136 



Influence of D^O on respiration. — The respiration of several 

 organisms has also been studied and found to be markedly re- 

 tarded in the higher concentrations of D2O. In one series of 

 experiments the oxygen consumption of luminous bacteria de- 

 creased rapidly as the concentration of D2O increased from 0.02 

 to 86 per cent and extrapolation of the curve representing respi- 

 ration as a function of D2O concentration indicated zero 

 respiration in 100 per cent heavy water (12). Other experiments 

 showed that the oxygen consumption of yeast (32) and green 

 algae (6) was reduced about one half in 100 per cent D2O but 

 that dilute D2O had no influence on the respiration of wheat 

 seedlings (7) or of yeast (32). 



Influence of D-^O on muscles and nerves. — There has been 

 much speculation concerning the effect of D2O on nerve and 

 muscle action, although few detailed experiments have been 

 reported. It is clear, however, that some muscles at least are 

 very sensitive to D2O. Carefully controlled experiments (36) 

 showed that the strength and frequency of the beat of isolated 

 frogs' hearts were progressively diminished as the concentration 

 of D2O in Ringer's solution was increased from 0.02 to 99.2 

 per cent. The deleterious effect on hearts and other muscles 

 was evident immediately, but D2O seemed to have no influence 

 on the nerves studied. 



Mechanism of the action of D2O on physiological processes. — 

 The influence of D2O on the rates of several physiological proc- 

 esses is well known, but the cause of the action is not clear. ^ 



Evidence in the literature indicates that, with the possible 

 exception of the membranes of mammalian erythrocytes (3, 21), 

 cell membranes do not differ significantly in permeability to 

 concentrated D2O and to H2O, and that the two liquids are not 

 markedly different in ability to participate in establishing os- 

 motic pressure (8, 17, 22). 



Numerous chemical studies have shown that when sugars, 

 cellulose, and many other organic and inorganic compounds are 

 placed in contact with D2O labile H atoms are quickly replaced 

 by D atoms, and that the activity of the deuterium compound 

 formed usually differs to a greater or lesser extent from that of 

 the original compound (22). Similar exchanges probably occur 



^ It should be observed that the sensitiveness of different species of organ- 

 isms to D2O varies considerably and is not clearly correlated with their phylo- 

 genetic positions. 



