Two US researchers have won the Nobel Prize in Chemistry for studies of protein receptors that let body cells sense and respond to outside signals, such as danger or the flavour of food.

Such studies are key for developing better drugs.

The Royal Swedish Academy of Sciences said Robert Lefkowitz and Brian Kobilka had made groundbreaking discoveries, mainly in the 1980s, on an important family of receptors, known as G-protein-coupled receptors.

About half of all medications act on these receptors, including beta blockers and antihistamines, so learning about them will help scientists to come up with better drugs.

The human body has about 1,000 kinds of such receptors, structures on the surface of cells, which let the body respond to a wide variety of chemical signals, like adrenaline.

Some receptors are in the nose, tongue and eyes, and allow humans to sense smells, tastes and light.

"They work as a gateway to the cell," Mr Lefkowitz told a news conference in Stockholm by phone.

"As a result they are crucial ... to regulate almost every known physiological process with humans."

Prof Lefkowitz, 69, is an investigator at the Howard Hughes Medical Institute and professor at Duke University Medical Center in Durham, North Carolina.

Prof Kobilka, 57, worked for Prof Lefkowitz at Duke before transferring to Stanford University School of Medicine in California, where he is now a professor.

Prof Lefkowitz said he was fast asleep when the Nobel committee called, but he did not hear it because he was wearing ear plugs. So his wife picked up the phone.

"I knew they ain't calling to find out what the weather is like in Durham today", he said.

Prof Kobilka said he found out around 2.30am, after the Nobel committee called his home twice.

He said he would put his half of the 8 million kronor (€928,000) award toward retirement or pass it on to his children.

The academy said it was long a mystery how cells interact with their environment and adapt to new situations, such as when they react to adrenaline by increasing blood pressure and making the heart beat faster.

Scientists suspected that cell surfaces had some type of receptor for hormones.

Using radioactivity ,Prof Lefkowitz managed to unveil receptors including the receptor for adrenaline, and started to understand how it works.

Prof Kobilka and his team realised that there is a whole family of receptors that look alike - a family that is now called G-protein-coupled receptors.

In 2011, Prof Kobilka achieved another breakthrough when his team captured an image of the receptor for adrenaline at the moment when it is activated by a hormone and sends a signal into the cell.

The academy called the image "a molecular masterpiece".

The award is "fantastic recognition for helping us further understand the intricate details of biochemical systems in our bodies," said Bassam Z Shakhashiri, president of the American Chemical Society.

"They both have made great contributions to our understanding of health and disease," Mr Shakhashiri said.

"This is going to help us a great deal to develop new pharmaceuticals, new medicines for combating disease."

This year's Nobel announcements started Monday with the medicine prize going to stem cell pioneers John Gurdon of Britain and Japan's Shinya Yamanaka.

Frenchman Serge Haroche and American David Wineland won the physics prize yesterday for work on quantum particles.

The Nobel Prizes were established in the will of 19th-century Swedish industrialist Alfred Nobel, the inventor of dynamite.

The awards are always handed out on 10 December, the anniversary of Nobel's death in 1896.