A study published in PLOS ONE on September 30, 2026, found that simulated space radiation was associated with delayed fibrosis around heart blood vessels in male rats. Hind-limb unloading, a method used to model microgravity, did not cause that fibrosis on its own or increase it in the study’s 1.5 Gy comparison.
Radiation was associated with collagen buildup around heart vessels
Researchers studied 72 male WAG/RijCmcr rats, 8–9 months old, assigned to six groups. They exposed the animals to a simplified simulation of galactic cosmic radiation called simGCRsim, delivered as a rapid, acute exposure using five ions and six beams. The team later examined heart tissue for perivascular collagen, meaning collagen around blood vessels.
The study reported a twofold increase in perivascular collagen after 0.75 Gy at 360 days and after 1.5 Gy at 270 days. Each result was compared with sham-irradiated rats that had not undergone hind-limb unloading. The 0.75 Gy result came from a follow-on experiment without hind-limb unloading.
| Experimental condition | Perivascular fibrosis finding |
| 0.75 Gy radiation, without hind-limb unloading; 360-day follow-up | Collagen was twice the level in sham-irradiated, non-unloaded controls. |
| 1.5 Gy radiation, without hind-limb unloading; 270-day follow-up | Collagen was twice the level in sham-irradiated, non-unloaded controls. |
| Hind-limb unloading alone | The study found no perivascular fibrosis. |
| 1.5 Gy radiation with hind-limb unloading | Unloading did not increase fibrosis beyond the radiation-alone result. |
The fibrosis finding was separate from the microgravity model
Hind-limb unloading suspends a rat by its tail so its rear limbs bear less weight. In this experiment, it represented a microgravity-like condition. The fibrosis results point to radiation exposure, rather than unloading alone, as the condition associated with the measured heart-tissue change. The study also reported that unloading affected immune measures, including spleen CD56+ areas and circulating lymphocytes and cytokines.
What the rat results say about human risk
The study did not assess heart function or measure heart attacks in people. Its findings therefore do not establish heart-attack risk for astronauts or people traveling to Mars. The experiment used only male rats, delivered radiation in one acute, high-dose-rate exposure, and used a simplified radiation field that did not fully represent the galactic cosmic-ray environment.
NASA’s December 16, 2020, discussion of its galactic cosmic-ray simulator also identifies circulatory and heart disease among radiation-related concerns, while noting uncertainty about the effects of chronic, low-dose-rate heavy-ion exposure.
