caffeine and theophylline are two methylxanthines that have been used to treat asthma , although their exact mechanisms of action are still unknown [ 1 , 2 ] . the bronchodilating property of theophylline is largely attributed to inhibition of phosphodiesterase ( pde ) activity or the release of catecholamines , thereby increasing camp in airway smooth muscle . in use since the 1930s , lack of specificity , a narrow therapeutic window , and negative side effects , plus the development of inhaled glucocorticoids , and short- and long - acting 2-agonists have decreased the clinical utility of theophylline [ 1 , 4 , 5 ] . however , the identification of airway - specific pde4 subtypes and subsequent development of pde4-selective inhibitors have resurrected this avenue as novel bronchodilators . the newest pde4 inhibitor approved for use in respiratory disease is roflumilast ( daxas ) [ 6 , 7 ] . in patients with copd , roflumilast has shown significant improvement of fev1 when taken in conjunction with long - acting 2-agonists or muscarinic antagonists . in preclinical studies , administration of selective pde4 inhibitors prevented bronchial hyperresponsiveness ( bhr ) in allergic mice , an in vivo model for asthma [ 9 , 10 ] . although no pde4-selective inhibitor is currently approved for the treatment of asthma in the united states , similar studies showed that administration of pde4 inhibitors attenuated bhr in patients with allergic asthma [ 1113 ] . with improved specificity for pde4 subtypes , pde4-selective inhibitors ( roflumilast , cdp840 , mk-0359 ) , have decreased side effects compared to theophylline although the therapeutic window is still narrow and still produces systemic effects due to oral administration . development of inhaled pde4-selective inhibitors has shown potential in reducing early- and late - phase asthmatic responses in mild allergic asthmatics . whether or not these improvements in fev1 are due to bronchodilation of airway smooth muscle or anti - inflammatory effects are yet to be determined ; however , inhalation will likely reduce plasma levels of drug and decrease side effects associated with oral delivery . further work on developing pde4 subtype - specific inhibitors ( a  d ) or combining various pde isoform inhibitors ( i.e. , pde1 , 3 , 7 with pde4 inhibitors )   may increase the efficacy of targeting this signaling pathway in treating asthma , providing a new application for a longstanding bronchodilator . of note , one pde4-selective inhibitor , quercetin , is a naturally occurring flavonol found in fruits , vegetables , and tea leaves . retrospective studies have shown increasing numbers of asthmatics self - treat their symptoms with herbal remedies [ 15 , 16 ] . in many cases , the exact mechanisms of action of these natural botanicals are unknown ; however recent work has focused on identifying the active constituents of herbal remedies and elucidating the signaling pathways involved in acute bronchodilation . given the advances in pde inhibition and the natural origin of many methylxanthines , many of these natural phytotherapies may possess pde inhibitory action .   recently , natural plant products have received accolades for the treatment of cough , respiratory infection , and bronchospasm . it is estimated that 10%42% of asthmatics use herbal therapies to self - treat their asthma symptoms [ 16 , 18 ] ; however the efficacy and safety of most herbal therapies have not been scientifically evaluated . the exact mechanism of action of most of these agents is unclear but may involve direct effects on airway smooth muscle , airway epithelium , airway nerves , inflammatory cytokines , and immune cells . moreover , the formulations of these herbal compounds are made up of many individual bioactive compounds . as such , it is important to define both the positive and potential negative impacts of these individual compounds on the airway as well as explore the interaction of herbal therapies with existing asthma therapies ( corticosteroids and -agonists ) . extensive preclinical , animal , and human studies have demonstrated that antiasthma simplified herbal medicine intervention ( ashmi ) , an extract of 3 plants ganoderma lucidum ( ling - zhi ) , sophora flavescens ( ku - shen ) and glycyrrhiza uralensis ( gan - cao ) , reduces lung inflammation , airway remodeling , and airway smooth muscle hyperresponsiveness [ 2022 ] . a blinded randomized trial in 91 subjects with moderate to severe allergic asthma demonstrated that 4 weeks of oral ashmi were nearly equivalent to oral prednisone in the improvement in fev1 , peak flows , serum ige levels , and eosinophilia . these clinical studies were followed by a series of pre - clinical studies that sought to identify the mechanism(s ) involved in the improvement of symptom and inflammatory profiles . both chronic and acute beneficial effects of ashmi were demonstrated on mouse lung inflammation and responsiveness . six weeks of oral administration of ashmi reduced inflammation and in vivo responses to acetylcholine [ 20 , 22 , 24 ] . acute treatment of isolated tracheal rings with ashmi from nave or ovalbumin sensitized mice demonstrated reduced acetylcholine - induced contractions in ex vivo organ bath experiments . a possible mechanism for these acute effects was elucidated in human airway smooth muscle cells that liberated prostaglandins in response to ashmi , which could mediate relaxation through activation of gs - coupled ep2 or ep4 receptors . current research is focused on identifying the specific purified chemical constituents of ashmi that mediate these chronic anti - inflammatory effects and acute airway smooth muscle relaxant effects . although pge2 relaxes airway smooth muscle in many species and benefits of inhaled pge2 have been shown in asthmatics , a specific agonist for the ep2 receptor failed to show benefit in human trials . however , newer studies suggest that targeting the ep4 receptor in human airway smooth muscle may be an alternative therapeutic target in patients with asthma . another potential therapeutic target in the treatment of bronchoconstrictive disease involves the bitter taste receptor family ( tas2r ) . recently , both qrt - pcr analysis and immunofluorescence microscopy of human airway smooth muscle ( asm ) cells revealed robust expression of several members of this g - protein - coupled receptor family ( tasr-10 , -14 , and -31 ) and showed increases in intracellular calcium ( [ ca]i ) in response to subsequent exposure to bitter tastants , the agonists to these receptor subtypes . despite increasing asm [ ca]i via the same pathway ( g plc ip3r ) shared by the classical contractile agonist acetylcholine , this group paradoxically found activation of tas2r in asm leads to a profound degree of asm bronchodilation in both isolated asm preparations as well as in vivo models of induced airway responsiveness . interestingly , the magnitude of bronchodilation achieved by high - dose tas2r agonists in many of these studies rivaled maximal -agonist treatments and mechanistically was found to be camp- and pkc - independent . this group has recently extended this observation to show that in relevant models of 2-adrenoceptor desensitization , chloroquine - mediated tas2r activation in asm retains its pro - bronchodilatory effects , a finding of considerable clinical relevance given the well - described concern of -agonist tachyphylaxis that occurs with repetitive -agonism . yet , it should be noted that tas2r activation in asm can lead to desensitization via a grk - mediated , -arrestin pathway , which may limit its therapeutic usefulness as it is seen currently with -adrenoceptor agonists . mechanistically , tas2r activation in asm is thought to achieve relaxation via a localized [ ca]-dependent activation of the large conductance ca - activated k channel ( bkca ) leading to membrane hyperpolarization . while other investigators have challenged the notion that bitter tastant - mediated asm relaxation is bkca - dependent , the evidence in human asm suggests at least a partial role of the bkca channel in what is likely a novel , multimodal mechanism leading to asm relaxation . the possibility of tas2r activation in asm ( in the context of localized calcium release ) leading to non - bkca - mediated asm relaxation via yet undescribed pathways is another exciting prospect behind this work that may uncover other potent targets to facilitate relaxation not susceptible to gpcr tachyphylaxis . chloride channels are expressed on airway smooth muscle and have been shown to effect airway smooth muscle force   and cell length . in 2005 , hirota et al . , described attenuation of acetylcholine - induced contractions in asm subsequent to calcium - activated chloride channel antagonism . additionally , activation of the ligand - gated chloride channel , gabaa , relaxed airway smooth muscle precontracted with the tachykinin , substance p . in 2011 , another ligand - gated chloride channel , the glycine receptor , was shown to relax airway smooth muscle contracted with a selective neurokinin 2 receptor agonist . these and other studies have led to the understanding that chloride channels may play a significant role in the airway smooth muscle contraction and relaxation mechanisms . calcium - activated chloride channels have been described functionally ; however , the true molecular identity of calcium - activated chloride channels have only recently been identified as belonging to the ano or tmem16 receptor family . the tmem16 receptors are membrane proteins with 8 transmembrane domains shown to allow chloride flux in the presence of increasing calcium while possessing voltage sensitive activity . tmem16a mrna expression has been described in airway smooth muscle   and its function in other cell types has been described as contributing to membrane depolarization during calcium increases . it has been hypothesized that acetylcholine- and caffeine - mediated release of calcium from the sarcoplasmic reticulum ( sr ) stimulates chloride efflux from the cell , leading to depolarization of the plasma membrane . force studies in ex vivo airway smooth muscle preparations examining the effects of chloride channel antagonists , 5-nitro-2-(3-phenylpropylamino)benzoic acid ( nppb ) , and niflumic acid ( nfa ) , showed a large attenuation of acetylcholine - induced contraction by nppb while nfa failed to have an effect . the differential effects of these chloride channel antagonists may be due to the effects on calcium - activated chloride channels located on the sr versus the plasma membrane . recently , members of the tmem family were shown to be expressed on various intracellular compartments and not exclusively on the plasma membrane . a possible mechanism of attenuated force generation in airway smooth muscle by calcium - activated chloride channel antagonism may be inhibition of chloride ion efflux during contractile agonist stimulus . ligand - gated chloride channels have been well described in the central nervous system with two families dominating the role as inhibitory inputs , gabaa , and glycine receptor channels . both gabaa and glycine receptors are expressed in asm and possess functional roles in the modulation of airway smooth muscle tone generation [ 35 , 36 ] . this inhibitory effect on asm contraction may be attributed to a relative hyperpolarization of the membrane potential after it has surpassed the chloride reversal potential following exposure to a contractile stimulus . this opening of the chloride channels causes an influx of chloride ions leading to a relative membrane hyperpolarization , eliminating , or attenuating the electromechanical component of contraction . gabaa channels containing alpha4 or alpha5 subunits can be selectively targeted in airway smooth muscle resulting in effects on membrane potential and airway tone . pharmacotherapies that are not gabaa subunit selective , such as the general anesthetic , propofol , have bronchodilatory capabilities . increased airway smooth muscle specificity will determine the viability of gabaa - related therapies as bronchorelaxants . gabaa function in airway smooth muscle has been studied in both rodent and human ex vivo models , as well as in vivo rodent models [ 42 , 43 ] , producing strong evidence that this channel , once thought to be exclusively neuronally expressed , may have direct effects on airway tone . in the last ten years , the existence of chloride channels in airway smooth muscle has been confirmed yet our current understanding of their mechanistic and functional roles remains incomplete . although poorly mechanistically understood [ 44 , 45 ] manipulation of chloride channels still remains a viable avenue of further research in the discovery of novel bronchodilators . continued research will uncover the exact mechanisms that dictate the role for chloride channels in the balance of contraction and relaxation in the airway . while bronchodilators will likely continue to be a mainstay of asthma therapy far into the future , receptor desensitization , -agonist insensitivity , -agonist refractory bronchoconstriction , and even death are all risks associated with prolonged use of traditional -agonists . as such , it is important to continually investigate new therapeutics for the treatment of asthma ; keeping in mind that acute bronchodilation is the first line therapy during an asthmatic episode . here we have illustrated 4 novel potential therapeutics that show functional bronchodilatory properties in the airway owing to a variety of mechanisms . these novel compounds may augment existing -agonist relaxant effects as in the case of pde inhibitors or provide complementary avenues for relaxation when combined with current therapies . table 1 summarizes beneficial aspects of traditional -agonists and these novel therapeutics as well as illustrating current limitations to implementing these novel bronchodilators . interestingly , compounds that transiently elevate [ ca]i such as phytotherapeutics , bitter taste ligands , gabaa receptor ligands , and chloride channel antagonists subsequently lead to functional relaxation of airways . this is counterintuitive in the face of decades of research closely linking global cellular calcium and smooth muscle contraction thus necessitating a broader understanding of complex calcium dynamics within cellular microdomains . while the mechanism of action of these potential therapeutics is still under investigation , they open the door for assessing new therapeutics and mechanisms leading to bronchodilation .