PRIMARY EVENT IN MUSCLE ACTION 



This conclusion should allay fears that our model is wasteful of 

 the "energetic wealth" of ATP. 



Implicit in the model under consideration is the idea 

 that myosin — more specifically, a myosin network structure — 

 behaves in its deformations like a flexible polyelectrolyte. 

 Amino acid analyses (2,43) have established that myosin is 

 a polyelectrolyte, but whether its mechanical properties reflect 

 this compositional feature is a matter for other investigations. 

 Early experimental confirmation was the result (9, 1 0,70) that for a 

 myosin thread immersed in a solution of low ionic strength 

 (c)^/d/)p,T < 0, and (dE/dl) p^T < 0, precisely as would be expected 

 from the idealized molecular chain model (vide supra) . * The bulk 

 of the remaining evidence consists of observations in which the 

 structural effect of ion-binding on myosin has been observed. 

 Thus variations of parameters such as ionic strength or pH (i.e., 

 extent of H+ binding) have been shown to produce profound 

 structural changes in myosin systems (5,31). Under conditions 

 in which ATP addition would cause contraction, it has been 

 shown by Churney (19) that other anions (e.g., 804^", Fe- 

 (CN)6^~, Fe(CN)6^~) will also bring about contraction, though 

 at concentrations higher than those of ATP. Similarly, Laki 

 and Bowen (48) have shown that I~ and SCN~ produce marked 

 contraction under these circumstances. As might be expected, 

 binding of ATP shows special features, but its effects seem, in 

 general, to be explainable in simple electrostatic terms. Again, 

 Churney (19) has shown that a myosin fiber pretreated with 

 acetic acid contracts upon ATP addition, whereas a fiber pre- 

 treated with ammonium hydroxide relaxes upon ATP addition. 

 If one considers that acid treatment induces positive charging of 

 the fiber, and alkali treatment induces negative charging, then 

 the oppositely directed ATP effects are quite comprehensible; 



* Here S is entropy, E is internal energy, and / is length. Considering 

 the overenthusiastic application of thermoelastic analysis to nonequilibrium 

 situations, it is wise to state that no ATP was present in these experiments, 

 so that the appropriate thermodynamic expression (9,10) could be expected to 

 apply. 



619 



