Energy-dependent transport of digoxin across renal tubular cell monolayers (LLC-PK1)

Abstract

Digoxin secretory transport across renal tubular cell monolayers (LLC-PK1) grown on permeable filters was characterized. Metabolic inhibitors reduced total and specific basolateral to apical (B–A) flux of digoxin and conversely increased the apical to basolateral (A–B) flux. The specific transport of digoxin from the basolateral to the apical compartment was saturable, with a maximum velocity of transport of 184.5 ± 38.0 pmol∙cm−2∙h−1 and a Michaelis–Menten constant (Km) of 14.1 ± 1.6 μM. In addition, B–A flux of digoxin resulted in accumulation of digoxin in the apical compartment against the concentration gradient. P-Glycoprotein inhibitors such as quinidine, verapamil, vincristine, and cyclosporine increased the net A–B flux and inhibited the total B–A flux without affecting the nonspecific flux significantly. Tetraethylammonium, a prototype substrate for an organic cation transport system, had no such effect. Our results suggest that digoxin undergoes transepithelial secretion by an energy-dependent, carrier-mediated process in renal tubules, a process that seems to be distinct from the tetraethylammonium transport system.Key words: renal tubular secretion of digoxin, quinidine, tetraethylammonium, verapamil, vincristine, cyclosporine, P-glycoprotein.