The effects of extrinsic and intrinsic nerves on ion and water transport by the intestine are considered and discussed in terms of their possible physiological function. Adrenergic nerves enter the small intestine via mesenteric nerves. Adrenergic tone is usually absent in tissues in vitro but is present in vivo. The nerves increase absorption in response to homeostatic changes associated with acute depletion of extracellular fluid. Cholinergic tone that reduces fluid absorption or causes secretion has been detected in the small intestine of humans, dogs, and cats and in the colon of humans. Extrinsic cholinergic fibers generally do not affect ion transport in small intestine but probably do so in colon. Whether peptides liberated in the mucosa affect enterocytes directly is not clear. Studies on humans and rabbits suggest that the role of substance P is minor. The physiological roles of vasoactive intestinal polypeptide (VIP) and somatostatin remain to be defined. Intraluminal factors also affect ion and water transport. Mucosal rubbing, distension, and cholera toxin cause fluid secretion; acid solutions in the duodenum cause alkaline secretion; these stimuli and hypertonic glucose liberate serotonin into the lumen, the mesenteric venous blood, or both. It has been proposed that the enterochromaffin cell is an epithelial sensory cell that responds to noxious stimuli within the lumen by liberating serotonin. The serotonin initiates a neural reflex through a nicotinic ganglion to liberate a secretagogue that acts on the enterocyte. The function of VIP in this proposed reflex is unclear. The variety of intraluminal stimuli that influence epithelial function implies that there is more than one type of epithelial sensory cell (or sensory mechanism). Prostaglandins may mediate the alkaline secretion caused by acid in the duodenum. There may be other effective substances. Although it has been known for years that intraluminal stimuli affect the coordination of smooth muscle functions, it is not known whether similar stimuli also influence salt and water transport as a meal traverses the alimentary canal.
[Ca2+] affects nerves and target cells in stimulus-secretion coupling. In flux-chamber studies of full-thickness rabbit ileum, we determined the effects of 1) ethylene glycol bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) 1.25 mM, 2) verapamil 0.1 mM and nifedipine 0.1 mM, and 3) trifluoperazine 0.1 mM on ion transport and its response to electrical field stimulation (EFS). EGTA increased JClm leads to s, JNam leads to s, Cl absorption and conductivity (G), and reduced Isc. In the absence of EGTA, EFS increased transmural PD and Isc and caused secretion of Na and Cl. EGTA prevented the responses to EFS, but the Isc responses to aminophylline and to glucose were normal. Verapamil reduced the response of Isc and Cl transport to EFS. Nifedipine reduced Isc but not the Isc response to EFS. Trifluoperazine reduced Isc and almost eliminated the Isc response to EFS. EFS did not increase the tissue concentration of cAMP. We conclude: 1) low extracellular [Ca2+] enhances net Cl absorption; 2) extracellular Ca2+ is required for the response of ion transport to EFS; 3) cAMP does not mediate Isc response to EFS; and 4) Isc response to EFS is blocked by trifluoperazine. The findings suggest that EFS stimulates secretion by increasing Ca entry into the epithelial cells, either directly or indirectly.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.