Stimulation of glycogenolysis in isolated hepatocytes by adenosine and one of its analogues is inhibited by caffeine

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Purinergic effects of a hydroalcoholic Agaricus brasiliensis (A. blazei) extract on liver functions

The effects of a hydroalcoholic extract of Agaricus brasiliensis (A. blazei) on functional parameters in the perfused rat liver were examined with emphasis on its content of nucleotides and nucleosides. Several nucleosides and nucleotides were identified in the A. brasiliensis extract, which was active on several liver functions. A significant part of the effects is the result of the purinergic action of nucleosides and nucleotides: pressure increment, glycogenolysis stimulation, transient inhibition of oxygen consumption, and redox state changes. Other phenomena such as the stimulation of gluconeogenesis, ureogenesis, and oxygen consumption are more likely consequences of the metabolic transformation of substrates contained within the extract, especially amino acids. It seems apparent that consumption of A. brasiliensis represents not only the ingestion of metabolic precursors but also the ingestion of substances that, even at low concentrations, can exert important signaling functions in the liver as well as in the organism as a whole.

Regulation of glycogen metabolism in hepatocytes through adenosine receptors. Role of Ca2+ and cAMP

The objective of this work is to identify the adenosine receptor subtype and the triggered events involved in the regulation of hepatic glycogen metabolism. Glycogenolysis, gluconeogenesis, cAMP, and cytosolic Ca2+ ([Ca2+](cyt)) were measured in isolated hepatocytes challenged with adenosine A1, A2A, and A3 receptor-selective agonists. Stimulation of adenosine receptor subtypes with selective agonists in Ca2+ media produced a dose-dependent increase in [Ca2+]cyt with A1>A2=A3, cAMP with A2A, glycogenolysis with A1>A2A>A3, and gluconeogenesis with A2A>A1>A3, in addition, a decrease in cAMP was observed with A1=A3. Comparatively, in Ca2+-free media or with a cell membrane-permeant Ca2+ chelator, activation of these adenosine receptors with the same selective agonists produced a smaller and transient rise in [Ca2+]cyt with A1=A3>A2, no rise in glycogenolysis and gluconeogenesis with A3>A1, but a full rise with A2A. Thus, in isolated rat hepatocytes activation of the adenosine A1 receptor triggered Ca2+-mediated glycogenolysis, activation of the adenosine A2A receptor stimulated cAMP-mediated gluconeogenesis, and activation of the adenosine A3 receptor increased [Ca2+]cyt and decreased cAMP with minor changes in glycogen metabolism.

Role of adenosine A1 receptors in inhibition of receptor-stimulated cyclic AMP production by ethanol in hepatocytes

Brief exposure of primary cultures of hepatocytes to ethanol had a biphasic effect on glucagon receptor-dependent cyclic AMP (cAMP) production: 25-50 mM ethanol decreased cAMP levels, whereas treatment with 100-200 mM ethanol increased cAMP. This biphasic effect was also observed after pretreatment with 10 microM 4-methylpyrazole, an inhibitor of alcohol dehydrogenase. Adenosine A1 and A2 receptors in primary cultures of rat hepatocytes are coupled to inhibition and stimulation of adenylyl cyclase, respectively. Since primary cultures of hepatocytes release adenosine into their extracellular media, we tested whether the acute effects of ethanol on cAMP were mediated by extracellular adenosine. Co-incubation with 2 U/mL adenosine deaminase prevented inhibition of cAMP production by 25-50 mM ethanol, but had no effect on stimulation by 100-200 mM ethanol. Pretreatment of hepatocytes with 110 nM 8-cyclopentyl-1,3-dimethylxanthine, an adenosine A1 receptor antagonist, also completely blocked the inhibitory effects of ethanol on cAMP production. Low concentrations of ethanol enhanced the inhibitory effects of R(-)N6-(2-phenylisopropyl)adenosine, an A1 receptor agonist, on cAMP production in cells pretreated with adenosine deaminase to remove endogenous adenosine. These data suggest that endogenously produced adenosine can be an important modulator of the effects of ethanol on receptor-stimulated cAMP production in primary cultures of rat hepatocytes.

Adenosine A1 receptors mediate chronic ethanol-induced increases in receptor-stimulated cyclic AMP in cultured hepatocytes

Cellular responses to adenosine depend on the distribution of the two adenosine receptor subclasses. In primary cultures of rat hepatocytes, adenosine receptors were coupled to adenylate cyclase via A1 and A2 receptors which inhibit and stimulate cyclic AMP production respectively. R-(-)-N6-(2-phenylisopropyl)-adenosine (R-PIA), the adenosine A1 receptor-selective agonist, inhibited glucagon-stimulated cyclic AMP production with an IC50 of 19 nM. This inhibition was blocked by the A1-specific antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPDX). 5'-N- Ethylcarboxamidoadenosine (NECA), an agonist which stimulates A2 receptors, increased cyclic AMP production with an EC50 of 0.6 microM. Treatment of primary cultures of rat hepatocytes with 100 mM ethanol for 48 h decreases the quantity and function of the inhibitory guanine-nucleotide regulatory protein (G(i)), resulting in a sensitization of receptor-stimulated cyclic AMP production [Nagy and deSilva (1992) Biochem. J. 286, 681-686]. When cells were cultured with 2 units/ml adenosine deaminase, to degrade extracellular adenosine, ethanol-induced increases in cyclic AMP production were completely prevented. Moreover, the specific A1-receptor antagonist, CPDX, also blocked the chronic effects of ethanol on receptor-stimulated cyclic AMP production. Treatment with adenosine deaminase or CPDX also prevented the decrease in quantity of the alpha subunit protein of G(i) observed in hepatocytes after chronic treatment with ethanol. Taken together, these results suggest that activation of adenosine A1 receptors on primary cultures of hepatocytes is involved in the development of chronic ethanol-induced sensitization of receptor-stimulated cyclic AMP production.

Nonparenchymal cell and hepatocellular injury to human liver grafts assessed by enzyme-release into the perfusate

Experimental studies have demonstrated preferential injury to the sinusoidal endothelium during liver preservation with University of Wisconsin (UW) or Euro-Collins solution. This endothelial cell injury has an unclear pathogenesis, and it has not yet been studied in the human liver. Therefore, we analyzed the effluent of 21 human liver allografts after cold storage. Markers of hepatocellular and nonparenchymal cell injury were assessed. After preservation with UW solution, early effluent samples contained 1823 +/- 1494 U/l lactate dehydrogenase (LDH), 493 +/- 516 U/l alanine aminotransferase (ALT) and 132 +/- 97 U/l creatine kinase (CK; 92 +/- 92 U/l CK-BB). The effluent of livers preserved in histidine-tryptophan-ketoglutarate (HTK) solution contained 3681 +/- 2009 U/l LDH, 1139 +/- 599 U/l ALT and 282 +/- 120 U/l CK (165 +/- 91 U/l CK-BB). Comparison of effluent enzyme activities with liver tissue enzyme activities indicates that the release of the endothelial cell/nonparenchymal cell marker creatine kinase was higher, by a factor of 7-8, than the release of hepatocellular enzymes. Effluent thrombomodulin concentrations were 123 +/- 248 ng/ml (UW) and 604 +/- 299 ng/ml (HTK), and effluent glucose concentrations, 40.3 +/- 27.0 mM (726 +/- 486 mg/dl; UW) and 10.4 +/- 4.5 mM (187 +/- 81 mg/dl; HTK). We conclude that prominent endothelial cell injury also occurs in human liver grafts after preservation with UW solution or HTK solution. This endothelial cell injury is unlikely to be caused by hypoxia-induced energy deficiency, as it affects a cell type with a high glycolytic capacity in the presence of high glucose levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of ecto-5'-nucleotidase in the rat liver: effect of anaemia

In the kidney a striking parallel exists between the expression of ecto-5'-nucleotidase and of erythropoietin by renal fibroblasts. It was therefore hypothesized that the expression of ecto-5'-nucleotidase in fibroblasts might be controlled by oxygen tension. In order to test this hypothesis, we examined the distribution of the enzyme in a tissue which displays a defined zonation in respect to oxygen tension, namely in the liver; anaemia was used in order to exaggerate this zonation. The distribution of ecto-5'-nucleotidase was investigated by light and electron microscopy using enzyme and immunohistochemical methods in the livers of healthy and of anaemic rats. Anaemia was produced by haemolysis combined with X-ray irradiation. The enzyme was detected in the bile canaliculi, in the connective tissue of the portal triads and of the central veins, and in fat-storing cells probably corresponding to a special form of fibroblasts. In healthy animals the perisinusoidal ecto-5'-nucleotidase activity was slightly higher in the pericentral than in the periportal area of the acinus whereas the inverse was observed for the staining of bile canaliculi. Anaemia provoked an increase of ecto-5'-nucleotidase in fat-storing cells in the pericentral zone of the acinus and in fibroblasts around the central veins, resulting in steepended gradients along the sinusoids. The intralobular gradient of ecto-5'-nucleotidase in perisinusoidal cells and the effect thereon of anaemia suggest that the expression of the ecto-5'-nucleotidase might be directly or indirectly controlled by local oxygen tension.

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.

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.

Adenosine deaminase and porcine meat quality. II. Effects of adenosine analogues on plasma free fatty acids, glucose and lactate in pigs representing high and low adenosine deaminase red cell activity

The adenosine analogues 5'-(N-ethyl) carboxamidoadenosine (NECA) and N6-(phenylisopropyl) adenosine (R-PIA) were shown to differ in their effect on the plasma level of free fatty acids (FFA), glucose and lactate in pigs representing low (Ada 0) and high (Ada A) red cell adenosine deaminase activity. At the same dosage range (0.001-0.005 mg/kg) R-PIA produced a much stronger suppression of the FFA level than NECA, indicating that A1 adenosine receptors predominate in porcine adipose tissue. Pretreatment with 8-phenyltheophylline completely abolished the antilipolytic effect of both adenosine analogues. NECA in contrast to R-PIA elevated the blood glucose concentration, suggesting that A2 adenosine receptors are involved in the stimulation of glycogenolysis. This effect of NECA was not altered by a beta-adrenoceptor blockade providing evidence for a direct effect of adenosine on glycogenolysis. Whereas the changes in plasma FFA following NECA administration were of similar magnitude in Ada A and Ada 0 pigs, the changes in the blood glucose concentration were different in these two groups of pigs.

Adenosine deaminase and porcine meat quality. II. Effects of adenosine analogues on plasma free fatty acids, glucose and lactate in pigs representing high and low adenosine deaminase red cell activity

The adenosine analogues 5'-(N-ethyl) carboxamidoadenosine (NECA) and N6-(phenylisopropyl) adenosine (R-PIA) were shown to differ in their effect on the plasma level of free fatty acids (FFA), glucose and lactate in pigs representing low (Ada 0) and high (Ada A) red cell adenosine deaminase activity. At the same dosage range (0.001-0.005 mg/kg) R-PIA produced a much stronger suppression of the FFA level than NECA, indicating that A1 adenosine receptors predominate in porcine adipose tissue. Pretreatment with 8-phenyltheophylline completely abolished the antilipolytic effect of both adenosine analogues. NECA in contrast to R-PIA elevated the blood glucose concentration, suggesting that A2 adenosine receptors are involved in the stimulation of glycogenolysis. This effect of NECA was not altered by a beta-adrenoceptor blockade providing evidence for a direct effect of adenosine on glycogenolysis. Whereas the changes in plasma FFA following NECA administration were of similar magnitude in Ada A and Ada 0 pigs, the changes in the blood glucose concentration were different in these two groups of pigs.

The role of nonparenchymal and parenchymal liver cells in the catabolism of extracellular purines

Adenosine-degrading enzymes within the liver lobule can modulate both vascular and metabolic effects of circulating adenosine in the liver. Since it has not been fully established whether nonparenchymal cells participate in the elimination of sinusoidal purines, isolated Kupffer cells and endothelial cells were tested for their capacity to degrade extracellular purines. After perfusion and digestion of rat livers by collagenase, the resulting mixed cell population was separated by centrifugal elutriation. The isolated parenchymal and nonparenchymal cells were incubated for up to 2 hr in the presence of [8(-14)C]adenosine, [8(-14)C]guanosine and [8(-14)C]hypoxanthine (50 mumoles per liter). In the deproteinized medium, adenosine, guanosine, inosine, adenine, guanine, xanthine, hypoxanthine, uric acid and allantoin were separated by reversed-phase high-performance liquid chromatography. Radioactive peaks were collected and counted. Nonparenchymal cells catalyzed the degradation of adenosine into inosine and hypoxanthine. However, the formation of xanthine, uric acid or allantoin from adenosine could only be detected in hepatocyte suspensions. Within 15 min, adenosine was completely eliminated from the medium by Kupffer cells, whereas endothelial cells catabolized only less than half of the initial amount of the adenine nucleoside during this time period. Accordingly, incubation of nonparenchymal cells in the presence of hypoxanthine did not result in the formation of further breakdown products of the purine, whereas its catabolites slowly accumulated in the medium of hepatocytes. Guanosine conversion into guanine and xanthine was much slower in endothelial cells as compared to Kupffer cells and hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)