Effect of myofilament Ca²⁺ sensitivity on Ca²⁺ wave propagation in rat ventricular muscle

Background: The propagation velocity of Ca²⁺ waves determines delayed afterdepolarization and affects the occurrence of triggered arrhythmias in cardiac muscle. We focused on myofilament Ca²⁺ sensitivity, investigating how the velocity of Ca²⁺ waves responds to its increased sensitivity resulting from muscle stretch or the addition of a myofilament Ca²⁺ sensitizer, SCH00013. We further investigated whether production of reactive oxygen species (ROS) may be involved in the change in velocity. Methods: Trabeculae were obtained from rat hearts. Force, sarcomere length, and [Ca²⁺]_i were measured. ROS production was estimated from 2',7'-dichlorofluorescein (DCF) fluorescence. Trabeculae were exposed to a 10mM Ca²⁺ jet for the induction of Ca²⁺ leak from the sarcoplasmic reticulum in its exposed region. Ca²⁺ waves were induced by 2.5-Hz stimulus trains for 7.5s (24°C, 2.0mM [Ca²⁺]_o). Muscle stretch of 5, 10, and 15% was applied 300ms after the last stimulus of the train. Results: Muscle stretch increased the DCF fluorescence, the amplitude of aftercontractions, and the velocity of Ca²⁺ waves depending on the degree of stretch. After preincubation with 3μM diphenyleneiodonium (DPI), muscle stretch increased only the amplitude of aftercontractions but not the DCF fluorescence nor the velocity of Ca²⁺ waves. SCH00013 (30μM) increased the DCF fluorescence, the amplitude of aftercontractions, and the velocity of Ca²⁺ waves. DPI suppressed these increases. Conclusions: Muscle stretch increases the velocity of Ca²⁺ waves by increasing ROS production, not by increasing myofilament Ca²⁺ sensitivity. In the case of SCH00013, ROS production increases myofilament Ca²⁺ sensitivity and the velocity of Ca²⁺ waves. These results suggest that ROS rather than myofilament Ca²⁺ sensitivity plays an important role in the determination of the velocity of Ca²⁺ waves, that is, arrhythmogenesis.