period of the interval. The recovery of ICl-Ca during the interval was faster than that of IK-Ca. The results indicate that the activation and decay time courses of ICl-Ca are slower but its recovery is faster than those of IK-Ca.

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

1. The effects of niflumic acid, an inhibitor of calcium-activated chloride channels, were compared with the actions of the calcium channel antagonist nifedipine on noradrenaline-evoked contractions in isolated preparations of the rat aorta. 2. The cumulative concentration-effect curve to noradrenaline (NA) was depressed by both nifedipine and niflumic acid in a reversible and concentration-dependent manner. The degree of inhibition of the maximal contractile response to NA (1 microM) produced by 10 microM niflumic acid (38%) was similar to the effect of 1 microM nifedipine (39%). 3. Contractions to brief applications (30 s) of 1 microM NA were inhibited by 55% and 62% respectively by 10 microM niflumic acid and 1 microM nifedipine. 4. In the presence of 0.1 microM nifedipine, niflumic acid (10 microM) produced no further inhibition of the NA-evoked contractions. Thus, the actions of niflumic acid and nifedipine were not additive. 5. In Ca-free conditions the transient contraction induced by 1 microM NA was not inhibited by niflumic acid (10 microM) and therefore this agent does not reduce the amount of calcium released from the intracellular store or reduce the sensitivity of the contractile apparatus to calcium. 6. Niflumic acid 10 microM did not inhibit the contractions produced by KCl (up to 120 mM) which were totally blocked by nifedipine. Contractions induced by 25 mM KCl were completely inhibited by 1 microM levcromakalim but were unaffected by niflumic acid. 7. It was concluded that niflumic acid produces selective inhibition of a component of NA-evoked contraction which is probably mediated by voltage-gated calcium channels. These data are consistent with a model in which NA stimulates a calcium-activated chloride conductance which leads to the opening of voltage-gated calcium channels to produce contraction.

Neurokinin A and Ca2+ current induce Ca(2+)-activated Cl(-) currents in guinea-pig tracheal myocytes

1. Membrane currents were recorded by a patch clamp technique in guinea-pig tracheal myocytes, using the whole cell mode with Cs(+) internal solution. 2. Both neurokinin A (NKA, 1 mu M) and caffeine (10 mM) evoked Ca(2+)-activated Cl- currents (I[Cl(Ca)]) transiently. In Ca(2+)-free bathing solution, the first application of NKA or caffeine elicited I[Cl(Ca)] but the second application of these substances failed to activate it. In addition, pretreatment with ryanodine in the presence of caffeine abolished the response to both NKA and caffeine whilst heparin (200 mu g ml(-1)) only blocked the NKA-induced response. I[Cl(Ca)] was also elicited by inositol 1,4,5-trisphosphate (IP(3)). 3. Command voltage pulses positive to 0 mV from a holding potential of -60 mV activated the voltage-dependent L-type Ca2+ current (I(Ca,L)) and late outward current. Upon repolarization to the holding potential, slowly decaying inward tail currents were recorded. The outward current during the depolarizing pulses and the inward tail current were enhanced by Bay K 8644, but completely blocked by Cd2+ or nifedipine. Replacement of external Ca2+ with Ba2+, removal of Ca2+ from the bath solution, or inclusion of EGTA (5 mM) in the patch pipette, also led to abolition of these currents, indicating that they were Ca2+ dependent, and that Ca2+ influx due to I(Ca,L) activated the currents. 4. When [Cl(-)](O) or [Cl(-)](i) was changed, the reversal potential (E(rev)) of the Ca2+-activated currents shifted, thus behaving like a Cl(-)-selective ion channel as predicted by the Nernst equation. DIDS (1 mM) completely abolished the currents, also suggesting that they were I[Cl(Ca)]. 5. NKA (1 mu M) and caffeine (30 mM) transiently activated I[Cl(Ca)], and after that both agents markedly reduced I[Cl(Ca)] induced by I(Ca,L). This is probably due to sarcoplasmic reticulum (SR) Ca2+ release induced by NKA or caffeine, followed by inhibition of the Ca(2+)-induced Ca2+ release from the SR. 6. The present results indicate that I[Cl(Ca)] can be activated by SR Ca2+ release due to NKA or caffeine (through IP(3) or ryanodine receptors) as well as by Ca2+ influx due to I(Ca,L). It also suggests that activation of I[Cl(Ca)] by NKA may be mediated by the production of IP(3), which releases Ca2+ from the SR.

Comparison of the effects of fenamates on Ca-activated chloride and potassium currents in rabbit portal vein smooth muscle cells

1. The perforated patch and conventional whole-cell recording techniques were used to study the action of flufenamic, mefenamic and niflumic acid on calcium-activated chloride and potassium currents in rabbit portal vein smooth muscle cells. 2. In K-conditions at a holding potential of -77 mV flufenamic acid and mefenamic acid decreased the amplitude of spontaneous transient inward currents (STICs, calcium-activated chloride currents, ICl(Ca)) in a concentration-dependent manner. The potency sequence was niflumic > flufenamic > mefenamic acid. 3. At -77 mV 1 x 10(-5) M flufenamic acid increased the STIC exponential decay time constant (tau). At higher concentrations the STIC decay was described by 2 exponentials with an initial decay (tau f) faster than the control tau value and a second exponential (tau s) which had a time constant slower than the control tau value. Low concentrations of mefenamic acid had no effect or decreased the tau value whereas in higher concentrations biphasic currents were recorded. 4. In K-free conditions the inhibitory effect of both flufenamic and mefenamic acid on STIC amplitude was greater at +50 mV compared to -50 mV, showing that the effect of these agents was voltage-dependent. 5. In cells held at 0 mV in K-containing conditions the fenamates reduced both the frequency and amplitude of spontaneous transient outward currents (STOCs, calcium-activated potassium currents, IK(Ca)). The concentration range to produce these effects was higher than that to decrease STIC amplitude and the potency sequence was flufenamic > niflumic > or = mefenamic acid. 6. All these compounds in concentrations greater than 5 x 10(-5) M evoked a 'noisy' potassium current at 0 mV which reached a maximum after approximately 3 min. This current was readily reversible on washout of the drug and could be elicited several times in the same cell. The current-voltage relationship of the fenamate-evoked current exhibited pronounced outward rectification characteristic of IK(Ca). 7. The current evoked by 2 x 10(-4) M flufenamic acid and 5 x 10(-4) M niflumic acid was not affected by 1 x 10(-5) M glibenclamide but was markedly inhibited by 1 x 10(-3) M tetraethylammonium. Furthermore, large currents were activated by flufenamic and niflumic acid in the presence of caffeine and cyclopiazonic acid (an inhibitor of the sarcoplasmic reticulum Ca-ATPase) to deplete intracellular Ca-stores. 8. Conventional whole-cell recording was performed with pipette solutions in which the ability to buffer changes in intracellular calcium was varied by altering the concentration of the calcium chelator (2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA). Flufenamic acid (2 x 10(-4) M) and niflumic acid (5 x 10(-4) M) both evoked large outward currents when recordings were made with either 1 x 10(-4) M or 1 x 10(-2) M BAPTA. Furthermore, bathing the cells in nominally calcium-free extracellular solution did not reduce the amplitude of the evoked currents. 9. It is concluded that both flufenamic and mefenamic acid inhibit ICl(Ca) by a mechanism similar to niflumic acid, possibly open channel blockade. Furthermore, at concentrations greater than 5 x 10(-5) M all three fenamates inhibited STOC activity and evoked directly an outward current which resembled IK(Ca).

Ca(2+)-dependent Cl- current at the presynaptic terminals of goldfish retinal bipolar cells

In ON-type bipolar cells dissociated from the goldfish retina, a slowly declining inward current (Itail) was observed after the termination of depolarizing voltage step commands, during which a Ca2+ current was elicited. The properties of Itail were investigated under the whole-cell voltage clamp. Introduction of the membrane permeant Ca2+ chelator, BAPTA/AM, into the cell suppressed Itail, indicating that Itail was activated by the increase of intracellular free Ca2+ concentration ([Ca2+]i). The major component of Itail was identified as the Ca(2+)-dependent Cl- current (ICl(Ca)), since the reversal potential of Itail was almost identical to the Cl- equilibrium potential at various extracellular Cl- concentrations ([Cl-]o). The contribution of the Na+/Ca2+ exchanger current to Itail was very small. ICl(Ca) was partially suppressed by 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) when it was locally applied to the axon terminal but not to the cell body region, suggesting that Ca(2+)-dependent Cl- channels were localized to the axon terminal. The relationship between the peak amplitude of ICl(Ca) and the amount of charge carried by the Ca2+ current was almost linear at levels less than ca. 50 pC, but became saturated at a higher Ca2+ charge.

Ca(2+)-dependent Cl- current in canine tracheal smooth muscle cells

Our goal was to investigate the role of Ca2+ entry in regulating Cl- current (ICl) in smooth muscle cells from canine trachealis. When studies were done using the perforated patch configuration, depolarization elicited a dihydropyridine-sensitive Ca2+ current (ICa), followed in many cells by a sustained current. This sustained current reversed direction close to the Cl- equilibrium potential, consistent with its representing ICl. The ICl was also apparent as slowly deactivating tail currents seen upon repolarization to negative potentials. The Cl- channel blocker niflumic acid abolished both the sustained and tail currents, without affecting ICa. Several observations indicated that the ICl was dependent on Ca2+ entry. ICl was increased in magnitude when Ca2+ influx was augmented [by prolonging the depolarization or using BAY K 8644 or acetylcholine (ACh)] and decreased in magnitude when Ca2+ influx was reduced (using nifedipine). Based on these findings, we conclude that depolarization causes Ca2+ entry, with resultant elevation of cytosolic free Ca2+ concentration leading to activation of ICl (ICl(Ca)). We investigated whether Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum was involved in activation of ICl(Ca), by depleting intracellular stores of Ca2+ using cyclopiazonic acid to block the sarcoplasmic Ca(2+)-adenosinetriphosphatase and repeated stimulation with ACh. In such Ca(2+)-depleted cells, depolarization-mediated Ca2+ entry continued to activate ICl(Ca), suggesting that Ca(2+)-induced Ca2+ release was not required for its activation. We conclude that Ca2+ entry can activate Cl- channels in tracheal smooth muscle. This represents a positive-feedback system, which would promote excitation and contraction of airway muscle.