But just 10 days after being returned to Earth, the heart cells returned to normal.
Once stem cells grew into heart cells aboard the International Space Station, their exposure to microgravity changed the expression of thousands of genes.
"Our study is novel because it is the first to use human-induced pluripotent stem cells to study the effects of spaceflight on human heart function," said senior study author Dr. Joseph Wu, from Stanford University School of Medicine in California.
"Microgravity is an environment that is not very well-understood, in terms of its overall effect on the human body, and studies like this could help shed light on how the cells of the body behave in space, especially as the world embarks on more and longer space missions such as going to the moon and Mars," Wu explained.
Past studies have shown that spaceflight triggers changes in heart function, including slower heart rate, lower arterial pressure and increased cardiac output, but relatively little is known about how microgravity affects human cardiac function on the cellular level.
"We're surprised about how quickly human heart muscle cells are able to adapt to the environment in which they are placed, including microgravity," Wu said.
The research was published Nov. 7 in the journal Stem Cell Reports.
"These studies may provide insight into cellular mechanisms that could benefit astronaut health during long-duration spaceflight, or potentially lay the foundation for new insights into improving heart health on Earth," Wu said in a journal news release.
The researchers plan to conduct further studies with different types of heart tissue.
"We also plan to test different treatments on the human heart cells, to determine if we can prevent some of the changes the heart cells undergo during spaceflight," Wu added.