Regulation of intracellular magnesium in ascites cells: involvement of different regulatory pathways

Extracellular ATP causes 40% stimulation of Mg2+ efflux from Ehrlich ascites tumor cells (EATC) incubated under 0-trans conditions. ATP also causes a threefold increase of arachidonic acid (AA) metabolite release from [3H]AA-preloaded EATC, indicating that, under these experimental conditions, it induces phospholipase A2 (PLA2) activation. ATP-induced Mg2+ efflux can be prevented by cyclooxygenase inhibition with indomethacin or lysine acetylsalicylate, but not by 5-lipooxygenase inhibition with BWA4C. Mg2+ efflux is also directly stimulated by exogenous AA in a concentration-dependent manner. This phenomenon involves PKA as it is virtually abolished by the specific inhibitor 8-bromoadenosine-3',5'-cyclic monophosphothioate. While stimulating Mg2+ efflux, exogenous AA also increases cAMP content of EATC and this effect can be prevented by cyclooxygenase inhibition. Measurements in mag-fura-2-loaded EATC reveal that stimulation of Mg2+ efflux does not correlate with significant fluctuations of [Mg2+]i. This suggests that Mg2+ efflux is compensated for by mobilization of bound Mg2+, leaving [Mg2+]i unaltered. Altogether these data indicate that Mg2+ efflux can be modulated by extracellular stimuli capable of activating PLA2. This modulation is triggered by cyclooxygenase products which, by activating adenylcyclase, determine an elevation of cAMP. This intracellular messenger upregulates Na-dependent Mg2+ efflux.