Potassium channels in gastrointestinal smooth muscle

Estrogen inhibits colonic smooth muscle contractions by regulating BKβ1 signaling

The estrogen inhibits colonic smooth muscle contractions, which may lead to constipation. However, the mechanisms of inhibition are poorly understood. Therefore, the present study examined the effect of estrogen on rat colonic smooth muscle contractions and its potential association with the large-conductance Ca2+-activated K+ channels β1 (BKβ1) subunit. Twenty-four female Sprague Dawley rats were randomly assigned to 4 groups. After 2 weeks of intervention, the contraction activity of isolated colonic smooth muscle and the expression of BKβ1 in colonic smooth muscle of rats were detected. Additionally, in order to investigate the effects of estrogen on BKβ1 expression and calcium mobilization, in vitro experiments were conducted using rat and human colonic smooth muscle cells (SMCs). BKβ1 shRNA was used to investigate whether calcium mobilization is affected by BKβ1 in colonic SMCs. To explore the relationship between ERβ and BKβ1, serial deletions, site-directed mutagenesis, a dual-luciferase reporter assay, and chromatin immunoprecipitation assays were employed. In response to E2, colonic smooth muscle strips showed a decrease in tension, while IBTX exposure transiently increased tension. Furthermore, in these muscle tissues, BKβ1 and α-SMA were found to be co-expressed. The E2 group showed significantly higher BKβ1 expression. In cultured colonic SMCs, the expression of BKβ1 was found to increase in the presence of E2 or DPN. E2 treatment reduced Ca2+ concentrations, while BKβ1 shRNA treatment increased Ca2+ concentrations relative to the control. ERβ-initiated BKβ1 expression appears to occur via binding to the BKβ1 promoter. These results indicated that E2 may upregulate BKβ1 expression via ERβ and inhibit colonic smooth muscle contraction through ERβ by directly targeting BKβ1.

Antispasmodic Effect of Valeriana pilosa Root Essential Oil and Potential Mechanisms of Action: Ex Vivo and In Silico Studies

Infusions of Valeriana pilosa are commonly used in Peruvian folk medicine for treating gastrointestinal disorders. This study aimed to investigate the spasmolytic and antispasmodic effects of Valeriana pilosa essential oil (VPEO) on rat ileum. The basal tone of ileal sections decreased in response to accumulative concentrations of VPEO. Moreover, ileal sections precontracted with acetylcholine (ACh), potassium chloride (KCl), or barium chloride (BaCl2) were relaxed in response to VPEO by a mechanism that depended on atropine, hyoscine butylbromide, solifenacin, and verapamil, but not glibenclamide. The results showed that VPEO produced a relaxant effect by inhibiting muscarinic receptors and blocking calcium channels, with no apparent effect on the opening of potassium channels. In addition, molecular docking was employed to evaluate VPEO constituents that could inhibit intestinal contractile activity. The study showed that α-cubebene, β-patchoulene, β-bourbonene, β-caryophyllene, α-guaiene, γ-muurolene, valencene, eremophyllene, and δ-cadinene displayed the highest docking scores on muscarinic acetylcholine receptors and voltage-gated calcium channels, which may antagonize M2 and/or M3 muscarinic acetylcholine receptors and block voltage-gated calcium channels. In summary, VPEO has both spasmolytic and antispasmodic effects. It may block muscarinic receptors and calcium channels, thus providing a scientific basis for its traditional use for gastrointestinal disorders.

Mechanistic insights of Cucumis melo L. seeds for gastrointestinal muscle spasms through calcium signaling pathway-related gene regulation networks in WGCNA and in vitro, in vivo studies

BACKGROUND

In addition to the nutritional benefits of Cucumis melo L., herbalists in Pakistan and India employ seeds to treat various ailments. This study aimed to determine the regulatory role of C. melo seeds in calcium-mediated smooth muscle contraction.

METHODS

We identified and quantified the phytochemicals of C. melo with LC ESI-MS/MS and HPLC, then conducted in vitro and in vivo tests to confirm the involvement in smooth muscle relaxation. Then, diarrhea-predominant irritable bowel syndrome gene datasets from NCBI GEO were acquired, DEGs and WGCNA followed by functional enrichment analysis. Next, molecular docking of key genes was performed.

RESULTS

The quantification of C. melo seeds revealed concentrations of rutin, kaempferol, and quercetin were 702.38 μg/g, 686.29 μg/g, and 658.41 μg/g, respectively. In vitro experiments revealed that C. melo seeds had a dose-dependent relaxant effect for potassium chloride (80 mM)-induced spastic contraction and exhibited calcium antagonistic response in calcium dose-response curves. In in vivo studies, Cm.EtOH exhibited antidiarrheal, antiperistaltic, and antisecretory effects. The functional enrichment of WGCNA and DEGs IBS-associated pathogenic genes, including those involved in calcium-mediated signaling, MAPK cascade, and inflammatory responses. MAPK1 and PIK3CG were identified as key genes with greater binding affinity with rutin, quercitrin, and kaempferol in molecular docking.

CONCLUSIONS

The bronchodilator and antidiarrheal effects of C. melo were produced by altering the regulatory genes of calcium-mediated smooth contraction.

Dose-dependent Spasmolytic, Bronchodilator, and Hypotensive Activities of Panicum miliaceum L

Panicum miliaceum L. is a medicinally effective plant used in indigenous system of medicine for a variety of ailments. However, there is no comprehensive study explaining its effectiveness in gastrointestinal tract, respiratory, and cardiovascular system ailments. This study was designed to validate the pharmacological basis for the folkloric use of Panicum miliaceum L. in diarrhea, asthma, and hypertension. Panicum miliaceum extract was analyzed to detect the presence of bioactive compounds by HPLC. The isolated rabbit jejunum, trachea, and aorta were used for in vitro experiments using tissue bath assembly coupled with Power Lab data acquisition system to explore their relative effects. In-vivo experiments were performed for anti-diarrheal activity. HPLC analysis revealed the presence of gallic acid, butylated hydroxytoluene, catechin, and quercetin. Concentration dependent activities were observed by relaxing K+ (low) induced contractions having spasmolytic effect with EC50 = .358 ± .052, bronchodilator (EC50 = 2.483 ± .05793), and vasorelaxant (EC50 = .383 ± .063), probably due to the ATP dependent potassium channel activation. It was confirmed through pre-exposure of glibenclamide (specific ATP-dependent K+ channel blocker) having similarities with cromakalim. Pm.Cr revealed its antidiarrheal via in vivo experiments on rats. This study indicates that Panicum miliaceum has antidiarrheal, spasmolytic, bronchodilator, and vasorelaxant activities probably due to the ATP dependent K+ channel activation.

Antispasmodic Effect of Asperidine B, a Pyrrolidine Derivative, through Inhibition of L-Type Ca2+ Channel in Rat Ileal Smooth Muscle

Antispasmodic agents are used for modulating gastrointestinal motility. Several compounds isolated from terrestrial plants have antispasmodic properties. This study aimed to explore the inhibitory effect of the pyrrolidine derivative, asperidine B, isolated from the soil-derived fungus Aspergillus sclerotiorum PSU-RSPG178, on spasmodic activity. Isolated rat ileum was set up in an organ bath. The contractile responses of asperidine B (0.3 to 30 µM) on potassium chloride and acetylcholine-induced contractions were recorded. To investigate its antispasmodic mechanism, CaCl2, acetylcholine, Nω-nitro-l-arginine methyl ester (l-NAME), nifedipine, methylene blue and tetraethylammonium chloride (TEA) were tested in the absence or in the presence of asperidine B. Cumulative concentrations of asperidine B reduced the ileal contraction by ~37%. The calcium chloride and acetylcholine-induced ileal contraction was suppressed by asperidine B. The effects of asperidine B combined with nifedipine, atropine or TEA were similar to those treated with nifedipine, atropine or TEA, respectively. In contrast, in the presence of l-NAME and methylene blue, the antispasmodic effect of asperidine B was unaltered. These results suggest that the antispasmodic property of asperidine B is probably due to the blockage of the L-type Ca2+ channel and is associated with K+ channels and muscarinic receptor, possibly by affecting non-selective cation channels and/or releasing intracellular calcium.

Molecular mechanism of apelin-13 regulation of colonic motility in rats

Apelin is a novel neuropeptide identified as the endogenous ligand for the apelin receptor. Apelin and its receptor are widely distributed in the gastrointestinal tract. Studies have reported that apelin-13 is involved in modulating gastrointestinal motility; however, the evidence is insufficient and the relevant mechanism is still not fully clear. Consequently, our study designed to explore the effect induced by exogenous apelin-13, to analyze the mechanism of action in isolated rat colons and colonic smooth muscle cells. The spontaneous contractions of colonic smooth muscle strips from rat were measured in an organ bath system. L-type calcium currents and large conductance Ca2+-activated K+ (BKCa) currents in rat colonic smooth muscle cells were investigated using the electrophysiological patch-clamp technique. Apelin-13 decreased the spontaneous contractile activity of colonic smooth muscle strips in a dose-dependent manner, and the inhibitory effect was not abolished by tetrodotoxin. The electrophysiological recordings revealed that apelin-13 reduced the crest currents of L-type calcium in a concentration-dependent manner in colonic smooth muscle cells at the test potential of 0 mV. Moreover, apelin-13 moved the current-voltage (I-V) curves of L-type calcium channels upward, but did not change their contour. Furthermore, the characteristics of L-type calcium channels with steady-state activation and steady-state inactivation were not significantly changed. Similarly, application of apelin-13 also significantly decreased BKCa currents in a concentration-dependent manner. In conclusion, apelin-13 inhibited the spontaneous contractile activity of isolated rat colons via the suppression of L-type calcium channels and BKCa channels in colonic smooth muscle cells.

Electrophysiological modeling of the effect of potassium channel blockers on the distribution of stimulation wave in the human gastric wall cells

The purpose of this study is to model the electrophysiological behavior of excitable membrane and wavefront propagation in the Stomach Wall in physiological and pharmacological states. The propagation of this wave is based on cellular electrophysiological activity and ionic channel properties. In this study, we arranged the stomach wall cells together using the Gap Junctions approach. Slow wave is generated by gastric pacemaker cells. This wave propagates via the interaction of cells with each other throughout the stomach wall. Potassium currents are one of the main factors in regulating the pattern of wavefront propagation. To investigate the effect of limiting the exchange of potassium currents from cell membranes, 10%, 50%, 90%, and complete blockade were applied on both non-inactivating potassium current (I_Kni) and fast-inactivating potassium current (I_Kfi). The results show that I_Kniion channel blockage has a considerable effect on the plateau phase in the propagation of the excitation wave. The maximum value of the action potential in the plateau phase in the excitation wave with complete obstruction from -40.92 mV in the physiological state reached -18.97 mV, which is about 54% higher than the physiological state. Also, compared to the physiological state, complete blockage of the I_Kfi causes a 15% increase in the slow-wave spike phase (from -36.72 mV to -31.36 mV). Using this model, the effect of ions in different phases of slow-wave can be investigated. In addition, by blocking ion channels, functional disorders and smooth muscle contraction can be improved.

Quercetin relaxes human gastric smooth muscles directly through ATP-sensitive potassium channels and not depending on the nitric oxide pathway

BACKGROUND

Quercetin has recently become a remarkably popular subject of research due to its broad beneficial pharmacological properties. The goal of our study was to observe its effects on contractility of human gastric smooth muscles in reference to the NO pathway and direct influence of potassium channels.

METHODS

Tissues were obtained from patients undergoing sleeve gastrectomy due to morbid obesity (n = 10 aged 24-56; BMI 47.16 ± 1.84). The following parameters were evaluated in the recordings: area under the curve (AUC), average baseline muscle tone, and relative change in muscle contraction.

KEY RESULTS

Quercetin induced noticeable, dose-dependent relaxation of the carbachol treated gastric strips. The substantial effect was noted at concentrations higher than 10^-7  mol/L and maximal at 10^-4  mol/L (81.82 ± 3.32%; n = 10; p < 0.0001) of the control. Neither NOS blockers nor guanylyl cyclase blockers had inhibitory effects on the relaxation of strips induced by examined polyphenol. Glibenclamide inhibited the relaxing effect of quercetin, significant at concentrations higher than 5⋅10^-5  mol/L. Preincubation with charybdotoxin or apamin extended the relaxing effect of quercetin (from 10^-6  mol/L). Tamoxifen, in turn, significantly increased muscle relaxation at all quercetin concentrations.

CONCLUSIONS & INFERENCES

In conclusion, the current study was the first to show that quercetin-induced relaxation of human gastric smooth muscle occurs directly through K⁺_ATP channels and independently to NO pathways. The present results suggest that quercetin is a potential nutraceutical in the treatment of functional gastrointestinal dyspepsia and other minor gastric muscle motility disturbance.

Aqueous Tuber Extracts of Tylosema fassoglense (Kotschy ex Schweinf.) Torre and Hillc. (Fabaceae). Possess Significant In-Vivo Antidiarrheal Activity and Ex-Vivo Spasmolytic Effect Possibly Mediated by Modulation of Nitrous Oxide System, Voltage-Gated Calcium Channels, and Muscarinic Receptors

Tylosema fassoglense (TFG) is used as an antidiarrheal traditional medicine in Western Kenya. This study aimed to investigate the antidiarrheal activity of its aqueous extracts in-vivo and the putative mechanism (s) of action ex-vivo using Sprague-Dawley rats and New Zealand white rabbits respectively. The in-vivo antidiarrheal effects of the extract were evaluated in castor oil-induced diarrhea, the castor oil-induced enteropooling, and phenol red gastric motility tests. On the other hand, isolated rabbit's jejunal segments were used to evaluate the spasmolytic effect of TFG on spontaneous contraction, in acetylcholine-induced contraction, in presence of 80mMK⁺, calcium chloride-induced contraction as well as in presence of the following antagonists: naloxone, methylene blue, L-NAME, prazosin, and propranolol in the ex-vivo studies. The data were express as Mean ± S.E.M and analyzed by one-way ANOVA and Tukey's post hoc test in cases of significance which was set at p < 0.05. The extract was phytochemically characterized using Liquid chromatography Mass spectroscopy (LC-MS).The extract possessed significant inhibitory effect in the in-vivo experiments. The extract exhibited significant spasmolytic effect on both spontaneous contraction and in jejunal segment pre-contracted acetylcholine as well as in presence of 80mMK⁺ solution. It also attenuated the spasmogenic effect of various concentration of calcium chloride. The extract's spasmolytic effect was, however, significantly attenuated in presence of several antagonists (methylene blue and L-NAME) but the adrenergic blockers (prazosin and propranolol) had no significant effect in the ex-vivo studies. LC-MS identified thirty compounds where Proathocyanidin (11.54%), Syringic acid (7.30%), and 4-Hydroxybenzoic acid (6.19%) had the highest percentage abundance. In conclusion, the results obtained in this study partially validate the traditional uses of the tubers of this plant species as an antidiarrheal. These antidiarrheal effects are probably mediated via modulation of nitrous oxide pathway, voltage gated calcium channels, and muscarinic receptors.

Toxicological and Pharmacological Activities of Leptohyptis macrostachys (Benth.) Harley and J.F.B.Pastore (Lamiaceae) on Intestinal Smooth Muscle

Leptohyptis macrostachys, previously known as Hyptis macrostachys Benth., is used in folk medicine to relieve the symptoms of asthma, cough, and bronchitis. Recently, we showed that the ethanol extract obtained from Leptohyptis macrostachys has selective spasmolytic activity on guinea pig ileum. Therefore, the aim of this study was to characterize the spasmolytic mechanism of this extract, investigated whether it presents toxicological and antidiarrheal activities. Therefore, the crude ethanolic extract of Leptohyptis macrostachys was analyzed by high-performance liquid chromatographic-diode array detection (HPLC-DAD). The spasmolytic effect was evaluated on guinea pig ileum, toxicological activity using rats and antidiarrheal activity using male and female mice. In HPLC-DAD analysis, Rosmarinic acid (5.44%) was the most abundant phenolic compound, being considered as a chemical marker. The spasmolytic potency of the extract on histamine-induced contraction was reduced in the presence of 1 mM TEA+, a selective big-conductance K+ channels blocker (BKCa). The extract produces a dose-dependent antidiarrheal activity, inhibiting equipotently defecation frequency and liquid stool formation. In addition, the extract has inhibited in a dose-dependent manner both castor oil-induced intestinal transit and intestinal fluid content. Thus, the spasmolytic activity of the extract involves positive modulation of BKCa and its antidiarrheal activity is related to inhibition of intestinal motility and secretion.

Depolarization of pacemaker potentials by caffeic acid phenethyl ester in interstitial cells of Cajal from the murine small intestine

Interstitial cells of Cajal (ICCs) are pacemaker cells in the gastrointestinal (GI) tract and generate pacemaker potentials. In this study, we investigated the effects of caffeic acid phenethyl ester (CAPE) on the pacemaker potentials of ICCs from the mouse small or large intestine. Using the whole-cell patch-clamp configuration, we found that CAPE depolarized the pacemaker potentials of cultured ICCs from the murine small intestine in a dose-dependent manner. The estrogen receptor (ER) β antagonist PHTPP completely inhibited CAPE-induced depolarization, but the ERα antagonist BHPI did not. Intracellular GDP-β-S and pretreatment with Ca²⁺-free solution or thapsigargin also blocked CAPE-induced depolarization. To investigate the mechanisms of CAPE-mediated depolarization of ICCs, we used the nonselective cation channel (NSCC) inhibitor flufenamic acid, the Cl^- channel blocker, mitogen-activated protein kinase (MAPK) inhibitors PD98059, SB203580, or SP600125, and PI3 kinase inhibitor LY294002. All inhibitors blocked the CAPE-induced pacemaker potential depolarization of ICCs. These results suggest that CAPE induces pacemaker potential depolarization through ERβ in a G protein, NSCC, Cl^- channel, MAPK- and PI3 kinase dependent manner via intracellular and extracellular Ca²⁺ regulation in the murine small intestine. CAPE may therefore modulate GI motility by acting on ICCs in the murine small intestine.

Involvement of Potassium Channels, Nitric Oxide Synthase, and Guanylate Cyclase in the Spasmolytic Effect of Simaba ferruginea A.St.-Hil on Rat Isolated Ileum

BACKGROUND AND AIM

Simaba ferruginea A.St.-Hil. Popularly known as "calunga," is a typical Brazilian cerrado plant whose rhizomes are popular for treating diarrhea.

AIMS

The aim of this study was to evaluate the spasmolytic activity and the antidiarrheal effect of the ethanolic extract obtained from S. ferruginea (Sf-EtOH).

METHODS

Ileal segments (1-2 cm) from male Wistar rats were mounted in isolated organ baths and connected to a force transducer, and then to an amplifier which was connected to a computer (AVS Projetos/São Paulo-SP). After stabilization for 60 min, under tension (1 gf), two submaximal contractions were induced with KCl 40 mM or carbachol 10^-6 M on ileal segments. During the third tonic and sustained contraction, Sf-EtOH was added in cumulative concentrations to the organ bath. Incubations with L-NAME (10^-4 M), ODQ (10^-5 M), TEA⁺ (5 or 1 mM), glibenclamide (10^-5 M), or apamine (100 nM) were prepared (n = 5), separately and used to verify the involvement of the nitric oxide synthase, guanylate cyclase, and potassium channels in the relaxing effect. The results were expressed as mean ± standard error of the mean and were statistically evaluated using one-way ANOVA followed by Bonferroni test, when necessary *p < 0.05.

RESULTS

Sf-EtOH promotes relaxation on rat isolated ileum pre-contracted with CCh and KCl in a concentration-dependent manner. Sf-EtOH also inhibited ileum contractions against cumulative concentrations of carbachol (CCh), KCl, and CaCl₂, shifting the curves to the right in a non-parallel manner with an E_max reduction. In the presence of potassium channel blockers, Sf-EtOH shifted the curves to the right with a reduction of E_max, suggesting the involvement of BK_Ca, K_ATP, and SK_Ca in its spasmolytic effect. In the presence of L-NAME or ODQ, the relaxation curves were shifted to the right, suggesting the involvement of this pathway in Sf-EtOH spasmolytic effect.

CONCLUSIONS

Sf-EtOH acts in a concentration-dependent manner, involving the positive modulation of K⁺ channels and NO pathway.

ATP-dependent potassium channels contribute to motor regulation of esophageal striated muscle in rats

The aim of the present study was to clarify roles of ATP-dependent potassium channels (K_ATP channels) in motility of the striated muscle portion in the esophagus. An isolated segment of the rat esophagus was placed in an organ bath and mechanical responses were recorded using a force transducer. Electrical stimulation of the vagus nerve evoked contractile response of striated muscle in the esophageal segment. Application of glibenclamide, an antagonist of K_ATP channels, increased amplitude of vagally mediated twitch contractions of the rat esophagus. On the other hand, minoxidil, an agonist of K_ATP channels, decreased amplitude of twitch contractions. RT-PCR revealed the expression of subunits of K_ATP channels in esophageal tissue. In addition, immunopositivity for subunits of K_ATP channels was observed in the striated muscle cells of the esophageal muscle layer. These findings indicate that K_ATP channels contribute to motor regulation of striated muscle in the rat esophagus.

Computational Modeling of Glucose Uptake in the Enterocyte

Absorption of glucose across the epithelial cells of the small intestine is a key process in human nutrition and initiates signaling cascades that regulate metabolic homeostasis. Validated and predictive mathematical models of glucose transport in intestinal epithelial cells are essential for interpreting experimental data, generating hypotheses, and understanding the contributions of and interactions between transport pathways. Here we report on the development of such a model that, in contrast to existing models, incorporates mechanistic descriptions of all relevant transport proteins and is implemented in the CellML framework. The model is validated against experimental and simulation data from the literature. It is then used to elucidate the relative contributions of the sodium-glucose cotransporter (SGLT1) and the glucose transporter type 2 (GLUT2) proteins in published measurements of glucose absorption from human intestinal epithelial cell lines. The model predicts that the contribution of SGLT1 dominates at low extracellular glucose concentrations (<20 mM) and short exposure times (<60 s) while the GLUT2 contribution is more significant at high glucose concentrations and long durations. Implementation in CellML permitted a modular structure in which the model was composed by reusing existing models of the individual transporters. The final structure also permits transparent changes of the model components and parameter values in order to facilitate model reuse, extension, and customization (for example, to simplify, or add complexity to specific transporter/pathway models, or reuse the model as a component of a larger framework) and carry out parameter sensitivity studies.

Estradiol mediates relaxation of porcine lower esophageal sphincter

Most pregnant women have symptoms of gastroesophageal reflux disease (GERD) during pregnancy. Postmenopausal hormone replacement therapy is associated with GERD. The effects of estradiol on lower esophageal sphincter (LES) motility and GERD are not clearly known. The purpose of this study is to investigate the effects of estradiol on the motility of the porcine LES. Relaxations of clasp and sling strips of porcine LES caused by estradiol were measured using isometric transducers. We investigated the mechanism of estradiol-induced relaxation of the porcine LES using tetraethylammonium, apamine, iberiotoxin, glibenclamide, KT5720, KT5823, NG-nitro-l-arginine, tetrodotoxin, and ω-conotoxin GVIA. Reverse transcription polymerase chain reaction (PCR) analysis and immunohistochemistry (IHC) were performed to determine the existence of the G protein-coupled estrogen receptor (GPER) in the porcine LES. In endothelin-1-precontracted porcine LES strips, estradiol caused marked relaxations in a concentration-dependent manner. The mechanism of estradiol-induced relaxation on the porcine LES was associated with the potassium channel. Reverse transcription PCR analysis and IHC revealed that GPER was expressed in the sling and clasp fibers of the porcine LES. This finding suggests that GPER mediates the relaxation of the porcine LES. Estradiol may play a role in LES motility.

Novel treatment strategies for smooth muscle disorders: Targeting Kv7 potassium channels

Smooth muscle cells provide crucial contractile functions in visceral, vascular, and lung tissues. The contractile state of smooth muscle is largely determined by their electrical excitability, which is in turn influenced by the activity of potassium channels. The activity of potassium channels sustains smooth muscle cell membrane hyperpolarization, reducing cellular excitability and thereby promoting smooth muscle relaxation. Research over the past decade has indicated an important role for Kv7 (KCNQ) voltage-gated potassium channels in the regulation of the excitability of smooth muscle cells. Expression of multiple Kv7 channel subtypes has been demonstrated in smooth muscle cells from viscera (gastrointestinal, bladder, myometrial), from the systemic and pulmonary vasculature, and from the airways of the lung, from multiple species, including humans. A number of clinically used drugs, some of which were developed to target Kv7 channels in other tissues, have been found to exert robust effects on smooth muscle Kv7 channels. Functional studies have indicated that Kv7 channel activators and inhibitors have the ability to relax and contact smooth muscle preparations, respectively, suggesting a wide range of novel applications for the pharmacological tool set. This review summarizes recent findings regarding the physiological functions of Kv7 channels in smooth muscle, and highlights potential therapeutic applications based on pharmacological targeting of smooth muscle Kv7 channels throughout the body.

The Modulation of Potassium Channels in the Smooth Muscle as a Therapeutic Strategy for Disorders of the Gastrointestinal Tract

Alterations of smooth muscle contractility contribute to the pathophysiology of important functional gastrointestinal disorders (FGIDs) such as functional dyspepsia and irritable bowel syndrome. Consequently, drugs that decrease smooth muscle contractility are effective treatments for these diseases. Smooth muscle contraction is mainly triggered by Ca(2+) influx through voltage-dependent channels located in the plasma membrane. Thus, the modulation of the membrane potential results in the regulation of Ca(2+) influx and cytosolic levels. K(+) channels play fundamental roles in these processes. The open probability of K(+) channels increases in response to various stimuli, including membrane depolarization (voltage-gated K(+) [K(V)] channels) and the increase in cytosolic Ca(2+) levels (Ca(2+)-dependent K(+) [K(Ca)] channels). K(+) channel activation is mostly associated with outward K(+) currents that hyperpolarize the membrane and reduce cell excitability and contractility. In addition, some K(+) channels are open at the resting membrane potential values of the smooth muscle cells in some gut segments and contribute to set the resting membrane potential itself. The closure of these channels induces membrane depolarization and smooth muscle contraction. K(V)1.2, 1.5, 2.2, 4.3, 7.4 and 11.1, K(Ca)1.1 and 2.3, and inwardly rectifying type 6K(+) (K(ir)6) channels play the most important functional roles in the gastrointestinal smooth muscle. Activators of all these channels may theoretically relax the gastrointestinal smooth muscle and could therefore be promising new therapeutic options for FGID. The challenge of future drug research and development in this area will be to synthesize molecules selective for the channel assemblies expressed in the gastrointestinal smooth muscle.

Inhibitory effects of SKF96365 on the activities of K(+) channels in mouse small intestinal smooth muscle cells

In order to investigate the effects of SKF96365 (SKF), which is a non-selective cationic channel blocker, on K⁺ channel currents, we recorded currents through ATP sensitive K⁺ (I_KATP), voltage-gated K⁺ (I_Kv) and Ca²⁺ activated K⁺ channels (I_BK) in the absence and presence of SKF in single small intestinal myocytes of mice with patch-clamp techniques. SKF (10 µM) reversibly abolished I_KATP that was induced by cromakalim (10 µM), which is a selective ATP sensitive K⁺ channel opener. These inhibitory effects were induced in a concentration-dependent and voltage-independent manner. The 50% inhibitory concentration (IC₅₀) was 0.85 µM, which was obviously lower than that reported for the muscarinic cationic current. In addition, SKF (1 µM ≈ the IC₅₀ value in I_KATP suppression) reversibly inhibited the I_Kv that was induced by repetitive depolarizing pulses from −80 to 20 mV. However, the extent of the inhibitory effects was only ~30%. In contrast, SKF (1 µM) had no significant effects on spontaneous transient I_BK and caffeine-induced I_BK. These results indicated that SKF inhibited ATP sensitive K⁺ channels and voltage-gated K⁺ channels, with the ATP sensitive K⁺ channels being more sensitive than the voltage-gated K⁺ channels. These inhibitory effects on K⁺ channels should be considered when SKF is used as a cationic channel blocker.

Flavonoid galetin 3,6-dimethyl ether attenuates guinea pig ileum contraction through K(+) channel activation and decrease in cytosolic calcium concentration

Flavonoid galetin 3,6-dimethyl ether (FGAL) has been isolated from the aerial parts of Piptadenia stipulaceae and has shown a spasmolytic effect in guinea pig ileum. Thus, we aimed to characterize its relaxant mechanism of action. FGAL exhibited a higher relaxant effect on ileum pre-contracted by histamine (EC50=1.9±0.4×10(-7) M) than by KCl (EC50=2.6±0.5×10(-6) M) or carbachol (EC50=1.8±0.4×10(-6) M). The flavonoid inhibited the cumulative contractions to histamine, as well as to CaCl2 in depolarizing medium nominally Ca(2+)-free. The flavonoid relaxed the ileum pre-contracted by S-(-)-Bay K8644 (EC50=9.5±1.9×10(-6) M) but less potently pre-contracted by KCl or histamine. CsCl attenuated the relaxant effect of FGAL (EC50=1.1±0.3×10(-6) M), but apamin or tetraethylammonium (1mM) had no effect (EC50=2.6±0.2×10(-7) and 1.6±0.3×10(-7) M, respectively), ruling out the involvement of small and big conductance Ca(2+)-activated K(+) channels (SKCa and BKCa, respectively). Either 4-aminopyridine or glibenclamide attenuated the relaxant effect of FGAL (EC50=1.8±0.2×10(-6) and 1.5±0.5×10(-6) M, respectively), indicating the involvement of voltage- and ATP-sensitive K(+) channels (KV and KATP, respectively). FGAL did not alter the viability of intestinal myocytes in the MTT assay and decreased (88%) Fluo-4 fluorescence, indicating a decrease in cytosolic Ca(2+) concentration. Therefore, the relaxant mechanism of FGAL involves pseudo-irreversible noncompetitive antagonism of histaminergic receptors, KV and KATP activation and blockade of CaV1, thus leading to a reduction in cytosolic Ca(2+) levels.

Effects of flavone on the contractile activity of the circular smooth muscle of the rabbit middle colon in vitro

The circular smooth muscles of the middle colon of the rabbit generate giant contractions of high amplitude and low frequency. Flavone, at various concentrations, reduces the giant contractions and the tonic contraction induced by 10 µM carbachol and 80 mM KCl. The contractions induced by dequalinium and tetraethylammonium are reduced by flavone (30 µM). At 100 µM, flavone decreases the contraction induced by 100 µM methylene blue and 1mM orthovanadate. These results suggest that flavone inhibit the giant contractions by (1) inhibition of voltage-dependent Ca(2+) channels, (2) activation of guanyl cyclase, (3) opening of K(+) channels and (4) inhibition of tyrosines kinases.

Antidiarrhoeal, antisecretory and antispasmodic activities of Matricaria chamomilla are mediated predominantly through K(+)-channels activation

Background

Matricaria chamomilla commonly known as “Chamomile” (Asteraceae) is a popular medicinal herb widely used in indigenous system of medicine for a variety of ailments. However, there is no detailed study available showing its effectiveness in hyperactive gut disorders like, abdominal colic and diarrhoea. This study was designed to determine the pharmacological basis for the folkloric use of Matricaria chamomilla in diarrhoea.

Methods

The crude aqueous-methanolic extract of Matricaria chamomilla (Mc.Cr) was studied for its protective effect in mice against castor oil-induced diarrhoea and intestinal fluid accumulation. The isolated rabbit jejunum was selected for the in-vitro experiments using tissue bath assembly coupled with PowerLab data acquisition system.

Results

Oral administration of Mc.Cr to mice at 150 and 300 mg/kg showed marked antidiarrhoeal and antisecretory effects against castor oil-induced diarrhoea and intestinal fluid accumulation, simultaneously, similar to the effects of cromakalim and loperamide. These effects of plant extract were attenuated in animals pretreated with K⁺ channel antagonist, glibenclamide (GB) or 4-aminopyridine (4-AP). When tested in isolated rabbit jejunum, Mc.Cr caused a dose-dependent (0.3-3 mg/ml) relaxation of spontaneous and low K⁺ (25 mM)-induced contractions, while it exhibited weak inhibitory effect on high K⁺ (80 mM). The inhibitory effect of Mc.Cr on low K⁺-induced contractions was partially inhibited in the presence of GB, while completely blocked by 4-AP. Cromakalim, an ATP-sensitive K⁺ channel opener, caused complete relaxation of low K⁺-induced contractions with little effect on high K⁺. Pretreatment of tissues with GB blocked the inhibitory effects of cromakalim on low K⁺, while the presence of 4-AP did not alter the original effect. Verapamil, a Ca⁺⁺ channel antagonist, caused complete relaxation of both low and high K⁺-induced contractions with similar potency. The inhibitory effect of verapamil was insensitive to GB or 4-AP. When assessed for Ca⁺⁺ antagonist like activity, Mc.Cr at high concentrations caused rightward shift in the Ca⁺⁺ concentration-response curves with suppression of the maximum response, similar to the effect of verapamil, while cromakalim did not show similar effect.

Conclusions

This study indicates that Matricaria chamomilla possesses antidiarrhoeal, antisecretory and antispasmodic activities mediated predominantly through K⁺-channels activation along with weak Ca⁺⁺ antagonist effect.

Exogenous H2 S enhances mice gastric smooth muscle tension through S-sulfhydration of KV 4.3, mediating the inhibition of the voltage-dependent potassium current

BACKGROUND

Hydrogen sulfide (H2 S) has been shown to have an excitatory effect on gastric motility, but the underlying molecular mechanism is unclear. In this study, we aimed to investigate the possible targets of H2 S and determine how H2 S affects its target proteins during H2 S-induced contraction.

METHODS

Patch-clamp and potentiometric fluorescence dye were utilized to measure the electrophysiological changes. The Biotin-switch assay was utilized to detect the protein S-sulfhydration. The isometric tension measurement was conducted too.

KEY RESULTS

Exogenous H2 S enhanced the tonic contraction of gastric antral smooth muscle, and voltage-dependent potassium channel (KV ) blocker and Dithiothreitol (DTT, a reducing agent) abolished the excitatory effect of NaHS. Exogenous H2 S inhibited the fast inactivation component of the voltage-dependent potassium channel current (IKVfast ) in isolated gastric antral smooth muscle cells. H2 S inhibited the KV 4.3 current in H293 cells with heterologous expression of KV 4.3, but did not inhibit the KV 4.1 and KV 4.2 currents, which together contribute greatly to IKVfast . NaHS significantly decreased the membrane potential in cultured gastric smooth muscle cells, but the NaHS-induced depolarization was suppressed by knockdown of KV 4.3 and N-ethylamaleimide (NEM), a free thiol group blocker. In addition, NaHS sulfhydrated KV 4.3 in H293 cells and in gastric smooth muscle tissue. However, this S-sulfhydration was inhibited by NEM and DTT. Meanwhile the NaHS-induced inhibition of IKVfast and KV 4.3 was also blocked by NEM and DTT.

CONCLUSIONS & INFERENCES

These results suggest that exogenous H2 S sulfhydrates KV 4.3 to decrease the membrane potential, thereby enhancing the basal tension of gastric antral smooth muscle.

Damage from dissection is associated with reduced neuro-musclar transmission and gap junction coupling between circular muscle cells of guinea pig ileum, in vitro

Excitatory and inhibitory junction potentials of circular smooth muscle cells in guinea pig ileum and colon are suppressed 30–90 min after setting up in vitro preparations. We have previously shown this “unresponsive” period is associated with a transient loss of dye coupling between smooth muscle cells, which subsequently recovers over the ensuing 30–90 min; junction potentials recover in parallel with dye coupling (Carbone et al., 2012). Here, we investigated which components of dissection trigger the initial loss of coupling. Intracellular recordings were made from circular muscle cells of guinea pig ileum with micropipettes containing 5% carboxyfluorescein. After allowing 90–120 min for junction potentials to reach full amplitude, we re-cut all 4 edges of the preparation more than 1 mm from the recording sites. This caused a reduction in the amplitude of IJPs from 17.2 ± 0.7 mV to 9.5 ± 1.5 mV (P < 0.001, n = 12) and a significant reduction in dye coupling. Both recovered within 60 min. We repeated this experiment (n = 4), recording both 1 and 4 mm from the cut edge: both sites were equally affected by re-cutting the sides of the preparation. Equilibrated preparations were stretched to 150% of their original length, this had no significant effect on junction potentials or dye coupling. Setting up preparations in low calcium solution did not prevent the initial suppression of IJPs and dye coupling. Application of 3 μM indomethacin (n = 3), 10 μM ketotifen (n = 4) or 10 μM forskolin during dissection did not prevent the suppression of IJPs and dye coupling. If dissection damage was reduced, by leaving the mucosa and submucosa attached to the circular muscle, IJPs showed less initial suppression than in preparations where the layers were dissected off. We conclude that physical damage to the gut wall triggers loss of gap junction coupling and neuromuscular transmission, this is not due to stretch, influx of calcium ions, release of prostaglandins or mast cell degranulation. The mechanisms underlying this potent effect remain to be determined.

K+ channels as potential targets for the treatment of gastrointestinal motor disorders

K(+) channels play important functional roles in excitable cells, as neurons and muscle cells. The activation or inhibition of K(+) channels hyperpolarizes or depolarizes the cell membrane, respectively. These effects determine in the smooth muscle decrease or increase in Ca(2+) influx through voltage-gated Ca(2+) (CaV1.2) channels and relaxation or contraction, respectively. Recent studies highlight the importance of voltage-dependent type 7 K(+) (KV7 or KCNQ) channels in regulating muscle tone and contractility in stomach and colon. KV7 channels, that include 5 subtypes (KV7.1-7.5), are activated at relatively negative potential values, close to those of the resting membrane potential for the smooth muscle cells of some segments of the gastrointestinal tract. Thus, they contribute to set the resting membrane potential and their blockade induces increase in smooth muscle contractility in stomach and colon. In addition, KV7 channel activation produces profound relaxations of gastric and colonic smooth muscle. Therefore, KV7 channel activators could be used to relax the smooth muscle and relieve symptoms in diseases such as functional dyspepsia and irritable bowel syndrome with prevalent diarrhea. The discovery of activators selective for the channel subtypes present in the smooth muscle, mainly KV7.4 and 7.5, would allow avoiding adverse cardiac and nervous system effects. A further step forward would be characterizing putative differences among the KV7 channel subtypes expressed in the various smooth muscles and synthesizing molecules specific for the gastrointestinal smooth muscle.

Voltage dependent potassium channel remodeling in murine intestinal smooth muscle hypertrophy induced by partial obstruction

Partial obstruction of the small intestine causes obvious hypertrophy of smooth muscle cells and motility disorder in the bowel proximate to the obstruction. To identify electric remodeling of hypertrophic smooth muscles in partially obstructed murine small intestine, the patch-clamp and intracellular microelectrode recording methods were used to identify the possible electric remodeling and Western blot, immunofluorescence and immunoprecipitation were utilized to examine the channel protein expression and phosphorylation level changes in this research. After 14 days of obstruction, partial obstruction caused obvious smooth muscle hypertrophy in the proximally located intestine. The slow waves of intestinal smooth muscles in the dilated region were significantly suppressed, their amplitude and frequency were reduced, whilst the resting membrane potentials were depolarized compared with normal and sham animals. The current density of voltage dependent potassium channel (K_V) was significantly decreased in the hypertrophic smooth muscle cells and the voltage sensitivity of K_V activation was altered. The sensitivity of K_V currents (IK_V) to TEA, a nonselective potassium channel blocker, increased significantly, but the sensitivity of IKv to 4-AP, a K_V blocker, stays the same. The protein levels of K_V4.3 and K_V2.2 were up-regulated in the hypertrophic smooth muscle cell membrane. The serine and threonine phosphorylation levels of K_V4.3 and K_V2.2 were significantly increased in the hypertrophic smooth muscle cells. Thus this study represents the first identification of K_V channel remodeling in murine small intestinal smooth muscle hypertrophy induced by partial obstruction. The enhanced phosphorylations of K_V4.3 and K_V2.2 may be involved in this process.

Exogenous interleukin-6 facilitated the contraction of the colon in a depression rat model

BACKGROUND

Gut dysmotility is closely associated with proinflammatory cytokines both in irritable bowel syndrome and inflammatory bowel disease. There is a dose-response relationship between depression and these inflammatory cytokines.

AIMS

In the present study, we aimed to investigate the effect of Interleukin-6 (IL-6) on colon motility in a rat model of depression induced by chronic unpredictable mild stress (CUMS).

METHODS

The contraction of the circular muscle strips of proximal colon was monitored by a polygraph. IL-6 and IL-6 receptor (IL-6R) mRNA was assayed by real-time quantitative PCR. Immunohistochemistry staining was used to locate the IL-6 and IL-6R in the rat colon.

RESULTS

IL-6 and IL-6R were expressed in the mucosal layer, smooth muscle cells, and myenteric plexus of the colon. Exogenous IL-6 (20 ng/ml) increased the contraction of the circular muscle strip. Pretreatment of tetrodotoxin (blocker of voltage-dependent Na(+) channel on nerve fiber) blocks the excitatory effect of IL-6 on the contraction of the colon in non-stressed rats, but partially inhibited IL-6-induced excitatory effect on the muscle strips in CUMS-treated rats.

CONCLUSIONS

These results suggest that IL-6-induced the contraction of the colonic strip by acting on the gut's nervous system and acting directly on the smooth muscle in rats with depression.

Species differences in the antidiarrheal and antispasmodic activities of Lepidium sativum and insight into underlying mechanisms

The aim of this study was to see if the crude extract of Lepidium sativum (Ls.Cr) exhibits species specificity in its antidiarrheal and antispasmodic activities along with insight into the underlying mechanisms using the in-vivo and in-vitro experiments. Ls.Cr inhibited castor oil-induced diarrhea in mice at doses (300 and 1000 mg/kg) three times higher dose than for rats. In isolated rat ileum and jejunum, Ls.Cr completely inhibited carbachol (CCh), low K⁺ (25 mM) and high K⁺ (80 mM)-induced contractions, while in guinea-pig tissues, Ls.Cr caused complete inhibition of only CCh-induced contraction. In rabbit tissues, Ls.Cr completely inhibited CCh and low K⁺-induced contractions sensitive to K⁺ channel antagonists. Pretreatment of guinea-pig and rat tissues with Ls.Cr caused a rightward shift in CCh-induced contractions in a pattern similar to dicyclomine, while in rabbit and rat tissues, Ls.Cr shifted isoprenaline curves to the left similar to papaverine. These data indicate that the antidiarrheal and antispasmodic activities of L. sativum are species dependent, mediating its antispasmodic effect through combinations of multiple pathways including activation of K⁺ channels, and inhibition of muscarinic receptors, Ca⁺⁺ channels and PDE enzyme. Rat tissues showed the highest potency. Based on the results, we recommend using multiple species to know the real pharmacological profile of medicinal products.

Plasma hormones facilitated the hypermotility of the colon in a chronic stress rat model

Objective

To study the relationship between brain-gut peptides, gastrointestinal hormones and altered motility in a rat model of repetitive water avoidance stress (WAS), which mimics the irritable bowel syndrome (IBS).

Methods

Male Wistar rats were submitted daily to 1-h of water avoidance stress (WAS) or sham WAS (SWAS) for 10 consecutive days. Plasma hormones were determined using Enzyme Immunoassay Kits. Proximal colonic smooth muscle (PCSM) contractions were studied in an organ bath system. PCSM cells were isolated by enzymatic digestion and IKv and IBKca were recorded by the patch-clamp technique.

Results

The number of fecal pellets during 1 h of acute restraint stress and the plasma hormones levels of substance P (SP), thyrotropin-releasing hormone (TRH), motilin (MTL), and cholecystokinin (CCK) in WAS rats were significantly increased compared with SWAS rats, whereas vasoactive intestinal peptide (VIP), calcitonin gene-related peptide (CGRP) and corticotropin releasing hormone (CRH) in WAS rats were not significantly changed and peptide YY (PYY) in WAS rats was significantly decreased. Likewise, the amplitudes of spontaneous contractions of PCSM in WAS rats were significantly increased comparing with SWAS rats. The plasma of WAS rats (100 µl) decreased the amplitude of spontaneous contractions of controls. The IKv and IBKCa of PCSMs were significantly decreased in WAS rats compared with SWAS rats and the plasma of WAS rats (100 µl) increased the amplitude of IKv and IBKCa in normal rats.

Conclusion

These results suggest that WAS leads to changes of plasma hormones levels and to disordered myogenic colonic motility in the short term, but that the colon rapidly establishes a new equilibrium to maintain the normal baseline functioning.

Targeting ion channels for the treatment of gastrointestinal motility disorders

Gastrointestinal (GI) functional and motility disorders are highly prevalent and responsible for long-term morbidity and sometimes mortality in the affected patients. It is estimated that one in three persons has a GI functional or motility disorder. However, diagnosis and treatment of these widespread conditions remains challenging. This partly stems from the multisystem pathophysiology, including processing abnormalities in the central and peripheral (enteric) nervous systems and motor dysfunction in the GI wall. Interstitial cells of Cajal (ICCs) are central to the generation and propagation of the cyclical electrical activity and smooth muscle cells (SMCs) are responsible for electromechanical coupling. In these and other excitable cells voltage-sensitive ion channels (VSICs) are the main molecular units that generate and regulate electrical activity. Thus, VSICs are potential targets for intervention in GI motility disorders. Research in this area has flourished with advances in the experimental methods in molecular and structural biology and electrophysiology. However, our understanding of the molecular mechanisms responsible for the complex and variable electrical behavior of ICCs and SMCs remains incomplete. In this review, we focus on the slow waves and action potentials in ICCs and SMCs. We describe the constituent VSICs, which include voltage-gated sodium (Na(V)), calcium (Ca(V)), potassium (K(V), K(Ca)), chloride (Cl(-)) and nonselective ion channels (transient receptor potentials [TRPs]). VSICs have significant structural homology and common functional mechanisms. We outline the approaches and limitations and provide examples of targeting VSICs at the pores, voltage sensors and alternatively spliced sites. Rational drug design can come from an integrated view of the structure and mechanisms of gating and activation by voltage or mechanical stress.

Evidence that endogenous hydrogen sulfide exerts an excitatory effect on gastric motility in mice

The present study was designed to investigate the effect of endogenous hydrogen sulfide (H₂S) on gastric motility in mice. Western blotting and immunocytochemistry were used to determine expression levels of the H₂S-producing enzymes cystathionine-β-synthase (CBS) and cystathionine-γ-lyase (CSE) in gastric tissues and cultured smooth muscle cells. Physiological and intracellular recordings and the whole-cell patch clamp technique were used to evaluate the effect of H₂S on the mechanical and electrical activities in muscle strips and in isolated smooth muscle cells, respectively. The results showed that CBS and CSE were expressed in mouse gastric smooth muscle. NaHS, a H₂S donor, inhibited the amplitude and frequency of spontaneous contraction at high concentrations (>200 μM). However, NaHS at low concentrations (<100 μM) enhanced the basal tension and increased the contractile amplitude of muscle strips. This excitatory effect was not altered by the blockade of the enteric nerve with TTX, but was abolished by tetraethylammonium (TEA) or 4-aminopyridine (4-AP). Aminooxyacetic acid (AOA), but not propargylglycine (PAG), caused a concentration-dependent inhibition of spontaneous contraction. This effect was restored by L-cysteine and NaHS. In addition, NaHS at low concentrations (<100 μM) produced a depolarization of the membrane potential, whereas AOA hyperpolarized the membrane potential and decreased the amplitude of slow waves. Furthermore, AOA increased whole-cell delayed rectifier K⁺ current (I(K(V))). These findings suggest that endogenous H₂S appears to be an excitatory gaseous mediator during physiological regulation of gastric motility and this excitable effect is mediated by depolarization of the membrane potential via inhibition of I(K(V)).

KV7 channels regulate muscle tone and nonadrenergic noncholinergic relaxation of the rat gastric fundus

Voltage-dependent type 7 K+ (KV7) channels play important physiological roles in neurons and muscle cells. The aims of the present study were to investigate the motor effects of KV7 channel modulators in the rat gastric fundus and the expression of KV7 channels in this tissue. Muscle tone and electrical field stimulation (EFS)-evoked relaxations of precontracted longitudinal muscle strips of the rat gastric fundus were investigated under nonadrenergic noncholinergic conditions by organ bath studies. Gene expression was studied by real-time PCR and tissue localization of channels was investigated by immunohistochemistry. The KV7 channel blocker XE-991 induced concentration-dependent contractions, with mean pD2 and Emax of 5.4 and 48% of the maximal U46619-induced contraction, respectively. The KV7 channel activators retigabine and flupirtine concentration-dependently relaxed U46619-precontracted strips, with pD2s of 4.7 and 4.4 and Emax of 93% and 91% of the maximal relaxation induced by papaverine, respectively. XE-991 concentration-dependently inhibited retigabine-induced relaxation with a pIC50 of 6.2. XE-991 and DMP-543, another KV7 channel blocker, increased by 13-25% or reduced by 11-21% the relaxations evoked by low- or high-frequency EFS, respectively. XE-991 also reduced the relaxation induced by vasoactive intestinal polypeptide (VIP) by 33% of controls. Transcripts encoded by all KV7 genes were detected in the fundus, with 7.4 and 7.5 showing the highest expression levels. KV7.4 and 7.5 channels were visualized by confocal immunofluorescence in both circular and longitudinal muscle layers. In conclusion, in the rat proximal stomach, KV7 channels appear to contribute to the resting muscle tone and to VIP- and high-frequency EFS-induced relaxation. KV7 channel activators could be useful relaxant agents of the gastric smooth muscle.

Role of TLR4 and MAPK in the local effect of LPS on intestinal contractility

OBJECTIVES

Lipopolysaccharide (LPS) has been shown to alter intestinal contractility. Toll-like receptor 4 (TLR4), K(+) channels and mitogen-activated protein kinases (MAPKs) have been proposed to be involved in the mechanism of action of LPS. The aim of this study was to determine the role of TLR4, K(+) channels and MAPKs (p38, JNK and MEK1/2) in the local effect of LPS on the acetylcholine (ACh)-induced contractions in rabbit small intestine in vitro.

METHODS

Segments of rabbit duodenum were suspended in the direction of longitudinal or circular smooth muscle fibres in a thermostatically controlled organ bath.

KEY FINDINGS

LPS (0.3 µg/ml) reduced the contractions induced by ACh (100 µm) in the longitudinal and circular smooth muscle of the duodenum after 90 min of incubation. Polymyxin (TLR4 inhibitor), SB203580 (p38 MAPK inhibitor), SP600125 (JNK1/2 inhibitor) and U0126 (MEK1/2 inhibitor) antagonized the effects of the LPS on ACh-induced contractions in duodenal smooth muscle. Incubation with the blockers of K(+) channels, TEA, apamin, charybdotoxin, iberiotoxin, glibenclamide or quinine, did not reverse the effect of LPS on ACh-induced contractions.

CONCLUSIONS

These results suggest that the effect of LPS on ACh-induced contractions in the rabbit duodenum might be mediated by TLR4 and p38, JNK1/2 and MEK1/2 MAPKs.

Pharmacology of mitochondrial potassium channels: dark side of the field

Mitochondrial potassium channels play an important role in cytoprotection. Potassium channels in the inner mitochondrial membrane are modulated by inhibitors and activators (potassium channel openers) previously described for plasma membrane potassium channels. The majority of mitochondrial potassium channel modulators exhibit a broad spectrum of off-target effects. These include uncoupling properties, inhibition of the respiratory chain and effects on cellular calcium homeostasis. Therefore, the rational application of channel inhibitors or activators is crucial to understanding the cellular consequences of mitochondrial channel inhibition or activation. Moreover, understanding their side-effects should facilitate the design of a specific mitochondrial channel opener with cytoprotective properties. In this review, we discuss the complex interactions of potassium channel inhibitors and activators with cellular structures.

Large conductance calcium-activated potassium channels (BKCa) modulate trigeminovascular nociceptive transmission

Migraine is a common, disabling, neurological problem whose acute management would benefit from the development of purely neurally acting therapies. The trigeminocervical complex is pivotal in nociceptive signaling in migraine, and is an accepted target for putative antimigraine agents. Whole-cell patch-clamp or extracellular recordings were made of trigeminal neurons identified in rat brainstem slices. Bath application of the large conductance calcium-activated potassium (BKCa) channel opener NS1619 caused a dramatic decrease of cell firing that could be reversed by the co-application of iberiotoxin. NS1619 hyperpolarized the resting membrane potential and reduced the frequency of spontaneous action potentials in these neurons. These data suggest the presence of BKCa channels in the trigeminocervical complex. In vivo in cat L-glutamate-evoked firing was facilitated in nociceptive neurons, also responding to stimulation of the superior sagittal sinus, in the trigeminal nucleus caudalis by the BKCa peptide antagonists, iberiotoxin and slotoxin. Of units tested, 70% responded to microiontophoretic application of the blockers, identifying a subpopulation of trigeminal neurons expressing toxin-sensitive BKCa channels. NS1619 inhibited 74% of cells tested, and this was reversed by slotoxin, suggesting that the action of NS1619 in these cells was mediated through BKCa channels. These data are consistent with the presence of BKCa channels in the trigeminal nucleus caudalis that are potential targets for the development of antimigraine treatments, and may also offer insights into receptor mechanisms involved in sensitization and thus allodynia, in migraine.

Involvements of PHI-nitric oxide and PACAP-BK channel in the sustained relaxation of mouse gastric fundus

The roles of nitric oxide (NO) and K(+) channels in sustained relaxation induced by electrical field stimulation (EFS) in the presence of atropine and guanethidine were studied in circular muscle strips of mouse gastric fundus. In the wild-type mouse, N(G)-nitro-l-arginine (l-nitroarginine), a nitric oxide synthase inhibitor, significantly inhibited the sustained relaxation in addition to the rapid relaxation. The sustained relaxation in pituitary adenylate cyclase-activating peptide (PACAP)-knockout mouse, which was smaller than that of the wild-type mouse, was also inhibited by l-nitroarginine. l-Nitroarginine inhibited the relaxation induced by the peptide histidine isoleucine (PHI), but not that induced by PACAP. S-Nitroso-N-acetyl-dl-penicillamine (SNAP), a NO donor, -induced relaxation was not affected by PACAP(6-38). EFS-induced sustained relaxation was inhibited by iberiotoxin, a big conductance calcium-activated K(+) (BK) channel inhibitor, but not by apamin, a small conductance calcium-activated K(+) (SK) channel inhibitor, and glibenclamide, an ATP-sensitive K(+) channel inhibitor. The relaxation that remained after the iberiotoxin-treatment was significantly inhibited by l-nitroarginine. Iberiotoxin inhibited PACAP-induced relaxation, while it had no effect on both PHI- and SNAP-induced relaxation. Immunoreactivities to anti-BK channel and anti-PHI antibodies were found in the circular muscle and the myenteric plexus layers, respectively. These results suggest interplay between PHI and NO in the sustained relaxation of the mouse gastric fundus, and that BK channels are involved in the PACAP-component of the sustained relaxation.

Sildenafil inhibits duodenal contractility via activation of the NO-K+ channel pathway

Phosphodiesterase type-5 (PDE5) specifically cleaves cyclic guanosine monophosphate (cGMP), a key intracellular secondary messenger. The PDE5 inhibitor sildenafil is a well-known vasodilator that also has gastrointestinal myorelaxant properties. In the present study, we further investigated sildenafil-induced myorelaxation in rat isolated duodenum, assessing its interaction with nitric oxide (NO) synthase and K(+) channel opening. The spontaneous contractions of duodenal strips were reversibly inhibited by sildenafil (0.1-300 microM) in a concentration-dependent manner [mean (95% confidence interval); EC(50) = 6.8 (2.7-17.3) microM]. The sildenafil-induced myorelaxation was significantly decreased by the NO synthase inhibitor N-nitro-L-arginine methyl ester [increasing the EC(50) value to 41.9 (26.1-67.3) microM]. Sodium nitroprusside or forskolin pretreatments enhanced the sildenafil-induced myorelaxation. In isolated strips pretreated with BaCl(2) (0.2 mM), 4-aminopyridine (4-AP, 3 mM), or glybenclamide (1 microM), the sildenafil-induced EC(50) value was significantly increased to 32.8 (19.1-56.4), 27.1 (15.2-48.3) and 20.1 (16.4-24.7) microM, respectively. Minoxidil (50 microM) or diazoxide (100 microM) also significantly attenuated the sildenafil-induced potency. In conclusion, the NO synthase/cyclic nucleotide pathway activation is involved in sildenafil-induced inhibition of spontaneous duodenal contractions. Its pharmacological action seems to be influenced by K(+) channel opening, especially the voltage-sensitive ones, being inhibited by 4-AP and K(ATP) channels, sensitive to glybenclamide.

Nitric oxide decreases the excitability of interstitial cells of Cajal through activation of the BK channel

Nitrergic nerves are structurally and functionally associated with ICC. To further understand mechanisms of communication, the hypothesis was investigated that NO might affect large conductance K channels. To that end, we searched for IbTX-sensitive currents in ICC obtained through explant cultures from the mouse small intestine and studied effects of the NOS inhibitor omega N-nitro-L-arginine (LNNA) and the NO donor sodium nitroprusside (SNP). IbTX-sensitive currents acquired in the whole-cell configuration through nystatin perforated patches exhibited high noise levels but relatively low amplitude, whereas currents obtained in the conventional whole-cell configuration exhibited less noise and higher amplitudes; depolarization from -80 to + 40 mV evoked 357 +/- 159 pA current in the nystatin perforated patch configuration and 1075 +/- 597 pA using the conventional whole-cell configuration. Immunohistochemistry showed that ICC associated with ganglia and Auerbach's plexus nerve fibers were immunoreactive to BK antibodies. The IbTX-sensitive currents were increased by SNP and inhibited by LNNA. BK blockers suppressed spontaneous transit outward currents in ICC. After block of BK currents, or before these currents became prominent, calcium currents were activated by depolarization in the same cells. Their peak amplitude occurred at -25 mV and the currents were increased with increasing extracellular calcium and inhibited by cobalt. The hypothesis is warranted that nitrergic innervation inhibits ICC excitability in part through activation of BK channels. In addition, NO is an intracellular regulator of ICC excitability.

Effects of washout following indomethacin administration on electrical field stimulation-induced isolated rat gastric fundus strip contractions

BACKGROUND AND AIM

Although gastric hypermotility is one of the mechanisms proposed to explain the ulcerogenic action of indomethacin, the drug has shown relaxatory effects on isolated longitudinal and transverse rat gastric fundus. To explain the above discrepancy, the aim of this study was to investigate the effects of washout following indomethacin administration on the contractile response of isolated rat gastric strips to electrical field stimulation.

METHODS

Transverse and longitudinal strips of stomach were suspended in organ baths containing oxygenated Krebs solution. Contractile responses to electrical field stimulation were recorded on a physiograph before and after administration of a single concentration of indomethacin. Recordings were also taken 15 min after the washout of the drug. To study the part played by K(ATP) channels on post-washout response, the effects of diazoxide as a channel opener and glybenclamide as a channel blocker were also studied.

RESULTS

The amplitude of the contractions was not changed following indomethacin administration but was significantly increased 15 min after washout of the drug. Diazoxide pretreatment inhibited the stimulatory post-washout response of both strips. Glybenclamide pretreatment showed different results depending on the type of strip. In the transverse strips the drug showed no effect while in the longitudinal strips it decreased the post-washout response.

CONCLUSIONS

The present data suggest that indomethacin has a delayed stimulatory effect on gastric smooth muscle, which will appear after the exposure of the strip to the drug followed by its washout. This effect seems to be under the influence of K(ATP) channel modulators.

Functional contribution of the endothelial component to the vasorelaxing effect of resveratrol and NS 1619, activators of the large-conductance calcium-activated potassium channels

Large-conductance calcium-activated potassium channels (BK) of smooth muscle play a role in the relevant modulation of vascular tone, due to their calcium- and voltage-dependent mechanisms of activation. A potential role of endothelial BK channels has also been suggested by approaches on endothelial cell cultures. However, no functional study, aimed at evaluating the contribution of endothelial BK channels to the effect of BK-openers, has been reported. Resveratrol and NS 1619, BK-openers, have been tested on endothelium-intact and -denuded aortic rings. Furthermore, the effects of high depolarisation of potassium channel blockers TEA (Tetraethylammonium), 4-AP ( 4-Aminopyridine) and IbTX (Iberiotoxin) and of inhibitors of NO-pathway (L-NAME and ODQ) have been evaluated. The presence of endothelium increased the vasorelaxing potency of BK-openers. This potentiation was eliminated by L-NAME and ODQ. TEA, 4-AP, IbTX and high depolarisation had modest or no antagonist influence on resveratrol in endothelium-denuded aortic rings. The effects of NS 1619 on endothelium-denuded aortic rings were not affected by IbTX, and were modestly antagonised by TEA, 4-AP and high depolarisation. In intact endothelium vessels, TEA, IbTX and 4-AP antagonised the vasorelaxing effect of the two BK-activators. A BK-mediated release of endothelial NO seems a very important factor, determining a strong influence on vasodilator profile of BK-openers. Therefore, an eventual therapy with a BK-opener could promote a series of cardiovascular impacts not confined to the only direct vasorelaxing effects, but also due to a significant contribution of endothelial NO.

K+ channels involved in contractility of rabbit small intestine

Most excitable cells, including gastrointestinal smooth muscle cells, express several types of K+ channels. The aim of this study was to examine the types of K' channels involved in the contractility of longitudinal smooth muscle of rabbit small intestine in vitro. Spontaneous contractions and KCl-stimulated contractions were reduced by atropine, phentolamine, propranolol, suramin, tetrodotoxin and indomethacin. The amplitude and tone of spontaneous contractions were increased by apamin, charybdotoxin, iberiotoxin, E4031, tetraetylammonium (TEA) and BaCl2. The frequency of contractions was reduced in the presence of apamin and TEA and increased by charybdotoxin. It was found that 4-aminopyridine increased the tone of spontaneous contractions and reduced the amplitude and frequency of contractions. Glibenclamide did not modify the amplitude, frequency or tone of contractions. KCl-stimulated contractions were increased by E4031, were not modified by apamin, glibenclamide, NS1619 or diazoxide, and were reduced by charybdotoxin, TEA, 4-aminopyridine or BaCl2. These results suggest that both Ca2+-activated K+ channels of small and high conductance, and HERG K+ channels and inward rectifier K+ channels participate in spontaneous contractions of small intestine. On the other hand, voltage-dependent K+ channels, HERG K+ channels, inward rectifier K+ channels and high conductance Ca2+-activated K+ channels are involved in KCl-stimulated contractions.

Effect of indomethacin on electrical field stimulation-induced contractions of isolated transverse and longitudinal rat gastric fundus strips

AIM: To study the effects of indomethacin on the isolated transverse and longitudinal rat gastric fundus strips.

METHODS: The strips were suspended in an organ bath containing oxygenated Krebs solution, and contractile responses to electrical field stimulation were recorded on a physiograph in an isotonic manner after administration of cumulative concentrations of indomethacin. The effects of indomethacin on the strips pretreated with K_ATP channel modulators, diazoxide and glybenclamide were studied.

RESULTS: Treatment of the transverse strips with indomethacin resulted in a concentration-dependent inhibitory response. In longitudinal strips, biphasic responses were seen, which included a stimulatory response at low concentrations of indomethacin, followed by an inhibitory response at higher concentrations. Diazoxide pre-treatment inhibited the stimulatory response of longitudinal strips. Glybenclamide pre-treatment not only blocked inhibitory effect of the low concentrations of indomethacin on transverse strips, but also increased the amplitude of contractions. Moreover, the drug decreased the amplitude of contractions in longitudinal strips.

CONCLUSION: Responses of the isolated longitudinal and transverse rat gastric fundus strips to indomethacin are not similar, and are influenced by K_ATP channel modulators.

Acute relaxation of mouse duodenum [correction of duodenun] by estrogens. Evidence for an estrogen receptor-independent modulation of muscle excitability

17-beta-Estradiol, the stereoisomer 17-alpha-estradiol and the synthetic estrogen diethylstilbestrol (DES), all caused a rapid (<3 min) dose-dependent reversible relaxation of mouse duodenal spontaneous activity, reduced basal tone and depressed the responses to CaCl(2) and KCl. The steroidal antiestrogen 7alpha-[9-[(4,4,5,5,5,-pentafluoropenty)sulphinyl]nonyl]-estra-1,3,5(19)-triene-3,17beta-diol (ICI182,780) failed to either mimic or prevent the effect of 17-beta-estradiol. The effect of estrogens was unrelated to activation of nitric oxide (NO), mitogen-activated protein kinase (MAPK), protein kinase A (PKA), protein kinase G (PKG) or protein kinase C (PKC). Estrogen-induced relaxation was partially reversed by 1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-pyridine-3-carboxilic acid methyl ester (BAY-K8644), depolarization, or by application of tetraethylammonium or 4-aminopyridine, but not by glibenclamide, apamin, charybdotoxin, paxilline or verruculogen. The effects of BAY-K8644 and K(+) channel blockers were synergistic, and allowed relaxed tissues to recover spontaneous activity and basal tone. We hypothesize that the rapid non-genomic spasmolytic effect of estrogens on mouse duodenal muscle might be triggered by an estrogen-receptor-independent mechanism likely involving activation of tetraethylamonium- and 4-aminopyridine-sensitive K(+) channels and inhibition of L-type Ca2(+) channels on the smooth muscle cells.

The therapeutic potential of neuronal KCNQ channel modulators

Neuronal KCNQ (Kv7) channels (KCNQ2-5 or Kv7.2-7.5, disclosed to date) were discovered by virtue of their homology with a known cardiac channel involved in long QT syndrome (KvLQT or KCNQ1, Kv7.1) and first disclosed in 1998. The involvement of KCNQ2 (Kv7.2) and KCNQ3 (Kv7.3) in a benign idiopathic neonatal epilepsy, KCNQ4 (Kv7.4) in a form of congenital deafness, and the discovery that neuronal KCNQ heteromultimers were among the molecular substrates of M-channels, resulted in a high level of interest for potential drug development strategies. A number of small-molecule modulators were quickly identified, including openers or activators such as the antiepileptic drug candidate retigabine and the structurally-related analgesic drug flupirtine (Katadolon trade mark Asta Medica), and a group of KCNQ channel inhibitors/blockers originally developed for cognition enhancement. All of these data have suggested a rich target profile for modulators of neuronal KCNQ channels, including a variety of neuronal hyperexcitability disorders and conditions for openers, such as the epilepsies, acute pain, neuropathic pain, migraine pain and some neurodegenerative and psychiatric disorders. KCNQ blockers could likewise have utility in disorders characterised by neuronal hypoactivity, including cognition enhancement and perhaps disorders of mood. Emerging patent literature suggests significant interest in neuronal KCNQ modulation in the pharmaceutical industry and significant chemical diversity concerning KCNQ modulation.