Salt's technique for tickling the taste buds.

The nose may know, but the taste bud testifies - especially when it comes to sodium chloride, commonly known as table salt.

Sodium's positively charged ion plays a crucial role in maintaining the body's water balance. Taste buds coated with clumps of sodium-seeking cells dot the tongues of most land animals, including humans, as nature's way of driving us to eat enough of the mineral,

But our sense of sodium's saltiness varies depending on whether chloride or some other negatively charged ion (anion) is coupled to it. Now, three physiologists at Virginia Commonwealth University in Richmond have explained this phenomenon.

In the Nov. 1 SCIENCE, John A. DeSimone, Qing Ye and Gerard L. Heck reveal the mechanism behind the so-called "anion effect." They discovered that sodium chloride tastes saltier than sodium linked to other anions because chloride is small enough to worm its way into taste buds, where it affects the way taste cells perceive sodium.

The work confirms earlier findings by others that chloride diffuses into taste buds through a dense network of filaments that swathe taste cells and link them to one another. But DeSimone's team also found that once the chloride anions traverse the barrier, they dilute the net positive charge created by sodium ions already drawn inside.

The dilution helps neutralize the charge difference between the outside and inside of the bud, making it easier for sodium-scavenging taste cells to haul in more of the mineral, Desimone says. Each time the taste cells gobble more sodium from outside the taste bud, he says, the process triggers nerves that register a salty sensation to the brain.

In their experiments, the Virginia team compared the effects of three different sodium salts on a patch of taste buds on the tongue of an anesthetized rat. Using electrodes, they measured the voltage difference between the patch's outer and inner surfaces after adding sodium chloride or sodium paired with the relatively large anions acetate or gluconate. They monitored the patch's response to the salts through other electrodes attached to nerve fibers relaying signals from the patch.

The group found that the chloride caused only half the voltage difference created by salts of the other two anions. This allowed the taste cells to pull in more sodium and elicit larger nerve impulses. When the researchers neutralized the voltage across the patch, it altered the diffusion rates of the three anions, eliminating the effect. But when they increased the positive charge on the patch's inner surface, even more chloride than acetate or gluconate anions flooded in, proving chloride's neutralizing role.

"I'm very impressed," says Joe Brand, a biophysicist at Monell Chemical Senses Center in Philadelphia. "This means that the functional unit of taste now has to include not just the taste cell, but its environment as well."