Force-pCa relation and troponin T isoforms of rabbit myocardium.

Abstract

We have previously reported the existence of at least four troponin T isoforms in rabbit ventricular muscle and described the changes in their distribution with development. In this report we test whether the proportions of the troponin T isoforms are related to the sensitivity of the myofilaments to calcium. We measured the force-pCa relations in 12 detergent-skinned ventricular strands of cardiac muscle from newborn (2-5-day-old) rabbits. We determined from each strand the amount of each troponin T isoform relative to the total amount of troponin T by using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and densitometric scans of Western blots probed with a cardiac-specific troponin T monoclonal antibody, MAb 13-11. To assess the presence of different relative amounts of cardiac and slow skeletal troponin I among the strands, we determined the amount of cardiac troponin I relative to tropomyosin. We determined the Hill coefficient and the pCa for half-maximal force, pCa50, for each strand. pCa50 was related directly to the relative amount of troponin T2 (pslope = 0.037). Our results do not indicate a relation between the Hill coefficient and troponin T2. We also did not find a relation between pCa50 and the cardiac troponin I/tropomyosin ratio, which suggests that the correlation between pCa50 and troponin T2 was not a result of changes in the relative amounts of cardiac and slow skeletal muscle troponin I. Our findings indicate that a relation exists between the force-pCa characteristics of rabbit myocardium and the troponin T isoforms that it expresses, suggesting a role for troponin T in modulating the sensitivity of cardiac myofilaments to calcium.

Role of myosin heavy chain composition in kinetics of force development and relaxation in rat myocardium

“…If this is the case, our observation of no significant change in maximum force with increased expression of â-MHC might indicate that fewer cross-bridges are bound during activation of thyroid deficient myocardium. The lack of change in pCa50 provides corroborating evidence that thyroid deficiency did not alter the relative expression of the thin-filament-associated regulatory proteins, since variations in expression of TnT (Nasser et al 1991;Reiser et al 1992;Hofmann et al 1995) have previously been shown to alter significantly the Ca¥ sensitivity of tension. Increased â-MHC expression did not significantly alter the steepness of the tension-pCa relationship (nµ) at least within the variability of the data ( Table 3), suggesting that MHC content has no effect on the apparent co-operativity of tension development.…”

Cardiac Troponin T Isoforms Affect the Ca2+ Sensitivity of Force Development in the Presence of Slow Skeletal Troponin I

“…Fibers were then transferred to a similar replacement solution without Triton X-100. The skinned cardiac muscle (ϳ120 m in diameter) was mounted using stainless steel clips to a force transducer and immersed in a relaxation solution (pCa 8 -ATP], 15 mM creatine phosphate, and 20 units/ml phosphocreatine kinase and between 76 and 92 mM potassium propionate to achieve a constant ionic strength of 150 mM in all solutions (17). The contraction solution (pCa 4.0) was the same composition as the pCa 8.0 solution except that the Ca 2ϩ concentration was 10 Ϫ4 M and was used to measure the initial force.…”

“…Recently, we have shown that the human cTnT isoforms affect the Ca 2ϩ sensitivity of force development and their ability to inhibit actomyosin ATPase activity in the presence of human cardiac troponin I (cTnI) (4). Other groups have also shown that TnT isoforms affect the Ca 2ϩ sensitivity of force development and ATPase activity (5)(6)(7)(8)(9). These results all suggest that the N terminus of TnT affects the physiological function of troponin (Tn).…”

Myofibrillar remodelling in cardiac hypertrophy, heart failure and cardiomyopathies

“…Isoform appearance is developmentally regulated: cTnT 3 predominates, while cTnT 4 is a minor (fetal) isoform in the adult human heart; the additional two fetal isoforms, cTnT 1 and cTnT 2 , are also present, but are barely detectable (86,87). Different studies have suggested that a significant amount of cTnT 4 is re-expressed in failing myocardium (84,85,87), which exhibits decreased myofibrillar ATPase activity (80,81,83) and a decrease in Ca 2+ sensitivity (88,89). Changes in the variable region of the cTnT molecule can alter interactions in the troponin complex; this may explain how a shift in cTnT isoforms alters myocardial contractility and Ca 2+ sensitivity of the cardiac contractile proteins (66,86).…”