Na⁺/Mg²⁺ transporter acts as a Mg²⁺ buffering mechanism in PC12 cells

Mg²⁺ buffering mechanisms in PC12 cells were demonstrated with particular focus on the role of the Na⁺/Mg²⁺ transporter by using a newly developed Mg²⁺ indicator, KMG-20, and also a Na⁺ indicator, Sodium Green. Carbonyl cyanide p-(trifluoromethoxy) phenylhydrazone (FCCP), a protonophore, induced a transient increase in the intracellular Mg²⁺ concentration ([Mg²⁺]_i). The rate of decrease of [Mg²⁺]_i was slower in a Na⁺-free extracellular medium, suggesting the coupling of Na⁺ influx and Mg²⁺ efflux. Na⁺ influxes were different for normal and imipramine- (a putative inhibitor of the Na⁺/Mg²⁺ transporter) containing solutions. FCCP induced a rapid increase in [Na⁺]_i in the normal solution, while the increase was gradual in the imipramine-containing solution. The rate of decrease of [Mg²⁺]_i in the imipramine-containing solution was also slower than that in the normal solution. From these results, we show that the main buffering mechanism for excess Mg²⁺ depends on the Na⁺/Mg²⁺ transporter in PC12 cells.