| The colon absorbs about one liter of fluid per day and its large transport capacity is attributed to the epithelium. Epithelial cells have the ability to absorb or secrete ions and water, but normally absorptive mechanisms predominate over secretory mechanisms. Colonic epithelial cells possess many ion channels and transporters working in concert to direct fluid movement. The balance of colonic fluid movement is tipped from absorption to secretion upon intestinal exposure to enterotoxins or inflammatory mediators. Alternatively, other pathophysiological conditions, such as the disease cystic fibrosis, can result in reduced secretion than normal. Many factors affect the behavior of epithelial ion channels and transporters, and ultimately colonic transport.; This thesis focuses on two mechanisms of regulating colonic epithelial ion transport: neurohormonal factors, via alpha-adrenergic receptors (absorptive) and via adenosine receptors (secretory), and cell membrane lipid composition (cholesterol and sphingomyelin). We used whole tissue transepithelial current measurements to study regulatory mechanisms of ion transport. Molecular techniques also elucidated the expression of certain potassium (K+) channels. Basolateral K+ channels are crucial for driving transepithelial anion secretion, and our studies emphasize their importance in colonic epithelial secretion.; Transepithelial ion transport studies showed that alpha2-adrenergic G protein-coupled receptors inhibit colonic Cl- secretion by inhibiting ATP-regulated K+ (KATP) channels. This inhibition was mediated by Gi/o proteins, but independent of Ca 2+ or cAMP signaling.; The cell membrane houses ion channels and transporters in a complex lipid environment, and we explored the role of cell membrane lipids in regulating ion transport. Using lipid-altering agents, we found that cholesterol and sphingomyelin are important regulators of colonic secretion. Lipid rafts are membrane microdomains that are rich in cholesterol and sphingolipids, and we showed that the basolateral large conductance Ca2+-regulated K+ (BK) channels are regulated by lipid raft integrity.; We have also characterized cholesterol, independent of its role in lipid raft structure, as an important mediator of adenosine-stimulated epithelial secretion. The intermediate conductance Ca2+-regulated basolateral K+ (IK) channel was involved in adenosine-mediated secretion.; Our studies characterize novel regulatory mechanisms that are important for colonic epithelial transport. These results expand our knowledge of colonic function and may have significant therapeutic implications. |
