Altered interaction of FKBP12.6 with ryanodine receptor as a cause of abnormal Ca2+ release in heart failure

Objective: Little information is available as to the Ca2+ release function of the sarcoplasmic reticulum (SR) in heart failure. We assessed whether the alteration in this function in heart failure is related to a change in the role of FK binding protein (FKBP), which is tightly coupled with the cardiac ryanodine receptor (RyR) and recently identified as a modulatory protein acting to stabilize the gating function of RyR. Methods: SR vesicles were isolated from dog LV muscles [normal (N), n=6; heart failure induced by 3-weeks pacing (HF), n=6]. The time course of the SR Ca2+ release was continuously monitored using a stopped-flow apparatus, and [H-3]ryanodine-binding and [H-3]dihydro-FK506-binding assays were also performed. Results: FK506, which specifically binds to FKBP12.6 and dissociates it from RyR, decreased the polylysine-induced enhancement of [H-3]ryanodine-binding by 38% in N (P<0.05) but it had no effect in HF. In HF, the rate constant for the polylysine-induced Ca2+ release from the SR was 61% smaller than in N. FK506 decreased the rate constant for the polylysine-induced Ca2+ release by 67% in N (P<0.05) but had no effect in HF. The [H-3]dihydro-FK506-binding assay revealed that the number (B-max) of FKBPs was decreased by 83% in HF (P<0.05), while the K-d, value was unchanged. FK506 did not significantly change SR Ca2+.-ATPase activity in either N or HF. Conclusions: In HF, the number of FKBPs showed a tremendous decrease; this may underlie the RyR-channel instability and the impairment of the Ca2+ release function of RyR seen in the failing heart. (C) 2000 Elsevier Science B.V. All rights reserved.