Abstract
Background: Secretin is a member of the secretin-glucagon-vasoactive intestinal peptide hormone superfamily and is a multifunctional gastrointestinal- (GI) and neuro- peptide hormone. Secretin is primarily secreted postprandially from the crypts of Lieberkühn of duodenal enteroendocrine S cells into circulation where it reaches secretin receptor targets in the central nervous system and periphery. This dynamic hormone has been shown to act as a key signaling molecule in the regulation of digestion, metabolism and energy expenditure, water retention, reproduction, thermogenesis in adipose tissue, and in gastric and intestinal motility. Secretin’s canonical role in the GI tract is to stimulate the secretion of bicarbonate and bile from pancreatic ducts and bile ducts to neutralize acidic chyme exiting the stomach. Secretin has also been shown to slow intestinal motility, but its targets and mechanism of action is poorly understood.
Aims: Several studies have proposed that secretin acts to slow intestinal motility primarily through secretin receptors present on vagal afferents in the GI tract. In fact secretin receptors (SCTR) are also highly expressed by interstitial cells of Cajal (ICC) in the small intestine, suggesting an alternate and complementary signaling pathway. We hypothesized that ICC act as a liaison to facilitate a reduction in contractile force through binding of SCRT and stimulation of the second messenger, cyclic adenosine monophosphate (cAMP) in ICC, and this mediates reduced contractile force generated by intestinal muscles.
Methods: Spinning-disk confocal microscopy was used monitor Ca2+ signaling in ICC from small intestinal muscles of GCaMP6f x KitiCre mice. Additionally, cAMP levels were evaluated using CAMPER mice. Intestinal muscle contractions were assessed using in vitro myography techniques.
Results: Secretin reduces small intestinal force of contraction in the presence of tetrodotoxin (TTX) and dampens effects of cholinergic neurotransmission. SCTR is expressed by ICC within the deep muscular plexus (ICC-DMP) in the small intestine and Ca2+ imaging confirmed the effects are primarily localized within ICC-DMP. Secretin reduced carbachol induced contractions and reduced Ca2+ transients in ICC-DMP in response to electrical field stimulation (EFS) in the presence of L-NNA (NO synthase inhibitor) and MRS2500 (P2Y1 antagonist). Secretin caused an increase in cAMP in ICC-DMP in muscles from Kit-iCre-CAMPER mice. PKA inhibitors rescued some of the inhibitory effects of secretin on ICC-DMP Ca2+ signaling. All experiments were conducted with a minimum sample size of n=6.
Conclusions: Secretin inhibits small intestinal motility by binding to SCTR on ICC-DMP and a cAMP-mediated mechanism. Intestinal ICC are a novel target for secretin that appears to mediate part of the inhibitory effects of this hormone on intestinal motility.
Funding: This project was supported by R01 DK-120759 from the National Institute of Diabetes and digestive and Kidney (NIDDK).