Patient Care Endocrinology & Cardiology - October 2007 - (Page 21)

FIGURE 1 IMAGE: CRAIG ZUCKERMAN The troponin complex is made up of 3 forms of troponin. Troponin I binds to actin and inhibits actin-myosin interaction. Troponin T binds to tropomyosin and attaches the troponin to the thin filament. Troponin C binds to calcium. In healthy patients, cTn is usually undetectable. Most of the available cTnI and cTnT is bound to cardiac myofilaments. Free cytosolic calcium stimulates myofibril contraction, but this process is regulated by the troponin complex. The calcium binds to a site at the N-terminal end of troponin C, producing a conformational change in this protein. This signals troponin I to release its inhibition on actin-myosin cross-coupling mediated predominantly by troponin T via its binding to the thin filament of actin. Up to 7% of troponin T and 5% of troponin I is present in the cytosol. This is potentially important because this pool of troponin is postulated to be that which is released in subnecrotic degrees of injury. It should be noted that, theoretically, troponin is undetectable in plasma with current assays. When the myocyte is injured and membrane integrity is lost, the cytosolic fraction leaks out. This occurs within 4 to 6 hours of injury and peaks at approximately 18 hours. Elevated levels can persist for up to 7 to 10 days as a result of the much slower release kinetics of structural troponin that occurs as a result of myofibrillar degradation. The release of only the soluble cytosolic pool in non-MI conditions ex- plains the more rapid return to baseline of the troponin level that is seen in this setting. Cardiac troponins are large protein molecules, so clearance occurs primarily via nonrenal mechanisms. The clearance half-life is approximately 2 hours. For this reason, renal insufficiency alone would not be expected to raise the cTn levels. Measurement Troponin is measured clinically by the use of monoclonal antibodies directed at epitopes of the troponin T and I proteins. Measurements of troponin C are not used because the identical protein is present in skeletal and cardiac muscle. The assay for troponin T is standardized, and results obtained from different institutions are comparable. This is not the case for troponin I; each institution must analyze and standardize its own results. The consensus definition of an abnormally increased level is one that exceeds that obtained in 99% of a reference control group. Ideally, the coefficient of variation should be less than 10%, although this is not always the case for all assays at all institutions. The corollary of the 99% cutoff rule is that 1% of specimens will have elevated or detectable levels OCTOBER 2007 PATIENT CARE ENDOCRINOLOGY & CARDIOLOGY 21

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Patient Care - Endocrinology & Cardiology - October 2007 Research Digest Contents Medicine in the News Recognizing and Managing Patients with the Metabolic Syndrome and Prediabetes Using Troponins to Evaluate Cardiac Injury The 15-Minute Visit Classified Advertising Clinical Clips

Patient Care Endocrinology & Cardiology - October 2007

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