Mice singly housed without bedding at 5° C clear carbon from the blood more slowly than animals similarly housed at 25° C. Increasing the time of exposure to cold to 2, 18 or 72 hours does not further alter the rate of clearance. Bacteria are also "cleared" uniformly at the two temperatures when a highly virulent strain (SR-11) of Salmonella typhimurium is injected intravenously, but not when one of low virulence (RIA) is used. The RIA strain disappears from blood more slowly in mice at 5° C than in those at 25° C. This was demonstrated both by dilution counts and by labeling the bacteria with p32 and following the decline in radioactivity of blood with time. Livers of mice were sampled at times postinfection for radioactivity and for viable bacterial counts. Housing temperature had no effect on radioactivity changes, but viable counts were higher and decreased more slowly in mice at 5° C than at 25° C. These findings are believed to account, in part, for the greater susceptibility to infection with RIA that was previously seen in mice exposed to cold compared to those at 25° C.
Mice housed at 25° C are protected by cortisone against endotoxin lethality when the hormone is given at the same time as the poison, but not an hour or two later. This is not true of mice housed at 5° C. Activity of liver tryptophan pyrrolase is lowered by endotoxin and elevated by cortisone only in animals at normal temperatures. When endotoxin and hormone are given concurrently, normal enzyme activity is maintained, but activity decreases when the hormone injection is given an hour or more after endotoxin. Actinomycin D, ethionine, 2-thiouracil, and 8-azaguanine (inhibitors of protein synthesis) when given in sublethal amount: potentiate endotoxin, prevent cortisone protection against endotoxin, and block the hormonal induction of tryptophan pyrrolase. Chloramphenicol has none of these effects. Mice infected with Salmonella typhimurium have lower than normal tryptophan pyrrolase activity and a smaller induction of enzyme by cortisone 18 hours postinfection than do normal mice or mice 42 hours postinfection. This occurs only at 25° C.