Thiamine and Quinine Differently Inhibit the Early Phase of Acetylcholine-Dependent Contraction of Mouse Ileum in vitro

Thiamine and quinine are popular bitter substances and their physiological effects have been studied; however, their impact on digestion remains unknown. Here, the physiological effects of thiamine and quinine was investigated for in vitro contraction of mouse ileum. Acetylcholine stimulates autonomous contraction of mouse ileum in a dose-dependent manner. The effect of Acetylcholine for contraction of ileum was partly suppressed by the adrenaline administration. Upon simultaneous treatment of the ileum by acetylcholine, thiamine, and quinine decreased the maximum contraction. The period till half maximum contraction was prolonged by the presence of thiamine and quinine but not by adrenaline. Because a physiological effect of thiamine and quinine was observed on acetylcholine-induced contraction of the ileum, the repertoire of human bitter taste receptors, TAS2R-1, -4, -7, -10, -14, -31, -39, -40, -43, and -46, were investigated to which thiamine and quinine may bind. These human bitter taste receptors were further analyzed among the database for mouse homologs using evolutionally conserved amino acid sequences. The only bitter receptor for both thiamine and quinine was TAS2R-39, the homology of TAS2R-139 to human TAS2R-39 was 74%. Importantly, the homology of mouse TAS2R-119 to human TAS2R-1 which interact with thiamine was 91%, and that of TAS2R-130 to human TAS2R-7 that interact with quinine was 81%. The present study indicated that thiamine and quinine changed the early phase of contraction of ileum in mice and suggested that TAS2R119 and TAS2R130 expressed in mouse enteroendocrine cells to modify the physiological effects of thiamine and quinine on the acetylcholine-induced contraction of the ileum.