Rat liver mitochondria were incubated in media of different osmolarities and in the presence of various substrates. Rates of oxygen consumption and mitochondrial matrix volumes were measured in the presence and absence of ADP and uncoupler. Duroquinol oxidation was insensitive to matrix volume, whereas other substrates tested showed increased rates of oxidation when the matrix volume increased from 1.0 to 1.5 microliter/mg of protein; this is the range of values measured in situ [Quinlan, Thomas, Armston & Halestrap (1983) Biochem. J. 214, 395-404]. Palmitoylcarnitine, octanoate and butyrate oxidations were particularly sensitive to the matrix volume, increasing from negligible rates to maximal rates within this range. Swelling induced by K+ uptake also stimulated palmitoylcarnitine oxidation. A similar effect of volume on substrate oxidation was seen when ferricyanide in the presence or absence of ubiquinone-1 replaced oxygen as terminal electron acceptor. Measurement of flavoprotein reduction (A 460-480) demonstrated that the locus of the effect of matrix volume is between the electron-transfer flavoprotein and ubiquinone. It is suggested that volume-mediated regulation of fatty acid and proline oxidation may be an important component of the hormonal stimulation of their oxidation.
