Abstract

The mechanism for hepatic uptake of temocaprilat, an angiotensin-converting enzyme inhibitor that is predominantly excreted into bile was studied using isolated rat hepatocytes and COS-7 cells expressing the organic anion transporting polypeptide (oatp1). The uptake of temocaprilat into isolated rat hepatocytes exhibited saturation with a K_m of 20.9 μM and a V_max of 0.21 nmol/min/mg protein. This uptake was temperature sensitive and was significantly reduced by metabolic inhibitors, a sulfhydryl-modifying reagent and an anion-exchange inhibitor, although the replacement of Na⁺ with Li⁺ in the medium did not affect the uptake. [³H]Temocaprilat uptake was inhibited by estradiol-17β-d-glucuronide and dibromosulphophthalein, typical substrates for the Na⁺-independent organic anion transporter, in a concentration-dependent manner, whereas excess estradiol-17β-d-glucuronide did not completely inhibit the uptake. Temocaprilat uptake into COS-7 cells transfected with oatp1 cDNA revealed a concentration-dependency with aK_m of 46.7 μM, a value comparable with that obtained in isolated hepatocytes. The contribution of oatp1 to carrier-mediated hepatic uptake of temocaprilat was less than 50% by correcting the uptake clearance with that of estradiol-17β-d-glucuronide. A good linear correlation was observed for the inhibitory effect of angiotensin-converting enzyme inhibitors (benazeprilat, cilazaprilat, delaprilat and enalaprilat) between isolated hepatocytes and oatp1-expressing cells. These data suggest that oatp1, along with another transporter(s), mediates the uptake of angiotensin-converting enzyme inhibitors into rat hepatocytes.