The loss of dystrophin in patients with Duchenne muscular dystrophy (DMD) causes devastating skeletal muscle degeneration and cardiomyopathy. Dystrophin-deficient (mdx) mice have a much milder phenotype, whereas double knockout (DKO) mice lacking both dystrophin and its homolog, utrophin, exhibit the clinical signs observed in DMD patients. We have previously shown that DKO and mdx mice have similar severities of histological features of cardiomyopathy, but no contractile functional measurements of DKO heart have ever been carried out. To investigate whether DKO mice display cardiac dysfunction at the tissue level, contractile response of the myocardium was tested in small, unbranched, ultrathin, right ventricular muscles. Under near physiological conditions, peak isometric active developed tension (F(dev), in mN/mm2) at a stimulation frequency of 4 Hz was depressed in DKO mice (15.3 +/- 3.7, n = 8) compared with mdx mice (24.2 +/- 5.4, n = 7), which in turn were depressed compared with wild-type (WT) control mice (33.2 +/- 4.5, n = 7). This reduced Fdev was also observed at frequencies within the murine physiological range; at 12 Hz, Fdev was (in mN/mm2) 11.4 +/- 1.8 in DKO, 14.5 +/- 4.2 in mdx, and 28.8 +/- 5.4 in WT mice. The depression of Fdev was observed over the entire frequency range of 4-14 Hz and was significant between DKO versus mdx mice, as well as between DKO or mdx mice versus WT mice. Under beta-adrenergic stimulation (1 micromol/l isoproterenol), Fdev in DKO preparations was only (in mN/mm2) 14.7 +/- 5.1 compared with 30.9 +/- 8.9 in mdx and 41.0 +/- 4.9 in WT mice. These data show that cardiac contractile dysfunction of mdx mice is generally worsened in mice also lacking utrophin.

Since omega-3 polyunsaturated fatty acids (n-3 PUFAs) can alter ventricular myocyte calcium handling, these fatty acids could adversely affect cardiac contractile function, particularly following myocardial infarction. Therefore, 4 wk after myocardial infarction, dogs were randomly assigned to either placebo (corn oil, 1 g/day, n = 16) or n-3 PUFAs supplement [docosahexaenoic acid (DHA) + eicosapentaenoic acid (EPA) ethyl esters; 1, 2, or 4 g/day; n = 7, 8, and 12, respectively] groups. In vivo, ventricular function was evaluated by echocardiography before and after 3 mo of treatment. At the end of the 3-mo period, hearts were removed and in vitro function was evaluated using right ventricular trabeculae and isolated left ventricular myocytes. The treatment elicited significant (P < 0.0001) dose-dependent increases (16.4-fold increase with 4 g/day) in left ventricular tissue and red blood cell n-3 PUFA levels (EPA + DHA, placebo, 0.42 +/- 0.04; 1 g/day, 3.02 +/- 0.23; 2 g/day, 3.63 +/- 0.17; and 4 g/day, 6.97 +/- 0.33%). Regardless of the dose, n-3 PUFA treatment did not alter ventricular function in the intact animal (e.g., 4 g/day, fractional shortening: pre, 42.9 +/- 1.6 vs. post, 40.1 +/- 1.7%; placebo: pre, 39.2 +/- 1.3 vs. post, 38.4 +/- 1.6%). The developed force per cross-sectional area, changes in length- and frequency-dependent behavior in contractile force, and the inotropic response to beta-adrenoceptor activation were also similar for trabeculae obtained from placebo- or n-3 PUFA-treated dogs. Finally, calcium currents and calcium transients were the same in myocytes from n-3 PUFA- and placebo-treated dogs. Thus dietary n-3 PUFAs did not adversely alter either in vitro or in vivo ventricular contractile function in dogs with healed infarctions.

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...

The body is from time to time exposed to nonnormothermic conditions; both hypo- and hyperthermia can occur as a result of external (environment) or internal (pathogens, allergens) stressors. To preserve life under hypo- and hyperthermic conditions, adequate perfusion of vital organs is mandated. Although cardiac output regulation under hyperthermic conditions has been studied, the mechanical response of basic contractile function of the myocardium itself is incompletely understood. Accordingly, we set out to test mechanical output of isolated myocardium under hyperthermic conditions and to compare the results with the hypo- and normothermic response in the same tissue. We observed that, in absence of a frequency change, developed force decreased markedly. At a physiological normal stimulation rate of 6 Hz, developed force decreases to 67.2 +/- 2.6% at 42 degrees C compared with 37 degrees C. In addition, twitch timing characteristics also accelerate, allowing for a faster relaxation; time from peak tension to 50% relaxation is approximately 23% faster (from 31.4 +/- 2.6 to 24.4 +/- 1.7 ms). Although this faster relaxation in turn prevents a steep increase in diastolic tension at high frequencies, the very fast calcium kinetics now prevent a more complete activation of the myofilaments, resulting in a lower twitch-force maximum at hyperthermic conditions. Even at maximal beta-adrenergic stimulation, developed force is well below levels reached at physiological temperature.

Hiranandani, Nitisha, Tepmanas Bupha-Intr, and Paul M. L. Janssen. SERCA overexpression reduces hydroxyl radical injury in murine myocardium. Am J Physiol Heart Circ Physiol 291: H3130 -H3135, 2006. First published June 23, 2006 doi:10.1152/ajpheart.01315.2005 are involved in the pathogenesis of ischemiareperfusion injury and are observed in clinical situations, including acute heart failure, stroke, and myocardial infarction. Acute transient exposure to ⅐OH causes an intracellular Ca 2ϩ overload and leads to impaired contractility. We investigated whether upregulation of sarcoplasmic reticulum Ca 2ϩ -ATPase function (SERCA) can attenuate ⅐OH-induced dysfunction. Small, contracting right ventricular papillary muscles from wild-type (WT) SERCA1a-overexpressing (transgenic, TG) and SERCA2a heterogeneous knockout (HET) mice were directly exposed to ⅐OH. This brief 2-min exposure led to a transient elevation of diastolic force (F dia) and depression of developed force (Fdev). In WT mice, Fdia increased to 485 Ϯ 49% and Fdev decreased to 11 Ϯ 3%. In sharp contrast, in TG mice F dia increased only to 241 Ϯ 17%, whereas Fdev decreased only to 51 Ϯ 5% (P Ͻ 0.05 vs. WT). In HET mice, Fdia rose more than WT (to 597 Ϯ 20%, P Ͻ 0.05), whereas F dev was reduced in a similar amount. After ϳ45 min after ⅐OH exposure, a new steady state was reached: F dev returned to 37 Ϯ 6% and 32 Ϯ 6%, whereas Fdia came back to 238 Ϯ 28% and 292 Ϯ 17% in WT and HET mice, respectively. In contrast, the sustained dysfunction was significantly less in TG mice: F dia and Fdev returned to 144 Ϯ 20% and 67 Ϯ 6%, respectively. Before exposure to ⅐OH, there is decrease in phospholamban (PLB) phosphorylation at Ser16 (pPLBSer16) and PLB phosphorylation at Thr17 (pPLBThr17) in TG mice and an increase in pPLBSer16 and pPLBThr17 in HET mice versus WT. After exposure to ⅐OH there is decrease in pPLBSer16 in WT, TG, and HET mice but no significant change in the level of pPLBThr17 in any group. The results indicate that SERCA overexpression can reduce the ⅐OH-induced contractile dysfunction in murine myocardium, whereas a reduced SR Ca 2ϩ -ATPase activity aggravates this injury. Loss of pPLB levels at Ser16 likely amplifies the differences observed in injury response. contractile function; calcium pump; ischemia; oxygen radicals HYDROXYL RADICALS (⅐OH) are one of the most aggressive species of oxygen free radicals that attack all molecules in the human body (8,29). These ⅐OH are involved in the pathogenesis of ischemia-reperfusion injury, which is observed in many clinical situations, including acute heart failure, stroke, and myocardial infarction. When ischemic myocardium is reperfused and oxygen reintroduced, there is a sudden burst of oxygen free radical production. This leads to the formation of damaging reactive species, such as hydroxyl radicals, hydrogen peroxide, and peroxynitrite (2, 28). These reactive oxygen species, especially hydroxyl radicals, interact with lipids, proteins, and nucleic acids and damage cell membranes and impair cellular fu...