MITOCHONDRIA. CELLS. AND TISSUES 



587 



way an increase in iodoacetate concentration accelerates the inhibition 

 rate. This might be due to different permeabilities of pacemaker and or- 

 dinary atrial cells to iodoacetate but is more likely the result of different 

 degrees of dependence of the functions on the susceptible metabolism. 

 Since the lowest concentration used (0.04 mM) stopped anaerobic activity 

 in the atria within 5 to 10 min, one would not expect an increase in con- 

 centration above this to increase the rate of inhibition under aerobic con- 

 ditions. It was suggested that iodoacetate might have a second action in 



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 60 

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 '' 20 



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10 20 30 

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Fig. 12-29. Changes in the frog central nervous system during perfusion 

 with iodoacetate. Inhibition of glycolysis is indicated by the curve for 

 lactate and this is followed by a fall in creatine phosphate (CrP). The ac- 

 tivity was measured by the response to strychnine. (From Holmes, 1933.) 



addition to the inhibition of glycolysis, and that this second action might 

 play the major role in functional depression aerobically. Since glycolysis 

 does not appear to be very important in cardiac tissue under aerobic con- 

 ditions and most of the energy for function is derived from the oxida- 

 tion of lipids, it is possible that this second action is on fatty acid oxi- 

 dation or more likely on the tricarboxylic acid cycle. These points will 

 be discussed in detail in the chapter devoted to iodoacetate inhibition. The 

 effect of increasing frequency of the beat on the rate of inhibition can be 



