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Abstract Genetically obese Zucker (Z) rats have been reported to display a body core temperature (T b ) that is consistently below that of their lean littermates. We asked the question whether the lower T b was a result of deficits in thermoregulation or a downward resetting of the set point for T b . For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (T a ) range of 15–35°C, while subjected to a 12:12-h light-dark cycle. T b was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (V˙ o 2 ) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred T a ) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower T b 37.1 ± 0.1°C (SD) vs. 37.3 ± 0.1°C, P < 0.001 and a lower V˙ o 2 (25.3 ± 1.9 ml · kg −1 · h −1 ) than lean controls 33.1 ± 3.7 (A) and 33.9 ± 3.9 (M) ml · kg −1 · h −1 , P < 0.001. Also, the obese Z rats consistently chose to occupy a cooler T a 20.9 ± 0.6°C vs. 22.7 ± 0.6°C (A) and 22.5 ± 0.7°C (M), P < 0.001 in the thermal gradient. This suggests a lower set point for T b in the obese Z rat, as they refused the option to select a warmer T a that might allow them to counteract any thermoregulatory deficiency that could lead to a low T b . Although all rats followed a definite circadian rhythm for both T b and V˙ o 2 , there was no discernible circadian pattern for preferred T a in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats. body temperature preferred ambient temperature metabolic rate Footnotes The work was supported in part by a grant to P. B. Frappell. Address for reprint requests and other correspondence: P. B. Frappell, Dept. of Zoology, La Trobe Univ., Melbourne, Victoria 3086, Australia (E-mail: P.Frappell@zoo.latrobe.edu.au ). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “ advertisement ” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Copyright © 2001 the American Physiological Society
AJP - Regulatory, Integrative and Comparative Physiology – The American Physiological Society
Published: Nov 1, 2001
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