Distribution and clearance of circulating endotoxin.

Bacterial endotoxins injected intravenously induce characteristic changes in body temperature and numbers of circulating leukocytes. A decrease in the duration and intensity of these effects occurs after repeated injection of similar doses (1, 2). This state of relative refractoriness to endo-toxin, called "tolerance," is nonspecific for a single endotoxin and is unrelated to the presence of anti-bodies (3-5). Tolerant animals given thorotrast or other colloids exhibit a nontolerant febrile response to endotoxin. This has led to the hypothesis that tolerance results from more rapid clearance of endotoxin by the reticuloendothelial system (4). Understanding the mechanisms of endotoxin action requires knowledge of the distribution and fate of injected endotoxin. Chromium51-labeled endotoxin was shown to localize rapidly in liver and the buffy coat of blood; smaller amounts were found in the lungs, spleen, and other organs (6). Blood clearance of large amounts of Cr51 endo-toxin was shown to be more rapid in tolerant than in nontolerant rabbits. Although clearance of small doses was slightly more rapid in tolerant animals, this was not considered significant (7). Endotoxin was given, however, in doses 10 to 600 times greater than those commonly used to study fever and leukocyte changes. The availability of chromium51 of high specific activity has made feasible the study of clearance of small doses of Cr5' endotoxin and its distribution among blood elements. This paper reports the results of such studies in nontolerant and tolerant animals. New Zealand white rabbits weighing 1.9 to 3.5 kg were studied. To produce febrile tolerance, unlabeled endo-toxin, 3 to 4 ttg per kg, was given intravenously to one group of animals for five consecutive days before administration of labeled endotoxin (2). Temperature measurements on four rabbits demonstrated definite febrile tolerance after the third injection; therefore it was considered unnecessary to test each animal for tolerance. and Zalesky (8). Endotoxin, 69 to 207 IAg per ml of sterile phosphate buffer (pH 7.0), was incubated with Cr5"Cl, (Chromitope, Squibb) at 370 C for 48 hours. The concentration of chromium ranged from 0.24 to 1.25 Ag per ml. After incubation the mixture was dialyzed at room temperature against phosphate buffer (pH 7.0) for 96.to 112 hours. After dialysis the labeled endotoxin was diluted with phosphate buffer to a final concentration of 10 Ag per ml (Table I). Labeled endotoxin, 3 to 4 ag per kg of body weight, was injected into a marginal ear vein. Each animal was lightly …