Possible involvement of eicosanoids in the zymosan and arachidonic-acid-induced oxygen uptake, glycogenolysis and Ca2+ mobilization in the perfused rat liver

Profibrotic Signaling and HCC Risk during Chronic Viral Hepatitis: Biomarker Development

Despite breakthroughs in antiviral therapies, chronic viral hepatitis B and C are still the major causes of liver fibrosis and hepatocellular carcinoma (HCC). Importantly, even in patients with controlled infection or viral cure, the cancer risk cannot be fully eliminated, highlighting a persisting oncogenic pressure imposed by epigenetic imprinting and advanced liver disease. Reliable and minimally invasive biomarkers for early fibrosis and for residual HCC risk in HCV-cured patients are urgently needed. Chronic infection with HBV and/or HCV dysregulates oncogenic and profibrogenic signaling within the host, also displayed in the secretion of soluble factors to the blood. The study of virus-dysregulated signaling pathways may, therefore, contribute to the identification of reliable minimally invasive biomarkers for the detection of patients at early-stage liver disease potentially complementing existing noninvasive methods in clinics. With a focus on virus-induced signaling events, this review provides an overview of candidate blood biomarkers for liver disease and HCC risk associated with chronic viral hepatitis and epigenetic viral footprints.

Behavioral and physiological responses to intraperitoneal injection of zymosan in chicks

Zymosan is a cell wall component of the yeast Saccharomyces cerevisiae and produces severe inflammatory responses in mammals. When zymosan is peripherally injected in mammals, it induces several behavioral and physiological changes including anorexia and hyperthermia. However, to our knowledge, behavioral and physiological responses to zymosan have not yet been clarified in birds. Therefore, the purpose of the present study was to determine if intraperitoneal injection of zymosan affects food intake, voluntary activity, cloacal temperature, plasma corticosterone (CORT) and glucose concentrations, and splenic gene expression of cytokines in chicks (Gallus gallus). Intraperitoneal injection of zymosan (2.5 mg) significantly decreased food intake, voluntary activity, and plasma glucose concentration, and increased plasma CORT concentration. The injection of 0.5 mg zymosan significantly increased cloacal temperature, while 2.5 mg zymosan had a tendency to increase it. Finally, 2.5 mg zymosan significantly increased the splenic gene expression of interleukin-1β (IL-1β), IL-6, IL-8, interferon-γ, and tumor necrosis factor-like cytokine 1A. The present results suggest that zymosan would be one of components which induces nonspecific symptoms including anorexia, hypoactivity, hyperthermia, and stress responses, under fungus infection in chicks.

Role of intrahepatic innervation in regulating the activity of liver cells

Liver innervation comprises sympathetic, parasympathetic and peptidergic nerve fibers, organized as either afferent or efferent nerves with different origins and roles. Their anatomy and physiology have been studied in the past 30 years, with different results published over time. Hepatocytes are the main cell population of the liver, making up almost 80% of the total liver volume. The interaction between hepatocytes and nerve fibers is accomplished through a wealth of neurotransmitters and signaling pathways. In this short review, we have taken the task of condensing the most important data related to how the nervous system interacts with the liver and especially with the hepatocyte population, how it influences their metabolism and functions, and how different receptors and transmitters are involved in this complex process.

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.

Role of Kupffer cells in host defense and liver disease

Kupffer cells (KC) constitute 80-90% of the tissue macrophages present in the body. They reside within the lumen of the liver sinusoids, and are therefore constantly exposed to gut-derived bacteria, microbial debris and bacterial endotoxins, known to activate macrophages. Upon activation KC release various products, including cytokines, prostanoides, nitric oxide and reactive oxygen species. These factors regulate the phenotype of KC themselves, and the phenotypes of neighboring cells, such as hepatocytes, stellate cells, endothelial cells and other immune cells that traffic through the liver. Therefore, KC are intimately involved in the liver's response to infection, toxins, ischemia, resection and other stresses. This review summarizes established basic concepts of KC function as well as their role in the pathogenesis of various liver diseases.

The hemodynamic effects of zymosan in the perfused rat liver

The actions of zymosan on hepatic microcirculation and on the cell membrane permeability were investigated using the multiple-indicator dilution technique. The experimental system was the perfused rat liver. [(3)H]Water, [(3)H]sucrose and [(14)C]urea or [(14)C]bicarbonate were simultaneously injected into the portal vein. Mean transit times, distribution spaces, variances, linear superpositions and transfer coefficients across the plasma membrane were calculated. Zymosan had no net effect on the great vessels space but increased the extracellular sucrose space and decreased the aqueous cell space. Zymosan impaired the flow-limited distribution and increased the normalized variances of all tracers. The increase in the portal pressure caused by zymosan results most probably from a constriction just after or at the exit of the sinusoids. Impairment of the flow-limited distribution of tracers in the sinusoidal bed indicates that zymosan induces the formation of permeability barriers, which could make the access of the solutes to transporters or enzymes located on the outer surface of the plasma membrane difficult.

Differences in the involvement of prostanoids from Kupffer cells in the mediation of anaphylatoxin C5a-, zymosan-, and lipopolysaccharide-dependent hepatic glucose output and flow reduction

Various inflammatory stimuli such as anaphylatoxin C5a, zymosan, and lipopolysaccharides (LPSs) have been reported both to enhance glucose output in the perfused rat liver and to induce prostanoid (ie, prostaglandin and thromboxane) release from Kupffer cells, the resident liver macrophages. Because prostanoids can enhance glucose output from hepatocytes, it was the aim of this study to compare the possible roles of prostanoids released after C5a, zymosan, and LPS in the mediation of hepatic glucose output. In perfused livers both C5a and zymosan immediately enhanced glucose output, reduced flow, and induced prostanoid overflow into the hepatic vein, but with different quantities and kinetics. Only the C5a-induced but not the zymosan-induced effects were abrogated by inhibitors of prostanoid signaling as the prostanoid synthesis inhibitor indomethacin and the thromboxane receptor antagonist daltroban. In contrast to C5a and zymosan, LPS had no effect on glucose output, flow rate, or prostanoid overflow. In isolated Kupffer cells, C5a and zymosan induced maximal release of prostaglandins D(2) and E(2) and of thromboxane A(2) within a period of 0 to 2 minutes and 5 to 15 minutes, respectively. In pulse-chase experiments, maximal prostanoid release was already observed after 2 minutes of continuous stimulation with C5a, but only after 10 to 15 minutes of continuous stimulation with zymosan. LPS-dependent prostanoid release was not seen before 1 hour. Thus, even though C5a, zymosan, and LPS induced prostanoid release from Kupffer cells, only C5a quickly regulated hepatic glucose metabolism in a prostanoid-dependent manner (due to the kinetics and quantities of prostanoids released).

Zymosan-induced changes in glucose release and fatty acid oxidation in the perfused rat liver

The aim of the present study was to investigate the actions of zymosan on glucose release and fatty acid oxidation in perfused rat livers and to determine if Kupffer cells and Ca2+ ions are implicated in these actions. Zymosan caused stimulation of glycogenolysis in livers from fed rats. In livers from fasted rats zymosan caused gradual inhibition of glucose production and oxygen consumption from lactate plus pyruvate. Ketogenesis, oxygen consumption, and [14C-]-CO2 production were inhibited by zymosan when the [1-14C]-palmitate was supplied exogenously. However, ketogenesis and oxygen consumption from endogenous sources were not inhibited. An interference with substrate-uptake by the liver may be the cause of the changes in gluconeogenesis and oxidation of fatty acids from exogenous sources. The pretreatment of the rats with gadolinium chloride and the removal of Ca2+ ions did not suppress the effects of zymosan on glucose release, a finding that argues against the participation of Kupffer cells or Ca2+ ions in the liver responses. The hepatic metabolic changes caused by zymosan could play a role in the systemic metabolic alterations reported to occur after in vivo zymosan administration.

Increase by anaphylatoxin C5a of glucose output in perfused rat liver via prostanoids derived from nonparenchymal cells: direct action of prostaglandins and indirect action of thromboxane A(2) on hepatocytes

In the perfused rat liver the anaphylatoxin C5a enhanced glucose output, reduced flow, and elevated prostanoid overflow. Because hepatocytes (HCs) do not express C5a receptors, the metabolic C5a actions must be indirect, mediated by e.g. prostanoids from Kupffer cells (KCs) and hepatic stellate cells (HSCs), which possess C5a receptors. Surprisingly, the metabolic C5a effects were not only impaired by the prostanoid synthesis inhibitor, indomethacin, but also by the thromboxane A(2) (TXA(2)) receptor antagonist, daltroban, even though HCs do not express TXA(2) receptors. TXA(2) did not induce prostaglandin (PG) or an unknown factor release from KCs or sinusoidal endothelial cells (SECs), which express TXA(2) receptors, because (1) daltroban did neither influence the C5a-induced release of prostanoids from cultured KCs nor the C5a-dependent activation of glycogen phosphorylase in KC/HC cocultures and because (2) the TXA(2) analog, U46619, failed to stimulate prostanoid release from cultured KCs or SECs or to activate glycogen phosphorylase in KC/HC or SEC/HC cocultures. In the perfused liver, Ca(2+)-deprivation inhibited not only flow reduction but also glucose output elicited by C5a to similar extents as daltroban. Similarly, in the absence of extracellular Ca(2+), flow reduction and glucose output induced by U46619 were almost completely prevented, whereas glucose output induced by the directly acting PGF(2alpha) was only slightly lowered. Thus, in the perfused rat liver PGs released after C5a-stimulation from KCs and HSCs directly activated glycogen phosphorylase in HCs, and TXA(2) enhanced glucose output indirectly mainly by causing hypoxia as a result of flow reduction.

Prevention of Kupffer cell-induced oxidant injury in rat liver by atrial natriuretic peptide

The generation of reactive oxygen species (ROS) by activated Kupffer cells contributes to liver injury following liver preservation, shock, or endotoxemia. Pharmacological interventions to protect liver cells against this inflammatory response of Kupffer cells have not yet been established. Atrial natriuretic peptide (ANP) protects the liver against ischemia-reperfusion injury, suggesting a possible modulation of Kupffer cell-mediated cytotoxicity. Therefore, we investigated the mechanism of cytoprotection by ANP during Kupffer cell activation in perfused rat livers of male Sprague-Dawley rats. Activation of Kupffer cells by zymosan (150 microgram/ml) resulted in considerable cell damage, as assessed by the sinusoidal release of lactate dehydrogenase and purine nucleoside phosphorylase. Cell damage was almost completely prevented by superoxide dismutase (50 U/ml) and catalase (150 U/ml), indicating ROS-related liver injury. ANP (200 nM) reduced Kupffer cell-induced injury via the guanylyl cyclase-coupled A receptor (GCA receptor) and cGMP: mRNA expression of the GCA receptor was found in hepatocytes, endothelial cells, and Kupffer cells, and the cGMP analog 8-bromo-cGMP (8-BrcGMP; 50 microM) was as potent as ANP in protecting from zymosan-induced cell damage. ANP and 8-BrcGMP significantly attenuated the prolonged increase of hepatic vascular resistance when Kupffer cell activation occurred. Furthermore, both compounds reduced oxidative cell damage following infusion of H2O2 (500 microM). In contrast, superoxide anion formation of isolated Kupffer cells was not affected by ANP and only moderately reduced by 8-BrcGMP. In conclusion, ANP protects the liver against Kupffer cell-related oxidant stress. This hormonal protection is mediated via the GCA receptor and cGMP, suggesting that the cGMP receptor plays a critical role in controlling oxidative cell damage. Thus ANP signaling should be considered as a new pharmacological target for protecting liver cells against the inflammatory response of activated Kupffer cells without eliminating the vital host defense function of these cells.

Aggravating action of zymosan on acute liver damage in perfused liver of rats treated with D-galactosamine

To study the role of Kupffer cells in the aggravation of liver injury, effects of zymosan on acute liver damage were explored using perfused livers of rats 24 h after intraperitoneal injection of D-galactosamine (800 mg/kg). The leakage of lactate dehydrogenase and aspartate aminotransferase into the effluent was used to indicate acute liver damage. Infusion of zymosan (30 microgram/ml) into the portal vein rapidly increased the leakage of lactate dehydrogenase and aspartate aminotransferase from galactosamine-treated liver with decreased perfusion flow. Pretreatment of animals with gadolinium, which diminished an immunostaining of resident macrophages in the injured liver, significantly attenuated the flow reduction induced by zymosan, whereas it did not affect the increases in enzyme leakage. Infusions of PGF2alpha, PGE2, and leukotriene D4, the eicosanoids mainly produced by Kupffer cells, decreased perfusion flow without rapid augmentation of enzyme leakage from galactosamine-treated liver. These results indicate that zymosan potentiates acute liver damage after galactosamine injection and suggest that certain types of nonparenchymal cells other than Kupffer cells are mainly involved in the action of zymosan.

Alteration of hepatic microcirculation by oxethazaine and some vasoconstrictors in the perfused rat liver

We previously reported that, in isolated perfused rat livers in a constant flow system, oxethazaine (OXZ) rapidly increased portal pressure (PP) accompanied by inhibition of oxygen uptake and the subsequent metabolic effects. In this study, hemodynamic changes were studied by using an indicator dilution technique and by microscopic observation of post-fixed liver samples stained with acridine orange or trapped fluorescence microspheres (FMSs). During the increase in PP induced by OXZ, the mean transit times of both red blood cells and azoalbumin were shortened markedly, and the vascular and extravascular albumin spaces decreased to 55 and 18% of the controls, respectively. With acridine orange, in the control livers, all the dye infused was taken up and the periportal zones were uniformly stained over all the liver sections, whereas in the OXZ-treated livers, about 30% of the dye drained out, and extensive staining was observed in the central portion of the liver mass, but the peripheral portions of the liver were much less stained. The staining was often localized around large portal vein branches and spread toward the hepatic veins. These changes were recoverable in the absence of OXZ. Distributions of 1-microm and 15-microm FMSs were likewise altered by OXZ. Thus, uneven perfusion may be the primary cause of decreased tissue spaces and also of the metabolic effects produced by OXZ. Endothelin 1 also produced OXZ-like changes, while U-46619 had lesser effects. The methodology used in this study may help delineate the hepatic perfusion disturbance caused by various vasoconstrictors.

Phagocytic function and metabolite production in thioacetamide-induced liver cirrhosis: a comparative study in perfused livers and cultured Kupffer cells

BACKGROUND/AIMS

The aim of the study presented here was to evaluate the basal and stimulated phagocytic activities and the metabolite production of isolated perfused livers, and also the phagocytic capacity of cultured Kupffer cells from rats with macronodular cirrhosis.

METHODS

Rats were made cirrhotic by oral administration of thioacetamide. The phagocytic activity was assessed by the rate of removal of colloidal carbon. The Kupffer cells were prepared by a pronase/collagenase digestion method followed by elutriation.

RESULTS

The phagocytic activity and production of glucose, lactate and pyruvate were reduced in cirrhotic livers when calculated per g liver. Due to hyperplastic-regenerative processes the mass of the cirrhotic livers was markedly augmented so that the colloidal carbon uptake calculated per cirrhotic liver was not significantly different from the controls. Colloidal carbon-induced glucose release increased more markedly in the controls than in cirrhotic livers. Isoproterenol considerably stimulated phagocytosis and glucose production in controls, whereas the response was clearly reduced in cirrhotic livers when calculated either per g liver or per total liver weight. The cyclic AMP analogue elicited a marked glycogenolytic response in the controls, whereas there was only a slight increase in glucose production in cirrhotic livers. Phagocytosis of cirrhotic livers was only moderately stimulated by opsonized zymosan when compared with the controls. Freshly isolated Kupffer cells exhibited a reduced phagocytic activity. Stimulation by zymosan was observed only in cell suspensions of the controls. In contrast, Kupffer cells from cirrhotic livers did not differ from controls with respect to basal or zymosan-stimulated phagocytic activity after 48-h cultivation.

CONCLUSION

The stimulated phagocytic function was disturbed in perfused macronodular-cirrhotic livers as compared to controls. In contrast, 48-h cultured Kupffer cells from cirrhotic livers exhibited the same basal and stimulated phagocytic capacity as controls. The glucose release from perfused livers, initiated by stimulation of Kupffer cells or hepatocytes, was significantly reduced in cirrhotic livers. Therefore, we postulate an impaired intra- and/or intercellular signalling in macronodular-cirrhotic livers.

The metabolic and hemodynamic effects of oxethazaine in the perfused rat liver

Alteration of hepatic microcirculation and its effects on hepatic metabolism were examined using oxethazaine (OXZ). The infusion of OXZ into isolated perfused livers rapidly increased the portal perfusion pressure (PP) and inhibited oxygen (O2) uptake, which was followed by a decrease in tissue ATP content and an increase in lactate, pyruvate and glucose release into the perfusate. P-450-dependent reductive drug metabolism was enhanced by OXZ, whereas oxidative drug metabolism was suppressed, and this was accompanied by a decrease in substrate uptake. During OXZ infusion, a time delay between the inhibition of O2 uptake and the release of cellular and xenobiotic metabolites was observed. The actions of OXZ required Ca2+. It is unlikely that the inhibition of O2 uptake is due to the inhibition of cellular respiration. The PP increase induced by OXZ was inhibited by papaverine, but not by prazosin, sodium nitroprusside and verapamil, whereas all of these vasodilators were effective against norepinephrine. Under retrograde perfusion, the PP increase by OXZ was abolished, but norepinephrine, uridine 5'-triphosphate, angiotensin II and endothelin 1 were still effective. The extrahepatic portal vein preparation contracted at high concentrations of OXZ. The results suggest that OXZ acts differently from other known vasoconstrictors and possibly narrows hepatic sinusoids to reduce the rate of substance exchange between the sinusoids and hepatocytes, including a reduction in O2 extraction.

Stimulation of glycogen phosphorylase in rat hepatocytes via prostanoid release from Kupffer cells by recombinant rat anaphylatoxin C5a but not by native human C5a in hepatocyte/Kupffer cell co-cultures

Human anaphylatoxin C3a had previously been shown to increase glycogenolysis in perfused rat liver and prostanoid formation in rat liver macrophages. Surprisingly, human C5a, which in other systems elicited stronger responses than C3a, did not increase glycogenolysis in perfused rat liver. Species incompatibilities within the experimental system had been supposed to be the reason. The current study supports this hypothesis: (1) In rat liver macrophages that had been maintained in primary culture for 72 h recombinant rat anaphylatoxin C5a in concentrations between 0.1 and 10 micrograms/ml increased the formation of thromboxane A2, prostaglandin D2, E2 and F2 alpha 6- to 12-fold over basal within 10 min. In contrast, human anaphylatoxin C5a did not increase prostanoid formation in rat Kupffer cells. (2) The increase in prostanoid formation by recombinant rat C5a was specific. It was inhibited by a neutralizing monoclonal antibody. (3) In co-cultures of rat hepatocytes and rat Kupffer cells but not in hepatocyte mono-cultures recombinant rat C5a increased glycogen phosphorylase activity 3-fold over basal. This effect was inhibited by incubation of the co-cultures with 500 microM acetylsalicyclic acid. Thus, C5a generated either locally in the liver or systemically e.g. in the course of sepsis, may increase hepatic glycogenolysis by a prostanoid-mediated intercellular communication between Kupffer cells and hepatocytes.

Endotoxin inhibits glucuronidation in the liver. An effect mediated by intercellular communication

Endotoxin [lipopolysaccharide (LPS) 50 micrograms/mL] added to the perfusion medium increased glucose production and inhibited the glucuronidation of p-nitrophenol in perfused mouse liver both in recirculating and non-recirculating systems, while sulfation of p-nitrophenol was unchanged. The effects of endotoxin could be prevented by the addition of cyclooxygenase inhibitors, while PGD2 and PGE2 also caused a decrease in p-nitrophenol glucuronidation in perfused liver. In isolated hepatocytes endotoxin failed to affect p-nitrophenol conjugation, while PGD2 and PGE2 decreased the rate of it. Our results suggest that endotoxin inhibits glucuronidation through an intercellular communication presumably mediated by eicosanoids.

Ethanol alters the metabolic response of isolated perfused rat liver to a phagocytic stimulus

Intercellular communication in the liver is a potentially important mechanism for the regulation of hepatic metabolism. Since alcohol (ethanol, ETOH) can interact with both parenchymal and nonparenchymal cells, the present study was performed to assess the possible effects of ETOH on the nonparenchymal cell-to-hepatocyte signal traffic by studying the glycogenolytic and glycolytic response of the perfused rat liver to colloidal carbon, a phagocytic stimulus for Kupffer and sinusoidal endothelial cells. Livers from fed rats were perfused with hemoglobin-free Krebs Ringer bicarbonate buffer containing ETOH (20 mM) or acetaldehyde (1 mM). Twenty minutes after initiating the infusion of ETOH or acetaldehyde, colloidal carbon was infused and the rate of carbon uptake, glucose, lactate and pyruvate output, and oxygen consumption were determined. In control livers, carbon stimulated the output of glucose (60%), lactate (25%), and pyruvate (53%), without affecting the lactate/pyruvate ratio. ETOH, but not acetaldehyde, enhanced the carbon effect on glucose output (38%), but suppressed the increased lactate and pyruvate output (48% and 91% respectively) resulting in a dramatic 10-fold increase in the lactate/pyruvate ratio. By using inhibitors of cyclooxygenase or alcohol dehydrogenase (indomethacin and 4-methylpyrazole, respectively) in the presence of carbon and/or ETOH, we determined that: (1) following carbon stimulation prostaglandins are the likely mediators secreted by nonparenchymal cells that increase carbohydrate output; and (2) the ETOH-induced enhancement of carbon-stimulated glycogenolysis is also mediated by prostaglandins and is not dependent on the oxidative metabolism of ETOH.

Transient activation of hepatic glycogenolysis by thrombin in perfused rat livers

Thrombin, a peptide with native protease activity, caused a rapid (less than 1 min) increase in glycogenolysis of about 30%, assessed from rates of production of glucose+lactate+pyruvate, and in oxygen uptake in perfused rat liver. These increases were followed by a rapid return to basal values within 5 min. The effect of thrombin on glycogenolysis was dose-dependent and was maximal at perfusate concentrations around 1 U/ml. Interestingly, the effect of thrombin on glycogenolysis could be elicited only once in any given liver. The activation of glycogenolysis by thrombin was diminished nearly 50% by prior infusion of the protease inhibitor, diisopropyl fluorophosphate (10 microM), and over 90% when thrombin was treated with diisopropyl fluorophosphate prior to infusion. The stimulation of glycogenolysis by thrombin could be detected in isolated hepatocytes or in livers stored for 24 h in cold Euro-Collins solution, a treatment which destroys endothelial cells. Further, thrombin stimulated production of prostaglandin D2 from arachidonic acid in cultured hepatic endothelial but not Kupffer cells. The effect of thrombin on carbohydrate output was also blocked by a phospholipase A2 inhibitor (quinacrine, 50 microM) and by an inhibitor of the cyclooxygenase (indomethacin, 20 microM), suggesting the involvement of cyclooxygenase in the mechanism of action of thrombin. In support of this idea, the transient kinetics of stimulation of glycogenolysis by thrombin and arachidonic acid was nearly identical to release of thromboxane B2 (80-420 pg/ml) and prostaglandin D2 (300-900 pg/ml) from the perfused liver. Further, a second addition of thrombin failed to increase thromboxane and prostaglandin D2 release as well as carbohydrate production, supporting a causal link between these phenomena. Taken together, these data support the hypothesis that thrombin interacts with receptors in the liver, possibly on endothelial cells, leading to activation of phospholipase A2 and subsequent transient production of prostaglandins and thromboxanes. These mediators subsequently interact with receptors on parenchymal cells, leading to a transient stimulation of glycogenolysis.

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.

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.

Eicosanoid-mediated increase in glucose and lactate output as well as decrease and redistribution of flow by complement-activated rat serum in perfused rat liver

Rat serum, in which the complement system had been activated by incubation with zymosan, increased the glucose and lactate output, and reduced and redistributed the flow in isolated perfused rat liver clearly more than the control serum. Heat inactivation of the rat serum prior to zymosan incubation abolished this difference. Metabolic and hemodynamic alterations caused by the activated serum were dose dependent. They were almost completely inhibited by the cyclooxygenase inhibitor indomethacin and by the thromboxane antagonist 4-[2-(4-chlorobenzesulfonamide)-ethyl]-benzene-acetic acid (BM 13505), but clearly less efficiently by the 5'-lipoxygenase inhibitor nordihydroguaiaretic acid and the leukotriene antagonist N-(3-[3-(4-acetyl-3-hydroxy-2-propyl-phenoxy)-propoxy]-4-chlorine-6-meth yl- phenyl)-1H-tetrazole-5-carboxamide sodium salt (CGP 35949 B). Control serum and to a much larger extent complement-activated serum, caused an overflow of thromboxane B2 and prostaglandin F2 alpha into the hepatic vein. It is concluded that the activated complement system of rat serum can influence liver metabolism and hemodynamics via release from nonparenchymal liver cells of thromboxane and prostaglandins, the latter of which can in turn act on the parenchymal cells.

Down-regulation of prostaglandin F2 alpha receptors in rat liver during chronic endotoxemia

Prostaglandin (PG) F2 alpha binding parameters were measured in purified plasma membrane preparations isolated from livers of chronically endotoxin-(ET) treated rats and corresponding controls. Two classes of binding sites were detected in both groups: high affinity, low capacity, with a KD of 44.4 +/- 8.8 nM for saline- and 28.6 +/- 11.3 nM for ET-treated rats (n = 5 for both, p greater than 0.05) and low affinity, high capacity with a KD of 1.12 +/- 0.49 microM for saline- and 1.24 +/- 0.43 microM for ET-treated rats (p greater than 0.05). Bmax values for high affinity sites were 1.01 +/- 0.18 fmol.mg-1 protein for saline- and 1.02 +/- 0.54 (same units) for ET-treated rats (p greater than 0.05). There was a significant difference (p less than 0.01) between the Bmax values for low affinity sites in saline- (675 +/- 332 fmol.mg-1 protein) and ET-treated rats (12 +/- 1, same units). This decrease in the amount of PGF2 alpha low affinity high capacity binding sites may underlie the depression of the PGF2 alpha stimulatory effect on hepatic gluconeogenesis induced by non-lethal, chronic ET treatment of rats, recently described by us (9).

Melittin stimulates liver glycogenolysis and the release of prostaglandin D2 and thromboxane B2

Melittin stimulates glycogenolysis and induces vasoconstriction in perfused rat liver. The effect was rapid and associated with production and release of prostaglandin D2 and thromboxane B2. Indomethacin blocked the release of these eicosanoids and the stimulation of glycogenolysis induced by melittin. Ibuprofen blocked the release of prostaglandin D2 induced by melittin and markedly attenuated that of thromboxane B2. Interestingly, the initial burst of glucose output induced by melittin was not inhibited by ibuprofen, although the duration of the glycogenolytic action of the peptide was greatly diminished.

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.

Direct activation by prostaglandin F2 alpha but not thromboxane A2 of glycogenolysis via an increase in inositol 1,4,5-trisphosphate in rat hepatocytes

In rat liver prostaglandin F2 alpha (PGF2 alpha) and thromboxane A2 (TXA2), released from non-parenchymal cells, have been implicated as mediators of the enhancement of glucose and lactate output from parenchymal cells caused by sympathetic nerve stimulation [Iwai, M. et al. (1988) Eur. J. Biochem. 175, 45-50]. In isolated rat hepatocytes PGF2 alpha, of which 75% were degraded within 10 min, but not the TXA2 analogue U46619 increased inositol 1,4,5-trisphosphate (IP3), glycogen phosphorylase a activity and glucose output like noradrenaline and vasopressin; cyclic AMP remained unaltered. The maximal increase in IP3 was reached within 20 s and in phosphorylase activity as well as glucose release within 1 min. The results indicate that only PGF2 alpha but not TXA2 can play a role as a direct mediator of the sympathetic metabolic nerve actions in rat liver and that hepatocytes contain also stimulatory prostaglandin receptors linked to phospholipase C in addition to the inhibitory receptors linked to adenylate cyclase known thus far.

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.

Increase of glucose and lactate output and decrease of flow by human anaphylatoxin C3a but not C5a in perfused rat liver

The complement fragments C3a and C5a were purified from zymosan-activated human serum by column chromatographic procedures after the bulk of the proteins had been removed by acidic polyethylene glycol precipitation. In the isolated in situ perfused rat liver C3a increased glucose and lactate output and reduced flow. Its effects were enhanced in the presence of the carboxypeptidase inhibitor DL-mercaptomethyl-3-guanidinoethylthio-propanoic acid (MERGETPA) and abolished by preincubation of the anaphylatoxin with carboxypeptidase B or with Fab fragments of an anti-C3a monoclonal antibody. The C3a effects were partially inhibited by the thromboxane antagonist BM13505. C5a had no effect. It is concluded that locally but not systemically produced C3a may play an important role in the regulation of local metabolism and hemodynamics during inflammatory processes in the liver.

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)

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

1. The metabolic and hemodynamic effects of prostaglandin F2 alpha, leukotriene C4 and the thromboxane A2 analogue U-46619 were studied during physiologically antegrade (portal to hepatic vein) and retrograde (hepatic to portal vein) perfusion and in a system of two rat livers perfused in sequence. 2. The stimulatory effects of prostaglandin F2 alpha (3 microM) on hepatic glucose release, perfusion pressure and net Ca2+ release were diminished by 77%, 95% and 64%, respectively, during retrograde perfusion when compared to the antegrade direction, whereas the stimulation of 14CO2 production from [1-14C]glutamate by prostaglandin F2 alpha (which largely reflects the metabolism of perivenous hepatocytes) was lowered by only 20%. Ca2+ mobilization and glucose release from the liver comparable to that seen during antegrade perfusion could also be observed in retrograde perfusions; however, higher concentrations of the prostaglandin were required. 3. The glucose, Ca2+ and pressure response to leukotriene C4 (20 nM) or the thromboxane A2 analogue U-46619 (200 nM) of livers perfused in the antegrade direction were diminished by about 90% during retrograde perfusion. Sodium nitroprusside (20 microM) decreased the pressure response to leukotriene C4 (20 nM) and U-46619 (200 nM) by about 40% and 20% in antegrade perfusions, respectively, but did not affect the maximal increase of glucose output. 4. When two livers were perfused antegradely in series, such that the perfusate leaving the first liver (liver I) entered a second liver (liver II), infusion of U-46619 at concentrations below 200 nM to the influent perfusate of liver I increased the portal pressure of liver I, but not of liver II. At higher concentrations of U-46619 there was also an increase of the portal pressure of liver II and with concentrations above 800 nM the pressure responses of both livers were near-maximal [19.6 +/- 0.8 (n = 7) cm H2O and 16.5 +/- 1.1 (n = 8) cm H2O for livers I and II, respectively]. There was a similar behaviour of glucose release from livers I and II in response to U-46619 infusion. When liver I was perfused in the retrograde direction, a significant pressure or glucose response of liver II (antegrade perfusion) could not be observed even with U-46619 concentrations up to 1000 nM. 5. Similarly, the perfusion pressure increase and glucose release induced by leukotriene C4 (10 nM) observed with liver II was only about 20% of that seen with liver I.(ABSTRACT TRUNCATED AT 250 WORDS)

Potential role for prostaglandin F2 alpha, D2, E2 and thromboxane A2 in mediating the metabolic and hemodynamic actions of sympathetic nerves in perfused rat liver

In isolated rat liver perfused at constant pressure perivascular nerve stimulation caused an increase of glucose and lactate output and a reduction of perfusion flow. The metabolic and hemodynamic nerve effects could be inhibited by inhibitors of prostanoid synthesis, which led to the suggestion that the effects of nerve stimulation were, at least partially, mediated by prostanoids [Iwai, M. & Jungermann, K. (1987) FEBS Lett. 221, 155-160]. This suggestion is corroborated by the present study. 1. Prostaglandin D2, E2 and F2 alpha as well as the thromboxane A2 analogue U46619 enhanced glucose and lactate release and lowered perfusion flow similar to nerve stimulation. 2. The extents, the kinetics and the concentration dependencies of the metabolic and hemodynamic actions of the various prostanoids were different. Prostaglandin F2 alpha and D2 caused relatively stronger changes of metabolism, while prostaglandin E2 and U46619 had stronger effects on hemodynamics. Prostaglandin F2 alpha elicited greater maximal alterations than D2 with similar half-maximally effective concentrations. Prostaglandin F2 alpha mimicked the nerve actions on both metabolism and hemodynamics best with respect to the relative extents and the kinetics of the alterations. 3. The hemodynamic effects of prostaglandin F2 alpha could be prevented completely by the calcium antagonist nifedipine without impairing the metabolic actions of the prostanoid. Apparently, prostaglandin F2 alpha influenced metabolism directly rather than indirectly via hemodynamic changes. The present results, together with the previously described effects of prostanoid synthesis inhibitors, suggest that prostanoids, probably prostaglandin F2 alpha and/or D2, could be involved in the actions of sympathetic hepatic nerves on liver carbohydrate metabolism. Since prostanoids are synthesized only in non-parenchymal cells, nervous control of metabolism appears to depend on complex intra-organ cell-cell interactions between the nerve, non-parenchymal and parenchymal cells.

Exposure to depolarizing concentrations of K+ inhibits hormonally-induced calcium influx in rat liver

The exposure of perfused rat livers to depolarizing concentrations of K+ (60 mM) by partial substitution of the NaCl in the medium with KCl induces glycogenolysis, respiratory changes and vasoconstriction. These responses were found to be inhibited 70-80% by 20 microM indomethacin and by 20 microM bromophenacyl bromide. This suggests that eicosanoids, namely prostaglandins, are involved in mediating these effects, and hence that the action of K+ involves primarily an effect on eicosanoid-producing cells (Kupffer and endothelial cells) within the liver. A 5 min pre-exposure of perfused livers to depolarizing concentrations of K+ (in the presence of indomethacin) was found to inhibit (by approx. 85%) the influx of Ca2+ induced by the co-administration of 10 nM glucagon and 10 nM vasopressin. A similar result was observed in isolated hepatocytes. The inhibition was probably not due to a decrease in the concentration of Na+ in the medium since the substitution of 80 mM NaCl with 80 mM choline chloride resulted in significantly less inhibition (30-40%). These results suggest that under these conditions the influx of Ca2+ in liver occurs through a pathway that is inhibited by high K+ concentration and/or a depolarization of the plasma membrane.

Net prostaglandin release by perfused rat liver after stimulation with phorbol 12-myristate 13-acetate

Phorbol myristate acetate, which was shown previously to elicit eicosanoid synthesis in primary cultures of Kupffer cells, led to a net release of prostaglandins (PG) D2 and E2 from the perfused rat liver. While a substantial amount of PGD2 (the major prostaglandin of Kupffer cells) left the liver, very little PGE2 was found in the effluent. Considerable amounts of immunologically reactive PGD2 and E2 were secreted with the bile. PGE2 rather than PGD2 was able to stimulate glycogenolysis and to increase perfusion pressure. These effects were, however, strongly dependent on the direction of the flow. If the liver was perfused in a retrograde fashion, i.e., from the vena cava to the portal vein, phorbol myristate acetate or PGE2 exerted only minor effects. These observations suggest a topological heterogeneity of producer and responder cells, respectively, in the liver sinusoid.

Leukotrienes increase glucose and lactate output and decrease flow in perfused rat liver

In isolated perfused rat liver leukotriene C4 and D4 but not B4 and E4 enhanced glucose and lactate output and lowered perfusion flow similar to the thromboxane A2 analogue U46619, extracellular ATP and prostaglandin F2 alpha. The kinetics of the metabolic changes caused by leukotriene C4 and D4 resembled those effected by U46619 and ATP but not those elicited by prostaglandin F2 alpha; the kinetics of the hemodynamic changes were similar only to those caused by U46619. The results show that leukotrienes could be important modulators of hepatic metabolism and hemodynamics and point to a complex intra-organ cell-cell communication between non-parenchymal and parenchymal cells.

Prostaglandin F2 alpha and the thromboxane A2 analogue ONO-11113 stimulate Ca2+ fluxes and other physiological responses in rat liver. Further evidence that prostanoids may be involved in the action of arachidonic acid and platelet-activating factor

The administration of prostaglandin F2 alpha (PGF2 alpha) and the thromboxane A2 analogue, ONO-11113, to rat livers perfused with media containing either 1.3 mM- or 10 microM-Ca2+ was followed by a stimulation of Ca2+ efflux, changes in O2 uptake and glucose output, and increase in portal pressure. The responses elicited by 5 microM-PGF2 alpha were similar to those induced by the alpha-adrenergic agonist phenylephrine. At both 1.3 mM and 10 microM extracellular Ca2+, PGF2 alpha induced Ca2+ efflux (70-90 nmol/g of liver), probably from the same source as that released by phenylephrine. Prostaglandin D2 (5 microM) and prostaglandin E2 (5 microM) also induced responses, but these were generally much smaller (less than 30%) than those induced by PGF2 alpha. Similarly to vasopressin and other Ca2+-mobilizing hormones, PGF2 alpha also interacted synergistically with glucagon (and cyclic AMP) in stimulating Ca2+ influx both in the perfused liver and in isolated hepatocytes. By comparison with phenylephrine and PGF2 alpha, ONO-11113 was much more potent in inducing vasoconstriction, and, at concentrations of 10-200 nM, induced a different pattern of changes in Ca2+ flux, respiration and glycogenolysis. There was first a rapid efflux of Ca2+ (45-60 nmol/g of liver), followed by a smaller Ca2+ influx, and a further release of Ca2+ (approx. 90 nmol/g of liver) when ONO-11113 was removed. Respiration was first stimulated but then markedly inhibited. At concentrations less than 5 nM, ONO-11113 induced a sustained stimulation of O2 uptake and a more prolonged efflux of Ca2+, with less Ca2+ efflux occurring upon the removal of the agent. Glycogenolysis followed a pattern which was similar to the Ca2+ response. Co-administration of glucagon did not potentiate Ca2+ influx by ONO-11113, but the action of ONO-11113 was inhibited (50%) by a few minutes' prior administration of 10 nM-vasopressin. The vasoconstrictive action of ONO-11113 was synergistically potentiated by the co-administration of phenylephrine. Since the actions of arachidonic acid, platelet-activating factor and lysophosphatidylcholine in liver were recently found to be cyclo-oxygenase-sensitive, the results provide strong evidence that at least PGF2 alpha and thromboxane A2 may be involved in mediating the action of these agents.

Phosphatidic acid and arachidonic acid each interact synergistically with glucagon to stimulate Ca2+ influx in the perfused rat liver

The administration of phosphatidic acid to rat livers perfused with media containing either 1.3 mM- or 10 microM-Ca2+ was followed by a stimulation of Ca2+ efflux, O2 uptake and glucose output. The responses elicited by 100 microM-phosphatidic acid were similar to those induced by the alpha-adrenergic agonist phenylephrine. Contrary to suggestions that phosphatidic acid acts like a Ca2+-ionophore, no net influx of Ca2+ was detected until the phosphatidic acid was removed. Sequential infusions of phenylephrine and phosphatidic acid indicate that the two agents release Ca2+ from the same intracellular source. The co-administration of glucagon (or cyclic AMP) and phosphatidic acid, and also of glucagon and arachidonic acid, led to a synergistic stimulation of Ca2+ uptake of the liver, a feature similar to that observed after the co-administration of glucagon and other Ca2+-mobilizing hormones [Altin & Bygrave (1986) Biochem. J. 238, 653-661]. A notable difference, however, is that the synergistic stimulation of Ca2+ uptake induced by the co-administration of glucagon and arachidonic acid was inhibited by indomethacin, whereas that induced by glucagon and phosphatidic acid, or glucagon and other Ca2+-mobilizing agents, was not. The results suggest that the synergistic action of glucagon and arachidonic acid in stimulating Ca2+ influx is mediated by prostanoids, but that of glucagon and phosphatidic acid is evoked by a mechanism similar to that of Ca2+-mobilizing agents.