Myosin heavy chain and cardiac troponin T damage is associated with impaired myofibrillar ATPase activity contributing to sarcomeric dysfunction in Ca(2+)-paradox rat hearts

This study aimed to explore the potential contribution of myofibrils to contractile dysfunction in Ca(2+)-paradox hearts. Isolated rat hearts were perfused with Krebs–Henseleit solution (Control), followed by Ca(2+)-depletion, and then Ca(2+)-repletion after Ca(2+)-depletion (Ca(2+)-paradox) by Langendorff method. During heart perfusion left ventricular developed pressure (LVDP), end-diastolic pressure (LVEDP), rate of pressure development (+ dP/dt), and pressure decay (-dP/dt) were registered. Control LVDP (127.4 ± 6.1 mmHg) was reduced during Ca(2+)-depletion (9.8 ± 1.3 mmHg) and Ca(2+)-paradox (12.9 ± 1.3 mmHg) with similar decline in +dP/dt and –dP/dt. LVEDP was increased in both Ca(2+)-depletion and Ca(2+)-paradox. Compared to Control, myofibrillar Ca(2+)-stimulated ATPase activity was decreased in the Ca(2+)-depletion group (12.08 ± 0.57 vs. 8.13 ± 0.19 µmol P(i)/mg protein/h), besides unvarying Mg(2+) ATPase activity, while upon Ca(2+)-paradox myofibrillar Ca(2+)-stimulated ATPase activity was decreased (12.08 ± 0.57 vs. 8.40 ± 0.22 µmol P(i)/mg protein/h), but Mg(2+) ATPase activity was increased (3.20 ± 0.25 vs. 7.21 ± 0.36 µmol P(i)/mg protein/h). In force measurements of isolated cardiomyocytes at saturating [Ca(2+)], Ca(2+)-depleted cells had lower rate constant of force redevelopment (k (tr,max), 3.85 ± 0.21) and unchanged active tension, while those in Ca(2+)-paradox produced lower active tension (12.12 ± 3.19 kN/m(2)) and k (tr,max) (3.21 ± 23) than cells of Control group (25.07 ± 3.51 and 4.61 ± 22 kN/m(2), respectively). In biochemical assays, α-myosin heavy chain and cardiac troponin T presented progressive degradation during Ca(2+)-depletion and Ca(2+)-paradox. Our results suggest that contractile impairment in Ca(2+)-paradox partially resides in deranged sarcomeric function and compromised myofibrillar ATPase activity as a result of myofilament protein degradation, such as α-myosin heavy chain and cardiac troponin T. Impaired relaxation seen in Ca(2+)-paradoxical hearts is apparently not related to titin, rather explained by the altered myofibrillar ATPase activity.