Cardiac Troponin T Release into Plasma after Acute Myocardial Infarction: Only Fractional Recovery Compared with Enzymes

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Abstract
After acute myocardial infarction (AMI) cardiac enzymes and proteins are released into plasma and are used as biochemical markers of cardiac muscle injury. We studied the completeness of the release of troponin T, a cardiac protein that is largely bound to myofibrillar structures and compared it with the release of cytoplasmic cardiac enzymes in 22 patients with AMI, who were treated with thrombolytic therapy. Creatine kinase (CK; EC 2.7.3.2), hydroxybutyrate dehydrogenase (HBDH), lactate dehydrogenase (LDH; EC 1.1.1.27) and troponin T were assayed serially in plasma samples obtained frequently and for at least 168 h after the start of thrombolytic therapy. Cumulative release of enzymes and troponin T in plasma were calculated by using a two-compartment model for circulating proteins. In order to express the cumulative plasma releases in gram equivalents (g-eq) healthy myocardium per litre plasma (infarct size), we determined HBDH, LDH and total troponin T contents per gram net weight of tissue in 17 human hearts obtained post-mortem from patients who died from non-cardiac causes. Mean (SD) tissue contents per gram wet weight of, respectively, 156 ± 25 U/g, 385 ± 59 U/g and 234 ± 65 μg/g were found. For the cardiac enzymes CK, HBDH and LDH the mean (SEM, n = 22) total release over 72 h, was, respectively, 5·9 ± 1·5, 5·9 ± 1·6 and 6·1 ± 1·7 g-eq/L. There was no further increase after 72 h and the differences between enzymes were not significant. The mean (SEM) cumulative troponin T release, expressed in gram equivalents of myocardium per litre of plasma was only 0·30 ± 0·09 g-eq/L after 72 h and 0·51 ± 0·61 g-eq/L after 168 h. After 72 h total recovery of troponin T in g-eq/L was only 5% and after 168 h only 8·5% of the total recovery of cytoplasmic cardiac enzymes after 72 h. Cumulative troponin T release after 72 h and after 168 h correlates well with infarct size, estimated from cumulative cytoplasmic enzyme release. However, quantification of infarct size should preferably be performed from plasma release curves of cytoplasmic cardiac enzymes or proteins in order to prevent underestimation of infarct size, caused by incomplete release of the non-cytoplasmic proteins.