Abstract

Sinusoidal entry is the first obligatory process preceding intracellular drug removal in liver. Transport of the angiotensin converting enzyme inhibitor enalapril (1–750 μM with [³H]enalapril), a substrate of Oatp1, the sodium-independent organic anion transporting polypeptide 1 cloned from rat liver, was studied in rat hepatocytes isolated from all zones of the liver (homogeneous) and from enriched periportal (PP) and perivenous (PV) hepatocytes prepared by collagenase perfusion and zone-selective destruction with digitonin, respectively. Uptake was linear over 1 min and was concentration-dependent. Transport by the homogeneous hepatocytes (in the presence and absence of Na⁺) and PP and PV cells was described by single saturable components of similar kinetic constants (K_mvalues of 344–461 μM and V_max values of 9.5–11.6 nmol/min/10⁶ cells; P > .05, ANOVA). The K_m value for enalapril uptake in hepatocytes was of the same order of magnitude compared with that for Oatp1 expressed in HeLa cells transfected with cDNA-Oatp1 and Western blot analysis revealed similar levels of immunoreactive Oatp1 expression in PP and PV hepatocytes. However, enalapril was not taken up by Oatp2 nor by the human OATP expressed in recombinant vaccinia systems.