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 (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 [³H]dihydro-FK506 to those for [³H]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 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.