SUMMARYCore 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 ,tHz) 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, kidneys 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.

The rhythms of aortic core temperature and overall heat production in Wistar rats was analyzed by using long series of recordings of temperature obtained from implanted thermocouple probes and heat release values from a chamber calorimeter. There was a very high degree of repetitiveness in the presentation of actual heat rhythms, with high cross-correlation values ascertained wit paired periodograms. No differences were observed between heat production between male and female adult rats. The cross-correlation for temperature gave similar figures. The cross-correlation study between heat production and aortic core temperature in the same animals was significant and showed a displacement of about 30 minutes between heat release and aortic core temperature. The analysis of heat production showed a strong predominance of rhythms with periods of 24 hours (frequencies < 11.6 microHz) or more; other rhythms detected (of roughly the same relative importance) had periods of 8 or 2.2 hours (35 or 126 microHz, respectively). The analysis of aortic core temperature showed a smaller quantitative contribution of the 8 or 2.2 hours (35 or 126 microHz) rhythms, with other harmonic rhythms interspersed (5.1 and 4.0 hours, i.e. 54 and 69 microHz). The proportion of 'noise' or cycles lower than 30 minutes (< 550 microHz) was higher in internal temperature than in the actual release of heat. The results are in agreement with the existence of a basic period of about 130 minutes (126 microHz) of warming/cooling of the blood, with a number of other harmonic rhythms superimposed upon the basic circadian rhythm.

A modification of the usual microsphere injection method is presented which is appropriate for rat lung blood flow measurement. The injection of microspheres into the abdominal cava vein, together with the sampling of a reference flow from the right ventricle, allowed to calculate pulmonary blood flows in the same range than the cardiac output. The differences between the two figures (about 30%) are attributed to arterio-venous shunts. The tissue distribution of the microspheres bypassing the lung capillary beds agree with this interpretation.