Thermogenic responses to body cooling during fever induced by Staphylococcus aureus cell walls in rabbits

Hypothalamic temperature (Thypo) and metabolic heat production (M) were measured in seven conscious rabbits injected intravenously with either saline or with Staphylococcus aureus, (8.10(7) cell walls.kg-1) while being subjected to a 3-h period of ramp-like total body cooling using a chronically implanted intravascular heat exchanger. In pyrogen-injected animals cooling started (1) at the time of injection or (2) 70 min after injection. In (1) the fall in Thypo induced by heat extraction was similar (1.0 degree C) in afebrile and febrile animals. In (2) there was a transient increase in Thypo of about 0.5 degree C at a time corresponding to the start of fever resulting in a significantly smaller fall in Thypo at the end of the 3-h cooling period (0.5 degree C vs 0.9 degree C, P < 0.05, n = 5). At this time in both (1) and (2) M was lower than theoretically expected from the increase in shivering threshold during fever. However, most of this effect can be explained when available data showing a decrease in thermosensitivity during S. aureus-induced fever are taken into account. After cessation of cooling in both groups of febrile animals Thypo rose to about 1 degree C higher than the precooling level, which is comparable to the fever level in a separate series of experiments with S. aureus injection without cooling (1.2 degrees C).