VOL. 4 (1950) CONTRACTILITY OF A NON-CONDUCTING FIBER 59 



chemical reaction is known to be more closely associated with the contractile mechanism. 

 A new development was initiated in 1939 by Engelhardt and Lyubimova^^' ^^ when 

 they tested this idea by studying the interaction between ATP and myosin, which at 

 that time, was the main protein considered to be associated with contraction. Under 

 the stimulus of their observations, the reaction between ATP and the muscle proteins has 

 been extensively studied and considerable progress has been achieved essentially by 

 the work of the Needhams and Szent-Gyorgyi and their associates^*' ^^' ^^. The 

 demonstration by Straub of a second protein, actin, which combines with myosin to 

 actomyosin, was an important advance in the study of the primary reactions which 

 may underly the contractile process^'^. 



However, if the interaction of ATP and actomyosin is studied in solution, the ele- 

 ment of organization of the protein is not included. Recently, Szent-Gyorgyi* has 

 described a muscle fiber preparation which contracts in the presence of ATP. The usual 

 electric stimulus is ineffective. This indicates that the conductive membrane is inactive. 

 In a normal muscle fiber, whether stimulated directly or indirectly, the activation of 

 the conductive membrane which envelopes the muscle cell intervenes between stimulus 

 and contraction. It is only through the activity of thi^ membrane that the contractile 

 process is initiated. Since in the preparation of Szent-Gyorgyi the conductive mecha- 

 nism is excluded but the contractile units are still functioning, as demonstrated by 

 the ATP induced contraction, this fiber offers a most suitable material for the study of 

 factors influencing contraction independent of the action of the conductive membrane. 

 Such a differentiation is of considerable interest for the understanding of some muscular 

 disorders, especially myotonia and familial periodic paralysis. It is with this problem 

 in mind that the present study has been initiated. 



material;' AND METHODS 



The psoas major of a rabbit was isolated by dissection and then tied at either end to an appli- 

 cator stick. This preserved the resting length. The muscle was removed in toto by severing its connec- 

 tion at origin and insertion. It was placed in 50% glycerol, kept in the icebox overnight and then 

 stored in 50% glycerol at — 10° C. The fibers of the psoas muscle of the rabbit pass throughout the 

 length of the muscle in a parallel fashion. For the present study this muscle appeared suitable but 

 other striated muscles may be used in a similar way. 



The main features of the glycerol preserved fibers are : the ease with which a small numbei of fibers 

 (about 3-10) can be stripped from the main bulk of the muscle; the retention of the structural organi- 

 zation of the fibrils; the modification of the cell membrane to an unexcitable state; and finally the 

 fiber's ability to contract on the addition of ATP. 



By grasping the desired amount of muscle fibers in a forceps, they can be peeled from the muscle 

 belly by exerting a slight tension. Forceful pulling on the fibers being detached causes partial inter- 

 ruptions in their continuity which can be noted by holding the fibers to the light and observing regions 

 of increased transparency. Fiber groups 0.5 to i mm in diameter were separated from the muscle 

 for study. 



The microscopic appearance of the unstained preserved fibers was similar to the normal un- 

 treated fibers from the same animal. However, the volume of sarcoplasm was diminished and the 

 diameters of the fibers were decreased appreciably. 



The contractions of the unloaded fibers were studied in various experiments. After a number 

 of preliminary observations, the experiments were carried out in the following way. The fibers were 

 suspended in a constant volume of mammalian Ringer's solution according to Krebs. The con- 

 tractions were recorded by an isotonic system on a kymograph moving at 3 cm per minute. The 

 suspended fibers were kept in a bath of constant temperature which could be varied according to 



* I am greatly indebted to Professor Szent-Gyorgyi for the demonstration of this preparation 

 which made this study possible. 



References p. 6y. 



