Mice on steroids

Long after they’ve stopped receiving steroids, mice continue to build muscle

Muscles in mice on steroids grew larger (right) than in mice that hadn’t been taking the drugs (left). And the effects of steroids lingered long after the mice stopped receiving the drug.


Taking steroids is a quick — and illegal — way for an athlete to improve muscle strength. These synthetic hormones boost the body’s ability to build muscle mass and power. That’s why sporting groups routinely test top athletes for signs they have used such doping agents to illegally enhance their performance. But a new study in mice suggests those tests may not work. The reason: Long after the body has rid itself of any sign of the drug, the effects of doping will persist.

In fact, if steroids have the same effect on people that they did in the study mice, steroid-taking athletes would have to be banned from competition for at least 10 years. The drugs’ effects may last at least that long.

Kristian Gundersen of the University of Oslo in Norway led the new study. As a physiologist, he and his coworkers have been studying muscle function. Cells in this tissue are long and stringy. Each muscle cell contains many nuclei, which operate like little control centers for their parent cell. Each nucleus also contains DNA, which instructs its parent cell on how to make protein. With multiple nuclei, a cell can pump out more proteins, which are critical ingredients in building new muscle.

In 2010, Gundersen’s team showed that the number of nuclei in muscle cells goes up with increased exercise. In the October 28 issue of the Journal of Physiology, the researchers now report that taking steroids boosts the number of nuclei in muscle cells even more.

The scientists gave female mice these steroids for two weeks. Compared with mice not on steroids, the treated mice packed more nuclei into their muscle cells. A lot more: Mice on steroids had 66 percent more nuclei in those cells than the undoped mice had. (That means that if an untreated mouse had an average of 100 nuclei per muscle cell, a doped mice would have 166 per cell.)

In another group of mice, none of which were doped, the scientists cut one muscle. This forced a neighboring muscle to work harder. These mice showed a 51 percent increase in the number of nuclei in the overworked muscle compared with mice with fully intact muscles.

Finally, some mice with a cut muscle were doped with steroids. Their overworked muscle cells really bulked up on new nuclei: They almost doubled the number of nuclei they started with.

The scientists reported that muscle cells of mice treated with steroids grew much bigger. After the scientists stopped doping the animals, those muscles cells shrank back to normal size. But the extra nuclei in these shrunken muscle cells persisted for at least three months. That time is comparable to about 10 years in the life of a person. And the effects might last even longer.

Three months after mice stopped receiving steroids, the scientists exercised the animals again. Those mice added muscle mass quickly. Untreated mice that were also exercised bulked up, too, but not nearly as fast. That finding suggests that the extra nuclei indeed allowed once-treated mice to more quickly add muscle long after the hormone was gone.

“In my career it has been rare to see such clear results,” Gundersen told Science News. “It is more dramatic than I thought it would be.”

Other scientists agree. Cell biologist Lawrence Schwartz, at the University of Massachusetts Amherst, did not work on the new study. He told Science News, “The implication is once you have these nuclei, you never lose them.”

But researchers don’t know if steroids will affect people to the same extent. And that will be tough to study. Steroids can cause harm, so scientists are unlikely to ever win approval to test them on large groups of people. Indeed, Schwartz said, “I don’t see any easy, or even an ethical way of doing this in humans.”

Power Words

biology  The study of living things. The scientists who study them are known as biologists.

hormone  A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. 

physiology  The branch of biology that deals with the everyday functions of living organisms and how their parts function.

steroid  (short for anabolic steroid) A compound that resembles the hormone testosterone, which promotes the growth in size of muscle cells. Doctors may prescribe such hormones to treat some forms of weight loss. Athletes and others sometimes take these hormones (illegally) to enhance physical performance, muscle strength and their appearance. 

Stephen Ornes lives in Nashville, Tenn., and his family has two rabbits, six chickens and a cat. He has written for Science News Explores since 2008 on topics including lightning, feral pigs, big bubbles and space junk.

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