Carrier-Mediated Active Transport of a Novel Thromboxane A2 Receptor Antagonist [2-(4-Chlorophenylsulfonylaminomethyl)indan-5-yl]acetate (Z-335) into Rat Liver

To elucidate the transport system by which [2-(4-chlorophenylsulfonylaminomethyl)indan-5-yl]acetate (Z-335) is taken up into the liver, we investigated the uptake characteristics of Z-335 in isolated rat hepatocytes. In addition, we estimated the hepatic uptake of Z-335 in intact rats under steady-state conditions and compared it with the in vitro uptake clearance. Uptake of Z-335 is highly concentrative (cell-to-medium concentration ratios were 21.2 at 0.5 min and 71.7 at 5 min), temperature-dependent, and sensitive to metabolic inhibitors, indicating that uptake is mediated by energy-dependent uphill transport. In the presence of metabolic inhibitors [carbonyl cyanide p -trifluoromethoxyphenylhydrazone and rotenone], uptake remained at 37 and 49% of the control value, respectively, suggesting that ATP-independent uptake contributes to the total uptake of Z-335. The concentration dependence of the initial uptake velocity indicated a two-component process, one saturable component, with a K m value of 45.6 μM and a V max value of 4.1 nmol/min/mg of protein, and a nonspecific diffusion clearance, with a P dif value of 8.3 μl/min/mg of protein. The contribution of the carrier-mediated uptake to the total uptake in a linear range was estimated as 91%. The in vivo hepatic intrinsic clearance (CL int, app ) was comparable with that in vitro uptake clearance (PS influx ) and indicated that the CL int, app of Z-335 at steady state is rate-limited by the uptake process. In conclusion, hepatic intrinsic clearance of Z-335 at steady state is rate-limited by the uptake process since Z-335 is efficiently taken up by an active transport mechanism, followed by metabolism or biliary excretion.