Carrier-mediated entry of 4-methylumbelliferyl sulfate: characterization by the multiple-indicator dilution technique in perfused rat liver

Interplay of phase II enzymes and transporters in futile cycling: influence of multidrug resistance-associated protein 2-mediated excretion of estradiol 17beta-D-glucuronide and its 3-sulfate metabolite on net sulfation in perfused TR(-) and Wistar rat liver preparations

The hepatic disposition of estradiol 17beta-D-glucuronide (E(2)17G), a substrate of the organic anion-transporting polypeptides Oatp1a1, Oatp1a4, and Oatp1b2, was investigated in Wistar and TR(-) [multidrug resistance-associated protein (Mrp) 2-mutant] rats to elucidate how absence of Mrp2, the major excretory transporter for both E(2)17G and its 3-sulfate metabolite (E(2)3S17G), affected the net sulfation. With absence of Mrp2, lower microsomal desulfation activity and higher Mrp3 but unchanged immunoreactive protein expression of other transporters (Oatps and Mrp4) and estrogen sulfotransferase were found in TR(-) rats. In recirculating, perfused liver preparations, the rapid decay of E(2)17G and sluggish appearance of low levels of E(2)3S17G in perfusate for Wistar livers were replaced by a protracted, biexponential decay of E(2)17G and greater accumulation of E(2)3S17G, whose levels reached plateaus upon the almost complete obliteration of biliary excretion of E(2)17G and E(2)3S17G in the TR(-) liver. Much higher amounts of E(2)17G (28x) and E(2)3S17G (11x) in liver and reduced net sulfation (40 +/- 6 from 77 +/- 6% dose, P < 0.05) were observed at 2 h for the TR(-) versus the Wistar rats. With use of a physiologically based pharmacokinetic model, analytical solutions for the areas under the curve for the precursor and metabolite were obtained to reveal how enzyme- and transporter-mediated processes affected the hepatic disposition of the precursor and metabolite in futile cycling. The analytical solutions were useful to explain transporter-enzyme interplay in futile cycling and predicted that a shutdown of Mrp2 function led to decreased net sulfation of E(2)17G by raising the intracellular concentration of the metabolite, E(2)3S17G, which readily refurnished E(2)17G via desulfation.

Influence of erythrocytes on the hepatic distribution kinetics of urea and thiourea

The role of erythrocyte on the hepatic distribution kinetics of urea and thiourea was investigated in the in situ isolated perfused rat liver. Perfusion experiments were conducted using Krebs-bicarbonate buffer delivered via the portal vein in a single pass mode at a total flow rate of 15 ml/min. With urea, superimposable unimodal effluent curves were obtained in the presence and absence of erythrocytes, indicating that its distribution kinetics is not affected by erythrocytes. With thiourea, effluent curves were unimodal in the absence of erythrocytes but bimodal in the presence of erythrocytes. The maximum frequency output at the first peak increased from 0.017+/-0.002 to 0.042+/-0.006 s(-1) with an increase in the bolus hematocrit from 0.40 to 0.75, indicating that some thiourea fraction is retained by the erythrocytes on transit through the liver. Although the fractional output associated with the first peak was very similar (11.9% versus 11.5%), whether the perfusate contained unlabelled thiourea or not, this fraction was reduced from 17 to 5% with a decrease in the incubation time before injection from 30 min to 40s. However, there was no evidence for a capacity limitation; a 30-min period of pre-incubation of either radiolabelled thiourea alone or combined with a high concentration of unlabelled thiourea had minimal effect on effluent profiles.

Differential Involvement of Mrp2 (Abcc2) and Bcrp (Abcg2) in Biliary Excretion of 4-Methylumbelliferyl Glucuronide and Sulfate in the Rat

The hepatic excretion of hydrophilic conjugates, end products of phase II metabolism, is not completely understood. In the present studies, transport mechanism(s) responsible for the biliary excretion of 4-methylumbelliferyl glucuronide (4MUG) and 4-methylumbelliferyl sulfate (4MUS) were studied. Isolated perfused livers (IPLs) from Mrp2-deficient (TR(-)) Wistar rats were used to examine the role of Mrp2 in the biliary excretion of 4MUG and 4MUS. After a 30-micromol dose of 4-methylumbelliferone, cumulative biliary excretion of 4MUG was extensive in wild-type rat IPLs (25 +/- 3 micromol) but was negligible in TR(-) livers (0.4 +/- 0.1 micromol); coadministration of the Bcrp and P-glycoprotein inhibitor GF120918 [N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide] had no effect on 4MUG biliary excretion in wild-type rat IPLs. In contrast, biliary excretion of 4MUS was partially maintained in Mrp2-deficient rat IPLs. Recovery of 4MUS in bile was approximately 50 to 60% lower in both control TR(-) (149 +/- 8 nmol) and wild-type IPLs with GF120918 coadministration (176 +/- 30 nmol) relative to wild-type control livers (378 +/- 37 nmol) and was nearly abolished in TR(-) IPLs in the presence of GF120918 (13 +/- 8 nmol). These changes were the result of decreased rate constants governing 4MUG and 4MUS biliary excretion. In vitro assays and perfused livers from Bcrp and P-glycoprotein gene-knockout mice indicated that 4MUS did not interact with P-glycoprotein but was transported by Bcrp in a GF120918-sensitive manner. In the rat liver, Mrp2 mediates the biliary excretion of 4MUG, whereas both Mrp2 and Bcrp contribute almost equally to the transport of 4MUS into bile.

Effect of erythrocytes on the hepatic distribution kinetics of antipyrine

The role of erythrocyte on the hepatic distribution kinetics of antipyrine was investigated in the in situ isolated perfused rat liver. Perfusion experiments were conducted using Krebs-bicarbonate buffer delivered via the portal vein in a single pass mode at a total flow rate of 15 ml/min. A bolus dose of antipyrine was administered in the presence and absence of erythrocytes. Almost identical moment analysis results (without erythrocytes mean transit time, MTT: 23 s; volume of distribution, VH: 0.57 ml/g liver and with erythrocytes, MTT: 24 s; VH: 0.60 ml/g liver) and superimposable unimodal effluent curves were obtained in the presence and absence of erythrocytes indicates that distribution kinetics of antipyrine within the liver is not affected by erythrocytes.

A microfabricated array bioreactor for perfused 3D liver culture

We describe the design, fabrication, and performance of a bioreactor that enables both morphogenesis of 3D tissue structures under continuous perfusion and repeated in situ observation by light microscopy. Three-dimensional scaffolds were created by deep reactive ion etching of silicon wafers to create an array of channels (through-holes) with cell-adhesive walls. Scaffolds were combined with a cell-retaining filter and support in a reactor housing designed to deliver a continuous perfusate across the top of the array and through the 3D tissue mass in each channel. Reactor dimensions were constructed so that perfusate flow rates meet estimated values of cellular oxygen demands while providing fluid shear stress at or below a physiological range (<2 dyne cm(2)), as determined by comparison of numerical models of reactor fluid flow patterns to literature values of physiological shear stresses. We studied the behavior of primary rat hepatocytes seeded into the reactors and cultured for up to 2 weeks, and found that cells seeded into the channels rearranged extensively to form tissue like structures and remained viable throughout the culture period. We further observed that preaggregation of the cells into spheroidal structures prior to seeding improved the morphogenesis of tissue structure and maintenance of viability. We also demonstrate repeated in situ imaging of tissue structure and function using two-photon microscopy.