DUGALD E. S. BROWN 99 



optimum temperature, it would be supposed that it would also be suf- 

 ficient at lower temperatures. In this event the tension at these lower 

 temperatures would depend merely on the capacity of the activated 

 units to develop tension. The tension-temperature relation should be 

 the same as that for the maximum tetanus of striated muscle. This, 

 however, is not the situation. 



In the Arrhenius plot of logio [y/(l — y)] the data for the turtle 

 auricle deviate from a clearly linear relation (fig. 7B-1) at the higher 

 temperatures. In the turtle the curve to the maximum covers such a 

 small temperature range that the measurements are subject to con- 

 siderable error. Recently, however, Dawson and Bartholomew (38) de- 

 scribed the tension-temperature relation in the heart of the desert iguana 

 (Dypsosaurus dorsalis). In this heart, tension in the twitch rises from 

 20% of full tension at 2°C to a maximum full tension at 28°-30°C. The 

 x\rrhenius plot of these data is similar to that for the turtle and establishes 

 clearly the non-linearity of the relation (fig. 7B-2). 



A decrease in intracellular pH by exposure to 80% CO2 as in the case 

 of the tetanus shifts the tension curve of the twitch as in the case of the 

 tetanus toward a higher temperature (fig. 7^4-3). Moreover, again as 

 in the tetanus, the Arrhenius plot of logio [y/(l — y)] for the twitch 

 now yields a straight line whose slope indicates a heat of 36,000 cal. 

 (fig. 75-3). 



The same shift in the tension-temperature relation results from com- 

 pression. Under a constant pressure in the range from 4,000 to 8,000 psi, 

 the Arrhenius plot is linear above 5°C and also indicates a heat or energy 

 of 36,000 cal. (fig. 75-4, 5). Thus, at a low intracellular pH or at high 

 pressure, the contractile system reacts as if all of the actomyosin were 

 in the active form and is being controlled with respect to temperature 

 primarily by equilibrium (D). 



Below 5°, however, the tension curve at high pressure deviates from 

 linearity. Referring to the control curve at normal pressure, the tension 

 may be considered as decreasing more rapidly with temperature than 

 would be so if only equilibrium (D) were in control. This would be the 

 result if the duration of the 'active state' at low temperatures was in- 

 sufficient for the full development of tension. There is the possibility, 

 therefore, that both low pH and high pressure prolong the 'active state' 

 sufficiently so that the tension dependence again comes under essentially 

 the sole control of equilibrium (D). the effect of such a situation in regard 

 to tension would be that at 0° the inhibition by pressure would be less 

 than expected since pressure would be tending to augment tension by 

 in-olonging the 'active state' as well as inhibiting the development of 

 tension by the active complex. 



