Would fish do swimmingly in outer space, or would their muscle tissue shrink like those of human astronauts floating in zero gravity?

Japanese scientists had this question in mind when they sent groups of medaka fish into space to learn what happens to their hearts. What they learned, according to a study being released Sunday at the American Heart Association’s Scientific Sessions, was that the tiny heart muscles of the fish atrophied the longer they stayed in space.

In a study conducted by researchers from Tokyo’s Keio University, different groups of 15 medaka fish, a transparent Japanese rice fish, spent varying time in an aquarium on the International Space Station. One group spent only two weeks on the station, while the other stayed in space for two months.

Researchers noted that the fish swam in screw-like loops, rather than in straight lines, once in space. But that was only initially, said the study’s lead author, Hirokazu Enomoto. After a month, the fish did not move at all except during feeding time.

The fish that spent only two weeks in the space aquarium had no difference in heart structure or tissue upon their return, when compared to a control set of fish. But those that stayed in space for two months were found to have thinner heart muscle fibers.

Enomoto said he hopes his findings will be able to provide the basis for future human research, specifically improving the health of astronauts.

NASA scientist Steven Platts, Ph.D., said space-based researchers often will experiment on animal specimens before expanding their scope.

“What often happens, people will send up fish, or mice or fruit flies to get a preliminary idea of what the lack of gravity might be doing, and then they propose human research after that,” he said.

That’s mainly because of limited resources: Only six crew members are aboard the ISS at any given time.

“U.S. researchers typically only have access to three of that six, so for us to do an experiment, it can take a long time – often many, many years,” said Platts, ISS medical project increment scientist based at the Johnson Space Center.

Astronauts, he said, have a limited number of hours they can do human research because they’re busy keeping the space station afloat and conducting planetary, physics and other types of research.

In recent studies of the hearts of astronauts, researchers found that ISS crew members who spent at least six months in space lost about 15 percent of their heart muscle. While part of the loss may be from dehydration, a great part of the shrinkage was from muscle atrophy, Platts said.

Past research has attributed such muscle loss to the fact that the heart doesn’t need to work as hard in space because it doesn’t have to fight against gravity to pump blood through the body. However, a recent study conducted by Benjamin Levine, M.D., F.A.C.C., a medical professor from the University of Texas Southwestern Medical Center, indicates that may not be the case.

Levine’s research showed that exercise routines conducted by astronauts in space may actually slow the loss of heart muscle, Platts said. The use of resistance exercise was seen to actually protect the heart from tissue loss.

Platts is currently overseeing a study of his own that examines the relationship between space-related oxidative stress– from radiation exposure, along with the sheer psychological and physical burdens – and any changes in vascular structure and function.

“We’re trying to figure out if all those kinds of stress can lead to higher risk of atherosclerosis down the road,” he said.

The impact of space flight experiments ultimately can be applied to people on Earth, he said.

He noted that one recent experiment examined treatment for orthostatic hypotension, which astronauts experience when they return to Earth. Subjects in the experiment were given Midodrine, a drug used to treat low blood pressure. Some subjects also were given another medication, Promethazine, to treat vomiting, another side effect crew members experience.

“We did a double-blind crossover study, and we found that those drugs interact and cause a condition called akathesia, which causes high anxiety and involuntary muscle twitches,” Platts said.

The findings were published in the Journal of the American Medical Association –and spotted by an emergency room doctor who saw its relevance on a patient on the verge of being admitted to the psychiatric unit. The patient had been taking Midodrine to control his blood pressure, but recently had been prescribed Promethazine to treat flu-related symptoms.

“Because of our publication, (the doctor) realized it was a drug interaction and the patient was treated more appropriately,” Platts said.

“There is so much we can learn from space flight that can be applied to people here.”