OSMOTIC NATURE OF ION-INDUCED SWELLING OF RAT-LIVER MITOCHONDRIA

Analytical techniques have been developed to measure the relation between water uptake and cation uptake in rat-liver mitochondria. The water content of inner and outer mitochondrial compartments were separately measured using [131I]albumin and [14C]sucrose. The cation contents of the same mitochondrial pellets were measured using flame photometry. These data were used to measure the water content of each compartment which was necessary, since the basic osmotic relationships are only revealed when movements into the inner and outer compartments are separately examined. The relation between the water uptake and K+ uptake, either spontaneous or valinomycin-induced, was dependent on medium osmolality. The concentration of the K+ solution taken up in the inner compartment was compatible with the hypothesis that transport-linked mitochondrial swelling is driven by osmotic pressure differences which have been induced by solute movement. The changes in volume of the water compartment and in ion content during phosphate-induced swelling and during incubation in the absence of substrate were analyzed using the same methods. Under these experimental conditions also, the mitochondrial inner compartment appears to be in osmotic equilibrium with the external medium. © 1969.