Abstract 497: Normalization of Intracellular Calcium Signaling by Chronic Cardiac Resynchronization Involves Redox Modulation of Ryanodine Receptors.

Cardiac resynchronization therapy (CRT) represents a promising treatment modality to alleviate LV dysfunction in humans with heart failure (HF). Using a canine chronic model of HF, we recently showed that CRT improves abnormal Ca2+ handling by reducing the diastolic Ca2+ leak from the SR through cardiac ryanodine receptors (RyR2s). Using the same models of HF and CRT therapy, we now demonstrate that the specific molecular mechanism for improved RyR2-mediated Ca2+ signaling involves partial restoration of RyR2s from post-translational oxidative modifications. RyR2 expression was markedly reduced in HF and restored to control levels after CRT. Single RyR2 channel activity from HF and CRT groups were recorded using the lipid bilayer technique. In both HF and CRT groups two functional types of RyR2s could be clearly distinguished based on their overall activity as well as sensitivities to luminal ( trans ) Ca2+ and cytosolic ( cis ) Mg2+: high-Po (open probability) and low-Po RyR2s; in contrast to normal controls (only low-Po). High-Po RyR2s in both groups exhibited enhanced sensitivity to trans Ca2+ and increased resistance to inhibition with cis Mg2+. These properties of the high Po RyR2s define the hyperactive, i.e. leaky RyR2 behavior in HF. Low-Po RyR2s had functional properties which did not differ from those observed in RyR2s from normal hearts. In HF ~80% of the RyR2s were high Po, while only ~20% were low Po RyR2s. Following CRT, the fraction of low Po RyR2s increased (to ~60%) while the remainder had high-Po behavior. High-Po channels could be partially normalized by the reducing agent DTT; whereas low-Po channels could be converted to the high Po type by treatment with the oxidative agent DTDP. These results suggest that RyR2 alterations in HF and normalization by CRT occur by redox modulation of a small number of critical sites susceptible to reversible oxidative modification. Additionally, the altered balance of the RyR2 functional fractions following CRT could reflect replacement of irreversibly modified RyR2s by newly synthesized protein, under condition of reduced oxidative stress. These data suggest that altered redox modulation contributes to abnormal function of the RyR2s in HF and that improved RyR2 redox balance may contribute to the efficacy of CRT.