Troponin I phosphorylation in the normal and failing adult human heart

2001--a biomarker odyssey

It is a time of transition in the area of biomarkers. Confusion is rampant, progress immense and the challenges invigorating. Better diagnosis is possible if we progress in a rational way and avoid the chaos so often characteristic of transitions by relying on the science of the discipline to keep us on track.

Increase in calcium responsiveness of cardiac myofilament activation in ovariectomized rats

To evaluate a possible role of ovarian sex hormones in the Ca2+ responsiveness of cardiac myofilament activation, the relations of pCa (-log Ca2+ molar concentration) to actomyosin adenosine triphosphatase (ATPase) activity of isolated myofibrillar preparations from 8-10 week ovariectomized (Ovx) rat hearts were compared with those from sham-operated hearts. Deficiency of ovarian sex hormones in plasma of ovariectomized rats was indirectly verified by a significant reduction in uterine weights. Body weights of the ovariectomized rats were significantly greater than those of sham-operated controls. Despite a significant increase in heart weight of 10 week ovariectomized animals, the percent of heart weight-to-body weight ratio was not different from control group. The maximum myofibrillar ATPase activity at pH 7.0 was significantly suppressed after ovariectomy in both eight and ten week groups. However, the maximum ATPase activity at pH 6.5 was significantly suppressed only in 10 week ovariectomized hearts. Surprisingly, in every condition with depressed maximum myofibrillar ATPase activity, the pCa-actomyosin ATPase relationships of ovariectomized cardiac myofilaments demonstrated a significant leftward shift in pCa50 (-log half-maximally Ca2+ activation) from those of sham-operated controls. There was, however, no change in the Hill-coefficient of these cardiac myofilaments after ovariectomy. Analysis of myofilament proteins using gel electrophoresis demonstrated neither change nor loss of any thin filament proteins. These results indicate a possible modulating effect of ovarian sex hormone deficiency on the Ca2+ responsiveness of cardiac myofilament activation by induction of myofilament Ca2+ hypersensitivity but suppression of maximum myofibrillar ATPase activity.

Characterization of cardiac troponin subunit release into serum after acute myocardial infarction and comparison of assays for troponin T and I

We examined the release of cardiac troponin T (cTnT) and I (cTnI) into the blood of patients after acute myocardial infarction (AMI). Three postAMI serum samples were applied in separate analytical runs onto a calibrated gel filtration column (Sephacryl S-200), and the proteins were separated by molecular weight. Using commercial cTnT and cTnI assays measured on collected fractions, we found that troponin was released into blood as a ternary complex of cTnT-I-C, a binary complex of cTnI-C, and free cTnT, with no free cTnI within the limits of the analytical methodologies. The serum samples were also examined after incubation with EDTA and heparin. EDTA broke up troponin complexes into individual subunits, whereas heparin had no effect on the assays tested. We added free cTnC subunits to 24 AMI serum samples and found no marked increase in the total cTnI concentrations, using an immunoassay that gave higher values for the cTnI-C complex than free cTnI. To characterize the cross-reactivity of cTnT and cTnI assays, purified troponin standards in nine different forms were prepared, added to serum and plasma pools, and tested in nine quantitative commercial and pre-market assays for cTnI and one approved assay for cTnT. All nine cTnI assays recognized each of the troponin I forms (complexed and free). In five of these assays, the relative responses for cTnI were nearly equimolar. For the remainder, the response was substantially greater for complexed cTnI than for free cTnI. Moreover, there was a substantial difference in the absolute concentration of results between cTnI assays. The commercial cTnT assay recognized binary and ternary complexes of troponin on a near equimolar basis. We conclude that all assays are useful for detection of cardiac injury. However, there are differences in absolute cTnI results due to a lack of mass standardization and heterogeneity in the cross-reactivities of antibodies to various troponin I forms.

Cardiac protein phosphorylation: functional and pathophysiological correlates

Protein phosphorylation acts a pivotal mechanism in regulating the contractile state of the heart by modulating particular levels of autonomic control on cardiac force/length relationships. Early studies of changes in cardiac protein phosphorylation focused on key components of the excitation-coupling process, namely phospholamban of the sarcoplasmic reticulum and myofibrillar troponin I. In more recent years the emphasis has shifted towards the identification of other phosphoproteins, and more importantly, the delineation of the mechanistic and signaling pathways regulating the various known phosphoproteins. In addition to cAMP- and Ca(2+)-calmodulin-dependent kinase processes, these have included regulation by protein kinase C and the ever-emerging family of growth factor-related kinases such as the tyrosine-, mitogen- and stress-activated protein kinases. Similarly, the role of protein dephosphorylation by protein phosphatases has been recognized as integral in modulating normal cardiac cellular function. Recent studies involving a variety of cardiovascular pathologies have demonstrated that changes in the phosphorylation states of key cardiac regulatory proteins may underlie cardiac dysfunction in disease states. The emphasis of this comprehensive review will be on discussing the role of cardiac phosphoproteins in regulating myocardial function and pathophysiology based not only on in vitro data, but more importantly, from ex vivo experiments with corroborative physiological and biochemical evidence.