Regulation of magnesium but not calcium transport by phorbol ester.
Phorbol esters in the presence of Ca2+ apparently mimic diacylglycerol in activating protein kinase C. Resulting phosphorylations alter multiple cellular processes including inhibition of the action of Ca2+-mobilizing agonists. In contrast to this inhibition of Ca2+ mobilization, addition of 4 beta-phorbol 12-myristate 13-acetate (PMA) to murine S49 lymphoma cells stimulated Mg2+ influx severalfold without any detectable alteration of Mg2+ efflux or of Ca2+ influx or efflux. Stimulation of Mg2+ influx did not require extracellular Ca2+, was half-maximal at 10 nM PMA, and was characterized by a marked increase in the Vmax of Mg2+ influx without change in the Ka for Mg2+. Stimulation of Mg2+ influx was not mimicked by 4 alpha-phorbol didecanoate, which does not activate protein kinase C and was not the result of Na+/H+ exchange activity. The effect of PMA on Mg2+ influx was inhibited by the beta-adrenergic agonist (-)-isoproterenol, which we have previously shown to inhibit Mg2+ influx by a non-cyclic AMP-dependent mechanism (Maguire, M. E., and Erdos, J. J. (1980) J. Biol. Chem. 255, 1030-1035). Forskolin, a direct activator of adenylate cyclase, also inhibited PMA stimulation of Mg2+ influx, suggesting the presence of both cyclic AMP-dependent and -independent influences on Mg2+ influx. We have also previously demonstrated that Mg2+ influx occurs solely into a small subcytoplasmic pool (Grubbs, R. D., Collins, S. D., and Maguire, M. E. (1984) J. Biol. Chem. 259, 12184-12192). PMA did not alter this compartmentation; rather, it almost doubled the content of this cytosolic Mg2+ pool. These data indicate that, in addition to phorbol ester modulation of intracellular Ca2+ mobilization, substantial changes in Mg2+ flux and content occur. They further demonstrate that Mg2+ influx is regulated by a variety of stimuli. It seems likely that such alterations in Mg2+ flux and content would have physiological consequences.