Muscles in space forfeit more than fibers.

Muscles in space forfeit more than fibers

Scientists and ex-football players know that muscles atrophy from disuse. But in space, where there is little gravity and muscles carry almost no load, the atrophy is compounded by other factors. After two weeks in space, rat muscles not only shrink but also lose blood vessels, nerve connections and even their own cells, according to a new study.

At the annual meeting last week of the American Society for Gravitational and Space Biology in Washington, D.C., Danny A. Riley of the Medical College of Wisconsin in Milwaukee reported findings from his research using five rats flown in the Soviet Cosmos Biosatellite 1887 last October. Soviet researchers dissected the rats and sent the chemically fixed and frozen tissues for study by Riley and his collaborators from San Jose (Calif.) State University and the University of Sydney, Australia.

In an environment with one-thousandth the gravity of Earth, muscles exert only a small amount of force to move the body. Because many muscle fibers are not used, muscles lose contractile proteins and shrink -- by 40 percent in the case of the rats flown for 12-1/2 days in the Cosmos. Their decrease in muscle mass was 10 percent greater than that of rats flown for a week in the 1985 Challenger's Spacelab 3 -- indicating that muscle atrophy in space is progressive, according to Riley.

Muscle atrophy during inactivity is "part of the normal turnover of muscle proteins," Riley says. But when his team examined inner thigh and calf muscles in the Cosmos rats, they found not just atrophy but serious damage in 4 to 7 percent of the muscle fibers. The damage was "much worse" than that found in Spacelab 3 rats, according to Riley.

The affected fibers were swollen and had been invaded by white blood cells that "clean up" infected or inflamed areas. Blood vessels had broken and red blood cells had entered the muscle. Half the muscles had damaged nerve endings.

"You see this kind of pathology in human nerve-muscle diseases," Riley says. He suggests that in space, such changes may result from several things, including cosmic radiation, stress and reduced circulation as well as muscle disuse.

Riley says his findings may explain why astronauts lose muscle strength and coordination despite vigorous exercise programs. In the 1973 Skylab missions, U.S. astronauts who exercised still lost 10 to 15 percent of their strength over three months in space. "Either it wasn't the right type of exercise," Riley says, "or there is pathology that is causing the muscle shrinkage and weakness."

Although astronauts' muscles -- like those of rats -- seem to regenerate after return to Earth, "that doesn't mean there's no pathology," Riley says. He warns muscle recovery may be compensatory and not actually regenerative.

It is not known whether the normal muscle-repair processes that work on Earth will work in space. If they do not, astronauts' muscles are likely to weaken progressively, and possibly irreversibly, on very long missions. "If someone goes to Mars, the regeneration process may be slow," Riley says. A potential more serious problem is that damaged blood supply to some muscles may affect supply to bone, he says.

Exercise can help prevent muscle atrophy by generating new muscle protein. But it won't stimulate muscle fibers disconnected from nerves, and it can't help those that remain unused in space. And fibers with damaged blood supply can contract for only a short time before they become exhausted.

"It appears that muscles have their own hormone that stimulates cell division and repair," Riley says. "It may be possible to identify that. And that [hormone] might benefit someone with damage from space flight."

Muscle regeneration might theoretically be promoted by hormones that stimulate protein production, such as growth hormone, or by anabolic steroids, which promote both protein synthesis and muscle repair. "I think the answer will be exercise plus anabolic steroids," Riley suggests.

He cautions that no one knows for sure whether the pathologies seen in rats occur in astronauts. Rats, for one thing, do not eat as well as humans in space. Nonetheless, says Riley, "the animal literature is saying we should biopsy humans."