Abstract

Binding, transport, and metabolism are factors that influence morphine (M) removal in the rat liver. For M and the morphine 3β-glucuronide metabolite (M3G), modest binding existed with 4% bovine serum albumin (unbound fractions of 0.89 ± 0.07 and 0.98 ± 0.09, respectively), and there was partitioning of M into red blood cells. Transport studies of M (<750 μM) showed similar, concentration-independent uptake clearances (CLs) of 1.5 ml min^-1 g^-1 among zonal and homogeneous, isolated rat hepatocytes. Transport of M3G, ascertained in multiple indicator dilution studies at various steady-state M3G concentrations (10-262 μM), uncovered a low and concentration-independent influx clearance (<10% of flow rate). The outflow dilution curve of [³H]M3G was superimposable onto that of [¹⁴C]sucrose, the extracellular reference, displaying similarity in transit times (23.5 and 22.2 s), negligible biliary excretion, and almost complete dose recovery from perfusate. In contrast, M3G occurred abundantly in both perfusate and bile in single-pass perfusion studies of the precursor, M, and revealed a biliary clearance of formed M3G that was 12.3-fold that of preformed M3G, suggesting a sinusoidal, diffusional barrier for M3G. With increasing concentrations of M (9-474 μM), clearance decreased, and metabolism and biliary excretion displayed concentration-dependent kinetics. Fitting of the data to a physiologically based liver model revealed that M removal mechanisms were saturable, with a K_m,met of 52.2 μM and V_max,met of 58.8 nmol min^-1 g^-1 for metabolism, and a K_m,ex of 41.2 μM and V_max,ex of 8.1 nmol min^-1 g^-1 for excretion. Sinusoidal transport was not rate-limiting for M removal.