Gastrointestinal (GI) tract plays the functions of digestion and absorption, which is important to the regulation of energy homeostasis. Gastric emptying is defined as the process of transforming food from the stomach into the duodenum. A fast speed of gastric emptying can result in a short time of fullness and being easy to feel hungry. In this study, rats with diet-induced obesity (DIO) or diet-induced obese resistant (DR) were screened from 100 SD male rats fed with high fat diet (HFD) for two weeks. The rats in the DIO group had a fast gastric emptying when compared to the DR or CON groups. The food-intake and intake energy of rats in the DIO group was much more than those of DR group. Body weight of rats in the DIO group were increased when compared to the DR or CON groups. Previous studies have been shown that the shape, properties, composition, energy density and total energy were correlated with the speed of gastric emptying[35, 36]. It suggests that gastric emptying is one of predominated factors of obese formation in the rats fed high fat diet. In addition, the measurement of intestinal transit and colon’s function was followed by gastric empting. The result of intestinal propulsion showed that the distance of liquid propulsion in the DIO group was smaller than that in the DR group. However, there did not have differences between DIO and DR groups. Fecal excretion can be considered as an index of the function of colonic transiting. Our results showed that the ratio of wet to dry fecal weight did not have differences between DIO and DR group. Thus, gastrointestinal transit and fecal output do not related to the obese rats fed with HFD in this study.
The GI tract, unlike other organs, is able to fulfill the functions of motility even when isolated from the body. It is necessary to measure the motility of the isolated small intestine-duodenum and ileum intestinal section. In this study, the peak values of duodenum were significantly decreased and the peak values of ileum were significantly increased in the DIO group when compared to the CON group. However, opposite results of peak values in the duodenum and ileum were found in the DR group in comparison with the DIO group. It means that the contract and relaxation of rat duodenum in the DIO group were weaker than that in the DR group. In the rat ileum, the stronger contraction and relaxation, the more stability of tension, the same speed of ileum movements and the more stability of speed were found in the DIO group. As known, compared to the ileum, duodenum mixes the gastric juice, pancreatic juice and bile, and plays a particularly prominent role in the function of digestion. The food clumps emptied from the stomach can be digested completely into small molecules which can be easily absorbed in duodenum[37, 38]. A more efficient absorption of nutrients definitely exists in the proximal intestine - duodenum other than ileum. It has been demonstrated that only small amounts of unabsorbed nutrients may transit into the ileum. In this study, the weaker contract and relaxation and the smaller speed of movement existing in the duodenum in the DIO group may result in a long term staying of food in the duodenum in order to digest the food more completely and easier to be absorbed. Thus, weight gain of rats was much more in the DIO group in comparison with the DR group.
In our previous study, improving chewing activity could reduce energy intake and modulate plasma gut hormone concentrations in obese and lean young Chinese men. However, in our study, the levels of gastrointestinal hormones such as CCK, motillin and gastrin in rats were not different between DIO and DR groups. It indicated that regulation of gastrointestinal hormones does not predominate in the rats from the DIO and DR groups fed with HFD. To further explain the mechanism of intestinal motility, interstitial cells of Cajal (ICC) in small intestine was determined. ICC, a kind of special mesenchymal cell in gastrointestinal tract and as the pacemaker cells of gastrointestinal motility and intermediary controller of neural of intestinal muscle, plays a series roles in not only slowing wave activity of gastrointestinal involved in slow wave conduction, but it also regulates neuromuscular signal conduction[33, 40, 41]. In a previous study, almost all ICCs could express the c-kit and almost all ICCs in gastrointestinal tract express ICCs. Thus, c-kit can be a marker to identify ICCs. In this study, the amount of c-kit expression in the duodenum, jejunum and ileum of rats was significantly high in the DIO group when compared to the DR group. It suggests that there have more ICCs in rat gastrointestinal tract from the DIO group to enhance the motility of GI.
In addition, in order to explain the intestinal motility, enteric nervous system (ENS) also was examined in this study. ENS is the specific nervous system which is able to make intestine to fulfill its motility function of GI which is also closely related to ICCs, ENS cells have synapse-like connections with ICC-IM cell bodies and gap-like junctions with smooth muscle cells. These specific structural basis make it possible that ICCs may be dominated by ENS through the way of transmission and regulation of various neurotransmitters and regulate GI smooth muscle contraction through the bridge role of ICCs existing in ENS and smooth muscle. The GI motility patterns were formed by the integration between the ENS and ICCs system (including the pacing ability of ICC-MY and the media role of ICC-IM). In ENS, especially in myenteric nerve plexus, there are stimulating neurons - choline acetyltransferase (ChAT) and substance P (SP) which respectively produce excitatory neurotransmitter – acetylcholine (Ach) and SP to make intestinal muscle contraction, and inhibitory neurons - nitric oxide synthase (NOS) and vasoactive intestinal peptides (VIP) which respectively produced inhibitory neurotransmitter - nitric oxide (NO) and VIP to make intestinal muscle relaxation[44, 45]. In this study, both stimulating and inhibitory neurons - ChAT, SP, NOS and VIP were significantly high expression in the DIO group in comparison with the DR group. These suggest that the contraction and relaxation - intestinal motility regulated by ENS in the rats from the DIO group were stronger than those in the DR group. In a previous study, the nutrients absorbed by GI tract had interacted with neurotransmitter produced by ENS, they enforced the functions of each other. The weaker motility in the duodenum and stability in all intestinal sections provided the process of food-digestion by duodenum and food-absorb by intestinal villus. Thus, neurotransmitters produced by ENS could promote the GI motility and enhance absorption of nutrients in high fat diet in this study.
In summary, rats were screened to diet-induced obesity (DIO) or diet-induced obese resistant rats (DR) from the rats fed with HFD for two weeks. A faster gastric emptying, a weaker contraction of duodenum movement, and a stronger contraction and relaxation of ileum movement were found in the rats from the DIO group when compared to the DR group. These changes of gastrointestinal motility might be regulated by increasing ICC amount, enhancing excitatory neurons and inhibitory neurons of enteric nervous system in the DIO group. However, no differences were found in the levels of serum gastrointestinal hormones such as CCK, motillin and gastrin. It suggested that diet intervention plays an important role in gastrointestinal motility, especially, in formation of obesity.