Altered stoichiometry of FKBP12.6 versus ryanodine receptor as a cause of abnormal Ca2+ leak through ryanodine receptor in heart failure

Background-In the pathogenesis of cardiac dysfunction in heart failure, a decrease in the activity of the sarcoplasmic reticulum (SR) Ca2+-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 (FK8P12.6) 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 [H-3]dihydro-FK506 to those for [H-3]ryanodine binding, In normal SR, the molar ratio was 3.6 (approximate to1 FKBP12.6 for each RyR monomer), whereas it was 1.6 in failing SR. In normal SR, FK506 caused a dose-dependent Ca2+ leak that showed a close parallelism with the conformational change in RyR. In failing SR, a prominent Ca2+ leak was observed even in the absence of FK506, and FK506 produced little or no further increase in Ca2+ 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 Ca2+ 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 Ca2+ leak might possibly cause Ca2+ overload and consequent diastolic dysfunction, as well as systolic dysfunction.