February 12, 2010

Salt Taste Cells Identified

Photo of salt pouring from a salt shaker

By identifying the cells that give us the ability to taste sodium, researchers have now pinpointed the taste receptor cells responsible for all 5 basic taste qualities.

The laboratories of Dr. Nicholas Ryba of NIH's National Institute of Dental and Craniofacial Research (NIDCR) and Dr. Charles Zuker from the Howard Hughes Medical Institute at the University of California at San Diego previously teamed up to identify the components responsible for sweet, bitter, savory (umami) and sour detection. In the new study, they turned their attention to the last of the 5 basic taste qualities: salty.

The sodium ion in dietary salt (sodium chloride) is an essential component of our body’s fluids. We’ve evolved to enjoy its taste. On the other hand, very high concentrations can be dangerous to us and taste far from palatable. To better understand how this taste system works, the research team studied taste receptor cells in mice. In mice, the taste for sodium ions at low concentrations is highly selective—other salts can’t substitute. It’s been known for some time that this taste is blocked by an ion-channel inhibitor called amiloride. Detection of high salt concentrations, in contrast, isn’t selective for sodium and isn’t blocked by amiloride.

The researchers developed a way to image taste receptor cells as the cells responded to salt stimulation. They reported their results in the advanced online edition of Nature on January 27, 2010. Some cells were only activated by high concentrations of salt, they found. These cells also responded to a wide range of non-sodium salts, and their activity was unaffected by the presence of amiloride. In contrast, low concentrations of sodium activated a completely separate population of cells that were blocked by amiloride and didn’t respond to non-sodium salts.

Amiloride is already known to block the epithelial sodium channel (ENaC), so the researchers tested whether taste cells with ENaC are responsible for the taste of salt at low concentrations. They made transgenic mice in which ENaC function was knocked out just in taste cells. Mice with taste cells lacking ENaC still responded to the other 4 basic taste qualities—as well as to high salt concentrations. But their taste cells had no detectable response to low concentrations of sodium chloride. The mice also showed no behavioral attraction to salt.

Further experiments showed that the taste receptor cells that detect sodium are distinct from those that respond to sweet, bitter, sour or savory stimuli. Thus, all 5 basic tastes are mediated by separate and dedicated cells. The detection of salts other than sodium chloride—and of very high salt concentrations—is made by yet another set of taste receptor cells.

Given the molecular similarities between mice and people in detecting other tastes, salt taste is likely similar as well. However, human studies of amiloride in the perception of saltiness have been inconclusive. "Perhaps that’s because people might also consider the taste of concentrated salt, which activates both types of salt receptor cells, when thinking of something salty," Ryba suggests. Further research will be needed to confirm whether ENaC plays the same role in human salt taste as it does in mice and drives our taste for salt.

—by Harrison Wein, Ph.D.

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