Troponin I and T

The troponin complex is the main regulatory protein of the thin filament of the myofibrils that regulate the Ca 2+-dependent ATP hydrolysis of actomyosin. The troponin complex consists of three subunits: an inhibitory subunit (troponin I), a tropomyosin binding subunit (troponin T), and a calcium-binding subunit (troponin C). Because each subunit has cardiac, slow-twitch, and fast-twitch skeletal isoforms, immunoassays based upon the significant heterogeneity in amino acid sequences can detect the specific isoforms. The cardiac isoform of troponin I is not found in skeletal muscle during any stage of development. As a result, elevations of cardiac troponin I do not occur in the setting of acute or chronic skeletal muscle damage in the absence of myocardial necrosis.

Following AMI, both cardiac troponin I and troponin T become elevated after approximately 6 h, peak at 12 h, and remain elevated for 7 to10 days. Both have a higher specificity for myocardial necrosis than CK-MB in selected subsets of patients, such as those presenting late in the course of MI, those with recent surgery, a cocaine habit, or skeletal muscle disease. Either troponin may be elevated in patients with renal failure. In ED patients with symptoms of possible ischemia, both troponins have been shown to have high sensitivity and specificity for AMI. Elevation of either cardiac troponin also predicts subsequent cardiovascular complications independent of CK-MB and the ECG.1 13

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