The effect of thyroxine and other substances on the swelling of isolated rat liver mitochondria.

After the demonstration by Harris (1) that mitochondris in intact cells are sensitive to changes in osmolarity and the work of Claude (2) which showed that isolated mitochondria swell in hypotonic sucrose, a number of investigators (3-14) have studied the changes in water content produced when mitochondria are exposed in vitro to various media. In particular, Raaflaub (7-10) has investigated t,he effect of various nucleotides, divalent cations, anions, and metal ion complexing agents on the swelling of rat liver mitochondria. Some connection between water content and ATPl or oxidative phosphorylation apparently exists (cf. (ll)), but the nature and significance of this relationship are still obscure because of the complexity of the interplay of factors known to affect the degree of swelling or water content of the mitochondria. This paper represents a study of the changes in water content of isolated rat liver mitochondria suspended in a sucrose medium to which various other compounds were added, particularly agents capable of uncoupling oxidative phosphorylation. In these experiments very little or no oxidation or net phosphorylation was taking place, thus excluding these events as major factors in the changes studied. Of particular interest is the observation that certain uncoupling agents, such as thyroxine and Ca+‘, cause rapid swelling of mitochondria, whereas other uncoupling agents such as DNP, pentachlorophenol, and Dicumarol inhibit the spontaneous swelling. Gramicidin, also an uncoupling agent, has no effect on either the rate or extent of swelling. The results suggest that thyroxine and related compounds may have their primary action on mitochondrial structure rather than on the enzymatic processes involved in coupling respiration and phosphorylation, a view which is directly supported in the following paper (15). A preliminary report of this work has been published (16).