Washington, Jan 23 : Drugs that bulk up muscles might have a troubling side effect - small, brittle tendons that could make muscle injuries more likely.
Such drugs, called myostatin inhibitors, restrict the protein myostatin, which normally prevents muscles from being overly bulky. They are stirring a lot of interest among body builders and athletes and can also help older people whose muscles naturally get smaller and weaker with age.
However, a new study in mice, has suggested that the findings are cautionary for a variety of people interested in the potential of myostatin inhibitors to increase muscle mass.
"Those interested in myostatin inhibitors need to be aware of the fact that by doing these things to muscles, they may be having negative effects on tendons," said John A. Faulkner, Ph.D., the study's senior author and professor in the Department of Molecular and Integrative Physiology at the U-M Medical School.
Christopher L. Mendias, Ph.D and study's first author said that when a person lifts weights at the gym, muscle tissue gets damaged. That sets off the release of myostatin, starting a process that clears away damaged proteins and sets the stage for muscle rebuilding.
The study suggests that people need normal myostatin action for other reasons, too.
"It also appears to make tendons bigger and more flexible," said Mendias, a U-M post-doctoral research fellow in the Regenerative Sciences Training Program in the Department of Surgery at the U-M Medical School.
Tendons are stiffer than muscles to begin with, and get stiffer with age. If tendons are brittle and short, as they were in myostatin-lacking mice in the study, they can't adequately do their important job of buffering against muscle injuries.
In the study, the research team conducted a series of examinations using a strain of laboratory mice that lacked the ability to produce myostatin.
They tested the mechanical properties of tendons, compared to tendons in a strain of normal laboratory mice. They isolated and treated tendon cells with myostatin and examined what genes control tendon activity.
The researchers were able to identify tendon genes that respond to myostatin, which is produced in muscles, showing that myostatin acts as a hormone to promote strong, flexible tendons.
The study is published in the Proceedings of the National Academy of Sciences.