DUGALD E. S. BROWN 



91 



the capacity of the contractetl unit AMj,,-^' to sustain tension without 

 returning to AM,,/'. As yet, a linear tension-temperature dependence of 

 this sort has not been observed in normal muscle but this may be due to 

 the failure to use a sufficiently low intracellular pH. 



Pressure. In the discussions which follow in this section, the data on 

 pressure all pertain to one temperature, 20°C. 



Compression of the relaxed n3 fiber at pH 5.6 with only ATP present 

 causes the development of tension proportional to the pressure, the con- 

 traction being sustained until tlie pressure is removed. The tension in- 



PRE SSU RE I X lO^psi 



Fig. 3. A : Tension induced by pressure in tlie glycerated psoas fiber and in retractor 

 jionis muscle of the turtle. B: Logi,, [y/(l — y)] in relation to pressure. Curve 1, 

 lOiiiM ATP, pH 5.6; curve 2, contracture tension in R. penis of turtle; curve 3, lOmM 

 ATP plus 16mM CP, pH 6.4; curve 4, lOmM ATP plus .16mM CP, pH 5.6, temperature 

 20°C. 



creases with pressure along an S-shaped curve, reaching an upper limit 

 at a pressure above 8,000 psi (fig. 3.4). When logio [y/(l — y)] is plotted 

 against pressure a linear curve results whose slope is aV/2.3 RT and 

 AV is —350 cc/mole (fig. 3B). Allowing for three active sites per unit 

 tension in accordance with equation 1, this would give a AV of —120 

 cc/moIe per unit change in pK. In line with the interpretation of the tem- 

 perature dependence, it is concluded that pressure is acting on equilibrium 

 (A), the formation of AMa,-^' proceeding with a decrease in volume. 



In regard to the reaction ( A I , the sensitivity to pressure depends on 

 ATP and CP (fig. 3.4 ) . When only ATP is present in the reaction mixture, 

 a pressure of 3,600 psi is required for the development of half maximal 

 tension. With the addition of CP at the same pH of 5.6, the system becomes 

 less sensitive to pressure and a pressure of 7,200 psi is required for half 



