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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.

molecular and cellular biochemistry

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