42 ALBERT L. LEHNINGER 



occurrence of active electron transport which may be atforded by endo- 

 genous substrates. The susceptibihty to swelHng agents can be conferred 

 by electron transfer in different segments of the respiratory chain. On the 

 other hand, data of Birt and Bartley among others, suggest [33] that both 

 the oxidation-reduction state of the carriers and the net electron flux may 

 be elements in susceptibility of mitochondria to swelling. 



Another piece of evidence implicating the coupled respiratory chain in 

 the swelling process is that sucrose and other polyhydroxylic compounds 

 inhibit swelling [29, 34]. These compounds also inhibit respiration and 

 uncouple phosphorylation in the osmotically insensitive digitonin fragments 

 of the mitochondrial membrane, suggesting they act as enzyme inhibitors 

 rather than in an osmotic sense [35]. In addition, dinitrophenol has been 

 found to inhibit swelling when added to fresh mitochondria [36], whereas 

 on delayed addition it becomes an activator of swelling [37]. 



Mitochondrial swelling /;/ vitro induced by thyroxine or phosphate 

 leads to an increase of volume of between 100 to 200 per cent over a 

 period of 10-15 min. at 20. Small-amplitude swelling of tightly-coupled 

 mitochondria has also been found to be dependent on respiration or 

 respiratory state by Holton [38] and Packer [39]. 



In large amplitude mitochondrial swelling taking place over longer 

 periods, a large part of the respiratory control by ADP is lost, as well as 

 ability to phosphorylate, possibly as a consequence of the "stretching" of 

 respiratory assemblies in the membranes. However, as is shown below, 

 such drastic mitochondrial swelling is still reversible by ATP [34, 40]. 



Mitochondrial contraction 



Price et al. first established in their thorough study [41] that re- 

 institution of phosphorylating respiration in swollen mitochondria by 

 appropriate supplements to the test medium would cause a contraction 

 with gravimetrically measurable extrusion of water. Similar observations 

 were reported by Beyer et al. [42]. Since the mechanism of oxidative 

 phosphorylation, at least in its terminal stages, has been thought to be 

 reversible, it would have appeared likely that ATP alone in the absence 

 of respiration might be able to effect mitochondrial contraction. However, 

 with the exception of a very limited contraction observed by Chappell 

 and Perry in pigeon breast muscle mitochondria by the addition of ATP 

 [43], no significant success was reported in effecting contraction by mere 

 addition of ATP to swollen mitochondria. In 1959 we found that the 

 failure of ATP to effect contraction could be traced to the presence of 

 sucrose in the test media ordinarily used in such experiments [29, 44]. 

 Sucrose in approximately isotonic concentrations completely inhibits con- 

 traction of swollen liver mitochondria by ATP, whereas mitochondria 



