H(2)O(2)-induced left ventricular dysfunction in isolated working rat hearts is independent of calcium accumulation

Reactive oxygen species (ROS) and intracellular Ca²⁺ overload play key roles in myocardial ischemia-reperfusion (IR) injury but the relationships among ROS, Ca²⁺ overload and LV mechanical dysfunction remain unclear. We tested the hypothesis that H₂O₂ impairs LV function by causing Ca²⁺ overload by increasing late sodium current (I_Na), similar to Sea Anemone Toxin II (ATX-II). Diastolic and systolic Ca²⁺ concentrations (dCa²⁺]_i and sCa²⁺]_i) were measured by indo-1 fluorescence simultaneously with LV work in isolated working rat hearts. H₂O₂ (100 μM, 30 min) increased dCa²⁺]_i and sCa²⁺]_i. LV work increased transiently then declined to 32% of baseline before recovering to 70%. ATX-II (12 nM, 30 min) caused greater increases in dCa²⁺]_i and sCa²⁺]_i. LV work increased transiently before declining gradually to 17%. Ouabain (80 μM) exerted similar effects to ATX-II. Late I_Na inhibitors, lidocaine (10 μM) or R56865 (2 μM), reduced effects of ATX-II on Ca²⁺]_i and LV function, but did not alter effects of H₂O₂. The antioxidant, N-(2-mercaptopropionyl)glycine (MPG, 1 mM) prevented H₂O₂-induced LV dysfunction, but did not alter Ca²⁺]_i. Paradoxically, further increases in Ca²⁺]_i by ATX-II or ouabain, given 10 min after H₂O₂, improved function. The failure of late I_Na inhibitors to prevent H₂O₂-induced LV dysfunction, and the ability of MPG to prevent H₂O₂-induced LV dysfunction independent of changes in Ca²⁺]_i indicate that impaired contractility is not due to Ca²⁺ overload. The ability of further increases in Ca²⁺]_i to reverse H₂O₂-induced LV dysfunction suggests that Ca²⁺ desensitization is the predominant mechanism of ROS-induced contractile dysfunction.