Current research interests in my lab focus on the response of intestinal epithelial cells to stimulation from neural and hormonal stimuli. Epithelial tissues form the barrier between the body interior and the external environment; and during digestion, intestinal epithelial cells secrete fluid to promote luminal enzyme action and motility, followed by reabsorption of this fluid to limit water loss from the body. Cells making up the epithelium produce fluid by secreting ions, such as Cl-, K+ and H+, into the lumen. Macromolecular secretions amend the composition of this fluid by adding digestive enzymes, mucus, immunoglobulins and other molecules that support the health of the digestive tract. Ion secretory mechanisms involve the action of specific ion transport proteins, such as Na+/K+-ATPases, Na+/K+/2Cl--cotransporters, K+ channels and Cl- channels, which guide flow from the interstitium into the lumen. Regulation of these transport proteins allows for an orderly production of fluid as circumstances change during the day.
Measurement of ion flow is a central activity in the lab, including patch-clamp recording of currents in epithelial cells as well as transepithelial currents. Biochemical assays complement these physiologic measurements by allowing detection of ion channel expression with RT-PCR, immunoblot and immunohistologic methods as well as quantifying changes in signaling molecules of specific regulatory pathways. And, imaging of epithelial structures permits visualization of morphological changes occurring during secretion, with incorporation of fluorescent dyes permitting detection of cellular ion composition (pH, Ca++, Cl-) during the time course of secretory activation. Together these techniques contribute to understanding the normal and pathophysiologic operation of epithelia.
Regulation of ion channels is key to controlling the rate of secretion, and physiologic signals lead both to activation and inhibition. Recent results indicate that sympathetic input activates sustained K+ secretion via b1 and b2-adrenergic receptors on the epithelial cells in addition to a transient Cl- secretory event [4, 5]. Divergent intracellular signaling via cAMP production allows these cells to differentially promote the two ion secretory events [3]. Most dramatically, the release of neuropeptide-Y (NPY) by sympathetic nerves leads to a severe blunting of the Cl- secretory component without altering the rate of sustained K+ secretion. Enteroendocrine cells release a related peptide, PYY, such that inhibition of Cl- secretion can be initiated through two distinct pathways. Both NPY and PYY act via the Y2-neuropeptide receptor present on epithelial cells, apparently by suppressing cAMP levels. The cAMP signaling producing the transient and sustained components of the b-adrenergic response is effectively compartmentalized by trafficking of receptor complexes in a manner promoting each secretory action.
Regulatory isolation allows the epithelial cells to activate groups of ion channels supporting each of the b-adrenergic secretory components. The transient Cl- secretion requires apical membrane Cl- channels and basolateral membrane K+ channels, whereas sustained K+ secretion operates via apical membrane K+ channels [1] and basolateral membrane Cl- channels [2, 8, 10, 11]. Other secretagogs produce higher and more sustained rates of Cl- secretion, with each secretagog using different types of K+ channels to support the secretory events [1, 6, 7, 12]. Clinically, inappropriate regulation of secretion hinders digestion by either restricted or excessive flow. Whereas restricted flow contributes to cystic fibrosis symptoms, excessive fluid secretion contributes to inflammatory bowel disease and irritable bowel syndrome. A greater understanding of secretory regulation will lead to better therapeutic solutions to these and other gastrointestional disorders.