Rats exposed to radiation similar to what a Mars-bound astronaut might face developed more scar tissue around the small blood vessels of their hearts than unexposed rats, Live Science reported. The research, published Sept. 30 in PLOS One, found that this scarring, a sign of cardiac fibrosis, was still present nearly a year after the exposure. Fibrosis is known to raise the risk of heart failure. Because the work was done in rats, it cannot definitively say whether the same effect will appear in people. The authors noted that humans live much longer than rats, so an effect seen in a year in rodents might, in a rough comparison they offered, take decades to show up in humans. "The message that we try to convey … is that the follow-up time is really important," co-author John Baker, a professor of surgery in the Radiation Bioscience Laboratory at the Medical College of Wisconsin, told Live Science. "When astronauts come back to the Earth, they expect — and we expect — that they're going to live for a long time. I think we have to be wary that they could well develop at least some type of cardiac disease."

The link between radiation and heart problems is well documented, but "no astronaut has ever been exposed to galactic cosmic radiation for long periods of time," Baker said. This study used NASA's Simulated Galactic Cosmic Ray Simulation, or SimGCRSim, which the researchers said offers a more precise comparison of the radiation an astronaut might face on a deep-space trip of more than a year. The team exposed rats to several minutes of radiation, scaled to their size, similar to an 18-month round trip to Mars with current technology. Some rats also had their hind legs suspended to simulate weightlessness. After 360 days, the animals exposed to the cosmic-ray simulation had twice as much scar tissue around the heart's small vessels as unexposed rats. "The heart can't relax to the point when it needs to," Baker said, increasing the risk of heart failure. Rats exposed only to simulated weightlessness did not show that scarring, which he said confirmed that "it was only the radiation that produced the damaging effect on the heart." Those animals did have lower levels of key immune cells in long-term testing, suggesting microgravity may still affect other aspects of health.

Rupak Pathak, an associate professor of pharmaceutical science at the University of Arkansas for Medical Sciences who was not involved in the study, told Live Science it was "a very valuable contribution to space biology." Rats and humans have clear differences, he said, but they share more than 90 percent of their genetic information and have similar cardiac systems. Because the rats were exposed only once, he said he would like to see further experiments on chronic radiation. Baker and colleagues hope later work can help NASA consider safeguards such as heart medications or better spacecraft shielding. "The more we know before we start, then the better NASA is going to be at working out what types of countermeasures can be developed," Baker said. The verified result is a lasting difference in rat heart tissue after a simulated deep-space dose, not a diagnosis of heart failure in astronauts.