EFFECTS ON THE HEART 



217 



75% of normal following addition of 0.04 mM iodoacetate. remains at this 

 level for 2 lir, and then falls suddenly along with the action potential (Fig. 

 1-22) (Gardner ct al., 1954). Earlier workers have reported reduction in 

 chronaxie (Gupta, 1933) and even some increase in the excitability as the 

 contractions fail (Van Ginkel, 1934), but in many cases there is confusion 

 between excitability and automaticity. The occurrence of fibrillation penul- 



FiG. 1-22. Effects of 0.04 mM iodoacetate on the 

 various characteristics of rabbit atrium. CR = con- 

 duction rate; MSR — maximal rate of stinuUation; 

 EE = electrical excitability; AP = action ])otential 

 magnitude; and DT = developed tension. .Since this 

 is only one experiment the results are not very quan- 

 titative and since there is no control, the spontaneous 

 clianucs during this interval are not known. (From 

 Gardner et al., 1954.) 



timately (Dobrowolski, 1933; Maltesos, 1934) has prompted some workers 

 to conclude that an increase in the excitability has occurred, which of course 

 is not necessarily so. The refractory period of frog ventricle (Gupta, 1933; 

 Dale, 1935), turtle atrium (Edwards and Sanger, 1933), turtle ventricle 

 (Marshall, 1955), and rat atrium (Liillmann, 1959 a) is shortened by iodo- 



