The researchers report that the enzyme called AMP-activated protein kinase (or AMPK) is directly involved in optimizing muscle activity. The team bred a mouse that genetically expressed AMPK in an activated state. Like a trained athlete, this mouse enjoyed increased capacity to exercise, manifested by its ability to run three times longer than a normal mouse before exhaustion.
One particularly striking feature of the finding was the accumulation of muscle glycogen, the stored form of carbohydrates, a condition that many athletes seek by "carbo-loading" before an event or game. The study appears in the Nov. 14 online issue of the American Journal of Physiology: Endocrinology and Metabolism.
"Our genetically altered mouse appears to have already been an exercise program," says Lee Witters, the Eugene W. Leonard 1921 Professor of Medicine and Biochemistry at Dartmouth Medical School and professor of biological sciences at Dartmouth College. "In other words, without a prior exercise regimen, the mouse developed many of the muscle features that would only be observed after a period of exercise training."
Witters, whose lab led the study, explains that this finding has implication for anyone with a muscle disease and especially for the growing proportion of the population that is aging. Deteriorating muscles often make the elderly much more prone to fall, leading to hip and other fractures. According to Witters, there is tremendous interest in the geriatric field to find ways to improve muscle performance.
"We now wonder if it's possible to achieve elements of muscular fitness without having to exercise, which in turn, raises many questions about possible modes of exercise performance enhancement, including the development of drugs that could do the same thing as we have done genetically," he says. "This also might raise to some the specter of 'gene doping,' something seriously being talked about in the future of high-performance athletes."
Witters says that the carbohydrate, glucose, is a major fuel that powers muscles, and this contributes directly to a muscle's ability to repetitively contract during exercise. The activated AMPK in the Dartmouth mouse appears to have increased glycogen content by actually switching on a gene that normally synthesizes liver glycogen.
"The switching on of this liver gene in muscles," he says, "is a shift in the conception of the biochemistry of muscle metabolism, since many enzyme genes are thought to only be active in just one tissue."
Other authors on the paper include Laura Barré, Christine Richardson, and Steven Fiering, all at Dartmouth; Michael Hirshman and Laurie Goodyear of Joslin Diabetes Center in Boston; Joseph Brozinick with Eli Lilly and Company; and Bruce Kemp of the St. Vincent's Institute in Australia.
This research is funded by the National Institutes of Health.
Sue Knapp | EurekAlert!
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