Animals

Twenty-four clean healthy male Sprague-Dawley rats aged 8–10 weeks and weighing 220–250 g were supplied by the Animal Experimental Center of Jilin University of China (animal license No. SCXK(Ji)2003-0002). Rats were housed in individual cages at 21 ± 2°C and 30–35% humidity with a 12-hour light/dark cycle, and allowed free access to food and water. No difference in exposure factors was detected among groups. All surgery was performed under anesthesia, and all efforts were made to minimize the pain and distress of animals. All animal experiments were carried out in accordance with the United States National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publication No. 85-23, revised 1986). The experiment was approved by the Animal Ethics Committee of Jilin University of China.