Relationship between rat heat stress mortality and alterations in reticuloendothelial carbon clearance function

It has recently been reported that, although bacterial endotoxins of intestinal origin are not associated with death after experimental rat heat stress, a state of endotoxin tolerance significantly decreases the heat stress mortality rate. To determine if this phenomenon were associated with the ability of endotoxins to stimulate clearance by the reticuloendothelial system (RES), the relationship between rat heat stress mortality and carbon clearance by the RES was examined. RES carbon clearance was stimulated by prior treatment with endotoxin, zymosan, or sublethal heat stress, as indicated by the significantly reduced (p less than 0.05) blood carbon concentrations 15 min after carbon injections. Prior treatment with injections of gelatin blocked RES carbon clearance. Rats subjected to endotoxin or sublethal heat treatment were significantly (p less than 0.05) resistant to the experimental heat stress, whereas zymosan treatment had no effect. Blockade of the RES with gelatin significantly (p less than 0.05) increased the heat stress mortality rate. These data compare favorably with previously reported studies evaluating RES function and mortality after experimental injury and shock and indicate that the RES may play a fundamental role in the pathogenesis of, and tolerance to, experimental heat stress.