H. A. S. VAN DEN BRENK AND RUTH MOORE 



Effect of Respired pOo and Methylene Blue on Cyanide Protection in Mice 

 It is known that methylene blue in vivo protects against cyanide poisoning of 

 animals. Methylene blue injected into the intact animal, acts as a hydrogen 

 acceptor and is rapidly reduced to a colourless form. It also combines with 

 haemoglobin to form methaemoglobin. The latter substance complexes 

 cyanide to foi'm a biologically inert compound. As the result of its oxidizing 

 properties, one might expect methylene blue to sensitize animals to irradiation 

 and that such sensitization may be increased further by increasing the tissue 

 oxygen tensions. Our preliminary results for mice support this conclusion 

 and are shown in Figure 5. On the other hand poisoning of oxidative meta- 

 bolism by cyanide, with a rise in tissue oxygen concentrations should give 

 similar results, but for mice a protective action results, which is annulled by 

 increasing the partial pressure of respired oxygen to 3 atmospheres absolute 

 in mice. This latter finding suggests that for mice, cyanide may be exerting 

 its protective effects largely due to respiratory inhibition, and anoxic 

 anoxaemia. By combining methylene blue pretreatment with cyanide, 

 raised respired pOa more effectively diminishes the protective action of 

 cyanide in mice. Again, further experiments along these lines are being 

 conducted, incorporating competitive pharmacological actions, as with 

 metabolite-antimetabolite inhibition, presented in the previous paper. 



CONCLUSIONS 



The analogy obtained for the respective effects of cyanide and artificially 

 raised tissue oxygen tensions, on radio-chemical protection in vivo, is striking. 

 The results point to the importance of an 'oxygen effect' in protective 

 mechanisms. 



If a substantial fraction of radiation damage is due to the production of 

 oxidizing radicals in irradiated tissue fluids, the levels of oxygen concen- 

 tration in cells is of paramount importance. A similar condition holds for 

 the interpretation of the 'oxygen effect' as due to sensitization (by oxygen) 

 of essential macromolecules to X-rays. 



From the data provided by Lefort^^ extrapolation of the curves for HgOg 

 production in water at pH 6 to 8 by X-rays, for different concentrations of 

 dissolved oxygen suggest, that for the low doses of X-rays used in mammalian 

 biological experiments, the increase in yield of oxidizing radicals for oxygen 

 concentrations beyond 358 [xmoles oxygen per litre of water {^i.e. water satur- 

 ated with air at atmospheric pressure) is small. The slope of the curve is 

 greatest over initial hypoxic levels. The respiration of oxygen at 3 to 4 

 atmospheres pOa, would not be expected to give a quantitative increase 

 in yield of such oxidizing radicals, accepting adequate oxygenation in 

 the lungs and blood flow sufficient to inactivate available protective free 

 radicals. On the other hand the oxygen effect in untreated animals is very 

 steep over levels of 5 to 7 per cent respired oxygen^''' (also see Figures 3 and 4), 

 and most rats exposed to such levels of oxygen tension suffer extreme collapse 

 and require resuscitation after irradiation exposures lasting 3 to 4 minutes. 

 ' Chemically protected ' animals respiring pure oxygen at 1 atmosphere and 

 even air, do not exhibit these extreme symptoms of anoxia. 



A more satisfactory explanation for the results obtained for pressurized, 

 protected animals, and supported by the results obtained ^vith cyanide 



185 



