Taste Cells of the Gut and Gastrointestinal Chemosensation

Humans consciously perceive only five distinct tastes: bitter, sweet, umami [the savory taste of monosodium glutamate (MSG)], salty, and sour. Taste cells of the tongue utilize G protein–coupled receptors (GPCRs), the heterotrimeric G-protein gustducin, and downstream signaling cascades to detect bitter, sweet, and umami compounds. The mechanisms of salty and sour taste transduction are less certain but may utilize the epithelial sodium channel (ENaC), polycystic kidney disease (PKD) channels, transient receptor potential channel M5 (Trpm5), or other ion channels. Sugars, such as glucose and fructose, and non-caloric sweeteners, such as sucralose, activate the tongue’s heterodimeric T1R2+T1R3 sweet receptor, whereas MSG and other savory amino acids are detected by the T1R1+T1R3 heterodimer. Toxic plant alkaloids, such as strychnine and other bitter compounds, activate one or more of approximately thirty divergent T2R bitter receptors.