Decreasing the osmolarity of the incubation media for rat liver mitochondria, by lowering the KCl or sucrose content, greatly increased the oxidation of palmitate, palmitoyl-CoA, palmitoyl-(–)-carnitine, octanoate and butyrate, but only slightly affected the oxidation of succinate, glutamate plus malate, and 3-hydroxybutyrate. Kinetic studies indicated that the increased rate offatty acid oxidation commenced at the onset of incubation in the hypotonic media and remained relatively constant. This was accompanied by an increase in the 3-hydroxy-butyrate: acetoacetate ratio, a higher rate of oxygen consumption, an increase in the extramitochondrial adenine-nucleotide phosphorylation-state ratio, and a greater state of reduction of the respiratory chain cytochromes b. c, and oxidase. Incubation of mitochondria in the hypotonic media caused a rapid decrease in the intermembrane and intracristal spaces and concurrent expansion of the matrix, without changing the total volume of the mitochondria.
These observations indicate that expansion of the inner-membrane–matrix compartment of mitochondria in vitro specifically activates fatty acid oxidation at the site of β-oxidation or at the site of its interaction with the respiratory chain, with a consequent increase in the states of reduction and phosphorylation of the mitochondria, Evidence is also presented which suggests that the activation of mitochondrial fatty acid oxidation by Ca2- is initiated by a Ca2+ -induced alteration of the structure of volume of the inner-membrane–matrix region.