DUGALD E. 8. BROWN 



95 



1 niM K+ and 1 niM Ca++ the first sign of contracture is at 7,500 to 

 8,000 psi. 



The withdrawal of Cl~ with substitution of sucrose also increases the 

 sensitivity to pressure. In the retractor penis muscle a contracture de- 

 velops at normal pressure in proportion to the amount of Cl~ withdrawn. 

 With 25 niM Cl~ remaining, the tension in the CI "-induced contracture 

 is about 30% of the maximum tension. When pressure is then applied, a 

 superimposed pressure contracture appears at 250 psi with half maximum 

 tension being reached at 3,000 psi. In contrast, a pressure of 9,000 psi is 

 required to produce half tension in the control, although otherwise the 

 tension-pressure curve is the same (27) (28). 



Another case of sensitization to pressure is that resulting from stimu- 

 lation. In the claw muscle of Callinectes sapidus compression to 3,000 psi 

 fails to produce a contracture. However, if the muscle is then stimulated 

 under this pressure a twitch develops which relaxes only about 25%, the 

 tension being sustained at the high level until the pressure is removed (29) . 

 On the basis of the results with the glycerated fiber, the appearance of the 

 contracture would be attributed to a lowering of the pK. Moreover, since it 

 seems certain that a stimulus initiates some positive changes to compensate 

 for the loss of K + , it seem appropriate to propose the establishment of a 

 new balance in the phosphate donor and acceptor system. 



As indicated above, the pressure contracture in muscle resembles in its 

 major characteristic the pressure induction of contraction in the glycer- 

 ated fiber. In both, the contraction involves a decrease in volume of 350 

 cc/mole and in both the pressure sensitivity is considerably modified by 

 chemical agents. It is proposed therefore that the contracture results from 

 the conversion of inactive AMi to active AMar^ (eciuilibrium A) with a 

 reduction in volume of 350 cc/mole. 



Tetanus. The maximum tetanus tension is generally considered as a 

 'steady state' condition in which the muscle is maintained in the fully 

 'active state' by repetitive stimulation. If the period of stimulation is 

 brief, the contraction is rapidly reversible, but continued stimulation leads 

 to various secondary situations in which the tension falls, relaxation is 

 delayed and a fatigue contracture or even rigor develops. In the light of 

 the data on the glycerated fibers, these sequential secondary phenomena 

 would be assigned to changes in pK due to new metabolic conditions 

 arising from the prolonged stimulation. In the present context only the 

 brief, reversible tetani will be considered. 



Temperature and pH. The maximum tetanus tension of striated 

 muscles increases with temperature, reaching an upper limit which is then 

 sustained over a considerable temperature range (30). With stigmatic 

 electrodes, the tension decreases at temperatures above 20 °C unless the 



