60 S. KOREV VOL. 4 (1950) 



the requirements of the experiment. The standard ATP solution or others tested were added at a 

 rapid and fairly constant rate reaching the suspended fiber almost instantaneously. 



Electrical stimulation applied directly to the fibers did not cause contraction. The fibers were 

 inert to supramaximal single and tetanic shocks. On the addition of ATP to the environment of the 

 fibers, a definite and easily recorded contraction developed. As the fibers shortened, their diameters 

 increased. In this respect the contractions resembled isotonic contractions of normal muscle. However, 

 the fibers did not readily relax following the contraction induced by ATP. It was, therefore, necessary 

 to use new fibers for each determination. The stability and constancy of fiber groups became all the 

 more important for this reason. During the first two weeks of preservation the libers were found to 

 be unstable and variable. On exposure to isotonic solutions of salts, e.g., contained in mammalian 

 Ringer's or saline, pseudo-contractile movements were occasionally observed. After the second week 

 of preservation, more dependence could be placed on the stability of the fibers. 



The ATPase activity of the homogenates of the fibers was determined by the method described 

 by Du Bois AND PotterI^. 



RliSULTS 



Addition of ATP 



Of the compounds tested, ATP and ADP alone elicited contraction of the libers*. 

 A particularly significant group of substances are listed in Table I. The fiber apparently 



TABLE I 



COMPOUNDS TESTED TO INDUCE CONTRACTION OF UNLOADED NON-CONDUCTIVE MUSCLE FIBERS 

 (rabbit), the ATP AND ADP FIGURES INDICATE THE LOWEST CONCENTRATION WITH WHICH CON- 

 TRACTION WAS OBSERVED. THE FIGURES OF THE OTHER COMPOUNDS INDICATE THE HIGHEST CON- 

 CENTRATION TESTED 



reacted in a selective manner to ATP and ADP. The threshold concentration of ATP 

 requisite for contraction was less than that of ADP. Moreover, with equimolar solutions 

 of ATP and ADP, the degree of shortening was greater in the case of the former. The 

 amount of shortening of fibers was found to depend on the concentration of the ATP 

 solution employed, approaching a maximum asymptotically (Figs i, 2). 



Unlike myosin threads, the loaded fibers contracted rather than extended in the 

 presence of ATP. Moreover, if the fibers were incapable of shortening because the load 

 was excessive, extension did not occur on the addition of ATP. 



Effect of temperature 



When the suspended fibers and the added solutions of standard ATP were main- 

 tained at 37° C, the extent of shortening was 5.4 times as great as observed under similar 

 conditions at 10° C (Fig. 3). Calculated on this basis there was an increase in the amount 

 of contraction by a factor of 1.9 for each lo*^ C rise in temperature between 10° and 37° C. 



* I am greatly obliged to Dr Harry G. Albaum, Brooklyn College, for supplying adenosine- 

 diphosphate and adenosinemonophosphate. The ADP was tested enzymatically b}" Dr Albaum and 

 found by his method to be free of ATP. 



References p. 6y. 



