Signalling pathway for histamine activation of non-selective cation channels in equine tracheal myocytes

Function and Role of Histamine H1 Receptor in the Mammalian Heart

Histamine can change the force of cardiac contraction and alter the beating rate in mammals, including humans. However, striking species and regional differences have been observed. Depending on the species and the cardiac region (atrium versus ventricle) studied, the contractile, chronotropic, dromotropic, and bathmotropic effects of histamine vary. Histamine is present and is produced in the mammalian heart. Thus, histamine may exert autocrine or paracrine effects in the mammalian heart. Histamine uses at least four heptahelical receptors: H₁, H₂, H₃ and H₄. Depending on the species and region studied, cardiomyocytes express only histamine H₁ or only histamine H₂ receptors or both. These receptors are not necessarily functional concerning contractility. We have considerable knowledge of the cardiac expression and function of histamine H₂ receptors. In contrast, we have a poor understanding of the cardiac role of the histamine H₁ receptor. Therefore, we address the structure, signal transduction, and expressional regulation of the histamine H₁ receptor with an eye on its cardiac role. We point out signal transduction and the role of the histamine H₁ receptor in various animal species. This review aims to identify gaps in our knowledge of cardiac histamine H₁ receptors. We highlight where the published research shows disagreements and requires a new approach. Moreover, we show that diseases alter the expression and functional effects of histamine H₁ receptors in the heart. We found that antidepressive drugs and neuroleptic drugs might act as antagonists of cardiac histamine H₁ receptors, and believe that histamine H₁ receptors in the heart might be attractive targets for drug therapy. The authors believe that a better understanding of the role of histamine H₁ receptors in the human heart might be clinically relevant for improving drug therapy.

Androgen Effects on the Adrenergic System of the Vascular, Airway, and Cardiac Myocytes and Their Relevance in Pathological Processes

INTRODUCTION

Androgen signaling comprises nongenomic and genomic pathways. Nongenomic actions are not related to the binding of the androgen receptor (AR) and occur rapidly. The genomic effects implicate the binding to a cytosolic AR, leading to protein synthesis. Both events are independent of each other. Genomic effects have been associated with different pathologies such as vascular ischemia, hypertension, asthma, and cardiovascular diseases. Catecholamines play a crucial role in regulating vascular smooth muscle (VSM), airway smooth muscle (ASM), and cardiac muscle (CM) function and tone.

OBJECTIVE

The aim of this review is an updated analysis of the role of androgens in the adrenergic system of vascular, airway, and cardiac myocytes. Body. Testosterone (T) favors vasoconstriction, and its concentration fluctuation during life stages can affect the vascular tone and might contribute to the development of hypertension. In the VSM, T increases α1-adrenergic receptors (α ₁-ARs) and decreases adenylyl cyclase expression, favoring high blood pressure and hypertension. Androgens have also been associated with asthma. During puberty, girls are more susceptible to present asthma symptoms than boys because of the increment in the plasmatic concentrations of T in young men. In the ASM, β ₂-ARs are responsible for the bronchodilator effect, and T augments the expression of β ₂-ARs evoking an increase in the relaxing response to salbutamol. The levels of T are also associated with an increment in atherosclerosis and cardiovascular risk. In the CM, activation of α _1A-ARs and β ₂-ARs increases the ionotropic activity, leading to the development of contraction, and T upregulates the expression of both receptors and improves the myocardial performance.

CONCLUSIONS

Androgens play an essential role in the adrenergic system of vascular, airway, and cardiac myocytes, favoring either a state of health or disease. While the use of androgens as a therapeutic tool for treating asthma symptoms or heart disease is proposed, the vascular system is warmly affected.

Canonical Transient Potential Receptor-3 Channels in Normal and Diseased Airway Smooth Muscle Cells

All seven canonical transient potential receptor (TRPC1-7) channel members are expressed in mammalian airway smooth muscle cells (ASMCs). Among this family, TRPC3 channel plays an important role in the control of the resting [Ca²⁺]_i and agonist-induced increase in [Ca²⁺]_i. This channel is significantly upregulated in molecular expression and functional activity in airway diseases. The upregulated channel significantly augments the resting [Ca²⁺]_i and agonist-induced increase in [Ca²⁺]_i, thereby exerting a direct and essential effect in airway hyperresponsiveness. The increased TRPC3 channel-mediated Ca²⁺ signaling also results in the transcription factor nuclear factor-κB (NF-κB) activation via protein kinase C-α (PKCα)-dependent inhibitor of NFκB-α (IκBα) and calcineurin-dependent IκBβ signaling pathways, which upregulates cyclin-D1 expression and causes cell proliferation, leading to airway remodeling. TRPC3 channel may further interact with intracellular release Ca²⁺ channels, Orai channels and Ca²⁺-sensing stromal interaction molecules, mediating important cellular responses in ASMCs and the development of airway diseases.

Synergistic interaction between diclofenac and pyrilamine on nociception, inflammation, and gastric damage in rats

Experiments using nonsteroidal anti-inflammatory drugs (NSAIDs) alone have produced limited antinociceptive effects in animal models. For this reason, the number of studies involving the administration of NSAIDs along with an adjuvant drug harboring different mechanisms of action has increased enormously. Here, combinations of diclofenac and pyrilamine were used to determine their influence on nociception (formalin test), inflammation (paw inflammation produced by carrageenan), and gastric damage in rodents. Diclofenac, pyrilamine, or combinations of diclofenac and pyrilamine produced antinociceptive and anti-inflammatory effects in the rat. The systemic administration of diclofenac alone and in combination with pyrilamine produced significant gastric damage. Effective dose (ED) values were determined for each individual drug, and isobolograms were prepared. The theoretical ED values for the antinociceptive (systemic, 35.4 mg/kg; local, 343.4 μg/paw) and the anti-inflammatory (37.9 mg/kg) effects differed significantly from the experimental ED values (systemic antinociception, 18.1 mg/kg; local antinociception, 183.3 μg/paw; anti-inflammation, 10.6 mg/kg). Therefore, it was concluded that the interactions between diclofenac and pyrilamine are synergistic. The data suggest that the diclofenac–pyrilamine combinations can interact at the systemic and local peripheral levels, thereby offering a therapeutic alternative for the clinical management of inflammatory pain.

Label-free versus conventional cellular assays: Functional investigations on the human histamine H1 receptor

A set of histamine H₁ receptor (H₁R) agonists and antagonists was characterized in functional assays, using dynamic mass redistribution (DMR), electric cell-substrate impedance sensing (ECIS) and various signaling pathway specific readouts (Fura-2 and aequorin calcium assays, arrestin recruitment (luciferase fragment complementation) assay, luciferase gene reporter assay). Data were gained from genetically engineered HEK293T cells and compared with reference data from GTPase assays and radioligand binding. Histamine and the other H₁R agonists gave different assay-related pEC₅₀ values, however, the order of potency was maintained. In the luciferase fragment complementation assay, the H₁R preferred β-arrestin2 over β-arrestin1. The calcium and the impedimetric assay depended on G_q coupling of the H₁R, as demonstrated by complete inhibition of the histamine-induced signals in the presence of the G_q inhibitor FR900359 (UBO-QIC). Whereas partial inhibition by FR900359 was observed in DMR and the gene reporter assay, pertussis toxin substantially decreased the response in DMR, but increased the luciferase signal, reflecting the contribution of both, G_q and G_i, to signaling in these assays. For antagonists, the results from DMR were essentially compatible with those from conventional readouts, whereas the impedance-based data revealed a trend towards higher pK_b values. ECIS and calcium assays apparently only reflect G_q signaling, whereas DMR and gene reporter assays appear to integrate both, G_q and G_i mediated signaling. The results confirm the value of the label-free methods, DMR and ECIS, for the characterization of H₁R ligands. Both noninvasive techniques are complementary to each other, but cannot fully replace reductionist signaling pathway focused assays.

Current Knowledge and Perspectives on Histamine H1 and H2 Receptor Pharmacology: Functional Selectivity, Receptor Crosstalk, and Repositioning of Classic Histaminergic Ligands

H₁ and H₂ histamine receptor antagonists, although developed many decades ago, are still effective for the treatment of allergic and gastric acid-related conditions. This article focuses on novel aspects of the pharmacology and molecular mechanisms of histamine receptors that should be contemplated for optimizing current therapies, repositioning histaminergic ligands for new therapeutic uses, or even including agonists of the histaminergic system in the treatment of different pathologies such as leukemia or neurodegenerative disorders. In recent years, new signaling phenomena related to H₁ and H₂ receptors have been described that make them suitable for novel therapeutic approaches. Crosstalk between histamine receptors and other membrane or nuclear receptors can be envisaged as a way to modulate other signaling pathways and to potentiate the efficacy of drugs acting on different receptors. Likewise, biased signaling at histamine receptors seems to be a pharmacological feature that can be exploited to investigate nontraditional therapeutic uses for H₁ and H₂ biased agonists in malignancies such as acute myeloid leukemia and to avoid undesired side effects when used in standard treatments. It is hoped that the molecular mechanisms discussed in this review contribute to a better understanding of the different aspects involved in histamine receptor pharmacology, which in turn will contribute to increased drug efficacy, avoidance of adverse effects, or repositioning of histaminergic ligands.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Effects of histamine on cultured interstitial cells of cajal in murine small intestine

Interstitial cells of Cajal (ICCs) are the pacemaker cells in the gastrointestinal tract, and histamine is known to regulate neuronal activity, control vascular tone, alter endothelial permeability, and modulate gastric acid secretion. However, the action mechanisms of histamine in mouse small intestinal ICCs have not been previously investigated, and thus, in the present study, we investigated the effects of histamine on mouse small intestinal ICCs, and sought to identify the receptors involved. Enzymatic digestions were used to dissociate ICCs from small intestines, and the whole-cell patch-clamp configuration was used to record potentials (in current clamp mode) from cultured ICCs. Histamine was found to depolarize resting membrane potentials concentration dependently, and whereas 2-PEA (a selective H1 receptor agonist) induced membrane depolarizations, Dimaprit (a selective H2-agonist), R-alpha-methylhistamine (R-alpha-MeHa; a selective H3-agonist), and 4-methylhistamine (4-MH; a selective H4-agonist) did not. Pretreatment with Ca²⁺-free solution or thapsigargin (a Ca²⁺-ATPase inhibitor in endoplasmic reticulum) abolished the generation of pacemaker potentials and suppressed histamine-induced membrane depolarization. Furthermore, treatments with U-73122 (a phospholipase C inhibitor) or 5-fluoro-2-indolyl des-chlorohalopemide (FIPI; a phospholipase D inhibitor) blocked histamine-induced membrane depolarizations in ICCs. On the other hand, KT5720 (a protein kinase A inhibitor) did not block histamine-induced membrane depolarization. These results suggest that histamine modulates pacemaker potentials through H1 receptor-mediated pathways via external Ca²⁺ influx and Ca²⁺ release from internal stores in a PLC and PLD dependent manner.

Identification of histamine receptors and effects of histamine on murine and simian colonic excitability

BACKGROUND

Inflammatory responses can include recruitment of cells of hematopoietic origin to the tunica muscularis. These cells can secrete a variety of factors which can reset the gain of smooth muscle cells (SMC) and influence motor patterns. Histamine (H), a major mediator in inflammation, is released by mast cells and exerts diverse effects in SMC by binding to H receptors. The profiles of H receptor expression in animal models used to study inflammatory diseases are unknown.

METHODS

Histamine receptor expression and electro-mechanical responses to H were tested in simian and murine colonic smooth muscle using qualitative and quantitative PCR, isometric force measurements, microelectrode recordings and patch clamp techniques.

KEY RESULTS

H1, H2, and H4 receptor transcripts were expressed at similar levels in simian colonic tissue whereas only the H2 receptor transcript was detected in murine colonic tissue. Stimulation of simian colonic muscles with H caused depolarization and contraction in the presence of tetrodotoxin. Histamine activated non-selective cation channels in simian SMC. In contrast, H caused hyperpolarization and inhibited contractions of murine colon. The hyperpolarization was inhibited by the K(ATP) channel blocker, glibenclamide. Histamine-activated K(+) currents were inhibited by glibenclamide in murine colonic SMC.

CONCLUSIONS & INFERENCES

Histamine receptor expression in simian SMC was similar to that reported in humans. However, H receptor profile and responses to H were considerably different in mice. Thus, monkey colon may be a more suitable model to study how inflammatory mediators affect the gain of smooth muscle excitability.

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

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

Ca2+ signaling during maturation of cumulus-oocyte complex in mammals

Under the influence of gonadotropins or growth factors, a close cooperation develops between cumulus cells and the oocyte that is implicated in transmitting signals involved in maintaining or releasing the meiotic arrest in the oocyte. While cyclic adenosine 5'-monophosphate (cAMP) is a key molecule in maintaining the meiotic arrest, calcium (Ca(2+)) may play a role in controlling either spontaneous or gonadotropin-induced oocyte maturation, possibly by modulating intracytoplasmic cAMP concentrations via Ca(2+)-sensitive adenylate cyclases. This review focuses on the mechanisms related to the origin of the Ca(2+) wave that travels from the cumulus cells to the oocyte, and discusses the source of variations affecting the dynamics of this wave.

Mechanism and regulatory function of CpG signaling via scavenger receptor B1 in primary B cells

It is well established that CpG promotes pro-inflammatory cytokine and antibody production by B cells via the Toll-like receptor 9 (TLR9)-dependent pathway. However, scavenger receptors (SRs) are also capable of binding such pathogen-derived molecules, yet their contribution to CpG-induced signaling events has not yet been evaluated. Here we identified a novel TLR9-independent mechanism of CpG-induced signaling and immune function that is mediated by the scavenger B1 receptor (SR-B1). Specifically, we show that CpG/SR-B1 triggers calcium entry into primary B lymphocytes via phospholipase C gamma-1-mediated activation of TRPC3 channels and also B cell adhesion to vascular cell adhesion molecule-1. CpG-induced calcium signals and vascular cell adhesion molecule-1 adhesion are TLR9-independent and are mediated exclusively by SR-B1. Although pro-inflammatory cytokine and Ig production induced by CpG require TLR9 expression, we also found that SR-B1 negatively regulates TLR9-dependent production of interleukin-6, interleukin-10, and IgM. Thus, our results provide a novel perspective on the complexity of CpG signaling within B cells by demonstrating that SR-B1 is an alternative pathway for nucleic acid-induced signaling that provides feedback inhibition on specific TLR9-dependent responses of B cells. Consequently, these results have wide implications for understanding the mechanisms regulating immune tolerance to nucleic acids and pathogen-associated molecules.

Role of phosphoinositol 4,5-bisphosphate and diacylglycerol in regulating native TRPC channel proteins in vascular smooth muscle

Stimulation of receptor-operated (ROCs) and store-operated (SOCs) Ca(2+)-permeable cation channels by vasoconstrictors has many important physiological functions in vascular smooth muscle. The present review indicates that ROCs and SOCs with diverse properties in different blood vessels are likely to be explained by composition of different subunits from the canonical transient receptor potential (TRPC) family of cation channel proteins. In addition we illustrate that activation of native TRPC ROCs and SOCs involves different phospholipase-mediated transduction pathways linked to generation of diacylglycerol (DAG). Moreover we describe recent novel data showing that the endogenous phospholipid phosphoinositol 4,5-bisphosphate (PIP(2)) has profound and contrasting actions on TRPC ROCs and SOCs. Optimal activation of a native TRPC6 ROC by angiotensin II (Ang II) requires both depletion of PIP(2) and generation of DAG which leads to stimulation of TRPC6 via a PKC-independent mechanism. The data also indicate that PIP(2) has a marked constitutive inhibitory action of TRPC6 and DAG and PIP(2) are physiological antagonists on TRPC6 ROCs. In contrast PIP(2) stimulates TRPC1 SOCs and has an obligatory role in activation of these channels by store-depletion which requires PKC-dependent phosphorylation of TRPC1 proteins. Finally, we conclude that interactions between PIP(2) bound to TRPC proteins at rest, generation of DAG and PKC-dependent phosphorylation of TRPC proteins have a fundamental role in activation mechanisms of ROCs and SOCs in vascular smooth muscle.

Salmeterol and cytokines modulate inositol-phosphate signalling in human airway smooth muscle cells via regulation at the receptor locus

Background

Airway hyper-responsiveness (AHR) is a key feature of asthma and a causal relationship between airway inflammation and AHR has been identified. The aim of the current study was to clarify the effect of proinflammatory cytokines and asthma medication on primary human airway smooth muscle (ASM) inositol phosphate (IPx) signalling and define the regulatory loci involved.

Methods

Primary Human ASM cells were isolated from explants of trachealis muscle from individuals with no history of respiratory disease. The effect of cytokine or asthma medication on histamine or bradykinin induced IPx signalling was assessed by [³H] inositol incorporation. Quantitative Real Time PCR was used to measure mRNA levels of receptors and downstream signalling components. Transcriptional mechanisms were explored using a combination of 5'Rapid Amplification of cDNA Ends (5'RACE) and promoter-reporter techniques.

Results

Treatment of Human ASM cells with IL-13, IFNγ or salmeterol for 24 hours lead to a modest augmentation of histamine induced IPx responses (144.3 +/- 9.3, 126.4 +/- 7.5 and 117.7 +/- 5.2%, p < 0.05). Similarly, TNFα, IFNγ or salmeterol treatment augmented bradykinin induced IPx responses (127.4 +/- 8.3, 128.0 +/- 8.4 and 111.7 +/- 5.0%, P < 0.05). No treatment significantly influenced sodium fluoride induced IPx responses suggesting regulation occurs at the receptor locus. Analyses of mRNA expression of components of the IPx pathway i.e. H1 Histamine Receptor (HRH1), B2 Bradykinin Receptor (BDKRB2), Gαq/11 and PLC-β1 identified that a significant induction of receptor mRNA (>2 fold) was a feature of these responses explaining the cytokine and spasmogen specificity. The HRH1 and BDKRB2 promoter regions were mapped in ASM and promoter-reporter analyses identified that salmeterol can induce HRH1 (>2 fold) and BDKRB2 (2–5 fold) transcription. The effect of cytokines on HRH1 and BDKRB2 promoter-reporter expression suggested a more complex regulation of mRNA expression involving additional loci to the core promoter.

Conclusion

Our results indicate that the spasmogen specific receptor locus may be a key site of regulation determining the magnitude of spasmogen mediated ASM IPx responses during airway inflammation or following asthma medication. These data provide further insight into the molecular basis of AHR and extend our understanding of potentially detrimental effects associated with existing therapies used in the treatment of asthma.

Functional coupling between the Na+/Ca2+ exchanger and nonselective cation channels during histamine stimulation in guinea pig tracheal smooth muscle

Airway smooth muscle (ASM) contracts partly due to an increase in cytosolic Ca(2+). In this work, we found that the contraction caused by histamine depends on external Na(+), possibly involving nonselective cationic channels (NSCC) and the Na(+)/Ca(2+) exchanger (NCX). We performed various protocols using isometric force measurement of guinea pig tracheal rings stimulated by histamine. We observed that force reached 53 +/- 1% of control during external Na(+) substitution by N-methyl-D-glucamine(+), whereas substitution by Li(+) led to no significant change (91 +/- 1%). Preincubation with KB-R7943 decreased the maximal force developed (52.3 +/- 5.6%), whereas preincubation with nifedipine did not (89.7 +/- 1.8%). Also, application of the nonspecific NCX blocker KB-R7943 and nifedipine on histamine-precontracted tracheal rings reduced force to 1 +/- 3%, significantly different from nifedipine alone (49 +/- 6%). Moreover, nonspecific NSCC inhibitors SKF-96365 and 2-aminoethyldiphenyl borate reduced force to 1 +/- 1% and 19 +/- 7%, respectively. Intracellular Ca(2+) measurements in isolated ASM cells showed that KB-R7943 and SKF-96365 reduced the peak and sustained response to histamine (0.20 +/- 0.1 and 0.19 +/- 0.09 for KB-R, 0.43 +/- 0.16 and 0.47 +/- 0.18 for SKF, expressed as mean of differences). Moreover, Na(+)-free solution only inhibited the sustained response (0.54 +/- 0.25). These data support an important role for NSCC and NCX during histamine stimulation. We speculate that histamine induces Na(+) influx through NSCC that promotes the Ca(2+) entry mode of NCX and Ca(V)1.2 channel activation, thereby causing contraction.

Angiotensin II activates two cation conductances with distinct TRPC1 and TRPC6 channel properties in rabbit mesenteric artery myocytes

Angiotensin II (Ang II) is a potent vasoconstrictor with an important role in controlling blood pressure; however, there is little information on cellular mechanisms underlying Ang II-evoked vasoconstrictor responses. The aim of the present study is to investigate the effect of Ang II on cation conductances in freshly dispersed rabbit mesenteric artery myocytes at the single-channel level using patch-clamp techniques. In cell-attached patches, bath application of low concentrations of Ang II (1 nM) activated cation channel currents (Icat1) with conductances states of about 15, 30 and 45 pS. At relatively high concentrations, Ang II (100 nM) inhibited Icat1 but evoked another cation channel (Icat2) with a conductance of approximately 2 pS. Ang II-evoked Icat1 and Icat2 were inhibited by the AT1 receptor antagonist losartan and the phospholipase C (PLC) inhibitor U73122. The diacylglycerol (DAG) lipase inhibitor RHC80267 initially induced Icat1 which was subsequently inhibited to reveal Icat2. The DAG analogue 1-oleoyl-2-acetyl-sn-glycerol (1 microM) activated Icat1 and Icat2 but inositol 1,4,5-trisphosphate did not evoke either conductance. The protein kinase C (PKC) inhibitor chelerythrine (3 microM) potentiated Ang II-evoked Icat1 and inhibited Icat2 whereas the PKC activator phorbol-12,13-dibutyrate (1 microM) reduced Ang II-induced Icat1 but activated Icat2. Moreover in cell-attached patches pretreated with chelerythrine, application of 100 nM Ang II activated Icat1. These data indicate that PKC inhibits Icat1 but stimulates Icat2. Agents that deplete intracellular Ca2+ stores also activated cation channel currents with similar properties to Icat2. Bath application of anti-TRPC6 and anti-TRPC1 antibodies to inside-out patches inhibited Icat1 and Icat2, respectively. Also flufenamic acid and zero external Ca2+ concentration, respectively, potentiated and reduced Ang II-evoked Icat1. Immunocytochemical studies showed TRPC6 and TRPC1 expression with TRPC6 preferentially distributed in the plasma membrane and TRPC1 expression located throughout the myocyte. These results indicate that Ang II activates two distinct cation conductances in mesenteric artery myocytes by stimulation of AT1 receptors linked to PLC. Icat1 is activated by DAG via a PKC-independent mechanism whereas Icat2 involves DAG acting via a PKC-dependent pathway. Higher concentrations of Ang II inhibit Icat1 by activating an inhibitory effect of PKC. It is proposed that TRPC6 and TRPC1 channel proteins are important components of Ang II-induced Icat1 and Icat2, respectively.

Evidence that CFTR is expressed in rat tracheal smooth muscle cells and contributes to bronchodilation

Background

The airway functions are profoundly affected in many diseases including asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF). CF the most common lethal autosomal recessive genetic disease is caused by mutations of the CFTR gene, which normally encodes a multifunctional and integral membrane protein, the CF transmembrane conductance regulator (CFTR) expressed in airway epithelial cells.

Methods

To demonstrate that CFTR is also expressed in tracheal smooth muscle cells (TSMC), we used iodide efflux assay to analyse the chloride transports in organ culture of rat TSMC, immunofluorescence study to localize CFTR proteins and isometric contraction measurement on isolated tracheal rings to observe the implication of CFTR in the bronchodilation.

Results

We characterized three different pathways stimulated by the cAMP agonist forskolin and the isoflavone agent genistein, by the calcium ionophore A23187 and by hypo-osmotic challenge. The pharmacology of the cAMP-dependent iodide efflux was investigated in detail. We demonstrated in rat TSMC that it is remarkably similar to that of the epithelial CFTR, both for activation (using three benzo [c]quinolizinium derivatives) and for inhibition (glibenclamide, DPC and CFTR_inh-172). Using rat tracheal rings, we observed that the activation of CFTR by benzoquinolizinium derivatives in TSMC leads to CFTR_inh-172-sensitive bronchodilation after constriction with carbachol. An immunolocalisation study confirmed expression of CFTR in tracheal myocytes.

Conclusion

Altogether, these observations revealed that CFTR in the airways of rat is expressed not only in the epithelial cells but also in tracheal smooth muscle cells leading to the hypothesis that this ionic channel could contribute to bronchodilation.

Muscarinic cationic current in gastrointestinal smooth muscles: signal transduction and role in contraction

1 The muscarinic receptor plays a key role in the parasympathetic nervous control of various peripheral tissues including gastrointestinal tract. The neurotransmitter acetylcholine, via activating muscarinic receptors that exist in smooth muscle, produces its contraction. 2 There is the opening of cationic channels as an underlying mechanism. The opening of cationic channels results in influxes of Ca2+ via the channels into the cell and also via voltage-dependent Ca2+ channels which secondarily opened in response to the depolarization, providing an amount of Ca2+ for activation of the contractile proteins. 3 Electrophysiological and pharmacological studies have shown that the cationic channels as well as muscarinic receptors exist in many visceral smooth muscle cells. However, the activation mechanisms of the cationic channels are still unclear. 4 In this article, we summarize the current knowledge of the muscarinic receptor-operated cationic channels, focusing on the receptor subtype, G protein and other signalling molecules that are involved in activation of these channels and on the molecular characteristics of the channel. This will improve strategies aimed at developing new selective pharmacological agents and understanding the activation mechanism and functions of these channels in physiological systems.

Airway smooth muscle excitation-contraction coupling and airway hyperresponsiveness

The primary complaints from patients with asthma pertain to function of airway smooth muscle (ASM) function including shortness of breath, wheezing, and coughing. Thus, it is imperative to better understand the mechanisms underlying excitation-contraction coupling in ASM. Here, we review the various signaling pathways underlying contraction in ASM, and then examine how these are altered in asthma and airway hyperresponsiveness (a hallmark feature of asthma). Throughout, we highlight how studies of vascular smooth muscle have helped or hindered progress in understanding ASM physiology and pathophysiology.

Ionic mechanisms of histamine-induced responses in guinea pig intracardiac neurons

Histamine, released from mast cells, can modulate the activity of intrinsic neurons in the guinea pig cardiac plexus. The present study examined the ionic mechanisms underlying the histamine-induced responses in these cells. Histamine evokes a small membrane depolarization and an increase in neuronal excitability. Using intracellular voltage recording from individual intracardiac neurons, we were able to demonstrate that removal of extracellular sodium reduced the membrane depolarization, whereas inhibition of K+ channels by 1 mM Ba2+, 2 mM Cs+, or 5 mM tetraethylammonium had no effect. The depolarization was also not inhibited by either 10 μM Gd3+ or a reduced Cl− solution. The histamine-induced increase in excitability was unaffected by K+ channel inhibitors; however, it was reduced by either blockage of voltage-gated Ca2+ channels with 200 μM Cd2+ or replacement of extracellular Ca2+ with Mg2+. Conversely, alterations in intracellular calcium with thapsigargin or caffeine did not inhibit the histamine-induced effects. However, in cells treated with both thapsigargin and caffeine to deplete internal calcium stores, the histamine-induced increase in excitability was decreased. Treatment with the phospholipase C inhibitor U73122 also prevented both the depolarization and the increase in excitability. From these data, we conclude that histamine, via activation of H1 receptors, activates phospholipase C, which results in 1) the opening of a nonspecific cation channel, such as a transient receptor potential channel 4 or 5; and 2) in combination with either the influx of Ca2+ through voltage-gated channels or the release of internal calcium stores leads to an increase in excitability.

Ion channels in smooth muscle: regulators of intracellular calcium and contractility

Smooth muscle (SM) is essential to all aspects of human physiology and, therefore, key to the maintenance of life. Ion channels expressed within SM cells regulate the membrane potential, intracellular Ca2+ concentration, and contractility of SM. Excitatory ion channels function to depolarize the membrane potential. These include nonselective cation channels that allow Na+ and Ca2+ to permeate into SM cells. The nonselective cation channel family includes tonically active channels (Icat), as well as channels activated by agonists, pressure-stretch, and intracellular Ca2+ store depletion. Cl--selective channels, activated by intracellular Ca2+ or stretch, also mediate SM depolarization. Plasma membrane depolarization in SM activates voltage-dependent Ca2+ channels that demonstrate a high Ca2+ selectivity and provide influx of contractile Ca2+. Ca2+ is also released from SM intracellular Ca2+ stores of the sarcoplasmic reticulum (SR) through ryanodine and inositol trisphosphate receptor Ca2+ channels. This is part of a negative feedback mechanism limiting contraction that occurs by the Ca2+-dependent activation of large-conductance K+ channels, which hyper polarize the plasma membrane. Unlike the well-defined contractile role of SR-released Ca2+ in skeletal and cardiac muscle, the literature suggests that in SM Ca2+ released from the SR functions to limit contractility. Depolarization-activated K+ chan nels, ATP-sensitive K+ channels, and inward rectifier K+ channels also hyperpolarize SM, favouring relaxation. The expression pattern, density, and biophysical properties of ion channels vary among SM types and are key determinants of electrical activity, contractility, and SM function.

The role of acetaldehyde in the neurobehavioral effects of ethanol: A comprehensive review of animal studies

Acetaldehyde has long been suggested to be involved in a number of ethanol's pharmacological and behavioral effects, such as its reinforcing, aversive, sedative, amnesic and stimulant properties. However, the role of acetaldehyde in ethanol's effects has been an extremely controversial topic during the past two decades. Opinions ranged from those virtually denying any role for acetaldehyde in ethanol's effects to those who claimed that alcoholism is in fact "acetaldehydism". Considering the possible key role of acetaldehyde in alcohol addiction, it is critical to clarify the respective functions of acetaldehyde and ethanol molecules in the pharmacological and behavioral effects of alcohol consumption. In the present paper, we review the animal studies reporting evidence that acetaldehyde is involved in the pharmacological and behavioral effects of ethanol. A number of studies demonstrated that acetaldehyde administration induces a range of behavioral effects. Other pharmacological studies indicated that acetaldehyde might be critically involved in several effects of ethanol consumption, including its reinforcing consequences. However, conflicting evidence has also been published. Furthermore, it remains to be shown whether pharmacologically relevant concentrations of acetaldehyde are achieved in the brain after alcohol consumption in order to induce significant effects. Finally, we review current evidence about the central mechanisms of action of acetaldehyde.

Distinct Calcium Channels Regulate Responses of Primary B Lymphocytes to B Cell Receptor Engagement and Mechanical Stimuli

Intracellular Ca(2+) plays a central role in controlling lymphocyte function. Nonetheless, critical gaps remain in our understanding of the mechanisms that regulate its concentration. Although Ca(2+)-release-activated calcium (CRAC) channels are the primary Ca(2+) entry pathways in T cells, additional pathways appear to be operative in B cells. Our efforts to delineate these pathways in primary murine B cells reveal that Ca(2+)-permeant nonselective cation channels (NSCCs) operate in a cooperative fashion with CRAC. Interestingly, these non-CRAC channels are selectively activated by mechanical stress, although the mechanism overlaps with BCR-activated pathways, suggesting that they may operate in concert to produce functionally diverse Ca(2+) signals. NSCCs also regulate the membrane potential, which activates integrin-dependent binding of B cells to extracellular matrix elements involved in their trafficking and localization within secondary lymphoid organs. Thus, CRAC and distinct Ca(2+) permeant NSCCs are differentially activated by the BCR and mechanical stimuli and regulate distinct aspects of B cell physiology.

Non-selective cationic channels of smooth muscle and the mammalian homologues of Drosophila TRP

Throughout the body there are smooth muscle cells controlling a myriad of tubes and reservoirs. The cells show enormous diversity and complexity compounded by a plasticity that is critical in physiology and disease. Over the past quarter of a century we have seen that smooth muscle cells contain--as part of a gamut of ion-handling mechanisms--a family of cationic channels with significant permeability to calcium, potassium and sodium. Several of these channels are sensors of calcium store depletion, G-protein-coupled receptor activation, membrane stretch, intracellular Ca2+, pH, phospholipid signals and other factors. Progress in understanding the channels has, however, been hampered by a paucity of specific pharmacological agents and difficulty in identifying the underlying genes. In this review we summarize current knowledge of these smooth muscle cationic channels and evaluate the hypothesis that the underlying genes are homologues of Drosophila TRP (transient receptor potential). Direct evidence exists for roles of TRPC1, TRPC4/5, TRPC6, TRPV2, TRPP1 and TRPP2, and more are likely to be added soon. Some of these TRP proteins respond to a multiplicity of activation signals--promiscuity of gating that could enable a variety of context-dependent functions. We would seem to be witnessing the first phase of the molecular delineation of these cationic channels, something that should prove a leap forward for strategies aimed at developing new selective pharmacological agents and understanding the activation mechanisms and functions of these channels in physiological systems.

Two types of non-selective cation channel opened by muscarinic stimulation with carbachol in bovine ciliary muscle cells

In the ciliary muscle, the tonic contraction requires a sustained influx of Ca2+ through the cell membrane. However, little has hitherto been known about the route(s) of Ca2+ influx in this tissue that lacks voltage-gated Ca2+ channels. To identify ion channels as the Ca2+ entry pathway we studied the effects of carbachol (CCh) on freshly isolated bovine ciliary muscle cells by whole-cell voltage clamp. Experiments were carried out using pipettes filled with K+ -free solution containing 100 mm caesium aspartate, 5 mm BAPTA and 180 microm GTP (pH 7.0; the intracellular free Ca2+ concentration, [Ca2+]i = 70 nm). CCh evoked an inward current showing polarity reversal at a holding potential near 0 mV. Analysis of the current noise distinguished two types of non-selective cation channel (NSCCL and NSCCS) with widely different unitary conductances (35 pS and 100 fS). The ratios of the permeabilities to Li+, Na+, Cs+, Mg2+, Ca2+, Sr2+ and Ba2+, estimated by cation replacement procedures, were 0.9: 1.0: 1.5: 0.2: 0.3: 0.4: 0.5 for NSCCL, and 1.0: 1.0: 1.8: 2.5: 2.6: 3.2: 5.0 for NSCCS. NSCCS, but not NSCCL, was strongly inhibited by elevation of [Ca2+]i. Both NSCCL and NSCCS were dose-dependently inhibited by 1-100 microm SKF96365, La3+ and Gd3+, which also inhibited the tonic component of the contraction produced in muscle bundles by CCh without markedly affecting the initial phasic component. NSCCL and/or NSCCS may serve as a major Ca2+ entry pathway required for sustained contraction of the bovine ciliary muscle. RT-PCR experiments in the bovine ciliary muscle (whole tissue) detected mRNAs of several transient receptor potential (TRP) channel homologues (TRPC1, TRPC3, TRPC4 and TRPC6), which are now regarded as possible molecular candidates for receptor-operated cation channels.

Phospholipase C, but not InsP3 or DAG, -dependent activation of the muscarinic receptor-operated cation current in guinea-pig ileal smooth muscle cells

1. In visceral smooth muscles, both M(2) and M(3) muscarinic receptor subtypes are found, and produce two major metabolic effects: adenylyl cyclase inhibition and PLCbeta activation. Thus, we studied their relevance for muscarinic cationic current (mI(CAT)) generation, which underlies cholinergic excitation. Experiments were performed on single guinea-pig ileal cells using patch-clamp recording techniques under conditions of weakly buffered [Ca(2+)](i) (either using 50 microm EGTA or 50-100 microm fluo-3 for confocal fluorescence imaging) or with [Ca(2+)](i) 'clamped' at 100 nm using 10 mm BAPTA/CaCl(2) mixture. 2. Using a cAMP-elevating agent (1 microm isoproterenol) or a membrane-permeable cAMP analog (10 microm 8-Br-cAMP), we found no evidence for mI(CAT) modulation through a cAMP/PKA pathway. 3. With low [Ca(2+)](i) buffering, the PLC blocker U-73122 at 2.5 microm almost abolished mI(CAT), in some cases without any significant effect on [Ca(2+)](i). When [Ca(2+)](i) was buffered at 100 nm, U-73122 reduced both carbachol- and GTPgammaS-induced mI(CAT) maximal conductances (IC(50)=0.5-0.6 microm) and shifted their activation curves positively. 4. U-73343, a weak PLC blocker, had no effect on GTPgammaS-induced mI(CAT), but weakly inhibited carbachol-induced current, possibly by competitively inhibiting muscarinic receptors, since the inhibition could be prevented by increasing the carbachol concentration to 1 mm. Aristolochic acid and D-609, which inhibit PLA(2) and phosphatidylcholine-specific PLC, respectively, had no or very small effects on mI(CAT), suggesting that these enzymes were not involved. 5. InsP(3) (1 microm) in the pipette or OAG (20 microm) applied externally had no effect on mI(CAT) or its inhibition by U-73122. Ca(2+) store depletion (evoked by InsP(3), or by combined cyclopiazonic acid, ryanodine and caffeine treatment) did not induce any significant current, and had no effect on mI(CAT) in response to carbachol when [Ca(2+)](i) was strongly buffered to 100 nm. 6. It is concluded that phosphatidylinositol-specific PLC modulates mI(CAT) via Ca(2+) release, but also does so independently of InsP(3), DAG, Ca(2+) store depletion or a rise of [Ca(2+)](i). Our present results explain the previously established 'permissive' role of the M(3) receptor subtype in mI(CAT) generation, and provide a new insight into the molecular mechanisms underlying the shifts of the cationic conductance activation curve.

Relationship between asynchronous Ca2+ waves and force development in intact smooth muscle bundles of the porcine trachea

Fluctuations in intracellular calcium concentration ([Ca2+]i) constitute the main link in excitation-contraction coupling (E-C coupling) in airway smooth muscle cells (ASMC). It has recently been reported that ACh induces asynchronous recurring Ca2+ waves in intact ASMC of murine bronchioles. With the use of a novel technique allowing us to simultaneously measure subcellular [Ca2+]i and force generation in ASMC located within an intact tracheal muscle bundle, we examined a similar pattern of Ca2+ signaling in the trachea. We found that application of ACh resulted in the generation of recurring intracellular Ca2+ waves progressing along the longitudinal axis of the ribbon-shaped intact ASMC. These Ca2+ waves were not synchronized between neighboring cells, and induction of wave-like [Ca2+]i oscillations was temporally associated with development of force by the tracheal muscle bundle. By comparing the concentration dependence of force generation and the parameters characterizing the [Ca2+]i oscillations, we found that the concentration-dependent increase in ACh-induced force development by the tracheal smooth muscle bundle is achieved by differential recruitment of intact ASMC to initiate Ca2+ waves and by enhancement in the frequency of [Ca2+]i oscillations and elevation of interspike [Ca2+]i once the cells are recruited. Our findings demonstrate that asynchronous recurring Ca2+ waves underlie E-C coupling in ACh-induced contraction of the intact tracheal smooth muscle bundle. Furthermore, in contrast to what was reported in enzymatically dissociated ASMC, Ca2+ influx through the L-type voltage-gated Ca2+ channel was not an obligatory requirement for the generation of [Ca2+]i oscillations and development of force in ACh-stimulated intact ASMC.

FKBP12.6 and cADPR regulation of Ca2+ release in smooth muscle cells

Intracellular Ca2+ release through ryanodine receptors (RyRs) plays important roles in smooth muscle excitation-contraction coupling, but the underlying regulatory mechanisms are poorly understood. Here we show that FK506 binding protein of 12.6 kDa (FKBP12.6) associates with and regulates type 2 RyRs (RyR2) in tracheal smooth muscle. FKBP12.6 binds to RyR2 but not other RyR or inositol 1,4,5-trisphosphate receptors, and FKBP12, known to bind to and modulate skeletal RyRs, does not associate with RyR2. When dialyzed into tracheal myocytes, cyclic ADP-ribose (cADPR) alters spontaneous Ca2+ release at lower concentrations and produces macroscopic Ca2+ release at higher concentrations; neurotransmitter-evoked Ca2+ release is also augmented by cADPR. These actions are mediated through FKBP12.6 because they are inhibited by molar excess of recombinant FKBP12.6 and are not observed in myocytes from FKBP12.6-knockout mice. We also report that force development in FKBP12.6-null mice, observed as a decrease in the concentration/tension relationship of isolated trachealis segments, is impaired. Taken together, these findings point to an important role of the FKBP12.6/RyR2 complex in stochastic (spontaneous) and receptor-mediated Ca2+ release in smooth muscle.

Phospholipase C-mediated signalling is not required for histamine-induced catecholamine secretion from bovine chromaffin cells

A possible role for signalling through phospholipase C in histamine-induced catecholamine secretion from bovine adrenal chromaffin cells has been investigated. Secretion evoked by histamine over 10 min was not prevented by inhibiting inositol-1,4,5-trisphosphate receptors with 2-APB, by blocking ryanodine receptors with a combination of ryanodine and caffeine, or by depleting intracellular Ca(2+) stores by pretreatment with thapsigargin. Inhibition of protein kinase C with Ro31-8220 also failed to reduce secretion. Inhibition of phospholipase C with ET-18-OCH(3) reduced both histamine- and K(+) -induced inositol phosphate responses by 70-80% without reducing their secretory responses. Stimulating phospholipase C with Pasteurella multocida toxin did not evoke secretion or enhance the secretory response to histamine. The secretory response to histamine was little affected by tetrodotoxin or by substituting extracellular Na(+) with N -methyl-d-glucamine(+) or choline(+), or by substituting external Cl(-) with nitrate(-). Blocking various K(+) channels with apamin, charybdotoxin, Ba(2+), tetraethylammonium, 4-aminopyridine, tertiapin or glibenclamide failed to reduce the ability of histamine to evoke secretion. These results indicate that histamine evokes secretion by a mechanism that does not require inositol-1,4,5-trisphosphate-mediated mobilization of stored Ca(2+), diacylglycerol-mediated activation of protein kinase C, or activation of phospholipase C. The results are consistent with histamine acting by depolarizing chromaffin cells through a phospholipase C-independent mechanism.

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.

The initial inositol 1,4,5-trisphosphate response induced by histamine is strongly amplified by Ca(2+) release from internal stores in smooth muscle

We have studied the Ca(2+)-dependence and wortmannin-sensitivity of the initial inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) response induced by activation of either histamine or muscarinic receptors in smooth muscle from guinea pig urinary bladder. Activation of H(1) receptors with histamine (100 microM) produced a significant elevation in Ins(1,4,5)P(3) levels with only 5s stimulation and in the presence of external Ca(2+). However, this response was abolished fully by either the prolonged absence of external Ca(2+) or the depletion of internal Ca(2+) stores with thapsigargin (100nM) or ryanodine (10 microM). In contrast, the same conditions only slightly reduced the initial Ins(1,4,5)P(3) response induced by carbachol. The prolonged incubation of smooth muscle in 10 microM wortmannin to inhibit type III PI 4-kinase abolished both the early histamine-evoked Ins(1,4,5)P(3) and Ca(2+) responses. Conversely, wortmannin did not alter Ca(2+) release induced by carbachol, despite a partial reduction of its Ins(1,4,5)P(3) response. Collectively, these data indicate that the detectable histamine-induced increase in Ins(1,4,5)P(3) is more the consequence of Ca(2+) release from internal stores than a direct activation of phospholipase C by H(1) receptors. In addition, the effect of wortmannin implies the existence of a Ca(2+)-dependent amplification loop for the histamine-induced Ins(1,4,5)P(3) response in smooth muscle.

Manipulation of chloride flux affects histamine-induced contraction in rabbit basilar artery

Cl(-) efflux induces depolarization and contraction of smooth muscle cells. This study was undertaken to explore the role of Cl(-) flux in histamine-induced contraction in the rabbit basilar artery. Male New Zealand White rabbits (n = 16) weighing 1.8-2.5 kg were euthanized by an overdose of pentobarbital sodium. The basilar arteries were removed for isometric tension recording. Histamine produced a concentration-dependent contraction that was attenuated by the H(1) receptor antagonist chlorpheniramine (10(-8) M) but not by the H(2) receptor antagonist cimetidine (3 x 10(-6) M) in normal Cl(-) Krebs-Henseleit bicarbonate solution (123 mM Cl(-)). The histamine-induced contraction was reduced by the following manipulations: 1) inhibition of Na(+)-K(+)-2Cl(-) cotransporter with bumetanide (3 x 10(-5) and 10(-4) M), 2) bicarbonate-free HEPES solution to disable Cl(-)/HCO exchanger, and 3) blockade of Cl(-) channels with the use of niflumic acid, 5-nitro-2-(3-phenylpropylamino) benzoic acid, and indoleacetic acid 94 R-(+)-methylindazone. In addition, substitution of extracellular Cl(-) (10 mM) with methanesulfonate acid (113 mM) transiently enhanced histamine-induced contraction. Manipulation of Cl(-) flux affects histamine-induced contraction in the rabbit basilar artery.

The developing relationship between receptor-operated and store-operated calcium channels in smooth muscle

Contraction of smooth muscle is initiated, and to a lesser extent maintained, by a rise in the concentration of free calcium in the cell cytoplasm ([Ca(2+)](i)). This activator calcium can originate from two intimately linked sources--the extracellular space and intracellular stores, most notably the sarcoplasmic reticulum. Smooth muscle contraction activated by excitatory neurotransmitters or hormones usually involves a combination of calcium release and calcium entry. The latter occurs through a variety of calcium permeable ion channels in the sarcolemma membrane. The best-characterized calcium entry pathway utilizes voltage-operated calcium channels (VOCCs). However, also present are several types of calcium-permeable channels which are non-voltage-gated, including the so-called receptor-operated calcium channels (ROCCs), activated by agonists acting on a range of G-protein-coupled receptors, and store-operated calcium channels (SOCCs), activated by depletion of the calcium stores within the sarcoplasmic reticulum. In this article we will review the electrophysiological, functional and pharmacological properties of ROCCs and SOCCs in smooth muscle and highlight emerging evidence that suggests that the two channel types may be closely related, being formed from proteins of the Transient Receptor Potential Channel (TRPC) family.

Mechanisms of leukotriene D4-induced constriction in human small bronchioles

We examined the mechanisms underlying leukotriene D4- (LTD4) induced constriction of human small (300 - 500 micron i.d.) bronchioles, and the effect of LTD4 on ion currents and Ca2+ transients in smooth muscle cells (SMC) isolated from these bronchioles. LTD4 caused a concentration-dependent bronchoconstriction with an EC50=0.58+/-0.05 nM (n=7) which was not easily reversible upon washout. This bronchoconstriction was entirely dependent on extracellular Ca2+. Blockade of L-type Ca2+ channels with nifedipine (10 microM) reduced LTD4 response by 39+/-2% (n=8), whilst La3+, Gd3+ and SK&F 96,365 abolished LTD4-induced bronchoconstriction completely and reversibly, suggesting the majority of Ca2+ entry was via non-selective cation channels. Antagonists of PI-PLC (U73,122 and ET-18-OCH3), PLD (propranolol) and PKC (cheleretrine and Ro31-8220) were without any effect on LTD4-induced bronchoconstriction, whilst the PC-PLC inhibitor D609 caused complete relaxation. Inhibition of protein tyrosine kinase with tyrphostin A23 (100 microM) caused about 50% relaxation, although the inactive analogue tyrphostin A1 was without effect. In freshly isolated SMC from human small bronchioles LTD4 caused a slow increase of intracellular Ca2+ concentration, with a consequent rise of the activity of large conductance Ca2+-dependent K+ channels and the amplitude of depolarization-induced outward whole-cell current. Again, no effect of LTD4 could be observed in the absence of extracellular Ca2+. We conclude that LTD4 causes constriction of these small bronchioles primarily by activating Ca2+ entry via non-voltage gated channels, possibly by a PC-PLC mediated pathway.

Differential expression and alternative splicing of TRP channel genes in smooth muscles

Nonselective cation channels (NSCC) are targets of excitatory agonists in smooth muscle, representing the nonselective cation current I(cat). Na(+) influx through NSCC causes depolarizations and activates voltage-dependent Ca(2+) channels, resulting in contraction. The molecular identity of I(cat) in smooth muscle has not been elucidated; however, products of the transient receptor potential (TRP) genes have characteristics similar to native I(cat). We have determined the levels of TRP transcriptional expression in several murine and canine gastrointestinal and vascular smooth muscles and have analyzed the alternative processing of these transcripts. Of the seven TRP gene family members, transcripts for TRP4, TRP6, and TRP7 were detected in all murine and canine smooth muscle cell preparations. TRP3 was detected only in canine renal artery smooth muscle cells. The full-length cDNAs for TRP4, TRP6, and TRP7, as well as one splice variant of TRP4 and two splice variants of TRP7, were cloned from murine colonic smooth muscle. Quantitative RT-PCR determined the relative amounts of TRP4, TRP6, and TRP7 transcripts, as well as that of the splice variants, in several murine smooth muscles. TRP4 is the most highly expressed, while TRP6 and TRP7 are expressed at a lower level in the same tissues. Splice variants for TRP7, deleted for exons encoding amino acids including transmembrane segment S1, predominated in murine smooth muscles, while the full-length form of the transcript was expressed in canine smooth muscles.

Muscarinic stimulation increases basal Ca(2+) and inhibits spontaneous Ca(2+) transients in murine colonic myocytes

Localized Ca(2+) transients in isolated murine colonic myocytes depend on Ca(2+) release from inositol 1,4,5-trisphosphate (IP(3)) receptors. Localized Ca(2+) transients couple to spontaneous transient outward currents (STOCs) and mediate hyperpolarization responses in these cells. We used confocal microscopy and whole cell patch-clamp recording to investigate how muscarinic stimulation, which causes formation of IP(3), can suppress Ca(2+) transients and STOCs that might override the excitatory nature of cholinergic responses. ACh (10 microM) reduced localized Ca(2+) transients and STOCs, and these effects were associated with a rise in basal cytosolic Ca(2+). These effects of ACh were mimicked by generalized rises in basal Ca(2+) caused by ionomycin (250-500 nM) or elevated external Ca(2+) (6 mM). Atropine (10 microM) abolished the effects of ACh. Pretreatment of cells with nicardipine (1 microM), or Cd(2+) (200 microM) had no effect on responses to ACh. An inhibitor of phospholipase C, U-73122, blocked Ca(2+) transients and STOCs but did not affect the increase in basal Ca(2+) after ACh stimulation. Xestospongin C (Xe-C; 5 microM), a membrane-permeable antagonist of IP(3) receptors, blocked spontaneous Ca(2+) transients but did not prevent the increase of basal Ca(2+) in response to ACh. Gd(3+) (10 microM), a nonselective cation channel inhibitor, prevented the increase in basal Ca(2+) after ACh and increased the frequency and amplitude of Ca(2+) transients and waves. Another inhibitor of receptor-mediated Ca(2+) influx channels, SKF-96365, also prevented the rise in basal Ca(2+) after ACh and increased Ca(2+) transients and development of Ca(2+) waves. FK-506, an inhibitor of FKBP12/IP(3) receptor interactions, had no effect on the rise in basal Ca(2+) but blocked the inhibitory effects of increased basal Ca(2+) and ACh on Ca(2+) transients. These results suggest that the rise in basal Ca(2+) that accompanies muscarinic stimulation of colonic muscles inhibits localized Ca(2+) transients that could couple to activation of Ca(2+)-activated K(+) channels and reduce the excitatory effects of ACh.