2.1. Animals and treatment

Male Sprague-Dawley rats aged from 6 to 7 weeks (180–220 g) were purchased from Experimental Animal Center of Zhejiang province (Hangzhou, China) (Production license: SCXK 2014-0001). They were housed in Laboratory Animal Care of Life Science College under controlled temperature (22 ± 2 °C) and relative humidity (55 ± 5%) with a normal 12-h light/dark cycle. Animal welfare and experimental procedures were carried out in accordance with the criteria outlined in the ‘Guide for the Care and Use of Laboratory Animals’ enacted by National Academy of Sciences and published by the National Institutes of Health (NIH publication 86-23 revised 1985) and the related ethnical regulations of Nanjing University [SYXK (SU) 2009-0017]. All animal experimental protocols were approved by the Institutional Animal Care and Use Committee of Nanjing University. All efforts were made to minimize animal suffering and to reduce the number of animals used.

Rats ad libitum accessed a standard chow and water for one week acclimatization before the experiment. To evaluate the protection of polydatin and pioglitazone in fructose-induced liver oxidative stress, inflammation and lipid deposition, rats were randomized into the following six groups (n = 8): control vehicle, fructose vehicle, fructose with polydatin (7.5, 15, 30 mg/kg) as well as fructose with pioglitazone (positive drug, 4 mg/kg). Each rat was given drinking water or 100 mL drinking water containing 10% fructose (wt/vol) for 6 weeks, and followed by the treatment of saline injection, polydatin, or pioglitazone table by intragastric administration for next 7 weeks. All drugs were administered once daily between 2:30 p.m. and 3:30 p.m.. Animal body weight was detected weekly. Doses of polydatin and pioglitazone were selected based on our preliminary studies and other reports [22], [23], [24], [25], [26], [27]. Polydatin significantly decreases liver TG, TC and TNF-α levels, as well as down-regulates SREBP-1c and SCD1 protein levels in SD rats with NAFLD at 30 mg/kg [22], decreases kidney IL-1β and TNF-α levels in fructose-fed mice at 12.5, 25 and 50 mg/kg (equivalent 8.75, 17.5, 35 mg/kg to rat) [23]. Pioglitazone is clinically used to improve liver steatosis and inflammation in patients with NAFLD [24], [25], [26]. Our previous study showed that pioglitazone at 4 mg/kg significantly alleviated hepatic inflammation and lipid deposition in fructose-fed rats [27]. Therefore, doses of 7.5, 15, and 30 mg/kg polydatin as well as 4 mg/kg pioglitazone were used in these animal experiments.