Evidence for a contribution by brown adipose tissue to the development of fever in the young rabbit

This study was undertaken to determine if brown adipose tissue was involved in heat production during fever produced by S. abortus equi (1 micrograms) in unanesthetized rabbits aged 19-26 days. The fever (0.9-1.6 degrees C) occurred after a delay of 20-30 min and was frequently biphasic. Radiolabelled microspheres for measuring tissue blood flow were injected intraventricularly into three groups of animals: rabbits not given pyrogen, rabbits in which the febrile response to pyrogen was developing, and rabbits in which the febrile response had peaked. Blood flow to brown fat deposits and other organs was calculated from the fractional distribution of the microspheres and the recovery of microspheres in a reference arterial blood sample. At the fever peak, blood flow to brown fat was not significantly different (p greater than 0.05) from the control value (0.9 +/- 0.2), but during the rising phase of the fever the flow increased significantly (p less than 0.01) to 2.6 +/- 0.4 mL min-1 g-1. The blood flow to muscles of the forelimbs and hind limbs was also increased significantly (p less than 0.05) during the rising phase of the fever. No significant change in blood flow to other organs or tissues was found during the rising phase of the fever. These results indicate that both nonshivering as well as shivering thermogenesis contribute to heat production during development of fever in the young rabbit. However, nonshivering thermogenesis was not involved in the maintenance of the elevated body temperature after the fever had peaked.