Cardiovascular and thermal responses to intravenous endotoxin in guinea pigs

Changes in blood pressure, heart rate, and heat balance during febrile response to E. coli endotoxin were studied in unanesthetized guinea pigs at an ambient temperature of 24 degrees C. The heat balance was measured with direct and indirect calorimetry. An intravenous injection of endotoxin (5 micrograms X kg-1) produced characteristic biphasic changes in the intra-aortic temperature (Tar), heat production (M), and heat loss (H). The response of arterial blood pressure (BP) to the endotoxin was monophasic and that of heart rate appeared triphasic. Phenylephrine (PHE: 20 micrograms X kg-1 X min-1) infused intravenously in the two rising phases of fever increased BP to the same extent, but the amount of the reflexive bradycardia seemed to be less during the first rise of Tar. The increased metabolism (shivering and nonshivering thermogenesis) during fever development was suppressed by PHE infusion at the two rising phases. The results conclude that, in guinea pigs, the baroreflexive suppression of metabolism is persistent during fever development, but the reflex sensitivity may be different at different phases of fever.