Involvement of ERK1/2 and p38 in Mg2+ accumulation in liver cells

Activation of PKC signaling induces Mg2+ accumulation in liver cells. To test the hypothesis that PKC induces Mg2+ accumulation via MAPKs activation, hepatocytes were incubated in the presence of PD98059 and SB202190 as specific inhibitors of ERK1/2 and p38, respectively, and stimulated for Mg2+ accumulation by addition of PMA or OAG. Accumulation of Mg2+ within the cells was measured by atomic absorbance spectrophotometry in the acid extract of cell pellet. The presence of either inhibitor completely abolished Mg2+ accumulation irrespective of the dose of agonist utilized while having no discernible effect on beta-adrenoceptor mediated Mg2+ extrusion. A partial inhibition on alpha(1)-adrenoceptor mediated Mg2+ extrusion was observed only in cells treated with PD98059. To confirm the inhibitory effect of PD98509 and SB202190, total and basolateral liver plasma membrane vesicles were purified in the presence of either MAPK inhibitor during the isolation procedure. Consistent with the data obtained in intact cells, liver plasma membrane vesicles purified in the presence of PD98509 or SB202190 lost the ability to accumulate Mg2+ in exchange for intra-vesicular entrapped Na+ while retaining the ability to extrude entrapped Mg2+ in exchange for extra-vesicular Na+. These data indicate that ERK1/2 and p38 are involved in mediating Mg2+ accumulation in liver cells following activation of PKC signaling. The absence of a detectable effect of either inhibitor on beta-adrenoceptor induced, Na+-dependent Mg2+ extrusion in intact cells and in purified plasma membrane vesicles further support the hypothesis that Mg2+ extrusion and accumulation occur through distinct and differently regulated transport mechanisms.