MANUEL F. MORALES AND JEAN BOTTS 



on interaction with ATP, in other words, to explain the Engel- 

 hardt experiment. The uhimate objective, of course, is beyond 

 this, i.e., it is to explain the mechanical deformation in living 

 muscles. It is to be expected that the situation in muscles will 

 differ from that of the model system in important respects (this 

 is already apparent from recent experimentation) ; so that one 

 must be prepared to modify or complicate the model corre- 

 spondingly; however, up to this time the similarities between 

 model and muscles outweigh the apparent differences. 



Although we are aware of the fallacies of gauging the 

 eventual importance of recent results, we are nonetheless im- 

 pressed by the following experimental observations on muscles. 

 (7) Repetitive activity (electrically induced) in muscle fibers 

 does not increase the rate of incorporation of orthophosphate 

 (radioactively labeled) into the ATP of such fibers compared 

 with the rate in resting fibers (8). (2) Under certain well 

 specified conditions unit activity (e.g., single twitches) in 

 muscles is not accompanied by changes in the concentra- 

 tion of ATP or ADP (changes in P seem to us uncertain), 

 under conditions in which regeneration is very probably ruled 

 out (22,23,53). (3) Microinjection of ATP into muscle fibers 

 does not cause contraction (21). An overly simple conclusion 

 from these findings is that ATP has nothing to do with the action 

 of living muscles, and that the Engelhardt-type of experiments 

 are but confounding coincidences. Such alarm should be 

 tempered, however, by three considerations: first, there exist 

 too many genuine similarities (70,71) between models and mus- 

 cles ; second, even in the model experiments it is reasonable — as 

 we have argued above — for the mechanical deformation (being 

 coupled with the ATP-adsorption process) not to require the 

 hydrolytic cleavage of ATP; and third, there have recently 

 been discovered relations between the models and other sub- 

 stances which may eventually resolve today's "discrepancies." 



The new relations to which we have alluded exist between 

 "relaxation factors"* and the ATP-myosin-Mg model systems, 



* As we have suggested elsewhere (57), it may be better to term these 

 substances "modifying factors," for in many instances they appear to enhance 

 contraction as well as relaxation. 



622 



