Troponin
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Overview
Troponin is a complex of three proteins that is integral to muscle contraction in skeletal and cardiac muscle, but not smooth muscle. 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.
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.
Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders.
Functional characteristics
Role of troponins
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 a component of thin filaments (along with actin and tropomyosin), and is the protein to which calcium binds to accomplish this regulation. Troponin has three subunits, TnC, TnI, and TnT. When calcium is bound to specific sites on TnC, tropomyosin rolls out of the way of the actin filament active sites, so that myosin (a molecular motor organized in muscle thick filaments) can attach to the thin filament and produce force and/or movement. In the absence of calcium, tropomyosin interferes with this action of myosin, and therefore muscles remain relaxed.
Troponin I has also been shown to inhibit angiogenesis in vivo and in vitro.
Individual subunits serve different functions:
- Troponin C binds to calcium ions to produce a conformational change in TnI
- Troponin T binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
- Troponin I binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place
Diagnostic use
Certain subtypes of troponin (cardiac troponin I and T) are very sensitive and specific indicators of damage to the heart muscle (myocardium). They are measured in the blood to differentiate between unstable angina and myocardial infarction (heart attack) in patients with chest pain. A patient who had suffered from a myocardial infarction would have an area of damaged heart muscle and so would have elevated cardiac troponin levels in the blood.[1]
It is important to note that cardiac troponins are a marker of all heart muscle damage, not just myocardial infarction. Other conditions that directly or indirectly lead to heart muscle damage can also therefore increase troponin levels:[1]
- Cardiac:
- Cardiac amyloidosis
- Cardiac contusion
- Cardiac surgery and heart transplant
- Defibrillation
- Closure of atrial septal defects
- Coronary vasospasm
- Dilated cardiomyopathy
- Heart failure
- Hypertrophic cardiomyopathy
- Myocarditis
- Percutaneous coronary intervention
- Radiofrequency ablation
- Supraventricular tachycardia
- Non-cardiac:
- Critical illness, e.g. sepsis
- High-dose chemotherapy
- Primary pulmonary hypertension
- Pulmonary embolism
- Renal failure
- Subarachnoid hemorrhage
- Scorpion venom
- Stroke
- Very heavy exercise (e.g. marathon)
Cardiac troponin T (cTnT) and I (cTnI) are measured by immunoassay methods. A single manufacturer distributes cTnT but a host of diagnostic companies make cTnI methods available on many different immunoassay platforms.[1]
Drug-induced cardiotoxicity is common to all classes of therapeutic drugs. It is essential that cardiotoxicity is detected with a high degree of sensitivity and specificity. The newly developed troponins are especially useful in this context[1]
References
fr:Troponine sv:Troponinvi:Troponin
Acknowledgement and Attribution Regarding Sources of Content
Some of the initial content on this page may be incorporated in part from copyleft sources in the public domain including wikis such as Wikipedia and AskDrWiki. Drug information for patients came from the The National Library of Medicine. Infectious disease information may have come from the Centers for Disease Control (CDC). Differential Diagnoses are drawn from clinicians as well as an amalgamation of 3 sources: 1.The Disease Database; 2. Kahan, Scott, Smith, Ellen G. In A Page: Signs and Symptoms. Malden, Massachusetts: Blackwell Publishing, 2004:3; 3. Sailer, Christian, Wasner, Susanne. Differential Diagnosis Pocket. Hermosa Beach, CA: Borm Bruckmeir Publishing LLC, 2002:7 .
