EFFECTS ON SKELETAL MUSCLE 205 



There is no doubt that in iodoacetate rigor the ATP and creatine-P are 

 very low or absent (Norpoth, 1931; Sacks, 1939). At the initial onset of 

 rigor the ATP and creatine-P are reasonably low (see the results of Her- 

 mans given on page 100), but the levels fall quite rapidly during the de- 

 velopment of rigor and thereafter. Hsu (1950) treated muscles with 0.54 mM 

 iodoacetate and 1.2 mM cyanide for 45 min, stimulating every 5 sec, and 

 analyzed for ATP throughout. He found initial levels to be 206 mg%, at 

 the onset of rigor 139 mg%, at maximal rigor 66 mg%, and after a period 

 of rigor 23 mg%. Hsu did not believe that the fall in ATP and the develop- 

 ment of rigor are parallel, however, but there is some doubt as to exactly 

 what relationship one would expect. He also postulated that ATP may 

 be utilized in the development of rigor, a point on which there is still no 

 definite information. Indirect evidence that the store of high-energy phos- 

 phate determines the rate of onset of rigor is the common observation that 

 stimulation of the muscle to greater activity accelerates the shortening. 

 Frog muscle, for example, which has been treated with 0.4 mM iodoacetate 

 exhibits four phases according to Marechal (1958): (1) the tetanic contrac- 

 ture during the 10- to 15-sec period of stimulation, (2) a relatively short 

 latent period during which the muscle is completely relaxed, (3) the de- 

 velopment and attaining of rigor, the maximal developed tension being 

 perhaps 60% of that generated by tetanus, and (4) the very slow disappear- 

 ance of the contracture over several hours. In general one may say that the 

 greater the activity of the muscle, the more rapidly will it pass into rigor; 

 furthermore, the rate of development of rigor is markedly dependent on 

 the temperature, being extremely slow and incomplete at 0°, much faster 

 at 18°, and very rapid at 30°, and this may or may not be related to the 

 rate of disappearance of ATP and creatine-P. Other observations are not 

 easily explained on this basis. If the muscle is stimulated during the de- 

 velopment of rigor, there follows a temporary relaxation before shortening 

 continues; also if the muscle during contracture is stretched briefly, it 

 relaxes immediately in an exponential manner and the tension does not 

 return to the initial contracture level for 30 min or longer (Marechal, 1960). 



(G) Creatine kinase. One important source for ATP regeneration in most 

 tissues is creatine-P, if the normal oxidative generation of ATP is blocked, 

 and for this reaction creatine kinase is necessary. If iodoacetate inhibited 

 this enzyme, it would presumably induce rigor sooner and more effectively, 

 assuming that ATP depletion is involved in rigor. As we have pointed out 

 (page 36), creatine kinase is often quite sensitive to iodoacetate in vitro, 

 and Mauriello and Sandow (1959) have suggested it as a possible site for 

 iodoacetate action. However, in vivo the enzyme is inhibited much less 

 strongly, perhaps due to protection by substrates present in the cell (Carl- 

 son and Siger, 1959, 1960; Padieu and Mommaerts, 1960). It is interesting 

 to speculate that, if this is true, the enzyme might become progressively 



