Glutaraldehyde has been used to fix mitochondria undergoing rapid volume changes associated with energized ion transport under oscillatory state conditions and valinomycin-induced potassium uptake. Fixation was found to prevent structural changes which normally occur during ion accumulation or loss. By correlating packed volume measurements with electron microscopy, it is shown that changes in volume associated with ion movements reflect changes in the inner membrane compartment and that this compartment can be related to the sucrose inaccessible space. The method can therefore be used to accurately determine volume changes that arise from ion translocation.

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