Bente Karlund Pedersen, professor of internal medicine at Rigshospitalet and leader of the Muscle Center at the University of Copenhagen, Denmark, and her team are part of that search. They found a cytokine, Interleukin-6, which is produced by contracting muscles and released into the blood, and demonstrated that IL-6 has many biologic roles, including:
In fact, Pedersen says the wide impact of muscle-derived IL-6 not only fulfills all the criteria for this "exercise factor" but that such classes of cytokines should be reclassified as "myokines." She points out that because of its diverse effects, IL-6 has potential as a therapeutic drug in treating such metabolic disorders as obesity, type 2 diabetes and atherosclerosis.
Many roles and sources prompt additional IL-6 research
Over the years, "increased levels of IL-6 after exercise is remarkably consistent," Pedersen and a collaborator, Mark A. Febbraio of the Dept. of Physiology at the University of Melbourne, Australia, noted in an article in the FASEB Journal. However, the actual appearance of IL-6 into circulation depends on several factors including exercise intensity, duration and mode. For instance, vigorous rowing doubles the amount of IL-6 in plasma relatively quickly, while in endurance activity IL-6 doesn't appear until later. Activation of the IL-6 gene seems to be enhanced when muscle glycogen content is low, while carbohydrate supplementation during exercise has been shown to inhibit the release of IL-6 from contracting muscle.
What does seem consistent is that IL-6 originates from the contracting limb and that skeletal muscle cells themselves are the likely source of production. The skeletal muscles seem to produce IL-6 in order to maintain metabolic homeostasis during periods of altered metabolic demand such as muscular exercise or insulin stimulation.
Such a powerful effector has also attracted some negative reaction, but Pedersen notes that IL-6 overall seems to have a "positive metabolic role in health and in the treatment of disease." For instance high levels of IL-6 have been found in patients with "metabolic syndrome," which may be explained by the fact that IL-6 is produced in adipose tissue. Similarly, increased levels of TNF-alpha and IL-6 have been observed in obese individuals, smokers and patients with non-insulin-dependent diabetes mellitus but there's no evidence that either is actually the source of these problems.
Indeed, it's possible, for instance, that IL-6 expression may be up-regulated in insulin resistant skeletal muscle in an attempt to overcome the impaired glucose uptake. And in other contexts, the current thinking is that IL-6 has primarily anti-inflammatory effects. It is this combination of effects that has prompted such widespread study.
Editors note: Pedersen is speaking at the American Physiological Society's 2004 Intersociety Meeting, "The Integrative Biology of Exercise," Oct. 6-9 in Austin. The basic meeting schedule can be found at (http://www.the-aps.org/meetings/aps/austin/tentative.pdf). The complete program, including abstracts, for the entire meeting is available upon request to members of the media.
Arrangements to attend the meeting, and for on-site or telephone interviews, can be made through APS Communications Officer Mayer Resnick (cell: 301-332-4402, mresnick@the-aps.org) or through Stacy Brooks, Communications Specialist, 301-634-7253 (sbrooks@the-aps.org). From Oct. 6 (2 p.m.) - Oct. 9, the onsite phone number in Austin is 512-482-8000, room 602, or 512-682-2950.
The 2004 Intersociety Meeting on the Integrated Biology of Exercise is cosponsored by APS, the American College of Sports Medicine and the Canadian Society for Exercise Physiology. Additional support as unrestricted educational grants came from: the National Aeronautics and Space Administration (NASA), the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMSD), Gatorade Sports Sciences Institute, Pfizer Inc. and U.S. Army Research Institute of Environmental Medicine (USARIEM).
The American Physiological Society was founded in 1887 to foster basic and applied bioscience. The Bethesda, Maryland-based society has more than 10,000 members and publishes 14 peer-reviewed journals containing almost 4,000 articles annually.
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