Thermoregulatory set point decreases after hemorrhage in rats.

Hemorrhage in rats causes a drop in body core temperature that is proportional to the hemorrhage volume. We tested the hypothesis that the hemorrhagic hypothermia is due to a downward shift in the thermoregulatory set point. If so, rats subjected to hemorrhage would prefer a cooler ambient temperature to enhance heat loss during the posthemorrhage period. Male Sprague-Dawley rats were fitted with carotid arterial catheters and biotelemetry temperature probes. Two days later, rats were placed in a temperature gradient chamber that allowed the rat to move between ambient temperatures of 15 degrees C to 40 degrees C. Rat location within the gradient was recorded as the selected ambient temperature. After 48 h, a 24 mL/kg hemorrhage was induced via the carotid cannula followed by a 24-h recovery period in the gradient. Body core and selected ambient temperatures significantly decreased after hemorrhage. Within 50 min, selected ambient temperature decreased by 11 degrees C, and returned to normal 100 min after hemorrhage. Within 80 min after hemorrhage, core temperature decreased by 2.3 degrees C, and returned to normal by 8 h after hemorrhage. Expanded analysis of the first hour after hemorrhage showed that reduction in selected ambient temperature preceded the drop in body core temperature. Importantly, the decrease in selected ambient temperature persisted even during the peak decrease in body core temperature. These results indicate that a decrease in thermoregulatory set point contributes to the drop in body core temperature after hemorrhage.