ATP stimulates Ca2+ oscillations and contraction in airway smooth muscle cells of mouse lung slices

Testosterone Enhances KV Currents and Airway Smooth Muscle Relaxation Induced by ATP and UTP through P2Y4 Receptors and Adenylyl Cyclase Pathway

Numerous studies suggest the involvement of adenosine-5'-triphosphate (ATP) and similar nucleotides in the pathophysiology of asthma. Androgens, such as testosterone (TES), are proposed to alleviate asthma symptoms in young men. ATP and uridine-5'-triphosphate (UTP) relax the airway smooth muscle (ASM) via purinergic P2Y2 and P2Y4 receptors and K+ channel opening. We previously demonstrated that TES increased the expression of voltage-dependent K+ (KV) channels in ASM. This study investigates how TES may potentiate ASM relaxation induced by ATP and UTP. Tracheal tissues treated with or without TES (control group) from young male guinea pigs were used. In organ baths, tracheas exposed to TES (40 nM for 48 h) showed enhanced ATP- and UTP-evoked relaxation. Tetraethylammonium, a K+ channel blocker, annulled this effect. Patch-clamp experiments in tracheal myocytes showed that TES also increased ATP- and UTP-induced K+ currents, and this effect was abolished with flutamide (an androgen receptor antagonist). KV channels were involved in this phenomenon, which was demonstrated by inhibition with 4-aminopyridine. RB2 (an antagonist of almost all P2Y receptors except for P2Y2), as well as N-ethylmaleimide and SQ 22,536 (inhibitors of G proteins and adenylyl cyclase, respectively), attenuated the enhancement of the K+ currents induced by TES. Immunofluorescence and immunohistochemistry studies revealed that TES did not modify the expression of P2Y4 receptors or COX-1 and COX-2, while we have demonstrated that this androgen augmented the expression of KV1.2 and KV1.5 channels in ASM. Thus, TES leads to the upregulation of P2Y4 signaling and KV channels in guinea pig ASM, enhancing ATP and UTP relaxation responses, which likely limits the severity of bronchospasm in young males.

Targeting calcium signaling by inositol trisphosphate receptors: A novel mechanism for the anti-asthmatic effects of Houttuynia cordata

Asthma is a chronic inflammatory disease characterized by airway hypersensitivity and remodeling. The current treatments provide only short-term benefits and may have undesirable side effects; thus, alternative or supplementary therapy is needed. Because intracellular calcium (Ca²⁺) signaling plays an essential role in regulating the contractility and remodeling of airway smooth muscle cells, the targeting of Ca²⁺ signaling is a potential therapeutic strategy for asthma. Houttuynia cordata is a traditional Chinese herb that is used to treat asthma due to its anti-allergic and anti-inflammatory properties. We hypothesized that H. cordata might modulate intracellular Ca²⁺ signaling and could help relieve asthmatic airway remodeling. We found that the mRNA and protein levels of inositol trisphosphate receptors (IP₃Rs) were elevated in interleukin-stimulated primary human bronchial smooth muscle cells and a house dust mite-sensitized model of asthma. The upregulation of IP₃R expression enhanced intracellular Ca²⁺ release upon stimulation and contributed to airway remodeling in asthma. Intriguingly, pretreatment with H. cordata essential oil rectified the disruption of Ca²⁺ signaling, mitigated asthma development, and prevented airway narrowing. Furthermore, our analysis suggested that houttuynin/2-undecanone could be the bioactive component in H. cordata essential oil because we found similar IP₃R suppression in response to the commercially available derivative sodium houttuyfonate. An in silico analysis showed that houttuynin, which downregulates IP₃R expression, binds to the IP₃ binding domain of IP₃R and may mediate a direct inhibitory effect. In summary, our findings suggest that H. cordata is a potential alternative treatment choice that may reduce asthma severity by targeting the dysregulation of Ca²⁺ signaling.

Starving a Cell Promotes Airway Smooth Muscle Relaxation: Inhibition of Glycolysis Attenuates Excitation-Contraction Coupling

Bronchomotor tone modulated by airway smooth muscle shortening represents a key mechanism that increases airway resistance in asthma. Altered glucose metabolism in inflammatory and airway structural cells is associated with asthma. Although these observations suggest a causal link between glucose metabolism and airway hyperresponsiveness, the mechanisms are unclear. We hypothesized that glycolysis modulates excitation-contraction coupling in human airway smooth muscle (HASM) cells. Cultured HASM cells from human lung donors were subject to metabolic screenings using Seahorse XF cell assay. HASM cell monolayers were treated with vehicle or PFK15 (1-(Pyridin-4-yl)-3-(quinolin-2-yl)prop-2-en-1-one), an inhibitor of PFKFB3 (PFK-1,6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3) that generates an allosteric activator for glycolysis rate-limiting enzyme PFK1 (phosphofructokinase 1), for 5-240 minutes, and baseline and agonist-induced phosphorylation of MLC (myosin light chain), MYPT1 (myosin phosphatase regulatory subunit 1), Akt, RhoA, and cytosolic Ca2+ were determined. PFK15 effects on metabolic activity and contractile agonist-induced bronchoconstriction were determined in human precision-cut lung slices. Inhibition of glycolysis attenuated carbachol-induced excitation-contraction coupling in HASM cells. ATP production and bronchodilator-induced cAMP concentrations were also attenuated by glycolysis inhibition in HASM cells. In human small airways, glycolysis inhibition decreased mitochondrial respiration and ATP production and attenuated carbachol-induced bronchoconstriction. The findings suggest that energy depletion resulting from glycolysis inhibition is a novel strategy for ameliorating HASM cell shortening and bronchoprotection of human small airways.

Ca2+ oscillations in rat carotid body type 1 cells in normoxia and hypoxia

We studied the mechanisms by which carotid body glomus (type 1) cells produce spontaneous Ca²⁺ oscillations in normoxia and hypoxia. In cells perfused with normoxic solution at 37°C, we observed relatively uniform, low-frequency Ca²⁺ oscillations in >60% of cells, with each cell showing its own intrinsic frequency and amplitude. The mean frequency and amplitude of Ca²⁺ oscillations were 0.6 ± 0.1 Hz and 180 ± 42 nM, respectively. The duration of each Ca²⁺ oscillation ranged from 14 to 26 s (mean of ∼20 s). Inhibition of inositol (1,4,5)-trisphosphate receptor and store-operated Ca²⁺ entry (SOCE) using 2-APB abolished Ca²⁺ oscillations. Inhibition of endoplasmic reticulum Ca²⁺-ATPase (SERCA) using thapsigargin abolished Ca²⁺ oscillations. ML-9, an inhibitor of STIM1 translocation, also strongly reduced Ca²⁺ oscillations. Inhibitors of L- and T-type Ca²⁺ channels (Ca_v; verapamil>nifedipine>TTA-P2) markedly reduced the frequency of Ca²⁺ oscillations. Thus, Ca²⁺ oscillations observed in normoxia were caused by cyclical Ca²⁺ fluxes at the ER, which was supported by Ca²⁺ influx via Ca²⁺ channels. Hypoxia (2-5% O₂) increased the frequency and amplitude of Ca²⁺ oscillations, and Ca_v inhibitors (verapamil>nifedipine>>TTA-P2) reduced these effects of hypoxia. Our study shows that Ca²⁺ oscillations represent the basic Ca²⁺ signaling mechanism in normoxia and hypoxia in CB glomus cells.

Transgelin-2 as a therapeutic target for asthmatic pulmonary resistance

There is a clinical need for new bronchodilator drugs in asthma, because more than half of asthmatic patients do not receive adequate control with current available treatments. We report that inhibition of metallothionein-2 protein expression in lung tissues causes the increase of pulmonary resistance. Conversely, metallothionein-2 protein is more effective than β₂-agonists in reducing pulmonary resistance in rodent asthma models, alleviating tension in tracheal spirals, and relaxing airway smooth muscle cells (ASMCs). Metallothionein-2 relaxes ASMCs via transgelin-2 (TG2) and induces dephosphorylation of myosin phosphatase target subunit 1 (MYPT1). We identify TSG12 as a nontoxic, specific TG2-agonist that relaxes ASMCs and reduces asthmatic pulmonary resistance. In vivo, TSG12 reduces pulmonary resistance in both ovalbumin- and house dust mite-induced asthma in mice. TSG12 induces RhoA phosphorylation, thereby inactivating the RhoA-ROCK-MYPT1-MLC pathway and causing ASMCs relaxation. TSG12 is more effective than β₂-agonists in relaxing human ASMCs and pulmonary resistance with potential clinical advantages. These results suggest that TSG12 could be a promising therapeutic approach for treating asthma.

Visualizing immune responses of the airway mucosa

The airway mucosa is the primary tissue site exposed to inhaled particulate matter, which includes pathogens and allergens. While most inhaled particles are eliminated from the airways via mucociliary clearance, some pathogens may penetrate the mucosal epithelial barrier and an effective activation of the mucosal immune system is required to prevent further pathogen spread. Similarly, inhaled environmental allergens may induce an aberrant activation of immune cells in the airway mucosa, causing allergic airway disease. During the last years, several investigators employed advanced microscopic imaging on both intravital and tissue explant preparations to observe the dynamic behavior of various immune cells within their complex tissue environment. In the respiratory tract, most imaging studies focused on immune responses of the alveolar compartment in the lung periphery. However, equally important immunological events occur more proximally in the mucosa of the conducting airways, both during infection and allergic responses, calling for a more detailed imaging analysis also at this site. In this review, I will outline the technical challenges of designing microscopic imaging experiments in the conducting airways and summarize our recent efforts in understanding airway mucosal immune cell dynamics in steady-state conditions, during infection and allergy.

The effect of extracellular ATP on rat uterine contraction from different gestational stages and its possible mechanisms of action

BACKGROUND

The mechanisms underlying the onset of labor are not fully understood. Extracellular adenosine 5'-triphosphate (ATP) is known to cause uterine contractions in different species but the exact underlying mechanisms are poorly investigated to date. The aims of this study were to investigate the effect of extracellular ATP on spontaneous uterine contractions from different gestational stages and to elucidate its possible underlying mechanisms.

METHODS

Longitudinal uterine strips were obtained from rats in different gestational stages (nonpregnant, late-pregnant, and term-pregnant). The effects of 1 mM ATP were examined on uterine contractions generated spontaneously, depolarized by high-KCl (60 mM), induced by oxytocin (5 nM), in the presence of high external Ca2+, or in the absence of external Ca2+.

RESULTS

Application of 1 mM extracellular ATP significantly increased the force of spontaneous contraction in uterine strips obtained from all gestational stages with prominent increase in term-pregnant rats compared to other gestations. ATP significantly increased the force induced by depolarization (122%, p=0.010, n=6), oxytocin (129%, p=0.001, n=7), high-Ca2+ (145%, p=0.005, n=6) and it was able to cause transient contraction in the absence of external Ca2+ (33%, p<0.01).

CONCLUSIONS

Extracellular ATP is able to increase the force and frequency of uterine contractions and its effect increases with the progression of pregnancy and it involves Ca2+ influx and release. These findings open a new window for clinicians to consider ATP as a therapeutic target to control the uterine activity during difficult labors.

Precision-cut Mouse Lung Slices to Visualize Live Pulmonary Dendritic Cells

Inhalation of allergens and pathogens elicits multiple changes in a variety of immune cell types in the lung. Flow cytometry is a powerful technique for quantitative analysis of cell surface proteins on immune cells, but it provides no information on the localization and migration patterns of these cells within the lung. Similarly, chemotaxis assays can be performed to study the potential of cells to respond to chemotactic factors in vitro, but these assays do not reproduce the complex environment of the intact lung. In contrast to these aforementioned techniques, the location of individual cell types within the lung can be readily visualized by generating Precision-cut Lung Slices (PCLS), staining them with commercially available, fluorescently tagged antibodies, and visualizing the sections by confocal microscopy. PCLS can be used for both live and fixed lung tissue, and the slices can encompass areas as large as a cross section of an entire lobe. We have used this protocol to successfully visualize the location of a wide variety of cell types in the lung, including distinct types of dendritic cells, macrophages, neutrophils, T cells and B cells, as well as structural cells such as lymphatic, endothelial, and epithelial cells. The ability to visualize cellular interactions, such as those between dendritic cells and T cells, in live, three-dimensional lung tissue, can reveal how cells move within the lung and interact with one another at steady state and during inflammation. Thus, when used in combination with other procedures, such as flow cytometry and quantitative PCR, PCLS can contribute to a comprehensive understanding of cellular events that underlie allergic and inflammatory diseases of the lung.

Dynamics of airway response in lung microsections: a tool for studying airway-extra cellular matrix interactions

The biological configuration of extracellular matrix (ECM) plays a key role in how mechanical interactions of the airway with its parenchymal attachments affect the dynamics of airway responses in different pulmonary disorders including asthma, emphysema and chronic bronchitis. It is now recognized that mechanical interactions between airway tissue and ECM play a key regulatory role on airway physiology and kinetics that can lead to the reorganization and remodeling of airway connective tissue. A connective tissue is composed of airway smooth muscle cells (ASM) and the ECM, which includes variety of glycoproteins and therefore the extent of interactions between ECM and ASM affects airway dynamics during exacerbations of major pulmonary disorders. Measurement of the velocity and magnitude of airway closure or opening provide important insights into the functions of the airway contractile apparatus and the interactions with its surrounding connective tissues. This review highlights suitability of lung microsection technique in studying measurements of airway dynamics (narrowing/opening) and associated structural distortions in airway compartments.

Increased airway reactivity in a neonatal mouse model of continuous positive airway pressure

BACKGROUND

Continuous positive airway pressure (CPAP) is a primary form of respiratory support used in the intensive care of preterm infants, but its long-term effects on airway (AW) function are unknown.

METHODS

We developed a neonatal mouse model of CPAP treatment to determine whether it modifies later AW reactivity. Unanesthetized spontaneously breathing mice were fitted with a mask to deliver CPAP (6 cmH2O, 3 h/day) for 7 consecutive days starting at postnatal day 1. AW reactivity to methacholine was assessed using the in vitro living lung slice preparation.

RESULTS

One week of CPAP increased AW responsiveness to methacholine in male, but not female mice, compared to untreated control animals. The AW hyper-reactivity of male mice persisted for 2 wk (at P21) after CPAP treatment ended. Four days of CPAP, however, did not significantly increase AW reactivity. Females also exhibited AW hyper-reactivity at P21, suggesting a delayed response to early (7 d) CPAP treatment. The effects of 7 d of CPAP on hyper-reactivity to methacholine were unique to smaller AWs whereas larger ones were relatively unaffected.

CONCLUSION

These data may be important to our understanding of the potential long-term consequences of neonatal CPAP therapy used in the intensive care of preterm infants.

Prostaglandin induced changes in the tone of porcine retinal arterioles in vitro involve other factors than calcium activity in perivascular cells

The cellular basis for the regulation of retinal blood flow is unknown, but recently a new type of perivascular cell (PVC) with pericyte characteristics was identified in the retinal arterial vascular wall located immediately external to the vascular smooth muscle cells. A possible involvement of this cell type in the regulation of retinal vascular tone might be elucidated by studying differences in the response after the addition of compounds stimulating respectively relaxation and contraction. The effects of PGE2 and PGF2α on vascular tone and calcium activity in PVCs in porcine retinal arterioles were studied in a confocal myograph after the addition of the ryanodine receptor blocker ryanodine, the L-type Ca(2+) channel blocker nifedipine, the non-specific cation channel blocker LOE908, the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) blocker CPA, and the inositol triphosphate receptor (IP3R) and transient receptor potential (TRP) ion channel blocker 2-APB. The Ca(2+) channel blockers nifedipine and LOE908 induced significant relaxation of retinal arterioles. After the addition of both PGE2 and PGF2α calcium activity in the PVCs was significantly reduced by both the SERCA inhibitor CPA and the IP3R antagonist 2-APB, but the changes in calcium activity were unrelated to the changes in tone induced by PGE2 and PGF2α. Changes in the tone of porcine retinal arterioles in vitro induced by PGE2 and PGF2α involve other factors than calcium activity in the perivascular cells.

Real-time imaging of ATP release induced by mechanical stretch in human airway smooth muscle cells

Airway smooth muscle (ASM) cells within the airway walls are continually exposed to mechanical stimuli, and exhibit various functions in response to these mechanical stresses. ATP acts as an extracellular mediator in the airway. Moreover, extracellular ATP is considered to play an important role in the pathophysiology of asthma and chronic obstructive pulmonary disease. However, it is not known whether ASM cells are cellular sources of ATP secretion in the airway. We therefore investigated whether mechanical stretch induces ATP release from ASM cells. Mechanical stretch was applied to primary human ASM cells cultured on a silicone chamber coated with type I collagen using a stretching apparatus. Concentrations of ATP in cell culture supernatants measured by luciferin-luciferase bioluminescence were significantly elevated by cyclic stretch (12 and 20% strain). We further visualized the stretch-induced ATP release from the cells in real time using a luminescence imaging system, while acquiring differential interference contrast cell images with infrared optics. Immediately after a single uniaxial stretch for 1 second, strong ATP signals were produced by a certain population of cells and spread to surrounding spaces. The cyclic stretch-induced ATP release was significantly reduced by inhibitors of Ca(2+)-dependent vesicular exocytosis, 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester, monensin, N-ethylmaleimide, and bafilomycin. In contrast, the stretch-induced ATP release was not inhibited by a hemichannel blocker, carbenoxolone, or blockade of transient receptor potential vanilloid 4 by short interfering RNA transfection or ruthenium red. These findings reveal a novel property of ASM cells: mechanically induced ATP release may be a cellular source of ATP in the airway.

Novel small airway bronchodilator responses to rosiglitazone in mouse lung slices

There is a need to identify novel agents that elicit small airway relaxation when β2-adrenoceptor agonists become ineffective in difficult-to-treat asthma. Because chronic treatment with the synthetic peroxisome proliferator activated receptor (PPAR)γ agonist rosiglitazone (RGZ) inhibits airway hyperresponsiveness in mouse models of allergic airways disease, we tested the hypothesis that RGZ causes acute airway relaxation by measuring changes in small airway size in mouse lung slices. Whereas the β-adrenoceptor agonists albuterol (ALB) and isoproterenol induced partial airway relaxation, RGZ reversed submaximal and maximal contraction to methacholine (MCh) and was similarly effective after precontraction with serotonin or endothelin-1. Concentration-dependent relaxation to RGZ was not altered by the β-adrenoceptor antagonist propranolol and was enhanced by ALB. RGZ-induced relaxation was mimicked by other synthetic PPARγ agonists but not by the putative endogenous agonist 15-deoxy-PGJ2 and was not prevented by the PPARγ antagonist GW9662. To induce airway relaxation, RGZ inhibited the amplitude and frequency of MCh-induced Ca(2+) oscillations of airway smooth muscle cells (ASMCs). In addition, RGZ reduced MCh-induced Ca(2+) sensitivity of the ASMCs. Collectively, these findings demonstrate that acute bronchodilator responses induced by RGZ are PPARγ independent, additive with ALB, and occur by the inhibition of ASMC Ca(2+) signaling and Ca(2+) sensitivity. Because RGZ continues to elicit relaxation when β-adrenoceptor agonists have a limited effect, RGZ or related compounds may have potential as bronchodilators for the treatment of difficult asthma.

Histamine, carbachol, and serotonin induce hyperresponsiveness to ATP in guinea pig tracheas: involvement of COX-2 pathway

Extracellular ATP promotes an indirect contraction of airway smooth muscle via the secondary release of thromboxane A2 (TXA2) from airway epithelium. Our aim was to evaluate if common contractile agonists modify this response to ATP. Tracheas from sensitized guinea pigs were used to evaluate ATP-induced contractions before and after a transient contraction produced by histamine, carbachol, or serotonin. Epithelial mRNA for COX-1 and COX-2 was measured by RT-PCR and their expression assessed by immunohistochemistry. Compared with the initial response, ATP-induced contraction was potentiated by pretreatment with histamine, carbachol, or serotonin. Either suramin (antagonist of P2X and P2Y receptors) plus RB2 (antagonist of P2Y receptors) or indomethacin (inhibitor of COX-1 and COX-2) annulled the ATP-induced contraction, suggesting that it was mediated by P2Y receptor stimulation and TXA2 production. When COX-2 was inhibited by SC-58125 or thromboxane receptors were antagonized by SQ-29548, just the potentiation was abolished, leaving the basal response intact. Airway epithelial cells showed increased COX-2 mRNA after stimulation with histamine or carbachol, but not serotonin, while COX-1 mRNA was unaffected. Immunochemistry corroborated this upregulation of COX-2. In conclusion, we showed for the first time that histamine and carbachol cause hyperresponsiveness to ATP by upregulating COX-2 in airway epithelium, which likely increases TXA2 production. Serotonin-mediated hyperresponsiveness seems to be independent of COX-2 upregulation, but nonetheless is TXA2 dependent. Because acetylcholine, histamine, and serotonin can be present during asthmatic exacerbations, their potential interactions with ATP might be relevant in its pathophysiology.

Determination of genotoxicity by the Comet assay applied to murine precision-cut lung slices

Precision-cut lung slices (PCLSs) are an organotypic lung model that is widely used in pharmacological, physiological, and toxicological studies. Genotoxicity testing, as a pivotal part of early risk assessment, is currently established in vivo in various organs including lung, brain, or liver, and in vitro in cell lines or primary cells. The aim of the present study was to provide the three-dimensional organ culture PCLS as a new ex vivo model for determination of genotoxicity using the Comet assay. Murine PCLS were exposed to increasing concentrations of ethyl methane sulfonate 'EMS' (0.03-0.4%) and formalin (0.5-5mM). Tissue was subsequently dissociated, and DNA single-strand breaks were quantified using the Comet assay. Number of viable dissociated lung cells was between 4×10(5) and 6.7×10(5)cells/slice. Even treatment with EMS did not induce toxicity compared to untreated tissue control. As expected, DNA single-strand breaks were increased dose-dependently and significantly after exposure to EMS. Here, tail length rose from 24μm to 75μm. In contrast, formalin resulted in a significant induction of DNA cross-links. The effects induced by EMS and formalin demonstrate the usefulness of PCLS as a new ex vivo lung model for genotoxicity testing in the early risk assessment of airborne substances in the future.

Adenosine A(1) and prostaglandin E receptor 3 receptors mediate global airway contraction after local epithelial injury

Epithelial injury and airway hyperresponsiveness are prominent features of asthma. We have previously demonstrated that laser ablation of single epithelial cells immediately induces global airway constriction through Ca(2+)-dependent smooth muscle shortening. The response is mediated by soluble mediators released from wounded single epithelial cells; however, the soluble mediators and signaling mechanisms have not been identified. In this study, we investigated the nature of the epithelial-derived soluble mediators and the associated signaling pathways that lead to the L-type voltage-dependent Ca(2+) channel (VGCC)-mediated Ca(2+) influx. We found that inhibition of adenosine A1 receptors (or removal of adenosine with adenosine deaminase), cyclooxygenase (COX)-2 or prostaglandin E receptor 3 (EP3) receptors, epidermal growth factor receptor (EGFR), or platelet-derived growth factor receptor (PDGFR) all significantly blocked Ca(2+) oscillations in smooth muscle cells and airway contraction induced by local epithelial injury. Using selective agonists to activate the receptors in the presence and absence of selective receptor antagonists, we found that adenosine activated the signaling pathway A1R→EGFR/PDGFR→COX-2→EP3→VGCCs→calcium-induced calcium release, leading to intracellular Ca(2+) oscillations in airway smooth muscle cells and airway constriction.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Connexin 43 hemichannels contribute to cytoplasmic Ca2+ oscillations by providing a bimodal Ca2+-dependent Ca2+ entry pathway

Many cellular functions are driven by changes in the intracellular Ca(2+) concentration ([Ca(2+)](i)) that are highly organized in time and space. Ca(2+) oscillations are particularly important in this respect and are based on positive and negative [Ca(2+)](i) feedback on inositol 1,4,5-trisphosphate receptors (InsP(3)Rs). Connexin hemichannels are Ca(2+)-permeable plasma membrane channels that are also controlled by [Ca(2+)](i). We aimed to investigate how hemichannels may contribute to Ca(2+) oscillations. Madin-Darby canine kidney cells expressing connexin-32 (Cx32) and Cx43 were exposed to bradykinin (BK) or ATP to induce Ca(2+) oscillations. BK-induced oscillations were rapidly (minutes) and reversibly inhibited by the connexin-mimetic peptides (32)Gap27/(43)Gap26, whereas ATP-induced oscillations were unaffected. Furthermore, these peptides inhibited the BK-triggered release of calcein, a hemichannel-permeable dye. BK-induced oscillations, but not those induced by ATP, were dependent on extracellular Ca(2+). Alleviating the negative feedback of [Ca(2+)](i) on InsP(3)Rs using cytochrome c inhibited BK- and ATP-induced oscillations. Cx32 and Cx43 hemichannels are activated by <500 nm [Ca(2+)](i) but inhibited by higher concentrations and CT9 peptide (last 9 amino acids of the Cx43 C terminus) removes this high [Ca(2+)](i) inhibition. Unlike interfering with the bell-shaped dependence of InsP(3)Rs to [Ca(2+)](i), CT9 peptide prevented BK-induced oscillations but not those triggered by ATP. Collectively, these data indicate that connexin hemichannels contribute to BK-induced oscillations by allowing Ca(2+) entry during the rising phase of the Ca(2+) spikes and by providing an OFF mechanism during the falling phase of the spikes. Hemichannels were not sufficient to ignite oscillations by themselves; however, their contribution was crucial as hemichannel inhibition stopped the oscillations.

Inositol trisphosphate receptors in smooth muscle cells

Inositol 1,4,5-trisphosphate receptors (IP(3)Rs) are a family of tetrameric intracellular calcium (Ca(2+)) release channels that are located on the sarcoplasmic reticulum (SR) membrane of virtually all mammalian cell types, including smooth muscle cells (SMC). Here, we have reviewed literature investigating IP(3)R expression, cellular localization, tissue distribution, activity regulation, communication with ion channels and organelles, generation of Ca(2+) signals, modulation of physiological functions, and alterations in pathologies in SMCs. Three IP(3)R isoforms have been identified, with relative expression and cellular localization of each contributing to signaling differences in diverse SMC types. Several endogenous ligands, kinases, proteins, and other modulators control SMC IP(3)R channel activity. SMC IP(3)Rs communicate with nearby ryanodine-sensitive Ca(2+) channels and mitochondria to influence SR Ca(2+) release and reactive oxygen species generation. IP(3)R-mediated Ca(2+) release can stimulate plasma membrane-localized channels, including transient receptor potential (TRP) channels and store-operated Ca(2+) channels. SMC IP(3)Rs also signal to other proteins via SR Ca(2+) release-independent mechanisms through physical coupling to TRP channels and local communication with large-conductance Ca(2+)-activated potassium channels. IP(3)R-mediated Ca(2+) release generates a wide variety of intracellular Ca(2+) signals, which vary with respect to frequency, amplitude, spatial, and temporal properties. IP(3)R signaling controls multiple SMC functions, including contraction, gene expression, migration, and proliferation. IP(3)R expression and cellular signaling are altered in several SMC diseases, notably asthma, atherosclerosis, diabetes, and hypertension. In summary, IP(3)R-mediated pathways control diverse SMC physiological functions, with pathological alterations in IP(3)R signaling contributing to disease.

Local small airway epithelial injury induces global smooth muscle contraction and airway constriction

Small airway epithelial cells form a continuous sheet lining the conducting airways, which serves many functions including a physical barrier to protect the underlying tissue. In asthma, injury to epithelial cells can occur during bronchoconstriction, which may exacerbate airway hyperreactivity. To investigate the role of epithelial cell rupture in airway constriction, laser ablation was used to precisely rupture individual airway epithelial cells of small airways (<300-μm diameter) in rat lung slices (∼250-μm thick). Laser ablation of single epithelial cells using a femtosecond laser reproducibly induced airway contraction to ∼70% of the original cross-sectional area within several seconds, and the contraction lasted for up to 40 s. The airway constriction could be mimicked by mechanical rupture of a single epithelial cell using a sharp glass micropipette but not with a blunt glass pipette. These results suggest that soluble mediators released from the wounded epithelial cell induce global airway contraction. To confirm this hypothesis, the lysate of primary human small airway epithelial cells stimulated a similar airway contraction. Laser ablation of single epithelial cells triggered a single instantaneous Ca(2+) wave in the epithelium, and multiple Ca(2+) waves in smooth muscle cells, which were delayed by several seconds. Removal of extracellular Ca(2+) or decreasing intracellular Ca(2+) both blocked laser-induced airway contraction. We conclude that local epithelial cell rupture induces rapid and global airway constriction through release of soluble mediators and subsequent Ca(2+)-dependent smooth muscle shortening.

Interleukin-13 enhanced Ca2+ oscillations in airway smooth muscle cells

Physiological mechanisms associated with interleukin-13 (IL-13), a key cytokine in asthma, in intracellular Ca(2+) signaling in airway smooth muscle cells (ASMCs) remain unclear. The aim of this study was to assess effects of IL-13 on Ca(2+) oscillations in response to leukotriene D4 (LTD4) in human cultured ASMCs. LTD4-induced Ca(2+) oscillations in ASMCs pretreated with IL-13 were imaged by confocal microscopy. mRNA expressions of cysteinyl leukotriene 1 receptors (CysLT1R), CD38, involved with the ryanodine receptors (RyR) system, and transient receptor potential canonical (TRPC), involved with store-operated Ca(2+) entry (SOCE), were determined by real-time PCR. In IL-13-pretreated ASMCs, frequency of LTD4-induced Ca(2+) oscillations and number of oscillating cells were significantly increased compared with untreated ASMCs. Both xestospongin C, a specific inhibitor of inositol 1,4,5-triphosphate receptors (IP(3)R), and ryanodine or ruthenium red, inhibitors of RyR, partially blocked LTD4-induced Ca(2+) oscillations. Ca(2+) oscillations were almost completely inhibited by 50 μM of 2-aminoethoxydiphenyl borate (2-APB), which dominantly blocks SOCE but not IP(3)R at this concentration. Pretreatment with IL-13 increased the mRNA expressions of CysLT1R and CD38, but not of TRPC1 and TRPC3. We conclude that IL-13 enhances frequency of LTD4-induced Ca(2+) oscillations in human ASMCs, which may be cooperatively modulated by IP(3)R, RyR systems and possibly by SOCE.

Uridine adenosine tetraphosphate induces contraction of airway smooth muscle

Contraction of airway smooth muscle (ASM) plays an important role in the regulation of air flow and is potentially involved in the pathophysiology of certain respiratory diseases. Extracellular nucleotides regulate ASM contraction via purinergic receptors, but the signaling mechanisms involved are not fully understood. Uridine adenosine tetraphosphate (Up(4)A) contains both pyrimidine and purine moieties, which are known to potentially activate P2X and P2Y receptors. Both P2X and P2Y receptors have been identified in the lung, including airway epithelial cells and ASM. We report here a study of purinergic signaling in the respiratory system, with a focus on the effect of Up(4)A on ASM contraction. Up(4)A induced contraction of rat isolated trachea and extrapulmonary bronchi as well as human intrapulmonary bronchioles. Up(4)A-induced contraction was blocked by di-inosine pentaphosphate, a P2X antagonist, but not by suramin, a nonselective P2 antagonist. Up(4)A-induced contraction was also attenuated by α,β-methylene-ATP-mediated P2X receptor desensitization. Several P2X receptors were detected at the mRNA level: P2X1, P2X4, P2X6, and P2X7, and to a lesser extent P2X3. Furthermore, the Up(4)A response was inhibited by removal of extracellular Ca(2+) and by the presence of the L-type Ca(2+) channel blocker, nifedipine, or the Rho-associated kinase inhibitor, H1152. We conclude that Up(4)A stimulates ASM contraction, and the underlying signaling mechanism appears to involve P2X (most likely P2X1) receptors, extracellular Ca(2+) entry via L-type Ca(2+) channels, and Ca(2+) sensitization through the RhoA/Rho-associated kinase pathway. This study will add to our understanding of the pathophysiological roles of extracellular nucleotides in the lung.

Characterization of P2Y receptors mediating ATP induced relaxation in guinea pig airway smooth muscle: involvement of prostaglandins and K+ channels

In airway smooth muscle (ASM), adenosine 5'-triphosphate (ATP) induces a relaxation associated with prostaglandin production. We explored the role of K(+) currents (I (K)) in this relaxation. ATP relaxed the ASM, and this effect was abolished by indomethacin. Removal of airway epithelium slightly diminished the ATP-induced relaxation at lower concentration without modifying the responses to ATP at higher concentrations. ATPγS and UTP induced a concentration-dependent relaxation similar to ATP; α,β-methylene-ATP was inactive from 1 to 100 μM. Suramin or reactive blue 2 (RB2), P2Y receptor antagonists, did not modify the relaxation, but their combination significantly reduced this effect of ATP. The relaxation was also inhibited by N-ethylmaleimide (NEM; which uncouples G proteins). In myocytes, the ATP-induced I (K) increment was not modified by suramin or RB2 but the combination of both drugs abolished it. This increment in the I (K) was also completely nullified by NEM and SQ 22,536. 4-Amynopyridine or iberiotoxin diminished the ATP-induced I (K) increment, and the combination of both substances diminished ATP-induced relaxation. The presence of P2Y(2) and P2Y(4) receptors in smooth muscle was corroborated by Western blot and confocal images. In conclusion, ATP: (1) produces relaxation by inducing the production of bronchodilator prostaglandins in airway smooth muscle, most likely by acting on P2Y(4) and P2Y(2) receptors; (2) induces I (K) increment through activation of the delayed rectifier K(+) channels and the high-conductance Ca(2+)-dependent K(+) channels, therefore both channels are implicated in the ATP-induced relaxation; and (3) this I (K) increment is mediated by prostaglandin production which in turns increase cAMP signaling pathway.

Exploring lung physiology in health and disease with lung slices

The development of therapeutic approaches to treat lung disease requires an understanding of both the normal and disease physiology of the lung. Although traditional experimental approaches only address either organ or cellular physiology, the use of lung slice preparations provides a unique approach to investigate integrated physiology that links the cellular and organ responses. Living lung slices are robust and can be prepared from a variety of species, including humans, and they retain many aspects of the cellular and structural organization of the lung. Functional portions of intrapulmonary airways, arterioles and veins are present within the alveoli parenchyma. The dynamics of macroscopic changes of contraction and relaxation associated with the airways and vessels are readily observed with conventional low-magnification microscopy. The microscopic changes associated with cellular events, that determine the macroscopic responses, can be observed with confocal or two-photon microscopy. To investigate disease processes, lung slices can either be prepared from animal models of disease or animals exposed to disease invoking conditions. Alternatively, the lung slices themselves can be experimentally manipulated. Because of the ability to observe changes in cell physiology and how these responses manifest themselves at the level of the organ, lung slices have become a standard tool for the investigation of lung disease.

In airways ATP refills sarcoplasmic reticulum via P2X smooth muscle receptors and induces contraction through P2Y epithelial receptors

In airway smooth muscle (ASM), ATP induces a contraction associated with the increase of [Ca(2+)](i). Cytosolic Ca(2+) is extruded to the extracellular space by the Na(+)/Ca(2+) exchanger (NCX) in its normal mode. Some agonists activate the reverse mode of the NCX (NCX(REV)), inducing Ca(2+) entry. We investigated whether ATP, via P2X receptors, activates the NCX(REV) and whether the increment in [Ca(2+)](i) is used for contraction or for the sarcoplasmic reticulum (SR) refilling in guinea pig ASM. ATP contracted the ASM and this effect was blocked by indomethacin. Suramin and RB2 diminished the contraction induced by ATP; PPADS did not modify this response. In myocytes, ATP produces an increase in [Ca(2+)](i) not modified by indomethacin. In tracheal strips, using simultaneous measurements, ATP induced a biphasic change in [Ca(2+)](i), (a Ca(2+) peak followed by a plateau) accompanied by a contraction. Indomethacin or epithelium removal abolished this contraction, but not the Ca(2+) peak, whereas the plateau was decreased by indomethacin. In myocytes, the ATP-induced [Ca(2+)](i) increment was inhibited by suramin (~96%), PPADS (~40%), and RB2 (~57%). ATP augmented the NCX(REV) and this effect was abolished by SKF 96365 and TNP-ATP (P2X(1) and P2X(3) receptors antagonist). P2X(1) and P2X(3) receptors were corroborated by immunoblotting of ASM. NCX(REV) activation and ATP in the presence of RB2 favor the SR Ca(2+) refilling. In tracheal rings, successive ATP stimulations were reduced with KB-R7943. Therefore, ATP: (1) indirectly promotes muscle contraction via epithelial P2Y receptors and prostaglandins release; (2) increases the [Ca(2+)](i) through a prostaglandin-independent manner by activating P2X and P2Y receptors in smooth muscle; and (3) activates P2X(1) and P2X(3) receptors and the NCX(REV) which refills the SR.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Human airway contraction and formoterol-induced relaxation is determined by Ca2+ oscillations and Ca2+ sensitivity

The etiology of airway hyperresponsiveness associated with asthma requires an understanding of the regulatory mechanisms mediating human airway smooth muscle cell (SMC) contraction. The objective of this study was to determine how human airway SMC contraction (induced by histamine) and relaxation (induced by formoterol) are regulated by Ca(2+) oscillations and Ca(2+) sensitivity. The responses of human small airways and their associated SMCs were studied in human lung slices cut from agarose-inflated lungs. Airway contraction was measured with phase-contrast video microscopy. Ca(2+) signaling and Ca(2+) sensitivity of airway SMCs were measured with two-photon fluorescence microscopy and Ca(2+)-permeabilized lung slices. The agonist histamine induced contraction of human small airways by stimulating both an increase in intracellular Ca(2+) concentration in the SMCs in the form of oscillatory Ca(2+) waves and an increase in Ca(2+) sensitivity. The frequency of the Ca(2+) oscillations increased with histamine concentration, and correlated with increased contraction. Formoterol induced airway relaxation at low concentrations by initially decreasing SMC Ca(2+) sensitivity. At higher concentrations, formoterol additionally slowed or inhibited the Ca(2+) oscillations of the SMCs to relax the airways. The action of formoterol was only slowly reversed. Human lung slices provide a powerful experimental assay for the investigation of small airway physiology and pharmacology. Histamine induces contraction by simultaneously increasing SMC Ca(2+) signaling and Ca(2+) sensitivity. Formoterol induces long-lasting relaxation by initially reducing the Ca(2+) sensitivity and, subsequently, the frequency of the Ca(2+) oscillations of the airway SMCs.

ATP stimulates Ca(2+)-waves and gene expression in cultured human pulmonary fibroblasts

Given that extracellular ATP is markedly elevated in inflammation and is known to modulate fibroblast function, we examined the effects of exogenously added ATP on Ca(2+)-handling and gene expression in human pulmonary fibroblasts. Cells were loaded with the Ca(2+)-indicator dye fluo-4 and studied using confocal fluorimetry. Standard RT-PCR was used to probe gene expression. ATP (10(-5)M) evoked recurring Ca(2+)-waves which were completely occluded by cyclopiazonic acid (depletes the internal Ca(2+)-store) or the phospholipase inhibitor U73122. Pretreatment with ryanodine (10(-5)M), however, had no effect on the ATP-evoked responses. Regarding the receptor through which ATP acted, we found the ATP-response to be mimicked by UTP or ADP but not by adenosine or alpha,beta-methylene-ATP, and to be blocked by the purinergic receptor blocker PPADS. The ATP-evoked response was greater and longer lasting within the nucleus than in the non-nuclear portion of the cytosol. RT-PCR showed that ATP also rapidly and dramatically increased gene expression of P2Y(4) receptors, the cytokine TGF-beta (an important modulator of wound repair) and two matrix proteins (collagen A1 and fibronectin) approximately 4-5 times above baseline: this increase was not significantly affected by ryanodine but was abolished by PPADS. We conclude that, in human pulmonary fibroblasts, ATP acts upon P2Y receptors to liberate internal Ca(2+) through ryanodine-insensitive channels, leading to a Ca(2+)-wave which courses throughout the cell and modulates gene expression.

Regulation of adenosine 5'-triphosphate (ATP)-gated P2X(4) receptors on tracheal smooth muscle cells

We examined the effects of extracellular adenosine 5'-triphosphate (ATP) on single airway smooth muscle (ASM) cells from porcine trachea using a patch-clamp technique. ATP induced a sustained inward current. Phospholipase C inhibitor U-73122 failed to inhibit the current, suggesting the involvement of P2X receptor. A specific effecter of P2X(4), ivermectin, augmented the current indicating the existence of P2X(4) receptors. Immunohistochemistry and reverse transcription/polymerase chain reaction analysis and Western blot analysis also showed the distribution of the P2X(4) receptors. The inward current was reduced by SKF-96365, an inhibitor of both voltage-dependent Ca(2+) channels (VDCCs) and voltage-independent Ca(2+) channels, although a VDCC antagonist, verapamil, did not affect the current. SKF-96365 caused complete suppression of both the increase in the intracellular Ca(2+) concentration and the contraction of ASM cells induced by ATP. Our results demonstrate that P2X(4) receptors exist on ASM and that the receptors are responsible for Ca(2+) influx. These findings suggest that the Ca(2+) influx regulated by P2X(4) receptors plays an important role in ASM contraction by a pathway distinct from VDCC.

Airway mechanics and methods used to visualize smooth muscle dynamics in vitro

Contraction of airway smooth muscle (ASM) is regulated by the physiological, structural and mechanical environment in the lung. We review two in vitro techniques, lung slices and airway segment preparations, that enable in situ ASM contraction and airway narrowing to be visualized. Lung slices and airway segment approaches bridge a gap between cell culture and isolated ASM, and whole animal studies. Imaging techniques enable key upstream events involved in airway narrowing, such as ASM cell signalling and structural and mechanical events impinging on ASM, to be investigated.

Mechanisms altering airway smooth muscle cell Ca+ homeostasis in two asthma models

BACKGROUND

Asthma is characterized by airway remodeling, altered mucus production and airway smooth muscle cell (ASMC) contraction causing extensive airway narrowing. In particular, alterations of ASMC contractility seem to be of crucial importance. The elevation of the cytoplasmic Ca(2+) concentration is a key event leading to ASMC contraction and changes in the agonist-induced Ca(2+) increase in ASMC have been reported in asthma.

OBJECTIVE

The aim of this study was to investigate mechanisms underlying these changes.

METHODS

Murine tracheal smooth muscle cells (MTSMC) from T-bet KO mice and human bronchial smooth muscle cells (HBSMC) incubated with IL-13 and IL-4 served as asthma models. Acetylcholine-induced changes in the cytoplasmic Ca(2+) concentration were recorded using fluorescence microscopy and the expression of Ca(2+) homeostasis regulating proteins was investigated with Western blot analysis.

RESULTS

Acetylcholine-induced Ca(2+) transients were elevated in both asthma models. This correlated with an increased Ca(2+) content of the sarcoplasmic reticulum (SR). In MTSMC from T-bet KO mice, the expression of the SR Ca(2+) buffers calreticulin and calsequestrin was higher compared to wild-type mice. In HBSMC incubated with IL-13 or IL-4, the expression of ryanodine receptors, inositol-3-phosphate receptors and sarcoplasmic/endoplasmic reticulum Ca(2+) ATPases 2 was increased compared to HBSMC without incubation with interleukins. The enlarged acetylcholine-induced Ca(2+) transients could be reversed by blocking inositol-3-phosphate receptors.

CONCLUSIONS

We conclude that in the murine asthma model the SR Ca(2+) buffer capacity is increased, while in the human asthma model the expression of SR Ca(2+) channels is altered. The investigation of the Ca(2+) homeostasis of ASMC has the potential to provide new therapeutical options in asthma.

Roles of P2X receptors and Ca2+ sensitization in extracellular adenosine triphosphate-induced hyperresponsiveness in airway smooth muscle

BACKGROUND

The release of adenosine triphosphate (ATP) from the airway epithelial cells during the inflammatory process is considered to play an important role in the pathophysiology of asthma and chronic obstructive pulmonary disease.

OBJECTIVE

This study was designed to determine whether extracellular ATP is involved in the bronchial hyperresponsiveness as an interaction between epithelium and smooth muscle in the airways.

METHODS

We examined the contractile response to methacholine (MCh) before and after exposure to low concentrations (< or = 10 microm) of ATP in isolated, epithelium-denuded guinea-pig tracheal smooth muscle by measuring isometric tension. Intracellular Ca2+ concentrations ([Ca2+]i) were assessed by fluorescent intensities of fura-2.

RESULTS

MCh-induced contractile force was increased with no change in [Ca2+]i after exposure to 10 microm ATP for 15 min. The ability of ATP to enhance the MCh-induced contraction was markedly attenuated by suramin, a non-selective P2 receptor inhibitor. Pre-incubation with ATPgammaS, a non-hydrolysable analogue of ATP and alpha,beta-meATP, a P2X agonist, also enhanced the MCh-induced contraction. In contrast, uracil triphosphate, a P2Y agonist, did not affect the MCh-induced contraction. Y-27632, a Rho-kinase inhibitor, suppressed the ability of ATP to enhance the MCh-induced contraction. Moreover, PP1 and PP2, Src tyrosin kinase inhibitors, suppressed the enhancement of MCh-induced contraction by ATP.

CONCLUSION

Pre-treatment with ATP induces hyperresponsiveness to MCh mediated by Ca2+ sensitization via the P2X receptor in airway smooth muscle. The present findings suggest the possible involvement of both the Rho-kinase and Src pathways in the intracellular mechanism of this phenomenon.

Role of platelet-activating factor in pneumolysin-induced acute lung injury

OBJECTIVE

Acute respiratory failure is a major complication of severe pneumococcal pneumonia, characterized by impairment of pulmonary microvascular barrier function and pulmonary hypertension. Both features can be evoked by pneumolysin (PLY), an important virulence factor of Streptococcus pneumoniae. We hypothesized that platelet-activating factor (PAF) and associated downstream signaling pathways play a role in the PLY-induced development of acute lung injury.

DESIGN

Controlled, ex vivo laboratory study.

SUBJECTS

Female Balb/C mice, 8-12 wks old.

INTERVENTIONS

Ventilated and blood-free-perfused lungs of wild-type and PAF receptor-deficient mice were challenged with recombinant PLY.

MEASUREMENTS AND MAIN RESULTS

Intravascular PLY, but not the pneumolysoid Pd-B (PLY with a Trp-Phe substitution at position 433), caused an impressive dose-dependent increase in pulmonary vascular resistance and increased PAF in lung homogenates, as detected by reversed-phase high-performance liquid chromatography coupled to tandem mass spectrometry. The pressor response was reduced in lungs of PAF receptor-deficient mice and after PAF receptor blockade by BN 50730. PLY and exogenous PAF increased thromboxane B2 in lung effluate, and thromboxane receptor inhibition by BM 13505 diminished the pressor response to PLY. Differential inhibition of intracellular signaling steps suggested significant contribution of phosphatidylcholine-specific phospholipase C and protein kinase C and of the Rho/Rho-kinase pathway to PLY-induced pulmonary vasoconstriction. Unrelated to the pulmonary arterial pressor response, microvascular leakage of PLY was diminished in lungs of PAF receptor-deficient mice as well.

CONCLUSIONS

PAF significantly contributed to PLY-induced acute injury in murine lungs. The PAF-mediated pressor response to PLY depends on thromboxane and on the downstream effectors phosphatidylcholine-specific phospholipase C, protein kinase C, and Rho-kinase.

Endothelin-induced contraction of bronchiole and pulmonary arteriole smooth muscle cells is regulated by intracellular Ca2+ oscillations and Ca2+ sensitization

Endothelin-1 (ET) induces increases in intracellular Ca(2+) concentration ([Ca(2+)](i)), Ca(2+) sensitization, and contraction of both bronchiole and pulmonary arteriole smooth muscle cells (SMCs) and may play an important role in the pathophysiology of asthma and pulmonary hypertension. However, because it remains unclear how changes in [Ca(2+)](i) and the Ca(2+) sensitivity regulate SMC contraction, we have studied mouse lung slices with phase-contrast and confocal microscopy to correlate the ET-induced contraction with the changes in [Ca(2+)](i) and Ca(2+) sensitivity of bronchiole and arteriole SMCs. In comparison with acetylcholine (ACh) or serotonin (5-HT), ET induced a stronger and long-lasting contraction of both bronchioles and arterioles. This ET-induced contraction was associated with prominent asynchronous Ca(2+) oscillations that were propagated as Ca(2+) waves along the SMCs. These Ca(2+) oscillations were mediated by cyclic intracellular Ca(2+) release and required external Ca(2+) for their maintenance. Importantly, as the frequency of the Ca(2+) oscillations increased, the extent of contraction increased. ET-induced contraction was also associated with an increase in Ca(2+) sensitivity. In "model" slices in which the [Ca(2+)](i) was constantly maintained at an elevated level by pretreatment of slices with caffeine and ryanodine, the addition of ET increased bronchiole and arteriole contraction. These results indicate that ET-induced contraction of bronchiole and arteriole SMCs is regulated by the frequency of Ca(2+) oscillations and by increasing the sensitivity of the contractile machinery to Ca(2+).

Effects of propofol and ketamine on ATP-induced contraction of the rat trachea

PURPOSE

ATP causes airway smooth-muscle contraction in patients with asthma and chronic obstructive pulmonary disease. Propofol and ketamine attenuate the airway smooth-muscle contraction induced by histamine and acetylcholine. However, it is not clear whether propofol and ketamine affect the ATP-induced airway smooth-muscle contraction.

METHODS

We examined the effects of propofol and ketamine on the ATP-induced contraction and ATP-P(2)-purinoceptor binding.

RESULTS

Propofol attenuated the ATP-induced contraction in a dose-dependent manner, with a 50% inhibitory concentration of 54 +/- 22 microM. Ketamine at 300 microM attenuated the ATP-induced contraction. In the binding study, propofol attenuated the binding of the P(2)-purinoceptor with [(3)H]-ATP in a dose-dependent manner, while ketamine did not attenuate this binding.

CONCLUSION

Propofol attenuates ATP-induced contraction through the inhibition of ATP-P(2)-purinoceptor binding.

A delayed ATP-elicited K+ current in freshly isolated smooth muscle cells from mouse aorta

Adenosine 5'-triphosphate (ATP) activated two sequential responses in freshly isolated mouse aortic smooth muscle cells. In the first phase, ATP activated Ca(2+)-dependent K(+) or Cl(-) currents and the second phase was the activation of a delayed outward current with a reversal potential of -75.9 +/- 1.4 mV. A high concentration of extracellular K(+) (130 mM) shifted the reversal potential of the delayed ATP-elicited current to -3.5 +/- 1.3 mV. The known K(+)-channel blockers, iberiotoxin, charybdotoxin, glibenclamide, apamin, 4-aminopyridine, Ba(2+) and tetraethylammonium chloride all failed to inhibit the delayed ATP-elicited K(+) current. Removal of ATP did not decrease the amplitude of the ATP-elicited current back to the control values. The simultaneous recording of cytosolic free Ca(2+) and membrane currents revealed that the first phase of the ATP-elicited response is associated with an increase in intracellular Ca(2+), while the second delayed phase develops after the return of cytosolic free Ca(2+) to control levels.ATP did not activate Ca(2+)-dependent K(+) currents, but did elicit Ca(2+)-independent K(+) currents, in cells dialyzed with ethylene glycol-bis (2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA). The delay of activation of Ca(2+)-independent currents decreased from 10.5 + 3.4 to 1.27 +/- 0.33 min in the cells dialyzed with 2 mM EGTA. Adenosine alone failed to elicit a Ca(2+)-independent K(+) current but simultaneous application of ATP and adenosine activated the delayed K(+) current. Intracellular dialysis of cells with guanosine 5'-O-(2-thiodiphosphate) transformed the Ca(2+)-independent ATP-elicited response from a sustained to a transient one. A phospholipase C inhibitor, U73122 (1 microM), was shown to abolish the delayed ATP-elicited response. These results indicate that the second phase of the ATP-elicited response was a delayed Ca(2+)-independent K(+) current activated by exogenous ATP. This phase might represent a new vasoregulatory pathway in vascular smooth muscle cells.

IL-13Ralpha2 reverses the effects of IL-13 and IL-4 on bronchial reactivity and acetylcholine-induced Ca+ signaling

BACKGROUND

The interleukins IL-4 and IL-13 play a key role in the pathophysiology of asthma. The interleukin receptor IL-13Ralpha2 is believed to act as a decoy receptor, but until now, the functional significance of IL-13Ralpha2 remains vague.

METHODS

Bronchial reactivity was quantified in murine lung slices by digital video microscopy and acetylcholine (ACH)-induced Ca(2+) signaling was measured in human airway smooth muscle cells (ASMC) using fluorescence microscopy.

RESULTS

IL-4 or IL-13 up to 50 ng/ml induced bronchial hyperreactivity. But after incubation with 100 ng/ml this effect was lost and bronchial responsiveness was again comparable to the control level. The effects of IL-4 and IL-13 on bronchial reactivity were paralleled by the effects on ASMC proliferation. Fifty nanograms per milliliter of IL-4 and IL-13 increased the Ca(2+) response of human ASMC to ACH. At 100 ng/ml, however, the effects of the cytokines on the Ca(2+) response were no longer evident. The expression of IL-13Ralpha2 increased with increasing concentrations of IL-4 or IL-13, reaching its maximum at 100 ng/ml. Blocking IL-13Ralpha2, the loss of the effect of IL-4 and IL-13 at 100 ng/ml on human ASMC proliferation and the ACH-induced Ca(2+) response were no longer present.

CONCLUSIONS

IL-4 and IL-13 induce bronchial hyperreactivity by changing the Ca(2+) homeostasis of ASMC. These effects are counteracted by IL-13Ralpha2. The biological significance of IL-13Ralpha2 might be a protective function by regulating IL-13- and IL-4-mediated signal transduction and thereby limiting pathological alterations in Th2-mediated inflammatory diseases.

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.

Purinergic signaling and kinase activation for survival in pulmonary oxidative stress and disease

Stimulus-induced release of endogenous ATP into the extracellular milieu has been shown to occur in a variety of cells, tissues, and organs. Extracellular ATP can propagate signals via P2 receptors that are essential for growth and survival of cells. Abundance of P2 receptors, their multiple isoforms, and their ubiquitous distribution indicate that they transmit vital signals. Pulmonary epithelium and endothelium are rich in both P2X and P2Y receptors. ATP release from lung tissue and cells occurs upon stimulation both in vivo and in vitro. Extracellular ATP can activate signaling cascades composed of protein kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3K). Here we summarize progress related to release of endogenous ATP and nucleotide signaling in pulmonary tissues upon exposure to oxidant stress. Hypoxic, hyperoxic, and ozone exposures cause a rapid increase of extracellular ATP in primary pulmonary endothelial and epithelial cells. Extracellular ATP is critical for survival of these cells in high oxygen and ozone concentrations. The released ATP, upon binding to its specific receptors, triggers ERK and PI3K signaling and renders cells resistant to these stresses. Impairment of ATP release and transmission of such signals could limit cellular survival under oxidative stress. This may further contribute to disease pathogenesis or exacerbation.

Evidence for both adenosine A1 and A2A receptors activating single vagal sensory C-fibres in guinea pig lungs

We addressed the hypothesis that single vagal afferent C-fibres can be stimulated via either the adenosine A1 or A2A receptor subtypes. The effect of adenosine on the nerve terminals of vagal sensory nerve subtypes was evaluated in an ex vivo perfused guinea pig lung preparation using extracellular recording techniques. Adenosine (10 microm) consistently evoked action potential discharge in lung C-fibre terminals arising from the nodose ganglia, but failed to evoke action potential discharge in most jugular ganglion C-fibres. Adenosine also failed to activate stretch-sensitive nodose A-fibres in the lungs. The selective A1 antagonist DPCPX (0.1 microm) or the selective A2A antagonist SCH 58261 (0.1 microm) partially inhibited the nodose C-fibre activation by adenosine, and the combination of both antagonists almost completely inhibited the response. The adenosine-induced action potential discharge in nodose C-fibres was mimicked by either the selective A1 agonist CCPA (1 microm) or the selective A2A agonist CGS 21680 (1 microm). Single cell PCR techniques revealed that adenosine A1 and A2A receptor mRNA was expressed in individual nodose neurons retrogradely labelled from the lungs. The gramicidin-perforated patch clamp technique on neurons retrogradely labelled from the lungs was employed to study the functional consequence of adenosine receptor agonists directly on neuronal membrane properties. Both the selective A1 agonist CCPA (1 microm) and the selective A2A agonist CGS 21680 (1 microm) depolarized the airway-specific, capsaicin-sensitive, nodose neurons to action potential threshold. The data support the hypothesis that adenosine selectively depolarizes vagal nodose C-fibre terminals in the lungs to action potential threshold, by stimulation of both adenosine A1 and A2A receptor subtypes located in the neuronal membrane.

Mechanism of ACh-induced asynchronous calcium waves and tonic contraction in porcine tracheal muscle bundle

Stimulation of the tracheal muscle bundle by acetylcholine (ACh) results in the generation of asynchronous repetitive Ca2+ waves (ACW) in intact tracheal smooth muscle (TSM) cells. We showed previously that ACW underlie cholinergic excitation-contraction coupling in porcine TSM and that Ca2+ entry through the L-type voltage-gated Ca2+ channel (VGCC) contributes partially to maintenance of the ACW. However, the mechanism of the ACW remains undefined. In this study, we pharmacologically characterized the mechanism of ACh-induced ACW in the intact porcine tracheal muscle bundle. We found that inhibition of receptor-operated channels/store-operated channels (ROC/SOC) by SKF-96365 completely abolished the nifedipine-insensitive component of ACh-mediated ACW and tonic contraction. Blockade of Na+/Ca2+ exchange with KB-R7943 or 2′,4′-dichlorobenzamil or removal of extracellular Na+ resulted in nearly complete inhibition of the nifedipine-insensitive component of ACh-mediated ACW and tonic contraction. Inhibition of the sarco(endo)plasmic reticulum Ca2+-ATPase by cyclopiazonic acid abolished the ongoing ACW. Application of 2-aminoethoxydiphenyl borate (2-APB) or xestospongin C to inhibit the inositol 1,4,5-trisphosphate-sensitive sarcoplasmic reticulum (SR) Ca2+ release channels produced no effect on ACh-mediated ACW and tonic contraction. However, pretreatment with caffeine or ryanodine inhibited ACh-induced ACW. Furthermore, application of procaine or tetracaine prevented the generation and abolished the ongoing ACh-mediated ACW and tonic contraction. Collectively, these results indicate that the ACh-stimulated ACW in porcine TSM are produced by repetitive cycles of Ca2+ release from SR through 2-APB- and xestospongin C-insensitive Ca2+ release channels, and plasmalemmal Ca2+ entry involving reverse-mode Na+/Ca2+ exchange, ROC/SOC, and L-type VGCC is required to refill the SR via SERCA to support the ongoing ACW.

Airway smooth muscle relaxation results from a reduction in the frequency of Ca2+ oscillations induced by a cAMP-mediated inhibition of the IP3 receptor

Background

It has been shown that the contractile state of airway smooth muscle cells (SMCs) in response to agonists is determined by the frequency of Ca²⁺ oscillations occurring within the SMCs. Therefore, we hypothesized that the relaxation of airway SMCs induced by agents that increase cAMP results from the down-regulation or slowing of the frequency of the Ca²⁺ oscillations.

Methods

The effects of isoproterenol (ISO), forskolin (FSK) and 8-bromo-cAMP on the relaxation and Ca²⁺ signaling of airway SMCs contracted with methacholine (MCh) was investigated in murine lung slices with phase-contrast and laser scanning microscopy.

Results

All three cAMP-elevating agents simultaneously induced a reduction in the frequency of Ca²⁺ oscillations within the SMCs and the relaxation of contracted airways. The decrease in the Ca²⁺ oscillation frequency correlated with the extent of airway relaxation and was concentration-dependent. The mechanism by which cAMP reduced the frequency of the Ca²⁺ oscillations was investigated. Elevated cAMP did not affect the re-filling rate of the internal Ca²⁺ stores after emptying by repetitive exposure to 20 mM caffeine. Neither did elevated cAMP limit the Ca²⁺ available to stimulate contraction because an elevation of intracellular Ca²⁺ concentration induced by exposure to a Ca²⁺ ionophore (ionomycin) or by photolysis of caged-Ca²⁺ did not reverse the effect of cAMP. Similar results were obtained with iberiotoxin, a blocker of Ca²⁺-activated K⁺ channels, which would be expected to increase Ca²⁺ influx and contraction. By contrast, the photolysis of caged-IP₃ in the presence of agonist, to further elevate the intracellular IP₃ concentration, reversed the slowing of the frequency of the Ca²⁺ oscillations and relaxation of the airway induced by FSK. This result implied that the sensitivity of the IP₃R to IP₃ was reduced by FSK and this was supported by the reduced ability of IP₃ to release Ca²⁺ in SMCs in the presence of FSK.

Conclusion

These results indicate that the relaxant effect of cAMP-elevating agents on airway SMCs is achieved by decreasing the Ca²⁺ oscillation frequency by reducing internal Ca²⁺ release through IP₃ receptors.

Ca2+-signaling in airway smooth muscle cells is altered in T-bet knock-out mice

BACKGROUND

Airway smooth muscle cells (ASMC) play a key role in bronchial hyperresponsiveness (BHR). A major component of the signaling cascade leading to ASMC contraction is calcium. So far, agonist-induced Ca2+-signaling in asthma has been studied by comparing innate properties of inbred rat or mouse strains, or by using selected mediators known to be involved in asthma. T-bet knock-out (KO) mice show key features of allergic asthma such as a shift towards TH2-lymphocytes and display a broad spectrum of asthma-like histological and functional characteristics. In this study, we aimed at investigating whether Ca2+-homeostasis of ASMC is altered in T-bet KO-mice as an experimental model of asthma.

METHODS

Lung slices of 100 to 200 microm thickness were obtained from T-bet KO- and wild-type mice. Airway contraction in response to acetylcholine (ACH) was measured by video-microscopy and Ca2+-signaling in single ASMC of lung slices was assessed using two-photon-microscopy.

RESULTS

Airways from T-bet KO-mice showed increased baseline airway tone (BAT) and BHR compared to wild-type mice. This could be mimicked by incubation of lung slices from wild-type mice with IL-13. The increased BAT was correlated with an increased incidence of spontaneous changes in intracellular Ca2+-concentrations, whereas BHR correlated with higher ACH-induced Ca2+-transients and an increased proportion of ASMC showing Ca2+-oscillations. Emptying intracellular Ca2+-stores using caffeine or cyclopiazonic acid induced higher Ca2+-elevations in ASMC from T-bet KO- compared to wild-type mice.

CONCLUSION

Altered Ca2+-homeostasis of ASMC contributes to increased BAT and BHR in lung slices from T-bet KO-mice as a murine asthma model. We propose that a higher Ca2+-content of the intracellular Ca2+-stores is involved in the pathophysiology of these changes.

Ciliary beat frequency is maintained at a maximal rate in the small airways of mouse lung slices

Ciliary beat frequency (CBF) is a key factor in the defense of the airways, and ATP can stimulate CBF by increasing intracellular calcium concentration ([Ca2+]i). However, the regulatory effects of ATP have been mainly studied in cultured or isolated epithelial cells from the large cartilaginous airways. The aim of this study was to evaluate the regulation of CBF in small airways of lung slices that are representative of in vivo tissue. Mice lungs were inflated with agarose and cut into thin slices with a vibratome. CBF in the small bronchioles was observed with differential interference contrast microscopy and quantified using high-speed digital imaging (at 240 images s(-1)). We found that the in situ organization of the ciliated cells was well preserved and that their CBF was high. We verified the fidelity of our recording system by analyzing rapid changes in CBF in response to temperature. However, we found that ATP had no effect on CBF, despite the fact that the [Ca2+]i, measured with confocal fluorescence imaging, was increased. Ionomycin and purinergic or beta-adrenergic agonists also failed to increase CBF. Similar results were obtained in outgrowths of cells cultured from lung slices. By contrast, ATP increased the slower CBF of outgrowths of ciliated cells cultured from tracheal rings. Therefore, we conclude that CBF in intrapulmonary airways of mice is maintained at a maximum rate and cannot be further increased by agonist stimulation. These conditions would ensure that mucociliary clearance is constantly active to provide continuous airway protection.

Modulation of the Ca2+ sensitivity of airway smooth muscle cells in murine lung slices

To investigate the phenomenon of Ca(2+) sensitization, we developed a new intact airway and arteriole smooth muscle cell (SMC) "model" by treating murine lung slices with ryanodine-receptor antagonist, ryanodine (50 microM), and caffeine (20 mM). A sustained elevation in intracellular Ca(2+) concentration ([Ca(2+)](i)) was induced in both SMC types by the ryanodine-caffeine treatment due to the depletion of internal Ca(2+) stores and the stimulation of a persistent influx of Ca(2+). Arterioles responded to this sustained increase in [Ca(2+)](i) with a sustained contraction. By contrast, airways responded to sustained high [Ca(2+)](i) with a transient contraction followed by relaxation. Subsequent exposure to methacholine (MCh) induced a sustained concentration-dependent contraction of the airway without a change in the [Ca(2+)](i). During sustained MCh-induced contraction, Y-27632 (a Rho-kinase inhibitor) and GF-109203X (a protein kinase C inhibitor) induced a concentration-dependent relaxation without changing the [Ca(2+)](i). The cAMP-elevating agents, forskolin (an adenylyl cyclase activator), IBMX (a phosphodiesterase inhibitor), and caffeine (also acting as a phosphodiesterase inhibitor), exerted similar relaxing effects. These results indicate that 1) ryanodine-caffeine treatment is a valuable tool for investigating the contractile mechanisms of SMCs while avoiding nonspecific effects due to cell permeabilization, 2) in the absence of agonist, sustained high [Ca(2+)](i) has a differential time-dependent effect on the Ca(2+) sensitivity of airway and arteriole SMCs, 3) MCh facilitates the contraction of airway SMCs by inducing Ca(2+) sensitization via the activation of Rho-kinase and protein kinase C, and 4) cAMP-elevating agents contribute to the relaxation of airway SMCs through Ca(2+) desensitization.

Biphasic effect of extracellular ATP on human and rat airways is due to multiple P2 purinoceptor activation

Background

Extracellular ATP may modulate airway responsiveness. Studies on ATP-induced contraction and [Ca²⁺]_i signalling in airway smooth muscle are rather controversial and discrepancies exist regarding both ATP effects and signalling pathways. We compared the effect of extracellular ATP on rat trachea and extrapulmonary bronchi (EPB) and both human and rat intrapulmonary bronchi (IPB), and investigated the implicated signalling pathways.

Methods

Isometric contraction was measured on rat trachea, EPB and IPB isolated rings and human IPB isolated rings. [Ca²⁺]_i was monitored fluorimetrically using indo 1 in freshly isolated and cultured tracheal myocytes. Statistical comparisons were done with ANOVA or Student's t tests for quantitative variables and χ² tests for qualitative variables. Results were considered significant at P < 0.05.

Results

In rat airways, extracellular ATP (10^-6–10^-3 M) induced an epithelium-independent and concentration-dependent contraction, which amplitude increased from trachea to IPB. The response was transient and returned to baseline within minutes. Similar responses were obtained with the non-hydrolysable ATP analogous ATP-γ-S. Successive stimulations at 15 min-intervals decreased the contractile response. In human IPB, the contraction was similar to that of rat IPB but the time needed for the return to baseline was longer. In isolated myocytes, ATP induced a concentration-dependent [Ca²⁺]_i response. The contractile response was not reduced by thapsigargin and RB2, a P2Y receptor inhibitor, except in rat and human IPB. By contrast, removal of external Ca²⁺, external Na⁺ and treatment with D600 decreased the ATP-induced response. The contraction induced by α-β-methylene ATP, a P2X agonist, was similar to that induced by ATP, except in IPB where it was lower. Indomethacin and H-89, a PKA inhibitor, delayed the return to baseline in extrapulmonary airways.

Conclusion

Extracellular ATP induces a transient contractile response in human and rat airways, mainly due to P2X receptors and extracellular Ca²⁺ influx in addition with, in IPB, P2Y receptors stimulation and Ca²⁺ release from intracellular Ca²⁺ stores. Extracellular Ca²⁺ influx occurs through L-type voltage-dependent channels activated by external Na⁺ entrance through P2X receptors. The transience of the response cannot be attributed to ATP degradation but to purinoceptor desensitization and, in extrapulmonary airways, prostaglandin-dependent PKA activation.

The contraction of smooth muscle cells of intrapulmonary arterioles is determined by the frequency of Ca2+ oscillations induced by 5-HT and KCl

Increased resistance of the small blood vessels within the lungs is associated with pulmonary hypertension and results from a decrease in size induced by the contraction of their smooth muscle cells (SMCs). To study the mechanisms that regulate the contraction of intrapulmonary arteriole SMCs, the contractile and Ca(2+) responses of the arteriole SMCs to 5-hydroxytrypamine (5-HT) and KCl were observed with phase-contrast and scanning confocal microscopy in thin lung slices cut from mouse lungs stiffened with agarose and gelatin. 5-HT induced a concentration-dependent contraction of the arterioles. Increasing concentrations of extracellular KCl induced transient contractions in the SMCs and a reduction in the arteriole luminal size. 5-HT induced oscillations in [Ca(2+)](i) within the SMCs, and the frequency of these Ca(2+) oscillations was dependent on the agonist concentration and correlated with the extent of sustained arteriole contraction. By contrast, KCl induced Ca(2+) oscillations that occurred with low frequencies and were preceded by small, localized transient Ca(2+) events. The 5-HT-induced Ca(2+) oscillations and contractions occurred in the absence of extracellular Ca(2+) and were resistant to Ni(2+) and nifedipine but were abolished by caffeine. KCl-induced Ca(2+) oscillations and contractions were abolished by the absence of extracellular Ca(2+) and the presence of Ni(2+), nifedipine, and caffeine. Arteriole contraction was induced or abolished by a 5-HT(2)-specific agonist or antagonist, respectively. These results indicate that 5-HT, acting via 5-HT(2) receptors, induces arteriole contraction by initiating Ca(2+) oscillations and that KCl induces contraction via Ca(2+) transients resulting from the overfilling of internal Ca(2+) stores. We hypothesize that the magnitude of the sustained intrapulmonary SMC contraction is determined by the frequency of Ca(2+) oscillations and also by the relaxation rate of the SMC.

The frequency of calcium oscillations induced by 5-HT, ACH, and KCl determine the contraction of smooth muscle cells of intrapulmonary bronchioles

Increased resistance of airways or blood vessels within the lung is associated with asthma or pulmonary hypertension and results from contraction of smooth muscle cells (SMCs). To study the mechanisms regulating these contractions, we developed a mouse lung slice preparation containing bronchioles and arterioles and used phase-contrast and confocal microscopy to correlate the contractile responses with changes in [Ca²⁺]_i of the SMCs. The airways are the focus of this study. The agonists, 5-hydroxytrypamine (5-HT) and acetylcholine (ACH) induced a concentration-dependent contraction of the airways. High concentrations of KCl induced twitching of the airway SMCs but had little effect on airway size. 5-HT and ACH induced asynchronous oscillations in [Ca²⁺]_i that propagated as Ca²⁺ waves within the airway SMCs. The frequency of the Ca²⁺ oscillations was dependent on the agonist concentration and correlated with the extent of sustained airway contraction. In the absence of extracellular Ca²⁺ or in the presence of Ni²⁺, the frequency of the Ca²⁺ oscillations declined and the airway relaxed. By contrast, KCl induced low frequency Ca²⁺ oscillations that were associated with SMC twitching. Each KCl-induced Ca²⁺ oscillation consisted of a large Ca²⁺ wave that was preceded by multiple localized Ca²⁺ transients. KCl-induced responses were resistant to neurotransmitter blockers but were abolished by Ni²⁺ or nifedipine and the absence of extracellular Ca²⁺. Caffeine abolished the contractile effects of 5-HT, ACH, and KCl. These results indicate that (a) 5-HT and ACH induce airway SMC contraction by initiating Ca²⁺ oscillations, (b) KCl induces Ca²⁺ transients and twitching by overloading and releasing Ca²⁺ from intracellular stores, (c) a sustained, Ni²⁺-sensitive, influx of Ca²⁺ mediates the refilling of stores to maintain Ca²⁺ oscillations and, in turn, SMC contraction, and (d) the magnitude of sustained airway SMC contraction is regulated by the frequency of Ca²⁺ oscillations.

Interplay between ryanodine and IP3 receptors in ATP-stimulated mouse luteinized-granulosa cells

In mouse luteinized-granulosa cells (MGLC), ATP induces an increase in intracellular Ca2+ concentration by stimulating phospholipase C (PLC) associated with purinergic receptors, leading to production of inositol 1,4,5-trisphosphate (IP3) and subsequent release of Ca2+ from intracellular stores. In this study, we examined the cross-talk between the ryanodine receptors (RyR) and IP3 receptors (IP3R) in response to ATP in MGLC. Specifically, the effect of RyR modulators on ATP response was examined. The results showed that ATP-induced intracellular calcium elevation was abolished by inhibitors of the RyR, such as dantrolene (25 microM) and ryanodine (80 microM). When the MGLC were stimulated with activators of RyR, 2 microM ryanodine and 10 mM caffeine, the ATP-elicited response was decreased. These actions were independent of IP3 production stimulated by ATP. Hence, ATP-induced intracellular Ca2+ mobilization involves the coordinated action of both types of calcium release channels (CRCs). Using fluorescent probes, it was shown that IP3R is uniformly distributed throughout the cell; in contrast, RyR is mainly found around the nuclei. It is concluded that the IP3R and the RyR are functionally associated, and both play a role in the pattern of Ca2+ increase observed during purinergic stimulation of MGLC. This coupling may provide a highly efficient amplification mechanism for ATP stimulation of Ca2+ mobilization.