title: Transport and epithelial secretion of the cardiac glycoside, digoxin, by human intestinal epithelial (Caco-2) cells.
abstract: 1. Human intestinal epithelial Caco-2 cells have been used to investigate the transepithelial permeation of the cardiac glycoside, digoxin. 2. Transepithelial basal to apical [3H]-digoxin flux exceeds apical to basal flux, a net secretion of [3H]-digoxin being observed. At 200 microM digoxin, net secretory flux (Jnet) was 10.8 +/- 0.6 nmol cm-2 h-1. Maximal secretory flux (Jmax) of vinblastine was 1.3 +/- 0.1 nmol cm-2 h-1. Cellular uptake of digoxin was different across apical and basal cell boundaries. It was greatest across the basal surface at 1 microM, whereas at 200 microM, apical uptake exceeded basal uptake. 3. Net secretion of [3H]-digoxin was subject to inhibition by digitoxin and bufalin but was not inhibited by ouabain, convallatoxin, and strophanthidin (all 100 microM). Inhibition was due to both a decrease in Jb-a and an increase in Ja-b. Uptake of [3H]-digoxin at the apical surface was increased by digitoxin and bufalin. All cardiac glycosides decreased [3H]-digoxin uptake at the basal cell surface (except for 100 microM digitoxin). 4. The competitive P-glycoprotein inhibitors, verapamil (100 microM), nifedipine (50 microM) and vinblastine (50 microM) all abolished net secretion of [3H]-digoxin due to both a decrease in Jb-a and an increase in Ja-b. Cellular accumulation of [3H]-digoxin was also increased across both the apical and basal cell surfaces. I-Chloro-2,4,-dinitrobenzene (10 microM), a substrate for glutathione-S-transferase and subsequent ATP-dependent glutathione-S-conjugate secretion, failed to inhibit net secretion of [3H]-digoxin. The increase in absorptive permeability Pa-b (= Ja-b/Ca) and cellular [3H]-digoxin uptake upon P-glycoprotein inhibition, showed that the intestinal epithelium was rendered effectively impermeable by ATP-dependent extrusion at the apical surface. 5. A model for [3H]-digoxin secretion by the intestinal epithelium is likely to involve both diffusional uptake and Na(+)-K+ pump-mediated endocytosis, followed by active extrusion at the apical membrane.
authors.affiliation: Department of Physiological Sciences, University of Newcastle upon Tyne, Medical School.
authors.forename: M E
authors.initials: ME
authors.lastname: Cavet
authors.forename: M
authors.initials: M
authors.lastname: West
authors.forename: N L
authors.initials: NL
authors.lastname: Simmons
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000037:antagonists & inhibitors / Q000378:metabolism; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D018938:Caco-2 Cells; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002316:Cardiotonic Agents / Q000378:metabolism* / Q000494:pharmacology; D004077:Digoxin / Q000378:metabolism* / Q000494:pharmacology; D004847:Epithelial Cells; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D007413:Intestinal Mucosa / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D011188:Potassium / Q000494:pharmacology; D012414:Rubidium Radioisotopes; D000254:Sodium-Potassium-Exchanging ATPase / Q000187:drug effects / Q000378:metabolism
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002121:Calcium Channel Blockers; D002316:Cardiotonic Agents; D012414:Rubidium Radioisotopes; D010042:Ouabain; D004077:Digoxin; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium