New Troponin T
Early Detector of Myocardial Infarction (MI)
Troponin is a complex of three regulatory proteins (subunits) that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle. These subunits are TnC, TnI, and TnT.
Troponin is attached to the protein tropomyosin and lies within the groove between actin filaments in muscle tissue. In a relaxed muscle, tropomyosin blocks the attachment site for the myosin crossbridge, thus preventing contraction. When the muscle cell is stimulated to contract by an action potential, calcium channels open in the sarcoplasmic reticulum and release calcium into the sarcoplasm. Some of this calcium attaches to troponin, causing a conformational change that moves tropomyosin out of the way so that the cross bridges can attach to actin and produce muscle contraction.
Both cardiac and skeletal muscles are controlled by changes in the intracellular calcium concentration. When calcium rises, the muscles contract, and when calcium falls the muscles relax.
Troponin is found in both skeletal muscle and cardiac muscle, but the specific versions of troponin differ between types of muscle. The main difference is that the TnC subunit of troponin in skeletal muscle has four calcium ion binding sites, whereas in cardiac muscle there are only three.
Cardiac troponin is specific to the myocardium and levels in the serum rise 3–4 hours after the occurrence of cardiac symptoms in patients with acute myocardial infarction (MI).
Because of its high sensitivity and specificity, elevated levels of troponin indicate myocardial damage but not the mechanism of damage. The diagnosis of MI has thus evolved following the introduction of routine troponin testing, resulting in the redefinition of MI by the joint European Society of Cardiology (ESC)/American College of Cardiology (ACC) in 2000. Based on this consensus document, any amount of myocardial damage, as detected by serum troponin and associated with evidence of ischemia, should be considered an MI.
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