FIGURE 3-4 An epithelial sodium channel (ENaC). The epithelium is represented as a lipid (fat) bilayer (round circles), the area above the lipid bilayer (oral cavity) represents the outside of the taste receptor cell, and the area below the lipid bilayer is the interior of the taste receptor cell. The channel itself is made up of three protein units (alpha, beta, and gamma) that are represented by the cylindrical structures. This channel is thought to form a tunnel through the taste receptor cell that allows Na+ ions outside the cell to move inside the cell. This channel is quite specific to sodium, which may explain why few compounds are purely salty. Once sodium is inside the taste receptor cell it causes a cascade of biochemical reactions that result in the release of neurotransmitters that signal salt taste to the brain.
monium) are present, rather than sodium or lithium. In addition, salt still elicits a taste in animal model studies, although to a lesser extent and with less specificity, when the ENaC is blocked by amiloride (DeSimone and Lyall, 2006; McCaughey, 2007). A full understanding of how salt taste is recognized by humans, a major gap in our understanding, could facilitate the discovery of effective and economically feasible salt taste enhancers.
EVOLUTION OF SALT TASTE PERCEPTION AND PREFERENCE
It is widely assumed that the ability to detect salt—hence, salt taste perception—arose in response to the need by plant-eating organisms to ensure an adequate intake of sodium (Denton, 1982; Geerling and Loewy, 2008). Sodium is crucial to many physiological processes, and the body cannot store large amounts. Moreover, outside the sea, salt is often hard to find or in low levels in the environment (Bloch, 1963).