Intrahepatic distribution of small unilamellar liposomes as a function of liposomal lipid composition

We investigated the intrahepatic distribution of small unilamellar liposomes injected intravenously into rats at a dose of 0.10 mmol of lipid per kg body weight. Sonicated liposomes consisting of cholesterol/sphingomyelin (1:1), (A); cholesterol/egg phosphatidylcholine (1:1), (B); cholesterol/sphingomyelin/phosphatidylserine (5:4:1), (C) or cholesterol/egg-phosphatidylcholine/phosphatidylserine (5:4:1), (D) were labeled by encapsulation of [3H]inulin. The observed differences in rate of blood elimination and hepatic accumulation (A much less than B approximately equal to C less than D) confirmed earlier observations and reflected the rates of uptake of the four liposome formulations by isolated liver macrophages in monolayer culture. Fractionation of the liver into a parenchymal and a non-parenchymal cell fraction revealed that 80-90% of the slowly clearing type-A liposomes were taken up by the parenchymal cells while of the more rapidly eliminated type-B liposomes even more than 95% was associated with the parenchymal cells. Incorporation of phosphatidylserine into the sphingomyelin-based liposomes caused a significant increase in hepatocyte uptake but a much more substantial increase in non-parenchymal cell uptake, resulting in a major shift of the intrahepatic distribution towards the non-parenchymal cell fraction. For the phosphatidylcholine-based liposomes incorporation of phosphatidylserine did not increase the already high uptake by the parenchymal cells while uptake by the non-parenchymal cells was only moderately elevated; this resulted in only a small shift in distribution towards the non-parenchymal cells. The phosphatidylserine-induced increase in liposome uptake by non-parenchymal liver cells was paralleled by an increase in uptake by the spleen. Fractionation of the non-parenchymal liver cells in a Kupffer cell fraction and an endothelial cell fraction showed that even for the slowly eliminated liposomes of type A endothelial cells do not participate to a measurable extent in the elimination process, thus excluding involvement of fluid-phase pinocytosis in the uptake process.     

Different types of mitochondria in parenchymal and non-parenchymal rat-liver cells.

1. Intact and pure parenchymal and non-parenchymal cells were isolated from rat liver. The specific activities of several mitochondrial enzymes were determined in both parenchymal and non-parenchymal cell homogenates to characterize the mitochondria in these liver cell types. 2.In general the activities of mitochondrial enzymes were lower in non-parenchymal liver cells than in parenchymal cells. The specific activity of pyruvate carboxylase in non-parenchymal cells expressed as the percentage of that in parenchymal cells was onlu 2% for glutamate dehydrogenase 4.3% and for cytochrome c oxidase 79.4%. Monoamine oxidase, as an exception, has an equal specific activity in both cell types. 3. The activity ratio of pyruvate carboxylase at 10 mM pyruvate over 0.1 mM pyruvate is 3.35 for parenchymal cells and 1.50 for non-parenchymal cells. This indicates that non-parenchymal liver cells only contain the high affinity form of pyruvate carboxylase in contrast to parenchymal cells. 4. The ratio of glycerol-3-phosphate cytochrome c reductase over succinate cytochrome c reductase activity differs from parenchymal (0.01) and non-parenchymal cells (0.10). This might indicate that the glycerol-3-phosphate shuttle, which is important for the transport of reduction equivalents for cytosol to mitochondria is relatively more active in non-parenchymal cells than in parenchymal cells. 5. The activity pattern of mitochondrial enzymes in parenchymal and non-parenchymal cell homogenates indicates that these cell types contain different types of mitochondria. The presence of these different cell types in liver will therefore contribute to the heterogeneity of isolated rat liver mitochondria in which the mitochondria from non-parenchymal cells might be considered as "non-gluconeogenic".     

Identity and activities of lysosomal enzymes in parenchymal and non-parenchymal cells from rat liver.

1. Intact parenchymal and non-parenchymal cells were isolated from rat liver. The parenchymal cells were purified by differential centrifugation, while non-parenchymal cells were obtained free of parenchymal cell contamination by preferentially destroying the parenchymal cells with the aid of pronase (0.25%). 2. The ability to isolate pure intact parenchymal and non-parenchymal cells permitted the characterization and measurement of specific activities of various lysosomal enzymes, representing the main functional hydrolytic activities of the lysosomes in these distinct cell types. 3. Lysosomal enzymes catalysing the hydrolysis of the terminal carbohydrate moiety of glycoproteins and glycolipids were not particularly enriched in the non-parenchymal cells as compared to parenchymal cells. The ratio of the specific activities of non-parenchymal cells over parenchymal cells varied between 0.7 for N-acetyl-beta-D-hexoseaminidase to 2.1 for alpha-glucosidase. This suggests no specific role of the non-parenchymal cells in the hydrolysis of terminal carbohydrate moieties of glycoproteins and glycolipids. 4. The enzymes acid phosphatase and aryl sulphatase, representing the phosphate and sulphate hydrolyzing activities, were enriched in the non-paranchymal cells as compared to the parenchymal cells by a factor of 2.5. 5. The most important peptidase cathepsin D, representing protein breakdown capacity, is enriched in the non-parenchymal cells as compared to parenchymal cells by a factor 6.0, suggesting a possible specific function of non-parenchymal cells in protein breakdown. 6. The most enriched lysosomal enzyme, representing lipid hydrolysis, is acid lipase, which is enriched in the non-parenchymal cells with a factor of 10. 7. The distribution of lysosomal enzymes between parenchymal and non-parenchymal cells suggests different functional roles of the lysosomes in these cell types. It can be concluded that the non-parenchymal cells possess a set of lysosomal enzymes which makes them extremely suitable for a phagocytic and antimicrobial function in the liver.     

Relative toxicities of particulate and soluble forms of beryllium to a rat liver parenchymal cell line in culture and possible mechanisms of uptake.

The relative toxicities of particulate beryllium phosphate, soluble beryllium sulphate and a beryllium sulphosalicylate complex to a rat liver parencymal derived cell line have been examined in culture. Due to the propensity of beryllium salts to form beryllium phosphate in solution the incubation medium used was free of inorganic phosphate. Cell death measured by the loss of cellular lactate dehydrogenase into the medium can be produced within 76 h from beryllium phosphate and beryllium sulphosalicylate or 48 h from beryllium sulphate provided the cells have, irrespective of the form of added beryllium, taken up a minimum of 2--5 nmol Be/10(6) cells. Whilst beryllium phosphate was readily taken up as a particle, beryllium complexed with excess sulphosalicylate was not so markedly accumulated by the cells except possibly by formation of small amounts of beryllium phosphate in the medium as a result of inorganic phosphate lost from the cells. The extent of beryllium uptake from beryllium sulphate quantitatively most resembled that observed for beryllium phosphate but was largely independent of beryllium phosphate formation in the medium and not accompanied by the uptake of the SO42- anion. However, the accumulation of beryllium derived from beryllium sulphate did appear to be associated with the production of a sedimentable from believed most probably to be colloidal beryllium hydroxide. The uptake of all forms of beryllium was temperature sensitive and metabolic inhibitor studies and treatment of the cells with trypsin or neuraminidase supported the view that the distinct behaviour of beryllium derived from beryllium sulphate may be related to the enhanced toxicity of this form both under the conditions used and when administered to experimental animals.     

Difference spectra, catalase- and peroxidase activities of isolated parenchymal and non-parenchymal cells from rat liver

The reduced minus oxidized difference spectra from isolated parenchymal and non-parenchymal cells from rat liver indicate that the non-parenchymal cells contain a considerable amount of peroxidase. This interpretation is favoured by the more than 30 times higher specific activity of peroxidase (EC 1.11.1.7) in the non-parenchymal cells as compared to the parenchymal cells. The catalase (EC 1.11.1.6) activity in the non-parenchymal cells is 4 times lower than in the parenchymal cells. These results are consistent with an antimicrobial function of the non-parenchymal cells in liver.     

Identification of metallothionein in parenchymal and non-parenchymal liver cells of the adult rat.

Parenchymal and non-parenchymal cells were isolated from the livers of control, starved, Zn2+-injected and Cd2+-injected rats. Parenchymal cells were prepared by differential centrifugation after perfusion of the liver with collagenase. Non-parenchymal cells were separated from parenchymal cells by unit-gravity sedimentation and differential centrifugation. Yields of 2 x 10(8) non-parenchymal cells with greater than 95% viability and less than 0.2% contamination with parenchymal cells were obtained without exposing cells to Pronase. Metallothioneins-I and -II were identified in parenchymal cells and non-parenchymal cells from Zn2+-treated rats. The metallothionein contents of parenchymal cells, non-parenchymal cells and intact liver were quantified by a competitive 203Hg-binding assay. Administration of heavy-metal salts significantly increased the metallothionein content of both cell populations, although the concentration of the protein was approx. 2.5-fold greater in parenchymal cells than in non-parenchymal cells. Overnight starvation increased the metallothionein content of parenchymal cells without altering that of non-parenchymal cells. The potential significance of this differential response by different liver cell types with regard to the influence of Zn2+ on stress-mediated alterations in hepatic metabolism is discussed.     

Alteration in the capacities as well as in the zonal and cellular distributions of pyruvate kinase L and M2 in regenerating rat liver

Pyruvate kinase L (PKL), the glucoregulatory isoenzyme of adult parenchymal cells, and M2 (PKM2), the isoenzyme of proliferating and non-parenchymal cells, were measured, using a specific anti-PKL antibody for differentiation, in total liver homogenates, in isolated parenchymal and non-parenchymal cells as well as in microdissected periportal and perivenous liver tissue from regenerating rat liver after two-thirds partial hepatectomy. Moreover, the zonal distribution of PKL was studied using immunohistochemical techniques. In total liver homogenates PKL activity per g liver decreased after partial hepatectomy, while PKM2 increased. Total PKL activity per 100 g body weight was restored to preoperational levels much more slowly than liver weight. During liver regeneration parenchymal cells acquired high PKM2 besides PKL activity. The isoenzyme outfit of non-parenchymal cells remained unchanged. Microdissection studies showed that PKL lost its normal perivenous to periportal gradient after partial hepatectomy and became evenly distributed within the liver acinus. PKM2 did not retain its even distribution, it became predominant in the periportal zone. Immunohistochemical staining revealed that after partial hepatectomy PKL was present in all parenchymal cells in an atypical non-zonal heterogeneous distribution. Normal specific activities as well as zonal and cellular distributions of both pyruvate kinase isoenzymes were restored 14-21 d after partial hepatectomy. During regeneration after 2/3 partial hepatectomy the liver loses its glucostat function as corroborated in this study by the decrease of the glycolytic capacity via the glucoregulatory PKL; this change of function is accompanied by a loss of PKL-zonation. This finding corroborates the view that zonation of carbohydrate-metabolizing enzymes is required only when the liver functions as a glucostat. The increase of PKM2 and the appearance of a zonal PKM2 heterogeneity are in line with the pattern of hepatocyte proliferation after partial hepatectomy.     

High density lipoprotein and low density lipoprotein catabolism by human liver and parenchymal and non-parenchymal cells from rat liver.

The capacity of the homogenates from human liver, rat parenchymal cells, rat non-parenchymal cells and total rat liver for the breakdown of human and rat high density lipoprotein (HDL) and human low density lipoprotein (LDL) was determined. Human HDL was catabolized by human liver, in contrast to human LDL, the protein degradation of which was low or absent. Human and rat HDL were catabolized by both the rat parenchymal and non-parenchymal cell homogenates with, on protein base, a 10-times higher activity in the non-parenchymal liver cells. This implies that more than 50% of the total liver capacity for HDL protein degradation is localized in these cell types. Human LDL degradation in the rat could only be detected in the non-parenchymal cell homogenates. These findings are discussed in view of the function of HDL and LDL as carriers for cholesterol.     

Characteristics of acid lipase and acid cholesteryl esterase activity in parenchymal and non-parenchymal rat liver cells

(1) Parenchymal and non-parenchymal cells were isolated from rat liver. The characteristics of acid lipase activity with 4-methylumbelliferyl oleate as substrate and acid cholesteryl esterase activity with cholesteryl[1-14C]oleate as substrate were investigated. The substrates were incorporated in egg yolk lecithin vesicles and assays for total cell homogenates were developed, which were linear with the amount of protein and time. With 4-methylumbelliferyl oleate as substrate, both parenchymal and non-parechymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 2.5 times higher than for parenchymal cells. It is concluded that 4-methylumbelliferyl oleate hydrolysis is catalyzed by similar enzyme(s) in both cell types. (2) With cholesteryl[1-14C]oleate as substrate both parenchymal and non-parenchymal cells show maximal activities at acid pH and the maximal activity for non-parenchymal cells is 11.4 times higher than for parenchymal cells. It is further shown that the cholesteryl ester hydrolysis in both cell types show different properties. (3) The high activity and high affinity of acid cholesteryl esterase from non-parenchymal cells for cholesterol oleate hydrolysis as compared to parenchymal cells indicate a relative specialization of non-parenchymal cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells in cholesterol ester hydrolysis. It is concluded that non-parenchymal liver cells possess the enzymic equipment to hydrolyze very efficiently internalized cholesterol esters, which supports the suggestion that these cell types are an important site for lipoprotein catabolism in liver.     

Polymerized albumin receptor on rat liver cells.

The polymerized albumin hypothesis was proposed for the mechanism of a hepatitis B virus (HBV) infection of human liver parenchymal cells on the basis that a receptor for polymerized albumin treated with glutaraldehyde was detected on isolated human liver parenchymal cells. However, some controversy exists regarding this hypothesis, because a receptor for formaldehyde-treated bovine serum albumin (f-BSA) has been found on liver non-parenchymal cells. Therefore, we characterized the uptake of polymerized rat serum albumin (p-RSA) and f-BSA by rat liver in vivo, and their bindings to liver cells in vitro. Most p-RSA and f-BSA was taken up by the liver after intravenous administration, and the uptake of p-RSA was inhibited by a 1,000-fold excess of f-BSA. In addition, more than 80% of p-RSA taken up by the liver was found in the non-parenchymal cells, and the remainder was found in the parenchymal cells. P-RSA as well as f-BSA could bind to isolated rat liver parenchymal and non-parenchymal cells. Furthermore, p-RSA and f-BSA could bind to isolated rat liver cell plasma membranes, and these bindings were completely inhibited by 1,000-fold excess of either f-BSA or p-RSA. These results indicate that there is a receptor, which can recognize both p-RSA and f-BSA, on not only rat liver non-parenchymal cells but also the parenchymal cells. It is also indicated that the receptor on the parenchymal cells as well as the non-parenchymal cells is involved in the in vivo uptake of p-RSA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Isoenzyme patterns in parenchymal and non-parenchymal cells isolated from regenerating and regenerated rat liver

Parenchymal and non-parenchymal cells were isolated from adult rat liver that had been fully regenerated after a 70% partial hepatectomy. The characteristics of the parenchymal cell preparations from regenerated rat liver indicated that they were a homogeneous population and comparable with parenchymal cells isolated from intact liver. The parenchymal cells from regenerated adult rat liver contain glucokinase, hexokinase, pyruvate kinase type I and aldolase B. The non-parenchymal cells contain hexokinase, pyruvate kinase type III and aldolase B. When cells were isolated at different times of the day from rats on controlled feeding schedules, variation of tyrosine aminotransferase activity and liver glycogen content were observed in the parenchymal cells in keeping with the reported diurnal oscillations found in whole liver extracts. When parenchymal cells were isolated from rats 48 and 72h after partial hepatectomy, different isoenzyme patterns were observed. These cells appeared to synthesize pyruvate kinase type III, a function that was assigned previously to non-parenchymal cells or to foetal rat liver hepatocytes.     

Role of parenchymal and non-parenchymal rat liver cells in the uptake of cholesterolester-labeled serum lipoproteins.

Very low density lipoprotein (VLDL)-remnants, prepared by extrahepatic circulation of VLDL, labeled biosynthetically in the cholesterol (ester) moiety, were injected intravenously into rats in order to determine the relative contribution of parenchymal and non-parenchymal liver cells to the hepatic uptake of VLDL-remnant cholesterol (esters). 82.7% of the injected radioactivity is present in liver, measured 30 min after injection. The non-parenchymal liver cells contain 3.1±0.1 times the amount of radioactivity per mg cell protein as compared to parenchymal cells. The hepatic uptake of biosynthetically labeled (screened) low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterolesters amounts to 26.8% and 24.4% of the injected dose, measured 6 h after injection. The non-parenchymal cells contain 4.3±0.8 and 4.1±0.7 times the amount of radioactivity per mg cell protein as compared to parenchymal cells for LDL and HDL, respectively. It is concluded that in addition to parenchymal cells, the non-parenchymal cells play an important role in the hepatic uptake of cholesterolesters from VLDL-remnants, LDL and HDL.     

Distribution of gangliosides in parenchymal and non-parenchymal cells of rat liver

Parenchymal and non-parenchymal cells were isolated from rat liver with purities of more than 90%. Total and ganglioside sialic acid contents were higher in non-parenchymal cells than in parenchymal cells. Thin-layer chromatography of gangliosides showed that the main component in rat liver was ganglioside GM3 and that this was abundant in non-parenchymal cells. Parenchymal cells had ganglioside GD1b as the main component and less GM3 than non-parenchymal cells. These results suggested that the main ganglioside of rat liver, GM3, arises mainly from non-parenchymal cells.     

Kupffer Cell Function in Thyroid Hormone-Induced Liver Oxidative Stress in the Rat

The influence of thyroid hormone (L-3, 3', 5-triiodothyronine, T₃) on Kupffer cell function was studied in the isolated perfused rat liver by colloidal carbon infusion. Rates of carbon uptake were determined from the influent minus effluent concentration difference and the flow rate, and the respective carbon-induced respiratory activity was calculated by integration of the area under the O₂ curves during carbon infusion. In the concentration range of 0.2 to 2.0 mg of carbon/ml, livers from euthyroid rats exhibited a sigmoidal-type kinetics of carbon uptake, with a Vmax of 4.8 mg/g liver/min and a concentration of 0.82 mg/ml for half-maximal rate; carbon-induced O₂ uptake presented a hyperbolic-type kinetics, with a Vmax of 4.57 μmol of O₂/g liver and a Km of 0.74 mg of carbon/ml, which significantly correlates with the carbon uptake rates. Light-microscopy showed that carbon was taken up exclusively by non-parenchymal cells, predominantly by Kupffer cells. Thyroid calorigenesis was found in parallel with increased rates of hepatic O₂ consumption and thiobarbituric acid reactive substances (TBARS) formation, glutathione (GSH) depletion, and higher sinusoidal lactate dehydrogenase (LDH) efflux compared to control values. In the concentration range of 0.25 to 0.75 mg/ml, carbon infusion did not modify liver LDH efflux in control rats, while it was significantly enhanced in T₃-treated animals. In this latter group, higher carbon concentrations (1 and 1.3 mg/ml) led to loss of viability of the liver. At 0.25 to 0.75 mg of carbon/ml, both the rates of carbon uptake and the associated carbon-induced respiratory activities were significantly increased by T₃ treatment, effects that were abolished by pretreatment of the rats with gadolinium chloride (GdCl₃). In addition, GdCl₃ decreased by 50% the changes induced by T₃ in hepatic GSH content and TBARS formation. It is concluded that hyperthyroidism enhances Kupffer cell function, correlated with the increased number of liver macrophages observed histologically, which may represent an alternate source of reactive O₂ species to that induced in parenchymal cells, thus contributing to the enhanced oxidative stress status developed.     

Uptake of native and deglycosylated ricin A-chain immunotoxins by mouse liver parenchymal and non-parenchymal cells in vitro and in vivo

The therapeutic activity of ricin A-chain immunotoxins is undermined by their rapid clearance from the bloodstream of animals by the liver. This uptake has generally been attributed to recognition of the mannose-terminating oligosaccharides present on ricin A-chain by receptors present on the non-parenchymal (Kupffer and sinusoidal) cells of the liver. However, we demonstrate here that, in the mouse, the liver uptake of a ricin A-chain immunotoxin occurs in both parenchymal and non-parenchymal cells in equal amounts. This is in contrast to the situation in the rat, where uptake of the immunotoxin is predominantly by the non-parenchymal cells. Recognition of sugar residues on the A-chain portion of the immunotoxin plays an important role in the liver uptake by both cell types in both species. However it is not the only mechanism since, firstly, an immunotoxin containing ricin A-chain which had been effectively deglycosylated with metaperiodate and cyanoborohydride was still trapped to a significant extent by hepatic non-parenchymal cells after it was injected into mice. Secondly, deglycosylation, while eliminating uptake of the free A-chain by parenchymal and non-parenchymal cells in vitro, only reduced the uptake of an immunotoxin by either cell type by about half. Thirdly, the addition of excess D-mannose or L-fucose inhibited the uptake of free A-chain by mouse liver cell cultures by more than 80% but only inhibited the uptake of the native A-chain immunotoxin by about half and had little effect on the uptake of the deglycosylated ricin A-chain immunotoxin. Recognition of the antibody portion of the immunotoxin by liver cells seems improbable, since antibody alone or an antibody-bovine serum albumin conjugate were not taken up in appreciable amounts by the cultures. Possibly attachment of the A-chain to the antibody exposes sites on the A-chain that are recognised by liver cells in vitro and in vivo.     

In vivo characteristics of a specific recognition site for LDL on non-parenchymal rat liver cells which differs from the 17 alpha-ethinyl estradiol-induced LDL receptor on parenchymal liver cells

Chemical modification of lysine or arginine residues of apolipoprotein B-100 in human low-density lipoprotein (LDL) with respectively reductive methylation (Me-LDL) or cyclohexanedione treatment (CHD-LDL) was applied to determine the role of these amino acids in LDL recognition by the various liver cell types. The cell association of native human LDL, Me-LDL and CHD-LDL to parenchymal and non-parenchymal cells was determined in vivo by isolating the various cell types 30 min after intravenous injection of the lipoproteins. In order to prevent degradation or release of cell-bound apolipoproteins during cell dissociation and purification, a low-temperature (8 degrees C) liver perfusion and cell isolation procedure was performed. It was found that reductive methylation of LDL inhibits the association of LDL to both parenchymal and non-parenchymal cells, indicating that lysine residues are important for recognition of LDL by both these cell types. In contrast, cyclohexanedione treatment of LDL did not influence the cell association of LDL to non-parenchymal cells. 17 alpha-Ethinyl estradiol treatment selectively increases the cell association of LDL by parenchymal cells (16-fold), leaving the non-parenchymal cell association uninfluenced. The increased cell-association of LDL to parenchymal cells is almost completely blocked by cyclohexanedione treatment of LDL (by 81%) or by methylation of LDL (by 97%). These data indicate that the arginine residues in LDL are not important for the recognition of LDL by non-parenchymal cells, whereas for the cell association of LDL to the estrogen-stimulated binding site on parenchymal cells both arginine and lysine residues are essential. The in vivo cell association of CHD-LDL or native LDL to non-parenchymal cells was lowered to the level of Me-LDL by ethyl oleate treatment of the rats, while no effect of ethyl oleate on parenchymal cells was noticed. These data suggest that the specific site for LDL on non-parenchymal cells, which need lysine residues on LDL for recognition, can be down-regulated by ethyl oleate treatment. The LDL, internalized by non-parenchymal cells, is effectively degraded. This degradation occurs at least partly in the lysosomes. It is suggested that the unique recognition site for LDL on non-parenchymal cells may be quantitatively important for serum LDL catabolism.     

Isolation of parenchymal cell-derived particles from non-parenchymal rat liver cell preparations

Rat liver was perfused with collagenase and the non-parenchymal cells were isolated by means of differential centrifugation. Low magnification microscopical examination indicated that in this non-parenchymal cell fraction less than 1 % are parenchymal cells, whereas the observed pyruvate kinase kinetics indicated that 50% of the total amount of pyruvate kinase in this fraction is of parenchymal cell origin. The non-parenchymal cell fraction was further purified by metrizamide density cushion centrifugation followed by centrifugal elutriation. A fraction that consisted of small particles, diameter < 5 μm, was collected. The pyruvate kinase activity in this fraction showed characteristics of absolute L-type kinetics and further examination of these particles, called blebs, indicated that they were of parenchymal cell origin. Determination of enzyme markers with regard to the different subcellular structures indicated that the blebs, as compared with parenchymal cells, contained lower specific activities of enzyme markers for the endoplasmic reticulum, mitochondria and especially peroxisomes. Electron micrographs indicated the complete absence of nuclei. It is suggested that the pure isolated blebs form a unique test material to study the involvement of the nucleus and/or peroxisomes in metabolic processes. The identification of these blebs in the non-parenchymal cell preparations might also explain some discrepancies in the literature about the presence of certain metabolic processes in non-parenchymal cells.     

Endotoxin stimulation of liver parenchymal cell phosphofructokinase activity requires nonparenchymal cells.

The rate of carbohydrate flux through phosphofructokinase (measured as the rate of [3-3H]glucose detritiation) was increased fourfold in rat liver parenchymal cells incubated with conditioned medium from lipopolysaccharide-stimulated adherent liver non-parenchymal cells. The rate was not affected in parenchymal cells incubated either with lipopolysaccharide directly or with conditioned medium from non-stimulated non-parenchymal cells. The stimulation of carbohydrate flux through phosphofructokinase by conditioned medium was not duplicated by peptide cytokines known to be released by lipopolysaccharide-activated liver non-parenchymal cells (interleukin-1, interleukin-6, tumor necrosis factor-alpha, and transforming growth factor-beta) or platelet activating factor. Furthermore, formation of the active conditioned medium was not prevented by inclusion of cycloheximide or dexamethasone to inhibit cytokine synthesis, or indomethacin or BW755c to inhibit arachidonic acid metabolism, during lipopolysaccharide-stimulation of the non-parenchymal cells. The results indicate that intercellular communication between lipopolysaccharide-stimulated liver non-parenchymal cells and parenchymal cells by soluble mediators is responsible for the stimulation of liver phosphofructokinase activity during endotoxin-induced shock. Studies to isolate and identify the factor(s) in the conditioned medium are currently in progress.     

Regulation of liver lipase. I. Evidence for several regulatory sites, studied in corticotrophin-treated rats

The activity of liver lipase, an enzyme that can be released from the liver by heparin, varies under several hormonal conditions. The site(s) at which regulation of the enzyme activity may occur was investigated in vitro. As a model, rats were used which had been treated with a corticotrophin analogue, to induce hypercortisolism, a condition in which liver lipase activity is lowered. Lipases isolated from heparin-containing perfusates of livers from ACTH or control rats were identical with respect to heat stability and specific activity as determined by immunotitration and binding to isolated non-parenchymal liver cells, indicating that the enzyme structure was not affected by the treatment. The secretion of liver lipase by isolated parenchymal liver cells was studied. During incubation of parenchymal cells derived from ACTH rats, less enzyme activity was found to be secreted when compared with hepatocytes isolated from control rats (ACTH rats, 2.30 +/- 0.2 mU/10(6) cells; control rats, 3.3 +/- 0.3 mU/10(6) cells). Liver lipase partially purified from control rats could be bound specifically to saturation by non-parenchymal cells, isolated from ACTH or control rats. Non-parenchymal cells from ACTH rats bound less lipase activity (29 mU/mg cell protein) than cells from control rats (50 mU/mg cell protein). This reduction in binding capacity seems to be due to a diminished number of binding sites, since the affinity based on Scatchard analysis and half-maximal binding was not different. These results suggest that the lowered liver lipase activity found during hypercortisolism may be due to an impaired synthesis and/or secretion of the enzyme by the parenchymal cells and to a reduced binding capacity of the non-parenchymal cells for liver lipase.     

The viability of cells grown or centrifuged in a new density gradient medium, Percoll(TM)

A new density gradient medium, Percoll (a modified colloidal silica), has been tested for toxicity in primary cultures of rat liver and calf testicle cells, and in continuous cultures of pig kidney and HeLa cells. The presence of Percoll did not appreciably affect the growth or viability of the cells as judged from cell counts and morphology. The various cells were also centrifugea in gradients of Percoll and subsequently cultured. The in vitro growth of the cells was similar to that of untreated cells. Rat liver cells were labelled in vivo with [¹²⁵I]asialoceruloplasmin (parenchymal cells) or heat-denatured [¹²⁵I]albumin (non-parenchymal cells). After dispersion of the cells and iso-pycnic centrifugation in Percoll the non-parenchymal cells banded preferentially at a lower density (1.04−1.05 g/ml) than parenchymal cells (1.07−1.09 g/ml). The two types of cells showed very different morphology in cell culture. The non-parenchymal cells retained their phagocytic properties during culture. Injured cells and cell debris band at the top of the Percoll gradients in contrast to their behaviour in gradients containing low molecular weight substances. Centrifugation in Percoll can be used to enrich viable cells.