The breakthrough suggests humans along with other mammals might harbor a mostly unused ability to hibernate on demand. Further research into the phenomenon could lead to medical advances, such as buying time for humans awaiting an organ transplant, scientists said.

"We are, in essence, temporarily converting mice from warm-blooded to cold-blooded creatures, which is exactly the same thing that happens naturally when mammals hibernate," said lead researcher Mark Roth of the Fred Hutchinson Cancer Research Center in Seattle.

During the induced hibernation, cells virtually stopped working, reducing the rodents' need for oxygen.

"We think this may be a latent ability that all mammals have - potentially even humans - and we're just harnessing it and turning it on and off, inducing a state of hibernation on demand," Roth said.

The results are detailed in the April 22 issue of the journal Science.

Squirrels, bears, snakes and many other animals hibernate naturally, some more deeply than others. Humans have been known to hibernate by accident, Roth and his colleagues point out.

A Norwegian skier was rescued in 1999 after being submerged in icy water for more than an hour. She had no heartbeat and her body temperature was 57 degrees Fahrenheit (normal is 98.6). She recovered.

Canadian toddler Erika Nordby wandered outside at night and nearly froze to death in 2001. She wore only a diaper and T-shirt. It was minus 11 Fahrenheit (-24 Celsius). When found, her heart had stopped beating for two hours and her body temperature was 61 degrees. She suffered severe frostbite but required no amputations and otherwise recovered.

"Understanding the connections between random instances of seemingly miraculous, unexplained survival in so-called clinically dead humans and our ability to induce - and reverse - metabolic quiescence in model organisms could have dramatic implications for medical care," Roth said. "In the end I suspect there will be clinical benefits and it will change the way medicine is practiced, because we will, in short, be able to buy patients time."

Already there are companies that will gladly freeze the dead in hopes some way of curing and reviving them might develop in the future. The field is called cryonics. So far, no one has been brought back.

The trick with the mice didn't require freezing. Instead, the rodents breathed air laced with hydrogen sulfide, a chemical produced naturally in the bodies of humans and other animals. Within minutes, they stopped moving and soon their cell functions approached total inactivity.

Humans use hydrogen sulfide to "buffer our metabolic flexibility," Roth explained. "It's what allows our core temperature to stay at 98.6 degrees, regardless of whether we're in Alaska or Tahiti."

In extreme doses, the hydrogen sulfide is thought to bind to cells in place of oxygen. The organism's metabolism shuts down. Upon breathing normal air again, the mice "quickly regained normal function and metabolic activity with no long-term negative effects," the researchers report. They plan to test the technique on larger mammals next.

"Hibernating humans and space travel aside," Roth told LiveScience, "we hope that 'metabolic flexibility' can be used to enhance trauma care, surgical outcome, and organ transplant."

Among the first applications in humans might be to reduce severe fevers, when a patient is near death. Clinical trials for such a procedure could begin in five years, the scientists say.

"We believe we know how to flip the breaker on the patient's furnace," Roth said. "If they have a fever, we believe we know how to stop it on a dime."

For cancer patients, Roth speculated that temporarily eliminating oxygen dependence in healthy cells could make them less vulnerable targets to radiation and chemotherapy.

"Right now in most forms of cancer treatment we're killing off the normal cells long before we're killing off the tumor cells," he said. "By inducing metabolic hibernation in healthy tissue we'd at least level the playing field."

Eric Blackstone, a graduate research assistant in Roth's laboratory, was lead author of the journal paper.

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