Muscle kept a memory of steroid-driven growth

Researchers at the University of Oslo gave female mice testosterone for two weeks, which added extra nuclei to muscle fibers, then withdrew the drug and watched what happened to the fibers as they shrank back down.

Kristian Gundersen's team found that although the mice lost muscle size after testosterone was withdrawn, the extra nuclei the fibers had gained stayed in place rather than being removed along with the muscle. When those same mice were later put through an overload exercise that builds muscle, they regained size faster than mice that had never been exposed to testosterone.

The extra nuclei functioned as a stored memory of earlier growth, giving muscle a head start the next time it had reason to grow. The mechanism raises anti-doping questions, since a past course of anabolic steroids could keep conferring an advantage long after the drug clears the body. Human studies since then have given mixed results.

The muscle remembers what it once built.