Xenin induces anion secretion via neural pathway in rat duodenum
Authors: Kaji Izumi , Akiba Yasutada , Kaunitz Jonathan D 
Published: November 2023

The duodenal mucosa, due to its strategic location, senses luminal nutrients, affects appetite, and regulates duodenal anion secretion, which in turn is important for nutrient absorption and mucosal protection from gastric acid. Xenin‐25 an anorexigenic gut hormone truncated from pro‐xenin, a coatomer protein complex, subunit α (COPA) gene product. Since the plasma concentration of xenin is increased after food intake and proton pump inhibitor treatment in humans, xenin may affect postprandial GI functions. Since the C‐terminal sequence of xenin is similar to that of neurotensin (NT), xenin activates NT receptors NTS1 and NTS2 with similar affinity to NT. In peripheral tissues, xenin is predominantly expressed in the gastric and duodenal mucosa, whereas NT expression is abundant in the terminal ileum. Since NT stimulates anion secretion in the hindgut, we hypothesized that xenin stimulates duodenal anion secretion.A duodenal loop was perfused in an anesthetized rat and luminal bicarbonate secretion was measured with flow‐through pH and CO2 electrodes. Electrogenic anion secretion was assessed by measuring short‐circuit current (Isc) using Ussing chamber with mucosa‐submucosal preparations of rat distal duodenum. Quantitative RT‐PCR and immunohistochemistry were used to confirm the expression of xenin and related receptors.Intravenous infusion of xenin‐8, a bioactive C‐terminal peptide of xenin‐25, dose‐dependently (1–10 nmol/kg) stimulates duodenal bicarbonate secretion. NTS1 mRNA was highly abundant in the enteric plexus, nodose and dorsal root ganglia, and in the lamina propria rather than in the epithelium; in contrast, NTS2 was abundant in isolated duodenocytes, as well as in neural tissues. The serosal application of xenin‐8 and xenin‐25 transiently increased Isc, which gradually decreased to baseline in a concentration‐dependent manner with an EC50 of 0.3 nM. The NTS1 antagonist SR48692 (10 μM) abolished the response to xenin‐8 (1 nM), whereas the NTS2 partial agonist levocabastine had no effect on the basal and xenin‐8‐evoked Isc, suggesting that NTS1 predominantly mediates xenin‐induced duodenal anion secretion. Xenin‐8‐evoked secretion was inhibited by tetrodotoxin (TTX) or by capsaicin, but not altered by atropine, indicating that a non‐cholinergic pathway mediates the xenin response. Pretreatment with the NK1 antagonist CP96345 (10 μM) or the 5‐HT3 antagonist ondansetron (1 μM), but not the 5‐HT4 antagonist GR113808 (1 μM), reduced the response to xenin‐8. NK1 was expressed in extrinsic afferent nerves and submucosal neurons, but not in myenteric neurons, confirmed by RT‐PCR and immunohistochemistry. Xenin‐25 was localized in a subset of enteroendocrine cells in the duodenum, but not in the ileum or the colon.In conclusion, serosal xenin induces duodenal bicarbonate secretion via neural pathways. Endogenous xenin released from endocrine cells may activate NTS1 present on intrinsic and extrinsic afferent nerves, followed by inducing anion secretion through release of substance P and 5‐HT as transmitters. Afferent nerve activation by postprandial, peripherally‐released xenin may account for the anorexigenic effect of xenin.Support or Funding InformationSupported by VA Merit Review, NIH R01 DK54221