The Relationship Between Ghrelin And High-fat Diet-induced Obesity In Rats

Objective: We investigated the possible involvement of ghrelin and GOAT in susceptibility to obesity in Sprague-Dawley rats and examined if there is metabolic change of ghrelin occurred in response to low fat diet intervention so as to reveal the effects of dietary intervention on regulation of appetite and explore the relationship between ghrelin and obese-prone and obese-resistant rats.Methods: Fifty-six male SD rats weighing 120 to 140g were acclimatized to specific-pathogen free animal house for ten days with free access to a low-fat diet and water. And then they were allocated to two groups based on different diet composition, low-fat diet and high-fat diet. Forty-six of the rats were fed with high-fat diet, and ten rats with low fat diet as control group. After 10 weeks feeding, the rats fed on high-fat diet whose body weights surpass average weight adding 1.96 times standard variance of control rats were defined as obesity-prone (OP) rats; while those less than average weight adding one time standard variance were considered as obesity-resistant (OR) rats.As for obesity-prone rats, six of rats were maintained on high-fat diet, and another six rats switched to low fat diet for an additional 8 weeks, and so were obese-resistant rats. At the end of the experiment, the rats killed by cervical dislocation after fasting 12 hours. Blood samples were collected, and serum was separated into three aliquots stored at -80°C until subsequent determination of serum glucose, triglyceride and insulin levels. Liver about 100mg was dissected fast and homogenized for determination of triglyceride concentration. Stomach was quickly removed, one part of which was homogenized for detection of stomach ghrelin peptide levels using EIA kit, another for extraction of stomach RNA in order to determine ghrelin and GOAT gene expression applying Real-time PCR method. Hypothalamus was dissected for analysis of GOAT mRNA expression. Perirenal- and epididymal- adipose tissues were also dissected and weighed. Food intake was determined daily from the amount of food preweighed and remaining in the food bowl. The weight of every rat was record once a week.Results: The OP rats gained more weights than OR and the weights of OR rats were comparable to those of control rats after 10 week high-fat diet. OP rats consumed more calories on the high-fat diet but OR rats did not ingest more calories than control rats. After another 8 weeks'dietary intervention, the average weight and percentage of body fat mass of OP-HF rats were significantly higher than those of OP-LF rats whose body fat mass have no difference in comparison to control rats. There were no differences in body weights between OR-LF and OR-HF rats. Food intakes in OP-LF and OR-LF rats were higher than OP-HF and OR-HF rats, while energy intakes in OP-LF and OR-LF rats were lower than OP-HF and OR-HF rats.After 8 weeks of low fat dietary intervention, serum glucose levels of OP-LF rats decreased compared with OP-HF, and those of OR-LF rats also decreased in comparison to OR-HF rats. There were no differences in serum triglycerides among different groups. The liver triglycerides of OP-HF and OR-HF rats were higher than those of control rats. OP-HF rats had significantly higher liver triglycerides than OR-HF. The liver triglycerides were markedly decreased in OP-LF and OR-LF rats as compared with OP-HF and OR-HF rats. OP-HF had significant higher insulin levels and lower insulin sensitivity than control rats. Shifting to low fat diet reduced insulin levels in OP-LF rats.There were no differences in stomach ghrelin mRNA expression among OP-HF, OR-HF, OP-LF, OR-LF and control rats. However, stomach ghrelin peptide levels were decreased in OP-HF rats compared to control rats, whereas ghrelin peptide levels of OR-HF rats were close to those of control rats and presented normal levels. The ghrelin peptide levels in OP-LF rats have a tendency to increase but not a significant difference observed compared with OP-HF. There was no difference in ghrelin peptide levels between OR-LF and OR-HF rats.OP rats had increased GOAT mRNA in hypothalamus in comparison to OR rats whether feeding a high-fat diet or a low fat diet.Conclusions: 1) The current data showed the differences in energy intake may account for the differential body weight of OP and OR rats fed on high fat diet. Low fat diet induced less body weight in OP rats.2) The results demonstrate the susceptibility to obesity may be in part due to the differences in stomach ghrelin peptide. Low fat diet may reverse reduced ghrelin peptide to normalize by improving insulin sensitivity in obese rats.3) Low Hypothalamic GOAT expression may account for lower energy intake and lower adiposity in OR rats in comparison to OP rats.