DUGALD E. S. BROWN 93 



linearly with temperature. In both cases the tension depends on the ac- 

 tivated unit with three active sites and there is good reason to conclude 

 that the inhibition of phosphorylation does not cause AMa/ to pass to the 

 inactive form. The fact that the phosphorylation reaction can act either 

 simultaneously with or independently of the activation equilibrium shows 

 clearly that the systems are distinct and it would be expected that they 

 have different pK values. 



In fibers prepared in a manner different from those discussed thus far, 

 namely, by soaking for four weeks in 50% glycerol before testing, tension 

 decreases with pressure in accordance with a volume change of 120 cc/mole 

 (fig. 4.4-1). These fibers also develop a sustained tension and, like the nl 

 fibers, are relatively insensitive to pH, indicating that the system is fully 

 activated. The prolonged soaking has evidently created a condition per- 

 mitting a new mechanism to assume control. 



In ft myosin gels (11) compression causes solation with a reduction in 

 volume of 120 cc/mole. Pressure also decreases the strength of the cortical 

 gel of the sea urchin egg with a similar decrease in volume (23) . In con- 

 siderations of the cortical gel Johnson et al. (3) pointed out that its reaction 

 to pressure could be accounted for in terms of an equilibrium between a 

 native globular and an unfolded fibrous form of the cortical protein. The 

 tension-pressure relation may be similarly interpreted, the decrease in 

 tension resulting from the conversion of the fibrous protein to the globular 

 form with a reduction in volume of 120 cc/mole. In terms of the schema, 

 this would be equilibrium (B) , AMa/ going to AMa,-^. 



In summarizing the results on the fiber, tension depends on three inter- 

 related, but discrete events represented by reactions A, B and D, any one 

 of which may, under suitable conditions, regulate tension. 



The equilibria have different pK values and, judging from the effects of 

 CP and ATP, it would be expected that the pK is highest in equilibrium 

 (A) and smallest in equilibrium (B). Thus a fiber relaxed by CP would 

 contract on removal of the CP, the \)K for D with respect to ATP then 

 taking over, to be replaced finally by the pK for B on the depletion of ATP 

 or other phosphate donors. 



MUSCULAR CONTRACTION 



In regard to muscle, the pressure contracture may be readily compared 

 to the pressure induction of contraction in the fiber. The tetanus tension 

 presents a situation where the muscle is apparently fully activated and 

 tension is dependent solely on the state of the activated units. The twitch 

 presents a new situation for here the discrete contractile events occur 

 sequentially. Following stimulation, depolarization produces the activated 

 state C(i , which persists for a short period; actomyosin undergoes activa- 



