40 A. SZENT-GYORGYI VOL. 4 (1950) 



In order to be able to discuss the stretching of the loaded actomyosin thread in 

 ATP, we have to give our attention for an instant to another effect of ATP, independent 

 of contraction. Fresh muscle is elastic. Post mortem the ATP is disintegrated and, 

 parallel to its disappearance, the muscle becomes inelastic, as shown by Th. Erdos, 

 Bate-Smith, and Bendall. It is possible to show that it was actually the disappearance 

 of ATP which induced this difference. A washed psoas fibre is inelastic. If suspended in 

 Ringer, containing ATP, it becomes elastic again. This shows that in absence of ATP, 

 links are developed between neighbouring micells which make the system rigid, 

 making slipping and relative motion impossible. These are abolished by ATP. This 

 effect of ATP is independent of its second effect, contraction. If ATP did not have the 

 first effect, it could not induce contraction at all because the system would be too rigid. 

 This effect of ATP was, in fact, the very first specific effect discovered of ATP on 

 "myosin" by Engelhardt, Ljubimowa, and Meitina who found that ATP makes 

 "myosin" threads more extensible. The decrease of dynamic softness of actomyosin 

 induced by ATP has also been studied extensively by Buchthal and his associates. 



After this short discussion we can consider now the extension of loaded actomyosin 

 threads. If an actomyosin thread is loaded, it will not stretch because it is rigid, its 

 particles being held together by the links or cohesive forces described before. If ATP is 

 added these forces will be abolished and, under action of the load, the short fragments 

 of filaments of which the thread is composed, will begin to slip under influence of the 

 load, and the thread will lengthen, even if at the same time these fragments shorten. 

 The situation will be different in an unloaded thread. There will be no force present to 

 cause slipping, and the shortening micells will make the thread contract or "shrink" 

 according to its co-axial or random distribution. In the muscle fibre there can be no 

 slipping because the filaments run continuously through the fibrils, and so the muscle 

 can shorten only if its filaments contract, whether loaded or unloaded. 



Perry, Reed, Astbury, and Spark stress one more difference between muscular 

 contraction and the contraction in actomyosin threads: the time factor. Muscle may 

 contract several hundred times per second, while even thin threads need seconds for 

 their contraction. Here again the difference lies in steric relations and not in principle. 

 If diffusion and friction are eliminated, the ATP contraction is instantaneous. This 

 can be shown in washed psoas-fibres suspended 0° C in a solution. At this temperature 

 the fibres develop only a very weak tension. If they are transferred into a Ringer of, say 

 25° C, the development of a high tension is instantaneous. Rapid reaction can also be 

 demonstrated in thin actomyosin threads, to which ATP is added in such a way as to 

 reach the thread from one side. On this side the actomyosin contracts and makes the 

 thread bend or curl up rapidly. 



The differences in behaviour of muscle and actomyosin can thus, in the instances 

 discussed, be explained satisfactorily by the rough structural differences of both for- 

 mations and need not be ascribed to the difference in underlying reactions. 



SUMMARY 



It is shown that the contraction of muscle, superprecipitation of its suspensions, superprecipita- 

 tion of actomyosin and contraction of actomyosin, ehcited by ATP, are related phenomena. 



Differences in behaviour, as for instance anisodiametry of shrinking in muscle and isodiametry 

 of shrinking in unoriented actomyosin gels, can be explained by the differences in structure. The 

 same is true for the difference of muscle and loaded actomyosin threads, the latter of which, contrary 

 to muscle, lengthen under influence of ATP. 



References p. 41. 



