Potassium channel openers: pharmacological and clinical aspects

Achillea fragrantissima Essential Oil: Composition and Detailed Pharmacodynamics Study of the Bronchodilator Activity

The bronchodilator effect of the Achillea fragrantissima essential oil (AFO) was studied in guinea pigs’ tracheas and the influence of drying on the quantity and composition of AFO was studied using GC-MS and GC analyses. AFO produced a complete and potent relaxation against carbachol (CCh), while lower potency and partial efficacy were observed against high K+ (80 mM), thus producing dual inhibitory effects similar to dicyclomine. The anticholinergic-like action was further confirmed when pre-incubation tracheal tissues were used at lower concentrations with AFO displacing the CCh concentration‒response curves (CRCs) to the right in a competitive manner similar to atropine. However, non-parallel shifts in CCh CRCs were observed with higher doses, similar to dicyclomine. Further confirmation of the CCB-like effect was obtained from the non-specific deflection of Ca++ CRCs toward the right using the pre-incubated tissues with AFO in Ca++ free medium, similar to verapamil. When AFO was tested against low K+-mediated contractions to explore the possible involvement of additional antispasmodic mechanism(s), AFO interestingly showed a complete inhibition with a higher potency. This inhibition was found to be sensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP), whereas glibenclamide (Gb) remained inactive. These results show that AFO possesses bronchodilator effects predominantly from its anticholinergic and K+ channel activation followed by weak Ca++ channels inhibition.

Evaluation of spasmolytic effects of naringenin on ileum contraction and intestinal charcoal meal transit: Involvement of ATP-sensitive K+ channels

Introduction: Naringenin is a flavonoid constituent of many herbal plants, including citreous fruits. Biological studies have suggested various therapeutic effects for naringenin, including protective effects on gastrointestinal (GI) motility. The present study was performed to investigate the involvement of ATP-sensitive K+ channels on the effect of naringenin in rat ileum motility. Methods: Ileum contractions were induced by either KCl or acetylcholine (ACh) in vitro. Inhibitory concentration-response curves were constructed for naringenin and diazoxide after exposure of rat isolated ileum to KCl (20mM) or ACh (500nM). The relaxant effects of naringenin and diazoxide were also examined in the presence of glibenclamide. Furthermore, oral effects of diazoxide (25 mg/kg) and naringenin (25, 50 mg/kg) were also assessed on the intestinal charcoal meal transit in mice (n=10) in the absence and presence of glibenclamide (50 mg/kg). Results: Diazoxide and naringenin in a concentration-dependent manner inhibited ileum contractions induced by low bath concentration of KCl (20mM). However, both drugs had no effect on contractions induced by a high concentration of KCl (160mM). The inhibitory effects of diazoxide and naringenin were blocked by glibenclamide. Oral administration of diazoxide and naringenin significantly reduced the intestinal transit of charcoal meal. The delay in the intestinal transit was blocked by the oral dose of glibenclamide. The effect of naringenin on the rat intestinal strip pre-contracted with the KCl was relatively similar to that of ATP-sensitive K+ channel opener (diazoxide). Conclusion: This research supports that ATP-sensitive K+ channels are involved in the rat small intestinal smooth muscles relaxation induced by naringenin.

In Silico and Ex Vivo Studies on the Spasmolytic Activities of Fenchone Using Isolated Guinea Pig Trachea

Fenchone is a bicyclic monoterpene found in a variety of aromatic plants, including Foeniculum vulgare and Peumus boldus, and is used in the management of airways disorders. This study aimed to explore the bronchodilator effect of fenchone using guinea pig tracheal muscles as an ex vivo model and in silico studies. A concentration-mediated tracheal relaxant effect of fenchone was evaluated using isolated guinea pig trachea mounted in an organ bath provided with physiological conditions. Sustained contractions were achieved using low K⁺ (25 mM), high K⁺ (80 mM), and carbamylcholine (CCh; 1 µM), and fenchone inhibitory concentration–response curves (CRCs) were obtained against these contractions. Fenchone selectively inhibited with higher potency contractions evoked by low K⁺ compared to high K⁺ with resultant EC₅₀ values of 0.62 mg/mL (0.58–0.72; n = 5) and 6.44 mg/mL (5.86–7.32; n = 5), respectively. Verapamil (VRP) inhibited both low and high K⁺ contractions at similar concentrations. Pre-incubation of the tracheal tissues with K⁺ channel blockers such as glibenclamide (Gb), 4-aminopyridine (4-AP), and tetraethylammonium (TEA) significantly shifted the inhibitory CRCs of fenchone to the right towards higher doses. Fenchone also inhibited CCh-mediated contractions at comparable potency to its effect against high K⁺ [6.28 mg/mL (5.88–6.42, n = 4); CCh] and [6.44 mg/mL (5.86–7.32; n = 5); high K⁺]. A similar pattern was obtained with papaverine (PPV), a phosphodiesterase (PDE), and Ca²⁺ inhibitor which inhibited both CCh and high K⁺ at similar concentrations [10.46 µM (9.82–11.22, n = 4); CCh] and [10.28 µM (9.18–11.36; n = 5); high K⁺]. However, verapamil, a standard Ca²⁺ channel blocker, showed selectively higher potency against high K⁺ compared to CCh-mediated contractions with respective EC₅₀ values of 0.84 mg/mL (0.82–0.96; n = 5) 14.46 mg/mL (12.24–16.38, n = 4). The PDE-inhibitory action of fenchone was further confirmed when its pre-incubation at 3 and 5 mg/mL potentiated and shifted the isoprenaline inhibitory CRCs towards the left, similar to papaverine, whereas the Ca²⁺ inhibitory-like action of fenchone pretreated tracheal tissues were authenticated by the rightward shift of Ca²⁺ CRCs with suppression of maximum response, similar to verapamil, a standard Ca²⁺ channel blocker. Fenchone showed a spasmolytic effect in isolated trachea mediated predominantly by K⁺ channel activation followed by dual inhibition of PDE and Ca²⁺ channels. Further in silico molecular docking studies provided the insight for binding of fenchone with Ca²⁺ channel (−5.3 kcal/mol) and K⁺ channel (−5.7), which also endorsed the idea of dual inhibition.

Multiple Pathway-Mediated Gut-Modulatory Effects of Maerua subcordata (Gilg) DeWolf

BACKGROUND

Gastrointestinal disorders are often poorly managed, especially in developing countries, where there are limited resources and therapeutic options. Despite the rich diversity of medicinal plants that offer effective treatment options with fewer side effects, studies that provide scientific verification are lacking. Maerua subcordata (Gilg) DeWolf is among the plants claimed to have wide traditional medicine, use, including as a remedy against gastrointestinal problems. Therefore, this work aimed to evaluate the gut-modulatory effects of a crude leaf extract of M. subcordata (MSL.Cr), as well as its possible mechanism of action.

METHODS

A castor oil (10 mL/kg)-induced diarrheal mouse model was used to evaluate the antidiarrheal effect of MSL.Cr, and the spasmodic/antispasmodic effect of the extract was assessed using isolated rabbit jejunum with and without addition of standard cholinergic agonists/antagonists to predict the possible mechanism of action.

RESULTS

MSL.Cr exhibited 40% and 80% protection against castor oil-induced diarrhea in mice at doses of 500 and 1,000 mg/kg, respectively. In isolated rabbit jejunum, the extract increased spontaneous contractions at low doses (0.01-0.1 mg/mL), and was sensitive to atropine, whereas it showed complete inhibition at higher doses (0.3-1 mg/mL). It was shown that the relaxant effect was possibly mediated by the involvement of phosphodiesterase-enzyme inhibition and K+-channel activation. The extract potentiated the control concentration-response curve of carbachol, shifting it to the left, similarly to the control drug papaverine. The potassium-channel opening-like activity of MSL.Cr was possibly mediated by the involvement of aspecific K+-channels inhibition, since tetraethylammonium, anunselective antagonist of K+ channels, significantly reversed its inhibitory effect.

CONCLUSION

This study showed that the M. subcordata leaf extract demonstrated gut-modulatory effects, possibly mediated by a combination of muscarinic-receptor stimulation, phosphodiesterase inhibition, and aspecific K+-channel activation.

First evidence of protective effects on stroke recovery and post-stroke depression induced by sortilin-derived peptides

Post-stroke depression (PSD) is the most common mood disorder following stroke with high relevance for outcome and survival of patients. The TREK-1 channel represents a crucial target in the pathogenesis of stroke and depression. Spadin and its short analog mini-spadin were reported to display potent antidepressant properties. We investigated the therapeutic effects of mini-spadin in a mouse model of focal ischemia and PSD. To activate TREK-1 and induce neuroprotection a single low dose of mini-spadin (0.03 μg/kg) was intraperitoneally injected 30  min after the onset of ischemia, once a day during 7 days post-ischemia. Then, to inhibit TREK-1 and induce antidepressant effect, the peptide was injected at higher concentration (3 μg/kg) once a day for 4 days/week until the sacrifice of animals. Electrophysiological studies showed that mini-spadin had a biphasic action on TREK-1. At low doses, the channel activity was increased whereas at higher doses it was inhibited. Mini-spadin prevented the loss of body weight and the delayed dopaminergic degeneration in substantia nigra and improved the motor and cognitive ischemia-induced deficits. Moreover, mini-spadin prevented PSD analyzed in the Forced Swim (FST) and Novelty Suppressed Feeding (NSF) tests. Finally, enhanced neurogenesis and synaptogenesis contributed to the beneficial effects of mini-spadin against stroke and PSD. This work reveals the first evidence that the modulation of TREK-1 channels in the early and chronic phases of stroke as well as the stimulation of brain plasticity by mini-spadin could play a key role in its brain protective effects against stroke and its deleterious consequences such as PSD.

Topical Minoxidil Versus Topical Diltiazem for Chemical Sphincterotomy of Chronic Anal Fissure: A Prospective, Randomized, Double-Blind, Clinical Trial

BACKGROUND

Anal fissure is a common anorectal problem causing severe pain and discomfort to the patients. Chemical sphincterotomy has emerged as a noninvasive alternative to the surgical methods of fissure treatment. The objective of this study was evaluation of the efficacy and the adverse effects of topically applied minoxidil in chemical sphincterotomy of chronic anal fissure in comparison with topical diltiazem.

METHODS

A total of 88 patients with chronic anal fissure aged between 15 and 65 years were included in this double-blind, randomized clinical trial and were randomly assigned to either 0.5% minoxidil cream or 2% diltiazem cream twice daily for 2 weeks. The pain intensity, bleeding, wound healing, itching, headache, dizziness, significant drop in blood pressure, allergy and fissure relapse were assessed on a monthly basis for 2 months.

RESULTS

Both diltiazem and minoxidil reduced the pain, bleeding and improved fissure healing with no significant difference. There were no between-groups differences in the frequencies of adverse effects, except for itching which was slightly higher with minoxidil during the first month. Allergy occurred in two patients in the minoxidil group, which was not severe and did not lead to discontinuation of the trial.

CONCLUSION

Topically administered minoxidil is of equal efficacy as diltiazem in the treatment of chronic anal fissure with low frequency of adverse effects. Thus, it can be considered as an agent for chemical sphincterotomy of anal fissure, but the itching at the beginning of the treatment can affect the adherence of the patient to treatment. Trial registration number IRCT2015041414483N6 (the full trial protocol could be accessed online at www.irct.ir ).

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Influence of Thromboxane A2 on the Regulation of Adenosine Triphosphate-Sensitive Potassium Channels in Mouse Ventricular Myocytes

Background and Objectives

Adenosine triphosphate (ATP)-sensitive potassium (K_ATP) channels play an important role in myocardial protection. We examined the effects of thromboxane A₂ on the regulation of K_ATP channel activity in single ventricular myocytes.

Subjects and Methods

Single ventricular myocytes were isolated from the hearts of adult Institute of Cancer Research (ICR) mice by enzymatic digestion. Single channel activity was recorded by excised inside-out and cell-attached patch clamp configurations at −60 mV holding potential during the perfusion of an ATP-free K-5 solution.

Results

In the excised inside-out patches, the thromboxane A₂ analog, U46619, decreased the K_ATP channel activity in a dose-dependent manner; however, the thromboxane A₂ receptor antagonist, SQ29548, did not significantly attenuate the inhibitory effect of U46619. In the cell-attached patches, U46619 inhibited dinitrophenol (DNP)-induced K_ATP channel activity in a dose-dependent manner, and SQ29548 attenuated the inhibitory effects of U46619 on DNP-induced K_ATP channel activity.

Conclusion

Thromboxane A₂ may inhibit K_ATP channel activity, and may have a harmful effect on ischemic myocardium.

Studies on Bronchodilator Activity of Salvia officinalis (Sage): Possible Involvement of K+ Channel Activation and Phosphodiesterase Inhibition

The aqueous methanolic extract of the aerial parts of Salvia officinalis (So.Cr) was studied to provide possible underlying mechanism(s) for its medicinal use in asthma using the in vivo bronchodilatory assay and isolated tracheal preparations. S. officinalis (1-10 mg/kg) dose-dependently inhibited carbachol (CCh)-induced bronchospasm in anesthetized rats with three-fold greater potency than the positive control, aminophylline. In tracheal preparations, So.Cr inhibited the low K⁺ (25 mM)-induced contractions. Pretreatment of the tissues with 4-aminopyridine reversed the inhibitory effect of the plant extract against low K⁺ , whereas glibenclamide did not show any effect, thus showing the involvement of voltage-sensitive K⁺ channels. When tested against the CCh-induced pre-contractions for the involvement of any additional mechanism, interestingly, the extract showed a dose-dependent (0.03-0.1 mg/mL) inhibitory effect and shifted the inhibitory concentration response curves of isoprenaline to the left, thus showing phosphodiesterase enzyme inhibitory-like action, similar to that of papaverine. These results indicate that the crude extract of S. officinalis possesses bronchodilatory activity mediated predominantly via activation of voltage-dependent K⁺ channels and inhibition of phosphodiesterase enzyme; thus, this study provides sound pharmacological basis for its medicinal use in hyperactive airways disorders such as asthma and cough. Copyright © 2015 John Wiley & Sons, Ltd.

Anticonvulsant effect of Diazoxide against Dichlorvos-induced seizures in mice

Dichlorvos, a synthetic organophosphate toxin, is used as pesticides. These toxins can be used as pesticides in farming and medicine for the devastation and/or elimination of ectoparasites of animals. Reports have shown that Dichlorvos generate seizure effects in various animals. Potassium channel opener is extensively used for medication of cardiovascular and other diseases. Studies have shown that potassium channel opener has anticonvulsant effects in different animal models. The goal of this study was to evaluate the effect of dizoxide on Dichlorvos-induced seizures in mice. In this research, the animals received different doses of Diazoxide (1, 2.5, 5, 10, and 20 mg/kg b.wt.) intraperitoneally 30 min before intraperitoneal injection of Dichlorvos (50 mg/kg b.w.t). After Dichlorvos injection, latency of clones, severity of seizure, and finally death as the fate were investigated. Results showed that Diazoxide dose-dependently decreased the severity of Dichlorvos-induced seizures, so that Diazoxide at a dose of 5 mg (the lowest, P < 0.05) and 20 mg/kg b.wt. (the highest, P < 0.001) has anticonvulsant effects. Thus, our data suggest that diazoxide as ATP-sensitive potassium channels opener has anticonvulsant activity against dichlorvas-induced seizure.

Smooth Muscle Relaxation Activity of an Aqueous Extract of Dried Immature Fruit of Poncirus Trifoliata (PF-W) on an Isolated Strip of Rat Ileum

We demonstrated that an aqueous extract of dried immature fruit of Poncirus trifoliate (PF-W) produces relaxation of intestinal smooth muscle using the ileac strips of a rat. Furthermore, the underlying mechanism of its relaxant activity was investigated. PF-W was prepared using the standard extraction protocol. A 1.5 – 2 cm long rat ileac strip was placed in an organ bath with Tyrode's solution and smooth muscle contractility was recorded by connecting it to a force transducer. Various compounds were added to the organ baths, and changes in muscular contractility were measured. PF-W concentration-dependently induced relaxation of rat ileac strips that were contracted both spontaneously and via acetylcholine treatment. Various potassium channel blockers did not inhibit the relaxation by PF-W. No difference in the effect of PF-W was observed between ileac strips treated with low (20 mM) and high concentrations (60 mM) of KCl. PF-W inhibited the contraction of rat ileac strips induced by extracellular calcium. PF-W acts as a potent smooth muscle relaxant, implicating its possible action as a rapid acting reliever for abdominal pains and a cure for intestinal convulsion. Considering that PF-W also exhibits prokinetic activity, its use in various gastrointestinal disorders seems promising.

Molecular expression and functional role of canonical transient receptor potential channels in airway smooth muscle cells

Multiple canonical or classic transient receptor potential (TRPC) molecules are expressed in animal and human airway smooth muscle cells (SMCs). TRPC3, but not TRPC1, is a major molecular component of native non-selective cation channels (NSCCs) to contribute to the resting [Ca(2+)](i) and muscarinic increase in [Ca(2+)](i) in freshly isolated airway SMCs. TRPC3-encoded NSCCs are significantly increased in expression and activity in airway SMCs from ovalbumin-sensitized/challenged "asthmatic" mice, whereas TRPC1-encoded channel activity, but not its expression, is largely augmented. The upregulated TRPC3- and TRPC1-encoded NSCC activity both mediate "asthmatic" membrane depolarization in airway SMCs. Supportively, tumor necrosis factor-α (TNFα), an important asthma mediator, increases TRPC3 expression, and TRPC3 gene silencing inhibits TNFα-mediated augmentation of acetylcholine-evoked increase in [Ca(2+)](i) in passaged airway SMCs. In contrast, TRPC6 gene silencing has no effect on 1-oleoyl-2-acetyl-sn-glycerol (OAG)-evoked increase in [Ca(2+)](i) in primary isolated cells. These findings provide compelling information indicating that TRPC3-encoded NSCCs are important for physiological and pathological cellular responses in airway SMCs. However, continual studies are necessary to further determine whether, which, and how TRPC-encoded channels are involved in cellular responses in normal and diseased (e.g., asthmatic) airway SMCs.

Selective bronchodilatory effect of Rooibos tea (Aspalathus linearis) and its flavonoid, chrysoeriol

BACKGROUND

Rooibos tea (Aspalathus linearis) is commonly used for hyperactive gastrointestinal, respiratory and cardiovascular disorders.

AIM OF STUDY

The aqueous extract of Rooibos tea (RT) was studied for the possible bronchodilator, antispasmodic and blood pressure lowering activities in an attempt to rationalize some of its medicinal uses.

METHODS

Isolated tissue preparations, such as rabbit jejunum, aorta and guinea-pig trachea and atria were set up in appropriate physiological salt solutions and aerated with carbogen. For in vivo studies rats were anesthetized with pentothal sodium and blood pressure was measured through carotid artery cannulation.

RESULTS

In jejunum, RT caused a concentration-dependent relaxation of low K(+) (25 mM)-induced contractions, with mild effect on the contractions induced by high K(+) (80 mM). In presence of glibenclamide, the relaxation of low K(+)-induced contractions was prevented. Similarly, cromakalim caused glibenclamide-sensitive inhibition of low K(+), but not of high K(+), while verapamil did not differentiate in its inhibitory effect on contractions produced by the two concentrations of K(+). Like in jejunum, RT caused glibenclamide-sensitive relaxation of low K(+)-induced contractions in trachea and aorta, but with a 20 times higher potency in trachea. In atria, RT was least potent with weak inhibitory effect on atrial force and rate of contractions. RT caused a dose-dependent fall in arterial blood pressure in rats under anesthesia. Among the tested pure compounds of Rooibos, chrysoeriol showed selective bronchodilator effect. Chrysoeriol (luteolin 3'-methyl ether) is a bioactive flavonoid known for antioxidant, antiinflammatory, antitumor, antimicrobial, antiviral, and free radical scavenging activities.

CONCLUSION

These results indicate that the bronchodilator, antispasmodic and blood pressure lowering effects of Rooibos tea are mediated predominantly through K(ATP) channel activation with the selective bronchodilatory effect. This study provides a sound mechanistic basis for the wide medicinal use of Rooibos tea, with the therapeutic potential to be developed for congestive respiratory ailments.

Investigation of the relaxant effects of pinacidil and cromakalim on the sheep sphincter of Oddi

BACKGROUND/AIMS

ATP-sensitive K+ (KATP) channels play an important role in the regulation of smooth muscle membrane potential. This study was designed to investigate the effects of pinacidil and cromakalim, KATP-sensitive channel activators, on sheep sphincters of Oddi (SO).

METHODS

SO rings were mounted in a tissue bath and tested for changes in isometric tension in response to pinacidil (10(-9)-10(-4)M) and cromakalim (10(-9)-10(-4)M) in the presence or absence of glibenclamide (10(-6)M), a blocker of KATP channels. Furthermore, concentration-dependent contraction responses of carbachol were obtained.

RESULTS

Carbachol (10(-9)-10(-5)M) induced concentration-dependent contraction responses in the SO rings. Pinacidil (10(-9)-10(-4)M) and cromakalim (10(-9)-10(-4)M) induced concentration-dependent relaxation in isolated SO rings precontracted with carbachol (10(-6)M). At their maximum effects, both pinacidil and cromakalim produced nearly full relaxation. In the presence of glibenclamide, concentration-relaxation curves for pinacidil and cromakalim underwent rightward parallel shifts. There were no significant differences between pEC50 and E(max) values of pinacidil and cromakalim in the absence of glibenclamide (10(-6)M) (p > 0.05), but pEC(50) values of pinacidil and cromakalim in the presence of glibenclamide (10(-6)M) were significantly reduced (p < 0.05).

CONCLUSION

These results suggest that the relaxation caused in sheep SO by pinacidil and cromakalim is mediated through the same glibenclamide-sensitive KATP channel. Pinacidil and cromakalim have an equipotent relaxing effect in isolated sheep SO and they can be beneficial as alternative drugs for obtaining selective relaxation during SO manometry after controlled clinical studies.

Antispasmodic and blood pressure lowering effects of Valeriana wallichii are mediated through K+ channel activation

Crude extract of Valeriana wallichii rhizome (Vw.Cr) and its fractions were studied for possible antispasmodic and blood pressure lowering activities to rationalize some of the folkloric uses. In rabbit jejunum preparations, Vw.Cr (0.1-3.0 mg/mL) caused relaxation of spontaneous contractions. When tested against high K(+) (80 mM)-induced contractions it produced weak inhibitory effect, while caused complete relaxation of the contractions induced by low K(+) (20 mM). In the presence of glibenclamide (3 microM), the inhibitory effect of low K(+) was shifted to the right, similar to that produced by cromakalim while, verapamil caused no differentiation in its inhibitory effect against low and high K(+)-induced contractions. In guinea pig ileum, the plant extract produced similar results as in rabbit jejunum. Intravenous administration of Vw.Cr, produced fall in arterial blood pressure in normotensive anaesthetized rats and this effect was partially blocked by glibenclamide. In rabbit aortic preparations, plant extract also caused a selective and glibenclamide-sensitive relaxation of low K(+) (20 mM)-induced contractions. Activity-directed fractionation studies revealed that the observed activity was distributed both in the chloroform and aqueous fractions. These results indicate that the antispasmodic and hypotensive effects of Valeriana wallichii are mediated possibly through K(ATP) channel activation, which justify its use in gastrointestinal and cardiovascular disorders.

Effect of thimerosal on arrhythmia induced by coronary ligation: the involvement of ATP-dependent potassium channels

Thiol-modifying agents induce the release of nitric oxide (NO) from endothelial epithelium and the release of reactive oxygen free radicals in the vascular system. Moreover, thiol groups are essential for the functioning of the ATP dependent potassium channel (K-ATP). The effects of thiol-modifying agents and their molecular mechanisms on arrhythmia have not been widely studied. In this study, we investigated the effect of the hydrophilic SH-group-oxidizing substance thimerosal on the arrhythmia induced by reperfusion/ischemia after coronary artery ligation in rats. We studied the possible involvement of the K-ATP and NOS on the effect of thimerosal. Thimerosal pretreatment (3, 30 mg/kg dose iv. 10 minutes before coronary occlusion) significantly decreased the length of total arrhythmia, ventricular tachycardia, and the arrhythmia score. This effect of thimerosal was reversed by the K-ATP opener pinacidil but not by the K-ATP blocker glibenclamide. The inhibition of iNOS by L-NAME did not alter the antiarrhythmic effect of thimerosal. These data clearly suggest that the antiarrhythmic effect of thimerosal is dependent upon the blockage of K-ATP.

150-kD oxygen-regulated protein is an essential factor for insulin release

OBJECTIVE

This study was designed to show the induction and function of 150-kD oxygen-regulated protein (ORP150) in insulin secretion in vitro.

METHODS

A mouse beta-cell line, MIN6 cells, was cultured in medium containing potassium channel openers or various concentrations of glucose (2-25 mmol/L). ORP150 expression was studied by reporter assay, Western blot, or Northern blot analysis, concomitantly with insulin secretion. In addition, MIN6 cells infected with ORP150 recombinant adenovirus were adopted to show the function of ORP150 in insulin release.

RESULTS

ORP150 expression in MIN6 cells was suppressed dose-dependently by the potassium channel opener diazoxide. Both low glucose (<2 mmol/L) and high glucose concentrations (25 mmol/L glucose) significantly induced more ORP150 expression compared with 10 mmol/L glucose. The treatment with diazoxide or infection of ORP150 antisense adenovirus suppressed ORP150 expression, and glucose-stimulated insulin secretion was effectively prevented.

CONCLUSION

These findings show the involvement of ORP150 in insulin secretion in MIN6 cells.

Contribution of Na+ -Ca2+ exchanger to pinacidil-induced relaxation in the rat mesenteric artery

1 Pinacidil relaxes blood vessels through opening the K(ATP) channels with a resultant membrane hyperpolarization and inhibition of Ca(2+) influx. The aim of this study was to examine the mechanisms thereby pinacidil induces K(+) channel-independent relaxation in isolated endothelium-denuded rat mesenteric artery. 2 Pinacidil-induced relaxation was inhibited by glibenclamide (1-10 micro M) in phenylephrine-preconstricted rings, but was unaffected by glibenclamide after inhibition of K(+) channels and VGCCs. Pinacidil-induced K(+) channel-independent relaxation remained unchanged after treatment with cyclopiazonic acid (10 micro M), thapsigargin (1 micro M), ouabain (100 micro M), propranolol (10 micro M), Rp-cAMPS triethylamine (30 micro M), L-NNA (100 micro M), or ODQ (10 micro M). 3 Pinacidil induced more relaxant effect in the presence of nifedipine than in the presence of 60 mM K(+) plus nifedipine. Pretreatment with Na(+)-Ca(2+) exchanger inhibitors, nickel (30-300 micro M) or benzamil (20 micro M) attenuated pinacidil-induced relaxation in normal or in nifedipine-containing solution. Pinacidil (1 micro M) produced less relaxant effect with decreasing extracellular Na(+) concentration. Na(+)-free condition abolished the inhibitory effect of benzamil. Both nickel and benzamil inhibited pinacidil-induced relaxation in the presence of glibenclamide (10 micro M). Nickel (300 micro M) did not affect the relaxant response to sodium nitroprusside. 4 Pinacidil relaxed the rings preconstricted by active phorbol and U46619 with similar potency. 5 The present results indicate that stimulation of the forward mode Na(+)-Ca(2+) exchange pathway is in part responsible for pinacidil-induced K(+) channel-independent vasorelaxation. Pinacidil also induces K(+) channel-dependent but VGCCs-independent relaxation. The PKC-mediated cellular pathway may be a target site for pinacidil only in higher concentrations.

Decreased alpha-endosulfine, an endogenous regulator of ATP-sensitive potassium channels, in brains from adult Down syndrome patients

Alpha-endosulfine has the ability to block ATP-sensitive potassium (K(ATP)) channels and stimulate insulin release in beta cells like sulfonylurea. Alpha-endosulfine is expressed in a wide range of tissue, including brain and endocrine tissues. Although K(ATP) channels are also present in brain and its regulators have been reported to be involved in the release of neurotransmitters such as acetylcholine that plays an important role in cognitive function, the neurobiological role of alpha-endosulfine has not been studied yet. We examined the expression levels of alpha-endosulfine protein in frontal cortex and cerebellum from patients with Down syndrome (DS) showing Alzheimer's disease (AD) pathology using Western blotting. In frontal cortex, alpha-endosulfine was detected in all of 10 controls, but only 1 (from female) out of 8 DS with weak density. In cerebellum, alpha-endosulfine was also detected in all of 9 controls, but only 1 (from male) out of 6 DS with weak density. The considerably decreased alpha-endosulfine could result in the continuous opening of K(ATP) channels and the subsequent decrease of neurotransmitters release associated with cognition. This study is of significance providing evidence for a biological role of alpha-endosulfine in brain and alpha-endosulfine protein could be a pharmacological target for therapeutic intervention.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

Potassium channel openers: therapeutic potential in cardiology and medicine

Potassium (K(+)) channel openers (KCOs) define a class of chemically diverse agents that share a common molecular target, the metabolism-regulated ATP-sensitive K(+) (K(ATP)) channel. In view of the unique function that K(ATP) channels play in the maintenance of cellular homeostasis, this novel class of ion channel modulators adds to existent pharmacotherapy with potential in promoting cellular protection under conditions of metabolic stress. Indeed, experimental studies have demonstrated broad therapeutic potential for KCOs, including roles as cardioprotective agents, vasodilators, bronchodilators, bladder relaxants, anti-epileptics, insulin secretagogues and promoters of hair growth. However, clinical experience with these drugs is limited and their place in patient management needs to be fully established.

Dual mechanism of action of nicorandil on rabbit corpus cavernosal smooth muscle tone

The potential of ATP-sensitive potassium channel openers (KCOs) for the treatment of male erectile dysfunction has recently been suggested based on positive clinical outcomes following intra-cavernosal administration of pinacidil. Agents that increase the levels of cGMP via elevation of nitric oxide (NO) nitroglycerin, for example, are also effective in improving erectile function preclinically and clinically. The aim of the present study was to determine the effects and mechanism of the action of nicorandil on rabbit corpus cavernosum. The in vitro regulation of smooth muscle tone was assessed in isolated cavernosal tissues pre-contracted with phenylephrine. Nicorandil, but not its major metabolite, relaxed phenylephrine-precontracted cavernosum smooth muscle with an EC(50) of 15 microM. The effects of nicorandil were only partially reversed by the K(ATP) channel blocker glyburide (10 microM) or by a soluble guanylate cyclase (sGC) inhibitor 1H-[1,2,4] oxadiazole [4,3-a] quinoxalin-1-one (ODQ, 3 microM). However, a combination of ODQ and glyburide completely blocked the relaxant effects of nicorandil. The results of the present study indicate that nicorandil can relax rabbit cavernosal tissue in vitro via a mechanism that involves activation of K(ATP) channels and stimulation of soluble guanylate cyclase.

Pharmacological evidence for the activation of potassium channels as the mechanism involved in the hypotensive and vasorelaxant effect of dioclein in rat small resistance arteries

The hypotensive and vasorelaxant effect of dioclein in resistance mesenteric arteries was studied in intact animals and isolated vessels, respectively. In intact animals, initial bolus administration of dioclein (2.5 mg kg(-1)) produced transient hypotension accompanied by an increase in heart rate. Subsequent doses of dioclein (5 and 10 mg kg(-1)) produced hypotensive responses with no significant change in heart rate. N(G)-nitro-L-arginine methyl ester (L-NAME) did not affect the hypotensive response. In endothelium-containing or -denuded vessels pre-contracted with phenylephrine, dioclein (5 and 10 mg kg(-1) produced a concentration-dependent vasorelaxation (IC(50)=0.3+/-0.06 and 1.6+/-0.6 microM, respectively) which was not changed by 10 microM indomethacin. L-NAME (300 microM) produced a shift to the right. Dioclein was without effect on contraction of vessels induced by physiological salt solution (PSS) containing 50 mM KCl and the concentration dependence of dioclein's effect on phenylephrine induced contraction was shifted to the right in vessels bathed in PSS containing 25 mM KCl. Tetraethylammonium (10 mM) and BaCl(2) (1 mM) increased the IC(50) for dioclein-induced vasorelaxation without affecting the maximal response (E(max)). Charybdotoxin (100 nM), 4-aminopyridine (1 mM) and iberiotoxin (100 nM) increased the IC(50) and reduced the E(max). Apamin (1 microM) reduced the E(max) without affecting the IC(50). Dioclein produced a hyperpolarization in smooth muscle of mesenteric arteries with or without endothelium (7.7+/-1.4 mV and 12.3+/-3.6 mV, respectively). In conclusion dioclein lowered arterial pressure probably through a decrease in peripheral vascular resistance. The underling mechanism implicated in the vasorelaxant effect of dioclein appears to be the opening of K(Ca) and Kv channels and subsequent membrane hyperpolarization.

K(ATP) channel openers, adenosine agonists and epileptic preconditioning are stress signals inducing hippocampal neuroprotection

Many models of induced ischemic and epileptic tolerance have now been described in the brain. Although detailed mechanisms underlying such protections still remain largely unknown, induction of heat shock proteins is amongst the endogenous responses believed to play an important role in cellular defense mechanisms. This study reveals that the development of epileptic tolerance also coincides with the induction of the 70,000 mol. wt heat shock protein expression within the time window of protection. Adenosine agonists or ATP-sensitive potassium channel openers have also been shown to exert strong neuroprotective effects when injected shortly prior to a severe ischemic or epileptic insult. The present work shows that adenosine receptor activation and ATP-sensitive potassium channel opening induce 70,000 mol. wt heat shock protein expression in the rat hippocampus and are able to mimic neuroprotection driven by preconditioning. R-phenylisopropyladenosine, a purine agonist, or (-)cromakalim, an ATP-sensitive potassium channel opener, was administered three days prior to a lethal ischemic or epileptic episode to mimic preconditioning. Neurodegeneration was assessed using Cresyl Violet staining and cellular DNA fragmentation visualized by the terminal deoxynucleotidyl transferase-mediated 2'-deoxyuridine 5'-triphosphate-biotin nick end labeling method. 70, 000 mol. wt heat shock protein expression was analysed by western blotting and immunohistochemistry. The results show a long-lasting neuroprotection induced by activation of adenosine receptors or ATP-sensitive K(+) channels as early as three days prior to induction of a severe ischemic or epileptic challenge. This protective effect is associated with enhanced 70,000 mol. wt heat shock protein expression also occurring three days following administration of R-phenylisopropyladenosine or (-)cromakalim. These findings support the idea that preconditioning doses of R-phenylisopropyladenosine and (-)cromakalim act as mild cellular stresses inducing neuroprotection in a manner similar to a mild kainate treatment prior to a lethal ischemic or severe epileptic insult three days later. They also suggest that a delayed 70,000 mol. wt heat shock protein expression induced by excitatory neuronal stresses such as short ischemia, mild kainic acid treatment or activation of adenosine receptors and ATP-sensitive potassium channels is predictive of neuronal survival against a subsequent lethal injury.

Polyunsaturated fatty acids are potent neuroprotectors

Results reported in this work suggest a potential therapeutic value of polyunsaturated fatty acids for cerebral pathologies as previously proposed by others for cardiac diseases. We show that the polyunsaturated fatty acid linolenic acid prevents neuronal death in an animal model of transient global ischemia even when administered after the insult. Linolenic acid also protects animals treated with kainate against seizures and hippocampal lesions. The same effects have been observed in an in vitro model of seizure-like activity using glutamatergic neurons and they have been shown to be associated with blockade of glutamatergic transmission by low concentrations of distinct polyunsaturated fatty acids. Our data suggest that the opening of background K(+) channels, like TREK-1 and TRAAK, which are activated by arachidonic acid and other polyunsaturated fatty acids such as docosahexaenoic acid and linolenic acid, is a significant factor in this neuroprotective effect. These channels are abundant in the brain where they are located both pre- and post-synaptically, and are insensitive to saturated fatty acids, which offer no neuroprotection.

Immune mechanisms of smooth muscle hyperreactivity in asthma

Asthma is characterized by bronchial hyperresponsiveness to a variety of bronchospasmogenic stimuli. To study the pathophysiologic mechanisms underlying the increased sensitivity and degree of maximal airway narrowing, various in vivo and in vitro models have been developed with methods of active and passive sensitization. These studies indicated a major role for alterations in the smooth muscle itself rather than neural dysfunction or airway inflammation as the underlying cause for the development of bronchial hyperresponsiveness. During the last years smooth muscle cells were found to exhibit not only the "classical" contractile phenotype but also a proliferative-synthetic phenotype, which is capable of producing proinflammatory cytokines, chemotaxins, and growth factors. Allergic sensitization can alter both contractile and secretory functions, thereby indicating that the smooth muscle cell could contribute directly to the persistence of airway inflammation in asthma. A better understanding of the changes within the smooth muscle cells and of the mechanisms that lead to their induction could contribute to the development of novel therapeutic approaches for the treatment of asthma.

Differential effects of pinacidil and levcromakalim on the contractions elicited electrically or by noradrenaline in the portal vein of the rabbit

The present study was undertaken to examine the antivasoconstrictor effects of pinacidil and levcromakalim, two potassium channel openers (PCOs), on the isolated rabbit portal vein and to define the role for different subtypes of pre- and/or post-synaptic K+ channels in the antivasoconstrictor action of the PCOs. The vein strips were contracted by electrical field stimulation (EFS) or by exogenous noradrenaline (NA). The results of this study showed that pinacidil produced a more potent inhibition of the neurogenic contractions (pD2 = 6.04 +/- 0.05) than of contractions induced by exogenous NA (pD2 = 4.90 +/- 0.10). Glibenclamide (1 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels (K(ATP)), did not affect the pinacidil-induced inhibition of contractions evoked by exogenous NA. In contrast, glibenclamide (0.1-10 microM) significantly antagonized the effect of pinacidil on EFS evoked contractions in a noncompetitive manner. There was no difference between the inhibitory effects of levcromakalim on neurogenic contractions (pD2 = 7.58 +/- 0.05) and contractions evoked by exogenous NA (pD2 = 7.64 +/- 0.08). Glibenclamide (1 microM) antagonized in the same manner the levcromakalim-induced inhibition of neurogenic contractions and contractions evoked by exogenous NA. Moreover, glibenclamide competitively antagonized the effect of levcromakalim on EFS induced contractions of the rabbit portal vein (pA2 = 6.40 +/- 0.10). Charybdotoxin (0.4 microM) and apamin (0.1 microM) did not influence the inhibitory effects of pinacidil and levcromakalim, both on contractions evoked by EFS and contractions evoked by exogenous NA. These results suggest that the antivasoconstrictor effect of levcromakalim might be postsynaptic and associated with opening of the smooth muscle K(ATP) channels. In contrast, it is hypothesized that the effect of pinacidil on neurogenic contractions is due to an interference with K(ATP) channels in the neuromuscular synapse. It seems that the action of pinacidil on the NA contractions is mediated by another still undefined mechanisms of pinacidil.

ATP-sensitive K+ channel openers prevent Ca2+ overload in rat cardiac mitochondria

1. Mitochondrial dysfunction, secondary to excessive accumulation of Ca2+, has been implicated in cardiac injury. We here examined the action of potassium channel openers on mitochondrial Ca2+ homeostasis, as these cardioprotective ion channel modulators have recently been shown to target a mitochondrial ATP-sensitive K+ channel. 2. In isolated cardiac mitochondria, diazoxide and pinacidil decreased the rate and magnitude of Ca2+ uptake into the mitochondrial matrix with an IC50 of 65 and 128 microM, respectively. At all stages of Ca2+ uptake, the potassium channel openers depolarized the mitochondrial membrane thereby reducing Ca2+ influx through the potential-dependent mitochondrial uniporter. 3. Diazoxide and pinacidil, in a concentration-dependent manner, also activated release of Ca2+ from mitochondria. This was prevented by cyclosporin A, an inhibitor of Ca2+ release through the mitochondrial permeability transition pore. 4. Replacement of extramitochondrial K+ with mannitol abolished the effects of diazoxide and pinacidil on mitochondrial Ca2+, while the K+ ionophore valinomycin mimicked the effects of the potassium channel openers. 5. ATP and ADP, which block K+ flux through mitochondrial ATP-sensitive K+ channels, inhibited the effects of potassium channel openers, without preventing the action of valinomycin. 6. In intact cardiomyocytes, diazoxide also induced mitochondrial depolarization and decreased mitochondrial Ca2+ content. These effects were inhibited by the mitochondrial ATP-sensitive K+ channel blocker 5-hydroxydecanoic acid. 7. Thus, potassium channel openers prevent mitochondrial Ca2+ overload by reducing the driving force for Ca2+ uptake and by activating cyclosporin-sensitive Ca2+ release. In this regard, modulators of an ATP-sensitive mitochondrial K+ conductance may contribute to the maintenance of mitochondrial Ca2+ homeostasis.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Comparison of the vascular relaxant effects of ATP-dependent K+ channel openers on aorta and pulmonary artery isolated from spontaneously hypertensive and Wistar-Kyoto rats

The vasorelaxant actions of adenosine 5'-triphosphate (ATP)-dependent K+ channel openers and sodium nitroprusside in isolated thoracic aorta and pulmonary artery of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats (14-18 weeks old) were investigated. Cumulative addition of sodium nitroprusside and different ATP-dependent K+ channel openers (pinacidil, cromakalim, nicorandil, 2-(2"(1",3"-dioxolone)-2-methyl-4-(2'-oxo-1'-pyrrolidinyl)-6-nitro -2H-1-benzopyren (KR-30450) and aprikalim) to these preparations caused a concentration-dependent relaxation of noradrenaline-pre-contracted aorta and pulmonary artery from both strains. The relative order of relaxation potency, estimated by comparing the IC50, was sodium nitroprusside > KR-30450 > aprikalim > or = cromakalim > pinacidil > nicorandil in pulmonary artery and aorta from both strains. At high concentrations (> or =1 microM), cromakalim, aprikalim and KR-30450 produced a greater percentage relaxation in SHR aorta than in WKY aorta. However, there was no apparent difference between SHR and WKY in the relaxation response to all drugs tested on the pulmonary artery. The effects of cromakalim, aprikalim, pinacidil and KR-30450 observed in aorta and pulmonary artery were significantly attenuated by 3 microM glibenclamide. 6-Anilino-5,8-quinolinequinone (LY 83583, 1 microM), a soluble guanylate cyclase inhibitor, abolished the vasorelaxant effects of nicorandil and sodium nitroprusside. In conclusion, sodium nitroprusside and ATP-dependent K+ channel openers cause relaxation of noradrenaline-pre-contracted aorta and pulmonary artery from both strains. However, all the drugs tested failed to cause selective relaxation of the pulmonary artery relative to the thoracic aorta.

Vascular nitric oxide may lessen Alzheimer's risk

Estrogen deficiency, hyperinsulinemia, type II diabetes, atherosclerosis, and a past history of elevated blood pressure may be associated with increased risk of Alzheimer's disease (AD). Common to all of these risk factors is a diminished capacity of vascular endothelium to generate nitric oxide (NO). Vascular NO has the potential to enhance the membrane polarization of cerebral neurons by increasing the open probability of calcium-activated potassium channels; this may protect neurons from the excessive calcium influx, potentiated by beta-amyloid peptides that is thought to mediate neuronal damage in AD. The possibility that NO/cyclic guanosine 3', 5'-phosphate (cGMP) may modulate the synthesis or processing of the amyloid precursor protein, also merits evaluation. Practical measures for promoting vascular NO production may include increased intakes of arginine, potassium, antioxidants, and fish-oil, as well as lifestyle measures that typically lower elevated blood pressure; potential benefits of chromium, glucosamine, and silicon should also be explored. In hypertensives, angiotensin-converting enzyme (ACE) inhibitors and sodium restriction may favorably influence endothelial function. Fish-oil should have the additional benefit of antagonizing the contribution of interleukin-1 to AD pathogenesis. Ancillary anti-excitotoxic measures such as magnesium, taurine, phenytoin, and vasodilators targeting ATP-dependent potassium (KATP) channels, may likewise reduce AD risk. Most of the nutritional measures suggested here would in any case be recommendable for preservation of vascular health.

Mg2+ and ATP dependence of K(ATP) channel modulator binding to the recombinant sulphonylurea receptor, SUR2B

1. The binding of modulators of the ATP-sensitive K+ channel (KATP channel) to the murine sulphonylurea receptor, SUR2B, was investigated. SUR2B, a proposed subunit of the vascular KATP channel, was expressed in HEK 293 cells and binding assays were performed in membranes at 37 degrees C using the tritiated KATP channel opener, [3H]-P1075. 2. Binding of [3H]-P1075 required the presence of Mg2+ and ATP. MgATP activated binding with EC50 values of 10 and 3 microM at free Mg2+ concentrations of 3 microM and 1 mM, respectively. At 1 mM Mg2+, binding was lower than at 3 microM Mg2+. 3. [3H]-P1075 saturation binding experiments, performed at 3 mM ATP and free Mg2+ concentrations of 3 microM and 1 mM, gave KD values of 1.8 and 3.4 nM and BMAX values of 876 and 698 fmol mg(-1), respectively. 4. In competition experiments, openers inhibited [3H]-P1075 binding with potencies similar to those determined in rings of rat aorta. 5. Glibenclamide inhibited [3H]-P1075 binding with Ki values of 0.35 and 2.4 microM at 3 Mm and 1 mM free Mg2+, respectively. Glibenclamide enhanced the dissociation of the [3H]-P1075-SUR2B complex suggesting a negative allosteric coupling between the binding sites for P1075 and the sulphonylureas. 6. It is concluded that an MgATP site on SUR2B with microM affinity must be occupied to allow opener binding whereas Mg2+ concentrations > or = 10 microM decrease the affinities for openers and glibenclamide. The properties of the [3H]-P1075 site strongly suggest that SUR2B represents the drug receptor of the openers in vascular smooth muscle.

Intracellular nucleotide-mediated gating of SUR/Kir6.0 complex potassium channels expressed in a mammalian cell line and its modification by pinacidil

1. We have examined the properties of intracellular nucleotide-mediated gating of K+ channel constructs composed of the sulphonylurea receptor 2B and the inwardly rectifying K+ channel subunits Kir6.1 and Kir6.2 (SUR2B/Kir6.1 and SUR2B/Kir6.2 complex K+ channels) heterologously expressed in human embryonic kidney (HEK) 293T cells. In the cell-attached form, both types of K+ channel were activated by pinacidil. 2. In inside-out (IO) patches, the SUR2B/Kir6.2 channels opened spontaneously and were inhibited by intracellular ATP (ATPi). Pinacidil attenuated the ATPi-mediated channel inhibition in a concentration-dependent manner. In contrast, the SUR2B/Kir6.1 channels required intracellular nucleoside di- or tri-, but not mono-, phosphates for opening. The potency of adenine, guanine or uracil nucleotides to activate SUR2B/Kir6.1 channels was enhanced by pinacidil. 3. In the presence of pinacidil, adenine and guanine, but not uracil, nucleotides exhibited bell-shaped concentration-dependent activating effects on SUR2B/Kir6.1 channels. This was due to channel inhibition caused by adenine and guanine nucleotides, which was unaffected by pinacidil. 4. From power density spectrum analysis of SUR2B/Kir6.1 currents, channel activation could be described by the product of two gates, a nucleotide-independent fast channel gate and a nucleotide-dependent slow gate, which controlled the number of functional channels. Pinacidil specifically increased the potency of nucleotide action on the slow gate. 5. We conclude that Kir6.0 subunits play a crucial role in the nucleotide-mediated gating of SUR/Kir6.0 complex K+ channels and may determine the molecular mode of pinacidil action.

Levcromakalim decreases vascular tone, cytoplasmic Ca2+ and Ca2+ sensitivity in canine basilar artery

The involvement of large conductance Ca(2+)-activated K+ channels (BK) and ATP-sensitive K+ (KATP) channels in the regulation of canine basilar arterial tone was estimated in the presence of the agonist and blockers of these channels, by simultaneously measuring the changes in intracellular Ca2+ concentration ([Ca2+]i) with the fura-2 microfluorimetric method. In the resting condition, levcromakalim reduced [Ca2+]i and vascular tone. Levcromakalim suppressed the serotonin-induced increases in [Ca2+]i and force of contraction, the maximum effects of which were much greater than those of nicardipine. The inhibitory effects of levcromakalim were blocked by glibenclamide but not by tetraethylammonium (TEA) or iberiotoxin (IbTX). In the presence of levcromakalim, the curve relating [Ca2+]i with force in the presence of serotonin at different extracellular Ca2+ concentration ([Ca2+]o) was shifted down- and right-ward compared with that in the absence of levcromakalim, suggesting that levcromakalim may reduce the Ca(2+)-sensitivity of the contractile proteins. Thus, levcromakalim may be a good candidate to suppress delayed cerebral vasospasm after subarachnoid hemorrhage.

Reversal of hypothermia-induced action potential lengthening by the KATP channel agonist bimakalim in isolated guinea pig ventricular muscle

1. ATP-sensitive potassium (KATP) channel openers shorten cardiac ventricular muscle action potential duration (APD), reduce resting and developed contractile force, and have been shown to provide cardioprotection when given before, during, and after either short-term ischemia or long-term hypothermia. The authors' aim was to determine the concentration-dependent effect of the potent KATP channel opener bimakalim on transmembrane action potential changes induced by mild (27 degrees C) and moderate (20 degrees C) hypothermia in isolated guinea pig ventricular muscle. 2. Conventional microelectrode techniques were used to record action potentials (APs) in single myocytes during normothermia (37 degrees C) and hypothermia in the presence and absence of 0.1 to 30 mumol.l-1 bimakalim. 3. Hypothermia alone increased APD and depolarized the diastolic membrane potential (DMP): APD90 = 141.7 +/- 7.0 msec and DMP -86.2 +/- 1.4 mV (n = 6) at 37 degrees C versus 235.7 +/- 7.8 msec and -75.6 +/- 1.0 mV at 20 degrees C (n = 7). At 37 degrees C, bimakalim (0.1-10 mumol.l-1) shortened APD in a concentration-dependent fashion. 4. APD90 was markedly reduced from 141.7 +/- 7.0 msec without bimakalim to 9.5 +/- 2.6 msec with 10 mumol.l-1 bimakalim (n = 6); this effect was blocked by glibenclamide. DMP was hyperpolarized by bimakalim. More bimakalim was required to shorten APs during mild and moderate hypothermia. The 50% effective concentration (EC50) of bimakalim required to maximally shorten APD90 was 0.96 +/- 0.10 mumol.l-1 at 37 degrees C; this increased to 3.96 +/- 0.24 mumol.l-1 at 27 degrees C, and to 12.34 +/- 0.72 mumol.l-1 at 20 degrees C. Relative to hypothermia-induced depolarization, bimakalim hyperpolarized DMP toward drug-free values obtained at 37 degrees C. 5. These results indicate that hypothermia shifts the bimakalim concentration APD90 response curve to the right such that 13 times more bimakalim is required at 20 degrees C shorten APD by the same amount as at 37 degrees C. Bimakalim also reverses hypothermia-induced AP lengthening and tends to reverse the hypothermia-induced decrease in DMP. 6. These findings aid in our understanding of the cardioprotective effects of KATP channel openers during hypothermia.

A neuronal two P domain K+ channel stimulated by arachidonic acid and polyunsaturated fatty acids

TWIK-1, TREK-1 and TASK K+ channels comprise a class of pore-forming subunits with four membrane-spanning segments and two P domains. Here we report the cloning of TRAAK, a 398 amino acid protein which is a new member of this mammalian class of K+ channels. Unlike TWIK-1, TREK-1 and TASK which are widely distributed in many different mouse tissues, TRAAK is present exclusively in brain, spinal cord and retina. Expression of TRAAK in Xenopus oocytes and COS cells induces instantaneous and non-inactivating currents that are not gated by voltage. These currents are only partially inhibited by Ba2+ at high concentrations and are insensitive to the other classical K+ channel blockers tetraethylammonium, 4-aminopyridine and Cs+. A particularly salient feature of TRAAK is that they can be stimulated by arachidonic acid (AA) and other unsaturated fatty acids but not by saturated fatty acids. These channels probably correspond to the functional class of fatty acid-stimulated K+ currents that recently were identified in native neuronal cells but have not yet been cloned. These TRAAK channels might be essential in normal physiological processes in which AA is known to play an important role, such as synaptic transmission, and also in pathophysiological processes such as brain ischemia. TRAAK channels are stimulated by the neuroprotective drug riluzole.

Nicorandil affects diurnal rhythms of body temperature, heart rate and locomotor activity in rats

The effects of nicorandil, a K+ channel opener with a potent vasodilator action, on diurnal rhythms of body temperature, heart rate and locomotor activity were assessed in rats. Transmitters were intraperitoneally implanted under ether anaesthesia. After recovery from surgery, body temperature, heart rate and locomotor activity were recorded during control, saline or nicorandil (10 mg x kg(-1) administered orally) treatment and for 5 days after treatment. For each period, Fourier analysis determined the predominant rhythmicity for body temperature, heart rate and locomotor activity while cosinor analysis assessed the corresponding mesors, acrophases and amplitudes and maxima and minima were directly plotted from raw data. The results indicated: (1) loss of the diurnal rhythmicity for all three rhythms after implantation; (2) stress-induced modifications of almost all the characteristics of the three rhythms after saline and (3) a loss of diurnal rhythmicity of heart rate after nicorandil, an effect that was not observed after saline and which was reversed when nicorandil administration was stopped. In conclusion, nicorandil perturbed the diurnal rhythmicity of heart rate while the rhythmicity of body temperature and locomotor activity was not affected.

Binding of K(ATP) channel modulators in rat cardiac membranes

1. The binding of [3H]-P1075, a potent opener of adenosine-5'-triphosphate-(ATP)-sensitive K+ channels, was studied in a crude heart membrane preparation of the rat, at 37 degrees C. 2. Binding required MgATP. In the presence of an ATP-regenerating system, MgATP supported [3H]-P1075 binding with an EC50 value of 100 microM and a Hill coefficient of 1.4. 3. In saturation experiments [3H]-P1075 binding was homogeneous with a KD value of 6+/-1 nM and a binding capacity (Bmax) of 33+/-3 fmol mg(-1) protein. 4. Upon addition of an excess of unlabelled P1075, the [3H]-P1075-receptor complex dissociated in a mono-exponential manner with a dissociation rate constant of 0.13+/-0.01 min(-1). If a bi-molecular association mechanism was assumed, the dependence of the association kinetics on label concentration gave an association rate constant of 0.030+/-0.003 nM(-1) min(-1). From the kinetic experiments the KD value was calculated as 4.7+/-0.6 nM. 5. Openers of the ATP-sensitive K+ channel belonging to different structural classes inhibited specific [3H]-P1075 binding in a monophasic manner to completion; an exception was minoxidil sulphate where maximum inhibition was 68%. The potencies of the openers in this assay agree with published values obtained in rat cardiocytes and are on average 3.5 times lower than those determined in rat aorta. 6. Sulphonylureas, such as glibenclamide and glibornuride and the sulphonylurea-related carboxylate, AZ-DF 265, inhibited [3H]-P1075 binding with biphasic inhibition curves. The high affinity component comprised about 60% of the curves with the IC50 value of glibenclamide being approximately 90 nM; affinities for the low affinity component were in the microM concentration range. The fluorescein derivative, phloxine B, showed a monophasic inhibition curve with an IC50 value of 6 microM, a maximum inhibition of 94% and a Hill coefficient of 1.5. 7. It is concluded that binding studies with [3H]-P1075 are feasible in rat heart membranes in the presence of MgATP and of an ATP-regenerating system. The pharmacological profile of the [3H]-P1075 binding sites in the cardiac preparation, which probably contains sulphonylurea receptors (SURs) from cardiac myocytes (SUR2A) and vascular smooth muscle cells (SUR2B), differs from that expected for SUR2A and SUR2B.

Pyridazine derivatives XIV. Study of the vasorelaxant action of 6-aryl-5-piperidino-3-hydrazinopyridazines in isolated rat thoracic aorta: comparison with hydralazine

1. For several years we have been working on the synthesis of modified hydrazinopyridazines which have proved to possess remarkable vasorelaxant and antihypertensive activity. We now report the vasodilator effects of three novel 6-aryl-5-piperidino-3-hydrazinopyridazines (1a, 1b and 1c), structurally related to the well-known antihypertensive drug hydralazine. 2. Hydralazine and the new hydrazinopyridazines relaxed, in a concentration-dependent and nonspecific way, the contractions elicited by noradrenaline or a high K+ concentration in rat aortic rings with or without endothelium. According to the IC50 (50% inhibitory concentrations) values obtained, the vasorelaxant potency of the new compounds was greater than that of hydralazine. 3. In a Ca2+-free medium, the contractions provoked by noradrenaline or caffeine were significantly inhibited by the new hydrazinopyridazines and by hydralazine. 4. Hydralazine and the novel molecules did not significantly modify basal, noradrenaline- or K+-induced 45Ca2+ uptake. 5. These results suggest that 1a, 1b and 1c have an endothelium-independent vasorelaxant activity greater than that of hydralazine in isolated rat aortic rings, which seems not to be mediated by a blockade of transmembrane Ca2+ movements through specific channels. This effect could be due, at least in part, to an intracellular mechanism of action.

Function, regulation, pharmacology, and molecular structure of ATP-sensitive K+ channels in the cardiovascular system

ATP-sensitive K+ (K[ATP]) channels are inhibited by intracellular ATP and activated by intracellular nucleoside diphosphates, and thus provide a link between cellular metabolism and excitability. K(ATP) channels are widely distributed in various tissues and may be associated with diverse cellular functions. In the heart, the K(ATP) channel appears to be activated during ischemic or hypoxic conditions and may be responsible for the increase of K+ efflux and shortening of the action potential duration. Therefore, opening of this channel may result in cardioprotective as well as proarrhythmic effects. In the vascular smooth muscle, the K(ATP) channel is believed to mediate the relaxation of vascular tone. Thus, K(ATP) channels play important regulatory roles in the cardiovascular system. Furthermore, K(ATP) channels are the targets of two important classes of drugs, i.e., the antidiabetic sulfonylureas, which block the channels, and a series of vasorelaxants called "K+ channel openers," which tend to maintain the channels in an open conformation. Recently, the molecular structure of K(ATP) channels has been clarified. The K(ATP) channel in pancreatic beta-cells is a complex composed of at least two subunits, a member of inwardly rectifying K+ channels and a sulfonylurea receptor. Subsequently, two additional homologs of the sulfonylurea receptor, which form cardiac and smooth muscle type K(ATP) channels, respectively, have been reported. Further works are now in progress to understand the molecular mechanisms of K(ATP) channel function.

Study of the in vivo and in vitro cardiovascular effects of a hydralazine-like vasodilator agent (HPS-10) in normotensive rats

1. In this work, the cardiovascular effects of HPS-10, a new vasodilator agent, were studied in rats. 2. In conscious normotensive rats, oral administration of HPS-10 (4-9 mg kg-1) produced a dose-related and long-lasting fall in systolic arterial blood pressure (ED30 of 5.32 mg kg-1), accompanied by an increase in heart rate (ED30 of 8.43 mg kg-1). This tachycardia was totally inhibited by pretreatment with (+/-)-propranolol (10 mg kg-1, p.o.). 3. In anaesthetized normotensive rats, HPS-10 (0.3-0.6 mg kg-1, i.v.) produced a gradual, dose-dependent and sustained decrease in systolic, diastolic and mean arterial pressure (MAP) (ED30 for MAP of 0.41 mg kg-1, i.v.), accompanied by a significant bradycardia at high doses (> 0.4 mg kg-1; ED20 of 0.61 mg kg-1, i.v.). HPS-10 (0.5 mg kg-1, i.v.) did not modify the positive chronotropic effects induced by intravenous administration of noradrenaline (NA; 5 micrograms kg-1), angiotensin II (AII; 0.2 microgram kg-1) and nicotine (200 micrograms kg-1) but markedly inhibited the hypertensive response produced by these agents. 4. In rat isolated rubbed aorta, HPS-10 (0.1-1 mM) non-competitively and with almost equal effectiveness antagonized the contractions induced by NA, AII (in normal Krebs solution) and Ca2+ (in depolarizing Ca(2+)-free high-K+ 50 mM solution). In the experiments in Ca(2+)-free medium, HPS-10 (1 mM) considerably inhibited the contractions induced by NA, AII and caffeine in rat aorta. 5. Furthermore, in the studies with radioactive Ca2+, HPS-10 (1 mM) did not modify the basal uptake of 45Ca2+ but strongly decreased the influx of 45Ca2+ induced by NA, AII and K+ in rat aortic rings. 6. In rat isolated atria, HPS-10 (1 mM) produced a positive inotropic/negative chronotropic effect. 7. HPS-10 (0.3 mM) significantly inhibited the sustained and transient Ba2+ inward current (IBa) recorded in whole-cell clamped rat aortic myocytes. 8. These results indicate that the non-selective vasorelaxant effects of HPS-10 in rat aortic rings can be attributed to transmembrane Ca(2+)-antagonist activity and an intracellular action on smooth muscle cells. The direct vasodilator action of HPS-10 observed in rat isolated aorta may be responsible for the HPS-10 hypotensive activity in anaesthetized normotensive rats.

Effects of pinacidil on K+ channels in human coronary artery vascular smooth muscle cells

We investigated pinacidil-activated K+ currents in vascular smooth muscle cells (VSMC) from human coronary arteries with the patch-clamp method. In 19 of 54 VSMC, pinacidil (1 and 20 microM) induced a large, nonrectifying, outward current [IK(ATP)] and increased voltage-dependent outward K+ currents [IK(Ca)] positive to voltages of -25 mV. The pinacidil-induced (1 microM) IK(ATP) was blocked by glibenclamide (3 microM) but was not affected by iberiotoxin (100-300 nM). Pinacidil activated up to 150 functionally active ATP-dependent K+ channels (KATP channels) per cell with a single-channel conductance of approximately 17 pS at physiological membrane potentials (between -80 and -30 mV) and K+ gradients (6 mM/130 mM). In 26 of 54 VSMC, on the other hand, pinacidil (1-20 microM) failed to induce IK(ATP) but increased IK(Ca). This current was completely blocked by iberiotoxin (100-300 nM) and tetraethylammonium (1 mM) but not by glibenclamide (3 microM). The single-channel conductance of the channel underlying IK(Ca) was approximately 150 +/- 16 pS between -10 and +30 mV, consistent with large-conductance, maxi Ca(2+)-activated, K+ channels (BKCa channels). We conclude that pinacidil is a nonselective K+ channel opener targeting KATP and BKCa channels. Furthermore, the conductance of KATP channels in human coronary arteries is likely to be small under physiological conditions.

Pharmacological characterization of the sulphonylurea receptor in rat isolated aorta

1. The binding of the sulphonylurea [3H]-glibenclamide, a blocker of adenosine 5'-triphosphate (ATP)-sensitive K+ channels (KATP channels), was studied in endothelium-denuded rings from rat aorta. 2. [3H]-glibenclamide labelled two classes of binding sites with KD values of 20 +/- 5 nM and 32 +/- 1 microM. The high affinity component, which comprised 17% of total binding at 1 nM [3H]-glibenclamide, had an estimated binding capacity of 150 fmol mg-1 wet weight. 3. Other sulphonylureas such as glipizide and glibornuride and the sulphonylurea-related carboxylate, AZ-DF 265, inhibited high affinity [3H]-glibenclamide binding with the potencies expected from their K+ channel activity. At very high concentrations, AZ-DF 265 and glipizide started to interact also with the low affinity component of [3H]-glibenclamide binding. 4. Openers of the ATP-sensitive K+ channel belonging to different structural groups inhibited only the high affinity [3H]-glibenclamide binding; the potencies in this assay were similar to those obtained in functional (i.e. vasorelaxation) studies. 5. High affinity [3H]-glibenclamide binding was abolished by prolonged hypoxia combined with metabolic inhibition. 6. The data indicate that the high affinity component of [3H]-glibenclamide binding mediates the block of the KATP channel by the sulphonylureas in rat aorta; hence, it represents the sulphonylurea receptor in this vessel. The pharmacological properties of this binding site resemble those of the binding site for the openers of the KATP channel; present evidence suggests that these two classes of sites are negatively allosterically coupled.

Cardiotoxicity of verrucotoxin, a protein isolated from the venom of Synanceia verrucosa

Freshly purified but unstable verrucotoxin (VTX) and a more stable proteic complex of the toxin (p-VTX) were isolated from the venom of the stonefish Synanceia verrucosa and applied to frog atrial fibres. VTX and p-VTX decreased the amplitude and the duration of the stimulated peak tension and accelerated the relaxation phase of the contraction. The negative inotropic effect of p-VTX decreased with increasing the external Ca concentration ([Ca]o) in the Ringer solution. The negative chronotropic effect induced by p-VTX was insensitive to change in [Ca]o. It is reversed by glibenclamide. p-VTX shortened the duration of the plateau and the repolarizing phase of the action potential. Glibenclamide but not tetraethylammonium reversed the p-VTX-induced shortening of the AP repolarizing phase. The data suggest that the toxin isolated from the venom of S. verrucosa inhibits Ca channels and might activate ATP-sensitive potassium channels in frog atrial heart muscle.

ATP-sensitive potassium channels in isolated rat aorta during physiologic, hypoxic, and low-glucose conditions

In arterial smooth muscle, adenosine triphosphate (ATP)-sensitive potassium channels are the targets of a variety of synthetic and endogenous vasodilators. In this study, we evaluated the influence of glibenclamide, an ATP-sensitive K(+)-channel blocker, on various vasodilator responses, including those by levcromakalim under hypoxic and low-glucose conditions in isolated rat aortic rings. The concentration-response curves induced by methacholine and sodium nitroprusside (after precontraction with 1 microM phenylephrine) were not affected by glibenclamide. Glibenclamide influenced neither the adenosine- nor the iloprost- (a stable prostacyclin) induced vasodilator effects. Glibenclamide caused a concentration-dependent rightward shift of the concentration-response curves of levcromakalim. The vascular tone induced by phenylephrine was not affected under low-glucose conditions, whereas hypoxia caused a decrease in the phenylephrine-induced contraction when compared with that under normal circumstances. Under all conditions, glibenclamide did not influence the phenylephrine-induced increase in vascular tone. Under low-glucose and hypoxic conditions, the concentration-response curves for levcromakalim showed a significantly less steep slope than under normal conditions, and higher concentrations of glibenclamide were necessary to inhibit the vasodilator response induced by levcromakalim under these experimental conditions adopted to mimic pathologic conditions. In conclusion, methacholine, sodium nitroprusside, adenosine, and iloprost appear not to induce vasodilation in the rat aorta by glibenclamide-sensitive K+ channels, whereas hypoxia and low-glucose levels cause an impaired function of the glibenclamide-sensitive K+ channels.