Ion transport by heart mitochondria. XX. Factors affecting passive osmotic swelling of isolated mitochondria.

The swelling of isolated bovine heart and rat liver mitochondria has been compared in various suspending media in the absence of a source of metabolic energy. It is concluded that mitochondria will swell under these conditions in the presence of a permeant anion and a permeant cation, provided that the internal pH is not restrictive. Mitochondria show a high selectivity for cation penetration in the absence of energy, but almost no selectivity in the presence of respiration or of exogenous ATP. It is concluded that no monovalent cations penetrate the mitochondrion by simple diffusion in the absence of energy, but that an exchange diffusion system permits Na+ to be exchanged for internal H+. Lipid-soluble amines, such as NH,, can penetrate the mitochondrion and, in the presence of reagents that acidify the interior, such as free acetic acid or the protonated form of an uncoupler, can generate a cation (NH*+). The combination of NH,+ plus an uncoupler, therefore, represents a means of allowing cation accumulation in the matrix which is equivalent to the presence of the valinomycin-K+ or the gramicidin-Na+ complex. Certain anions, such as nitrate, appear to diffuse into the mitochondrion at neutral pH when cation penetration is induced by one of the above mechanisms. Bovine heart mitochondria show little, if any, tendency to admit substrate anions by the specific exchange diffusion systems found in rat liver mitochondria. All anions tested, however, are able to penetrate the bovine heart mitochondrion in the presence of a penetrating cation by a pH-dependent mechanism. This pathway for anion penetration can also be demonstrated in liver mitochondria at high pH, but it is not as striking as in the heart mitochondria in the range of pH 7.2 to 7.5. Uncoupled mitochondria swollen in the ammonium salts of nitrate, trichloracetate, fumarate, succinate, or malate can be contracted by either exogenous ATP or respiratory substrate as previously described for NH&l.