Altered Stoichiometry of FKBP12.6 Versus Ryanodine Receptor as a Cause of Abnormal Ca 2+ Leak Through Ryanodine Receptor in Heart Failure

Abstract

Background —In the pathogenesis of cardiac dysfunction in heart failure, a decrease in the activity of the sarcoplasmic reticulum (SR) Ca 2+ -ATPase is believed to be a major determinant. Here, we report a novel mechanism of cardiac dysfunction revealed by assessing the functional interaction of FK506–binding protein (FKBP12.6) with the cardiac ryanodine receptor (RyR) in a canine model of pacing-induced heart failure. Methods and Results —SR vesicles were isolated from left ventricular muscles (normal and heart failure). The stoichiometry of FKBP12.6 per RyR was significantly decreased in failing SR, as assessed by the ratio of the B max values for [ 3 H]dihydro-FK506 to those for [ 3 H]ryanodine binding. In normal SR, the molar ratio was 3.6 (≈1 FKBP12.6 for each RyR monomer), whereas it was 1.6 in failing SR. In normal SR, FK506 caused a dose-dependent Ca 2+ leak that showed a close parallelism with the conformational change in RyR. In failing SR, a prominent Ca 2+ leak was observed even in the absence of FK506, and FK506 produced little or no further increase in Ca 2+ leak and only a slight conformational change in RyR. The level of protein expression of FKBP12.6 was indeed found to be significantly decreased in failing SR. Conclusions —An abnormal Ca 2+ leak through the RyR is present in heart failure, and this leak is presumably caused by a partial loss of RyR-bound FKBP12.6 and the resultant conformational change in RyR. This abnormal Ca 2+ leak might possibly cause Ca 2+ overload and consequent diastolic dysfunction, as well as systolic dysfunction.