Troponins, Cardiac
Basic Information 
Physiology
• Cardiac troponins are a group of intracellular, contractile proteins within the cardiomyocyte that are part of the troponin-tropomyosin regulatory complex. They regulate the calcium dependent interaction of actin and myosin during excitation-contraction coupling.
• The troponin complex is comprised of three distinct subunits. Troponin C (cTnC) binds calcium, troponin I (cTnI) is the inhibitory component that prevents interaction between actin and myosin until intracellular calcium is bound by troponin C, allowing heart muscle relaxation, and troponin T (cTnT) binds troponin I to the actin filament.
• Approximately 95% of cTnI exists as structurally bound, with ~5% in the cytosolic pool.
• Cardiac troponins are released into circulation with ischemia, necrosis, or damage to the cardiomyocyte.
• The cardiac troponins are well-conserved across mammalian species and have been used with increasing frequency to aid in the diagnosis of certain cardiac diseases.
• The cardiac isoforms of troponin I and T are specific for cardiac muscle because they are antigenically distinct from the skeletal muscle isoforms, showing minimal cross-reactivity. They are the preferred biochemical diagnostic tools for detecting certain myocardial diseases in people.
• Troponin C is not used clinically because of homology (cross reactivity) to other isoforms.
• In people, cardiac troponins are released into the circulation within 4 to 6 hours after myocardial injury or damage, and may persist for 1 to 2 weeks.
• cTnI has been studied the most in horses. Its half-life in horses is not known; however, it appears to be released within 3 to 4 hours of myocardial injury.
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