REACTIVATION OF EXTRACTED SPERM CELL MODELS 265 



make the reciprocal change. The situation parallels that of a pair of 

 flexor-extensor muscle fibers inseparably fixed in opposition to one 

 another. 



This coupled process makes it difficult to assign a precise role of 

 ATP, for example, to the flagellation cycle. Aside from the question 

 whether the nucleotide is bound or split during the active phase, the 

 problem concerning the exact time when ATP exerts its effect cannot 

 be simply resolved (Weber, 1958; Hoffmann-Berling, 1959). On a 

 priori grounds, we may assume this to occur during active contrac- 

 tion or shortening of motile elements of the sperm flagellum, but 

 Hoffmann-Berling (1958a) has shown that in at least one oscillating 

 system, the vorticellid myoneme, ATP activity is associated with 

 elongation and Ca++ excess with contraction (see also Tibbs, this 

 symposium). 



Further assumptions along these lines suggest the interesting possi- 

 bility that ATP might serve as both contracting and relaxing sub- 

 stance depending on its rate of splitting, on the one hand, and ex- 

 cessive accumulation, on the other. On a basis of the studies reported 

 here, it seems more likely that nucleotide activity represents part of 

 the cycle and is balanced against a component or process which effects 

 the completion of the oscillation. Dr. Nelson has presented evidence 

 during this symposium which suggests that the contractile phase is 

 dominated by ATPase activity and the relaxation phase by IPPase 

 action. The evidence from our models indicates that a relaxing sub- 

 stance or factor, possibly unique to these cells but replaceable by 

 muscle relaxing factor, is present which counterbalances the ATPase 

 complex. One way in which such a system might operate is by the 

 interchange and momentary binding of the cations, Ca++ and Mg+ + , 

 with the two components during the oscillation cycle. During con- 

 traction or shortening, Mg++ is bound to the ATPase system and 

 Ca++ is bound to and inhibits the relaxing system; during elongation 

 the affinities of the cations are reversed. A much more rapid shift of 

 Ca++ and Mg++ over the distances that might be here involved has 

 been envisaged by Csapo and Suzuki (1957) for the excitation-con- 

 traction coupling of muscle fibers. The forces which evoke these 

 ionic migrations and the velocities of response certainly differ in 

 these two systems. Nevertheless, the concept of the regulation of 



