Characteristics of swelling of skeletal muscle mitochondria in the absence of Mg ++

Abstract

A specific Na +-activated ATPase becomes rate-limiting for swelling when mitochondrial membrane permeability is modified in the absence of exogenous Mg ++. When valinomycin is superimposed in such a Mg ++-free medium, a specific K +-activated ATPase becomes rate-limiting for swelling. Uncoupled substrate oxidation also facilitates diffusion in the absence of Mg ++. Substrate facilitates diffusion in the presence of the alkali metal salt of either Na + or K +. It fails to facilitate diffusion in the absence of alkali metal salt or when the alkali metal cation is replaced by Tris +, even although its oxidation is uncoupled in such circumstances. Since substrate support of swelling is not alkali metal cation specific although each specific proton-producing ATPase is so, it would appear that the swelling process itself is independent of permeant cation species. The possibility is suggested therefore that proton exchange may be the rate-limiting factor involved in the facilitated diffusion process. The observed permeable anion dependency for this swelling would appear to suggest charge compensation of ion pairs as the mode of transport through the less polar regions of membrane. The findings support exchange diffusion as the basis for ion translocation after the establishment of a proton gradient. Incubation in the presence of Mg ++ prevents both ATP- and substrate-supported swelling. Mg ++ also imposes control of respiration by phosphorylation. The addition of Mg ++ to mitochondria actively swollen in its absence causes contraction with ATP but not with substrate as energy source. Active swelling and contraction were confirmed as an absorbance change and as a change in particle volume, measured in the the Coulter Electronic Counter. The findings suggest that the presence of a contractile principle in the mitochondrial structure may play the role of a regulator of the extent to which generated high-energy bonds are exposed to hydrolysis and may thus constitute a basis for metabolic control.