MUSCLE RESEARCH AND FLAGELLAR MOVEMENT 281 



information as obtained from muscle studies. It may be mentioned 

 here that the discovery by Marsh (1952) of "relaxing factor" involved 

 a counteraction by the factor of the ATP-induced volume change in 

 muscle homogenates. In other words, this synaeresis was (and is) as- 

 sumed to be equivalent to muscle contraction, and its counteraction 

 equivalent to relaxation. Subsequently, other workers found that the 

 factor caused a loss of tension (isometric) or an extension (isotonic) 

 of ATP-contracted, glycerinated muscle cells, which brought its con- 

 sideration much closer to that of relaxation in the living muscle. 



The point that I would like to make is that, in the postulation of 

 molecular mechanisms, the terms "relaxation'* and "relaxing factor" 

 are not precise enough. To make this clear, we may ask the simple 

 question, "What sort of processes will bring about the end result of 

 relaxation?" and it is easy to see that at least two kinds of processes, 

 possibly more, can each produce what is called relaxation. These are 

 (1) the reversal of a contractile process and (2) the "loosening" (plas- 

 ticizing) of the contractile structure. 



To use a crude analogy, picture a crank-and-pulley arrangement 

 which pulls up a rope with a weight attached at the bottom end. The 

 energetic upward cranking by which the weight is lifted is analogous 

 to the contraction process. Cranking down, then, is the reversal of 

 this process, and is analogous to relaxation. But suppose the bearing 

 on the pulley loosens as the rope is being cranked up; the crank can 

 continue to turn up, but the rope and weight drop. In other words, 

 the effect of relaxation is achieved by two different mechanisms as 

 given above. The question then arises, "Which of these two mecha- 

 nisms does the 'relaxing factor' bring about?" 



Experimental evidence to point up this dilemma can be cited. A 

 single fiber of glycerinated psoas muscle can be isolated and mounted 

 in a simple isotonic lever system. A load of about 100 mg can be im- 

 posed and ATP added to the surrounding bath, whereupon the fiber 

 will contract and lift the load a distance equal to about 50% of the 

 length of the fiber. Now "relaxing factor" is added to the bath, where- 

 upon the fiber "relaxes," elongating to the original length. So far 

 everything is as expected. However, if now half of the load is re- 

 moved, reducing the load on the fiber to 50 mg, the fiber promptly 

 contracts again! It is quite clear, therefore, that under conditions 

 where "relaxation" occurs with a heavier load, the contractile proc- 



