Matrix magnesium and the permeability of heart mitochondria to potassium ion

Isolated beef heart mitochondria were treated with A23187 in the presence of different concentrations of Mg2+ or EDTA to establish varying levels of total mitochondrial Mg2+. The Mg2+ content was related to the rate of passive swelling of the mitochondria in potassium acetate and other potassium salts in which swelling is presumed to depend on K+ entry via an endogenous K+/H+ antiport. Swelling in these salts does not commence until Mg2+ has been depleted from an initial value of 36 nmol X mg-1 of protein to 8 nmol/mg-1, or less. Below this level, swelling increases linearly with decreasing Mg2+ to a maximum rate at 2 nmol of Mg2+ X mg-1. Rotenone-treated heart mitochondria suspended in 75 mM potassium acetate at pH 7.80 show no delta pH by 5,5-dimethyl-2,4-oxazolidinedione distribution. Distribution of methylamine also shows essentially no delta pH under these conditions when allowance is made for binding of [14C]methylamine by mitochondrial membranes under these conditions. Addition of A23187 results in a small and transient delta pH (delta pH less than 0.14, acid interior) as measured by methylamine distribution. Estimation of the maximum matrix free Mg2+ concentration from the maximum delta pH observed and the external free Mg2+ concentration at equilibrium with A23187 shows that swelling is not initiated until matrix free Mg2+ is decreased to below 150 microM. An independent estimate of free Mg2+ using a null-point procedure gives a lower, but quite similar value (50 microM) for maximum matrix free Mg2+ when swelling commences. The large depletion of total and free Mg2+ that is required to activate swelling in potassium acetate (and presumably K+/H+ antiport activity) does not appear to be compatible with previous indications that free Mg2+ acts as a "carrier brake" to regulate K+ extrusion from the mitochondrion on such an antiport (Garlid, K. D. (1980) J. Biol. Chem. 255, 11273-11279). The removal of a tightly bound component of mitochondrial Mg2+ is closely related to increased K+ permeability and increased passive swelling in potassium salts. This Mg2+ appears to play a role in the maintenance of mitochondrial membrane structure and integrity.