STC is the most common type of chronic functional constipation, which has a high incidence in the population and is difficult to treat. STC has become one of the important factors affecting people's quality of life worldwide. Data show that the prevalence of constipation in Chinese adult population is 3% ~ 17%, which is higher in females, 4 times as high as that in males [18, 19]. AABW, as a component from Anemarrhena asphodeloides Bge., has been reported to play multiple effects, including relieving constipation. However, the detailed mechanisms through which AABW exerted laxative effects remain to be elucidated. The results of this study showed that AABW could promote defecation and increase fecal water content in constipated rats, and reduce fecal passage time in the intestine. These results suggested that AABW has the effect of relieving constipation (Fig. 2–3).
Loperamide directly stimulates enteric-wall µ-receptors, inhibits the production of Ache and PEG2, and reduces intestinal peristalsis and secretion of gastrointestinal hormones, thus playing an anti-diarrhea role [20]. The rat constipation model induced by loperamide has a short establishment period and a long duration of symptoms, and can well simulate the pathophysiological characteristics of constipation. It should be pointed out that gender had a great influence on the establishment of the constipation model, and female rats had a more obvious difference in modeling effect. In order to avoid the interference of unknown factors, male rats were finally selected [21, 22]. In this study, it was found that rats were injected with loperamide, combined with symptoms, colonic fecal volume and fecal water content, to preliminarily determine the establishment of loperamide induced constipation. Above all, it could be concluded that the rat model of constipation induced by lomeramide was successfully replicated (Fig. 2).
Constipation is associated with an enteric nervous System (ENS) disorder. In ENS, 5-hydroxytryptamine (5-HT), vasoactive intestinal peptide (VIP) and NO are all neurotransmitters related to peristaltic activity of gastrointestinal tract [23–25]. 5-HT is an excitatory neurotransmitter, which can directly act on the 5-HT4 receptor in the intestinal pheochromocytoma cells in the superior colonic mucosa to cause intestinal smooth muscle contraction and promote intestinal peristalsis to cause defecation [23]. VIP could promote gastric empties, promote digestive secretion, inhibit gastrointestinal smooth muscle relaxation, and play a promoting role in gastrointestinal movement [24, 29, 30]. MTL and Gas could promote the secretion of pepsin, contract gastrointestinal smooth muscle, promote gastric peristalsis, and play an exciting role in gastrointestinal movement [25]. NO is a non-specific inhibitory neurotransmitter in ENS. Excessive NO could be diffused into intestinal smooth muscle cells to reduce intracellular Ca2+ concentration, thus causing excessive relaxation of intestinal stage smooth muscle and excessive intestinal stage peristalsis (intestinal spasm) [26, 27]. AABW could improve the symptoms of constipation by relieving the above gastrointestinal hormone and neurotransmitter abnormalities (Fig. 5)
At present, the etiology of STC is not clear, but slow intestinal peristalsis and excessive absorption of intestinal water are two important factors for the occurrence of constipation. Recent studies have shown that ICC and AQPs are closely related to intestinal dynamics and intestinal fluid metabolism, respectively. ICC is closely related to intestinal peristalsis and is considered as a pacemaker cell of intestinal peristalsis, which can initiate rhythmic intestinal myoelectric activity and simultaneously produce contraction rhythm of gastrointestinal smooth muscle [30]. A study showed that the ICC density of constipation patients was lower than that of normal people, and the decrease of ICC density resulted in the loss of ICC spontaneous rhythmic slow wave effect and colonic motor disorder. C-Kit and its ligand SCF are the main factors for ICC growth, development and maintenance [31–34]. AABW significantly upregulated the levels of both c-Kit and SCF in mice with Lop-induced constipation (Fig. 6). These results suggested that AABW increased the numbers of ICCs in mice with Lop-induced constipation.
Aquaporin (AQPs) is closely related to intestinal water absorption. Abnormal expression of AQPs in the intestinal tract will have an impact on intestinal hydrologic metabolism, and is closely related to the occurrence and development of constipation [35]. At present, 13 subtypes of AQP family have been found (AQP0-AQP12), among which AQP3 plays an important role in intestinal hydrohydration metabolism. VIP is a neuropeptide regulating intestinal smooth muscle contraction and intestinal fluid metabolism, while AQP3 is mainly distributed in human intestinal and colon epithelial cells, and plays an important role in intestinal epithelial cell fluid reabsorption. VIP might bind to the receptor of intestinal epithelial cells and regulate the expression of AQP3 in the intestine of rats through the cAMP-PKA signaling pathway [36]. Recent studies have found that a variety of inflammatory factors have regulation on AQP3, such as IL-6, PEG2. PEG2 is associated with increased AQP endocytosis and degradation [37]. Stimulating colon to release PEG2 and VIP and down-regulating AQP3 expression in epithelial cytoplasm may be the key link of AABW in regulating the water metabolism of hemp canal. By regulating the content of functional AQP3 in the plasma membrane of colonic epithelial cells, the efficiency of water transport can be regulated, which further influences the hydrologic metabolism of colon. This might be the biological basis for the role of AABW (Figs. 1D, 6 and 7).