Abstract 1539: Redox-Modification of Ryanodine Receptor Underlies Sarcoplasmic Reticulum Luminal Calcium- Dependent Cardiac Alternans in a Canine Model of Sudden Cardiac Death
Cardiac alternans is a known predictor of cardiac arrhythmias and sudden cardiac death (SCD). While many factors have been proposed in the genesis of alternans, underlying causes remain to be defined in disease settings. We investigated the potential role and mechanisms of myocyte calcium alternans in a canine post-myocardial infarction (MI) model of SCD using a combination of approaches including in vivo cardiac function measurements and imaging of cytosolic Ca ([Ca]c) and sarcoplasmic reticulum (SR) luminal Ca ([Ca]SR) in isolated patch-clamped cardiac myocytes. The MI dogs used in this study were studied 6 – 8 weeks after the MI, had preserved left ventricular function, and were susceptible to ventricular fibrillation during exercise. In myocytes isolated from MI dogs, the frequency-dependence of beat-to-beat alternations in the amplitude of systolic [Ca]c transients (Ca alternans) was shifted towards slower rates compared to control. In both groups of cells, cytosolic Ca transients alternated in phase with changes in diastolic [Ca]SR; however the dependence of cytosolic Ca transient amplitude on [Ca]SR was steeper in MI cells than in controls. Abnormal ryanodine receptor (RyR) function in MI cells was indicated by increased fractional Ca release (systolic [Ca]c/diastolic [Ca]SR) for a given amplitude of Ca current, and elevated diastolic RyR-mediated SR Ca leak. SR Ca uptake did not differ between MI and control cells. MI myocytes had a significantly increased rate of reactive oxygen species production and increased RyR oxidation. Treatment of MI myocytes with the reducing agents dithiothreitol or mercaptopropionylglycine, normalized parameters of calcium handling and shifted the threshold of calcium alternans to higher frequencies. Conversely, application of the oxidizing agent 4,4′-dithiodipyridine to control cells mimicked the abnormal Ca cycling phenotype of MI myocytes. We suggest that redox modulation of RyRs promotes generation of Ca alternans by enhancing the steepness of the Ca release-load relationship thereby providing a substrate for arrhythmia in MI hearts.
This research has received full or partial funding support from the American Heart Association, AHA National Center.