Fura-2 antagonises calcium-induced calcium release

Calcium-induced calcium release (CICR) from the endoplasmic reticulum (ER) takes place through ryanodine receptors (RyRs) and it is often revealed by an increase of the cytosolic Ca2+ concentration ([Ca2+](c)) induced by caffeine. Using fura-2-loaded cells, we find such an effect in bovine adrenal chromaffin cells, but not in cerebellar granule neurones or in HEK-293 cells. In contrast, a caffeine-induced [Ca2+](c), increase was clearly visible with either fluo-3 or cytosolic acquorin. Simultaneous loading with fura-2 prevented the [Ca2+](c) increase reported by the other Ca2+ probes. Caffeine-induced Ca2+ release was also measured by following changes of [Ca2+] inside the ER ([Ca2+](ER)) with ER-targeted aequorin in IEK-293 cells. Fura-2 loading did not modify Ca2+ release from the ER. Thus, fura-2, but not fluo-3, antagonises the generation of the cytosolic Ca2+ signal induced by activation of RyRs. Cytosolic Ca2+ buffering and/or acceleration of Ca2+ diffusion through the cytosol may contribute to these actions. Both effects may interfere with the generation of microdomains of high [Ca2+](c), near the ER release channels, which are essential for the propagation of the Ca2+ wave through the cytosol. In any case, our results caution the use of fura-2 to study CICR. (C) 2003 Elsevier Science Ltd. All rights reserved.