Free Radical-Dependent Ca 2+ Signaling: Role of Ca 2+ -Induced Ca 2+ Release

Previously we have shown that Fe³⁺/ascorbate-induced Ca²⁺ release from scallop sarcoplasmic reticulum (SR) is due to Ca²⁺-channel gating by free radicals. This study is aimed at demonstrating that Ca²⁺-induced Ca²⁺ release (CICR) plays a role in this kind of Ca²⁺ release. Scallop SR vesicles were incubated with fluo-3 and exposed to Fe³⁺/ascorbate. Fluorimetric recordings showed massive Ca²⁺ release, with maximum rate and 50% release occurring at 30 min after exposure. Conversely, the use of the probe for reactive oxygen species dihydrorhodamine or the assay of malondialdehyde allowed oxyradical production to be traced for ∼5 min only. Hence, although Ca²⁺ release started just after exposure to Fe³⁺/ascorbate, most release occurred after free radical exhaustion. Ruthenium red addition after Fe³⁺/ascorbate slowed down the Ca²⁺ release, whereas cyclic adenosine 5′-diphosphoribose addition accelerated it, indicating that the free radical-induced Ca²⁺ release from SR vesicles triggers a mechanism of CICR that dramatically increases the initial effect.