Enhanced Ryanodine Receptor-Mediated Calcium Leak Determines Reduced Sarcoplasmic Reticulum Calcium Content in Chronic Canine Heart Failure

In this study, we investigated the role of elevated sarcoplasmic reticulum (SR) Ca2+ leak through ryanodine receptors (RyR2s) in heart failure (HF)-related abnormalities of intracellular Ca2+ handling, using a canine model of chronic HF. The cytosolic Ca2+ transients were reduced in amplitude and slowed in duration in HF myocytes compared with control, changes paralleled by a dramatic reduction in the total SR Ca2+ content. Direct measurements of [Ca2+]SR in both intact and permeabilized cardiac myocytes demonstrated that SR luminal [Ca2+] is markedly lowered in HF, suggesting that alterations in Ca2+ transport rather than fractional SR volume reduction accounts for the diminished Ca2+ release capacity of SR in HF. SR Ca2+ ATPase (SERCA2)-mediated SR Ca2+ uptake rate was not significantly altered, and Na+/Ca2+ exchange activity was accelerated in HF myocytes. At the same time, SR Ca2+ leak, measured directly as a loss of [Ca2+]SR after inhibition of SERCA2 by thapsigargin, was markedly enhanced in HF myocytes. Moreover, the reduced [Ca2+]SR in HF myocytes could be nearly completely restored by the RyR2 channel blocker ruthenium red. The effects of HF on cytosolic and SR luminal Ca2+ signals could be reasonably well mimicked by the RyR2 channel agonist caffeine. Taken together, these results suggest that RyR2-mediated SR Ca2+ leak is a major factor in the abnormal intracellular Ca2+ handling that critically contributes to the reduced SR Ca2+ content of failing cardiomyocytes.