The effect of probenecid on the in vitro absorption of cardiac glycosides

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 [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.

Intestinal secretion of digoxin in the rat. Augmentation by feeding activated charcoal

Secretion of tritium-labeled digoxin was studied in rats using noneverted gut sacs, in ligated and perfused intestinal preparations in bile duct ligated rats, and by measuring 4 day fecal excretion of total radioactivity in bile duct ligated rats. Serosal to mucosal transfer was proportional to substrate concentration in vitro. In ligated intestinal loops radioactivity was concentrated in the lumen relative to serum. In perfused intestinal preparations the fraction of dose increased with time and was similar over a 100-fold range of doses. Bile duct ligated rats excreted 13.4 +/- 5.8 (S.D.) % of parenterally administered label in 4 day stool collections. Bile duct ligated rats treated with p.o. activated charcoal excreted significantly more radioactivity (33.4 +/- 7.9%). The results suggest that net nonbiliary intestinal secretion of digoxin and metabolites can be augmented by intraluminal binding. A role for this phenomenon in accounting for some effects of diseases and drug interactions is suggested.

Kinetics and inotropic action of probenecid in guinea-pig heart in vitro

Probenecid (100-750 microgram . ml-1) was found to inhibit cardiac contraction force in untreated and digoxin-treated (100 microgram . ml-1) isolated right guinea-pig atrium in vitro by a reversible process, without influencing beating frequency. At low concentrations (1.5--60 microgram . ml-1), 14C-probenecid was accumulated into right atrium by an oxygen-dependent process. Correlation between uptake and negative inotropic action of probenecid could not be found.

Drug interactions in intestinal absorption of 3H-digitoxin in rats

The absorption of 3H-digitoxin from perfused rat small intestine was inhibited by probenecid (1.0 x 10-2 M), ethacrynic acid (0.5 x 10-3 M), and mersalyl (8.0 x 10-3 M) indicating that digitoxin absorption is at least partly an active process.

Intestinal transport of 3H-digitoxin in vitro incompatible with simple diffusion

On everted jejunal segments of mice the transfer and tissue uptake of 3H-digitoxin, over a concentration range from 2 times 10(-10)--1 times 10(-4)M, was investigated from the mucosal ("m") to the serosal ("s") side as well as in the opposite direction. 1. The time course of the absorption of 3H-digitoxin and some other compounds investigated (glucose, urea, p-aminohippurate) gave evidence of functional integrity throughout the 75 min-periods of the experiments. 2. When 3H-digitoxin was applied to the mucosal side the permeability coefficient showed a dose-dependent increase but returned to lower values at higher concentrations. When 3H-digitoxin was administered to the serosal side the permeability coefficient showed a dose-dependent decrease at high concentrations. The ratio of both coefficients "m" leads to "s"/"s" leads to "m" increased dose-dependently from 0.4--2.6. 3. The uptake of 3H-digitoxin--applied on the serosal side--into the tissue was independent of dose. However, having administered 3H-digitoxin on the mucosal side the tissue accumulation was 2--5 fold higher and the tissue/medium (T/M) ratio increased within the concentration range from 3.0-9.0 4. Under DNP (1 mM) the asymmetry and dose dependence of the permeability and tissue uptake up 3H-digitoxin observed in controls were almost abolished. Therefore it is likely that the transfer of 3H-digitoxin in the intact intestine involves a mechanism more complex than simple diffusion. The existence of more than a two compartment system and/or the contribution of an active transport mechanism is suggested.