VOL. 4 (1950) 



CONTRACTILITY OF A NON-CONDUCTING FIBER 



61 



Fig. I. The series of tracings represent the ATP induced isotonic contraction of fibers 8.5 cm in length 



recorded on a kymograph moving 3.0 cm per minute. Concentrations of ATP decreasing from 0.04 M 



in the first to o.ooi M in the last tracing. Magnification 6 X . 



g>75 



10 





0.05 .01 .02 .03 Oh 



Molarity ATP 



Fig. 2. Degree of shortening of fibers suspended 

 in an isotonic recording system as function of 

 increasing concentrations of ATP. Ordinates: 

 Degree of shortening at a given time in arbi- 

 trary units. Abscissae: M ATP concentration. 

 All fibers were of equal length (8.5 cm). 



Fig. 3. Effect of temperature on the ATP 



induced isotonic contraction. Lower curve 



contraction at 10° C, upper curve at 37° C. 



ATP concentration 0.002 M. 



Effect of pH 



The present experiments were carried out at p^ 7.4-7.6. It was observed that the 

 fibers deteriorated rapidly in solutions beyond the limits of p^ 6.8 and 7.8. 



Effect of ions 



Sodium ATP caused contraction of the libers in the absence of other ions. However, 

 magnesium ion activated the reaction of ATP with the contractile proteins of the fibers 

 as shown by the increased extent of shortening in equimolar solutions of ATP (Fig. 4). 

 The optimal concentration of magnesium ion was i • io~^ M. Potassium in similar 

 concentrations did not manifest the activating effect of magnesium. In the presence of 

 References p. 6y. 



