Transport and epithelial secretion of the cardiac glycoside, digoxin, by human intestinal epithelial (Caco-2) cells

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 [H-3]-digoxin flux exceeds apical to basal flux, a net secretion of [H-3]- digoxin being observed. At 200 mu M digoxin, net secretory flux (J(net)) was 10.8+/-0.6 nmol cm(-2) h(-1). Maximal secretory flux (J(max)) 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 mu M, whereas at 200 mu M, apical uptake exceeded basal uptake. 3 Net secretion of [H-3]-digoxin was subject to inhibition by digitoxin and bufalin but was not inhibited by ouabain, convallatoxin, and strophanthidin (all 100 mu M). Inhibition was due to both a decrease in J(b-a) and an increase in J(a-b). Uptake of [H-3]-digoxin at the apical surface was increased by digitoxin and bufalin. All cardiac glycosides decreased [H-3]-digoxin uptake at the basal cell surface (except for 100 mu M digitoxin). 4 The competitive P-glycoprotein inhibitors, verapamil (100 mu M), nifedipine (50 mu M) and vinblastine (50 mu M) all abolished net secretion of [H-3]-digoxin due to both a decrease in J(b-a) and an increase in J(a-b). Cellular accumulation of [H-3]-digoxin was also increased across both the apical and basal cell surfaces. 1-Chloro-2,4,-dinitrobenzene (10 mu M), a substrate for glutathione-S-transferase and subsequent ATP-dependent glutathione-S-conjugate secretion, failed to inhibit net secretion of [H-3]-digoxin. The increase in absorptive permeability P-a-b (=J(a-b)/C-a) and cellular [H-3]-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 [H-3]-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.