92 



INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



tension. If the pH is raised to 6.4 with ATP and CP present, the system 

 becomes more sensitive to pressure. In view of these large changes in 

 sensitivity, it is possible that a situation could exist relevant to substrate 

 concentration and pK wherein pressure would have no effect over the 

 range of physiological interest, i.e. up to 10,000 psi. 



The foregoing remarks apply to fully relaxed n3 fibers. In the partially 

 contracted fibers at pH 6.8, tension increases with pressure, reaching a 

 maximum at 3,500 psi but thereafter decreases. In contrast, at pH 7.0 

 where the fiber is fully contracted, pressure always causes a fall in tension 

 with a aV of 180 cc/mole (fig. 4.4-3). This is interpreted as being due to 

 an inhibition of the phosphorylation equilibrium (D) with a reduction in 

 volume of 60 cc/mole per active site or 30 cc/mole with respect to the 



PRE'.':(JKC pSiXlO-^ PRESSURE psiXIO- 



FiG. 4. Logio [y/(l — y)] in relation to pressure. For description see text. 



phosphate ion. This may represent the dissociation of the phosphorylated 

 complex. 



In the response at pH 6.8, where pressure first causes contraction and 

 then relaxation, the actomyosin is only partially activated. As a result, 

 the tension at any pressure depends both on the activation equilibrium 

 (A) involving a volume decrease of 350 cc/mole in the formation of the 

 activated unit and on the phosphorylation equilibrium (D) involving a 

 volume increase of 180 cc/mole in maintaining the contracted state. 



In nl fibers, which develop a sustained tension at pH 7.2, the tension- 

 pressure relationship also has a aV of 180 cc/mole which remains constant 

 from pH 6.15 to 7.2. In these fibers there is no sign of the activation equi- 

 librium and the fall in tension is most likely due solely to an inhibition 

 of the phosphorylating mechanism (D). As mentioned earlier, the tension- 

 temperature relation for these fibers at pH 7.0 is controlled by reactions 

 characterized by a AH of 36,000 cal., while at pH 6.1 tension decreases 



