MANUEL F. MORALES AND JEAN BOTTS 



muscles at rest ATP*~ and anionic factor are both adsorbed to 

 Mg-myosin in such quantitative relations that the anions to- 

 gether "overcharge" negatively appropriate regions of the myo- 

 sin structure. We may then imagine that excitation makes Ca^"^ 

 (or possibly some other metallic cation) available. By any one 

 of several reactions {vide supra, p. 614), e.g., simple electrostatic 

 discharge, spatial removal of ATP, the Ca^+ may temporarily 

 abolish factor, thereby effecting contraction. Subsequent with- 

 drawal of Ca2+ would bring about relaxation. This suggestion 

 sets the energetic problem back one step, i.e., poses the question 

 as to what energy source releases and withdraws Ca^^ from the 

 contractile system. It also leaves it uncertain whether condi- 

 tions would or would not permit ATP hydrolysis during, say, a 

 very short exposure to Ca^"^. At the same time, the suggestion 

 is not without advantage. The ingredients required for a 

 straightforward physicochemical explanation of the operation 

 of in vitro model systems are retained, and with them the funda- 

 mental connection between models and muscle. The new in- 

 gredients added — "factor" and Ca^"^ — are added because experi- 

 ment has already shown how important they are to the operation 

 of the simple systems. Yet this simple elaboration suggests an 

 explanation of the three in vivo observations cited early in this 

 section, for this more elaborate system (7) could operate in the 

 absence of ATP hydrolysis, and (2) would be insensitive to ATP 

 (inasmuch as ATP would already be there). Curiously enough, 

 such a system would also behave in keeping with two further 

 observations: First (72), microinjection of Mg2+ into a living 

 fiber does not cause contraction (Mg2+ is already there and 

 bound), but microinjection of Ca^^ does cause contraction 

 (Ca^^ abolishes factor action). Second, in a fatiguing fiber (62) 

 it is relaxation that progressively slows (rate of refilling sites falls 

 as ATP concentration falls), while contraction continues unaf- 

 fected (so long as only Ca^^ needs to be presented). 



In closing an essay which, perhaps prematurely, has sought 

 to explain a physiological phenomenon, the authors are moved 

 to philosophical comment. An "explanation" is certainly more 



624 



