The human lungs can "taste" bitter substances in the air, according to a study that could lead to new types of drugs designed to make it easier for asthma sufferers to breathe.
It is the first time that taste receptors, normally found on the tongue, have been discovered in the involuntary smooth muscle that controls the flow of air into the bronchi, the narrow airways of the lungs.
Scientists said the taste receptors are identical to those designed to detect the presence of bitter compounds in the mouth but, unlike the taste buds on the tongue, the receptors in the lungs do not send nerve impulses to the brain.
The discovery, published in the journal Nature Medicine, was so surprising that the researchers who made it initially thought they had made a mistake. They also discovered that, rather than narrow the tubes of the lungs to protect them against bitter, poisonous gases, stimulating the receptors actually widened the airways.
"Nobody expected that taste receptors, considered to be for the tongue, would be found on the smooth muscle of the bronchi," said Professor Stephen Liggett of the University of Maryland Medical Centre in Baltimore. "When we [stimulated] these cells, we figured that they would constrict the muscle, signaling a person to leave a toxic environment. Instead, they open the airways better than any known drug for treating asthma."
Many poisons made by plants are bitter, so the scientists thought that the receptors must play a role in warning the lungs of a potentially toxic environment. "I initially thought the bitter-tasting receptors in the lungs would prompt a 'fight or flight' response to a noxious inhalant, causing chest tightness and coughing so you leave a toxic environment, but that's not what we found," Professor Liggett said.
"It turns out that the bitter compounds worked the opposite way. They opened the airway more profoundly than any known drug that we have for treatment of asthma or chronic obstructive pulmonary disease," he said.
"New drugs to treat asthma, emphysema or chronic bronchitis are needed. This could replace or enhance what is now in use, and represents a completely new approach."
There is no possibility that the lungs could be subconsciously used to "taste" air, Professor Liggett said. "In the tongue, there is a neural pathway from the taste bud to the brain. There is no such pathway in the lungs. The receptors are on the airway smooth muscle, and signal to the interior of the cell to cause it to relax," he said.
One explanation for their presence is that the taste receptors detect bacterial infections which secrete bitter substances, thereby ensuring that the vital airways are kept open.