Hepatic uptake of bromosulfophthalein-glutathione in perfused Eisai hyperbilirubinemic mutant rat liver: A multiple-indicator dilution study
The hepatocellular uptake of the glutathione conjugate of bromosulfophthalein (BSPGSH) was examined in Eisai hyperbilirubinemic rats (EHBR; originating from Sprague-Dawley rats), which lacked the ATP-dependent canalicular transport for non-bile acid organic anions, a trend common to other mutant rat strains (TR- and GY, originating from Wistar rats). Single-pass perfused rat liver experiments were conducted with BSPGSH (26-257 microM) using the multiple indicator dilution technique. The steady-state extraction ratio of BSPGSH was close to zero due to lack of biliary excretion. After the introduction of a bolus dose containing vascular (51Cr-labeled red blood cells), interstitial (125I-labeled albumin and [14C]sucrose) and cellular space (D2O) indicators and [3H]BSPGSH into the portal vein, the outflow dilution profile of [3H]BSPGSH was found to display a protracted declining profile (tailing) at low input BSPGSH concentrations; the tail disappeared at higher BSPGSH concentrations. When data were fitted with the barrier-limited model of Goresky as used previously for BSPGSH for the Sprague-Dawley rat (SDR), model fitting was found to evoke an additional "deep pool" within the hepatocyte to account for the "tail" component. The deep pool became evident for the EHBR because biliary excretion of BSPGSH was absent and the rate of return from the deep pool was slow. The concentration of BSPGSH within the deep pool was estimated to be 12 +/- 8 times that in the cytosol. The binding of BSPGSH to EHBR S9 (effective binding concentration of 53 microM and a binding association constant KA of 2.4 x 10(4) M-1), however, was found to be lower than that of SDR S9 and could not account for the late-in-time data. The influx permeability-surface area product was concentration dependent and decreased from 0.27 to 0.01 ml.sec-1.g-1 with increasing BSPGSH concentration; the throughput component, or the portion of the dose that goes through the liver without entering the hepatocyte, increased with increasing concentration. The trends were characteristic of carrier-mediated transport and were similar to those found for the uptake of BSPGSH in SDR.
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