The Effect Of Warm Exposure On Metabolic Thermogenesis And Body Fat Contene In Striped Hamsters

Proper adjustments of both physiology and behavior are required for small mammals living in a temperate zone to cope with considerable changes in seasonal environment. The variations in body mass and/or fat concent are important adaptive strategies for small mammals to cope with the changes of environment temperature. It has been observed that many small mammals show notable increases in energy intake, basal metabolic rate and non-shivering thermogenesis, but significant reductions in body weight and body fat content in winter. In contrary, energy intake and metabolic thermogenesis decrease significantly, but body weight and /or body fat increase significantly in summer. These studies show that metabolic thermogenesis may play an important role in the adaptive regulation of fat content, but the mechanism is not clear. In order to further explore the relationship between metabolic heat production and body fat in small mammals and its regulation mechanism, 1) the striped hamsters were exposed to different temperature conditions, during which several physiological,hormonal, and biochemical measures indicative of metabolic thermogenesis were measured. The data showed that the warm-acclimated (30?) hamsters had significantly lower energy intake,BMR and NST, while they were fatter than their counterpart acclimated to the cold (5?). After being transferred from cold to warm temperature,hamsters decreased food intake, but significantly increased body fat content. BAT COX activity and UCP1 mRNA expression were significantly down-regulated in the hamsters acclimated to the warm and those transferred from cold to warm. Furthermore, body fat content was significantly negatively correlated with BAT COX activity and UCP1 expression. 2) The striped hamster were acclimated to a warm condition(30?) for 1, 3 and 4 months. The results sowed that the warm-acclimated groups significantly decreased energy intake, and simultaneously decreased nonshivering thermogenesis compared to those housed at 21?. But body fat content increased by 29.9%, 22.1% and 19.6% in the hamsters acclimated to 1, 3 or 4 months, respectively relative to their counterparts maintain at 21? (P<0.01). The cytochrome c oxydase (COX) activity of brain, liver, heart and skeletal muscle, and the ratio of serum tri-iodothyronine to thyroxine significantly decreased in warm-acclimated groups compared with 21? group. COX activity and uncoupling protein 1 (UCP1)mRNA expression of brown adipose tissue(BAT) were significantly down-regulated under he warm conditions.COX activity of BAT, liver, heart and muscle were significantly negatively correlated with body fat content, and the correlation between UCPi expression and body fat content tended to be negative. These findings suggest that the decrease in the energy expended on metabolic thermogenesis contributes to the increased body fat content in animals at warm temperature. The attenuation of COX and UCP1-based BAT activity may be involved in body fat accumulation in animals under warm conditions.