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What we usually think of as salty is actually two kinds of taste-low concentration of salt and high concentration of salt. Scientists are also exploring the perception mechanism of high concentrations of salty taste. In fact, the deeper they studied saltiness, the weirder they found it.

Compile | Chen Qiang

We generally think that our tongues can sense sweet, sour, bitter, fresh and salty tastes, but in fact, there are six kinds of flavors because we have two separate salty systems. One system senses low concentrations of salt that make foods such as potato chips delicious; the other senses high levels of salt-enough to make food disgusting and prevent people from eating it.

Over the past 20 years, we have solved a lot of taste perception mechanisms. For sweetness, bitterness and delicacy, the perception mechanism is that the receptor molecules on some taste buds recognize specific molecules in the food and trigger a series of responses when activated, eventually sending signals to the brain to give us a different taste. The sour taste is slightly different. Scientists have recently learned that it is detected by taste bud cells that respond to acidity.

Salty taste is the most complicated. Scientists have basically mastered the mysteries of the low-salt taste system, but little is known about the high-salt taste system, and it is not clear which taste buds are responsible for detecting high salt. However, the study of salty taste is not only out of scientific curiosity, but also because a high-salt diet brings cardiovascular risks to some of us, so it is important to understand the process.

Which molecule can make us feel moderately salty? When we eat too much salt, the body tries to dilute the salinity in the blood by retaining water. For many people, extra moisture can lead to higher blood pressure. Too much water can also put pressure on arteries, which in the long run can damage arteries and increase the risk of heart disease or stroke.

However, our bodies need a certain amount of salt to maintain the normal functioning of muscles, nerves and other tissues. For example, eating too little salt can lead to nausea and muscle convulsions, and if it lasts too long, it can lead to shock or death. This is why athletes often drink drinks containing electrolytes to replenish the lost salt.

In order to accurately regulate the amount of sodium in the body, the body controls the amount of sodium excreted in the urine and the amount of sodium consumed through the mouth. And our two perception systems of salty taste can help us maintain the balance of sodium intake.

Scientists have found that there are "ion channels" (porous proteins) in various parts of our bodies that allow sodium ions to pass through nerve cell membranes to produce nerve impulses. But they speculate that the taste buds in our mouths must have special mechanisms to sense saltiness.

Scientists discovered an important clue to the mechanism of salt perception in the 1980s. At the time, they used a drug that blocked sodium from entering kidney cells. If the drug is applied to the tongues of mice, it will interfere with their perception of saltiness. Studies have shown that kidney cells use an ion channel called "ENaC" to absorb excess sodium from the blood, helping to regulate salt levels in the blood. The findings suggest that salt-sensing taste buds also use ENaC.

To test this hypothesis, some scientists used genetic engineering in 2010 to make some mouse taste buds lack ENaC. Experiments show that these mice no longer have a normal preference for low-salt solutions, thus confirming that ENaC is a receptor for low-salt taste.

Which taste buds are responsible for feeling moderately salty? However, to really understand how low-salt taste is produced, scientists also need to know how sodium is converted into neural signals when it enters the taste buds, so that we have "delicious, salty!" What it feels like. To reveal the process of this signal transmission, scientists need to find where the signal begins in the mouth.

The answer seems obvious: the signal should come from a class of taste buds that contain ENaC and are sensitive to moderate sodium levels. But these cells are not easy to find. Studies have shown that ENaC is made up of three different components, and although a single part of ENaC can be found everywhere in the mouth, it is difficult for scientists to find cells containing all three components.

In 2020, a team from Kyoto Prefecture Medical University in Japan announced that they had found sodium-sensing cells. They first hypothesized that sodium-sensing cells produce electrical signals in the presence of salt, but not if ENaC blockers are present. Eventually, they found a group of cells in the taste buds in the middle of the mouse tongue that were able to produce all three parts of ENaC.

Specifically, when there are enough sodium ions outside these taste buds, these ions can enter the taste buds through the three-part ENaC channel. This rebalances the concentration of sodium ions inside and outside the cell, but it also changes the distribution of positive and negative charges on both sides of the cell membrane. This change triggers the electrical signals in the cells, and the taste bud cells send "delicious, salty!" to the brain. It's a message.

The production of oversalty taste is still an unsolved mystery, but the system cannot explain why we sometimes feel "Bah, too salty!" . This feeling usually occurs when we taste something more than twice as salty as the blood.

Some studies suggest that another component of salt, chloride ion, may be a key factor. We all know that the chemical composition of salt is sodium chloride, but when dissolved in water, it breaks down into positively charged sodium ions and negatively charged chloride ions. Chlorine and sodium can produce a high salt taste, while sodium combines with other larger, polyatomic anions with a lighter taste. This suggests that chloride may be an important factor in producing high-salt taste, but scientists do not have a clear answer as to how it causes high-salt taste.

In 2013, scientists at the National Dental and Craniofacial Institute found a clue through a study of mustard oil. They reported that a compound in mustard oil reduced the sensitivity of mice to high salt. Oddly, the mustard oil compound also almost eliminated the mouse's perception of bitterness, as if the high-salt system was attached to the bitterness system.

More strangely, the study also found that cells that sense sour taste also seem to respond to high salt. If the mice lacked any of the bitter or sour systems, they were less resistant to extremely salty water. If both systems are missing, the mice will happily drink extremely salty water.

The findings are not shared by all scientists, but if confirmed, it raises an interesting question: why don't extremely salty things taste bitter or sour at the same time? Some scientists think this may be because the salty taste is formed by the superposition of multiple signals rather than a single signal.

Although the study of mustard oil provides some clues, scientists have so far found no receptor molecules responsible for sensing the taste of high salt. In 2021, a Japanese team reported that cells containing TMC4, an ion channel that allows chloride ions to enter cells, normally produce signals when exposed to high salt water in a laboratory petri dish. But when scientists created mice without TMC4, their aversion to extremely salty water did not change significantly.

What's even more tricky is that we can't be sure whether mice perceive saltiness in the same way as humans. Humans can clearly distinguish between pleasantly low-salt tastes and disgusting high-salt tastes, and like mice, ENaC receptors seem to be involved. However, the study of human use of ENaC blockers is confusing-sometimes blockers seem to weaken the salty taste, sometimes they enhance the salty taste.

One possible explanation is that humans have a fourth ENaC component that mice do not have, called delta subunit. It can replace one of the other components and may form an ion channel that is less sensitive to ENaC blockers.

In short, so far, scientists have not figured out how our tongues detect the high salt taste. Scientists hope to solve this mystery in the near future. They even hope to develop better salt enhancers or substitutes that can create "delicious" without harming their health. But obviously, scientists still need a lot of research to do this.

References:

Https://www.science.org/doi/abs/10.1126/science.6691151

Https://www.nature.com/articles/nature08783

Https://academic.oup.com/chemse/article/45/4/235/5719542

Https://www.sciencedirect.com/science/article/pii/S0896627320301926

Https://www.science.org/doi/abs/10.1126/science.1948054

Https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3587117/

Https://www.annualreviews.org/doi/10.1146/annurev-physiol-031522-075853

Https://link.springer.com/article/10.1186/s12576-021-00807-z

Https://link.springer.com/article/10.1007/s12078-008-9006-4

Https://www.sciencedirect.com/science/article/abs/pii/0031938494003297

Https://www.jbc.org/article/S0021-9258(19)76642-3/fulltext

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