Regulation by extracellular Na+ of cytosolic Mg2+ concentration in Mg2+‐loaded rat sublingual acini

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Abstract
The regulation of cytosolic free Mg2+ concentration ([Mg2+]i) in Mg2+-loaded rat sublingual mucous acini was examined using the Mg2+-sensitive fluorescent indicator mag-fura-2. Loading sublingual acini with 5 mM Mg2+ elevated the [Mg2+]i from 0.35 ± 0.01 mM to 0.66 ± 0.01 mM. Removal of extracellular Mg2+ resulted in a significantly faster [Mg2+]i decrease in Mg2+-loaded rcini than in unloaded acini. Membrane depolarization with high extracellular [K+] and inhibition of P-type ATPases by vanadate did not alter the [Mg2+]i decrease, indicating that the Mg2+ efflux mechanism is not electrogenic. Na+-free medium inhibited 80% of the [Mg2+]i decrease suggesting that a Na+-dependent Mg2+ efflux pathway mediates the [Mg2+]i decrease. Accordingly, the Na+-dependent antiport inhibitor quinidine reduced > 80% of the [Mg2+]i decrease, suggesting that the Na+-dependent Mg+ efflux was also partly driven by K+. The [Mg+]i system. Mg2+ efflux was also partly driven by K+. The [Mg2+]i decrease was significantly inhibited by carbachol, a muscarinic agonist, but not by cAMP. These results indicate that in sublingual acinar cells a Na+-dependent pathway mediates Mg2+ efflux and that muscarinic stimulation may regulate Mg2+ extrusion