Diphenylamine-2-carboxylate analogues block Cl- conductances in A7r5 cells by affecting cellular Ca2+ homeostasis

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.

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.

Involvement of chloride channels in the receptormediated activation of longitudinal colonic muscle

In gastrointestinal smooth muscle, intracellular Cl- is maintained at levels higher than its electrochemical equilibrium. Therefore, Cl- efflux through receptor-mediated opening of Cl- channels should result in membrane depolarization and may be sufficient to activate voltage-sensitive calcium channels (VSCCs). To determine the contribution of Cl- channels to receptor-mediated contraction of the longitudinal muscle layer of the rabbit distal colon, we studied the mechanical response of muscle strips to substance P, carbachol and potassium depolarization following the depletion of Cl- i, and in the presence of the Cl- channel blocker 5-nitro-2-(3-phenylpropyl-amino)-benzoate (NPPB). A 60-min incubation of tissues in a HEPES-buffered solution in which NaCl had been replaced by Na isethionate (or Na gluconate) in equimolar amounts resulted in disappearance of phasic contractions, and in a partially reversible reduction of the tonic response to substance P and carbachol, but not to KCl depolarization. When the agonist was applied to tissues in control solution, or to Cl-(-)depleted tissues in a solution in which Na+ was acutely replaced in equimolar amounts by N-methyl-D-glucosamine, the mechanical response to substance P and carbachol was almost abolished. Acute Na+ replacement alone without prior Cl- depletion did not abolish phasic contractions, but reduced the tonic response to substance P and carbachol. Similar to the effect of Cl- depletion, incubation of tissues in NPPB (6.6 x 10(-5) M) reduced the tonic response to substance P and carbachol, and abolished phasic contractions. These findings are consistent with a contribution of a Cl- channel to the receptor-mediated activation of colonic smooth muscle. In addition, the data suggest that transient Cl- channel mediated depolarizations may play a role in the generation of phasic contractions.

Investigation of the effects of 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) on membrane currents in rat portal vein

1. The effects of 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) were investigated on evoked and spontaneous currents in freshly-isolated cells from the rat portal vein by use of conventional whole-cell recording and perforated-patch techniques. 2. At a holding potential of -60 mV in potassium-free, caesium-containing solutions, NPPB (10 microM) inhibited calcium (Ca)-sensitive chloride currents (ICl(Ca)) evoked by caffeine (10 mM) and by noradrenaline (10 microM) by 58% and 96%, respectively. 3. At a holding potential of -2 mV in potassium (K)-containing solutions, NPPB (10 microM) inhibited charybdotoxin-sensitive K-currents (IBK(Ca)) induced by noradrenaline (10 microM) and acetylcholine (10 microM) by approximately 90%. In contrast, IBK(Ca) induced by caffeine (10 mM) was unaffected in the presence of NPPB (10 microM). Conversely, IBK(Ca) elicited by caffeine (2 mM) was reduced by approximately 50% whereas IBK(Ca) evoked by noradrenaline (50 microM) was not significantly inhibited by NPPB. 4. In K-containing solutions, NPPB (10 microM) abolished spontaneous transient outward currents (STOCs) and induced a slowly-developing outward K-current. Bath application of glibenclamide (10 microM) abolished the outward current but did not antagonize the inhibitory effects of NPPB on STOCs or on IBK(Ca) evoked by noradrenaline. 5. In caesium-containing solutions, NPPB (30 microM) inhibited voltage-sensitive Ca-currents. 6. In Ca-free, K-containing solutions and in the presence of glibenclamide (5 microM), IBK(Ca) induced by 20 microM NS1619 was enhanced by NPPB (10 microM). 7. It is concluded that NPPB inhibits agonist-induced ICl(Ca) in rat portal vein smooth muscle. However, this agent also inhibits agonist-evoked IBK(Ca) and STOCs. Moreover, NPPB inhibits voltage-sensitive Ca-currents and stimulates a glibenclamide-sensitive K-current and IBK(Ca). The effects of this agent on evoked ICl(Ca) and IBK(Ca) and on STOCs probably involves an inhibitory action on intracellular Ca-stores.