Martijn Manson

The bitter taste receptor (TAS2R) agonists denatonium and chloroquine display distinct patterns of relaxation of the guinea pig trachea

Activation of taste receptors (TAS2Rs) by bitter taste agonists has been reported to cause bronchodilation. The aim of this study was to extend the information on the effects of bitter taste agonists on responses induced by different contractile mediators in a standard airway physiology preparation. Isometric responses were assessed in guinea pig trachea (GPT). TAS2R agonists were administered either to segments precontracted with different agonists for contraction or given before challenge with the different contractile stimuli, including antigen in tissues from ovalbumin-sensitized animals. TAS2R mRNA expression on GPT epithelium and smooth muscle was measured with real-time PCR. Denatonium, chloroquine, thiamine, and noscapine induced concentration-dependent relaxations (R(max): 98.3 ± 1.6, 100.0 ± 0.0, 100.0 ± 0.0, and 52.3 ± 1.1% of maximum, respectively, in the presence of indomethacin) in segments precontracted with carbachol. The receptors for denatonium (TAS2R4, TAS2R10) and chloroquine (TAS2R3, TAS2R10) were expressed in GPT. Whereas denatonium selectively inhibited contractions induced by carbachol, chloroquine uniformly inhibited contractions evoked by prostaglandin E(2), the thromboxane receptor agonist U-46619, leukotriene D(4), histamine, and antigen. The effects of denatonium, but not those of chloroquine, were partly inhibited by blockers of the large Ca(2+)-activated K(+) channels and decreased by an increase of the level of precontraction. In conclusion, TAS2R agonists mediated strong relaxations and substantial inhibition of contractions in GPT. Chloroquine and denatonium had distinct patterns of activity, indicating different signaling mechanisms. The findings reinforce the hypothesis that TAS2Rs are potential targets for the development of a new class of more efficacious agonists for bronchodilation.

Prostaglandin E2 inhibits mast cell-dependent bronchoconstriction in human small airways through the E prostanoid subtype 2 receptor.

BACKGROUND Inhaled prostaglandin (PG) E2 might inhibit asthmatic responses, but the mechanisms involved remain undefined. OBJECTIVE We sought to characterize the direct and indirect effects of PGE2 on human small airways with particular reference to the receptors mediating the responses. METHODS Contraction and relaxation were studied in isolated human bronchi with an inner diameter of 1 mm or less. RESULTS Low concentrations of PGE2 (0.01-1 μmol/L) relaxed the bronchi precontracted by histamine. The bronchodilator response was inhibited by the E prostanoid (EP) subtype 4 receptor antagonist ONO-AE3-208 but unaffected by the EP2 receptor antagonist PF-04418948. Higher concentrations of PGE2 (10-100 μmol/L) contracted the small airways. However, the TP receptor agonists U-46,619, PGF2α, and PGD2 were more potent than PGE2. Moreover, the bronchoconstrictor responses to PGE2 and all other tested prostanoids, including the EP1/EP3 receptor agonist 17-phenyl trinor PGE2 and the partial FP receptor agonist AL-8810, were uniformly abolished by the TP receptor antagonist SQ-29,548. In the presence of TP and EP4 antagonists, PGE2 inhibited the mast cell-mediated bronchoconstriction resulting from anti-IgE challenge. Measurement of the release of histamine and cysteinyl leukotrienes documented that this bronchoprotective action of PGE2 was mediated by the EP2 receptor, unrelated to bronchodilation, and increased with time of exposure. CONCLUSION The pharmacology of PGE2 in isolated human small airways was different from its profile in animal models. This first demonstration of powerful EP2 receptor-mediated inhibition of IgE-dependent contractions in human airways introduces a new selective target for the treatment of asthma. This EP2 control of mast cell-mediated bronchoconstriction is presumably exaggerated in patients with aspirin-exacerbated respiratory disease.

Bitter taste receptor agonists mediate relaxation of human and rodent vascular smooth muscle.

Taste-sensing type 2 receptors (TAS2Rs) have been implicated in extraoral functions. Airway smooth muscle expresses TAS2Rs and is strongly relaxed by TAS2R agonists. We hypothesised that TAS2R agonists might affect vascular smooth muscle as well. Moreover, the general pharmacological profile of TAS2R agonists, which are used to investigate the functions of TAS2R׳s, are undefined. The aim of this study was to pharmacologically characterise the effects of five prototype TAS2R agonists in vascular smooth muscle. Responses to the TAS2R agonists were investigated in guinea-pig aorta and taenia coli, mouse aorta (wild-type and caveolin-1-/- mice) and human pulmonary arteries. Chloroquine, denatonium, dextromethorphan, noscapine and quinine, agonists for TAS2R3, TAS2R4, TAS2R10 and TAS2R14, induced strong endothelium-independent relaxations (responses between 82-96% of maximal relaxations) in phenylephrine pre-contracted guinea-pig aorta that persisted in the presence of L-type Ca2+ and KCa1.1-channel blockers. Experiments in guinea-pig taenia coli revealed that denatonium and quinine also inhibited relaxations to phenylephrine, indicating antagonism of α-adrenoceptors. Only chloroquine and noscapine mediated relaxations when the guinea pig aorta was pre-contracted by U-46619 or PGF2α. Relaxations to chloroquine and noscapine after U-46619 pre-contractions were however markedly impaired in aortae from caveolin-1-/- mice. Chloroquine and noscapine mediated relaxations of human pulmonary arteries that expressed also mRNA for TAS2R3, TAS2R4, TAS2R10 and TAS2R14, at levels similar to that of the α1A adrenoceptor. Notwithstanding whether TAS2Rs are involved or not, TAS2R agonists have profound effects on vascular smooth muscle. Chloroquine and noscapine are of special interest as their effects cannot be accounted for by conventional pathways.

The TLR7 agonist imiquimod induces bronchodilation via a nonneuronal TLR7-independent mechanism: a possible role for quinoline in airway dilation

Toll-like receptor (TLR) 7 agonists are known to reduce allergic airway inflammation. Their recently reported ability to rapidly relax airways has further increased their interest in the treatment of pulmonary disease. However, the mechanisms behind this effect are not fully understood. The present study, therefore, aimed to determine whether airway smooth muscle (ASM)-dependent mechanisms could be identified. TLR7 agonists were added to guinea pig airways following precontraction with carbachol in vitro or histamine in vivo. Pharmacological inhibitors were used to dissect conventional pathways of bronchodilation; tetrodotoxin was used or bilateral vagotomy was performed to assess neuronal involvement. Human ASM cells (HASMCs) were employed to determine the effect of TLR7 agonists on intracellular Ca(2+) ([Ca(2+)]i) mobilization. The well-established TLR7 agonist imiquimod rapidly relaxed precontracted airways in vitro and in vivo. This relaxation was demonstrated to be independent of nitric oxide, carbon monoxide, and cAMP signaling, as well as neuronal activity. A limited role for prostanoids could be detected. Imiquimod induced [Ca(2+)]i release from endoplasmic reticulum stores in HASMCs, inhibiting histamine-induced [Ca(2+)]i The TLR7 antagonist IRS661 failed to inhibit relaxation, and the structurally dissimilar agonist CL264 did not relax airways or inhibit [Ca(2+)]i This study shows that imiquimod acts directly on ASM to induce bronchorelaxation, via a TLR7-independent release of [Ca(2+)]i The effect is paralleled by other bronchorelaxant compounds, like chloroquine, which, like imiquimod, but unlike CL264, contains the chemical structure quinoline. Compounds with quinoline moieties may be of interest in the development of multifunctional drugs to treat pulmonary disease.

The bronchodilatory capacity of imiquimod: the existence of two mechanisms

to the editor: We recently published a study investigating the bronchodilatory capacity of the Toll-like receptor 7 (TLR7) agonist imiquimod, establishing the presence of a nonneuronal, TLR7-independent mechanism of dilation ([9][1]). Our study adds new information to the previously published