Polymyositis is a skeletal muscle disease that causes muscle weakness and inflammation. Your signs and symptoms might come on gradually, but your muscle weakness is likely to be progressive. The disease can make it tough for you to stand from a seated position, reach overhead and, in some cases, swallow or speak. This condition is more common among women. Problems with metabolic processes that involve ATP may be a factor in this disease.
ATP
The cellular processes in muscle tissue require energy. ATP, or adensosine triphosphate, is the most common type of energy molecule that your muscles use. ATP interacts with many other substances in a complex chemical process when your muscles contract and relax. Your body does not keep much ATP on hand, however.
Metabolic Issue
The physical cause of polymyositis hasn't been pinpointed yet, but researchers have some theories. Geisinger Medical Center researchers E.D. Newman and R.J. Kurland believe it may be a metabolic issue related to changes in the way the body utilizes energy. During exercise, people who have polymyositis have higher than normal ratios of inorganic phosphate to phosphocreatine. Also, their loss of ATP is disproportional to their loss of loss of phosphocreatine, the Pennsylvania researchers say in a 1992 study published in "Arthritis and Rheumatism."
Phosphocreatine and ATP
The relationship between phosphocreatine and ATP in your body is important, as is your body's ratio of inorganic phosphate to phosphocreatine. Phosphocreatine is found in muscle cells. Since your body doesn't have a stockpile of ATP to utilize, it most often synthesizes this energy currency via a chemical reaction, or pathway, that utilizes phosphocreatine. Phosphocreatine cannot directly provide energy for muscle contractions, but it "donates" a high-energy phosphate group to a chemical called adenosine diphosphate, or ADP, to form ATP. A 2002 study by University of Oxford, England researchers G. Cea et al. found that people with polymyositis have recovery half-times for phosphocreatine and adenosine diphosphate that are almost twice as long as normal. In addition, in a person with polymyositis the maximum rate of ATP production is half that of a normal person's. The study was published in the neurology journal, "Brian"
Inorganic Phosphate
Meanwhile, inorganic phosphate levels rise during muscle fatigue due to breakdown of creatine phosphate. Researchers used to believe that lactic acid build-up was the most significant factor in muscle fatigue. However, new research points to inorganic phosphate as a major cause of muscle fatigue, according to a 2002 study by Håkan Westerblad et al. published in the journal "Physiology." Thus, if your ratio of inorganic phosphate to phosphocreatine is higher than normal, you have less of the chemical you need to create ATP for energy and more of the substance that is believed to cause fatigue.
References
- Mayo Clinic: Polymyositis
- PubMed.gov: P-31 magnetic resonance spectroscopy in polymyositis and dermatomyositis. Altered energy utilization during exercise
- "Physiology"; Muscle Fatigue: Lactic Acid or Inorganic Phosphate the Major Cause?; Håkan Westerblad, et al.; 2002
- PubMed.gov: Reduced oxidative phosphorylation and proton efflux suggest reduced capillary blood supply in skeletal muscle of patients with dermatomyositis and polymyositis: a quantitative 31P-magnetic resonance spectroscopy and MRI study
- Peak Performance: Phosphocreatine -- Your Body's Primary Energy Currency
- University of California: Energy Sources
