4 OLOV LINDBHRG et al. 



at the expense of thermodynamic efficiency. A somewhat similar hypo- 

 thesis, based on a chemical relationship between thyroxine and dinitro- 

 phenol (both being substituted phenols), was also put forward by Martius 

 [7]. In the many attempts [812] to prove these hypotheses experimentally, 

 it has been possible, in some instances, to demonstrate a partial uncoupling 

 of respiration from phosphorylation in isolated mitochondria due to thy- 

 roxine treatment, both /// vitro and in vivo. However, these effects did not 

 appear in a consistent manner, nor could a preferential uncoupling of one 

 of the three respiratory chain phosphorylations be established. 



In 1954 Hoch and Lipmann [13] reported that a consistent decrease 

 of the P/0 ratio in isolated rat liver mitochondria by thyroxine could be 

 obtained, if the mitochondria were preincubated with thyroxine for a 

 period of time before the addition of substrate. Hamster liver mitochondria, 

 on the other hand, required no preincubation. However, even in this case, 

 the results were rather inconsistent from one experiment to another. The 

 significance of the loss of respiratory control without an actual loss of 

 phosphorylating capacity, found in earlier work [4], was re-emphasized. In 

 parallel papers by Bain [14] and by Mudd et al. [15] it was shown that the 

 effect of thyroxine on the P/0 ratio could be prevented by magnesium ions. 



By this time, attention became directed towards the effect of thyroxine 

 on mitochondrial structure. In 1953 Aebi and Abelin [16] reported that 

 liver mitochondria from thyrotoxic rats exhibited an increased tendency 

 to spontaneous swelling /// vitro. Subsequently Klemperer [17] found an 

 increased water-content in thyroxine-treated mitochondria. Tapley et al. 

 [18] demonstrated in 1955 that thyroxine added /// vitro enhances the 

 swelling of KCl-suspended normal rat liver mitochondria. A similar effect 

 was obtained with kidney mitochondria, while the swelling was much 

 weaker with mitochondria from muscle, brain and testes [19]. It was also 

 shown [20] that the P/0 ratio of phosphorylating mitochondrial fragments, 

 prepared with digitonin from liver mitochondria, was not affected 

 by thyroxine whereas it was still sensitive to dinitrophenol. From these 

 findings it was concluded (cf. also [21, 22]) that thyroxine, in contrast to 

 dinitrophenol, exhibits its effect on oxidative phosphorylation by a 

 secondary mechanism which is somehow correlated with the mitochondrial 

 structure. 



The swelling effect of thyroxine /// vitro has been subsequently studied 

 in great detail in a series of papers by Lehninger and associates ([23-28] ; 

 for review, cf. [29]). It emerged from these studies that this effect is similar 

 to that obtained when mitochondria are incubated for a period of time 

 ("aged") in a phosphate-containing medium ([30-46] for review, cf. [47]). 

 In both cases, the swelling seems to be the result of an active process, 

 which is typically temperature- and time-dependent. It requires the 

 presence of an oxidizable substrate, and is prevented by respiratory 



