Chloride channel blockers decrease intracellular pH in cultured renal epithelial LLC-PK1 cells

Carbocisteine stimulated an increase in ciliary bend angle via a decrease in [Cl-]i in mouse airway cilia

Carbocisteine (CCis), a mucoactive agent, is widely used to improve respiratory diseases. This study demonstrated that CCis increases ciliary bend angle (CBA) by 30% and ciliary beat frequency (CBF) by 10% in mouse airway ciliary cells. These increases were induced by an elevation in intracellular pH (pH_i; the pH_i pathway) and a decrease in the intracellular Cl^- concentration ([Cl^-]_i; the Cl^- pathway) stimulated by CCis. The Cl^- pathway, which is independent of CO₂/HCO₃^-, increased CBA by 20%. This pathway activated Cl^- release via activation of Cl^- channels, leading to a decrease in [Cl^-]_i, and was inhibited by Cl^- channel blockers (5-nitro-2-(3-phenylpropylamino) benzoic acid and CFTR(inh)-172). Under the CO₂/HCO₃^--free condition, the CBA increase stimulated by CCis was mimicked by the Cl^--free NO₃^- solution. The pH_i pathway, which depends on CO₂/HCO₃^-, increased CBF and CBA by 10%. This pathway activated HCO₃^- entry via Na⁺/HCO₃^- cotransport (NBC), leading to a pH_i elevation, and was inhibited by 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid. The effects of CCis were not affected by a protein kinase A inhibitor (1 μM PKI-A) or Ca²⁺-free solution. Thus, CCis decreased [Cl^-]_i via activation of Cl^- channels including CFTR, increasing CBA by 20%, and elevated pH_i via NBC activation, increasing CBF and CBA by 10%.

Growth inhibition of fungus Phycomyces blakesleeanus by anion channel inhibitors anthracene-9-carboxylic and niflumic acid attained through decrease in cellular respiration and energy metabolites

Increasing resistance of fungal strains to known fungicides has prompted identification of new candidates for fungicides among substances previously used for other purposes. We have tested the effects of known anion channel inhibitors anthracene-9-carboxylic acid (A9C) and niflumic acid (NFA) on growth, energy metabolism and anionic current of mycelium of fungus Phycomyces blakesleeanus. Both inhibitors significantly decreased growth and respiration of mycelium, but complete inhibition was only achieved by 100 and 500 µM NFA for growth and respiration, respectively. A9C had no effect on respiration of human NCI-H460 cell line and very little effect on cucumber root sprout clippings, which nominates this inhibitor for further investigation as a potential new fungicide. Effects of A9C and NFA on respiration of isolated mitochondria of P. blakesleeanus were significantly smaller, which indicates that their inhibitory effect on respiration of mycelium is indirect. NMR spectroscopy showed that both A9C and NFA decrease the levels of ATP and polyphosphates in the mycelium of P. blakesleeanus, but only A9C caused intracellular acidification. Outwardly rectifying, fast inactivating instantaneous anionic current (ORIC) was also reduced to 33±5 and 21±3 % of its pre-treatment size by A9C and NFA, respectively, but only in the absence of ATP. It can be assumed from our results that the regulation of ORIC is tightly linked to cellular energy metabolism in P. blakesleeanus, and the decrease in ATP and polyphosphate levels could be a direct cause of growth inhibition.

Single-cell intracellular nano-pH probes

Within a large clonal population, such as cancerous tumor entities, cells are not identical, and the differences between intracellular pH levels of individual cells may be important indicators of heterogeneity that could be relevant in clinical practice, especially in personalized medicine. Therefore, the detection of the intracellular pH at the single-cell level is of great importance to identify and study outlier cells. However, quantitative and real-time measurements of the intracellular pH of individual cells within a cell population is challenging with existing technologies, and there is a need to engineer new methodologies. In this paper, we discuss the use of nanopipette technology to overcome the limitations of intracellular pH measurements at the single-cell level. We have developed a nano-pH probe through physisorption of chitosan onto hydroxylated quartz nanopipettes with extremely small pore sizes (~100 nm). The dynamic pH range of the nano-pH probe was from 2.6 to 10.7 with a sensitivity of 0.09 units. We have performed single-cell intracellular pH measurements using non-cancerous and cancerous cell lines, including human fibroblasts, HeLa, MDA-MB-231 and MCF-7, with the pH nanoprobe. We have further demonstrated the real-time continuous single-cell pH measurement capability of the sensor, showing the cellular pH response to pharmaceutical manipulations. These findings suggest that the chitosan-functionalized nanopore is a powerful nano-tool for pH sensing at the single-cell level with high temporal and spatial resolution.

Cell swelling-induced ATP release is tightly dependent on intracellular calcium elevations

Mechanical stresses release ATP from a variety of cells by a poorly defined mechanism(s). Using custom-designed flow-through chambers, we investigated the kinetics of cell swelling-induced ATP secretion, cell volume and intracellular calcium changes in epithelial A549 and 16HBE14o- cells, and NIH/3T3 fibroblasts. Fifty per cent hypotonic shock triggered transient ATP release from cell confluent monolayers, which consistently peaked at around 1 min 45 s for A549 and NIH/3T3, and at 3 min for 16HBE14o- cells, then declined to baseline within the next 15 min. Whereas the release time course had a similar pattern for the three cell types, the peak rates differed significantly (294 +/- 67, 70 +/- 22 and 17 +/- 2.8 pmol min(-1) (10(6) cells)(-1), for A549, 16HBE14o- and NIH/3T3, respectively). The concomitant volume changes of substrate-attached cells were analysed by a 3-dimensional cell shape reconstruction method based on images acquired from two perpendicular directions. The three cell types swelled at a similar rate, reaching maximal expansion in 1 min 45 s, but differed in the duration of the volume plateau and regulatory volume decrease (RVD). These experiments revealed that ATP release does not correlate with either cell volume expansion and the expected activation of stretch-sensitive channels, or with the activation of volume-sensitive, 5-nitro-2-(3-phenylpropylamino) benzoic acid-inhibitable anion channels during RVD. By contrast, ATP release was tightly synchronized, in all three cell types, with cytosolic calcium elevations. Furthermore, loading A549 cells with the calcium chelator BAPTA significantly diminished ATP release (71% inhibition of the peak rate), while the calcium ionophore ionomycin triggered ATP release in the absence of cell swelling. Lowering the temperature to 10 degrees C almost completely abolished A549 cell swelling-induced ATP release (95% inhibition of the peak rate). These results strongly suggest that calcium-dependent exocytosis plays a major role in mechanosensitive ATP release.

The non-steroidal anti-inflammatory drug niflumic acid inhibits Candida albicans growth

The non-steroidal anti-inflammatory drug niflumic acid was found to inhibit growth of the yeast form of Candida albicans. Niflumic acid inhibited respiratory oxygen uptake and it is hypothesised that this was achieved by cytosolic acidification and block of glycolysis. Inhibitory concentrations are compatible with current practice of topical application.

Chloride channel antagonists perturb growth and morphology of Neurospora crassa

The chloride channel antagonists anthracene-9-carboxylic acid, ethacrynic acid and niflumic acid were found to be fungistatic and morphogenic when tested against the ascomycete Neurospora crassa. Potency increased with decreasing pH, suggesting that the protonated forms of the compounds were active. Niflumic acid produced the most pronounced growth aberrations which may reflect an ability to acidify the cytoplasm and block the plasma membrane anion channel of N. crassa.

MAP kinase activation by hypoosmotic stress of tobacco cell suspensions: towards the oxidative burst response?

Hypoosmotic stress activates a phosphorylation-dependent oxidative burst. In-gel kinase assays were performed to characterize the protein kinases that could be implicated in osmoregulation and in the activation of the oxidative burst. Hypoosmotic stress activated several kinases among which 50 and 46 kDa proteins displayed mitogen-activated protein kinase (MAP kinase) properties. They phosphorylated myelin basic protein in the absence of calcium, were recognized by antibodies directed against human MAP kinases, and were phosphorylated on tyrosine. Immunoprecipitation with an antibody directed against the tobacco MAP kinase Ntf4 showed that at least one of the activated kinases would be Ntf4-like. Apigenin, a MAP kinase and cyclin-dependent kinase inhibitor which prevents the hypoosmotically induced oxidative burst (Cazale et al. 1998; Plant Physiol. 116, 659-669), inhibited these kinases in vitro suggesting that they may play a role in the activation of the oxidative burst. Like the oxidative response, activation of the kinases depended on extracellular calcium influx and protein kinases sensitive to staurosporine and 6-DMAP. However, kinase activation did not depend on effluxes through anion channels or on the oxidative burst. Two-dimensional in-gel kinase assays revealed the presence of three protein kinases with an apparent molecular mass of 50 kDa and one of 46 kDa, all four being activated by hypoosmotic stress. The same kinases were also activated by oligogalacturonides and salicylic acid, underlying the importance of these MAP kinases as common components of different signaling pathways triggered by different extracellular stimuli.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Effects of UTP on Na+, Cl- and K+ transport in primary cultures from human sweat gland coils

Extracellular ATP and UTP can increase membrane permeability in the sweat gland, but the intracellular signalling regulating the response to these agonists is poorly understood. Stimulation of Cl- transport by nucleotides has been suggested as a pharmacological therapy to improve Cl- secretion in patients with cystic fibrosis. In the present study, regulation of Na+, Cl- and K+ transport in primary cultures of cells from the secretory coil of human sweat glands was investigated by electron probe X-ray microanalysis. Stimulation with 200 microM UTP for 2 min at room temperature caused a significant increase in intracellular Na but did not affect Cl and K. After 5 min, the Na concentration was still increased, but now also a significant decrease in Cl and K was observed, indicating an increase in Cl- and K+ permeability. The effect of UTP on Cl- secretion was enhanced in Mg2+-deficient buffer, indicating that the response is elicited by the extracellular fully ionized form of UTP (UTP4+), but not by MgUTP2+. The effects of UTP were abolished in Ca2+-deficient buffer supplemented with EGTA. Alloxan, an adenylate cyclase inhibitor, did not inhibit the response to UTP. These results indicate that the membrane Cl- and K+ permeability elicited by UTP in primary coil cell cultures is Ca2+-dependent. The response to UTP did not attenuate at 8 degrees C, suggesting that it could be activated, in part, via ligand-gated ion channels. The effect of UTP was not decreased in the presence of ouabain. Pre-treatment of the cells with pertussis toxin (24 h) had minor effects on Cl- secretion activated by UTP, indicating a role for G proteins in the UTP activation of Cl- secretion.

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.