Postischemic myocardial contractile dysfunction is in part mediated by the burst of reactive oxygen species (ROS), which occurs with the reintroduction of oxygen. We hypothesized that tissue oxygen tension modulates this ROS burst at reperfusion. After 20 min of global ischemia, isolated rat hearts were reperfused with temperature-controlled (37.4°C) Krebs-Henseleit buffer saturated with one of three different O2 concentrations (95, 20, or 2%) for the first 5 min of reperfusion and then changed to 95% O 2. Additional hearts were loaded with 1) allopurinol (1 mM), a xanthine oxidase inhibitor, 2) diphenyleneiodonium (DPI; 1 M), an NAD(P)H oxidase inhibitor, or 3) Tiron (10 mM), a superoxide scavenger, and were then reperfused with either 95 or 2% O 2 for the first 5 min. ROS production and tissue oxygen tension were quantitated using electron paramagnetic resonance spectroscopy. Tissue oxygen tension was significantly higher in the 95% O2 group. However, the largest radical burst occurred in the 2% O 2 reperfusion group (P Ͻ 0.001). Recovery of left ventricular (LV) contractile function and aconitase activity during reperfusion were inversely related to the burst of radical production and were significantly higher in hearts initially reperfused with 95% O2 (P Ͻ 0.001). Allopurinol, DPI, and Tiron reduced the burst of radical formation in the 2% O2 reperfusion groups (P Ͻ 0.05). Hypoxic reperfusion generates an increased ROS burst originating from multiple pathways. Recovery of LV function during reperfusion is inversely related to this oxygen radical burst, highlighting the importance of myocardial oxygen tension during initial reperfusion.reactive oxygen species; contractile function; cardiac arrest REPERFUSION, FOLLOWING GLOBAL ISCHEMIA of the heart, restores oxygen delivery to the ischemic tissue. During ischemia, there is a strong reductive pressure, which, with the reintroduction of oxygen, results in the burst of reactive oxygen species (ROS), which are thought to primarily be responsible for the postischemic myocardial contractile dysfunction (2, 4, 5). This phenomenon of postischemic contractile dysfunction may result from either cell injury or myocardial stunning (6), or a combination of both. This postischemic myocardial dysfunction is seen after a number of clinical syndromes including both regional (acute myocardial infarction) and global (cardiac arrest) myocardial ischemia (15,20).ROS, particularly superoxide, may be generated from multiple sources and mechanisms depending on the duration of preceding ischemia. Earlier studies identified endothelial cells as a major source of ROS at reperfusion that could be inhibited by xanthine oxidase blockers (31) and react with iron to form the very reactive hydroxyl radical (30). After prolonged periods of ischemia sufficient to result in necrosis, a major source of ROS appears to be neutrophils, which can be blocked with anti-neutrophil interventions causing a reduction of infarct size (7). However, after shorter periods of ischemia resulting in myocardial s...

Background-Ventricular hypertrophy is a physiological response to pressure overload that, if left untreated, can ultimately result in ventricular dysfunction, including diastolic dysfunction. The aim of this study was to test the hypothesis that frequency-dependent myofilament desensitization, a physiological response of healthy myocardium, is altered in hypertrophied myocardium. Methods and Results-New Zealand white rabbits underwent a pulmonary artery banding procedure to induce pressure overload. After 10 weeks, the animals were euthanized, hearts removed, and suitable trabeculae harvested from the free wall of the right ventricle. Twitch contractions, calibrated bis-fura-2 calcium transients, and myofilament calcium sensitivity (potassium contractures) were measured at frequencies of 1, 2, 3, and 4 Hz. The force frequency response, relaxation frequency response, and calcium frequency relationships were significantly blunted, and diastolic tension significantly increased with frequency in the pulmonary artery banding rabbits compared with sham-operated animals. Myofilament calcium sensitivity was virtually identical at 1 Hz in the treatment versus sham group (pCa 6.11Ϯ0.03 versus 6.11Ϯ0.06), but the frequency-dependent desensitization that takes place in the sham group (⌬pCa 0.14Ϯ0.06, PϽ0.05) was not observed in the pulmonary artery banding animals (⌬pCa 0.02Ϯ0.05). Analysis of myofilament protein phosphorylation revealed that the normally observed frequency-dependent phosphorylation of troponin-I is lost in pulmonary artery banding rabbits. Conclusions-The frequency-dependent myofilament desensitization is significantly impaired in right ventricular hypertrophy and contributes to the frequency-dependent elevation of diastolic tension in hypertrophy. (Circ Heart Fail. 2009;2:472-481.)Key Words: hypertrophy Ⅲ calcium sensitivity Ⅲ heart rate Ⅲ EC-coupling Ⅲ myofilaments V entricular hypertrophy can occur as a result of sustained pressure overload on the ventricles arising from hypertension, valvular stenosis, or ventricular dysfunction. The hypertrophic response is thought to be compensatory at first, but in later stages can result in ventricular dysfunction and eventually pump failure. 1 The normal myocardial responses to increases in heart rate can begin to change during the transition from compensatory hypertrophy to decompensation. The force frequency relationship (FFR), normally positive in healthy myocardium, is usually severely blunted or even negative in cases of decompensated hypertrophy and becomes worse as the heart approaches failure. 2,3 Clinical Perspective on p 481Although it is incompletely understood how the FFR changes with disease, it is clear that alterations in calcium handling play a major role. [3][4][5] The role (if any) the myofilaments play in the contractile dysfunction of decompensating ventricular hypertrophy remains unresolved, in particular as it relates to changes in heart rate. Myofilament calcium sensitivity has been reported to be unaltered in LV myocytes from rapid paced d...

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Pyruvate is a metabolic fuel that is a potent inotropic agent. Despite its unique inotropic and antioxidant properties, the molecular mechanism of its inotropic mechanism is still largely unknown. To examine the inotropic effect of pyruvate in parallel with intracellular calcium handling under near physiological conditions, we measured pH, myofilament calcium sensitivity, developed force, and calcium transients in ultra thin rabbit heart trabeculae at 37 °C loaded iontophoretically with the calcium indicator bis-fura-2. By contrasting conditions of control versus sarcoplasmic reticulum block (with either cyclopiazonic acid and ryanodine or with thapsigargin) we were able to characterize and isolate the effects of pyruvate on sarcoplasmic reticulum calcium handling and developed force. A potassium contracture technique was subsequently utilized to assess the force-calcium relationship and thus the myofilament calcium sensitivity. Pyruvate consistently increased developed force whether or not the sarcoplasmic reticulum was blocked (16.8±3.5 to 24.5±5.1 vs. 6.9±2.6 to 12.5±4.4 mN/mm2, non-blocked vs. blocked sarcoplasmic reticulum respectively, p<0.001, n = 9). Furthermore, the sensitizing effect of pyruvate on the myofilaments was demonstrated by potassium contractures (EC50 at baseline versus 20 minutes of pyruvate infusion (peak force development) was 701±94 vs. 445±65 nM, p<0.01, n = 6). This study is the first to demonstrate that a leftward shift in myofilament calcium sensitivity is an important mediator of the inotropic effect of pyruvate. This finding can have important implications for future development of therapeutic strategies in the management of heart failure.

In vivo LGE-CMR sequences and high-density voltage mapping using a conventional linear catheter fail to provide accurate characterization of post-MI scar, limiting the specificity of voltage-based strategies and imaging-guided procedures.