Matrix free Mg(2+) 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 Mg(2+) ([Mg(2+)]) serves as the sensor of matrix volume and regulates a K(+)-extruding K(+)/H(+) antiport (K. D. Garlid.  J. Biol.  Chem.  255: 11273-11279, 1980).  To test this hypothesis, the fluorescent probe furaptra was used to monitor [Mg(2+)] and free Ca(2+) concentration ([Ca(2+)]) in the matrix of isolated beef heart mitochondria, and K(+)/H(+) antiport activity was measured by passive swelling in potassium acetate.  Concentrations that result in 50% inhibition of maximum activity of 92 microM matrix [Mg(2+)] and 2.2 microM [Ca(2+)] were determined for the K(+)/H(+) antiport.  Untreated mitochondria average 670 microM matrix [Mg(2+)], a value that would permit <1% of maximum K(+)/H(+) antiport activity.  Hypotonic swelling results in large decreases in matrix [Mg(2+)], but swelling due to accumulation of acetate salts does not alter [Mg(2+)].  Swelling in phosphate salts decreases matrix [Mg(2+)], but not to levels that permit appreciable antiport activity.  We conclude that 1) it is unlikely that matrix [Mg(2+)] 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 [Mg(2+)], and 3) alternative mechanisms for mitochondrial volume control should be considered.