Ashley, C. C., Ellory, J. C. 1972. The efflux of magnesium from single crustacean muscle fibres. J. Physiol. 226:653-74
DiPolo, R., Beauge, L.1988. An ATPdependent Na+/Mg2+ countertransport is the only mechanism for Mg extrusion in squid axons.Biochim. Biophys. Acta 946:424-28
All these references and more were taken from:
MECHANISMS OF MAGNESIUM TRANSPORT by Peter W. Flatman (Annu. Rev. Phvsiol. 1991.53:259-71).
the exact molecular nature of the Magnesium efflux System is still unknown. CNNM2 is more likely a Magnesium sensor rather than a Transport System itself, another potential candidates are the SLC41A1-3 Proteins, see also:
Kolisek M, Nestler A, Vormann J, Schweigel-Röntgen M. Human gene SLC41A1 encodes for the Na+/Mg²+ exchanger. Am J Physiol Cell Physiol. 2012 Jan 1;302(1):C318-26.