The bitter taste receptor agonist-induced negative chronotropic effects on the Langendorff-perfused isolated rat...

Bitter taste receptors (Tas2rs), the members of the G-protein-coupled receptors, mediate the bitter taste and express in extra-oral tissues. Previous studies have shown that Tas2r mRNAs are expressed in the whole heart and cultured cardiomyocytes of neonatal rats. This study aimed to determine the expression of Tas2rs and their function in the adult rat hearts by using RT-qPCR techniques, Langendorff-perfused isolated hearts, and isolated sinoatrial (SA) nodes. The data presented here revealed the mRNA expression of Tas2rs and their coupled G-protein subunits in the SA node and left ventricle of adult rat hearts. Tas2r agonists, quinine and chloroquine, decreased the heart rate and increased the RR interval and QRS duration in Langendorff-perfused isolated rat hearts; they reduced the spontaneous beating rate of isolated SA nodes with pEC₅₀ values of 4.907 ± 0.045 and 4.968 ± 0.030, respectively. The blockade of Tas2r108 with abscisic acid, the inhibition of phosphodiesterases (PDEs) with 3-isobutyl-1-methylxanthine (IBMX), or the selective inhibition of PDE3 and PDE4 with a cocktail of cilostamide and rolipram, attenuated the negative chronotropic effects of quinine and chloroquine on the SA node. Furthermore, quinine and chloroquine suppressed the tachycardia effect of isoprenaline on the SA node and shifted the concentration-response curve of isoprenaline rightward. In summary, we provided a few lines of evidence that Tas2r agonists, quinine and chloroquine, decreased the heart rate by prolonging ventricular depolarization, and by attenuating the SA node pace in a PDE-dependent manner; they can counteract with β-adrenergic receptor activation and eliminate isoprenaline-induced tachycardia.