Heat stress increases the rate of tolerance development to lipopolysaccharide in rats

We have investigated the effect of prior exposure of rats to either a hot environment (40°C, 2.6 kPa water vapor pressure) or a control environment (24°C, 1.9 kPa water vapor pressure) on the development of tolerance to lipopolysaccharide. Sixty minutes of heat exposure significantly raised the abdominal temperature of the heat-stressed rats above that of the control animals for a period of 80 min (P<0.001). However, neither the magnitude nor the time-course of the febrile response of the heat-stressed rats was significantly different from that of the control animals when an initial injection of lipopolysaccharide was given 24 h after temperature exposure (P>0.05). Nevertheless, heat exposure did increase the rate of tolerance development to successive lipopolysaccharide injections, spaced 3 days apart. Heat-stressed animals were tolerant by the second injection of lipopolysaccharide, whilst the control animals were tolerant only by the third injection of lipopolysaccharide. Therefore, heat stress increases the rate of tolerance development to lipopolysaccharide without affecting the response of heat-stressed animals to their first lipopolysaccharide injection.