Troponin is a protein complex that confers calcium sensitivity to muscle cells. Calcium fluctuations are pivotal to each muscle contraction.

Troponin is found in both skeletal muscle and cardiac muscle, but the specific versions of troponin differ between types of muscle, due to the expression of different genes (in the heart, for example).

Discussions of troponin often pertain to its functional characteristics and/or to its usefulness as a diagnostic marker for various heart disorders.

Functional charactertistics

Both cardiac and skeletal muscles are exquisitely controlled by changes in the intracellular calcium concentration. When calcium rises, the muscles contract. When calcium falls the muscles relax. Troponin is a component of thin filamemts (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, the structure of the thin filament changes in such a manner that myosin (a molecular motor organized in muscle thick filaments) attaches to thin filaments and produces force and/or movement. In the absence of calcium, tropomyosin interferes with this action of myosin, and therefore muscles remain relaxed.

Diagnostic use

Certain subtypes of troponin (Troponin I and T) are quite specific to the heart muscle (myocardium) and are measured in blood to differentiate between unstable angina pectoris and myocardial infarction in patients with chest pain.

Other conditions in which cardiac troponins can be elevated (Ammann P et al 2004):

• 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 (marathon)

Sources

• Ammann P, Pfisterer M, Fehr T, Rickli H. Raised cardiac troponins. BMJ 2004;328:1028-9. PMID 15117768