588 



12. RATES OF INHIBITION 



interpreted in two immediately obvious ways. Either the increased number 

 of membrane depolarizations allows the iodoacetate anion to penetrate 

 more rapidly, or the increased functional activity exerts a greater metabolic 

 demand and consequently depletes the tissue more rapidly of energy- 

 rich substances. 



240 



TIME FOR 50% 

 REDUCTION IN 

 CONTRACTILITY 



TEMP- 



Fig. 12-30. Effects of various factors on the rate of action 

 of iodoacetate on rabbit atria. (From Gardner et at., 1954.) 

 Curve A, effect of iodoacetate concentration on the atrial 

 rate. Cin-ve B, effect of iodoacetate concentration on the 

 amplitude of the contraction. Curve C, effect of stimulus 

 frequency on the response of contraction to iodoacetate. 

 Curve D, effect of temperature on the response of con- 

 traction to iodoacetate. 



{D) Effects of heavy metal ions on diaphragm respiratio)) . One of the most 

 thorough studies of the factors involved in the kinetics of a cellular inhi- 

 bition was done l)y Demis and Rothstein (1955). They found that the up- 

 take of glucose by rat diaphragm muscle was very sensitive to Hg+"^ (it 

 will be convenient to designate mercury in this way although most of the 

 metal in solution exists in complexes with anions), 0.2 mil/ inhiluting al- 

 most completely in 20 min (Fig. 12-31). Respiration, on the other hand, 

 was much less sensitive to Hg"*""^; a lag period of over 30 min was observed 

 before any inhilntion occurred and after 2 lir the inhibition had reached 



