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

Abstract

In this study, we investigated the role of elevated sarcoplasmic reticulum (SR) Ca 2+ leak through ryanodine receptors (RyR2s) in heart failure (HF)-related abnormalities of intracellular Ca 2+ handling, using a canine model of chronic HF. The cytosolic Ca 2+ 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 Ca 2+ content. Direct measurements of [Ca 2+] SR in both intact and permeabilized cardiac myocytes demonstrated that SR luminal [Ca 2+] is markedly lowered in HF, suggesting that alterations in Ca 2+ transport rather than fractional SR volume reduction accounts for the diminished Ca 2+ release capacity of SR in HF. SR Ca 2+ ATPase (SERCA2)-mediated SR Ca 2+ uptake rate was not significantly altered, and Na +/Ca 2+ exchange activity was accelerated in HF myocytes. At the same time, SR Ca 2+ leak, measured directly as a loss of [Ca 2+] SR after inhibition of SERCA2 by thapsigargin, was markedly enhanced in HF myocytes. Moreover, the reduced [Ca 2+] 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 Ca 2+ signals could be reasonably well mimicked by the RyR2 channel agonist caffeine. Taken together, these results suggest that RyR2-mediated SR Ca 2+ leak is a major factor in the abnormal intracellular Ca 2+ handling that critically contributes to the reduced SR Ca 2+ content of failing cardiomyocytes.