Matrix free Mg2+ and the regulation of mitochondrial volume

Mitochondria must maintain volume homeostasis in order to carry out oxidative phosphorylation. It has been postulated that the concentration of free Mg2+ ([Mg2+]) serves as the sensor of matrix volume and regulates a K+-extruding K+/H+ antiport (K. D. Garlid. J. Biol. Chem. 255: 11273-11219, 1980). To test this hypothesis, the fluorescent probe furaptra was used to monitor [Mg2+] and free Ca2+ concentration ([Ca2+)] in the matrix of isolated beef heart mitochondria, and K+/H+ antiport activity mas measured by passive swelling in potassium acetate. Concentrations that result in 50% inhibition of maximum activity of 92 mu M matrix [Mg2+] and 2.2 mu M [Ca2+] were determined for the K+/H+ antiport. Untreated mitochondria average 670 mu M matrix [Mg2+], a value that would permit <1% of maximum K+/H+ antiport activity. Hypotonic swelling results in large decreases in matrix [Mg2+], but swelling due to accumulation of acetate salts does not alter [Mg2+]. Swelling in phosphate salts decreases matrix [Mg2+], hut not to levels that permit appreciable antiport activity. We conclude that 1) it is unlikely that matric [Mg2+] serves as the mitochondrial volume sensor, 2) if K+/H+ antiport functions as a volume control transporter, it is probably regulated by factors other than [Mg2+], and 3) alternative mechanisms for mitochondrial volume control should be considered.