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Mature male Sprague-Dawley (SD) and Long-Evans (LE) rats were instrumented with telemetry transmitters for measurement of heart rate (HR) and housed in room calorimeters for assessment of food intake and oxygen consumption (V O2) at standard laboratory temperatures (23°C) to examine physiological responses to caloric restriction (CR; 60% of baseline ad libitum calories for 2 wk) and refeeding. Ad libitum controls had stable food intake (84 -88 kcal/day) and gained weight at rates of 3-4 g/day. Groups from both strains assigned to CR exhibited similar patterns of weight loss and reductions in V O2 and HR. Upon refeeding, SD rats exhibited a mild, transient hyperphagic response (1 day) accompanied by sustained suppression of V O2 and HR that remained evident 8 days after refeeding. In contrast, LE rats exhibited sustained daily hyperphagia that persisted 8 days after refeeding and was accompanied by a complete restoration of HR and V O2. The lower HR and V O2 observed during refeeding in SD rats were not due to reduced locomotor activity. The results reveal a strain-dependent divergent response to recovery from CR. We conclude that during recovery from CR, homeostatic stimulation of appetite or suppression of energy expenditure may occur selectively to restore body weight. hyperphagia; thermogenesis; indirect calorimetry; energy homeostasis CALORIC RESTRICTION (CR) produces a multifaceted set of adaptations that reduce energy expenditure and increase appetite in an attempt to defend body fat and body weight (29,41,44). Frequently, sexually mature but rapidly growing male rats are used to examine mechanisms of energy homeostasis. These animals are in positive energy balance, consuming excess calories that are used to increase both lean and fat mass at rates of weight gain often exceeding 25 g/wk. Thus growing Sprague-Dawley (SD) rats treated with CR (rats fed 60% of control caloric intake) lose very little weight and reach a fairly stable weight within a week, whereas ad libitum control rats continue to rapidly grow. As a result, the weight differences are due more to growth of controls than weight loss in treated rats. When SD rats are refed, this strain of rats exhibits a sustained reduction in energy expenditure that appears to contribute to restoration of fat mass (3, 7).The primary purpose of this study was to examine physiological responses to CR in another commonly used rat strain.As the results indicate, we observed that outbred Long-Evans (LE) rats and SD rats exhibit similar physiological responses during CR but exhibit unexpectedly divergent strategies for normalization of weight during refeeding. The findings indicate that suppressed thermogenesis is not always observed after CR. METHODSAll experimental procedures and protocols were approved by the Institutional Animal Care and Use Committee at Florida State University. Male LE [(Crl:LE)BR; Charles River, Raleigh, NC] and SD rats [Crl:CD(SD)IGSBR; Charles River Laboratories] were purchased at 5-6 wk of age and individually housed in polycarbonate cages ...

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