DUGALD E. S. BROWN 85 



raent of tension following a quick release from maximum tetanus tension 

 where the 'active state' is fully developed. 



In the liglit of these observations, Goodall (9) has reinvestigated the 

 relation between activation and the rate of development of tension. As 

 a result of these studies he has concluded that activation should be con- 

 sidered a function of tension or length as well as time, the dependence being 

 expressed by a rate equation. "The time course of isometric tension de- 

 velopment is then determined by a function which measures the initial 

 activation." The discovery that a rate equation describes the dependence 

 of tension on activation is most significant since it presents the possibility 

 of treating contraction in terms of tlie theory of absolute reaction rates. 

 As Goodall has indicated, it also presents the opportunity of identifying 

 the physico-chemical meaning of the constants a and b in Hill's funda- 

 mental equation. In view of Goodall's proposals, the contractile unit in 

 the schema is shown as a phosphorylated activated unit AMa,-^' in which 

 phosphorylation depends on reaction D which, for example, may involve 

 the hydrolysis of adenosine triphosphate by myosin ATPase. 



The effects of temperature and pressure on the sequential events in 

 contraction may be conveniently considered in terms of the above schema. 

 In producing contraction, it is supposed that the depolarization of the 

 cell creates a condition, Cj , which causes inactive actomyosin AMi^ to 

 pass rapidly to an active contractile form, AMa,-^', which then shortens 

 to AMac^^ In either the relaxed or contracted states j^hosphorylases and 

 transphorylases are believed to be active in contributing to the phos- 

 phorylation of AMa .^ 



If the state of depolarization is sustained by repetitive stimulation, the 

 'active state," in the Hill sense, persists and the maximum tension develops. 

 However, if the depolarization is transient as in the twitch, deactivational 

 processes intervene and the active state rapidly disappears. As a result, 

 the tension developed in a twitch is less than the tension developed in a 

 maximum tetanus. 



In terms of the above schema: the steady state of tension requires the 

 active moiety AMa,-^ and depends both on the state of an equilibrium 

 reaction (B) and on the rate of reactions involving the hydrolysis of ATP 

 by myosin ATPase (D). In becoming active through equilibrium (A), 

 it is thought that actomyosin undergoes a molecular reconfiguration to a 

 form capable of shortening and that concomitantly myosin ATPase changes 

 from an inactive to an active enzyme. 



As a result of the studies reported by Guthe (10), it seems certain that 



^ The term phosphorylation refers to the phosphate potential as defined by equation 

 3. 



