PHYSIOLOGY OF RADIATION INJURY 981 



Plasma histamine rises in the rat and rabbit and this is related in time 

 to the fall in blood pressure (Painter et al., 1947; Weber and Steggerda, 

 1949). The presence of depressor substances receives further support 

 from transfusion experiments (Painter et al., 1947). It is of interest that 

 vagotomy, atropine, or benadryl reduces the blood pressure response in 

 the rabbit by about half (Painter et al., 1947). It might be expected that 

 vagotomy or atropine on the one hand and the antihistamine benadryl, 

 on the other, would be additive in their effect on blood pressure. This is 

 not the case, which may imply that benadryl is acting as a parasympatho- 

 lytic agent or that histamine effects are decreased in the absence of 

 parasympathetic activity. The temporal relation between histamine 

 appearance and hypotension may, of course, be fortuitous, and other 

 factors could be responsible for the lowering of blood pressure (Mont- 

 gomery and Warren, 1951). 



After the initial hypotension, arterial pressure recovers to a level 

 somewhat below normal for a few days and then declines gradually. In 

 the terminal period, blood pressure is frequently below 50 mm Hg 

 (Painter et al., 1947). Relatively few measurements have been made in 

 the dog. Blood pressure is said to be constant in this species until the 

 acute terminal period (several days before death), when it is reduced by 

 about 25 per cent. Similar changes occur in acutely and chronically 

 X-irradiated animals (Prosser, Painter, Lisco, et al., 1947; Prosser and 

 Moore, 1946). 



Heart rate is unchanged in the dog, but it increases in the rabbit during 

 the first few hours (Prosser, Painter, Lisco, et al., 1947; Painter et al., 

 1947). The latter probably represents a compensatory response to the 

 initial hypotension. During the terminal period preceding death, pulse 

 rate is increased in the dog but not always in the rabbit. Myocardial 

 hemorrhages and abnormal electrocardiographic findings, e.g., lowered 

 take-off level and sometimes reversal of the T wave, are described in dogs 

 dying after acute or chronic X irradiation (Prosser, Painter, Lisco, et al., 

 1947; Prosser and Moore, 1946). Terminal myocardial damage has also 

 been seen in a small series of dogs following exposure to cyclotron neu- 

 trons (Ross and Ely, 1947). 



While radiation may exert a direct local action on blood vessels, the 

 myocardium appears to be quite resistant (Leach and Sugiura, 1942) ; 

 changes that are observed after irradiation of the whole body are probably 

 consequences of autonomic and humoral influences initially, and of local 

 extravasation of blood and anoxia terminally (Prosser, Painter, Lisco, 

 et al., 1947). On the whole, the evidence for a generalized circulatory 

 disturbance after median lethal irradiation is not impressive, although 

 this seems to be a factor in relation to massive dosages. After smaller 

 amounts of radiation, early severe hypotension and death are seen in 

 some species, and failure of the circulation may develop during the last 



