294 RADIATION BIOLOGY 



clTccl l>ut irradiated paramenia added to the compounds died more rapidly 

 tliaii the controls. Resorcinol and sodium hypo.sulfite protected against 

 the combined action of radiation ;iii(l these compounds. These authors 

 believed that the death was due to the easier penetration of toxic sub- 

 stances broufiiht about by a radiation-induced increase in the permeability 

 of the membrane. lilack (llK^ti) lias studied the effects of a numl)er of 

 salts on the cytolysis of .1 mocha proteus by ultraviolet irradiation. 

 Koehring (1940) has shown that the ameba, Chaos chaos, is more readily 

 killed by a combination of neutral red and the radiation from radon than 

 by either alone. Bohn (1941) found that paramecia in various salt solu- 

 tions, dyes, etc., survived quite normally in the dark but were killed in a 

 few hours on exposure to visible light. It is possible to consider all these 

 effects as due to increased permeability to injurious substances, but it 

 should be emphasized that there is no complete agreement that permea- 

 bility is changed by irradiation (see Table 8-4). 



Levin and Piifault (1934a, b, c) have found that Paramecium aurelia 

 placed in suspensions of lecithin or of cholesterol become resistant to the 

 immediate killing action of X rays. Thus, after exposure for 3 days to a 

 mixture of 1 part of lecithin emulsion to (^00 parts of culture fluid, a dose 

 of X rays three and a half times the normal was needed to kill the animals. 

 It is, of course, of interest that the substances concerned are considered to 

 be important constituents of the cell membrane. However, it must be 

 kept in mind that animals kept in emulsions of this sort for some days ma}^ 

 change their nutritive condition. Giese and Heath (1948) have shown 

 the importance of the nutritive condition for sensitivity to X rays. Like- 

 wise, cholesterol and lecithin may have a protective action of the sort 

 found by Evans et at. (1942) for sea-urchin sperm. 



Halberstaedter and Back (1943) found that pretreatment of Para- 

 mecium caudatum with sublethal concentrations of colchicine for 2 days 

 lowered the resistance to X rays. In controls, the dose required to 

 produce immediate death of 50 per cent of the animals lay between 

 250,000 and 300,000 r, while in the colchicine-treated animals it was 

 between 100,000 and 150,000 r. They found no effect of colchicine on 

 resistance to arsenic or ultraviolet. No explanation is offered for these 

 findings. 



Effect on the Medium. There is considerable evidence that radiation 

 may act indirectly on cells by way of an effect upon the medium surround- 

 ing them. To what extent, then, can these effects on the medium account 

 for the total effect of radiation upon the cell? It seems obvious that this 

 is not a matter of mutually exclusive alternatives. Rather, it is a ques- 

 tion of the relati\'e impoi'taiice, under the conditions employed, of dif- 

 ferent mechanisms by which the radiation effect could be brought about. 

 All discussion of effects on the medium will be inchided in this section 

 even though other than lethal effects are involved. 



