The time-dependent laxative effect of sennoside A, the main functional component of rhubarb, has been attributed to gut microbiota and aquaporins (2023)

Rhubarb is one of the most effective laxatives and is used worldwide as it is listed not only in the Chinese Pharmacopoeia and Russian Pharmacopoeia (Shikov et al., 2021) but also in the British Pharmacopoeia and European Pharmacopoeia (Sun and Yeh , 2005). . Sennoside A, the main component of anthraquinone in rhubarb, is said to have a strong laxative effect, which manifests itself in increased mucus and watery stools (Yokoyama et al., 2017). However, long-term use of sennoside A can lead to drug resistance, so higher doses of sennoside A must be given to maintain normal bowel movements. Secondary constipation and weight loss after anthraquinone discontinuation can also lead to intestinal dysmotility (Willems et al., 2003). Furthermore, prolonged use of sennoside A in high doses may even produce some side effects, including the occurrence of melanosis coli and colon cancer carcinogenesis. To avoid side effects of anthraquinone-based ingredients, final evaluation reportRheum palmatumTerraRheum officinaleThe Baillon Review issued by the European Medicines Agency (EMA) in 2020 suggested that herbal preparations in doses of 10 to 30 mg/day, converted from hydroxyanthraquinone derivatives, should be taken for less than 7 consecutive days. In 1996, the Federal Institute for Drugs and Medical Devices (BfArM) stated that the maximum daily dose of anthraquinone-containing laxatives should not exceed 30 mg and should not be taken continuously for more than 1-2 weeks (Xiao, 1998). The above suggests that dosage and timing must be considered to achieve the effects of anthraquinones. However, there is still uncertainty about the dose and timing at which sennoside A exerts a good laxative effect, and the underlying mechanism remains unclear.

Previous studies have reported some mechanisms by which sennoside A exerts its laxative effects. First, senoside A can shorten the transit time of luminal contents and reduce water absorption in the colon, which are related to luminal prostanoid levels and only partially related to cholinergic nerve mediation (Kobayashi et al., 2007). Second, sennoside A can inhibit the absorption of water and electrolytes in the colonic epithelium and stimulate the secretion of water and electrolytes into the colonic lumen, resulting in increased concentration of fluids and electrolytes in the colonic lumen (Yagi et al., 1990) . Furthermore, sennoside A can regulate intraluminal water volume through the regulation of aquaporins (AQPs), thus causing a laxative effect (Kon et al., 2014; Na et al., 2022).

The vast majority of gut microbes are located in the large intestine (Sender et al., 2016) and have important functions in host metabolic, physiological, and health homeostasis (Shreiner et al., 2015; Sommer and Backhed, 2013), including functional constipation and gastrointestinal inflammatory reactions (Chang and Lin, 2016; Keller and Andresen, 2015). Previous studies have shown that the intestinal microbiota is involved in the laxative effect of anthraquinones, sennoside A can be metabolized by the intestinal microflora into reinanthrone, thus exerting a laxative effect. And the laxative effect of sennoside A is susceptible to dietary changes in the gut microbiota (Kon et al., 2018; Takayama et al., 2019). However, whether long-term administration of sennoside A has an effect on the gut microbiota, and whether changes in the gut microbiota are further implicated in the laxative effects and side effects of sennoside A, remains worthy of investigation.

Furthermore, abnormal expression of AQPs in the colon leads to disruption of water uptake and secretion (Cao et al., 2018). There are currently 13 human AQPs, of which aquaporin 1 (AQP1), aquaporin 3 (AQP3), aquaporin 7 (AQP7) and aquaporin 8 (AQP8) are highly expressed in the colon. AQP3 has been shown to be involved in the laxative effect induced by sennoside A, a single dose of sennoside A (50 mg/kg) caused an immediate down-regulation of AQP3 in the colon of rats (Kon et al., 2014). In another model of sennoside A-induced diarrhea, most AQPs were down-regulated, while AQP3 was up-regulated in rat colon following 6-day administration of 50 mg/kg sennoside A (Cao et al., 2018). Although the results are conflicting, they all point to a role for AQPs in the effect induced by sennoside A. Therefore, it is important to investigate how highly expressed AQPs in the colon change during long-term administration of sennoside A and whether they are related to their effects collateral.

The above suggests the importance of gut microbiota and AQPs for the laxative effects of sennoside A. In this study, we chose the median dose of sennoside A recommended by the EMA and administered it for 21 consecutive days to monitor the laxative effect during this period (Fig. . 1). In addition, we investigated whether prolonged use of sennoside A caused functional and pathological changes in intestinal structure, as well as effects on intestinal microbiota and colonic AQPs. In this way, the safe duration of administration and the mechanism of the laxative effect of sennoside A in the treatment of constipation can be determined, which should guide the clinical use of sennoside A.