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 μM 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 μM, whereas at 200 μM, 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 μM). Inhibition was due to both a decrease in J(b-a), and an increase in J(a-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 μM digitoxin). 4 The competitive P-glycoprotein inhibitors, verapamil (100 μM), nifedipine (50 μM) and vinblastine (50 μM) all abolished net secretion of [3H]-digoxin due to both a decrease in J(b-a) and an increase in J(a-b). Cellular accumulation of [3H]-digoxin was also increased across both the apical and basal cell surfaces. 1-Chloro-2,4,-dinitrobenzene (10 μM), 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 P(a-b) (=J(a-b)/C(a)) 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.