g6 PHYSIOLOGICAL TRIGGERS 



protein forms a four-way complex with relaxing factor, iMg"*""*" and ATP. Initia- 

 tion of the active state can then be pictured as an introduction of Ca"*^ which 

 removes or inactivates the relaxing factor and allows contraction to take place. 

 As long as Ca++ is present, relaxation is inhibited. Various other schemes have 

 been proposed for relaxing factor action (4, 5, 9, 88, 89). 



Considerable attention was devoted in the earlier part of this chapter to 

 substances which act near the membrane surface and in some way lead to a 

 prolongation of the normal activating process. It has been- suggested that one 

 step in this activating process is the release of Ca ions in the fiber interior. If the 

 normal activating route is bypassed and Ca ions are supplied directly through 

 artificial means, contraction might also be expected to take place. It is found 

 that a reversible localized contraction does occur at the site of intrafiber injec- 

 tion of Ca++ although similar injections of ]Mg++ and ATP are ineffective (30, 

 75, 91). A further indication that calcium may be released in the fiber interior 

 during stimulation is the finding that stimulation causes an increased rate of 

 outflow of Ca'*^ from frog sartorius muscle previously soaked in Ca'*^ Ringer's 

 solution (94). 



Another means of inducing or prolonging the active state would be to apply to 

 the fiber some other substance which directly inhibits relaxing factor action. 

 Ryanodine appears to be such a substance (55). It is found that muscle (frog 

 rectus abdominis) soaked in Boyle-Conway solution containing a small amount 

 of ryanodine contracts irreversibly. The action potential (27) and the ATPase 

 activity of myosin B (55) are unaffected by ryanodine. In glycerinated rabbit 

 psoas fibers (in which presumably the naturally occurring relaxing factors are no 

 longer present), ryanodine does not appear to affect ATP-induced contraction 

 or EDTA-induced relaxation (55), suggesting that ryanodine does not obscure 

 enzymatic sites or relaxing factor binding sites on the contractile protein. These 

 findings appear to be consistent with the view that ryanodine directly inhibits a 

 naturally occurring relaxing factor. 



If it is supposed that the active state is normally precipitated by a release of 

 calcium in the fiber interior, the question arises as to the source of this calcium. 

 The micro-incineration studies of Draper and Hodge (25) indicate that some 

 bound metals, probably calcium and magnesium, occur at 400-A intervals 

 throughout the A and I bands, except in the immediate vicinity of the Z and M 

 bands. As Weber and Portzehl (89) have pointed out in connection with their 

 theory of the contraction-relaxation cycle, movement of ions through a 100- to 

 200-A distance as required here is not an unreasonable demand. Recently 

 Hodge (48) has found a similar spacing of about 400 A in the protein supporting 

 structure between filaments. He has tentatively suggested that this protein may 

 be tropomyosin. It is of interest that tropomyosin has an exceptionally high 

 negative charge, which would make it a particularly attractive haven for 

 cations, and also has a particle length approximately equal to the estimated 



