Intra-sarcoplasmic reticulum Ca2+ oscillations are driven by dynamic regulation of ryanodine receptor function by luminal Ca2+ in cardiomyocytes.

During the cardiac cycle, the release of Ca(2+) from the sarcoplasmic reticulum (SR) through the ryanodine receptor (RyR2) channel complex is controlled by the levels of cytosolic and luminal Ca(2+) and alterations in these regulatory processes have been implicated in cardiac disease including arrhythmia. To better understand the mechanisms of regulation of SR Ca(2+) release by Ca(2+) on both sides of the SR membrane, we investigated SR Ca(2+) release in a wide range of cytosolic Ca(2+) concentrations ([Ca(2+)](cyt); 1-100 microm) in permeabilized canine ventricular myocytes by monitoring [Ca(2+)] inside the SR ([Ca(2+)](SR)). Exposing myocytes to activating [Ca(2+)](cyt) resulted in spontaneous oscillations of [Ca(2+)](SR) due to periodic opening and closing of the RyR2s. Elevating [Ca(2+)](cyt) (up to 10 microm) increased the frequency of [Ca(2+)](SR) oscillations; however at higher [Ca(2+)](cyt) (>50 microm) the oscillations diminished due to RyR2s staying perpetually open, resulting in depleted SR. Ablation of cardiac calsequestrin (CASQ2) altered the [Ca(2+)](cyt) dependence of Ca(2+) release oscillations such that oscillations were highly frequent at low [Ca(2+)](cyt) (100 nm) but became diminished at moderate [Ca(2+)](cyt) (10 microm), as determined in myocytes from calsequestrin-null versus wild-type mice. Our results suggest that under conditions of continuous activation by cytosolic Ca(2+), RyR2s can periodically cycle between open and deactivated states due to effects of luminal Ca(2+). Deactivation at reduced [Ca(2+)]SR appears to involve reduction of sensitivity to cytosolic Ca(2+) and might be mediated by CASQ2. Inactivation by cytosolic Ca(2+) plays no detectable role in controlling SR Ca(2+) release.