10 October 2006

Farty Gas Triggers Suspended Animation State

by Kate Melville

Hydrogen sulfide gas, a favorite of laboratory pranksters, appears to also induce a state of suspended animation in mice. Interestingly, it manages to lower the metabolic rate while maintaining normal blood pressure, something the researchers didn't expect, and which could have all sorts of applications in medicine. Known for its rotten egg smell, hydrogen sulfide gas occurs naturally in swamps, springs and volcanoes. While usually harmless, it can be toxic if breathed in sufficient quantity.

The new study, presented at the recent American Physiological Society conference, built on earlier research which found that when mice breathed the gas, they went into a hibernation-like state. Their metabolic rate dropped by 90 percent and their body temperature decreased dramatically.

"We wanted to confirm the [earlier] study and record the effects the gas has on blood pressure, heart rate, respiratory rate and the activity level of the mice," researcher Gian Paolo Volpato explained. To that end, they administered 80 parts-per-million of hydrogen sulfide gas to their mice and then assessed their metabolic function. They found that the heart rate of the mice fell from 500 beats per minute to 200 beats per minute and the respiration rate decreased from 120 breaths to 25 breaths per minute. Additionally, their core body temperature fell from 38& #176; C to 30& #176; C and their activity levels declined dramatically; moving only when directly stimulated. The mice returned to normal once they recommenced breathing normal air.

These changes were expected, based on the earlier research. But the researchers also found something they did not expect. Normally, as oxygen consumption goes down and heart rate decreases, blood pressure decreases also. Since the heart rate of the mice fell by more than 50 percent, the researchers expected blood pressure to fall, but surprisingly, blood pressure stayed normal. An animal's blood pressure needs to remain at a certain level to ensure blood reaches the vital organs. Normally, a mouse with a very low heart rate would have very low blood pressure and would be close to death. But these mice returned to normal two hours after the gas was discontinued.

Measuring the cardiac output of the mice, the researchers found that stroke volume (the amount of blood the mice pumped with each beat) was unchanged by the gas. However, total cardiac output decreased because the heart was beating much slower. "These findings demonstrate that mice that breathe 80 parts per million of hydrogen sulfide become hypothermic and decrease their respiration rate, heart rate and cardiac output without affecting stroke volume or mean arterial pressure," Volpato said.

The new findings could have a variety of helpful applications, including sustaining the function of organs of critically ill people. Patients undergoing surgery could also benefit, as anesthesia usually causes blood pressure to drop. "Currently, hypothermia is the only proven way to decrease metabolic rate and confer some protection when blood flow to the organs is impaired or intentionally reduced, such as during complex cardiac surgery," co-researcher Fumito Ichinose explained. "However, hypothermia has some adverse effects, including depressing cardiovascular functions and blood clotting. If we can figure out how hydrogen sulfide reduces metabolic rate without depressing myocardial function, we may be able to reduce metabolism and protect organs without using hypothermia."

Source: American Physiological Society