The regulation of the oxidation of fatty acids and other substrates in rat heart mitochondria by changes in the matrix volume induced by osmotic strength, valinomycin and Ca2+.

1. The rate of ADP-stimulated respiration with various substrates and the matrix volume of rat heart mitochondria were measured over a range of osmolarities of the medium. 2. The rate of oxidation of palmitoylcarnitine (in the presence of malate) was stimulated 7-fold by increasing the matrix volume from 0.6 to 1.0 microliter/mg of protein. Oxidation of octanoate showed a similar sensitivity to the matrix volume, whereas oxidation of other substrates showed little sensitivity until the volume fell below 0.7 microliter/mg of protein. 3. The matrix volume of heart mitochondria incubated under physiological conditions was about 0.8 microliter/mg of protein. 4. Low concentrations of valinomycin added to mitochondria incubated under such physiological conditions could activate the rate of ADP-stimulated palmitoylcarnitine oxidation by at least 100%. 5. Decreasing the matrix volume increased the reduction of the electron-transferring flavoprotein (ETF), suggesting an effect on electron flow between ETF and ubiquinone, as has been observed for liver mitochondria [Halestrap & Dunlop (1986) Biochem. J. 239, 559-565]. 6. A rapid decrease in light-scattering by heart mitochondria incubated in State 4 was induced by addition of Ca2+, reaching 50% of the maximal effect after about 30 s at 30 degrees C and with K0.5 for Ca2+ of 0.3 microM. This was not associated with a change in matrix volume, and is discussed in terms of a conformational change whose identity remains to be determined. 7. However, incubation of heart mitochondria at 37 degrees C in the presence of 0.65 microM-Ca2+ for 4 min did increase the matrix volume significantly, by 0.181 +/- 0.029 microliter/mg of protein (n = 7, P less than 0.001), similar to the Ca2+-induced changes observed with liver mitochondria [Halestrap, Quinlan, Whipps & Armston (1986) Biochem. J. 236, 779-787]. 8. The possible significance of these results in the co-ordinate regulation of fatty acid oxidation and the citric acid cycle in the heart responding to increased work load or hormonal stimulation is discussed.