Characterization Of Relaxations Of The Guinea Pig Aorta In Response To Bitter Taste Receptor Agonists

Martijn Manson ML, Sven-Erik Dahlen SE, Mikael Adner M. Karolinska Institutet, Institute for Environmental Medicine, Center for Allergy Research, Experimental Asthma and Allergy Research, Stockholm, Sweden

Bitter taste-sensing type 2 receptors (TAS2Rs) belong to a family of G-protein coupled receptors. TAS2Rs are responsible for gustatory taste perception in the oral cavity, where they recognize a wide range of compounds with relatively low specificity and affinity. Recently, expression of TAS2Rs has been reported in the respiratory tract and airway smooth muscle from mice, guinea pigs and humans relax in response to TAS2R agonists. The aim of this study was to examine the possible effects of TAS2Rs agonists on vascular smooth muscle.

Male Dunkin Hartley guinea pigs were killed by an overdose of pentobarbital. The thoracic aorta was quickly removed and placed into ice-cold Krebs Henseleit (KH) buffer, dissected into equally sized rings and mounted in 5 ml organ baths containing KH buffer (37ºC) which was continuously bubbled with carbon gas (5% CO2 in O2.) Isometric smooth muscle force was detected using a force-displacement transducer. Aorta rings were stretched to 8mN during one hour equilibration phase followed by validation of the viability by two consecutive administrations of KCl (60mM). After reaching a stable pre-contraction with either 10μM phenylephrine, 30nM U-46619 or 3μM PGF2α the effects of the TAS2R agonists; chloroquine, denatonium, dextromethorphan, noscapine and quinine were assessed by cumulative administration (0.1μM–300μM). All experiments were performed on endothelium denuded preparations, verified by the absence of acetylcholine induced relaxations (0.1–10μM). Data are presented in percentage decrease of pre-contractions (Mean ± SEM) with 4-8 observations for each experimental set-up. Statistical analysis was performed using One-Way ANOVA followed by Dunnett’s multiple comparison test.

The level of pre-contractions induced by phenylephrine, U-46619 and PGF2α were in a similar range (114±3%, 99± 4% and 93±4%, respectively, related to the KCl-induced contraction). After pre-contractions with phenylephrine; chloroquine (96.2±1.9%), denatonium (91.4±1.8%), dextromethorphan (90.2±1.4%), noscapine (82.6±3.2%) and quinine (91.3±1.6%) induced strong maximal relaxations with comparable potencies (pEC50:4.5˜4.8) as what has been observed in airway smooth muscle. The noscapine-mediated relaxations remained unaltered by U-46619 and PGF2α pre-contractions (96.2±2.7% and 80.3±3.1%, respectively). After pre-contractions with U-46619 and PGF2α, chloroquine mediated relaxations were still strong but slightly reduced (85.6±1.8%, p<0.05 and 79.1±3.2%, p<0.001 respectively). In contrast, in with U-46619 or PGF2α pre-contracted aorta’s, the effects of denatonium, dextromethorphan and quinine were markedly smaller. Both denatonium and dextromethorphan were completely unable to relax these pre-contractions, whereas quinine mediated relaxations were reduced to 42.0±5.7% and 39.1±1.5%, respectively (p<0.001). Moreover the potency of quinine decreased by one log unit (p<0.001).

Findings of this study are comparable to our recent study in the guinea pig trachea, where we also observed different patterns of relaxations for the TAS2R agonists depending upon the agent used for pre-contraction. By use of additional agonists in this study we could confirm that signaling through the TAS2R10 receptor were more dependent on the agonist used for pre-contraction.

This study introduces vascular smooth muscle as another target tissue where TAS2R agonists may induce relaxations, suggesting that TAS2Rs may have a general action to inhibit smooth muscle function.