In squid nerves intracellular Mg2+promotes deactivation of the ATP-upregulated Na+/Ca2+ exchanger

We investigated the role of intracellular Mg(2+) (Mg(i)(2+)) on the ATP regulation of Na(+)/Ca(2+) exchanger in squid axons and bovine heart. In squid axons and nerve vesicles, the ATP-upregulated exchanger remains activated after removal of cytoplasmic Mg(2+), even in the absence of ATP. Rapid and complete deactivation of the ATP-stimulated exchange occurs upon readmission of Mg(i)(2+). At constant ATP concentration, the effect of intracellular Mg(2+) concentration ([Mg(2+)](i)) on the ATP regulation of exchanger is biphasic: activation at low [Mg(2+)](i), followed by deactivation as [Mg(2+)](i) is increased. No correlation was found between the above results and the levels of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] measured in nerve membrane vesicles. Incorporation of PtdIns(4,5)P(2) into membrane vesicles activates Na(+)/Ca(2+) exchange in mammalian heart but not in squid nerve. Moreover, an exogenous phosphatase prevents MgATP activation in squid nerves but not in mammalian heart. It is concluded that 1) Mg(i)(2+) is an essential cofactor for the deactivation part of ATP regulation of the exchanger and 2) the metabolic pathway of ATP upregulation of the Na(+)/Ca(2+) exchanger is different in mammalian heart and squid nerves.