90 LESTER PACKER 



with 1-25 X io~^ M ATP (curve B), The results show that the tension 

 response is faster than ATP production by mitochondrial phosphorylation. 

 Escape of ATP from the mitochondria may be the rate-limiting react'on, 

 and this process may be connected with the extensive mitochondrial 

 shrinkage states present. 



In summary, it was shown that changes in mitochondrial volume or 

 membrane structure are brought about by changes in the activity of the 

 respiratory chain. Certain evidence suggests that these structural changes 

 may lead to altered reaction rates of ATP-requiring systems which react 

 at or near the membrane surface. 



The second point which I would like to consider is the locus of the 

 coupling mechanism involved in the swelling-shrinking phenomenon, 

 which has been raised by some experiments we have done with p-chloro- 

 mercuribenzoate (PCMB). Certain striking similarities between this 



o 0-5 



200 400 600 800 



Concentration of ADP or ATP (//M) 



Fig. 4. Contraction of glj'cerol-treated fibres of rabbit psoas muscle with ATP 

 synthesized by rabbit cardiac muscle mitochondria (a) or ATP only (b). (Courtesy 

 of the Journal of Biological Chemistry.) 



system and the contractile protein of muscle, myosin B, are apparent. 

 Tapley [25] and Lehninger and Ray [26] have studied the action of PCMB 

 on mitochondrial volume and report that it enhanced swelling. Significantly 

 Lehninger and Ray [26] found that swelling was more rapid under aerobic 

 than anaerobic conditions. Tapley [25] suggested that sulphydryl groups 

 may be important in determining mitochondrial structure. In Fig. 5 the 

 time course is shown of the effect of 83 /xM PCMB on shrinking and swelling 

 of a suspension of cardiac mitochondria respiring different substrates. In 

 the absence of substrate, PCMB exerts no appreciable effect upon mito- 

 chondrial shrinkage for over 2 min., and then extensive swelling occurs. 

 When, however, PCMB is added when substrates are present it immediately 

 initiates an enormous shrinkage. Later a reversal occurs and swelling ensues, 

 resulting in a decrease in light-scattering similar to that in absence of 

 substrates. PCMB-induced shrinkage is dependent on the presence of 

 substrate and on the concentration of the SH-binding reagent. With 



