DUGALD E. S. BROWN 97 



at 0°C is only 30% of the maximum, which maximum is now attained at 

 about 25 °C. 



The tension-temperature relation for the tetanus is curvilinear and 

 resembles that obtaining above pH 6.8 in all types of glycerated fibers 

 studied. Letting y equal the fractional tension and plotting logio 

 [y/(l — y)] against the reciprocal of the absolute temperature, a straight 

 line results with AH of 36,000 cal. (fig. 6B). This AH is the same as that 

 for the fiber under conditions where tension depends on the degree of 

 phosphorylation of the activated unit AMar^ with AH of 12,000 cal. per 

 active site. 



As in the glycerated fiber, a decrease in pH shifts the tension curve into 

 a higher temperature range. On the other hand, the curve is shifted to a 

 lower temperature by acclimatization at low temperatures. In the light 

 of the results on the fiber, this latter phenomenon could be attributed, 

 within a given species, to a decrease in the pK for tension consequent on 

 a new balance in the phosphate donor-acceptor system. 



Pressure and Tension. The relation between tetanus tension and 

 pressure is quite complex. Cattell and Edwards (34) reported insignifi- 

 cant changes in maximal tetanus tension on compression. In our labora- 

 tories this observation has been confirmed but, in addition, either an 

 increase or a decrease of about 2% per 1,000 psi or no change at all has 

 been observed. In a frog sartorius at 3°C where a contracture appears at 

 3,500 psi, tetanus tension decreases with pressure in accordance with 

 a volume decrease of 60 cc/mole (fig. 4B-1). The b constant of Hill's 

 fundamental equation, calculated from these myograms, is also reduced 

 by pressure with a volume decrease of 60 cc/mole (fig. 4B-2). 



Under certain conditions where the contracture does not appear until 

 7,500 psi, the tetanus tension may be unchanged or slightly increased 

 under pressure. Apparently in brief tetani, pressure tends both to induce 

 contraction (equilibrium A) and to inhibit tension (equilibria B and D) 

 indicating that in the maximum tetanus the actomyosin is not entirely 

 in the active form. It is probable that in a prolonged tetanus, the cellular 

 changes would exclude equilibria (A) and (C) and that tension could 

 then come under the control of equilibrium (B) and diminish with pres- 

 sure with a volume decrease of 120 cc/mole. 



Isometric Twitch. For purposes of comparison with the glycerated fiber 

 the relevant aspects of the twitch are the tension-temperature relation 

 in respect to pH and pressure and the influence of pressure on activation. 

 For several reasons the auricular muscle of the turtle is ideally suited 

 for such studies. The slow speed of contraction renders the relatively 

 abrupt application of pressure a simple matter while the long refractory 

 period excludes the possibility of a re-excitation resulting from the abrupt 



