Rat endogenous pyrogen and fever.

Rat endogenous pyrogen (EP), prepared from the white blood cells of lipopolysaccharide-pretreated rats, produced a fever in both rabbits and rats. The effects of human, rabbit, and rat EP on the body temperatures of rabbits and rats were investigated to clarify differences in the febrile response to each kind of EP. The latency to fever onset and the time to peak fever induced by rat EP in rabbits and rats were significantly greater than those induced by human and rabbit EP. The maximum elevation of the body temperature induced by rat EP in both animals was almost identical to that induced by human and rabbit EP. In a comparison of the febrile responses to various EPs between rabbits and rats, the latency to fever onset, the time to peak, and the maximum elevation of fever in rats were approximately half those observed for rabbits. Furthermore, the threshold dosage to induce a fever response in rats was greater than that necessary for rabbits. It is concluded that rat EP could be obtained by in vitro techniques and that rats have the physiological mechanisms to develop a fever through production of EP.