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Physiology

Tropical solution

Lizards and squirrels deal with oxygen variations during hibernation in different ways

CELIO HADDAD/UNESPStranger in the nest: teiid survives awakening from hibernation without producing antioxidantsCELIO HADDAD/UNESP

Brazilian winters are relatively mild, but this does not keep one of the country’s most common lizards, the teiu or teiid lizard, from hibernating from four to five months a year, normally as of the late autumn. Under laboratory studies, the tropical reptile’s hibernation is providing researchers with new clues about the way in which many animals go about resuming their normal life after long periods of extreme cold or heat, during which their body runs at the lowest possible level. Upon returning from this state of suspended animation – called hibernation when it occurs during cold periods and estivation during hot periods – the body faces strong oxidative stress, caused by the production of a large amount of molecules that are highly reactive to oxygen (the free radicals) and that damage cells.

Alexis Fonseca Welker, a biologist at the Federal University of Goiás (UFG) in Catalão, demonstrated in his doctoral work that the Tupinambis merianae teiid is a sort of stranger in the nest among those animals that freeze their metabolism. Common in Brazil and in northern Argentina, this lizard that can grow as long as 1.4 meters and as heavy as 5 kilograms does not seem to prepare to resume its normal life by producing antioxidants, which help to avoid the worst effects of free radicals upon the organism. In another study, biologist Marcelo Hermes-Lima, from the University of Brasília (UnB) and Welker’s doctoral thesis advisor, analyzed what happens with an animal that is a lot closer to the stereotype of hibernating creatures: the arctic squirrel (Spermophilus parryii), a rodent with small ears and brown further speckled with white, found in Canada and in Alaska. Weighing less than 1 kilogram and 40 centimeters long, these squirrels effectively fight oxidative damage to their tissues upon coming out of hibernation – the exception is the brown fatty tissue, chiefly responsible for producing the energy that helps them to wake up.

Hermes-Lima, who describes himself as a stress hunter, has studied the biochemical effects of oxidative stress on almost all types of animal metabolical changes: hibernation, estivation, high salinity changes, partial and total reduction of oxygen (hypoxia and anoxia, respectively) and diapause, a long pause in the development of certain insects. The appearance of free radicals is an inevitable consequence of the fact that live beings use oxygen as the basis of their metabolism to produce energy. Over time, the so-called reactive types of oxygen appear (hydrogen peroxide being one of the most common), which chemically change molecules that are important for the organism, such as DNA, proteins and lipids.

“Sometimes, this damage is reversible,” says Hermes-Limja. “However, in 80% of the cases at least, it cannot be mended.” In practice, therefore, oxidative damage renders the affected molecules useless, which explains the close relation between the phenomenon and aging. Oxidative damage is also common following a blood flow restriction (ischemia), common in cardiovascular problems. The abrupt return of the oxygen supply to the tissues, known as reperfusion, goes hand in hand with an increase in the production of free radicals, which, in turn, causes a considerable number of cells to die.

Alligators and seals
Animals that hibernate, estivate or temporarily submit themselves to low oxygenation – such as alligators, turtles or seals, or that dive and do not breathe for long spells – suffer something akin to ischemia followed by reperfusion. In all these cases, there is a marked reduction (as much as 90%) of the metabolic rate, associated with the reduction of tissue oxygenation – except that during dives, the organs that are essential for survival, such as the heart, brain and liver, generally maintain their normal metabolic rate. Therefore, it is believed that the oxidative stress connected with waking up from hibernation might be a good model of what happens in more prosaic circumstances.

Incidentally, Welker chose the teiids from Brazil’s south and southeast as an interesting model because, contrary to squirrels, frogs, serpents and other animals from the temperate regions of the Northern hemisphere, they do not undergo a drastic drop in body temperature in order to hibernate. Arctic squirrels, for example, can even reach negative temperatures. “I was lucky to be able to work with teiids,” says Welker. Temperatures of some 17 degrees Celsius, as well as the change in natural lighting that comes with the approach of winter are sufficient to cause these reptiles to hibernate. “The classic explanation is that this is linked to diminished food supplies, such as fewer insects, but this idea is yet to be proven,” he states.

DUBHE/WIKIMEDIA COMMONSSquirrel: Metabolism as much as 90% slower during winter monthsDUBHE/WIKIMEDIA COMMONS

In his doctorate, Welker compared indicators of oxidative stress – the presence of proteins or lipids modified by the free radicals, and the antioxidant enzymes that attack them – in the intestine of the hibernating teiids and other teiids submitted to famine. “This is a distinguishing feature of the work, as it had not been done before,” says Welker. “Since hibernation involves fasting, we were interested in finding out whether oxidative stress was similar in the two sets of circumstances.”

Studies with other animal species have shown that most of them, during hibernation, anoxia or freezing, prepare for oxidative stress, producing a sizeable amount of antioxidant enzymes to avoid the worst effects of waking up. Teiids, however, do not seem to follow this pattern. “In general, their metabolism drops overall during hibernation, reducing the production of enzymes and of free radicals. The animal’s body does not appear to face stressful challenge when it wakes up,” says the biologist. In animals submitted to fasting, however, oxidative stress was far more visible. According to Welker, this indicates that the general drop in metabolism only occurs when these reptiles are really hibernating.

In Alaska
Hermes-Lima’s research on arctic squirrels, published in June in the journal Comparative Biochemistry and Physiology, showed that rodents can efficiently restrict oxidative stress when they awaken from hibernation. He conducted the work with colleagues from the University of Alaska at Fairbanks, United States, including two members of the Inuit and indigenous community in the region, Adrienne Orr and Lonita Lohse, two young researchers that benefited from an affirmative action program.

Indirectly, the group found that upon awakening, the hibernating squirrels produced levels of free radicals similar to those of rodents that had not hibernated. Of the body tissues analyzed, only the brown adipose tissue (BAT), whose activity is responsible for the rapid heat generation that causes the squirrels’ body temperature to shoot up from 2 to 37 degrees Celsius in just half an hour, presented increased free radicals production. “In the liver and the brain, we saw practically no signs of this oxidative stress,” says Hermes-Lima.

The researcher himself, however, highlights the chief criticism one could voice about the results. “The animals that we regard as being awake are actually squirrels that, for unknown reasons, did not hibernate during winter. This happens with about one third of the squirrels raised in the lab – in nature it would not occur, because if they didn’t hibernate, they’d simply die,” explains Hermes-Lima. As the metabolism undergoes changes during the different seasons of the year, the researchers preferred not to make any comparisons with active squirrels in summer.

The impressive metabolic feats observed among the animals that hibernate or estivate seem to have inspired scenes in science fiction books and films in which human beings are maintained in states of suspended animation. The idea is certainly intriguing, but likely to continue limited to science fiction only, say the researchers.

“One of the chief financers of this type of research in the United States used to be the army,” Hermes-Lima tells us. “The dream was to get astronauts to hibernate during long-term space travel. Sincerely, I very much doubt that this will ever happen,” states the UnB researcher. “It would be necessary to radically modify human biology to achieve anything similar.” To understand the complexity of such a feat, it is enough to consider the following: while sleeping, human metabolism drops by only about 3%, whereas the metabolism of hibernating squirrels and bears slows down by 90% and 70% respectively.

However, this does not mean that acquiring an understanding of hibernation and estivation cannot generate interesting results for medicine. Preliminary data suggest that lowering the body temperature of patients in a coma diminishes the risk of neurological damage connected with strokes. If this is confirmed, the strategy may help to minimize, at least temporarily, the worst consequences of lack of brain oxygenation. “But of course, this is a highly specific medical situation,” states Hermes-Lima.

For Welker, the biology of hibernation may also provide clues about how to improve organ transplant efficiency. “Generally, drastically reducing the temperature of an organ, such as the heart, leads to problems in its functioning. Both mammals and reptiles that hibernate seem to have a specific lipids composition in their heart that keeps this problem from occurring,” he explains. It is possible that understanding this phenomenon may help to preserve an organ for a longer time prior to its transplant.

Scientific article
ORR, A. L. et al. Physiological oxidative stress after arousal from hibernation in Arctic ground squirrel. Comparative Biochemistry and Physiology, Part A, v. 153, p. 213-221, June 2009.

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