Abstract: | 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. |