Cardiac troponin T composition in normal and regenerating human skeletal muscle

Cardiac troponin T (cTnT), measurement of which has been recommended for diagnosing myocardial infarction, was initially believed to be specific for the heart. However, recent publications have reported cTnT in sera of patients without cardiac disease; therefore, we investigated whether cTnT could be found in human skeletal muscle tissues. Using immunohistochemistry, Western blot, and quantitative cTnT ELISA, we assayed human heart (n = 3), normal human skeletal muscle (n = 6), and diseased skeletal muscle samples from patients with polymyositis (PM, n = 13) and Duchenne muscular dystrophy (DMD, n = 6). All heart specimens contained cTnT, but the expression of cTnT in normal skeletal muscle samples varied widely, ranging from no expression (quadriceps femoris) to expression by up to 20% of the muscle fibers (diaphragm). Immunohistochemistry detected cTnT in skeletal muscle of 8 of the PM patients and all of the DMD patients. Mean myofibrillar cTnT concentrations (mg/g myofibrillar protein) were: cardiac = 10.0, normal skeletal = 0.8, PM skeletal = 0.7, and DMD skeletal = 4.37, confirming the results of immunohistochemistry. Western blot analysis also confirmed the expression of cTnT in muscle from DMD patients. These findings provide evidence that cTnT is not 100% cardiac-specific but also is expressed in regenerating (PM and DMD) as well as in normal (nonregenerating) skeletal muscle.