VOL. 4 (1950) ACTOMYOSIN AND MUSCULAR CONTRACTION 39 



is, in its essence, identical with muscular contraction, having been elicited by the same 

 specific substance under a similar condition, the only difference being the destruction 

 of the fibrillary architecture. 



We can go one step further and dissolve out of the freshly minced psoas the con- 

 tractile matter, actomyosin, by prolonged extraction by means of Weber's alkaline 

 0.6 M KCl. This actomyosin behaves like the suspended muscle giving flocculation in 

 presence of salts and superprecipitation in presence of ATP. The last step of degradation 

 of the muscle may be the isolated extraction of actin and myosin. The two proteins, 

 if mixed, unite to actomyosin which gives the same reactions as actomyosin extracted 

 or the suspended psoas. This stepwise decomposition of the psoas thus gives identical 

 results all the way, and the reactions, elicited by the h'ghly specific ATP, are, in all 

 phases, so similar that there can be little doubt about the essential identity of these 

 reactions. Naturally, we must bear in mind that the fibril has its specific architecture 

 which is present no more in suspensions. . 



Instead of making a suspension out of our actomyosin, we can also bring it into 

 the form of a gel and make of this gel, by the method of Weber, a fibre again. Suspended 

 in pure water, the thread will swell. Addition of salts will make this swelling regress, 

 a reaction which evidently corresponds to the flocculation of our actomyosin or muscle 

 suspensions. On addition of ATP the thread, if thin enough, will shorten rapidly, a 

 reaction which evidently corresponds to the superprecipitation of our suspensions and 

 corresponds thus, also, in its essence, to contraction in muscle. 



After having pointed out these analogies of actomyosin and muscle, let us consider 

 the dissimilarities, quoted above. 



Muscle shortens; actomyosin shrinks. This is certainly true, and our problem is 

 whether this difference is due to a difference in the very essence of the reaction or whether 

 it is due merely to the rough structural difference between fibril and actomyosin thread. 

 In the former, as shown by the electron microscopic studies of Hall, Jacus, and 

 ScHMiTT*, the contractile filaments run all along the muscle fibril continuously, parallel 

 to the axis. On extraction these filaments are broken up into fragments which are 

 distributed at random in the actomyosin thread. If, in contracting muscle the filaments 

 become shorter and wider, the muscle will have to do the same — become shorter and 

 wider without changing volume. If the same shortening of filaments occur in the acto- 

 myosin thread which contains the fragments unoriented, at random distribution, the 

 shortening of the very same filaments has to make the thread contract equally in all 

 directions, that is make it shrink. 



That this difference is actually due only to this difference in orientation can easily 

 be shown. If the thread is gently stretched, as shown by Gerendas, the filaments 

 become oriented parallel to the axis similarly to muscle. If ATP is made to act on such* 

 an oriented thread, this thread will shorten and become wider, thus contract without 

 changing volume, similarly to muscle. The same is true, as shown by Buchthal and 

 his associates after drying which acts as stretching. 



Perry, Reed, Astbury, and Spark explain the "synaeresis" of actomyosin bj^ 

 a lateral association of particles. That this explanation cannot be correct is shown by 

 the anisodiameteral contraction of the oriented actomyosin threads. In this structure 

 the filaments are oriented parallel to the axis. Their lateral association could only make 

 the thread thinner and never shorter, while the experiment shows that actually the 

 opposite happens and the thread becomes shorter and wider. 

 References p. 41. 



