Protein kinase a dependent phosphorylation activates mg2+ efflux in the basolateral region of the liver

Isolated hepatocytes in physiological [Na+] (0) tightly maintain [Mg2+] (i) . Upon beta-adrenergic stimulation or in the presence of permeable cAMP, hepatocytes release 5-10% (1-3 mM Mg2+) of their total Mg2+ content. However, isolated basolateral liver plasma membranes (bLPM), release Mg2+ in the presence of [Na+] (o) even in the absence of catecholamine stimulation. The data indicate that a physiological brake for Mg2+ efflux is present in the hepatocyte and is removed upon cellular signaling. In contrast, this regulation "brake" is absent in purified bLPM thus rendering them fully active. The present study was carried out to reconstruct the missing regulatory component. Activation of Mg2+ extrusion in intact cells is consistent with cAMP dependent phosphorylation of the transporter or a regulatory protein. Treatment of bLPM with a non-specific phosphatase such as alkaline phosphatase (AP), decreased Mg2+ efflux by 70% compared to untreated bLPM. When AP-treated bLPM were loaded with protein kinase A (PKA), and stimulated with permeable cAMP, Mg2+ transport fully recovered. These data suggest that phosphorylation of the Na+/Mg2+ exchanger or a nearby protein activates the Mg2+ transport mechanism in hepatocytes.