QO PHYSIOLOGICAL TRIGGERS 



Calcium also plays a role at the fiber membrane. The effect of increased 

 extracellular potassium, in increasing membrane permeability to ions, can be 

 counteracted by increasing the extracellular Ca++ concentration (86), whereas 

 lowering the extracellular Ca+"^ concentration causes a lowering of the resting 

 potential (13) and a general increase in membrane permeability (17). 



RELATION OF FIBER SURFACE EVENTS TO CONTRACTION 



Fleckenstein (36) expresses the view that the energy available from the 

 inflow of sodium and outflow of potassium across the fiber membrane during 

 excitation is sufficient to account for the work of contraction. In such a picture, 

 the entire amplification of the energy input is accomplished in the events at the 

 fiber surface during excitation. In support of this view, Fleckenstein cites the 

 work of Wilde and O'Brien (92) on the slowly contracting turtle heart. Injection 

 with K"*- /;/ vivo was followed, after death, by perfusion of the heart with non-K'*^ 

 Ringer's solution via the coronary artery. The amount of K^- recovered in the 

 venous blood per systole was compared with the amount of work performed by 

 such a heart per systole. The work done was found to be approximately equiv- 

 alent to the maximum work available from the transfer of the recovered amount 

 of K'*'^ — and an equivalent amount of sodium — from higher to lower concentra- 

 tions across the muscle cell membrane. The difiference between this calculation 

 and that based on the membrane electrical properties is certainly striking and 

 illustrates the basis for some of the divergent views on the triggering of muscle 

 contraction. 



Sandow (83) has recently reviewed the literature on contracture and has 

 stressed the essential similarity between muscle contracture and contraction. 

 Of considerable interest are the 'slow' muscle fibers (somewhat akin to smooth 

 muscle) present to a greater or lesser degree in several skeletal muscles of the 

 frog. These fibers are characterized, in part, by sluggish response to mild stimu- 

 lation, by the absence of any propagated action potential, and by the pres- 

 ence of multiple innervation per fiber with graded local contractures in response 

 to stimulation (62). Study of these slow fibers has suggested a simple relation- 

 ship between the degree of contracture and the extent of depolarization of the 

 fiber membrane. In such fibers contracture can be sustained for long periods of 

 time and can be reversed by repolarization of the membrane (61,35). O^i the other 

 hand, Niedergerke (76), studying potassium-induced contracture in strips of frog 

 ventricle, has found that a change in Ringer calcium concentration can greatly 

 increase the contracture tension and rate of tension development without 

 significantly altering the extent and time course of depolarization. Other fibers 

 also give evidence of an indirect connection between surface electrical events 

 and contraction. In a crustacean muscle the mechanical response can be 

 completely abolished by long soaking in the presence of tetrabutylammonium 

 ion, while the action potential remains apparently unimpaired though pro- 



