Levosimendan improves contractility in vivo and in vitro in a rodent model of post-myocardial infarction heart failure.

AIM: As few studies have presented a thorough analysis of the effect of levosimendan (LEV) on contractility, our purpose was to investigate in vivo cardiac function as well as in vitro cardiomyocyte function and calcium (Ca(2+) ) handling following LEV treatment. METHODS: Rats with post-myocardial infarction heart failure (HF) induced by ligation of the left anterior descending coronary artery and sham-operated animals were randomized to the infusion of LEV (2.4 µg kg(-1) min(-1) ) or vehicle for 40 min. Echocardiographic examination was coupled to pressure-volume sampling in the left ventricle before (B) and after (40 min) infusion. Isolated left ventricular cardiomyocytes were studied in an epifluorescence microscope. RESULTS: HF LEV (n = 6), HF vehicle (n = 7), sham LEV (n = 5) and sham vehicle (n = 6) animals were included. LEV infusion compared to vehicle in HF animals reduced left ventricular end-diastolic pressure and mean arterial pressure (both P < 0.001) and improved the slope of the preload-recruitable stroke work (P < 0.05). Administrating LEV to HF cardiomyocytes in vitro improved fractional shortening and Ca(2+) sensitivity index ratio, and increased the diastolic Ca(2+) (all P < 0.01). CONCLUSION: In HF animals, LEV improved the contractility by increasing the Ca(2+) sensitivity. Furthermore loading conditions were changed, and LEV could consequently change organ perfusion. An observed increase in diastolic Ca(2+) following LEV treatment and clinical implications of this should be further addressed.

Reduced aerobic capacity causes leaky ryanodine receptors that trigger arrhythmia in a rat strain artificially selected and bred for low aerobic running capacity.

AIM: Rats selectively bred for inborn low capacity of running (LCR) display a series of poor health indices, whereas rats selected for high capacity of running (HCR) display a healthy profile. We hypothesized that selection of low aerobic capacity over generations leads to a phenotype with increased diastolic Ca(2+) leak that trigger arrhythmia. METHODS: We used rats selected for HCR (N = 10) or LCR (N = 10) to determine the effect of inborn aerobic capacity on Ca(2+) leak and susceptibility of ventricular arrhythmia. We studied isolated Fura-2/AM-loaded cardiomyocytes to detect Ca(2+) handling and function on an inverted epifluorescence microscope. To determine arrhythmogenicity, we did a final experiment with electrical burst pacing in Langendorff-perfused hearts. RESULTS: Ca(2+) handling was impaired by reduced Ca(2+) amplitude, prolonged time to 50% Ca(2+) decay and reduced sarcoplasmic reticulum (SR) Ca(2+) content. Impaired Ca(2+) removal was influenced by reduced SR Ca(2+) ATP-ase 2a (SERCA2a) function and increased sodium/Ca(2+) exchanger (NCX) in LCR rats. Diastolic Ca(2) leak was 87% higher in LCR rats. The leak was reduced by CaMKII inhibition. Expression levels of phosphorylated threonine 286 CaMKII levels and increased RyR2 phosphorylation at the serine 2814 site mechanistically support our findings of increased leak in LCR. LCR rats had significantly higher incidence of ventricular fibrillation. CONCLUSION: Selection of inborn low aerobic capacity over generations leads to a phenotype with increased risk of ventricular fibrillation. Increased phosphorylation of CaMKII at serine 2814 at the cardiac ryanodine receptor appears as an important mechanism of impaired Ca(2+) handling and diastolic Ca(2+) leak that results in increased susceptibility to ventricular fibrillation.

High inborn aerobic capacity does not protect the heart following myocardial infarction.

Maximal oxygen uptake (Vo2max) is a strong prognostic marker for morbidity and mortality, but the cardio-protective effect of high inborn Vo2max remains unresolved. We aimed to investigate whether rats with high inborn Vo2max yield cardio-protection after myocardial infarction (MI) compared with rats with low inborn Vo2max. Rats breed for high capacity of running (HCR) or low capacity of running (LCR) were randomized into HCR-SH (sham), HCR-MI, LCR-SH, and LCR-MI. Vo2max was lower in HCR-MI and LCR-MI compared with respective sham (P < 0.01), supported by a loss in global cardiac function, assessed by echocardiography. Fura 2-AM loaded cardiomyocyte experiments revealed that HCR-MI and LCR-MI decreased cardiomyocyte shortening (39%, and 34% reduction, respectively, both P < 0.01), lowered Ca(2+) transient amplitude (37%, P < 0.01, and 20% reduction, respectively), and reduced sarcoplasmic reticulum (SR) Ca(2+) content (both; 20%, P < 0.01) compared with respective sham. Diastolic Ca(2+) cycling was impaired in HCR-MI and LCR-MI evidenced by prolonged time to 50% Ca(2+) decay that was partly explained by the 47% (P < 0.01) and 44% (P < 0.05) decrease in SR Ca(2+)-ATPase Ca(2+) removal, respectively. SR Ca(2+) leak increased by 177% in HCR-MI (P < 0.01) and 67% in LCR-MI (P < 0.01), which was abolished by inhibition of Ca(2+)/calmodulin-dependent protein kinase II. This study demonstrates that the effect of MI in HCR rats was similar or even more pronounced on cardiac- and cardiomyocyte contractile function, as well as on Ca(2+) handling properties compared with observations in LCR. Thus our data do not support a cardio-protective effect of higher inborn aerobic capacity.

Reduced pH and contractility in failing rat cardiomyocytes.

AIM: To determine whether reduced cardiomyocyte contractility in heart failure is associated with reduced intracellular pH (pH(i)). Involvement of the Na(+)/H(+) exchanger and the H(+)/K(+) ATPase were investigated with specific blockers. METHODS: Myocardial infarction and subsequent heart failure in Sprague-Dawley rats were induced by chronic occlusion of the left coronary artery. 6 weeks post-ligation, contractility (cell shortening) and pH(i) (BCECF fluorescence) were recorded in freshly dissociated cardiomyocytes during 2-10 Hz electrical stimulation, with or without either Na(+)/H(+) exchanger or H(+)/K(+) ATPase inhibition. RESULTS: Elevated end-diastolic and reduced peak systolic pressures confirmed heart failure. Increased heart weights (20-30%; P < or = 0.01) and cardiomyocyte lengths and widths (22-25%; P < or = 0.01) confirmed substantial cardiac hypertrophy. In myocytes isolated from sham operated rats, a positive staircase response occurred with stimulation rates from 2 to 7 Hz; further increases in stimulation rate up to 10 Hz reduced contractility. In contrast, pH(i) fell progressively over the entire stimulation range. In failing myocytes, pH(i) was consistently 0.07 pH units lower and contractility 40% lower (P < or = 0.01) than sham control values; the shape of the contractility staircase remained similar to controls. At all stimulation frequencies, Na(+)/H(+) exchanger inhibition reduced pH(i) by 0.05 pH units (P < or = 0.01) and contractility by 22% (P < or = 0.05) in cardiomyocytes from the heart failure group. A significantly smaller decrease of pH(i) and reduction in contractility was observed after inhibition of Na(+)/H(+) exchanger (10 micro m HOE694) in sham myocytes. H(+)/K(+) ATPase inhibition (100 micro m SCH28080) had no effect on pH(i). CONCLUSION: Reduced pH(i) is accompanied by reduced cardiomyocyte contractility in isolated myocytes from post-MI heart failure. The data suggest compensatory Na(+)/H(+) exchanger activation in heart failure, whereas H(+)/K(+) ATPase does not appear to contribute significantly to pH(i) maintenance.