Energy-linked respiration causes a net movement of Mg2+ between rat heart mitochondria and the ambient medium. When the extramitochondrial concontration of Mg2+ is less that about 2.5 mM the net movement of Mg2+ constitutes an efflux, whereas a net influx of Mg2+ occurs when the external concentration of Mg2+ is greater than this. Both the efflux and the influx are induced to only a very small degree by externally added ATP. Evidence suggests that Pi may be required for the respiration-induced efflux of Mg2+.
