92 PHYSIOLOGICAL TRIGGERS 



similar to that of the fiber stimulated in NOs-Ringer, the twitch tension de- 

 creases along approximately the same time course as that of the fiber in NOs" 

 Ringer (57). Thus, the increased heat production and the early fatiguing of 

 muscle in anion-substituted Ringer solution do not appear to be specific anion 

 effects but rather a reflection of a more rapid depletion of metabolic energy 

 reserves. 



Since the tetanus tension is unaffected by the abnormal anions, it is presumed 

 that the increase in twitch tension does not reflect an over-all intensification of 

 the 'active state' but rather a prolongation of this state (45). A. V. Hill (44) sug- 

 gests that elastic components in series with active contractile components in 

 muscle act as 'shock-absorbers' so that some of the force of contraction is ex- 

 pended in stretching these passive elastic elements. In a normal twitch, all the 

 contractile components are presumed to become activated, but this active state 

 does not persist long enough to be manifested as a full tension on the ends of the 

 fiber. However, if the active state can be made to last beyond the period in 

 which the elastic elements are being rapidly extended, an increased external 

 tension appears. 



Some years ago it was found that application of pressure (of the order of 100 

 atmospheres) during the early stages of contraction causes an increase in ten- 

 sion and heat production (10, 11), similar to that observed with the abnormal 

 anions. Podolsky (79) has recently suggested that the effects of hydrostatic 

 pressure and of the anion series may be manifestations of the same phenomenon. 

 It is found, for example, that a twitch of a frog sartorius muscle in chloride 

 Ringer's solution at 80 atmospheres appears to be identical with a twitch in 

 nitrate Ringer's at one atmosphere. The temperature dependence of the tension 

 is also similar in the two cases. It is proposed that an increase in hydrostatic 

 pressure may increase the hydration shell of the chloride ion in such a way that 

 the muscle fiber finds this hydrated anion indistinguishable from, say, a hy- 

 drated nitrate ion at one atmosphere pressure. It is known that the rate of 

 sodium efflux in cat erythrocytes decreases when chloride is replaced by Br~", 

 NOs", or I~, the greatest reduction occurring with iodide (19). When a hydro- 

 static pressure of 80 atmospheres was applied to these cells in the presence of 

 chloride ion, the rate of sodium efflux was reduced to that in a corresponding 

 nitrate medium at one atmosphere (79). Thus, in two different systems (muscle 

 fiber and erythrocyte) a nitrate response is mimicked by application of 80 

 atmospheres of pressure to cells in a chloride medium. With certain other anions 

 (acetate, propionate, or butyrate), pressure was found to have little effect on 

 the rate of sodium efflux in erythrocytes (79). Although these results are not 

 conclusive, they indicate that in the range of pressure used (< 100 aim) there is 

 a specific pressure effect on the chloride ion rather than a general alteration of 

 some property of the fiber in response to pressure. 



The studies with abnormal anions and with pressure suggest that normally 



