There is evidence that the enterochromaffin-like cells (ECL cells) of the intestinal mucosa secrete serotonin, or 5-hydroxytryptamine, in response to the stimuli of pressure and various chemicals. Serotonin then stimulates intrinsic afferents, which conduct impulses into the submucosal and myenteric plexuses and there activate motor neurons. Motor neurons that terminate in the muscularis can stimulate contractions; those that terminate in the intestinal crypts can stimulate the secretion of salt and water into the lumen. The ECL cells have also been shown to produce another paracrine regulator, termed motilin, which stimulates contraction in the duodenum and stomach antrum.
Guanylin is a recently discovered paracrine regulator produced by the ileum and colon. It derives its name from its ability to activate the enzyme guanylate cyclase, and thus to cause the production of cyclic GMP (cGMP) within the cytoplasm of intestinal epithelial cells. Acting through cGMP as a second messenger, guanylin stimulates the intestinal epithelial cells to secrete Cl- and water and inhibits their absorption of Na+. These actions increase the amount of salt and water lost from the body in the feces. A related polypeptide, called uroguanylin, has been found in the urine. This polypeptide appears to be produced by the intestine, and may therefore function as a hormone that stimulates the kidneys to excrete salt in the urine.
Certain Escherichia colibacteria produce heat-stable en-terotoxins that are responsible for traveler's diarrhea. The enterotoxins act by stimulating the same receptors on the apical membranes of the intestinal epithelial cells that are activated by guanylin. By mimicking the actions of guanylin, the enterotoxins stimulate intestinal Cl- and water secretion to produce diarrhea.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.