Thermoregulation in complex situations: combined heat exposure, infectious fever and water deprivation.

Abstract

Heat exposure, infectious fever and water deprivation are stressors that, individually, produce disturbances in more than one regulated system, calling for diverse compensatory responses. A potential conflict is created when these stimuli are combined and impose concurrent stressful loads on the body because the homeostatic defenses mobilized against one are also partly needed against the other stressors. To learn how the competing demands of combined stressors for shared regulatory systems are met, rabbits were exposed to 32 degrees C and 37 degrees C (heat), administered lipopolysaccharide (Salmonella enteritidis LPS, 2 lg/kg, i.v.) in temperatures of 22 degrees C or 27 degrees C, or water-deprived for 1 or 2 days in 22 degrees C or 27 degrees C, in separate experiments. The corresponding controls were exposed to 22 degrees C or 27 degrees C, administered pyrogen-free saline i.v. in 22 degrees C or 27 degrees C, or normally hydrated in 22 degrees C or 27 degrees C. In subsequent experiments, two or all three of these treatments were applied concurrently. Core and ear skin temperatures and respiratory rates were monitored continuously. The results indicated that the concomitant needs of moderate heat exposure, fever and 1 day of water deprivation were generally met by the regulatory systems involved, but different patterns of thermoeffector activities were evoked and the eventual body temperature changes produced were different under each condition. However, when the test conditions were severe, their combined needs were not met adequately and the eventual compensatory response depended not only on the particular stimulus intensity, but also on the immediate importance for survival of the functions being defended. Thus, dehydration was the most dangerous factor to the physiological integrity of the animals. In sum, conflicting physiological stimuli appear to result in responses that are different from the responses to a single perturbation, the eventual output representing the resultant of the inputs rather than a singular output dictated by one dominant drive to the exclusion of the others.