Core and organ temperatures in the rat have been found to be subject to regular periodic oscillations (with a frequency of ca 12 cycles per day, i.e. 143 mu Hz) within set limits, by using a chronic thermocouple implant procedure. These cycles consisted of consecutive warming (the temperature increased steadily) and cooling periods (the temperature decreased regularly). There were sustained temperature differences between several tissues and that of the aortic blood (core temperature), higher for brown adipose tissue, kidney and liver, and lower for muscle, white adipose tissue and skin. Organ blood flows and cardiac output were measured by a radioactive microsphere retention method applied to unanaesthetized rats. Blood flow measurements were done on unaware rats (see Methods), during either the warming or cooling phases. During the warming phases, there was a higher blood flow across the brown adipose tissue, kidney and skin, with a cardiac output about twice that found in the cooling periods. The haemodynamic changes observed, as well as the organ temperature differences observed with respect to the blood, suggest that periodic changes in blood flow are an essential part of the operation of the thermogenic system.

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