Measurement of mitochondrial inner membrane cation
permeability was performed as in [6]. This involves measuring
the rate of swelling of respiring mitochondria in the acetate salt
of a cation at different membrane potentials (set by titrating
with the respiratory chain inhibitor malonate). Beavis et al. [lo]
and Garlid and Beavis [l l] have shown that the reciprocal absorbance
of mitochondrial suspensions can be used as a quantitative
measure of mitochondrial volume (and thus rate of
solute uptake). Selwyn [12] has shown that transmittance may
be used instead of reciprocal absorbance without introducing
much error when relative rates of solute transport are required
(as here). The rate of mitochondrial swelling (and thus relative
rate of solute transport) was therefore followed from the
change in %transmittance at 540 nm, in a 3 ml cuvette continuously
stirred by an air driven paddle in a Perkin Elmer
Lambda 5 spectrophotometer. Initial values of Votransmittance
were approximately 1%. Measurements of swelling rate were
made in the range 1_3%transmittance.