Myosin heavy chain and cardiac troponin T damage is associated with impaired myofibrillar ATPase activity contributing to sarcomeric dysfunction in Ca2+-paradox rat hearts

This study aimed to explore the potential contribution of myofibrils to contractile dysfunction in Ca2+-paradox hearts. Isolated rat hearts were perfused with Krebs-Henseleit solution (Control), followed by Ca2+-depletion, and then Ca2+-repletion after Ca2+-depletion (Ca2+-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 Ca2+-depletion (9.8 +/- 1.3 mmHg) and Ca2+-paradox (12.9 +/- 1.3 mmHg) with similar decline in +dP/dt and -dP/dt. LVEDP was increased in both Ca2+-depletion and Ca2+-paradox. Compared to Control, myofibrillar Ca2+-stimulated ATPase activity was decreased in the Ca2+-depletion group (12.08 +/- 0.57 vs. 8.13 +/- 0.19 A mu mol P-i/mg protein/h), besides unvarying Mg2+ ATPase activity, while upon Ca2+-paradox myofibrillar Ca2+-stimulated ATPase activity was decreased (12.08 +/- 0.57 vs. 8.40 +/- 0.22 A mu mol P-i/mg protein/h), but Mg2+ ATPase activity was increased (3.20 +/- 0.25 vs. 7.21 +/- 0.36 A mu mol P-i/mg protein/h). In force measurements of isolated cardiomyocytes at saturating [Ca2+], Ca2+-depleted cells had lower rate constant of force redevelopment (k (tr,max), 3.85 +/- 0.21) and unchanged active tension, while those in Ca2+-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, alpha-myosin heavy chain and cardiac troponin T presented progressive degradation during Ca2+-depletion and Ca2+-paradox. Our results suggest that contractile impairment in Ca2+-paradox partially resides in deranged sarcomeric function and compromised myofibrillar ATPase activity as a result of myofilament protein degradation, such as alpha-myosin heavy chain and cardiac troponin T. Impaired relaxation seen in Ca2+-paradoxical hearts is apparently not related to titin, rather explained by the altered myofibrillar ATPase activity.