The Effect of Tetrahydrocortisol–Apolipoprotein A-I Complex on the RNA Polymerase Interaction with Eukaryotic DNA and the Rate of Protein Biosynthesis in Hepatocytes

A mechanism of activation of protein biosynthesis in hepatocytes was proposed as effected by the conditioned medium of nonparenchymal liver cells incubated in the presence of high density lipoproteins, cortisol, and lipopolysaccharides. It was found that the increase in the biosynthesis rate was associated with the formation of the tetrahydrocortisol–apolipoprotein A-I (THC–apoA-I) complex in macrophages, which display 5- and 5-reductase activity and are constituents of nonparenchymal liver cell. Using the small-angle X-ray scattering technique, it was shown that the THC–apoA-I–eukaryotic DNA interaction may break hydrogen bonds between pairs of complementary nucleic bases and cause the formation of single-stranded DNA fragments capable of binding to DNA-dependent RNA polymerase. The interaction is highly cooperative and has a saturating mode, up to six enzyme molecules being bound per DNA molecule.     

Stimulation of growth of primary cultured adult rat hepatocytes without growth factors by coculture with nonparenchymal liver cells

DNA synthesis of adult rat parenchymal hepatocytes alone in primary culture can be stimulated only by the addition of humoral growth factors to the culture medium. However, when parenchymal hepatocytes were cocultured with nonparenchymal liver cells from adult rats, their DNA synthesis was markedly stimulated in the absence of added growth factors or calf serum. DNA synthesis of parenchymal hepatocytes was not stimulated by conditioned medium from nonparenchymal liver cells and was greatest when the parenchymal cells were plated on 24-h cultures of nonparenchymal liver cells. A dead feeder layer of nonparenchymal cells was almost as effective as a feeder layer of viable nonparenchymal cells. These results suggest that the stimulation of DNA synthesis in parenchymal hepatocytes was not due to some soluble factors secreted by nonparenchymal liver cells but to an insoluble material(s) produced by the nonparenchymal liver cells. This insoluble material(s) was collagenase- and acid-sensitive, suggesting that it was a protein containing collagen. The effect of nonparenchymal liver cells was specific: coculture with hepatoma cells, liver epithelial cells, or Swiss 3T3 cells did not stimulate DNA synthesis in parenchymal hepatocytes. Added insulin and epidermal growth factor showed additive effects with nonparenchymal cells in the cocultures. These results suggest that DNA synthesis in parenchymal hepatocytes is stimulated not only by various humoral growth factors but also by cell-cell interaction between parenchymal and nonparenchymal hepatocytes, possibly endothelial cells. This cell-cell interaction may be important in repair of liver damage and liver regeneration.     

Species differences in the toxicity of p-chloro-o-toluidine to rats and mice. Covalent binding to hepatic macromolecules and hepatic non-parenchymal cell DNA and an investigation of effects upon the incorporation of [3H] thymidine into capillary endothelial cells

The interaction of p-[14C] chloro-o-toluidine with hepatic macromolecules of rats and mice has been investigated. At all time points after single administration the extent of binding decreased in the order protein greater than RNA greater than DNA in both species. The level of binding to mouse liver DNA was greater than that to rat liver DNA after both single and repeated administration. In vitro studies showed that mouse liver fractions catalysed the binding of p-chloro-o-toluidine to calf thymus DNA more readily than rat liver fractions. Conversely, binding to protein and RNA was more marked in the rat than in the mouse. Species differences in DNA repair rates were not observed. The results failed to demonstrate a preferential persistence of binding to mouse liver nonparenchymal cell DNA. Autoradiographic determinations did not demonstrate any effect of p-chloro-o-toluidine upon the incorporation of [3H] thymidine into subcutaneous capillary endothelial cells. The results suggest that different reactive metabolites are responsible for binding to DNA and protein, and that the pattern of reactive metabolites formed from p-chloro-o-toluidine in the mouse differs from that formed in rats.     

Regulation of the components of the 150 kDa IGF binding protein complex in cocultures of rat hepatocytes and Kupffer cells by 3',5'-cyclic adenosine monophosphate

In the circulation, most of IGFs are bound to a high molecular mass complex of 150 kDa that consists of IGF-I (or IGF-II), IGFBP-3 and the acid-labile subunit (ALS). Within rat liver, biosynthesis of these components has been localized to different cell populations with hepatocytes as source of ALS and nonparenchymal cells (endothelial and Kupffer cells (KC)) as source of IGFBP-3. In the present study, the regulatory effects of the cAMP analogs dibutyryl-cAMP (db-cAMP) and 8-bromo-cAMP (8-br-cAMP) on IGF-I, ALS, and IGFBP expression were evaluated in primary cultures of rat hepatocytes, KC as well as in cocultures of hepatocytes and KC. In cocultures, biosynthesis of IGFBP-3 and ALS was inhibited dose-dependently by db-cAMP and 8-br-cAMP while that of IGF-I, IGFBP-1, and -4 was stimulated as demonstrated by ligand and Northern blotting. IGFBP-3 expression in primary cultures of pure KC did not respond to cAMP treatment indicating the importance of a cellular interaction between KC and hepatocytes for the decreased IGFBP-3 synthesis. The inhibition of IGFBP-3 in db-cAMP-treated cocultures was due to a decrease of IGFBP-3 mRNA level accompanied by a reduced cellular degradation of IGFBP-3. We conclude that cAMP stimulate the biosynthesis of IGF-I, IGFBP-1, and -4 in cocultures of hepatocytes and KC thereby enabling the formation of binary IGF/IGFBP complexes while the formation of the 150 kDa complex is impaired through downregulation of IGFBP-3 and ALS. This complex regulation may be a prerequisite for the effects of cAMP-dependent hormones on the transfer of IGFs from circulation to peripheral tissues.     

肝脏细胞在生物活性物质代谢调控中的协作作用

肝实质细胞和非实质细胞是肝脏功能的物质和结构基础.肝实质细胞是肝脏的主要细胞类型之一,承担和执行肝脏代谢、信号转导、解毒和稳态调节等多种功能.而非实质细胞也必不可少,主要包括星型细胞、肝窦内皮细胞、枯否细胞、自然杀伤细胞和隐窝细胞等,具有物质和信号转运、吞噬、抗原提呈、免疫耐受等功能.目前,有关肝脏生理功能和病理机制的研究主要集中在组织、细胞和差异分子的水平,单细胞的生理和病理功能研究也越来越深入,而肝脏细胞协作的研究比较少,系统性总结也鲜有报道.本文从肝脏细胞相互协作的角度,对肝脏的生理功能及病理机制进行探讨,为全面了解肝细胞生理功能及肝脏疾病的致病机理提供参考.     

Distribution and some properties of NADPH and NADH oxidase in parenchymal and nonparenchymal liver cells

The activities of NADPH and NADH oxidase were determined in homogenates of isolated pure parenchymal and nonparenchymal rat liver cells at neutral (7.4) and acid (5.5) pH. The NADPH oxidase at pH 7.4 is about equally active in parenchymal and nonparenchymal cells and in both cell types is rather insensitive to KCN (1 mm) inhibition. By lowering the pH to 5.5, the NADPH oxidase of the nonparenchymal cells is stimulated (twofold) while the activity in parenchymal cells is decreased. The NADH consumption at neutral pH in parenchymal cells is 75% inhibited by KCN, while this activity in nonparenchymal cells is relatively insensitive to KCN. The NADH oxidase in both parenchymal and nonparenchymal liver cells is less active when the pH is lowered from 7.4 to 5.5. The distribution of NAD(P)H oxidases between parenchymal and nonparenchymal liver cells and the effect of pH on their activities suggest that in the nonparenchymal cells, the NADPH oxidase might play a role in the synthesis of H₂O₂ within the phagocytic vacuole. A scheme is proposed which describes the metabolic events involved in H₂O₂ formation and catabolism of endo(phago)cytosed particles in nonparenchymal liver cells.     

Some kinetic properties of Mucor rouxii phosphofructokinase. Effect of cyclic adenosine 3', 5'-monophosphate.

Intact and pure parenchymal and nonparenchymal cells were isolated from rat liver. The activities of Superoxide dismutase in these cell types were determined by two different methods. With both methods the specific activity of this enzyme is 1.5 times higher in parenchymal than in nonparenchymal liver cells. It can be calculated that about 7% of the total rat liver Superoxide dismutase activity is localized in the nonparenchymal liver cells. Electrophoresis on polyacrylamide gels indicates that the isolated parenchymal cells contain both cytosolic and mitochondrial isoenzymes, whereas with nonparenchymal cells only the cytosolic enzyme could be detected. The mitochondrial band observed in isolated parenchymal cells is absent in the original total liver homogenate. This isoenzyme seems to be activated during the parenchymal cell isolation procedure. Isoelectrofocusing indicates that the cytosolic Superoxide dismutase consists in four different isoelectric forms in both parenchymal and nonparenchymal cells. With the mitochondrial isoenzyme two bands are obtained. The possibility that O₂^? is an important intermediate in H₂O₂ formation in nonparenchymal liver cells is discussed. In this respect, Superoxide dismutase might not only protect the cell against a toxic reagent as O₂^t-, but might also help to regulate the level of the important antimicrobial agent, H₂O₂.     

Specific targeting of high density lipoproteins to liver hepatocytes by incorporation of a tris-galactoside-terminated cholesterol derivative

A triantennary galactose-terminated cholesterol derivative, N-(tris(beta-D-galactopyranosyloxymethyl) methyl)-N alpha-(4(5-cholesten-3 beta-yloxy)succinyl)glycinamide (Tris-Gal-Chol), which dissolves easily in water, was added to human apolipoprotein E-free high density lipoproteins (HDL) in varying quantities. Incorporation of 5 or 13 micrograms of Tris-Gal-Chol into HDL (20 micrograms of protein) stimulates the liver association of the HDL apoprotein radioactivity 24- and 55-fold, respectively, at 10 min after intravenous injection into rats. The increased interaction of Tris-Gal-Chol HDL with the liver is blocked by preinjection of asialofetuin or N-acetylgalactosamine but not influenced by N-acetylglucosamine. The parenchymal liver cell uptake of HDL is stimulated 42- or 105-fold, respectively, by incorporation of 5 or 13 micrograms of Tris-Gal-Chol into HDL (20 micrograms of protein), while the association with nonparenchymal cells is stimulated only 1.7- or 5-fold. It can be calculated that 98.0% of the Tris-Gal-Chol HDL is associated with parenchymal cells. In contrast, incorporation of 13 micrograms of Tris-Gal-Chol into LDL (20 micrograms of protein) leads to a selective association of LDL with nonparenchymal cells (92.3% of the total liver uptake). It is concluded that Tris-Gal-Chol incorporation into HDL leads to a specific interaction of HDL with the asialoglycoprotein (galactose) receptor on parenchymal cells whereas Tris-Gal-Chol incorporation into LDL leads mainly to an interaction with a galactose receptor from Kupffer cells. Probably this highly selective cellular targeting of LDL and HDL by Tris-Gal-Chol is caused by the difference in size between these lipoproteins. The increased interaction of HDL with the parenchymal cells upon Tris-Gal-Chol incorporation is followed by degradation of the apolipoprotein in the lysosomes. It is concluded that Tris-Gal-Chol incorporation into LDL or HDL leads to a markedly increased catabolism of LDL by way of the Kupffer cells and HDL by parenchymal cells which might be used for lowering serum cholesterol levels. The use of Tris-Gal-Chol might also find application for targeting drugs or other compounds of interest to either Kupffer or parenchymal liver cells.     

Retinol and retinyl esters in parenchymal and nonparenchymal rat liver cell fractions after long-term administration of ethanol

Chronic ethanol consumption reduces the liver retinoid store in man and rat. We have studied the effect of ethanol on some aspects of retinoid metabolism in parenchymal and nonparenchymal liver cells. Rats fed 36% of total energy intake as ethanol for 5-6 weeks had the liver retinoid concentration reduced to about one-third, as compared to pair-fed controls. The reduction in liver retinoid affected both the parenchymal and the nonparenchymal cell fractions. Plasma retinol level was normal. Liver uptake of injected chylomicron [3H]retinyl ester was similar in the experimental and control group. The transport of retinoid from the parenchymal to the nonparenchymal cells was not found to be significantly retarded in the ethanol-fed rats. Despite the reduction in total retinoid level in liver, the concentrations of unesterified retinol and retinyl oleate were increased in the ethanol fed rats. Hepatic retinol esterification was not significantly affected in the ethanol-fed rats. Since our study has demonstrated that liver uptake of chylomicron retinyl ester is not impaired in the ethanol-fed rat, we suggest that liver retinoid metabolism may be increased.     

Effect of cell-cell interactions in preservation of cellular phenotype: cocultivation of hepatocytes and nonparenchymal cells.

Heterotypic cell interaction between parenchymal cells and nonparenchymal neighbors has been reported to modulate cell growth, migration, and/or differentiation. In both the developing and adult liver, cell-cell interactions are imperative for coordinated organ function. In vitro, cocultivation of hepatocytes and nonparenchymal cells has been used to preserve and modulate the hepatocyte phenotype. We summarize previous studies in this area as well as recent advances in microfabrication that have allowed for more precise control over cell-cell interactions through 'cellular patterning' or 'micropatterning'. Although the precise mechanisms by which nonparenchymal cells modulate the hepatocyte phenotype remain unelucidated, some new insights on the modes of cell signaling, the extent of cell-cell interaction, and the ratio of cell populations are noted. Proposed clinical applications of hepatocyte cocultures, typically extracorporeal bioartificial liver support systems, are reviewed in the context of these new findings. Continued advances in microfabrication and cell culture will allow further study of the role of cell communication in physiological and pathophysiological processes as well as in the development of functional tissue constructs for medical applications.     

Uptake of beta-hexosaminidase by nonparenchymal liver cells and peritoneal macrophages

The uptake of beta-hexosaminidase (EC 3.2.1.30) in nonparenchymal liver cells (i.e. endothelial and Kupffer's cells) and peritoneal macrophages has been determined by an enzymatic assay. A considerable uptake was noted in nonparenchymal liver cells, whereas no measurable uptake was seen in peritoneal macrophages. The endothelial cells were more active in the uptake of beta-hexosaminidase than were the Kupffer's cells. The uptake of beta-hexosaminidase by nonparenchymal liver cells showed saturation kinetics and was competitively inhibited by mannan. These findings support the concept that a cell surface receptor on nonparenchymal liver cells mediates uptake of beta-hexosaminidase and suggests a difference in the receptor mechanisms on liver and peritoneal macrophages.     

Liver takes up retinol-binding protein from plasma

Retinol is transported in plasma bound to a specific transport protein, retinol-binding protein. We prepared 125I-tyramine cellobiose-labeled rat retinol-binding protein and studied its tissue uptake 1, 5, and 24 h after intravenous injection into rats. The liver was the organ containing most radioactivity at all time points studied. After 5 and 24 h, 30 and 22% of the injected dose were recovered in liver, respectively. After separating the liver into parenchymal and nonparenchymal cells in the 5-h group, we found that both cell fractions contained approximately the same amount of radioactivity (per gram of liver). Most of the retinol-binding protein radioactivity in the nonparenchymal cell fraction was in the stellate cells. The implication of these results for a possible transfer mechanism for retinol between parenchymal and stellate cells is discussed.     

Receptor phenotype underlies differential response of hepatocytes and nonparenchymal cells to heparin-binding fibroblast growth factor type 1 (aFGF) and type 2 (bFGF)

Summary Heparin-binding fibroblast growth factors (HBGF) have been implicated in the regeneration of both parenchymal and nonparenchymal cells of the liver. The response to and phenotype of hepatocyte receptors for HBGF-1 (acidic fibroblast growth factor) and HBGF-2 (basic fibroblast growth factor) were compared to keratinocytes, fibroblasts, and endothelial cells. HBGF-1 stimulated DNA synthesis in hepatocytes, keratinocytes, fibroblasts, and endothelial cells whereas activity of HBGF-2 was limited to fibroblasts and endothelial cells. HBGF-2 antagonized the mitogenic activity of HBGF-1 for hepatocytes and keratinocytes. Hepatocytes and keratinocytes exhibited both high- and low-affinity, nonmatrix receptor sites for HBGF-1, but only low-affinity sites for HBGF-2. The mesenchymal cells displayed only high-affinity sites for both HBGF-1 and HBGF-2. Northern blot and immunochemical analysis revealed that the expression of HBGF receptor genesbek andflg are partitioned between normal hepatocytes and nonparenchymal cells, respectively. Expression of epithelial cell-specific, mesenchymal cell-derived HBGF-7 (keratinocyte growth factor) mRNA in regenerating liver tissue was undetectable relative to HBGF-1. The results support a multifunctional role of HBGF-1 acting through different receptor phenotypes in hepatocyte and nonparenchymal cells during liver regeneration.     

Uptake and 25-hydroxylation of vitamin D3 by isolated rat liver cells

The physiological roles played by hepatocytes and nonparenchymal cells of rat liver in the metabolism of vitamin D3 have been investigated. Tritium-labeled vitamin D3 dissolved in ethanol was administered intravenously to two rats. Isolation of the liver cells 30 and 70 min after the injection showed that vitamin D3 had been taken up both by the hepatocytes and by the nonparenchymal liver cells. The relative proportion of vitamin D3 that accumulated in the nonparenchymal cells increased with time. Perfusion of the isolated rat liver with [3H] vitamin D3 added to the perfusate confirmed the ability of both cell types to efficiently take up vitamin D3 from the circulation. By a method based on high pressure liquid chromatography and isotope dilution-mass fragmentography it was found that isolated liver cells in suspension had a considerable capacity to take up vitamin D3 from the medium. About 2.5 fmol of vitamin D3 were found to be associated with each hepatocyte or nonparenchymal cell after 1 h of incubation. 25-Hydroxylation in vitro was found to be carried out only by the hepatocytes. The rate of hydroxylation was about the same whether the cells were isolated from normal or rachitic rats (3.5 and 4 pmol of 25-hydroxyvitamin D3 formed per h per 10(6) cells, respectively). The possibility that the nonparenchymal cells might serve as a storage site for vitamin D3 in the liver is discussed.     

Altered distribution of hexokinase and glucokinase between parenchymal and nonparenchymal cells of rat liver after methapyrilene intoxication

The cell number as well as the hexokinase and glucokinase activity of liver parenchymal and nonparenchymal cells were studied in methapyrilene treated rats. The number of nonparenchymal cells was doubled after treatment with methapyrilene for two weeks while that of hepatocytes remained constant. The hexokinase activity was increased fourfold in the nonparenchymal cell fraction while it was unchanged in the parenchymal cells. The glucokinase activity was decreased in the hepatocytes to one third. Hence, the increased hexokinase activity was due to a proliferation of nonparenchymal cells rather than to a toxic dedifferentiation of hepatocytes.     

Incorporation of labelled amino acids into proteins of isolated parenchymal and nonparenchymal rat liver cells in the absence and presence of ethanol.

Parenchymal and nonparenchymal cells were isolated from perfused rat livers and incubated at 37 degrees C in the absence and presence of ethanol (50 mM). 1. Nonparenchymal cells prepared by means of centrifugation showed a higher rate of incorporation of L-[U-14C]valine into protein than nonparenchymal cells prepared by means of pronase. Cells prepared by the former method were used for further studies. 2. Protein degradation was present in suspensions of both parenchymal and nonparenchymal cells evidenced by increasing levels of branched amino acids in the intracellular and extracellular compartment during cell incubation. 3. The rate of cellular protein synthesis (corrected for precursor pool specific radioactivity) was of the same order of magnitude in nonparenchymal and parenchymal cells when expressed as nmol valine incorporated per mg protein. This rate was also close to the value found in intact liver by other workers. 4. Approximately 25% of the total radioactivity incorporated during incubation for 2 h was found in proteins released to the medium from parenchymal cells, while the corresponding figure for nonparenchymal cells was 3.5%. 5. Ethanol inhibited incorporation of labelled valine into stationary and medium proteins of parenchymal cells. No such effects were found in nonparenchymal cells. 6. Nonparenchymal cells did not metabolize ethanol while parenchymal cells did, shown by changes in lactate/pyruvate ratio and medium pH. It was concluded that nonparenchymal cells are capable of synthesizing proteins at a rate comparable to that found in parenchymal cells. Protein synthesis in parenchymal cells was inhibited by ethanol, but nonparenchymal protein synthesis was unaffected. This difference may be linked to the ability of the former cell type to metabolize ethanol.     

Differential abilities of mouse liver parenchymal and nonparenchymal cells in HDL and LDL (native and oxidized) association and cholesterol efflux.

The aim of this study was to quantify the abilities of mouse liver parenchymal and nonparenchymal cells with respect to (i) cholesteryl ester (CE) selective uptake from low-density lipoproteins (LDL), oxidized LDL (OxLDL), and high-density lipoprotein (HDL); and (ii) their free cholesterol efflux to HDL. The preparations of cells were incubated with lipoproteins labelled either in protein with iodine-125 or in CE with 3H-cholesterol oleate, and lipoprotein-protein and lipoprotein-CE associations were measured. The associations of LDL-protein and LDL-CE with nonparenchymal cells were 5- and 2-fold greater, respectively, than with parenchymal cells. However, in terms of CE-selective uptake (CE association minus protein association) both types of cell were equivalent. Similar results were obtained with OxLDL, but both types of cell showed higher abilities in OxLDL-CE than in LDL-CE selective uptake (on average by 3.4-fold). The association of HDL-protein with nonparenchymal cells was 3x that with parenchymal cells; however, nonparenchymal cells associated 45% less HDL-CE. Contrary to parenchymal cells, nonparenchymal cells did not show HDL-CE selective uptake activity. Thus parenchymal cells selectively take CE from the 3 types of lipoproteins, whereas nonparenchymal cells exert this function only on LDL and OxLDL. Efflux was 3.5-fold more important in nonparenchymal than in parenchymal cells.     

The effects of 1-amino-D-proline on the production of carnitine from exogenous protein-bound trimethyllysine by the perfused rat liver

Following receptor-mediated endocytosis of trimethyllysine-labeled asialofetuin and agalacto-orosomucoid by liver parenchymal and nonparenchymal cells, respectively, the glycoproteins are degraded and the methylated lysine residues released. The free intracellular trimethyllysine is then converted, in addition to 2-N-acetyl-6-N-trimethyllysine, to 4-N-trimethylaminobutyrate, carnitine, and acetylcarnitine. In the presence of 1-amino-D-proline, a vitamin B6 antagonist, the total production from protein-bound trimethyllysine of 4-N-trimethylaminobutyrate, the immediate precursor of carnitine, carnitine, and its acetylated derivative was depressed by as much as 60-80% in perfused rat liver. The decreased synthesis of carnitine was accompanied by an accumulation of 3-hydroxy-6-N-trimethyllysine, and intermediate in the carnitine biosynthetic pathway. The extent of 3-hydroxy-6-N-trimethyllysine accumulation, which was not evident in the absence of added 1-amino-D-proline, depended on the dose of 1-amino-D-proline perfused through the liver. In addition, those effects of 1-amino-D-proline were almost completely reversed by inclusion of pyridoxine in the perfusing medium. These results support the suggestion of a requirement for pyridoxal 5'-phosphate in the biosynthesis of carnitine by the liver.     

Cholecystokinin binding and degradation by isolated rat liver cells

The present studies were directed to examine and quantify binding and degradation of radiolabelled cholecystokinin (CCK) peptides by isolated rat liver cells. After incubation with liver cells (4.5 x 10(6) cells/ml) at 14 degrees C, minimal binding (less than 5%) of labelled CCK33 was detected. When labelled nonsulfated (nsCCK8) and sulfated CCK8 (sCCK8) were incubated, 16.2 +/- 1.8% (mean +/- S.E.) and 7.2 +/- 0.1% of 125I-nsCCK8 and 125I-sCCK8, respectively, were bound to the cell fraction. However, no inhibition of binding of either labelled nsCCK8 or sCCK8 was observed when incubated in the presence of excess unlabelled peptide (10 ng-10 micrograms). Preferential binding of labelled sCCK8, the biologically active form of the octapeptide, appeared to be to the nonparenchymal liver cell, rather than the hepatocyte, fraction; when corrected for cell size and protein content, binding of sCCK8 was approximately 15-times greater by the nonparenchymal cell population. When incubated with hepatocytes at 37 degrees C for 60 min, no degradation of labelled sCCK8 was detected by high pressure liquid chromatography. In contrast, progressive degradation of sCCK8 was observed when the peptide was incubated with the nonparenchymal cells. The results of these studies confirm previous observations that CCK33 is not bound by the liver. They further demonstrate that to some degree CCK8 is preferentially bound and degraded by hepatic nonparenchymal cells; however, this binding appears to be noncompetitive and, therefore, probably not receptor-mediated.     

Uptake of yeast invertase by rat liver cells in vivo and in vitro.

Yeast invertase injected intravenously in rats is rapidly taken up by the liver, reaching levels in that organ of 20% or more of the injected dose in about 12 h. At early time points, the bulk of the liver invertase appears in the sedimentable homogenates but, with time, there is a progressive increase in the fraction in the soluble phase, which remains at a constant proportion as the total hepatic invertase declines. The uptake of polyvinylpyrrolidone by the liver is much slower, as is its redistribution to the soluble fraction of homogenates. Separation of cell types from livers containing the markers revealed that the invertase was almost exclusively in the nonparenchymal cell population, while polyvinylpyrrolidone was distributed relatively indiscriminately between parenchymal and nonparenchymal cells. Measurements of uptake of invertase by liver cell preparations in vitro confirmed that nonparenchymal cells were much more active than parenchymal cells in this regard. Furthermore, the process was saturable with the former cell types and inhibitable by α-methylmannoside. Thus, it may be concluded that the uptake of invertase is via fluid pinocytosis in parenchymal cells and adsorptive pinocytosis in the nonparenchymal cells.