Matrix free Mg2+ and the regulation of mitochondrial volume

Mitochondria must maintain volume homeostasis inorder to carry out oxidative phosphorylation. It has been postulatedthat the concentration of freeMg²⁺(Mg²⁺]) serves as thesensor of matrix volume and regulates aK⁺-extrudingK⁺/H⁺antiport (K. D. Garlid. J. Biol. Chem.255: 11273-11279, 1980). To test this hypothesis, the fluorescentprobe furaptra was used to monitorMg²⁺] and freeCa²⁺ concentration (Ca²⁺]) in the matrix ofisolated beef heart mitochondria, andK⁺/H⁺antiport activity was measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 µM matrix Mg²⁺] and 2.2 µMCa²⁺] were determined for theK⁺/H⁺ antiport. Untreated mitochondria average670 µM matrix Mg²⁺], a value that would permit <1%of maximumK⁺/H⁺antiport activity. Hypotonic swelling results in large decreases inmatrix Mg²⁺], butswelling due to accumulation of acetate salts does not alterMg²⁺]. Swelling inphosphate salts decreases matrixMg²⁺], but not tolevels that permit appreciable antiport activity. We conclude that1) it is unlikely that matrixMg²⁺] serves as themitochondrial volume sensor, 2) ifK⁺/H⁺antiport functions as a volume control transporter, it is probably regulated by factors other thanMg²⁺], and3) alternative mechanisms formitochondrial volume control should be considered.