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.