Modification of sarcoplasmic reticulum (SR) Ca²⁺release by FK506 induces defective excitation-contraction coupling only when SR Ca²⁺recycling is disturbed

This study examined whether the effects of FK506-binding protein dissociation from sarcoplasmic reticulum (SR) Ca(2+) release channels on excitation-contraction (EC) coupling changed when SR Ca(2+) reuptake and (or) the trans-sarcolemmal Ca(2+) extrusion were altered. The steady-state twitch Ca(2+) transient (CaT), cell shortening, post-rest caffeine-induced CaT, and Ca(2+) sparks were measured in rat ventricular myocytes using laser-scanning confocal microscopy. In the normal condition, 50 micromol FK506/L significantly increased steady-state CaT, cell shortening, and post-rest caffeine-induced CaT. When the cells were solely perfused with thapsigargin, FK506 did not reduce any of the states, but when low [Ca(2+)](0) (0.1 mmol/L) was perfused additionally, FK506 reduced CaT and cell shortening, and accelerated the reduction of post-rest caffeine-induced CaT. FK506 significantly increased Ca(2+) spark frequency in the normal condition, whereas it mainly prolonged duration of individual Ca(2+) sparks under the combination of thapsigargin and low [Ca(2+)](0) perfusion. Modification of SR Ca(2+) release by FK506 impaired EC coupling only when released Ca(2+) could not be taken back into the SR and was readily extruded to the extracellular space. Our findings could partly explain the controversy regarding the contribution of FK506-binding protein dissociation to defective EC coupling.