Sustained Ca2+ Transfer across Mitochondria Is Essential for Mitochondrial Ca2+ Buffering, Store-operated Ca2+ Entry, and Ca2+ Store Refilling

Abstract

Mitochondria have been found to sequester and release Ca2+ during cell stimulation with inositol 1,4,5-triphosphate-generating agonists, thereby generating subplasmalemmal microdomains of low Ca2+ that sustain activity of capacitative Ca2+ entry (CCE). Procedures that prevent mitochondrial Ca2+ uptake inhibit local Ca2+ buffering and CCE, but it is not clear whether Ca2+ has to transit through or remains trapped in the mitochondria. Thus, we analyzed the contribution of mitochondrial Ca2+ efflux on the ability of mitochondria to buffer subplasmalemmal Ca2+, to maintain CCE, and to facilitate endoplasmic reticulum (ER) refilling in endothelial cells. Upon the addition of histamine, the initial mitochondrial Ca2+ transient, monitored with ratiometric-pericam-mitochondria, was largely independent of extracellular Ca2+. However, subsequent removal i)f extracellular Ca2+ produced a reversible decrease in [Ca2+] mito, indicating that Ca2+ was continuously taken up and released by mitochondria, although [Ca2+]mito had returned to basal levels. Accordingly, inhibition of the mitochondrial Na +/Ca2+ exchanger with CGP 37157 increased [Ca 2+]mito and abolished the ability of mitochondria to buffer subplasmalemmal Ca2+, resulting in an increased activity of BKCa channels and a decrease in CCE. Hence, CGP 37157 also reversibly inhibited ER refilling during cell stimulation. These effects of CGP 37157 were mimicked if mitochondrial Ca2+ uptake was prevented with oligomycin/antimycin A. Thus, during cell stimulation a continuous Ca 2+ flux through mitochondria underlies the ability of mitochondria to generate subplasmalemmal microdomains of low Ca2+, to facilitate CCE, and to relay Ca2+ from the plasma membrane to the ER.