Pharmacokinetic analysis of factors determining elimination pathways for sulfate and glucuronide metabolites of xenobiotics. III: Mechanisms for sinusoidal efflux of 4-methylumbelliferone sulfate

1. To elucidate the mechanisms involved in the sinusoidal efflux of sulfate and glucuronide metabolites of 4-methylumbelliferone (4MU), isolated rat liver perfusion studies were performed under several conditions. 2. The effect of sodium azide on the hepatic handling of both conjugates was examined. The net sinusoidal efflux clearance (CL_eff) based on the unbound concentration in the liver did not change for 4MU glucuronide (4MUG) or significantly increase for 4MU sulfate (4MUS), suggesting that the sinusoidal efflux of both conjugates is not mediated by the transport systems dependent on adenosine triphosphate. 3. Under CP-depleted conditions, the CL_eff of 4MUG significantly decreased, but the saturation of its sinusoidal efflux rather than the transport system dependent on CP might be involved because the hepatic concentration of 4MUG was extensively higher than that of the control study due to the extremely attenuated biliary excretion. The CL_eff of 4MUS also significantly decreased, but its hepatic concentration was not different from that in the control study, suggesting that the transport system using CP is involved in the sinusoidal efflux of 4MUS. 4. The effect of glutathione was examined. CL_eff of 4MUG was not affected by the additional glutathione, but CL_eff of 4MUS decreased significantly, suggesting that some transport system sensitive to glutathione is involved in the sinusoidal efflux of 4MUS, but not of 4MUG. 5. Transporters such as Oatp1, Oatp2 and/or Npt1 might be involved in the sinusoidal efflux of 4MUS, but 4MUG is secreted from the sinusoidal membrane via the systems that are totally different from those for 4MUS.