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) Ca2+ release channels on excitation-contraction (EC) coupling changed when SR Ca2+ reuptake and (or) the trans-sarcolemmal Ca2+ extrusion were altered. The steady-state twitch Ca2+ transient (CaT), cell shortening, post-rest caffeine-induced CaT, and Ca2+ sparks were measured in rat ventricular myocytes using laser-scanning confocal microscopy. In the normal condition, 50 mu mol 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 [Ca2+](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 Ca2+ spark frequency in the normal condition, whereas it mainly prolonged duration of individual Ca2+ sparks under the combination of thapsigargin and low [Ca2+](0) perfusion. Modification of SR Ca2+ release by FK506 impaired EC coupling only when released Ca2+ 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.