Muscarinic excitation-contraction coupling mechanisms in tracheal and bronchial smooth muscles

Investigation of the relaxing effect of a camphor nanoemulsion on rat isolated trachea

Asthma is a chronic inflammatory disease that targeting lower airways, being characterized by bronchial smooth muscle hyper responsiveness and mucus hypersecretion. Asthma is considered the most common respiratory disease in the world, affecting approximately 235 million individuals. The main therapy sometimes fails to establish clinical improvement in patients, which leads to a constant search for new alternatives. Camphor is a transparent solid monoterpene with a strong aroma, which due to its high lipophilicity is insoluble in water. Nanostructured carrier systems have shown promise as a delivery system for lipophilic compounds such as monoterpenes. Therefore, the objective of this work was to evaluate the relaxant effect of nanoemulsified camphor (NEC), as well as the mechanism of action of that monoterpene, in isolated rat trachea. The results obtained demonstrated that NEC promote relaxation of the isolated rat trachea when smooth muscle contraction was induced by both carbachol (CCh) and KCl, presenting a pCE50 of 2.25 ± 0.27 and 3.30 ± 0.07, respectively. In the presence of dexamethasone (DEXA), tetraethylammonium (TEA), glibenclamide (GLIB), 1H-[1,2,4]-oxadiazole-[4,3,-a]-quinoxaline-1-one (ODQ) and ruthenium red (RR) there was a significant difference in at least one of the evaluated pharmacological parameters, such as concentration-response curves shape, Emax or pCE50. As conclusion, NEC may be involved with β-adrenergic receptors, channels for K+ sensitive to ATP (KATP) or Channels for K+ opened by Ca2+ (KCa), increase in prostanoids and with receptor channel with transient potential (TRPv). In conclusion, β-adrenergic receptors, prostanoids, nitric oxide (NO), ATP-sensitive K+ channels (KATP), Ca2+-opened K+ channels (KCa), and transient receptor potential cation channel subfamily V (TRPV) are involved in the relaxing effect of NEC. In addition, the mechanism of action of NEC may be involved with the signal transduction pathway Nitric Oxide/soluble guanylyl cyclase/cGMP/cGMP-activated protein kinase. NEC, therefore, demonstrates spasmolytic activity when presenting tracheal relaxation compared to CCh and KCl contracturants.

Mechanopharmacology of Rho-kinase antagonism in airway smooth muscle and potential new therapy for asthma

The principle of mechanopharmacology of airway smooth muscle (ASM) is based on the premise that physical agitation, such as pressure oscillation applied to an airway, is able to induce bronchodilation by reducing contractility and softening the cytoskeleton of ASM. Although the underlying mechanism is not entirely clear, there is evidence to suggest that large-amplitude stretches are able to disrupt the actomyosin interaction in the crossbridge cycle and weaken the cytoskeleton in ASM cells. Rho-kinase is known to enhance force generation and strengthen structural integrity of the cytoskeleton during smooth muscle activation and plays a key role in the maintenance of force during prolonged muscle contractions. Synergy in relaxation has been observed when the muscle is subject to oscillatory length change while Rho-kinase is pharmacologically inhibited. In this review, inhibition of Rho-kinase coupled to therapeutic pressure oscillation applied to the airways is explored as a combination treatment for asthma.

Lippia alnifolia essential oil induces relaxation on Guinea-pig trachea by multiple pathways

ETHNOPHARMACOLOGICAL RELEVANCE

Lippia alnifolia Mart. & Schauer, known as "alecrim-do-mato", "alecrim-de-vaqueiro" and "pedrécio", is used in folk medicine as antiseptic and to treat diseases that affect respiratory system, like bronchitis and asthma.

AIM OF THE STUDY

The aim of this work was to investigate the spasmolytic activity and relaxant mechanism of the Lippia alnifolia essential oil (EOLA) on isolated guinea-pig trachea and to correlate with its use in folk medicine.

MATERIALS AND METHODS

Leaves from L. alnifolia were collected in Pico das Almas, Chapada Diamantina, situated in the city of Rio de Contas, Bahia, Brazil. EOLA was extracted by hydrodistillation, analyzed by GC/FID and GC/MS and the volatile constituents were identified. Spasmolytic activity was assayed in isolated guinea-pig trachea pre-contracted with carbachol 1 μM or histamine 10 μM. Relaxant mechanism of EOLA was determined comparing concentration-response curves in the presence or absence of different blockers.

RESULTS

Chemical analysis revealed the presence of carvone (60 ± 0.8%) as major constituent. EOLA (1-243 μg/mL) relaxed isolated guinea-pig trachea pre-contracted with carbachol 1 μM [EC₅₀ = 53.36 (44.75-63.51) μg/mL] or histamine 10 μM [EC₅₀ = 5.42 (4.42-6.65) μg/mL]. The pre-incubation of 4-aminopyridine in histamine-induced contractions did not alter significantly the relaxant effect of EOLA. However, the presence of cesium chloride, glibenclamide, tetraethylammonium, propranolol, indomethacin, dexamethasone, hexamethonium, atropine, L-NAME, methylene blue or ODQ reduced EOLA relaxant effect. EOLA 18 μg/mL pre-incubation in calcium-free medium reduced histamine-evoked contractions, but did not alter histamine contractions in the presence of nifedipine.

CONCLUSIONS

Lippia alnifolia essential oil has spasmolytic activity on isolated guinea-pig trachea and its mechanism of action possibly involves the activation of multiple signal transduction pathways, which culminate in potassium channels activation and cytosolic calcium reduction.

Relaxant effect of Lippia origanoides essential oil in guinea-pig trachea smooth muscle involves potassium channels and soluble guanylyl cyclase

ETHNOPHARMACOLOGICAL RELEVANCE

Lippia origanoides H.B.K. is an aromatic species used in folk medicine to treat respiratory diseases, including asthma.

AIM OF THE STUDY

The aim of this work was to evaluate the relaxing potential and mechanism of action of the L. origanoides (LOO) essential oil in isolated guinea-pig trachea.

MATERIALS AND METHODS

Leaves from L. origanoides were collected at experimental fields under organic cultivation, at the Forest Garden of Universidade Estadual de Feira de Santana. Essential oil was extracted by hydrodistillation, analyzed by GC/FID and GC/MS and the volatile constituents were identified. Spasmolytic activity and relaxant mechanism of LOO were assayed in isolated guinea-pig trachea contracted with histamine, carbachol or hyperpolarizing KCl.

RESULTS

Chemical analysis revealed the presence of carvacrol (53.89%) as major constituent. LOO relaxed isolated guinea-pig trachea pre-contracted with KCl 60 mM [EC₅₀ = 30.02 μg/mL], histamine 1 µM [EC₅₀ = 9.28 μg/mL] or carbachol 1 µM [EC₅₀ = 51.80 μg/mL]. The pre-incubation of glibenclamide, CsCl, propranolol, indomethacin, hexamethonium, aminophylline or L-NAME in histamine-induced contractions did not alter significantly the relaxant effect of LOO. However, the presence of 4-aminopyridine, tetraethylammonium or methylene blue reduced LOO effect, while the presence of dexamethasone or atropine potentialized the LOO relaxant effect. LOO pre-incubation inhibited carbachol-evoked contractions, with this effect potentialized in the presence of sodium nitroprusside and blocked in the presence of ODQ.

CONCLUSIONS

The relaxant mechanism of LOO on the tracheal smooth muscle possibly involves stimulating of soluble guanylyl cyclase with consequent activation of the voltage-gated and Ca²⁺-activated K⁺ channels.

Contractile function of smooth muscle retained after overnight storage

The functional responses of different overnight-stored in vitro tissues are not clearly described in any animal model. The influence of overnight storage in an animal model may vary between tissue types. We employed Sprague-Dawley rat as our animal model and investigated the functional changes of rat aorta, trachea, bronchus and bladder that were used (i) immediately after surgical removal (denoted as fresh) and (ii) after storage in aerated (95% O2, 5% CO2) Krebs-Ringer bicarbonate solution at 4 °C for 24 h (denoted as stored). The aorta ring was pre-contracted with phenylephrine, and the functional response of the tissue was investigated using isoprenaline, forskolin and carbachol. Carbachol was also used to increase the tone in trachea, bronchus rings and bladder strips. A clear reduced function of endothelium, with a minor if any effect in the smooth muscle function in rat aorta was observed after overnight storage. The contractile response of overnight-stored rat airway (trachea and bronchus) and bladder smooth muscles remained unchanged. Among all tested tissues, only bronchus showed a reduced response rate (only 40% responded) after storage. In vitro rat tissues that are stored in Krebs solution at 4 °C for 24 h can still be used to investigate smooth muscle responses, however, not endothelium-mediated responses for aorta. The influence of overnight storage on different tissues from an animal model (Sprague-Dawley rat in our study) also provides an insight in maximising the use of sacrificed animals.

Emerging airway smooth muscle targets to treat asthma

Asthma is characterized in part by variable airflow obstruction and non-specific hyperresponsiveness to a variety of bronchoconstrictors, both of which are mediated by the airway smooth muscle (ASM). The ASM is also involved in the airway inflammation and airway wall remodeling observed in asthma. For all these reasons, the ASM provides an important target for the treatment of asthma. Several classes of drugs were developed decades ago which targeted the ASM - including β-agonists, anti-cholinergics, anti-histamines and anti-leukotrienes - but no substantially new class of drug has appeared recently. In this review, we summarize the on-going work of several laboratories aimed at producing novel targets and/or tools for the treatment of asthma. These range from receptors and ion channels on the ASM plasmalemma, to intracellular effectors (particularly those related to cyclic nucleotide signaling, calcium-homeostasis and phosphorylation cascades), to anti-IgE therapy and outright destruction of the ASM itself.

Caffeine relaxes smooth muscle through actin depolymerization

Caffeine is sometimes used in cell physiological studies to release internally stored Ca(2+). We obtained evidence that caffeine may also act through a different mechanism that has not been previously described and sought to examine this in greater detail. We ruled out a role for phosphodiesterase (PDE) inhibition, since the effect was 1) not reversed by inhibiting PKA or adenylate cyclase; 2) not exacerbated by inhibiting PDE4; and 3) not mimicked by submillimolar caffeine nor theophylline, both of which are sufficient to inhibit PDE. Although caffeine is an agonist of bitter taste receptors, which in turn mediate bronchodilation, its relaxant effect was not mimicked by quinine. After permeabilizing the membrane using β-escin and depleting the internal Ca(2+) store using A23187, we found that 10 mM caffeine reversed tone evoked by direct application of Ca(2+), suggesting it functionally antagonizes the contractile apparatus. Using a variety of molecular techniques, we found that caffeine did not affect phosphorylation of myosin light chain (MLC) by MLC kinase, actin-filament motility catalyzed by MLC kinase, phosphorylation of CPI-17 by either protein kinase C or RhoA kinase, nor the activity of MLC-phosphatase. However, we did obtain evidence that caffeine decreased actin filament binding to phosphorylated myosin heads and increased the ratio of globular to filamentous actin in precontracted tissues. We conclude that, in addition to its other non-RyR targets, caffeine also interferes with actin function (decreased binding by myosin, possibly with depolymerization), an effect that should be borne in mind in studies using caffeine to probe excitation-contraction coupling in smooth muscle.

Epac as a novel effector of airway smooth muscle relaxation

Dysfunctional regulation of airway smooth muscle tone is a feature of obstructive airway diseases such as asthma and chronic obstructive pulmonary disease. Airway smooth muscle contraction is directly associated with changes in the phosphorylation of myosin light chain (MLC), which is increased by Rho and decreased by Rac. Although cyclic adenosine monophosphate (cAMP)-elevating agents are believed to relieve bronchoconstriction mainly via activation of protein kinase A (PKA), here we addressed the role of the novel cAMP-mediated exchange protein Epac in the regulation of airway smooth muscle tone. Isometric tension measurements showed that specific activation of Epac led to relaxation of guinea pig tracheal preparations pre-contracted with methacholine, independently of PKA. In airway smooth muscle cells, Epac activation reduced methacholine-induced MLC phosphorylation. Moreover, when Epac was stimulated, we observed a decreased methacholine-induced RhoA activation, measured by both stress fibre formation and pull-down assay whereas the same Epac activation prevented methacholine-induced Rac1 inhibition measured by pull-down assay. Epac-driven inhibition of both methacholine-induced muscle contraction by Toxin B-1470, and MLC phosphorylation by the Rac1-inhibitor NSC23766, were significantly attenuated, confirming the importance of Rac1 in Epac-mediated relaxation. Importantly, human airway smooth muscle tissue also expresses Epac, and Epac activation both relaxed pre-contracted human tracheal preparations and decreased MLC phosphorylation. Collectively, we show that activation of Epac relaxes airway smooth muscle by decreasing MLC phosphorylation by skewing the balance of RhoA/Rac1 activation towards Rac1. Therefore, activation of Epac may have therapeutical potential in the treatment of obstructive airway diseases.

Genistein, a competitive PDE1-4 inhibitor, may bind on high-affinity rolipram binding sites of brain cell membranes and then induce gastrointestinal adverse effects

The affinities of genistein on phosphodiesterase (PDE)1-4 and cause of gastrointestinal adverse effects of genistein remain unclear. Female BALB/c mice were actively sensitized by intraperitoneal injections of ovalbumin and challenged by aerosolized ovalbumin (1%). After secondary challenge, aerosolized methacholine (6.25-50mg/ml) induced increases of enhanced pause (P(enh)) values in conscious mice in a concentration-dependent manner. Genistein (30-100 micromol/kg, i.p.) markedly inhibited methacholine (12.5-50mg/ml)-induced increase of P(enh) value in the sensitized and challenged mice. In addition, genistein significantly reduced total inflammatory cells, macrophages, lymphocytes, neutrophils, and eosinophils in bronchoalveolar lavage fluid, with the exception that lymphocytes and neutrophils were not significantly inhibited by genistein at the lowest dose (10 micromol/kg). Genistein also markedly attenuated the release of cytokines, including interleukin (IL)-2, IL-4, IL-5, interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha. Genistein competitively inhibited PDE1-4, with a K(i) value ranging from 4.3 to 13.7 microM. Genistein (3-300 microM) concentration-dependently displaced 2nM [(3)H]-rolipram bound on high-affinity rolipram binding sites of brain cell membranes. The therapeutic ratio of genistein was calculated to be 7.9. Genistein (100 micromol/kg, s.c.) significantly shortened xylazine/ketamine-induced anesthesia, suggesting that genistein administered at a higher dose may have gastrointestinal adverse effects. In conclusion, owing to the low therapeutic ratio of genistein, the gastrointestinal adverse effects may be induced via the binding of genistein on high-affinity rolipram binding sites of brain cell membranes, when it is used for a long term or at higher doses for treating allergies, asthma or chronic obstructive pulmonary disease.

The NADPH oxidase inhibitor diphenyleneiodonium is also a potent inhibitor of cholinesterases and the internal Ca(2+) pump

BACKGROUND AND PURPOSE

Diphenyleneiodonium (DPI) is often used as an NADPH oxidase inhibitor, but is increasingly being found to have unrelated side effects. We investigated its effects on smooth muscle contractions and the related mechanisms.

EXPERIMENTAL APPROACH

We studied isometric contractions in smooth muscle strips from bovine trachea. Cholinesterase activity was measured using a spectrophotometric assay; internal Ca(2+) pump activity was assessed by Ca(2+) uptake into smooth muscle microsomes.

KEY RESULTS

Contractions to acetylcholine were markedly enhanced by DPI (10(-4) M), whereas those to carbachol (CCh) were not, suggesting a possible inhibition of cholinesterase. DPI markedly suppressed contractions evoked by CCh, KCl and 5-HT, and also unmasked phasic activity in otherwise sustained responses. Direct biochemical assays confirmed that DPI was a potent inhibitor of acetylcholinesterase and butyrylcholinesterase (IC(50) approximately 8 x 10(-6) M and 6 x 10(-7) M, respectively), following a readily reversible, mixed non-competitive type of inhibition. The inhibitory effects of DPI on CCh contractions were not mimicked by another NADPH oxidase inhibitor (apocynin), nor the Src inhibitors PP1 or PP2, ruling out an action through the NADPH oxidase signalling pathway. Several features of the DPI-mediated suppression of agonist-evoked responses (i.e. suppression of peak magnitudes and unmasking of phasic activity) are similar to those of cyclopiazonic acid, an inhibitor of the internal Ca(2+) pump. Direct measurement of microsomal Ca(2+) uptake revealed that DPI modestly inhibits the internal Ca(2+) pump.

CONCLUSIONS AND IMPLICATIONS

DPI inhibits cholinesterase activity and the internal Ca(2+) pump in tracheal smooth muscle.

Effect of low level laser therapy on bronchial hyper-responsiveness

The objective of this study was to investigate whether low level laser therapy (LLLT) could reduce bronchial hyper-responsiveness (BHR) induced by tumour necrosis factor-alpha (TNF-alpha) modulating the metabolism of inositol phosphate (IP) in bronchial smooth muscle cells (BSMCs). The study was on 28 Wistar rats, randomly divided into four groups. Irradiation (1.3 J/cm(2)) was administered 5 min and 4 h after bronchial smooth muscle (BSM) had been suspended in TNF-alpha baths, and the contractile response-induced calcium ion (Ca(2+)) sensitization was measured. The BSMCs were isolated, and the IP accumulation was measured before and after TNF-alpha immersion in the groups that had been irradiated or not irradiated. BSM segments significantly increased contraction 24 h after TNF-alpha immersion when exposed to carbachol (CCh) as Ca(2+), but it was significantly reduced by 64% and 30%, respectively, after laser treatment. The increase in IP accumulation induced by CCh after TNF-alpha immersion was reduced in the BSMCs by LLLT. The dose of 2.6 J/cm(2) reduced BHR and IP accumulation in the rats' inflammatory BSMCs.

Tachyphylaxis to the inhibitory effect of L-type channel blockers on ACh-induced [Ca2+]i oscillations in porcine tracheal myocytes

Discrepancies about the role of L-type voltage-gated calcium channels (VGCC) in acetylcholine (ACh)-induced [Ca(2+)](i) oscillations in tracheal smooth muscle cells (TSMCs) have been seen in recent reports. We demonstrate here that ACh-induced [Ca(2+)](i) oscillations in TMCS were reversibly inhibited by three VGCC blockers, nicardipine, nifedipine and verapamil. Prolonged (several minutes) application of VGCC blockers, led to tachyphylaxis; that is, [Ca(2+)](i) oscillations resumed, but at a lower frequency. Brief (15-30 s) removal of VGCC blockers re-sensitized [Ca(2+)](i) oscillations to inhibition by the agents. Calcium oscillations tolerant to VGCC blockers were abolished by KB-R7943, an inhibitor of the reverse mode of Na(+)/Ca(2+) exchanger (NCX). KB-R7943 alone also abolished ACh-induced [Ca(2+)](i) oscillations. Enhancement of the reverse mode of NCX via removing extracellular Na(+) reversed inhibition of ACh-induced [Ca(2+)](i) oscillations by VGCC blockers. Inhibition of non-selective cation channels using Gd(3+) slightly reduced the frequency of ACh-induced [Ca(2+)](i) oscillations, but did not prevent the occurrence of tachyphylaxis. Altogether, these results suggest that VGCC and the reverse mode of NCX are two primary Ca(2+) entry pathways for maintaining ACh-induced [Ca(2+)](i) oscillations in TSMCs. The two pathways complement each other, and may account for tachyphylaxis of ACh-induced [Ca(2+)](i) oscillations to VGCC blockers.

Airway smooth muscle as a target of asthma therapy: history and new directions

Ultimately, asthma is a disease characterized by constriction of airway smooth muscle (ASM). The earliest approach to the treatment of asthma comprised the use of xanthines and anti-cholinergics with the later introduction of anti-histamines and anti-leukotrienes. Agents directed at ion channels on the smooth muscle membrane (Ca2+ channel blockers, K+ channel openers) have been tried and found to be ineffective. Functional antagonists, which modulate intracellular signalling pathways within the smooth muscle (beta-agonists and phosphodiesterase inhibitors), have been used for decades with success, but are not universally effective and patients continue to suffer with exacerbations of asthma using these drugs. During the past several decades, research energies have been directed into developing therapies to treat airway inflammation, but there have been no substantial advances in asthma therapies targeting the ASM. In this manuscript, excitation-contraction coupling in ASM is addressed, highlighting the current treatment of asthma while proposing several new directions that may prove helpful in the management of this disease.

Involvement of reduced sensitivity to Ca in beta-adrenergic action on airway smooth muscle

BACKGROUND

It is well known that beta-adrenoceptor agonists (beta-agonists) cause relaxation in airway smooth muscle mediated by a reduction in the concentration of intracellular Ca2+ ([Ca2+](i)). However, little is currently known regarding whether reduced sensitization to Ca2+ is involved in the beta-adrenergic relaxation.

OBJECTIVE

This study was designed to determine the intracellular mechanisms underlying suppression of Ca2+ sensitization in beta-adrenergic relaxation (Ca(2+)-independent relaxation by beta-agonists). Methods Isometric tension and [Ca2+](i) were simultaneously measured in fura-2-loaded strips isolated from guinea-pig tracheal smooth muscles. The relationships between tension and [Ca2+](i) were examined in the inhibitory action of isoprenaline (ISO) and other cAMP-related agents against methacholine-induced contraction.

RESULTS

The concentration-inhibition curve for ISO against methacholine in tension was significantly dissociated from the curve for ISO in [Ca2+](i). In ISO-induced relaxation, a reduction in tension was significantly greater than that in [Ca2+](i.) This phenomenon was mimicked by other cAMP-related agents: forskolin and dibutyryl-cAMP. In contrast, the inhibitory action of SKF-96365, a non-selective inhibitor of Ca(2+) channels, was associated with that in [Ca2+](i). In the presence of Rp-cAMPS, an inhibitor of protein kinase A (PKA), ISO caused an equivalent relaxation with less reduction in [Ca2+](i). The effects of ISO were not affected by Y-27632, an inhibitor of Rho-kinase, or by bisindolylmaleimide, an inhibitor of protein kinase C. ISO failed to inhibit contraction elicited by calyculin A, an inhibitor of myosin phosphatase. Conclusion beta-Adrenergic action antagonizes not only Ca2+ mobilization but also Ca2+ sensitization in methacholine-induced contraction. The cAMP/PKA-independent, G(s)-direct action is more potent in Ca(2+)-independent relaxation by beta-agonists than the cAMP/PKA-dependent pathway. Moreover, myosin phosphatase is a fundamentally affected protein in the reduced response to Ca2+ mediated by beta-agonist. Our results may provide evidence that this Ca2+ desensitization is a novel target for a reliever medication using rapid-acting beta-agonists in acute asthma management.

Agonist-induced cyclic ADP ribose production in airway smooth muscle

Cyclic ADP-ribose (cADPR) triggers sarcoplasmic reticulum (SR) Ca(2+) release in airway smooth muscle (ASM). SR Ca(2+) release is an important component of the intracellular Ca(2+) ([Ca(2+)](i)) response of ASM to agonists. Whether cADPR is endogenously produced in ASM during agonist stimulation has not been established. In this study, cADPR production was examined in acutely dissociated porcine ASM cells. ACh stimulation (> or = 1 microM) significantly increased cADPR levels, peaking between 30s and 1 min. This effect was inhibited by M(2) and M(3) muscarinic receptor antagonists. Histamine ((> or = 5 microM) increased cADPR levels to a greater extent than ACh, while diphenhydramine blocked histamine-induced cADPR elevation. Both bradykinin (100 nM) and endothelin-1 (100 nM) also increased cADPR levels to a greater extent than ACh or histamine. These results indicate that in porcine ASM, certain agonists acting via receptors increase cADPR levels. Furthermore, the extent of cADPR responses to agonist varies, possibly reflecting differences in G-protein coupling.

Isoprostane-induced airway hyperresponsiveness is dependent on internal Ca2+ handling and Rho/ROCK signaling

We previously reported the ability of isoprostanes to induce airway hyperresponsiveness (AHR). In this study, we examined the signaling mechanisms underlying that phenomenon with the standard muscle bath technique. Responses to a threshold concentration of carbachol (CCh, 3 x 10(-9) M) were significantly augmented by pretreatment for 20 min with 8-isoprostaglandin E(2) (15-E(2t)-IsoP, 10(-6) M): this AHR was obliterated in tissues pretreated with the selective Rho kinase (ROCK) inhibitor Y-27632 added 20 min before isoprostane, but not by cyclopiazonic acid (CPA). Increasing the CCh concentration to 3 x 10(-8) M (still considerably less than the half-maximally effective concentration of CCh) evoked larger contractions that were also augmented significantly by 15-E(2t)-IsoP: this AHR was completely abolished in tissues pretreated with CPA as well as those pretreated with Y-27632. We noted, however, that Y-27632 and CPA profoundly effect baseline tone and the cholinergic response per se, which confounds the interpretation of the data summarized above. We therefore modified the protocol by using combinations of CCh and blocker (CPA, Y-27632, or nifedipine) that were equieffective. In this way, we found that AHR could not be demonstrated under conditions in which Rho/ROCK signaling or Ca(2+) release was abolished (by Y-27632 and CPA, respectively). Likewise, other autacoids that act through G protein-coupled receptors via Rho/ROCK and Ca(2+) release (serotonin, histamine) mimicked this effect of isoprostane, whereas bradykinin did not. We conclude that isoprostane-induced AHR is mediated in part through an action on Rho/ROCK signaling. This novel finding may contribute to a better understanding of the mechanisms underlying AHR and asthma.

Muscarinic receptor signaling in the pathophysiology of asthma and COPD

Anticholinergics are widely used for the treatment of COPD, and to a lesser extent for asthma. Primarily used as bronchodilators, they reverse the action of vagally derived acetylcholine on airway smooth muscle contraction. Recent novel studies suggest that the effects of anticholinergics likely extend far beyond inducing bronchodilation, as the novel anticholinergic drug tiotropium bromide can effectively inhibit accelerated decline of lung function in COPD patients. Vagal tone is increased in airway inflammation associated with asthma and COPD; this results from exaggerated acetylcholine release and enhanced expression of downstream signaling components in airway smooth muscle. Vagally derived acetylcholine also regulates mucus production in the airways. A number of recent research papers also indicate that acetylcholine, acting through muscarinic receptors, may in part regulate pathological changes associated with airway remodeling. Muscarinic receptor signalling regulates airway smooth muscle thickening and differentiation, both in vitro and in vivo. Furthermore, acetylcholine and its synthesizing enzyme, choline acetyl transferase (ChAT), are ubiquitously expressed throughout the airways. Most notably epithelial cells and inflammatory cells generate acetylcholine, and express functional muscarinic receptors. Interestingly, recent work indicates the expression and function of muscarinic receptors on neutrophils is increased in COPD. Considering the potential broad role for endogenous acetylcholine in airway biology, this review summarizes established and novel aspects of muscarinic receptor signaling in relation to the pathophysiology and treatment of asthma and COPD.

Neurotrophin effects on intracellular Ca2+ and force in airway smooth muscle

Neurotrophins [e.g., brain-derived neurotrophic factor (BDNF), neurotrophin 4 (NT4)], known to affect neuronal structure and function, are expressed in nonneuronal tissues including the airway. However, their function is unclear. We examined the effect of acute vs. prolonged neurotrophin exposure on regulation of airway smooth muscle (ASM) intracellular Ca(2+) concentration ([Ca(2+)](i)): sarcoplasmic reticulum (SR) Ca(2+) release and Ca(2+) influx (specifically store-operated Ca(2+) entry, SOCE). Human ASM cells were incubated for 30 min in medium (control) or 1 or 10 nM BDNF, NT3, or NT4 (acute exposure) or overnight in 1 nM BDNF, NT3, or NT4 (prolonged exposure) and imaged after loading with the Ca(2+) indicator fura-2 AM. [Ca(2+)](i) responses to ACh, histamine, bradykinin, and caffeine and SOCE following SR Ca(2+) depletion were compared across cell groups. Force measurements were performed in human bronchial strips exposed to neurotrophins. Basal [Ca(2+)](i), peak responses to all agonists, SOCE, and force responses to ACh and histamine were all significantly enhanced by both acute and prolonged BDNF exposure (smaller effect of NT4) but decreased by NT3. Inhibition of the BDNF/NT4 receptor trkB by K252a prevented enhancement of [Ca(2+)](i) responses. ASM cells showed positive immunostaining for BDNF, NT3, NT4, trkB, and trkC (NT3 receptor). These novel data demonstrate that neurotrophins influence ASM [Ca(2+)](i) and force regulation and suggest a potential role for neurotrophins in airway diseases.

Cyclic nucleotide regulation of store-operated Ca2+ influx in airway smooth muscle

Sarcoplasmic reticulum (SR) Ca2+ release and plasma membrane Ca2+ influx are key to intracellular Ca2+ ([Ca2+]i) regulation in airway smooth muscle (ASM). SR Ca2+ depletion triggers influx via store-operated Ca2+ channels (SOCC) for SR replenishment. Several clinically relevant bronchodilators mediate their effect via cyclic nucleotides (cAMP, cGMP). We examined the effect of cyclic nucleotides on SOCC-mediated Ca2+ influx in enzymatically dissociated porcine ASM cells. SR Ca2+ was depleted by 1 microM cyclopiazonic acid in 0 extracellular Ca2+ ([Ca2+]o), nifedipine, and KCl (preventing Ca2+ influx through L-type and SOCC channels). SOCC was then activated by reintroduction of [Ca2+]o and characterized by several techniques. We examined cAMP effects on SOCC by activating SOCC in the presence of 1 microM isoproterenol or 100 microM dibutryl cAMP (cell-permeant cAMP analog), whereas we examined cGMP effects using 1 microM (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO nitric oxide donor) or 100 microM 8-bromoguanosine 3',5'-cyclic monophosphate (cell-permeant cGMP analog). The role of protein kinases A and G was examined by preexposure to 100 nM KT-5720 and 500 nM KT-5823, respectively. SOCC-mediated Ca2+ influx was dependent on the extent of SR Ca2+ depletion, sensitive to Ni2+ and La3+, but not inhibitors of voltage-gated influx channels. cAMP as well as cGMP potently inhibited Ca2+ influx, predominantly via their respective protein kinases. Additionally, cAMP cross-activation of protein kinase G contributed to SOCC inhibition. These data demonstrate that a Ni2+/La3+-sensitive Ca2+ influx in ASM triggered by SR Ca2+ depletion is inhibited by cAMP and cGMP via a protein kinase mechanism. Such inhibition may play a role in the bronchodilatory response of ASM to clinically relevant drugs (e.g., beta-agonists vs. nitric oxide).

Role of contractile prostaglandins and Rho-kinase in growth factor-induced airway smooth muscle contraction

Background

In addition to their proliferative and differentiating effects, several growth factors are capable of inducing a sustained airway smooth muscle (ASM) contraction. These contractile effects were previously found to be dependent on Rho-kinase and have also been associated with the production of eicosanoids. However, the precise mechanisms underlying growth factor-induced contraction are still unknown. In this study we investigated the role of contractile prostaglandins and Rho-kinase in growth factor-induced ASM contraction.

Methods

Growth factor-induced contractions of guinea pig open-ring tracheal preparations were studied by isometric tension measurements. The contribution of Rho-kinase, mitogen-activated protein kinase (MAPK) and cyclooxygenase (COX) to these reponses was established, using the inhibitors Y-27632 (1 μM), U-0126 (3 μM) and indomethacin (3 μM), respectively. The Rho-kinase dependency of contractions induced by exogenously applied prostaglandin F_2α (PGF_2α) and prostaglandin E₂ (PGE₂) was also studied. In addition, the effects of the selective FP-receptor antagonist AL-8810 (10 μM) and the selective EP₁-antagonist AH-6809 (10 μM) on growth factor-induced contractions were investigated, both in intact and epithelium-denuded preparations. Growth factor-induced PGF_2α-and PGE₂-release in the absence and presence of Y-27632, U-0126 and indomethacin, was assessed by an ELISA-assay.

Results

Epidermal growth factor (EGF)-and platelet-derived growth factor (PDGF)-induced contractions of guinea pig tracheal smooth muscle preparations were dependent on Rho-kinase, MAPK and COX. Interestingly, growth factor-induced PGF_2α-and PGE₂-release from tracheal rings was significantly reduced by U-0126 and indomethacin, but not by Y-27632. Also, PGF_2α-and PGE₂-induced ASM contractions were largely dependent on Rho-kinase, in contrast to other contractile agonists like histamine. The FP-receptor antagonist AL-8810 (10 μM) significantly reduced (approximately 50 %) and the EP₁-antagonist AH-6809 (10 μM) abrogated growth factor-induced contractions, similarly in intact and epithelium-denuded preparations.

Conclusion

The results indicate that growth factors induce ASM contraction through contractile prostaglandins – not derived from the epithelium – which in turn rely on Rho-kinase for their contractile effects.

Rho-kinase inhibition suppresses bladder hyperactivity in spontaneously hypertensive rats

AIMS

Spontaneously hypertensive rats (SHR) exhibit overactive bladder (OAB) symptoms and have an up-regulated calcium sensitizing RhoA/Rho-kinase pathway in their vascular smooth muscle tissues. This study examined the role of RhoA/Rho-kinase pathway in bladder hyperactivity by evaluating the effect of a specific Rho-kinase inhibitor (Y-27632) on SHR bladder function.

METHODS

Adult male SHR (n = 9) and their normotensive controls (Wistar-Kyoto; WKY) (n = 8) were anesthetized and the carotid artery cannulated for blood pressure monitoring. A catheter was fixed into the bladder dome and connected to a pressure transducer and an infusion pump. After equilibration, systemic and bladder pressure were recorded. Continuous filling cystometrograms (CMGs) were performed and threshold pressure (TP), peak pressure (PP), and number of voids and non-voiding contractions (NVCs) per unit time recorded. Each SHR then received Y-27632, 10 mumol intra-arterially. After 10 min, CMG was repeated and the same measurements recorded. Bladder tissues were evaluated immunohistochemically (IHC) for RhoA protein expression.

RESULTS

SHR exhibited significantly higher number of voids and NVCs than normotensive WKY rats (P < 0.05). In SHR, Y-27362 administration significantly decreased the number of voids (29%, from 0.83 +/- 0.3 to 0.63 +/- 0.17 voids/min) and NVCs (61%, from 1.8 +/- 0.54 to 0.64 +/- 0.167 NVC/min). IHC showed significantly higher RhoA protein expression in SHR bladder tissues.

CONCLUSIONS

Overexpression of RhoA may play a role in hypertension-related OAB. Inhibition of Rho-kinase activity with Y-27632 produced a significant suppression of bladder overactivity. Identification of Rho-kinase isoforms that are bladder-tissue specific and their selective inhibitors may help to disassociate the unwanted hypotensive effects of this approach.

Regulation of Rho/ROCK signaling in airway smooth muscle by membrane potential and [Ca2+]i

Recently, we have shown that Rho and Rho-activated kinase (ROCK) may become activated by high-millimolar KCl, which had previously been widely assumed to act solely through opening of voltage-dependent Ca(2+) channels. In this study, we explored in more detail the relationship between membrane depolarization, Ca(2+) currents, and activation of Rho/ROCK in bovine tracheal smooth muscle. Ca(2+) currents began to activate at membrane voltages more positive than -40 mV and were maximally activated above 0 mV; at the same time, these underwent time- and voltage-dependent inactivation. Depolarizing intact tissues by KCl challenge evoked contractions that were blocked equally, and in a nonadditive fashion, by nifedipine or by the ROCK inhibitor Y-27632. Other agents that elevate intracellular calcium concentration ([Ca(2+)](i)) by pathways independent of G protein-coupled receptors, namely the SERCA-pump inhibitor cyclopiazonic acid and the Ca(2+) ionophore A-23187, evoked contractions that were also largely reduced by Y-27632. KCl directly increased Rho and ROCK activities in a concentration-dependent fashion that paralleled closely the effect of KCl on tone and [Ca(2+)](i), as well as the voltage-dependent Ca(2+) currents that were measured over the voltage ranges that are evoked by 0-120 mM KCl. Through the use of various pharmacological inhibitors, we ruled out roles for Ca(2+)/calmodulin-dependent CaM kinase II, protein kinase C, and protein kinase A in mediating the KCl-stimulated changes in tone and Rho/ROCK activities. In conclusion, Rho is activated by elevation of [Ca(2+)](i) (although the signal transduction pathway underlying this Ca(2+) dependence is still unclear) and possibly also by membrane depolarization per se.

Activation of the SPHK/S1P signalling pathway is coupled to muscarinic receptor-dependent regulation of peripheral airways

Background

In peripheral airways, acetylcholine induces contraction via activation of muscarinic M2-and M3-receptor subtypes (M₂R and M₃R). Cholinergic hypersensitivity is associated with chronic obstructive pulmonary disease and asthma, and therefore the identification of muscarinic signaling pathways are of great therapeutic interest. A pathway that has been shown to be activated via MR and to increase [Ca²⁺]_i includes the activation of sphingosine kinases (SPHK) and the generation of the bioactive sphingolipid sphingosine 1-phosphate (S1P). Whether the SPHK/S1P signaling pathway is integrated in the muscarinic control of peripheral airways is not known.

Methods

To address this issue, we studied precision cut lung slices derived from FVB and M₂R-KO and M₃R-KO mice.

Results

In peripheral airways of FVB, wild-type, and MR-deficient mice, SPHK1 was mainly localized to smooth muscle. Muscarine induced a constriction in all investigated mouse strains which was reduced by inhibition of SPHK using D, L-threo-dihydrosphingosine (DHS) and N, N-dimethyl-sphingosine (DMS) but not by N-acetylsphingosine (N-AcS), a structurally related agent that does not affect SPHK function. The initial phase of constriction was nearly absent in peripheral airways of M₃R-KO mice when SPHK was inhibited by DHS and DMS but was unaffected in M₂R-KO mice. Quantitative RT-PCR revealed that the disruption of the M₂R and M₃R genes had no significant effect on the expression levels of the SPHK1-isoform in peripheral airways.

Conclusion

These results demonstrate that the SPHK/S1P signaling pathway contributes to cholinergic constriction of murine peripheral airways. In addition, our data strongly suggest that SPHK is activated via the M₂R. Given the important role of muscarinic mechanisms in pulmonary disease, these findings should be of considerable therapeutic relevance.

Ryanodine receptors in muscarinic receptor-mediated bronchoconstriction

Ryanodine receptors (RyRs), intracellular calcium release channels essential for skeletal and cardiac muscle contraction, are also expressed in various types of smooth muscle cells. In particular, recent studies have suggested that in airway smooth muscle cells (ASMCs) provoked by spasmogens, stored calcium release by the cardiac isoform of RyR (RyR2) contributes to the calcium response that leads to airway constriction (bronchoconstriction). Here we report that mouse ASMCs also express the skeletal muscle and brain isoforms of RyRs (RyR1 and RyR3, respectively). In these cells, RyR1 is localized to the periphery near the cell membrane, whereas RyR3 is more centrally localized. Moreover, RyR1 and/or RyR3 in mouse airway smooth muscle also appear to mediate bronchoconstriction caused by the muscarinic receptor agonist carbachol. Inhibiting all RyR isoforms with > or = 200 microM ryanodine attenuated the graded carbachol-induced contractile responses of mouse bronchial rings and calcium responses of ASMCs throughout the range of carbachol used (50 nM to > or = 3 microM). In contrast, inhibiting only RyR1 and RyR3 with 25 microM dantrolene attenuated these responses caused by high (>500 nM) but not by low concentrations of carbachol. These data suggest that, as the stimulation of muscarinic receptor in the airway smooth muscle increases, RyR1 and/or RyR3 also mediate the calcium response and thus bronchoconstriction. Our findings provide new insights into the complex calcium signaling in ASMCs and suggest that RyRs are potential therapeutic targets in bronchospastic disorders such as asthma.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

KCl evokes contraction of airway smooth muscle via activation of RhoA and Rho-kinase

Airway smooth muscle (ASM) cells express voltage-dependent Ca2+ channels, primarily of the L-subtype. These may play a role in excitation-contraction coupling of ASM, although other signaling pathways may also contribute: one of these includes Rho and its downstream effector molecule Rho-associated kinase (ROCK). Although voltage-dependent Ca2+ influx and Rho/ROCK signaling have traditionally been viewed as entirely separate pathways, recent evidence in vascular smooth muscle suggest differently. In this study, we monitored contractile activity (muscle baths) in bronchial and/or tracheal preparations from the pig, cow, and human, and further examined Rho and ROCK activities (Western blots and kinase assays) and cytosolic levels of Ca2+ (fluo 4-based fluorimetry) in porcine tracheal myocytes. KCl evoked substantial contractions that were suppressed in tracheal preparations by removal of external Ca2+ or using the selective L-type Ca2+ channel blocker nifedipine; porcine bronchial preparations were much less sensitive, and bovine bronchi were essentially unaffected by 1 microM nifedipine. Surprisingly, KCl-evoked contractions were also highly sensitive to two structurally different ROCK inhibitors: Y-27632 and HA-1077. Furthermore, the inhibitory effects of nifedipine and of the ROCK inhibitors were not additive. KCl also caused marked stimulation of Rho and ROCK activities, and both these changes were suppressed by nifedipine or by removal of external Ca2+. KCl-induced elevation of [Ca2+]i was not affected by Y-27632 but was reversed by NiCl2 or by BAPTA-AM. We conclude that KCl acts in part through stimulation of Rho and ROCK, possibly secondary to voltage-dependent Ca2+ influx.

Tyrosine kinase inhibitors: a new approach for asthma

The pathogenesis of allergic asthma involves the interplay of inflammatory cells and airway-resident cells, and of their secreted mediators including cytokines, chemokines, growth factors and inflammatory mediators. Receptor tyrosine kinases are important for the pathogenesis of airway remodeling. Activation of epidermal growth factor (EGF) receptor kinase and platelet-derived growth factor (PDGF) receptor kinase leads to hyperplasia of airway smooth muscle cells, epithelial cells and goblet cells. Stimulation of non-receptor tyrosine kinases (e.g. Lyn, Lck, Syk, ZAP-70, Fyn, Btk, Itk) is the earliest detectable signaling response upon antigen-induced immunoreceptor activation in inflammatory cells. Cytokine receptor dimerization upon ligand stimulation induces activation of Janus tyrosine kinases (JAKs), leading to recruitment and phosphorylation of signal transducer and activator of transcription (STAT) for selective gene expression regulation. Activation of chemokine receptors can trigger JAK-STAT pathway, Lck, Fyn, Lyn, Fgr, and Syk/Zap-70 to induce chemotaxis of inflammatory cells. Inhibitors of tyrosine kinases have been shown in vitro to block growth factor-induced hyperplasia of airway-resident cells; antigen-induced inflammatory cell activation and cytokine synthesis; cytokine-mediated pro-inflammatory gene expression in inflammatory and airway cells; and chemokine-induced chemotaxis of inflammatory cells. Recently, anti-inflammatory effects of tyrosine kinase inhibitors (e.g. genistein, tyrphostin AG213, piceatannol, tyrphostin AG490, WHI-P97, WHI-P131, Syk antisense) in animal models of allergic asthma have been reported. Therefore, development of inhibitors of tyrosine kinases can be a very attractive strategy for the treatment of asthma.

Enhanced Myosin Phosphatase and Ca2+-Uptake Mediate Adrenergic Relaxation of Airway Smooth Muscle

We examined the mechanisms underlying relaxations evoked by isoproterenol (Iso) in isolated porcine, bovine, or human tracheal and bronchial tissues (TSM and BSM, respectively). Iso had little effect against contractions evoked by high KCl, indicating that it does not directly suppress voltage-dependent Ca(2+)-influx nor directly inhibit myosin light chain kinase. Furthermore, Iso was equally potent against carbachol (CCh) contractions in the presence versus absence of nifedipine (10(-6) M), establishing that the primary action of Iso is not through membrane hyperpolarization. However, Iso relaxations in porcine/bovine BSM were significantly suppressed by inhibitors of the internal Ca(2+) pump (cyclopiazonic acid; 10(-5) M) or of myosin light chain phosphatase (calyculin; 10(-6) M). Myosin light chain phosphatase activity was assayed directly (using (32)P-labeled myosin) and found to be enhanced in a time- and concentration-dependent fashion by Iso. Iso relaxations in human airway tissues, on the other hand, were not significantly affected by either calyculin or cyclopiazonic acid. Thus, we conclude that Iso acts largely in a voltage-independent fashion: in nonhuman airways, this involves enhanced Ca(2+) pump activity (to decrease [Ca(2+)](i)) and myosin light chain phosphatase activation (to decrease Ca(2+)-sensitivity of the contractile apparatus), whereas in human airways the underlying mechanisms are still unclear.

TNF-[alpha] modulates murine tracheal rings responsiveness to G-protein-coupled receptor agonists and KCl

Although the mechanisms that underlie airway hyperresponsiveness in asthma are complex and involve a variety of factors, evidence now suggests that intrinsic abnormalities in airway smooth muscle (ASM) may play an important role. We previously reported that TNF-alpha, a cytokine involved in asthma, augments G-protein-coupled receptor (GPCR) agonist-evoked calcium responses in cultured ASM cells. Here we have extended our previous studies by investigating whether TNF-alpha also modulates the contractile and relaxant responses to GPCR activation using cultured murine tracheal rings. We found that in tracheal rings treated with 50 ng/ml TNF-alpha, carbachol-induced isometric force was significantly increased by 30% compared with those treated with diluent alone (P < 0.05). TNF-alpha also augmented KCl-induced force generation by 70% compared with rings treated with diluent alone (P < 0.01). The enhancing effect of TNF-alpha on carbachol-induced isometric force generation was completely abrogated in the tracheal rings obtained from TNF-alpha receptor (TNFR)1-deficient mice and in control rings treated with a TNF-alpha mutant that solely activates TNFR2. TNF-alpha also attenuated relaxation responsiveness to isoproterenol but not to PGE2 or forskolin. TNF-alpha modulatory effects on GPCR-induced ASM responsiveness were completely abrogated by pertussis toxin, an inhibitor of Gialpha proteins. Taken together, these data suggest that TNF-alpha may participate in the development of airway hyperresponsiveness in asthma via the modulation of ASM responsiveness to both contractile and beta-adrenoceptor GPCR agonists.

Expression and functional role of Rho-kinase in rat urinary bladder smooth muscle

(1) The involvement of Rho-kinase (ROCK) in the contractile mechanisms mediating smooth muscle contraction of the rat urinary bladder was investigated using expression studies and the ROCK inhibitor Y-27632. (2) Both isoforms of ROCK (ROCK I and ROCK II) were detected in high levels in rat urinary bladder. (3) Y-27632 (10 micro M) significantly attenuated contractions of rat urinary bladder strips evoked by the G-protein coupled receptor agonists carbachol (58.1+/-10.5% at 0.3 micro M) and neurokinin A (68.6+/-12.7% at 1 micro M) without affecting contractions to potassium chloride (10-100 mM). In addition, basal tone was reduced by 47.8+/-2.0% by 10 micro M Y-27632 in the absence of stimulation. (4) Contractions of urinary bladder strips evoked by the P2X receptor agonist alpha,beta-methylene ATP (alpha,beta-mATP; 10 micro M) were also attenuated by Y-27632 (30.0+/-7.2% at 10 micro M). (5) Y-27632 (10 micro M) significantly attenuated contractions evoked by electrical field stimulation (2-16 Hz). The effect of Y-27632 on the tonic portion of the neurogenic response (4-16 Hz) was not significantly different from the effect of atropine (1 micro M) alone. (6) While the mechanism underlying the ability of Y-27632 to inhibit alpha,beta-mATP-evoked contractions remains undetermined, the results of the present study clearly demonstrate a role for ROCK in the regulation of rat urinary bladder smooth muscle contraction and tone.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

Modulation of calcium homeostasis as a mechanism for altering smooth muscle responsiveness in asthma

Airway hyperresponsiveness remains a defining characteristic of asthma. Traditional views assert that airway smooth muscle is an important structural effector cell in the bronchi that modulates bronchomotor tone induced by contractile agonists. New evidence, however, suggests that abnormalities in airway smooth muscle functions, induced by variety of extracellular stimuli, may play an important role in the development of airway hyperresponsiveness. Studies using isolated bronchial preparations or cultured cells show that inflammatory mediators and cytokines may alter calcium homeostasis in airway smooth muscle and render the cells nonspecifically hyperreactive to agonists.