DUGALD E. S. BROWN 89 



to pH as evidenced by 7i = 3. In fibers which develop and sustain the maxi- 

 mum tension with only ATP added, and which are insensitive to CP, n is 

 close to unity (the nl fibers). Goodall and Andrew Szent Gyorgyi (15) 

 observed in earlier studies that, with CP as a relaxing factor, an extreme 

 sensitivity to pH existed. This suggests /) = 3 may only obtain when a 

 relaxing factor is present. 



In these experiments pH may be considered in the role of an activator 

 inducing the formation of an activated unit with three active sites. Recently 

 Moos and Lorand (211 have studied the induction of contraction by cal- 

 cium in fibers relaxed by phospho-enol pyruvate at pH 7. On analysis, their 

 data show that here also three active sites are involved per unit tension. 



The important fact in these studies is the constancy of n with either 

 pH or Ca++ as an activator. With pH as an activator, n remains constant 

 even though the pK value undergoes extensive changes when the con- 

 centration of ATP, CP or A]\IP is varied. The chief efi"ect of these agents, 

 therefore, is to shift the tension curve along the pH axis in a direction and 

 to a degree determined by their effects on the pK. In regard to 

 tension, the significant concept develops that at constant pH the tension 

 is altered primarily by changes in the pK. Thus in the presence of a 

 suitable concentration of ATP, an increase or decrease in the concentration 

 of CP may cause either an increase or a decrease in tension. Since these 

 phosjihorylated compounds are involved in enzymatic reactions, their 

 action on tension is subject to modification by those chemical and physical 

 agents which influence enzymatic kinetics. The shift in pK of the activated 

 unit as a whole thus becomes the focal point through which a variety of 

 phosphate donors and acceptors influence tension. Conceptually this is 

 equivalent to considering tension as dependent on a phosphate potential 

 as defined by equation 3. 



Temperature. The variation in tension with temperature has been in- 

 vestigated on standard n3 fibers both in the pH range above 6.8 where 

 the tension is maximal and independent of pH and in the lower range 

 where fractional tensions exist that depend on pH (fig. 1.4) (22). At 

 all values of pH from 6.15 to 7.3, equal tensions develop depending on the 

 temperature, a higher temperature being required the lower the pH (fig. 

 2/1). In the pH range above 6.8 the tension decreases curvilinearly with 

 decrease in temperature but at lower pH values, for example pH 6.15, the 

 curvilinear relation persists only to about 20°C. and thereafter tension 

 decreases linearly with temperature (fig. 2.4-1). 



In the nl fibers the above relations arc modified. Although behaving 

 similarly to the above at pH values higher than j^H 6.8, the tension at 

 liH 6.15 varies linearly with temperature from 0° to its maximum at 30°C. 

 In both types of fiber, the linear and curvilinear components are present, 



