222 1. lODOACETATE AND lODOACETAMIDE 



taneous relaxation, even though the atria or ventricle strips are under 

 tension. 



Recent unpublished work on rat atria by Dr. J. Lacuara will be discussed 

 briefly inasmuch as it sheds light on the mechanism of rigor. Concentrations 

 of iodoacetate from 0.2 mM to 2 mM depress the contractile tension of 

 electrically driven atria progressively over periods of 30 min, complete fail- 

 ure occurring with the higher concentrations at 20 min. Simultaneously one 

 observes the development of rigor and by 30 min it is often quite marked, 

 although there is variation with regard to the rates at which the dias- 

 tolic tension rises. When 5 mM pyruvate is present with the iodoacetate, 

 the contractile depression is definitely less (not over 30% even with 2 mM 

 iodoacetate) and rigor does not occur. Lactate acts similarly but is some- 

 what less effective, while acetate is only slightly effective. The time curves 

 for rigor usually consist of two phases, an early phase during which the 

 tension increases to 120-130% of the control value and a later phase during 

 which the tension rises rather suddenly to 170-180% of the control value. 

 The omission of Ca++ in the medium tends to abolish the first phase but 

 has little or no effect on the later rigor. Removal of Ca++ after the develop- 

 ment of the initial rigor causes a slow relaxation of the atria, but the later 

 rigor occurs at the same time and to the same degree as in the normal 

 medium. There seem to be two different types of rigor, one dependent on 

 Ca++ and one not. Atrial ATP was determined under various conditions 

 with iodoacetate at 0.5 mM (see accompanying tabulation). Iodoacetate 



% Change of contraction 



87 



+ 17 



^ 27 



" The numbers in parentheses indicate the number of atria used. 



* These atria developed rigor somewhat faster than those in the line above. 



causes a severe fall in ATP, which is about 35% of normal soon after the 

 initiation of rigor, but from these preliminary values it seems that the ATP 

 does not continue to decrease as the rigor progresses. In other words, these 

 data suggest that the fall in ATP may be responsible for rigor but that ATP 

 is not utilized in the rigor process. Pyruvate is fairly well able to maintain 

 the ATP level in the presence of iodoacetate and rigor does not occur. 



