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 con...
