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The immunophilin FK506-binding protein modulates Ca2+ release channel closure in rat heart
R P Xiao1, H H Valdivia, K Bogdanov
1Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, MD 21224, USA.
Insights
FK506-binding protein (FKBP) may terminate calcium release from the cardiac sarcoplasmic reticulum. Inhibiting FKBP prolonged calcium sparks and RyR openings, suggesting a negative feedback role in heart cells.
Area of Science:
- Cardiology
- Molecular Biology
- Cell Physiology
Background:
- The signal terminating calcium (Ca2+) release from cardiac sarcoplasmic reticulum is not well understood.
- FK506-binding protein (FKBP) is associated with the ryanodine receptor (RyR)/Ca2+ release channel in the heart.
Purpose of the Study:
- To investigate the role of FKBP in terminating Ca2+-induced Ca2+ release (CICR) in cardiac cells.
- To determine if FKBP acts as a negative feedback mechanism on RyR activity.
Main Methods:
- Confocal microscopy with fluo-3 to visualize Ca2+ sparks in rat ventricular myocytes.
- Electrophysiological single-channel recordings of cardiac RyRs in planar lipid bilayers.
- Inhibition of FKBP using FK506 or rapamycin.
Main Results:
- Inhibiting FKBP with FK506 or rapamycin increased Ca2+ spark duration 6-7 fold.
- FK506 prolonged electrically stimulated Ca2+ transients and contractions without affecting L-type Ca2+ channel current.
- Single-channel recordings showed FK506 prolonged RyR mean open lifetime ~7-fold and slowed spontaneous decay of activity.
Conclusions:
- FKBP may provide an intrinsic mechanism to terminate RyR openings.
- FKBP likely exerts negative feedback on CICR in heart cells.
Abstract:
1. The nature of the signal that terminates the release of Ca2+ from the cardiac sarcoplasmic reticulum has remained elusive. This study was intended to examine whether FK506-binding protein (FKBP), which is tightly associated to the ryanodine receptor (RyR)/Ca2+ release channel, plays a role in the termination of Ca(2+)-induced Ca2+ release (CICR) in heart. 2. Confocal microscopy and the Ca2+ indicator fluo-3 were used to visualize the elementary release events, i.e. 'Ca2+ sparks' in rat ventricular myocytes under resting or voltage-clamped conditions. Additionally, electrophysiological single-channel recordings, at constant [Ca2+] or during [Ca2+] steps produced by photorelease of caged Ca2+, were obtained from rat cardiac RyRs incorporated in planar lipid bilayers. 3. Inhibition of FKBP by the immunosuppressants FK506 or rapamycin increased the duration of spontaneous or depolarization-evoked Ca2+ sparks 6- to 7-fold. In addition, Ca2+ sparks were seen with two-level amplitudes, corresponding to full and half normal spark amplitude. 4. FK506 potentiated and prolonged electrically stimulated [Ca2+]i transients and contractions, but did not affect the amplitude and kinetics of the L-type Ca2+ channel current. 5. In planar lipid bilayers, FK506 (15 microM) prolonged approximately 7-fold the mean open lifetime of reconstituted single RyRs, induced the appearance of long-lasting subconductance states, and markedly slowed the spontaneous decay of RyR activity elicited by fast and sustained Ca2+ stimuli. The time constant of the spontaneous decay of activity increased from 1.8 s in control to > or = 20 s in the presence of FK506. 6. We conclude that FKBP may afford an intrinsic mechanism to terminate RyR openings and it may thus exert a negative feedback on CICR in heart cells.