Much as most people would like to, nobody can get that fit, firm body they're aiming for by concentrating on just one type of exercise. Our muscles won't let us off that easy. But scientists have never really been able to explain why muscles develop the way they do.

UT Southwestern researchers, however, recently discovered a genetic switch that tells muscles how to behave, opening the door for the possible creation of a drug that would mimic some of the health-promoting benefits of regular exercise. The scientists found the molecular pathway that tells muscle fiber to be either the fast, strength muscle seen in weight lifters or the slow, endurance muscle developed by aerobic enthusiasts.

Using cultured muscle cells, investigators, led by Dr. R. Sanders Williams, chief of cardiology, delineated a biochemical-signaling mechanism that converts one muscle-fiber type to another. The findings were reported in the journal Genes and Development.

The discovery could make it possible to restore endurance muscle tissue in people who have lost it due to congestive heart failure. People with diabetes might also benefit from a drug that would enhance slow endurance-promoting muscle, which is more sensitive to insulin.

"We believe this pathway provides a molecular explanation for the important effects of aerobic exercise in increasing physical endurance and reducing risk for cardiovascular disease," Dr. Williams said. "When people go jogging, molecular events happen in the muscles they are exercising that both enhance their capability to exercise further and improve their health.

"We have shown both in cultured cells and in animals that there is a signaling pathway we can modify to stimulate or reverse what exercise does naturally. We believe it is possible to design a drug which would have this effect."

The scientists will investigate further to clarify how this calcium-regulated, calcineurin-dependent pathway affects other muscle types and how it interacts with other cellular activities.