Hepatocyte heterogeneity in response to icosanoids. The perivenous scavenger cell hypothesis

ADSCs-derived exosomes ameliorate hepatic fibrosis by suppressing stellate cell activation and remodeling hepatocellular glutamine synthetase-mediated glutamine and ammonia homeostasis

Background

Hepatic fibrosis is a common pathologic stage in chronic liver disease development, which might ultimately lead to liver cirrhosis. Accumulating evidence suggests that adipose-derived stromal cells (ADSCs)-based therapies show excellent therapeutic potential in liver injury disease owing to its superior properties, including tissue repair ability and immunomodulation effect. However, cell-based therapy still limits to several problems, such as engraftment efficiency and immunoreaction, which impede the ADSCs-based therapeutics development. So, ADSCs-derived extracellular vesicles (EVs), especially for exosomes (ADSC-EXO), emerge as a promise cell-free therapeutics to ameliorate liver fibrosis. The effect and underlying mechanisms of ADSC-EXO in liver fibrosis remains blurred.

Methods

Hepatic fibrosis murine model was established by intraperitoneal sequential injecting the diethylnitrosamine (DEN) for two weeks and then carbon tetrachloride (CCl₄) for six weeks. Subsequently, hepatic fibrosis mice were administrated with ADSC-EXO (10 μg/g) or PBS through tail vein infusion for three times in two weeks. To evaluate the anti-fibrotic capacity of ADSC-EXO, we detected liver morphology by histopathological examination, ECM deposition by serology test and Sirius Red staining, profibrogenic markers by qRT-PCR assay. LX-2 cells treated with TGF-β (10 ng/ml) for 12 h were conducted for evaluating ADSC-EXO effect on activated hepatic stellate cells (HSCs). RNA-seq was performed for further analysis of the underlying regulatory mechanisms of ADSC-EXO in liver fibrosis.

Results

In this study, we obtained isolated ADSCs, collected and separated ADSCs-derived exosomes. We found that ADSC-EXO treatment could efficiently ameliorate DEN/CCl₄-induced hepatic fibrosis by improving mice liver function and lessening hepatic ECM deposition. Moreover, ADSC-EXO intervention could reverse profibrogenic phenotypes both in vivo and in vitro, including HSCs activation depressed and profibrogenic markers inhibition. Additionally, RNA-seq analysis further determined that decreased glutamine synthetase (Glul) of perivenous hepatocytes in hepatic fibrosis mice could be dramatically up-regulated by ADSC-EXO treatment; meanwhile, glutamine and ammonia metabolism-associated key enzyme OAT was up-regulated and GLS2 was down-regulated by ADSC-EXO treatment in mice liver. In addition, glutamine synthetase inhibitor would erase ADSC-EXO therapeutic effect on hepatic fibrosis.

Conclusions

These findings demonstrated that ADSC-derived exosomes could efficiently alleviate hepatic fibrosis by suppressing HSCs activation and remodeling glutamine and ammonia metabolism mediated by hepatocellular glutamine synthetase, which might be a novel and promising anti-fibrotic therapeutics for hepatic fibrosis disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03049-x.

Glutamine synthetase as a central element in hepatic glutamine and ammonia metabolism: novel aspects

Glutamine synthetase (GS) in the liver is expressed in a small perivenous, highly specialized hepatocyte population and is essential for the maintenance of low, non-toxic ammonia levels in the organism. However, GS activity can be impaired by tyrosine nitration of the enzyme in response to oxidative/nitrosative stress in a pH-sensitive way. The underlying molecular mechanism as investigated by combined molecular simulations and in vitro experiments indicates that tyrosine nitration can lead to a fully reversible and pH-sensitive regulation of protein function. This approach was also used to understand the functional consequences of several recently described point mutations of human GS with clinical relevance and to suggest an approach to restore impaired GS activity.

Characterization of the scavenger cell proteome in mouse and rat liver

The structural-functional organization of ammonia and glutamine metabolism in the liver acinus involves highly specialized hepatocyte subpopulations like glutamine synthetase (GS) expressing perivenous hepatocytes (scavenger cells). However, this cell population has not yet been characterized extensively regarding expression of other genes and potential subpopulations. This was investigated in the present study by proteome profiling of periportal GS-negative and perivenous GS-expressing hepatocytes from mouse and rat. Apart from established markers of GS+ hepatocytes such as glutamate/aspartate transporter II (GLT1) or ammonium transporter Rh type B (RhBG), we identified novel scavenger cell-specific proteins like basal transcription factor 3 (BTF3) and heat-shock protein 25 (HSP25). Interestingly, BTF3 and HSP25 were heterogeneously distributed among GS+ hepatocytes in mouse liver slices. Feeding experiments showed that RhBG expression was increased in livers from mice fed with high protein diet compared to standard chow. While spatial distributions of GS and carbamoylphosphate synthetase 1 (CPS1) were unaffected, periportal areas constituted by glutaminase 2 (GLS2)-positive hepatocytes were enlarged or reduced in response to high or low protein diet, respectively. The data suggest that the population of perivenous GS+ scavenger cells is heterogeneous and not uniform as previously suggested which may reflect a functional heterogeneity, possibly relevant for liver regeneration.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Stoichiometry based steady-state hepatic flux analysis: computational and experimental aspects

: The liver has many complex physiological functions, including lipid, protein and carbohydrate metabolism, as well as bile and urea production. It detoxifies toxic substances and medicinal products. It also plays a key role in the onset and maintenance of abnormal metabolic patterns associated with various disease states, such as burns, infections and major traumas. Liver cells have been commonly used in in vitro experiments to elucidate the toxic effects of drugs and metabolic changes caused by aberrant metabolic conditions, and to improve the functions of existing systems, such as bioartificial liver. More recently, isolated liver perfusion systems have been increasingly used to characterize intrinsic metabolic changes in the liver caused by various perturbations, including systemic injury, hepatotoxin exposure and warm ischemia. Metabolic engineering tools have been widely applied to these systems to identify metabolic flux distributions using metabolic flux analysis or flux balance analysis and to characterize the topology of the networks using metabolic pathway analysis. In this context, hepatic metabolic models, together with experimental methodologies where hepatocytes or perfused livers are mainly investigated, are described in detail in this review. The challenges and opportunities are also discussed extensively.

Kupffer cell activation in normal and fibrotic livers increases portal pressure via thromboxane A(2)

BACKGROUND/AIMS

Cirrhotic patients show an increased risk of variceal bleeding upon bacterial infections. Kupffer cells (KC) constitute the first macrophage population to become activated by bacterial beta-glucans and endotoxins derived from the gut. We therefore investigated whether and how KC activation increases portal pressure.

METHODS

KC in normal and fibrotic livers from bile duct ligated (BDL) rats were activated by the beta-glucan component of zymosan in vivo and during isolated rat liver perfusion.

RESULTS

Activation of KC in normal livers resulted in a severalfold increase of portal pressure in vivo as well as in isolated perfused liver preparations. This increase and the accompanying 40-fold stimulation of hepatic prostaglandin F(2alpha)/D(2) and thromboxane A(2) (TxA(2)) production in isolated perfused livers were attenuated by KC blockade. The TxA(2) synthase inhibitor furegrelate and the TxA(2) receptor antagonist BM 13.177 reduced the increase of portal perfusion pressure supporting TxA(2) as pivotal vasoconstrictor released by activated KC. Importantly, a more pronounced vasopressor response in fibrotic livers was related to a raise in KC density and a 10-fold increase of TxA(2) production after KC activation.

CONCLUSIONS

KC activated by beta-glucans increase portal pressure through the release of TxA(2). This vasopressor response is augmented in BDL induced fibrosis.

Control of hepatocyte metabolism by sympathetic and parasympathetic hepatic nerves

More than any other organ, the liver contributes to maintaining metabolic equilibrium of the body, most importantly of glucose homeostasis. It can store or release large quantities of glucose according to changing demands. This homeostasis is controlled by circulating hormones and direct innervation of the liver by autonomous hepatic nerves. Sympathetic hepatic nerves can increase hepatic glucose output; they appear, however, to contribute little to the stimulation of hepatic glucose output under physiological conditions. Parasympathetic hepatic nerves potentiate the insulin-dependent hepatic glucose extraction when a portal glucose sensor detects prandial glucose delivery from the gut. In addition, they might coordinate the hepatic and extrahepatic glucose utilization to prevent hypoglycemia and, at the same time, warrant efficient disposal of excess glucose.

Enhanced expression of basolateral multidrug resistance protein isoforms Mrp3 and Mrp5 in rat liver by LPS

Lipopolysaccharide (LPS) induces hepatocellular down-regulation and endocytic retrieval of multidrug resistance protein 2 (Mrp2, Abcc2). Basolateral Mrp isoforms may compensate for the intracellular metabolic changes in cholestasis. Therefore, the effect of LPS on the zonal localization of Mrp2 and Mrp3 and the expression of Mrp3, Mrp4, Mrp5, and Mrp6 mRNA were investigated in rat liver. In normal rat liver Mrp3 was found in pericentral hepatocytes also expressing glutamine synthetase. In LPS-treated rat liver the decrease in Mrp2 protein was most pronounced in pericentral hepatocytes, with only minor down-regulation in periportal hepatocytes. Conversely, induction of Mrp3 was found in pericentral hepatocytes with a low expression of Mrp2. Furthermore, we found a strong induction of Mrp5 mRNA. Likewise, Mrp6 mRNA was up-regulated, however Mrp6 protein expression was not significantly altered. It is concluded that Mrp3 is inversely regulated to Mrp2 in a zonal pattern and may compensate for the LPS-induced loss of Mrp2 in the perivenous area. Induction of pericentral Mrp3 and up-regulation of Mrp5 mRNA may play an important role in the hepatocellular clearance of cholephilic substances and cyclic nucleotides accumulating after LPS treatment.

Hepatic glutamine transport and metabolism

Although the liver was long known to play a major role in the uptake, synthesis, and disposition of glutamine, metabolite balance studies across the whole liver yielded apparently contradictory findings suggesting that little or no net turnover of glutamine occurred in this organ. Efforts to understand the unique regulatory properties of hepatic glutaminase culminated in the conceptual reformulation of the pathway for glutamine synthesis and turnover, especially as regards the role of sub-acinar distribution of glutamine synthetase and glutaminase. This chapter describes these processes as well as the role of glutamine in hepatocellular hydration, a process that is the consequence of cumulative, osmotically active uptake of glutamine into cells. This topic is also examined in terms of the effects of cell swelling on the selective stimulation or inhibition of other far-ranging cellular processes. The pathophysiology of the intercellular glutamine cycle in cirrhosis is also considered.

Zonation of hepatic cytochrome P-450 expression and regulation

The CYP genes encode enzymes of the cytochrome P-450 superfamily. Cytochrome P-450 (CYP) enzymes are expressed mainly in the liver and are active in mono-oxygenation and hydroxylation of various xenobiotics, including drugs and alcohols, as well as that of endogenous compounds such as steroids, bile acids, prostaglandins, leukotrienes and biogenic amines. In the liver the CYP enzymes are constitutively expressed and commonly also induced by chemicals in a characteristic zonated pattern with high expression prevailing in the downstream perivenous region. In the present review we summarize recent studies, mainly based on rat liver, on the factors regulating this position-dependent expression and induction. Pituitary-dependent signals mediated by growth hormone and thyroid hormone seem to selectively down-regulate the upstream periportal expression of certain CYP forms. It is at present unknown to what extent other hormones that also affect total hepatic CYP activities, i.e. insulin, glucagon, glucocorticoids and gonadal hormones, act zone-specifically. The expression and induction of CYP enzymes in the perivenous region probably have important toxicological implications, since many CYP-activated chemicals cause cell injury primarily in this region of the liver.

Inhibition by the protein kinase C activator 4 beta-phorbol 12-myristate 13-acetate of the prostaglandin F2 alpha-mediated and noradrenaline-mediated but not glucagon-mediated activation of glycogenolysis in rat liver

In perfused rat livers, infusion of prostaglandin F2 alpha (PGF2 alpha) or noradrenaline increased glucose and lactate output and reduced flow. Glucagon increased glucose output and decreased lactate output without influence on flow. Infusion of phorbol 13-myristate 14-acetate (PMA) for 20 min prior to these stimuli strongly inhibited the metabolic and hemodynamic effects of noradrenaline, reduced the metabolic actions of PGF2 alpha but did not alter the effects of glucagon. In isolated rat hepatocytes PGF2 alpha, noradrenaline and glucagon activated glycogen phosphorylase but only PGF2 alpha and noradrenaline increased intracellular inositol 1,4,5-trisphosphate (InsP3). The noradrenaline- or PGF2 alpha-elicited activation of glycogen phosphorylase and increase in InsP3 were largely reduced after preincubation of the cells for 10 min with PMA, whereas the glucagon-mediated enzyme activation was not affected. In contrast to PMA, the phorbol ester 4 alpha-phorbol 13,14-didecanoate, which does not activate protein kinase C, did not attenuate the PGF2 alpha- and noradrenaline-elicited stimulation of glucose output, glycogen phosphorylase and InsP3 formation. Stimulation of InsP3 formation by AlF4-, which activates phospholipase C independently of the receptor, was not attenuated by prior incubation with PMA. Plasma membranes purified from isolated hepatocytes had both a high-capacity, low-affinity and a low-capacity, high-affinity binding site for PGF2 alpha. The Kd of the high-capacity, low-affinity binding site was close to the concentration of PGF2 alpha that increased glycogen phosphorylase activity half-maximally. Binding to the high-capacity, low-affinity binding site was enhanced by guanosine 5'-O-(3-thio)triphosphate (GTP[S]). This high-capacity, low-affinity site might thus represent the receptor. The Bmax and Kd of the high-capacity site, as well as the enhancement by GTP[S] of PGF2 alpha binding to this site, remained unaffected by PMA treatment. It is concluded that, in hepatocytes, activation of protein kinase C by PMA interrupted the InsP3-mediated signal pathway from PGF2 alpha via a PGF2 alpha receptor and phospholipase C to glycogen phosphorylase at a point distal of the receptor prior to phospholipase C.

Hepatocyte heterogeneity in response to extracellular adenosine

Metabolic and haemodynamic effects of adenosine were studied in antegrade and retrograde rat liver perfusions with influent nucleoside concentrations either below (i.e. 20 microM) or exceeding (i.e. 200-300 microM) the single-pass clearance capacity of the liver. Adenosine (20 microM) increased in antegrade perfusions the perfusion pressure and markedly stimulated prostaglandin D2, thromboxane B2 and glucose output, whereas in retrograde perfusions no pressure and eicosanoid response occurred and glucose output was stimulated only slightly. The perfusion-direction-dependent differences in the glucose and pressure response to adenosine (20 microM) were fully abolished in presence of ibuprofen (50 microM). When the adenosine concentration in influent was raised to 200-300 microM, i.e. to a concentration exceeding single-pass clearance of the nucleoside, the adenosine-induced prostaglandin D2 release was about 10-fold higher in retrograde perfusions than in antegrade perfusions. On the other hand, both adenosine (20-300 microM)-induced cyclic AMP (cAMP) and K+ release from the liver were not affected by the direction of perfusion, and maximal effects on cAMP release were observed at influent adenosine concentrations of 100 microM. The basal rate (adenosine absent) of prostaglandin D2 and thromboxane B2 release was about 10-fold higher in retrograde than in antegrade perfusion experiments, whereas the basal cAMP release from the liver was not affected by the direction of perfusion. Maximal adenosine-stimulated glucose output was significantly higher in antegrade than in retrograde perfusions at all adenosine concentrations tested (range 10-300 microM). Ibuprofen abolished this difference, indicating that eicosanoids liberated under the influence of adenosine contribute to the glycogenolytic response in antegrade, but not in retrograde, perfusion. Desensitization occurred following repetitive adenosine infusion; this was more pronounced for adenosine-induced prostaglandin release than for cAMP or K+ efflux. The data suggest the following. (i) Both cAMP and eicosanoids are involved in the stimulation of glycogenolysis by adenosine. (ii) Eicosanoids are probably liberated under the influence of extracellular adenosine from a portal pre-sinusoidal compartment and accordingly stimulate glycogenolysis only in antegrade perfusions. Thus signals derived from portal vein structures can modulate hepatocellular function. (iii) Contractile elements are probably located also inside the liver acinus. (iv) Eicosanoids released into the hepatic vein reflect less than 10% of hepatic eicosanoid formation, because of marked clearance by perivenous hepatocytes.

Hydroperoxide metabolism in rat liver. K+ channel activation, cell volume changes and eicosanoid formation

Addition of t-butylhydroperoxide (0.2 mM) to isolated perfused rat liver led to a net K+ release of 7.2 +/- 0.2 mumol/g within 8 min and a net K+ reuptake of 6.6 +/- 0.4 mumol/g following withdrawal of the hydroperoxide, in line with earlier findings by Sies et al. [Sies, H., Gerstenecker, C., Summer, K. H., Menzel, H. & Flohé, R. (1974) in Glutathione (Flohé, L., Benöhr, C., Sies, H., Waller, H. D., eds) pp. 261-276, G. Thieme Publ. Stuttgart]. Net K+ release roughly paralleled the amount of GSSG released from the liver under the influence of the hydroperoxide. The t-butylhydroperoxide-induced K+ efflux was inhibited by approximately 70% in the presence of Ba2+ (1 mM), by 30% in Ca(2+)-free perfusions and was decreased by 50-60% when the intracellular Ca2+ stores were simultaneously depleted by repeated additions of phenylephrine. t-Butylhydroperoxide-induced K+ efflux was accompanied by a decrease of the intracellular water space by 58 +/- 14 microliter/g (n = 4), corresponding to a 10% cell shrinkage. The effect of t-butylhydroperoxide on cell volume was inhibited by 70-80% in the presence of Ba2+. In isolated rat hepatocytes treatment with t-butylhydroperoxide led to a slight hyperpolarization of the membrane at concentrations of 100 nM, but marked hyperpolarization occurred at t-butylhydroperoxide concentrations above 10 microM. t-Butylhydroperoxide (0.2 mM) transiently increased the portal-perfusion pressure by 3.3 +/- 0.6 cm H2O (n = 18), due to a slight stimulation of prostaglandin-D2 release under the influence of the hydroperoxide. In the presence of Ba2+ (1 mM), t-butylhydroperoxide increased the perfusion pressure by 12.7 +/- 1.2 cm H2O (n = 9) and produced an approximately tenfold increase of prostaglandin-D2 and thromboxane-B2 release. Under these conditions, glucose output from the liver rose from 0.9 +/- 0.03 to 2.9 +/- 0.7 mumol.g-1.min-1 (n = 4) with a time course roughly resembling that of portal-pressure increase and prostaglandin-D2 overflow. These effects were largely abolished in the presence of ibuprofen or the thromboxane-receptor-antagonist BM 13.177. The t-butylhydroperoxide effects on perfusion pressure, glucose and eicosanoid output were also enhanced in the presence of insulin or during hypotonic exposure; i.e. conditions known to swell hepatocytes, but not during hyperosmotic exposure. The data suggest that t-butylhydroperoxide induces liver-cell shrinkage and hyperpolarization of the plasma membrane due to activation of Ba(2+)-sensitive K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Different preparations of zymosan induce glycogenolysis independently in the perfused rat liver. Involvement of mannose receptors, peptide-leukotrienes and prostaglandins

Zymosan (non-boiled) induced glycogenolysis biphasically, with no lag time, in the perfused rat liver. After the zymosan was boiled, it could be separated into two fractions, both of which stimulated glycogenolysis independently. The soluble fraction of boiled zymosan (zymosan sup) showed homologous desensitization, indicating that zymosan sup-induced glycogenolysis is a receptor-mediated event. Mannan (polymannose), which is known to be a biologically active component of zymosan, induced a glycogenolytic response similar to that produced by zymosan sup, and desensitized the response to the latter. Preinfusion of platelet-activating factor (PAF, 20 nM) or isoprenaline (10 microM) did not extinguish the glycogenolytic response to zymosan sup, while the response to a secondary infusion of PAF was blocked. The glycogenolytic response to zymosan sup was completely inhibited by nordihydroguaiaretic acid (NDGA, 10 microM), a lipoxygenase inhibitor, and by ONO-1078 (100 ng/ml), a leukotriene (LT) D4 receptor antagonist. On the other hand, the glycogenolytic effect of zymosan pellet (the particulate fraction of boiled zymosan) was not affected by preinfusion of zymosan sup, and was inhibited by ibuprofen (20 microM), a cyclo-oxygenase inhibitor. Prostaglandins (PGs) detected in the perfusate were augmented with infusion of zymosan pellet. Opsonization of the zymosan pellet by serum (complement) enhanced the glycogenolytic response without a lag period, and with a concomitant enhancement of PG output. Correlations between glucose production and PGs were r = 0.832 (PGD2), r = 0.872 (PGF2 alpha), r = 0.752 (PGE2) and r = 0.349 (6-oxo-PGF1 alpha). The glycogenolytic response to non-boiled zymosan was delayed and the biphasic glycogenolytic response was not observed when mannan was infused first. NDGA mimicked the effects of the preinfusion of mannan, while ibuprofen had no effect on the non-boiled-zymosan-induced glycogenolysis. These results suggest: (1) that non-boiled zymosan stimulates glycogenolysis through a mannose receptor-dependent, but unidentified, pathway, (2) that zymosan sup induces glycogenolysis via mannose receptor activation through the production of peptide-LTs but not PAF, and (3) that zymosan pellet causes glycogenolysis through the production of prostanoids, which is enhanced in the presence of complement.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Modulation of maximal glycogenolysis in perfused rat liver by adenosine and ATP

Rat livers perfused at constant flow via the portal vein with dibutyryl cyclic AMP produced glucose equivalents at a steady maximal rate (6 mumol/min per g of liver). Addition of adenosine (150 microM) caused a biphasic effect. (i) First, the glycogenolytic rate rose transiently, to a mean peak of 150% of control levels after 2 min. This glycogenolytic burst was reproduced by two P1-receptor agonists, but not by ATP, and was blocked by a P1-antagonist (8-phenyltheophylline), as well as by inhibitors of eicosanoid synthesis (indomethacin, ibuprofen or aspirin). It did not occur in phosphorylase-kinase-deficient livers. The adenosine-induced glycogenolytic burst coincided with moderate and transient changes in portal pressure (+6 cmH2O) and O2 consumption (-20%), but it could not be explained by an increase in cytosolic Pi, since the n.m.r. signal fell precipitously. (ii) Subsequently, the rate of glycogenolysis decreased to one-third of the preadenosine value, in spite of persistent maximal activation of phosphorylase. The decrease could be linked to the decline in cytosolic Pi: both changes were prevented by the adenosine kinase inhibitor 5-iodotubercidin, whereas they were not affected by ibuprofen or 8-phenyltheophylline, and were not reproduced by non-metabolized adenosine analogues. In comparison with adenosine, ATP caused a slower decrease of Pi and of glycogenolysis. The fate of the cytosolic Pi was unclear, especially with administered ATP, which did not increase the n.m.r.-detectable intracellular ATP.

Role of eicosanoids, inositol phosphates and extracellular Ca2+ in cell-volume regulation of rat liver

1. In isolated perfused rat liver, the time-course of volume-regulatory K+ efflux following exposure to hypoosmolar perfusate resembled the leukotriene-C4-induced K+ efflux in normotonic perfusion. Omission of Ca2+ from the perfusion fluid had no effect on volume-regulatory K+ efflux, but abolished completely the leukotriene-C4-induced K+ efflux. 2. Volume-regulatory K+ fluxes following hypoosmolar exposure (225 mOsmol l-1) and subsequent reexposure to normotonic media (305 mOsmol l-1) were not significantly affected by the cyclooxygenase inhibitors indomethacin (5 mumol l-1) or ibuprofen (50 mumol l-1), the leukotriene D4/C4-receptor antagonist 1-[2-hydroxy-3-propyl-4-[4-(1H-tetrazol-5-yl)butoxy]phenyl]etha none (YL 171883, 50 microM), the lipoxygenase inhibitor nordihydroguaiaretic acid (20 microM), the phospholipase-A2 inhibitor bromophenacyl bromide (50 microM) or the thromboxane-receptor antagonist 4-[2-(benzenesulfonamido)ethyl]-phenoxyacetic acid (BM 13.177, 20 microM). Also the effects of hypoosmotic cell swelling on lactate, pyruvate and glucose balance across the liver remained largely unaffected in presence of these inhibitors. Neither exposure of perfused rat liver to hypoosmolar (225 mOsmol l-1) nor to hyperosmolar (385 mOsmol l-1) perfusion media affected hepatic prostaglandin-D2 release. 3. When livers were 3H-labeled in vivo by an intraperitoneal injection of myo-[2-3H]inositol about 16 h prior to the perfusion experiment, cell swelling due to lowering the perfusate osmolarity from 305 mOsmol l-1 to 225 mOsmol l-1 led to about a threefold stimulation of [3H]inositol release. The maximum of hypotonicity-induced [3H]inositol release preceded maximal volume-regulatory K+ efflux by about 30 s, but came after the maximum of water shift into the cells. Hypotonicity-induced [3H]inositol release was largely prevented in presence of Li+ (10 mM), but simultaneously inositol monophosphate accumulated inside the liver within 10 min and a small, but significant increase of inositol trisphosphate 1 min after onset of hypoosmolar exposure was detectable. No stimulation of [3H]inositol release was observed during cell shrinkage by switching the perfusate osmolarity from 225 mOsmol l-1 to 305 mOsmol l-1 or from 305 mOsmol l-1 to 385 mOsmol l-1. No stimulation of [3H]inositol release was observed upon swelling of preshrunken livers by lowering the osmolarity from 385 mOsmol l-1 to 305 mOsmol l-1, although the volume-regulatory K+ efflux under these conditions was almost identical to that observed after lowering the osmolarity from 305 mOsmol l-1 to 225 mOsmol l-1. 4.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential effects of human anaphylatoxin C3a on glucose output and flow in rat liver during orthograde and retrograde perfusion: the periportal scavenger cell hypothesis

1) During orthograde perfusion of rat liver human anaphylatoxin C3a caused an increase in glucose and lactate output and reduction of flow. These effects could be enhanced nearly twofold by co-infusion of the carboxypeptidase inhibitor MERGETPA, which reduced inactivation of C3a to C3adesArg. 2) During retrograde perfusion C3a caused a two- to threefold larger increase in glucose and lactate output and reduction of flow than in orthograde perfusions. These actions tended to be slightly enhanced by MERGETPA. 3) The elimination of C3a plus C3adesArg immunoreactivity during a single liver passage was around 67%, irrespective of the perfusion direction and the presence of the carboxypeptidase inhibitor MERGETPA; however, less C3adesArg and more intact C3a appeared in the perfusate in the presence of MERGETPA in orthograde and retrogade perfusions. It is concluded that rat liver inactivated human anaphylatoxin C3a by conversion to C3adesArg and moreover eliminated it by an additional process. The inactivation to C3adesArg seemed to be located predominantly in the proximal periportal region of the liver sinusoid, since C3a was less effective in orthograde perfusions, when C3a first passed the proximal periportal region before reaching the predominant mass of parenchyma as its site of action, than in retrograde perfusions, when it first passed the perivenous area. These data may be evidence for a periportal scavenger mechanism, by which the liver protects itself from systemically released mediators of inflammation that interfere with the local regulation of liver metabolism and hemodynamics.

Stimulation of release of prostaglandin D2 and thromboxane B2 from perfused rat liver by extracellular adenosine

In isolated perfused rat liver, adenosine infusion (50 microM) led to increases in glucose output and portal pressure and a net K+ release of 3.7 +/- 0.21 mumol/g, which was followed by an equivalent net K+ uptake after cessation of the nucleoside infusion. These effects were accompanied by a transient stimulation of hepatic prostaglandin D2 and thromboxane B2 release. The Ca2+ release observed upon adenosine infusion (50 microM) was 23.5 +/- 5.2 nmol/g, i.e. 10-20% of the Ca2+ release observed with extracellular ATP (50 microM). Indomethacin (10 microM) prevented the adenosine-induced stimulation of glucose output and the increase in portal pressure by 79 and 63% respectively, and completely abolished the stimulation of prostaglandin D2 release. The thromboxane A2 receptor antagonist BM 13.177 (20 microM), the phospholipase A2 inhibitor 4-bromophenacyl bromide (20 microM) and the cyclo-oxygenase inhibitor ibuprofen (50 microM) also decreased the glycogenolytic and vasoconstrictive responses of the perfused rat liver upon adenosine infusion by 50-80%. When the indomethacin inhibition of adenosine-induced prostaglandin D2 release was titrated, a close correlation between prostaglandin D2 release and the metabolic and vascular responses to adenosine was observed. These findings suggest an important role for eicosanoids in mediating the nucleoside responses in the perfused rat liver. Since eicosanoids are known to be formed by non-parenchymal cells in rat liver [Decker (1985) Semin. Liver Dis. 5, 175-190], the present study gives further evidence for an important role of eicosanoids as signal molecules between the different liver cell populations.

Liver glutamine metabolism

A fundamental conceptional change in the field of hepatic glutamine metabolism is derived from an understanding of the unique regulatory properties of hepatic glutaminase, the occurrence of glutamine cycling, and the discovery of marked hepatocyte heterogeneities in nitrogen metabolism, with metabolic interactions between differently localized subacinar hepatocyte populations. This change provided new insight into the role of the liver in maintaining ammonia and bicarbonate homeostasis under physiologic and pathologic conditions. Glutamine synthetase is present only in a specialized cell population at the hepatic venous outflow of the liver acinus; these cells act as scavengers for ammonia and probably also for various signal molecules ("perivenous scavenger cell hypothesis"). The function of mitochondrial glutaminase is that of a pH- and hormone-modulated ammonia amplification system that controls carbamoylphosphate synthesis and urea cycle flux in periportal hepatocytes. Not only is hepatic glutamine metabolism essential for maintenance of bicarbonate and ammonia homeostasis, but glutamine itself can act in the liver as a signal modulating hepatic metabolism. This article summarizes some major aspects of hepatic glutamine metabolism, based on previous reviews.

Hypoxia and CCl4-induced liver injury, but not acidosis, impair metabolism of cysteinyl leukotrienes in perfused rat liver

Uptake, metabolism and biliary elimination of infused cysteinyl leukotrienes were investigated in single-pass perfused rat liver. Hypoxia did not impair uptake of infused [3H]leukotriene C4, but inhibited biliary excretion of radioactivity by about 50% compared with normoxic control experiments. In addition, the leukotriene metabolite pattern in bile was profoundly altered and was characterized in hypoxia by a 75% to 80% decrease of both leukotriene C4 and polar metabolites, representing omega-oxidation products, whereas the appearance of leukotriene D4 in bile was not affected. Reoxygenation was followed by a marked increase of biliary excretion of polar metabolites, indicating that leukotrienes taken up and stored in the liver cells during the hypoxic period now underwent omega-oxidation with subsequent elimination of the omega-oxidized products. Hypoxia also inhibited the biliary excretion of radioactivity after [3H]leukotriene E4 addition because of an almost complete absence of omega-oxidation products in bile, whereas N-acetyl-leukotriene E4 excretion was not affected. Induction of liver injury by carbon tetrachloride treatment decreased single-pass uptake of [3H]leukotriene C4 by 30%, and only 36% of the radioactivity taken up by the liver was eliminated into bile within 1 hr, compared with 78% in normal livers. The pattern of biliary leukotriene metabolites, however, was not significantly different. Lowering the pH in the perfusion medium from 7.4 to 7.1 had no effect on uptake, metabolism or biliary elimination of infused [3H]leukotriene C4. The data show that hypoxia and experimental liver injury, but not acidosis, impair hepatic processing of cysteinyl leukotrienes. Thus, in leukotriene-induced shock syndromes, leukotriene elimination and inactivation may be impaired giving rise to a "vicious circle."

Hepatic uptake and metabolic disposition of leukotriene B4 in rats

1. In isolated perfused rat liver and in vivo, up to 25% of [3H]leukotriene B4 was eliminated from the circulation via hepatic uptake and biliary excretion within 1 h. Total body recovery of 3H amounted to about 60% of infused [3H]leukotriene B4. 2. Hepatobiliary excretion of leukotriene B4 and its metabolites exceeded renal elimination by about 4-fold and depended, in contrast with excretion of cysteinyl leukotriene E4, upon continuous taurocholate supply. 3. Analyses of bile, liver and recirculated perfusate using h.p.l.c. indicated that the liver metabolized leukotriene B4 extensively to omega-carboxyleukotriene B4 and its beta-oxidized derivatives, and no unmetabolized leukotriene B4 appeared in bile. These results substantiate the important contribution of the hepatobiliary system with respect to the metabolic fate of leukotriene B4.

Inhibition of leukotriene omega-oxidation by isonicotinic acid hydrazide (isoniazid)

Metabolism of leukotrienes via omega-oxidation represents a major degradative and inactivating pathway of these biologically active icosanoids. Isonicotinic acid hydrazide (isoniazid) inhibited this process in rats in vivo, in the isolated perfused rat liver, and in hepatic microsomes. The in vivo catabolism of leukotriene E4 via N-acetyl-leukotriene E4 to its omega-oxidized metabolites was inhibited by 50% or 71% using single intravenous isoniazid doses of 0.6 mmol or 1.0 mmol/kg body mass, respectively. Isoniazid interfered with leukotriene catabolism at the initial omega-oxidation step, resulting in an accumulation of N-acetyl-leukotriene E4. Analogous although weaker inhibition of leukotriene omega-oxidation in vivo was observed by pretreatment with isonicotinic acid 2-isopropylhydrazide and monoacetyl hydrazine. In the isolated perfused liver, isoniazid at concentrations varying over 0.2-10 mM decreased the omega-oxidation of cysteinyl leukotrienes dose-dependently by up to 94%. omega-Oxidation of both leukotriene E4 and leukotriene B4 by rat liver microsomes was inhibited by isoniazid, isonicotinic acid 2-isopropylhydrazide, and monoacetyl hydrazine with half-maximal concentrations in the range of 5-15 mM. Our measurements indicate that the impairment of leukotriene omega-oxidation by isoniazid involves both cytochrome-P450-dependent enzyme systems responsible for omega-oxidation of leukotriene E4 and leukotriene B4. In effect, under isoniazid treatment one can expect a prolongation of the proinflammatory actions of endogenously produced leukotrienes.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.

Hemodynamic and metabolic responses to leukotriene C4 in isolated perfused rat liver

Responses of isolated perfused rat liver to leukotriene C4 were studied in order to assess the mechanisms involved in leukotriene-mediated liver injury. Infusion of leukotriene C4 (11 and 44 pmoles per min per gm liver weight) into the portal vein resulted in a rise in portal pressure, a decrease in oxygen consumption, an increase in hepatic glucose and lactate efflux and lactate/pyruvate ratio in the perfusate and a small decrease in bile flow. Isoproterenol (1 microM) counteracted the effects of leukotriene C4 on respiration and portal pressure, whereas bile flow and glucose efflux were reversibly stimulated. The same changes were observed upon withdrawal of leukotriene C4. The release of glucose was correlated with the increase in oxygen consumption upon both isoproterenol addition and withdrawal of leukotriene C4. These results are indicative of leukotriene C4-induced microcirculatory redistribution of perfusate flow. Since, in the presence of nitroprusside (50 microM), both the effects of leukotriene C4 and their reversal by isoproterenol were diminished, a vascular site of action can be assumed. Accordingly, the accompanying metabolic responses can be explained by gradual changes in oxygen supply to parts of the liver. Reversibility of the leukotriene C4 effects and lack of short-term impairment of viability of the isolated liver suggest that leukotriene-mediated liver injury is a long-term effect related to events subsequent to microcirculatory changes.

Uptake, production and metabolism of cysteinyl leukotrienes in the isolated perfused rat liver. Inhibition of leukotriene uptake by cyclosporine

1. The isolated perfused rat liver efficiently takes up cysteinyl leukotrienes (LTs) C4, D4, E4 and N-acetyl-LTE4 from circulation. More than 70% of these cysteinyl LTs are excreted from liver into bile within 1 h of onset of a 5 min infusion, while about 5% remain in the liver. About 20% of infused N-acetyl-LTE4 escapes hepatic first-pass extraction under our conditions. 2. Metabolites of LTC4 appearing in bile within 20 min of the onset of infusion include mainly LTD4 and N-acetyl-LTE4, but also omega-hydroxy-N-acetyl-LTE4 and omega-carboxy-N-acetyl-LTE4. Metabolites generated from omega-carboxy-N-acetyl-LTE4 by beta-oxidation from the omega-end represent the major biliary LTs secreted at later times. 3. Stimulation of the isolated perfused liver by the combined infusion of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and the Ca2+ ionophore A23187 results in a transient increase of endogenous cysteinyl LT production, which is independent of extrahepatic cells. 4. The immunosuppressive drug cyclosporine causes a dose-dependent inhibition of hepatobiliary cysteinyl LT excretion, probably by interference with the sinusoidal uptake system for cysteinyl LTs.

Functional hepatocyte heterogeneity. Vascular 2-oxoglutarate is almost exclusively taken up by perivenous, glutamine-synthetase-containing hepatocytes

1. In isolated perfused rat liver maximal rates of 2-[1-14C]oxoglutarate uptake were about 0.4 mumol.g-1 .min-1; half-maximal rates of 2-[14C]oxoglutarate uptake were observed with influent concentrations of about 100 microM. 2-[14C]Oxoglutarate uptake by the liver was not affected by the direction of perfusion, but was decreased by about 80-90% when Na+ in the perfusion fluid was substituted by choline+, suggesting a Na+-dependence of hepatic 2-oxoglutarate uptake. In the absence of added ammonia, [14C]oxoglutarate uptake by the liver was about twice the net oxoglutarate uptake, indicating a simultaneous release of unlabeled oxoglutarate from perfused rat liver. 2. 14C-Labeled metabolites derived from [1-14C]oxoglutarate and recovered in the effluent perfusate were 14CO2 and 14C-labeled glutamate and glutamine; they accounted for 85-100% of the radiolabel taken up by the liver. 14CO2 was the major product (more than 70%) from [1-14C]oxoglutarate taken up the liver, provided glutamine synthesis was either inhibited by methionine sulfoximine or the endogenous rate of glutamine production was below 40 nmol.g-1.min-1. 3. Stimulation of glutamine synthesis by ammonia did not affect [14C]oxoglutarate uptake by the liver, but considerably increased net hepatic oxoglutarate uptake, indicating a decreased release of unlabeled oxoglutarate from the liver. Stepwise stimulation of hepatic glutamine synthesis led to a gradual decrease of 14CO2 production and radiolabel was recovered increasingly as [14C]glutamine in the effluent. At high rates of glutamine formation (i.e. about 0.6 mumol.g-1.min-1), about 60% of the [1-14C]oxoglutarate taken up by the liver was recovered in the effluent as [14C]glutamine. 14CO2 and [14C]glutamine production from added [1-14C]oxoglutarate were dependent on the rate of hepatic glutamine synthesis but not on the direction of perfusion. Extrapolation of 14C incorporation into glutamine to maximal rates of hepatic glutamine synthesis yielded an about 100% utilization of the [14C]oxoglutarate taken up by the liver for glutamine synthesis. This was again true for both the antegrade and the retrograde perfusion directions. On the other hand, addition of ammonia did not affect 14CO2 production from labeled oxoglutarate, when glutamine synthetase was inhibited by methionine sulfoximine. 4. The data suggest that vascular oxoglutarate is almost exclusively taken up by the small perivenous hepatocyte population containing glutamine synthetase, i.e. a cell population comprising only 6-7% of all hepatocytes. Thus, the findings demonstrate the existence of a, to date, uniquely zonally distributed oxoglutarate transport system which is probably Na+-dependent in the plasma membrane.(ABSTRACT TRUNCATED AT 400 WORDS)

Metabolism of cysteinyl leukotrienes in non-recirculating rat liver perfusion. Hepatocyte heterogeneity in uptake and biliary excretion

1. The uptake, metabolism and biliary excretion of the cysteinyl leukotrienes LTC4, LTD4 and LTE4, were studied in a non-recirculating rat liver perfusion system at constant flow in both antegrade (from the portal to the caval vein) and retrograde (from the caval to the portal vein) perfusion directions. During a 5-min infusion of [3H]LTC4, [3H]LTD4 and [3H]LTE4 (10 nmol/l each) in antegrade perfusions single-pass extractions of radioactivity from the perfusate were 66%, 81% and 83%, respectively. Corresponding values for LTC4 and LTD4 in retrograde perfusions were 83% and 93%, respectively, indicating a more efficient uptake of cysteinyl leukotrienes in retrograde than in antegrade perfusions. The concentrations of unmetabolized leukotrienes in the effluent perfusate were 8-12% in antegrade and 2-4% in retrograde perfusions. [14C]Taurocholate extraction from the perfusate was inhibited by LTC4 by only 3%, suggesting that an opening of portal-venous/hepatic-venous shunts does not explain the effects of perfusion direction on hepatic LTC4 uptake. 2. Following infusion of [3H]LTC4 and [3H]LTD4, in the antegrade perfusion direction, about 80% and 87%, respectively, of the radiolabel taken up by the liver was excreted into bile. In retrograde perfusions, however, only 40% and 57%, respectively, was excreted into bile and the remainder was slowly redistributed into the perfusate, indicating that leukotrienes were taken up into a hepatic compartment with less effective biliary elimination or converted to metabolites escaping biliary excretion. The metabolite pattern found in bile was not affected by the direction of perfusion. Biliary products of LTC4 were polar metabolites (31-38%), LTD4 (27-30%), LTE4 (about 1%) and N-acetyl-LTE4 (3-4%) in addition to unmodified LTC4 (17-18%). 3. LTC4 was identified as a major metabolite of [3H]LTD4 in bile, amounting to about 20% of the total radioactivity excreted into bile. This is probably due to a gamma-glutamyltransferase-catalyzed glutamyl transfer from glutathione in the biliary compartment, as demonstrated in in vitro experiments. The presence of sinusoidal gamma-glutamyltransferase activity in perfused rat liver was shown in experiments on the hydrolysis of infused gamma-glutamyl-p-nitroanilide. 90% inhibition of this enzyme activity by AT-125 did not affect the metabolism of LTC4. 4. When [3H]LTE4 was infused in the antegrade perfusion direction, biliary metabolites comprised N-acetyl-LTE4 (24%) and polar components (60%).(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of action of cysteinyl leukotrienes on glucose and lactate balance and on flow in perfused rat liver. Comparison with the effects of sympathetic nerve stimulation and noradrenaline

Rat livers were perfused at constant pressure via the portal vein with media containing 5 mM glucose, 2 mM lactate and 0.2 mM pyruvate. 1. Leukotrienes C4 and D4 enhanced glucose and lactate output and reduced perfusion flow to the same extent and with essentially identical kinetics. They both caused half-maximal alterations (area under the curve) of carbohydrate metabolism at a concentration of about 1 nM and of flow at about 5 nM. The leukotriene-C4/D4 antagonist CGP 35949 B inhibited the metabolic and hemodynamic effects of 5 nM leukotrienes C4 and D4 with the same efficiency, causing 50% inhibition at about 0.1 microM. 2. Leukotriene C4 elicited the same metabolic and hemodynamic alterations with the same kinetics as leukotriene D4 in livers from rats pretreated with the gamma-glutamyltransferase inhibitor, acivicin. 3. The calcium antagonist, nifedipine, at a concentration of 50 microM did not affect the metabolic and hemodynamic changes caused by 5 nM leukotriene D4. The smooth-muscle relaxant, nitroprussiate, at a concentration of 10 microM reduced flow changes, without significantly affecting the metabolic alterations. 4. Leukotriene D4 not only reduced flow; it also caused an intrahepatic redistribution of flow, restricting some areas from perfusion. Thus, leukotrienes increased glucose and lactate output directly in the accessible parenchyma and, in addition, indirectly by washout from restricted areas during their reopening upon termination of application. 5. The phospholipase A2 inhibitor, bromophenacyl bromide, but not the cyclooxygenase inhibitor, indomethacin, at a concentration of 20 microM reduced the metabolic and hemodynamic effects of 5 mM leukotriene D4. 6. Stimulation of the sympathetic hepatic nerves with 2-ms rectangular pulses at 20 Hz and infusion of 1 microM noradrenaline increased glucose and lactate output and decreased flow, similar to 10 nM leukotrienes C4 and D4. The kinetics of the metabolic and hemodynamic changes caused by the leukotrienes differed, however, from those due to nerve stimulation and noradrenaline. 7. The leukotriene-C4/D4 antagonist, CGP 35949 B, even at very high concentrations (20 microM) inhibited the metabolic and hemodynamic alterations caused by nerve stimulation or noradrenaline infusion only slightly and unspecifically.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of hepatic parenchymal and non-parenchymal cell function by the diadenine nucleotides Ap3A and Ap4A

The diadenine nucleotides diadenosine 5',5"-P1,P3-triphosphate (Ap3A) and diadenosine 5',5"-P1,P4-tetraphosphate (Ap4A) can be released from platelets and were shown to act as long-lived signal molecules. Accordingly, we studied their potential effect on hepatic metabolism. In isolated perfused rat liver, Ap3A and Ap4A increase the portal pressure, lead to a transient net release of Ca2+, complex net K+ movement across the liver plasma membrane and stimulate hepatic glucose output and 14CO2 production from [1-14C]glutamate. These responses resemble that obtained with extracellular ATP. This and studies on the additivity of ATP and Ap4A effects suggest similar mechanisms mediating the ATP and diadenine nucleotide effects in the liver. Ap3A and Ap4A increased the activity of glycogen phosphorylase a in isolated hepatocyte suspensions by about 100%, pointing to a direct effect of these nucleotides on hepatic parenchymal cells. A response of hepatic non-parenchymal cells to diadenine nucleotide infusion is suggested by a marked stimulation of thromboxane and prostaglandin D2 release from perfused liver. Studies with the thromboxane A2 receptor antagonist BM 13.177 (20 microM) show that the pressure and glucose response to the diadenine nucleotides is partially mediated by this thromboxane formation. Studies with retrograde and sequential liver perfusions suggest a less efficient degradation of the diadenine nucleotides during a single liver passage compared to extracellular ATP. The data suggest that Ap3A and Ap4A are potential regulators of hepatic hemodynamics and metabolism, involving complex interactions between hepatic parenchymal cells and hepatic non-parenchymal cells, including eicosanoids as signal molecules.

Stimulation of thromboxane release by extracellular UTP and ATP from perfused rat liver. Role of icosanoids in mediating the nucleotide responses

1. In isolated perfused rat liver, infusion of UTP (20 microM) led to a transient, about sevenfold stimulation of thromboxane release (determined as thromboxane B2), which did not parallel the time course of the UTP-induced stimulation of glucose release. An increased thromboxane release was also observed after infusion of ATP (20 microM). Although the maximal increase of portal pressure following ATP was much smaller than with UTP (4.2 vs 11.5 cm H2O), the peak thromboxane release was similar with both nucleotides. 2. Indomethacin (10 microM) inhibited the UTP-induced stimulation of thromboxane release and decreased the UTP-induced maximal increase of glucose output and of portal pressure by about 30%. The thromboxane A2 receptor antagonist BM 13.177 (20 microM) completely blocked the pressure and glucose response to the thromboxane A2 analogue U-46619 (200 nM) and decreased the ATP- and UTP-induced stimulation of glucose output by about 25%, whereas the maximal increase of portal pressure was inhibited by about 50% and 30%, respectively. BM 13.177 and indomethacin inhibited the initial nucleotide-induced overshoot of portal pressure increase, but had no effect on the steady-state pressure increase which is obtained about 5 min after addition of ATP or UTP. 3. The leukotriene D4/E4 receptor antagonist LY 171883 (50 microM) inhibited not only the glucose and pressure response of perfused rat liver to leukotriene D4, but also to leukotriene C4 by about 90%. This suggests that leukotriene D4 (not C4) is the active metabolite in perfused liver and the effects of leukotriene C4 are probably due to its rapid conversion to leukotriene D4. LY 171883 also inhibited the response to the thromboxane A2 analogue U-46619 by 75-80%, whereas the response of perfused liver to leukotriene C4 was not affected by the thromboxane receptor antagonist BM 13.177 (20 microM). The glucose and pressure responses of the liver to extracellular UTP were inhibited by LY 171883 and by BM 13.177 by about 30%. This suggests that the inhibitory action of LY 171883 was due to a thromboxane receptor antagonistic side-effect and that peptide leukotrienes do not play a major role in mediating the UTP response. 4. In isolated rat hepatocytes extracellular UTP (20 microM), ATP (20 microM), cyclic AMP (50 microM) and prostaglandin F2 alpha (3 microM) increased glycogen phosphorylase a activity by more than 100%.(ABSTRACT TRUNCATED AT 400 WORDS)