Vascular binding, blood flow, transporter, and enzyme interactions on the processing of digoxin in rat liver

The roles of vascular binding, flow, transporters, and enzymes as determinants of the clearance of digoxin were examined in the rat liver. Digoxin is metabolized by Cyp3a and utilizes the organic anion transporting polypeptide 2 (Oatp2) and P-glycoprotein (Pgp) for influx and excretion, respectively. Uptake of digoxin was found to be similar among rat periportal (PP) and perivenous (PV) hepatocytes isolated by the digitonin-collagenase method. The K_m values for uptake were 180 ± 112 and 390 ± 406 nM, V_max values were 13 ± 8 and 18 ± 4.9 pmol/min/mg protein, and nonsaturable components were 9.2 ± 1.3 and 10.7 ± 2.5 μl/min/mg for PP and PV, respectively. The evenness of distribution of Oatp2 and Pgp was confirmed by Western blotting and confocal immunofluorescent microscopy. When digoxin was recirculated to the rat liver preparation in Krebs-Henseleit bicarbonate (KHB) for 3 h in absence or presence of 1% bovine serum albumin (BSA) and 20% red blood cell (rbc) at flow rates of 40 and 10 ml/min, respectively, biexponential decays were observed. Fitted results based on compartmental analyses revealed a higher clearance (0.244 ± 0.082 ml/min/g) for KHB-perfused livers over the rbc-albumin-perfused livers (0.114 ± 0.057 ml/min/g) (P < 0.05). We further found that binding of digoxin to 1% BSA was modest (unbound fraction = 0.64), whereas binding to rbc was associated with slow on (0.468 ± 0.021 min^-1) and off (1.81 ± 0.12 min^-1) rate constants. We then used a zonal, physiologically based pharmacokinetic model to show that the difference in digoxin clearance was attributed to binding to BSA and rbc and not to the difference in flow rate and that clearance was unaffected by transporter or enzyme heterogeneity.