The role of mitochondria for Ca2+ refilling of the endoplasmic reticulum

Endoplasmic reticulum (ER) Ca2+ refilling is an active process to ensure an appropriate ER Ca2+ content under basal conditions and to maintain or restore ER Ca2+ concentration during/after cell stimulation. The mechanisms to achieve successful ER Ca2+ refilling are multiple and built on a concerted action of processes that provide a suitable reservoir for Ca2+ sequestration into the ER. Despite mitochondria having been found to play an essential role in the maintenance of capacitative Ca2+ entry by buffering subplasmalemmal Ca2+, their contribution to ER Ca2+ refilling was not subjected to detailed analysis so far. Thus, this study was designed to elucidate the involvement of mitochondria in Ca2+ store refilling during and after cell stimulation. ER Ca2+ refilling was found to be accomplished even during continuous inositol 1,4,5-trisphosphate (IP3)-triggered ER Ca2+ release by an agonist. Basically, ER Ca2+ refilling depended on the presence of extracellular Ca2+ as the source and sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) activity. Interestingly, in the presence of an IP3-generating agonist, ER Ca2+ refilling was prevented by the inhibition of trans-mitochondrial Ca2+ flux by CGP 37157 (7-chloro-5-(2-chlorophenyl)-1,5-dihydro4,1-benzothiazepin-2(3H)-one) that precludes the mitochondrial Na+/Ca2+ exchanger as well as by mitochondrial depolarization using a mixture of oligomycin and antimycin A. In contrast, after the removal of the agonist, ER refilling was found to be largely independent of trans-mitochondrial Ca2+ flux. Under these conditions, ER Ca2+ refilling took place even without an associated Ca2+ elevation in the deeper cytosol, thus, indicating that superficial ER domains mimic mitochondrial Ca2+ buffering and efficiently sequester subplasmalemmal Ca2+ and consequently facilitate capacitative Ca2+ entry. Hence, these data point to different contribution of mitochondria in the process of ER Ca2+ refilling based on the presence or absence of IP3, which represents the turning point for the dependence or autonomy of ER Ca2+ refilling from trans-mitochondrial Ca2+ flux.