THE IMMUNOPHILIN FK506-BINDING PROTEIN MODULATES CA2-HEART( RELEASE CHANNEL CLOSURE IN RAT)

1. The nature of the signal that terminates the release of Ca2+ from t he cardiac sarcoplasmic reticulum has remained elusive. This study was intended to examine whether FK506-binding protein (FKBP), which is ti ghtly associated to the ryanodine receptor (RyR)/Ca2+ release channel, plays a role in the termination of Ca2+-induced Ca2+ release (CICR) i n heart. 2. Confocal microscopy and the Ca2+ indicator fluo-3 were use d to visualize the elementary release events, i.e. 'Ca2+ sparks' in ra t ventricular myocytes under resting or voltage-clamped conditions. ad ditionally, electrophysiological single-channel recordings, at constan t [Ca2+] or during [Ca2+] steps produced by photorelease of caged Ca2, were obtained from rat cardiac RyRs incorporated in planar lipid bil ayers. 3. Inhibition of FKBP by the immunosuppressants FK506 or rapamy cin increased the duration of spontaneous or depolarization-evoked Ca2 + sparks 6- to 7-fold. In addition, Ca2+ sparks were seen with two-lev el amplitudes, corresponding to full and half normal spark amplitude. 4. FK506 potentiated and prolonged electrically stimulated [Ca2+](i) t ransients and contractions, but did not affect the amplitude and kinet ics of the L-type Ca2+ channel current. 5. In planar lipid bilayers, F K506 (15 mu M) prolonged similar to 7-fold the mean open lifetime of r econstituted single RyRs, induced the appearance of long-lasting subco nductance states, and markedly slowed the spontaneous decay of RyR act ivity elicited by fast and sustained Ca2+ stimuli. The time constant o f the spontaneous decay of activity increased from 1.8 s in control to greater than or equal to 20 s in the presence of FK506. 6. We conclud e that FKBP may afford an intrinsic mechanism to terminate RyR opening s and it may thus exert a negative feedback on CICR in heart cells.