Bitter taste receptor agonists induced chronotropic effects on rat heart

Background 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 rat. Aim This study investigates the expression of Tas2rs in the sinoatrial node (SA node) and left ventricle of rat heart, and the mechanism of bitter compounds regulation of the heart rate. Methods Rats were excised by thoracotomy and the sinoatrial node and left ventricle tissues were isolated. These tissues were processed to study the mRNA expression levels of Tas2rs and their coupled G‐protein subunits by real time quantitative PCR (qPCR). Langendorff isolated heart models were used to record the heart rate and electrocardiogram. SA node beating recordings were used to measure the beating rate in combination with bitter compounds, quinine and chloroquine. The statistic dada are expressed as mean ± SEM. Results We detected the mRNA expression of 7 subtypes of Tas2rs (Tas2r108, 120, 121, 126, 135, 137, 143) in the left ventricle and 8 subtypes in the SA node including Tas2r38 in addition to the 7 subtypes in the left ventricle, in which the mRNA level of Tas2r120 is the highest among them followed by Tas2r143 and 121. In addition, Tas2r coupled G‐protein subunits also expressed in the SA node and left ventricle. Tas2r agonists, quinine (1–50 μM) and chloroquine (1–20 μM), decreased the heart rate and increased the RR interval and QRS duration in Langendoff‐perfused isolated rat hearts; they reduced the spontaneous beating rate of isolated SA nodes with pEC 50 values of 4.907 ± 0.045 and 4.968 ± 0.030, respectively. The blockade of Tas2r108 with abscisic acid (500 μM), the inhibition of PDEs with IBMX (50 μM), or the selective inhibition of PDE3 and PDE4 with a cocktail of cilostamide (0.5 μM) and rolipram (10 μM), attenuated the negative chronotropic effects of quinine and chloroquine on the SA node. Furthermore, quinine (20 μM) and chloroquine (10 μM) suppressed the tachycardia effect of isoprenaline on the SA node and shifted the concentration‐response curve of isoprenaline rightward. Conclusion Tas2r agonists, quinine and chloroquine, induce negative ionotropic effects in rat hearts. Our data provide mechanistic evidence that Tas2r agonists decreased the heart rate by prolonging ventricular depolarization, and by attenuating the SA node pace in a PDE‐dependent manner. Tas2r agonists can counteract with β ‐adrenergic receptor activation and eliminate isoprenaline‐induced tachycardia, suggesting Tas2rs could be the potential targets of drugs for treating sinus or ventricular tachycardia and elevated heart rate related to heart failure. Support or Funding Information This work was supported by Funding from Southwest University [Grant number SWU 115086, 104290/22300503] and Administration of Foreign Experts Affairs of China [Grant number BC2018042].