Sinusoidal uptake

As far as sinusoidal uptake is concerned, drug-drug interactions have also been reported between antituberculosis agents (rifamycin SV and rifampicin) and bromosulfophthalein in humans both drugs reduce the clearance of bromosul-fophthalein and also induce hyperbilirubinemia [108]. These results may be ac-... 

Always a determinant of net hepatic clearance (regardless of other mechanisms involved). Changes in sinusoidal uptake (e.g., due to transport inhibition) will affect overall hepatic clearance, even for drugs that are highly metabolized. 

Since this facilitated transport system allows the equilibrium of bilirubin across the sinusoidal membrane of the hepatocyte, the net uptake of bilirubin will be dependent upon the removal of bilirubin via subsequent metabolic pathways. 

Meier, P. J., et al. Substrate specificity of sinusoidal bile acid and organic anion uptake systems in rat and human liver. Hepatology 1997, 26, 1667-1677. 

Zimmerli, B., J. Vaiantinas, and P. J. Meier. Multispecificity of Na+-dependent taurocholate uptake in basolateral (sinusoidal) rat liver plasma membrane vesicles. J. Pharmacol. Exp. Ther. 1989, 250, 301-318. 

The liver plays an important role in determining the oral bioavailability of drags. Drag molecules absorbed into the portal vein are taken up by hepatocytes, and then metabolized and/or excreted into the bile. For hydrophilic drugs, transporters located on the sinusoidal membrane are responsible for the hepatic uptake [1, 2]. Biliary excretion of many drags is also mediated by the primary active transporters, referred to as ATP-binding cassette transmembrane (ABC) transporters, located on the bile canalicular membrane [1, 3-5], Recently, many molecular biological... 

MDCK II cells (Fig. 12.3) [93], Kinetic analysis revealed that the Km value for transcellular transport (24 pM) was similar to the Km for OATP2 (34 pM) [93], Moreover, the efflux across the bile canalicular membrane was not saturated under these experimental conditions. These in vitro observations are consistent with in vivo experimental results in rats which showed that the rate-determining process for the biliary excretion of pravastatin is uptake across the sinusoidal membrane. By normalizing the expression level between the double transfectant and human hepatocytes, it might be possible to predict in vivo hepatobiliary excretion. 

Kamps, J.A., Morselt, H.W., and Scherphof, G.L., 1999, Uptake of hposomes containing phosphatidylserine by hver cehs in vivo and by sinusoidal hver ceUs in primary culture in vivo - in vitro differences, Siochem. Biophys. Res. Commun. 256 57-62. 

Hepatocytes make up 60-70% of the total number of liver cells. They have a well-organized intracellular structure with huge numbers of cell organelles to maintain the high metabolic profile. At the apical side or canalicular membrane the cell is specialized for the secretion of bile components. There are several ATP-dependent transport carriers located on this side of the membrane, which transport bile salts, lipids and xenobiotics into the canaliculus. On the sinusoidal side, the cells specialize in uptake and secretion of a wide variety of components. To increase the surface of the membrane for this exchange with the bloodstream, the sinusoidal domain of the membrane is equipped with irregular microvilli. The microvilli are embedded into the fluid and matrix components of the space of Disse and are in close contact with the sinusoidal blood because of the discontinuous and fenestrated SECs. To facilitate its metabolic functions numerous membrane transport mechanisms and receptors are situated in the membrane. 

The viability and function tests described above are used to evaluate the hepatocytes within the slice. Up to now, tests to measure the viability of the non-parenchymal cells have not been reported. The presence of the latter cell types is one of the conceptual advantages of slices as compared to isolated hepatocytes. As some drug targeting devices are designed to target non-parenchymal cells in the liver, the development of tests for the sinusoidal cell types deserves more attention. For example, the uptake of substrates such as succinylated human serum albumin (Suc-HSA,which is specifically endocytosed by endothelial cells [79]), or hyaluronic acid [80], can be used to assess the functionality of endocytotic pathways in the endothelial cells in the liver [81]. Other modified proteins that are specifically taken up by Kupffer cells such as mannosylated HSA, may be used to assess the functionality of the endocytotic pathway in Kupffer cells [79]. Another parameter which can be used to assess the functionality of these non-parenchymal liver cells, is the excretion of cytokines in response to pro-inflammatory stimuli. Non-parench5mal cell function in liver slices will be described in more detail in the Section 12.7. 

Figure 12.5. Uptake of I-Suc-HSA in liver slices from humans and rats at 37°(0) and 4°C ( ). The accumulation factor is defined as the concentration of the compound in the slices divided hy the concentration in the medium. Each point is the mean of 5-6 separate experiments SEM. n = number of livers. p < 0.05 versus 4°C. The dotted line represents the accumulation factor if I-Suc-HSA is exclusively distrihuted within the sinusoids.

Organic anions have frequently been implicated as substrates for transporters in the sinusoidal membrane of the liver. This was illustrated for a series of TxRAs, where hepatic uptake was identified as the rate-determining step in the clearance process [22]. A representative compound from this series, UK-147,535 (Figure 9.3), was progressed to clinical trials [23]. It is thus possible to contrast clearance of this compound between a number of species including man (Figure 9.4). 

Muro H, Shirasawa H, Kosugi I, et al. 1990. Defect of sinusoidal Fc receptors and immune complex uptake in CCKinduced liver cirrhosis in rats. Gastroenterology 99 200-210. 

Figure 19.5 Physiological pharmacokinetic model for hepatic uptake of drug constantly infused in the isolated rat liver perfusion system. Q, flow rate (mL/min) Cb, inflow concentration (pg/mL) Cs, sinusoidal concentration (pg/mL) Vs, sinusoidal volume (mL) X, binding constant (pg) Xm, maximum binding amount (pg) K, binding constant (mL/pg) kmt, internalization rate constant (min-1).

Parenchymal cells (PC), or hepatocytes, originate from epithelial cells and represent most of the total number of liver cells (65%). Because of their relatively large size, hepatocytes are microscopically clearly visible after staining liver sections with hematoxylin and eosin (HE). Also, the hepatocytes store glycogen, which can be identified histochemically with periodic acid-Schiff (PAS) reagent. The hepatocytes are primarily responsible for the uptake of endogenous products and xenobiotics at the sinusoidal membrane of the cell and their subsequent metabolism and excretion into bile by means of the canalicular membrane. 

M. G. Irving, F. J. Roll, S. Huang, and D. M. Bissell, Characterization and culture of sinusoidal endothelium from normal rat fiver Lipoprotein uptake and collagen phenotype, Gastroenterology 697 239-246 (1995). 

Liver Uptake Blood Parenchymal cells Isolated, cultured cryopreserved hepatocytes, sinusoidal membrane vesicles, transporter expressions system... 

Oatplal was isolated from rat liver as a candidate for sodium-independent uptake of organic anions (65). Oatplal is localized to the sinusoidal membrane in the rat liver and the brush border membrane in the male kidney (66). Cumulative studies have elucidated its broad substrate specificity, including... 

NaPi-1 (SLC17A1), alternatively referred to as NPT1, was originally cloned as a transporter involved in the reabsorption of phosphate in the body. Expression of NaPi-1 in X. laevis oocytes induced saturable uptake of benzylpenicillin (189). This uptake does not depend on Na+ and H+, but on Cl- (190), and increasing extracellular concentration of chloride reduced the uptake of benzyl-penicillin (190). The substrates include faropenem, foscamet, and mevalonate, as well as benzylpenicillin (190). In contrast to the kidney, the expression is localized to the sinusoidal membrane of the liver (190). When the direction of the concentration gradient of Cl- is taken into consideration, the transport direction mediated by NaPi-1 is efflux from inside the cells to the blood and urine in the liver and kidney, respectively. 

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