Objective and Methods: PC12 cells were loaded with a novel Mg indicator KMG-104 and Ca indicator fura-2, and intracellular Mg was studied in the endoplasmic reticulums (ERs), mitochondria, and Mg-ATP. Under coexistence of the two indicators, fluorescent signals of Mg and Ca can be measured separately. Mg release from the ER was investigated by photolysis of caged compounds. Results: Transient [Ca] i increase by uncaging of caged Ca or caged IP 3 or bath-application of caffeine (10 mM) induced no [Mg] i increase. These results suggest that there is no mechanism for Mg release from the ER through ryanodine receptors or IP 3 receptors. In order to investigate the possibility of Mg release from Mg-ATP by energy consumption, we depleted ATP by oligomycin, an inhibitor of mitochondrial ATP synthase. Treating with oligomycin (4 μM) for several minutes showed no change of [Mg] i and [Ca] i. Conclusions: This result shows that Mg-ATP is not a Mg store. Since, when cells were treated by an uncoupler FCCP (3 μM), [Mg] i and [Ca] i increased, we concluded that mitochondria participate in maintenance of intracellular Mg stores.
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