G protein coupled receptors (GPCRs) comprise a superfamily of proteins capable of transducing a wide range of extracellular signals across the plasma membrane of the cell into discrete intracellular messages capable of regulating numerous, diverse cell functions. Over 800 GPCRs have been cloned to date and over 1000 are suspected in the human genome 1. The majority of all prescribed drugs target either activation of GPCRs or their downstream signals. This holds true for drugs used in the management of airway diseases such as asthma; it is generally accepted that GPCRs on airway smooth muscle (ASM) are the direct targets of the majority of anti-asthma drugs.

Until recently most research efforts examining GPCR expression, function, and regulation in ASM have focused on those receptors capable of dynamic regulation of ASM contractile state and consequently, airway resistance. However, the growing appreciation of ASM as a pleiotropic cell capable of regulating airway resistance via "synthetic functions" has provided a much wider context in which to consider the relevance of numerous ASM GPCRs. GPCRs whose activation has little or no direct impact on contractile state may instead modulate ASM growth or the secretion of various cytokines, chemokines, eicosanoids, or growth factors that orchestrate airway inflammation through actions on both mesenchymal and infiltrating cells. These effects may ultimately influence airway resistance by: 1) promoting airway remodeling that impacts the mechanics of ASM contraction in vivo; or 2) regulating the inflammatory response to either disrupt the balance of local pro-contractile/relaxant molecules or alter electro- or pharmaco-mechanical coupling in ASM. Accordingly, it is no longer permissible to judge the relevance of a given ASM GPCR based on its ability to dynamically modulate ASM contractile state and airway resistance. Indeed, our newfound appreciation of multiple experimental endpoints defining ASM function has aided efforts to identify relevant ASM GPCRs and their signaling properties.

In this review we will summarize the signaling and functional effects of various GPCRs that have been identified in ASM cells. In addition, we will consider how the regulation (or dysregulation) of GPCR signaling potentially impacts asthma pathogenesis and treatment.

Models for analyzing GPCR signaling in ASM run the spectrum of integrative to reductionist approaches, each having certain advantages and disadvantages. Integrative in vivo models in which GPCR ligands are administered systemically or through inhalation can suggest the presence of ASM GPCRs capable of mediating bronchoconstrictive or relaxant effects. Such experiments can provide important insight into the role of a given GPCR in regulating lung resistance, and suggest the utility of targeting a receptor in order to control bronchospasm. However, the direct target cell of delivered agents is often unclear, and frequently the response of ASM is secondary to actions on other cell types. For example, inhaled agents can provoke the release of bronchoreactive substances from multiple cell types that in turn engage ASM GPCRs, or regulate autonomic control of ASM contraction through actions on pre- or post-ganglionic neurons or reflex arcs 234.

A more controlled environment in which to characterize ASM GPCRs is provided by ex vivo analyses of tracheal or bronchial smooth muscle isolated as strips or as part of a complex including cartilaginous ring. This approach reduces, but does not eliminate, neural or paracrine effects on ASM that can dominate functional ASM responses in vivo. Such effects can persist because preparations still include autonomic effector and sensory nerve fiber endings, epithelium, fibroblasts, and blood cells capable of releasing constricting/relaxing agents in response to exogenous agents or, possibly, mechanical forces 5. Consequently, intelligent design of such ex vivo analyses can help clarify the in vivo effects of numerous agents and identify their target cells. For example, immunohistochemical analysis and tissue bath mechanics of excised ASM strips suggest that the pronounced bronchoconstriction elicited by inhaled adenosine or adenosine monophosphate in asthmatic subjects or sensitized animals can be attributed primarily to histamine release from mast cells in close proximity to or imbedded in ASM tissue 67891011.

Arguably, the development of ASM cell cultures has provided the most reliable system for identifying and characterizing ASM GPCRs. Typically generated by enzymatic dissociation of ASM cells from sections of tracheae or bronchi, ASM cultures provide a pure population of ASM cells that can be greatly expanded, and thus are amenable to extensive pharmacological, biochemical, and molecular analyses not possible in vivo or with tissues 1213. Cells of ASM cultures of several species (including human, canine, bovine, guinea pig, and mouse) have been shown to be morphologically and functionally similar to ASM in vivo; they stain for smooth muscle-alpha-actin and myosin heavy chain, and exhibit signaling and functional responses that are consistent with ASM function observed or suspected in vivo 12131415.

The power of ASM cultures as an experimental model capable of verifying existing and identifying new signaling paradigms, while also establishing their physiologic relevance, is under-appreciated. This power is largely attributed to the fact that ASM cells possess physiologic levels of most signaling components (e.g., receptors, effectors, and downstream signaling intermediates), yet many signaling pathways are readily characterized with robust signal to noise ratios. Most importantly, numerous ASM cell functions (including growth, synthesis/secretion of autocrine/paracrine factors, and to a limited extent, contraction) are also easily quantified and can be linked to their associated signaling events. In many other cell culture systems such linkage of signaling to relevant cell function cannot be achieved. For example, the majority of studies revealing novel receptor-mediated signaling paradigms have utilized expression systems such as COS or HEK293 cells to express recombinant receptors or signaling components in order to delineate pathway interactions and their modes of regulation. It is unclear whether such paradigms occur under relevant conditions in which most signaling components are expressed at low levels and their actions may be constrained by compartmentalization 1617. Moreover, whether such signaling has any relevance to cell function is unclear, because such cells typically either lack discrete measurable functions or their functions are known to be dysregulated (e.g., physiologic regulation of growth cannot be studied in a transformed cell). Recent studies 181920 have begun testing the applicability and physiologic relevance of various GPCR signaling paradigms in cultured ASM cells.

However, ASM cultures as a model system are far from perfect. That ASM cells in culture lack the context of the in vivo condition is not only a strength but also an inherent limitation of this reductionist model. Moreover, like most primary cells grown in culture, ASM cells undergo a degree of de-differentiation that coincides with a loss or increase in various signaling elements and functional apparatus 3. Specific changes in ASM cells relevant to GPCR signaling that are known to occur in culture include a rapid and progressive decrease in the expression of Gq-coupled receptors such as the m3 muscarinic acetylcholine receptor (m3 mAChR) 21 and the cysteinyl leukotriene type 1 receptor (CLT1R; Stuart Hirst, personal communication). In addition, contractile function of cultured ASM cells is rapidly diminished, coinciding with reduced expression of smooth muscle alpha-actin and myosin heavy chain, calponin, h-caldesmon, beta-tropomyosin, and myosin light chain kinase (MLCK) 22. However, Shore, Fredberg, and colleagues have developed a model for examining agonist-induced changes in stiffness of cultured ASM cells that has provided useful information linking regulation of GPCR signaling with ASM contractile state 23. Interestingly, Stephens 24, Halayko, Solway 252627, and colleagues have demonstrated that prolonged serum starvation of cultured canine ASM cells can beget a subpopulation of cells that reacquire high m3 mAChR and contractile/cytoskeletal protein expression and thus contractile function. These findings suggest a potentially powerful strategy for delineating elements critical to Gq-coupled receptor signaling and pharmaco-mechanical coupling in ASM.

Although numerous GPCRs have the ability to couple to more than one heterotrimeric G protein, a given GPCR is typically classified based on the G protein subfamily (e.g., Gs, Gi/o, or Gq/11) it preferentially activates. A diagram of Gq-coupled receptor signaling, and the associated functional outcomes in ASM, is provided in Fig. 1. Signaling via Gq-coupled receptors in ASM is of particular interest due to its prominent role in promoting ASM contraction. Transmembrane signaling occurs in the classical GPCR-G protein-effector protein paradigm. An agonist-bound receptor undergoes a conformational change that promotes its association with and activation of the heterotrimeric G protein Gq. The extreme C-terminus of the G alpha subunit is the receptor recognition domain and dictates receptor-Gα specificity. Receptor-Gα association promotes the release of GDP from Gα and binding of GTP. The active GTP-bound Gα dissociates from Gβγ and in turn activates an effector molecule. The Gβγ heterodimer (numerous combinations of 7 different β and 12 different γ subunits exist) also has the capacity to regulate the activity of various effectors and numerous other signaling elements (discussed below). The duration of one cycle of receptor activation of effector is dictated by the GTPase activity of Gα, as the hydrolysis of GTP to GDP promotes reconstitution and membrane localization of the Gαβγ trimer. Traditionally, alpha subunit GTPase activity was presumed "intrinsic", but it is now appreciated that this activity can be regulated by GTPase proteins (GAPs) in a manner similar to that demonstrated for small G proteins 28. Phospholipase C (PLC) is the principal effector of Gq-mediated signaling. Eleven different isoforms of PLC exist and exhibit distinct patterns of regulation; members of the PLCβ subfamily tend to mediate the actions of activated Gq 29. Activated PLC hydrolyzes phosphoinositol 4,5-bisphosphate (PIP₂) into 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). The net effect of increased IP₃ and DAG levels is to increase intracellular Ca²⁺ through release from internal stores and influx from membrane-bound channels 3, and in ASM to activate the cell's contractile machinery through both Ca²⁺ and protein kinase C (PKC) -dependent mechanisms 30313233 (see Fig. 1 Legend for details).

Studies of agonist-induced increases in airway resistance, smooth muscle contraction ex vivo, and receptor binding and second messenger analyses of cultured ASM cells have helped identify numerous Gq-coupled receptors in ASM (Table 1). Resting ASM tone in vivo is determined primarily by parasympathetic cholinergic innervation acting on ASM m3 mAChRs. Other ASM Gq-coupled receptors capable of inducing significant ASM contraction (in vivo or ex vivo) include the H1 histamine receptor, CLT1R, B2 bradykinin receptors, and ET-A endothelin receptor. Additional Gq-coupled receptors such as the A3 adenosine, NK-1, NK-2 (Neurokinin-1 and -2) and P2 purinergic have been identified, but their importance in mediating contraction under physiologic or pathologic conditions is unclear. In some cases the evidence for their expression in ASM is either indirect or is difficult to interpret given the labile nature of Gq-coupled receptor expression in ASM cultures.

However, as noted above ASM cells do more than contract and studies of other functional outcomes in ASM suggest a potentially important role for numerous Gq-coupled receptors in modulating ASM synthetic functions. Both thrombin (capable of activating Gq through protease-activated receptors (PARs) 34) and lysophosphatidic acid (LPA) (capable of activating Gq through endothelium differentiation gene (EDG) receptors) are strong stimulators of cultured ASM DNA synthesis and cell proliferation. These effects appear in part Gq-dependent (Billington and Penn, unpublished observations) and may be mediated by the capacity of Gq signaling to stimulate the p42/p44 MAPK (via PKC-mediated phosphorylation of Raf-1) and p70S6K pathways and therefore induce promitogenic transcription factor activation, cyclin D1 induction, and upregulate the translational machinery necessary for cell cycle progression 3637. Moreover, numerous Gq-coupled receptor agonists including thrombin, lysophosphatidic acid, leukotriene D4 (LTD4), endothelin, histamine, thromboxane (activating Thromboxane A2 / Prostaglandin (TP) receptors)19, and sphingosine-1-phosphate (SPP) (activating EDG receptors) have been shown to potentiate the mitogenic effects of receptor tyrosine kinase signaling, although it has not been established that Gq activation per se mediates this effect.

Gq-dependent activation of PKC and p42/p44 also promotes phosphorylation and activation of phospholipase A2 (PLA2), which contributes to rapid eicosanoid synthesis in ASM cells stimulated with bradykinin (acting on B2 bradykinin receptors)39. Other effects reported to involve Gq activation by ASM GPCRs include actin polymerization induced by LPA, endothelin, or carbachol, which appears to occur via a Rho-dependent mechanism 40. This suggests that effectors other than PLC can be directly activated by Gq in ASM.

Whereas Gq-coupled receptors are the principal mediators of ASM contraction, Gs-coupled receptors on ASM play a central role in promoting relaxation of contracted ASM and in conferring prophylactic "bronchoprotection". Inhaled beta-agonists, which activate the Gs-coupled beta-2-adrenergic receptor (β₂AR) on ASM, are the most widely used agents in asthma therapy and are universally recognized as the treatment of choice for acute asthma attacks. Several other Gs-coupled receptors, including the E-Prostanoid 2 (EP2) prostaglandin E2 (PGE₂) 20, IP prostacyclin (41 and Pascual and Penn, unpublished observations), A2b adenosine 42, and vasoactive intestinal peptide (VIP)43 receptors have been identified in ASM and represent intriguing, albeit elusive, therapeutic targets (Table 1).

Gs-coupled receptor signaling and its regulation have been extensively characterized in numerous cells types, including ASM 44. The overwhelming majority of studies delineating the basic tenets of Gs-coupled receptor signaling have examined β₂AR signaling, based on the prevalence of endogenously expressed β₂ARs, the established relevance of β₂ARs in the function of several organ systems, the existence of highly selective β₂AR ligands, and the early cloning of the β₂AR enabling heterologous expression of the receptor in various cell systems. Figure 2 depicts the most prominent features of Gs-coupled receptor signaling and functional consequences in ASM cells.

Adenylyl cyclase (AC) is the principal effector of Gs-coupled receptor transmembrane signaling. Nine isoforms (type I through IX) of AC are known to exist 45. RT-PCR has identified transcripts of all AC subtypes except III and VIII in human ASM cultures, although immunoblot analysis suggests the presence of only V/VI (existing antibodies do not distinguish between type V and VI), and analyses of AC regulation in human ASM cultures (discussed below) are consistent with the expression of AC V and VI 4647. Interestingly, AC subtype expression in ASM cultures may be species specific, as regulatory features of AC in bovine, canine, and guinea pig ASM suggest prominent expression of AC II 48495051, whereas a minimal 46 or no 47 level of AC II transcripts were detected in human ASM (see below).

Adenylyl cyclase isoforms are subject to multiple forms of regulation (discussed below), although dynamic activation of AC under physiologic conditions occurs almost exclusively by interaction with Gαs 52. Gαs activation of AC catalyzes ATP to cyclic AMP (cAMP), which in turn binds to the regulatory subunits of the cAMP-dependent protein kinase (protein kinase A or PKA). The cAMP-bound regulatory subunits then dissociate from and thereby activate the catalytic subunits of the enzyme, which in turn phosphorylate and regulate the activity of numerous proteins, including the transcription factor CREB. PKA activity is presumed responsible for the majority of cellular actions elicited by Gs-coupled receptor activation, which in ASM include relaxation, altered transcription of numerous genes that impact airway inflammation and remodeling 5354, inhibition of cell growth, and ion channel gating 3. However, cAMP/PKA -independent signaling by Gs-coupled receptors has also been proposed and may have important functional consequences in ASM. These include beta-agonist-induced actin depolymerization 55, direct activation of Ca²⁺-sensitive K⁺ channels by Gαs subunits 56, and possibly other ill-defined signaling events that promote relaxation and are unaffected by exposure of ASM to pharmacological inhibitors of PKA 57.

The majority of known GPCRs preferentially couple to members of the Gi family, and Gi appears to be the most abundantly expressed heterotrimeric G protein in most cell types. Members of the Gαi family expressed in ASM include Gαi-1, Gαi-2, and Gαi-3 5859. Gαi activation is typically associated with inhibition of Gαs-stimulated AC activity (for certain AC isoforms) and thus reduced cAMP generation, the functional consequences of which should be predictable but are often difficult to identify in a wide range of experimental models 260. However, numerous other signaling events elicited by Gαi activation, with clear functional consequences, have recently been identified (Fig. 3). Gi appears capable of activating Rho through activation of Rho guanine nucleotide exchange factors (GEFs), and in ASM this can mediate both actin polymerization and possibly contractile sensitization 4061. Whether Gi activation of Rho is mediated by α or βγ subunits is unclear. βγ subunits released due to Gi activation are believed to promote many of the βγ effects identified to date, perhaps reflecting the relatively high levels of Gi in most cells that could provide the levels of free βγ required for its signaling effect 52. In in vitro systems βγ subunits have been shown to enhance the activity of selected AC isoforms stimulated by Gαs. Moreover, βγ may also mediate, through what may be an indirect mechanism 62, the AC sensitization observed in neuronal cells chronically exposed to opioids (contributing to tolerance to morphine 636465) and in human ASM cells chronically treated with carbachol and other ligands capable of activating Gi-coupled receptors 46. The purpose of such AC sensitization in ASM is unclear, but may involve the need to maintain a degree of Gs-coupled receptor signaling in the face of persistent Gi-coupled receptor activation.

A role for Gi-coupled receptors in modulating growth in ASM is suggested by studies that demonstrate that pertussis toxin (which ADP-ribosylates and inhibits Gαi) partially inhibits ASM DNA synthesis stimulated by numerous GPCR ligands including carbachol (activating the m2 mAChR), LPA, SPP, endothelin, and thrombin 1838. The mechanism mediating Gi-stimulated growth of ASM is unclear, although actions of both α and βγ subunits may be involved. Gβγ has the potential to stimulate p42/p44 MAPK via activation of PLC and PKC, and can also mediate p42/p44 activation through Src-dependent transactivation of the epidermal growth factor (EGF) receptor 66. However, none of these mechanisms has been established in ASM. On the contrary, transactivation of the EGF receptor is not induced by thrombin, carbachol, or LPA in human ASM cultures, and increased p42/p44 MAPK signaling does not appear to mediate the synergistic effect of several GPCR agonists on EGF-stimulated ASM growth 1819. These latter findings suggest potentially novel mitogenic signaling events and define cooperativity between GPCRs and receptor tyrosine kinases in mediating ASM growth.

Signaling via activation of the G_12/13 family has not been characterized as extensively as has that by other heterotrimeric G proteins. The effector molecules that interact directly with G₁₂ and G₁₃ are not well established, with the exception of members of a family of guanine nucleotide exchange factors for the small G protein Rho 67. The GPCRs capable of activating G₁₂ or G₁₃ are also unclear. Immunoblot analysis demonstrates Gα₁₂ and Gα₁₃ protein in rat bronchial smooth muscle tissue, and levels are elevated by repeated antigen challenge (see below)68. In ASM cells, those GPCRs activating G_12/13 have not been characterized, although SPP/LPA-activated EDG receptors, thrombin-activated PAR receptors, and TP receptors are candidates. The profound effect of inhibitors of Rho and Rho kinase on GPCR-mediated changes in contractile sensitization 6970 and actin polymerization 40 strongly suggest a physiologic role for G_12/13 signaling in ASM.

Signaling by GPCRs is a highly regulated process. One critical way in which a cell controls its response to extracellular GPCR ligands is through regulation of the expression and activity of each component of the GPCR-G protein-effector pathway. Either a loss (desensitization) or increase (sensitization) in responsiveness of transmembrane signaling components can be evoked to presumably preserve the cell/organism from excessive signals or ensure detection and reaction to infrequent or minimal signals. In ASM, studies of regulation of GPCR signaling have focused on changes that occur in receptor and G protein expression and second messenger generation in cells, or on altered contractile/relaxant effects on ASM in vivo or ex vivo. No studies to date have considered the effect of desensitization or sensitization of GPCR signaling on GPCR-mediated functions in ASM other than contraction.

Changes in airway structure and ASM contractile state are the principal causes of increased airway resistance in asthma. Altered airway composition and architecture affect airway resistance through mechanisms that are both independent of and complimentary to changes in ASM contractile state. Excessive mucous production and edema are physical impediments to conductance, whereas edema and increased ASM mass alter airway geometry to amplify the effect of ASM contraction on airway lumen diameter 148149150151152153154.

ASM contractile state can be viewed as a function of: 1) the net sum of GPCR-mediated signals that result in establishing the level of the key contractile signaling molecule, calcium; and 2) the response of the cell's contractile machinery to calcium. Figure 5 offers a model that proposes 3 levels at which regulation of GPCR-mediated ASM contraction is altered in asthma.

To further complicate the relationship between GPCR responsiveness and asthma, evidence suggests that both glucocorticoids and beta-agonists, the two most widely used drugs in the treatment of asthma, also regulate GPCR responsiveness, primarily via changes in receptor expression and coupling. Glucocorticoids have been shown to up-regulate β₂AR and Gαs expression 8991199, counteract the β₂AR down-regulation induced by beta-agonist 92, and reverse increases in GRK activity and β₂AR desensitization induced in a rat model of airway inflammation 58. Conversely, glucocorticoids inhibit expression of NK2 receptors in bovine ASM 94, inhibit m2 mAChR expression in the airway 200, and inhibit the IL-1β-mediated up-regulation of B2 bradykinin (BK) receptors in the airway 201. Pretreatment of human ASM cells with glucocorticoids significantly inhibits histamine-stimulated phosphoinositide production 202. Thus the sum of effects of glucocorticoids on GPCR signal transduction components tends to render ASM less responsive to procontractile stimuli and more responsiveness to beta-agonists.

Beta-agonist therapy, on the other hand, tends to promote the sensitization of procontractile GPCR signaling and desensitization of prorelaxant GPCR signaling, with uncertain clinical relevance. Although conflicting data exist as to whether beta-agonist therapy exacerbates bronchial hyperresponsiveness in asthmatics, Mak and colleagues have recently demonstrated that exposure of ASM ex vivo to beta-agonist up-regulates both NK2 95 and H1 histamine 90 receptors, suggesting a mechanism whereby enhanced procontractile GPCR signaling promotes bronchial hyperresponsiveness.

Numerous studies have also demonstrated that repeated use of inhaled beta-agonists results in a loss of the prophylactic bronchoprotection conferred by beta-agonists 203204205206. In many respects this could be considered a normal and predictable response, consistent with a physiologic/teleologic role of β₂AR desensitization and the demonstration of homologous desensitization of ASM β₂ARs in multiple in vivo, ex vivo, and in vitro models. However, Finney et al. recently observed that lung GRK2 levels were elevated in rats chronically treated with beta-agonists 110. Thus, in a manner similar to that invoked by IL-1β (see above), chronic beta-agonist therapy may up regulate the GPCR desensitization "machinery" to further limit the effect of therapy and possibly exacerbate disease.

Although the clinical relevance of the observed loss of bronchoprotection has been questioned 207, the collective evidence suggests that homologous β₂AR desensitization does occur as a consequence of beta-agonist therapy. Accordingly, therapies that minimize or counteract β₂AR desensitization, such as glucocorticoids and salmeterol, may benefit from this property. Glucocorticoids preserve or enhance β₂AR function in the airway through both their anti-inflammatory actions as well as their direct effects on ASM β₂AR expression and regulation noted above. These effects may explain in part the positive cooperativity exhibited by combined beta-agonist and glucocorticoids therapy. As a low intrinsic activity beta agonist, salmeterol has limited capacity to promote homologous β₂AR desensitization in in vitro models 208209; this property in addition to its lipophilic nature appears largely responsible for its long-lasting effect. Moreover, daily salmeterol treatment has little effect on the rescue or prophylactic ability of albuterol 203210.

Within the last decade the field of GPCR signaling has experienced an epiphany with the realization that GPCRs do more than subserve restricted functions in fully differentiated cells; they also play important roles in mediating diverse cell functions such as embryogenesis, tissue regeneration, and cell proliferation 211212. Interestingly, this realization coincided with a similarly profound discovery in the field of asthma research – that ASM not only contracts, but also performs numerous "synthetic" functions that modulate both airway structure and airway inflammation. Not surprisingly, ASM GPCRs are important regulators of many ASM synthetic functions.

The newfound respective focuses of GPCR signaling and ASM research suggest an exciting direction for the study of GPCRs in ASM over the next decade. The current challenge (or curse) confronting the student of ASM signal transduction extends beyond defining the myriad intracellular signaling pathways, their regulation, and their degree of "cross-talk" with each other, to understanding how these events occur within an equally complex, dynamic airway environment.

Such an understanding should not only greatly improve our knowledge of asthma pathogenesis, but also redefine asthma therapy. With the possible exception of steroids, asthma drugs have been developed and prescribed to prevent or reverse acute bronchospasm with little consideration of their effects on ASM synthetic functions and the chronic nature of asthma. As the roles of airway remodeling and ASM synthetic functions in asthma pathogenesis become more clearly established, agents that target the activation or signaling of various GPCRs that mediate these phenomena will undoubtedly receive greater consideration as prophylactic and therapeutic asthma drugs.

AA = arachadonic acid

AC = adenylyl cyclase

ASM = airway smooth muscle

BK = bradykinin

β₂AR = beta-2-adrenergic receptor

CLT1R = cysteinyl leukotriene type I receptor

COX2 = cyclo-oxygenase-2

DAG = 1,2-diacylglycerol

EDG = endothelium differentiation gene

EGF = epidermal growth factor

GAP = GTPase protein

GEF = Guanine nucleotide exchange factor

GPCR = G protein-coupled receptor

GRK = G protein-coupled receptor kinase

IL-1β = interleukin-1β

IP₃ = inositol 1,4,5-trisphosphate

LPA = lysophosphatidic acid

LPS = lipopolysaccharide

LTD4 = leukotriene D4

mAChR = muscarinic acetylcholine receptor

MLC = myosin light chain

MLCK = myosin light chain kinase

MLCP = myosin light chain phosphatase

NK = neurokinin

PAR = protease-activated receptor

PDGF = platelet derived growth factor

PGE₂ = prostaglandin E₂

PI = phosphoinositide

PIP2 = phosphoinositol 4,5-bisphosphate

PKA = protein kinase A

PKC = protein kinase C

PLA2 = phospholipase A2

PLC = phospholipase C

RGS = regulators of G protein signaling

SNP = single nucleotide polymorphism

SSP = sphingosine-1-phosphate

TGF-β = transforming growth factor beta

TNF-α = tumor necrosis factor alpha

TP = thromboxane A2 / prostaglandin

VIP = vasoactive intestinal peptide