Inverse effects of D-galactosamine and inorganic phosphate on glycogenolysis in isolated rat hepatocytes

Prostaglandin cytoprotection of galactosamine-incubated hepatocytes isolated from young and old rats

The aim of the study was to investigate the effect of aging on cytoprotective properties of prostaglandins. Hepatocytes were obtained by collagenase perfusion of livers of young (4-6 mo) and old (24-28 mo) male Wistar rats. Cells were incubated for 1.5 h in Krebs-Ringer-bicarbonate buffer containing glucose and 3H-leucine in the presence of galactosamine (2.5-100 mM), PGE1, or two prostacyclin analogues: 9 beta-methylcarbacyclin and TRK-100. Cell damage was assessed by decrease in the rate of protein synthesis measured as 3H-leucine incorporation into acid precipitable material, and by increase in lactate dehydrogenase release into the medium. Hepatocytes from old rats were more susceptible to suppression of protein synthesis by GalN than cells of young ones. Preincubation of cells for 15 min with 9MC (41-560 nM) or PGE1 (10-100 nM), but not with TRK-100, before adding 10 mM GalN, led to a partial recovery of protein synthesis in both age groups. GalN increased LDH release and decreased ATP/ADP ratio to a similar extent in hepatocytes of young and old rats; both parameters were not altered by preincubation of cells with PGs. PGE1 and 9MC, but not TRK-100, elevated cyclic AMP content in hepatocytes of young but not old rats. Glucagon and forskolin similarly increased cyclic AMP content in cells of both young and old animals. These in vitro results suggest that PGE1 and some prostacyclin analogues might protect hepatocytes of both young and old rats from chemical damage, and stress the necessity for further research on cyto- and hepato-protection in the elderly.

Toxicity of D-galactosamine for rat hepatocytes in monolayer culture

Hepatocellular injury was induced by exposure of primary cultures of rat hepatocytes to 4 mM D-galactosamine. The cell damage was very similar to that seen in vivo and in the isolated perfused rat liver, both in biochemical and in structural terms. The severity of the lesions caused by D-galactosamine was dependent on the age of the culture being treated. Less severe damage was found with older cultures. Since the primary metabolic effects of D-galactosamine were age-independent, the reduction in cell damage seems to be due to progressive cell dedifferentiation. Dexamethasone (1 microM) suppressed the full development of the injury, while 1 microM triiodo-L-thyronine enhanced it. A protection of hepatocytes by alpha 2-macroglobulin against the effects of D-galactosamine could be observed neither in vivo nor in vitro. Direct cytotoxic effects of endotoxin from Salmonella minnesota R 595 could be demonstrated only on hepatocytes in the early phases of primary culture using rather high doses of the purified lipopolysaccharide. It is unlikely that they play a major role in the hepatocellular injury seen following endotoxinemia in vivo. Lowering of extracellular Ca2+ concentration and additions of calcium/calmodulin inhibitors did not prevent cell injury after treatment with D-galactosamine. The results suggest that cell death is not due to an increased influx of Ca2+ into the cells.

Effect of D-galactosamine in vitro on [U-14C]palmitate oxidation, triacylglycerol synthesis and secretion in isolated hepatocytes

Isolated rat hepatocytes were used to study in vitro effects of 10 mM D-galactosamine (GalN) on hepatic fatty acids metabolism. At this concentration, membrane integrity and biochemical competence (i.e., gluconeogenesis and ureogenesis) remained unaffected. Protein synthesis and secretion, as measured by the incorporation of [U-14C]leucine into total and medium protein, was significantly inhibited when incubated for more than 2 h. GalN activated the incorporation of [U-14C]palmitate into triacylglycerols and depressed its utilization in the formation of labelled ketone bodies and 14CO2. Hepatocytes isolated from fasted rats exposed to GalN in vitro did not show any variation in prelabelled triacylglycerol secretion. GalN induced a rapid inhibition of prelabelled triacylglycerol secretion by hepatocytes isolated from fed rats in which this secretion occurred to a larger extent than in hepatocytes isolated from fasted rats. The data reported here suggest that GalN induces a rise of triacylglycerol synthesis by inhibiting the palmitate oxidation pathway and a decrease of triacylglycerol secretion through an early derangement of the secretory pathway.

Methionine metabolism and ultrastructural changes with D-galactosamine in isolated rat hepatocytes

The biochemical and morphological effects of 2, 10 and 100 mM of D-galactosamine (GalN) were studied in isolated rat hepatocytes during 2 h of incubation. Lactate dehydrogenase (LDH), alanine aminotransferase (ALAT) and cell viability did not change, whatever the concentration used. The variations observed, which were dose dependent, included a large drop in ATP levels and inhibition of RNA and protein synthesis. A very high concentration of GalN was necessary, however, to induce a significant decline in methionine adenosyltransferase activity compared to control cells. The use of L-[methyl-14C]methionine during cell incubation with GalN demonstrated a decrease of S-adenosyl-L-methionine (SAMe) and an accumulation of L-methionine content related to the GalN concentration. These results suggested that an hepatotoxic agent such as GalN was able to induce disturbances of methionine metabolism. Some of the ultrastructural changes observed were different from those previously found in vivo, in rats given GalN intraperitoneally, underlining the marked difference between in vivo and in vitro intoxication.

Possible role of Pi supply in mitochondrial actions of glucagon

Glucagon is able to diminish the net release of inorganic phosphate (Pi) occurring on incubation of isolated hepatocytes from 48-h-starved rats. Concomitantly the hormone increases the cellular Pi content. This is associated with a rise of Pi in the cytosolic fraction. Other hormonal effectors like phenylephrine, vasopressin and angiotensin II exert a smaller and transient effect as compared to glucagon. It is proposed that this increase in Pi availability to the mitochondria, by favouring substrate level phosphorylation at the succinyl-CoA synthetase step plays a role in the development of the metabolite pattern found in the mitochondrial matrix space after exposure of hepatocytes to glucagon or the above agents. With regard to the glutamate level this view is evidenced by the finding that its hormone-dependent decrease was inversely correlated to the respective increase in the cytosolic Pi concentration. Further evidence is provided by experiments with isolated mitochondria incubated under state-3 conditions at medium Pi concentrations corresponding to those metabolically active in the cytosolic compartment of control and glucagon-stimulated hepatocytes, being 2 mM and 3 mM, respectively. Increasing medium phosphate concentration from 2 mM to 3 mM caused a marked decrease in the level of succinyl-CoA and increased the rates of 2-oxoglutarate utilization and of malate and phosphoenolpyruvate production. Citrulline synthesis also was found to be stimulated at 3 mM Pi. Taken together our results suggest a role of Pi supply in mitochondrial actions of glucagon in intact hepatocytes. Moreover, they could contribute to a better interpretation of glucagon effects on isolated mitochondria from hormone-pretreated liver cells.

On the mechanism of hepatic glycogenolysis induced by anoxia or cyanide

Addition of glucagon to isolated hepatocytes increased glycogenolysis and phosphorylase a in a proportional manner. KCN caused slightly more glycogenolysis at considerably lower levels of phosphorylase a; the discrepancy was most pronounced after pretreatment of the hepatocytes with EGTA. When incubated with tagatose, the hepatocytes accumulated tagatose 1-phosphate, a presumed inhibitor of phosphorylase a. In these conditions the glucagon-induced glycogenolysis was blocked, but the glycogen loss caused by KCN or anoxia was not affected. Cyanide and anoxia may allow phosphorylase b and a to become equally active, or they may trigger a non-phosphorolytic glycogenolysis.

Ordered synthesis and degradation of liver glycogen involving 2-amino-2-deoxy-D-glucose

The incorporation of 2-amino-2-deoxy-D-glucose from precursor 2-amino-2-deoxy-D-galactose into liver glycogen has been shown to be a metabolically inhomogeneous process after starvation. The protein-to-polysaccharide ratio is also heterogeneous with respect to molecular size, and enhanced overall as compared to normal glycogen. The results are discussed from the viewpoint of a molecular order in the synthesis and degradation of liver glycogen.

Influence of D-galactosamine upon the NAD-metabolism in rat liver

After application of D-galactosamine a hepatitis develops in the rat liver. This can be prevented by different agents, including tryptophan. Yet it has not been possible to give definitive conclusions about the mechanism of galactosamine hepatitis. In this paper we report about the influence of galactosamine on the NAD metabolism. D-galactosamine inhibits the NAD synthesis initiated by nicotinamide in normal and adrenalectomized animals. The NAD synthesis from tryptophan is prevented in normal animals, in adrenalectomized ones however there is an increase of NAD in the presence of D-galactosamine reduces the activity of the ADPR transferase. Inhibitors of the ADPR transferase prevent the galactosamine hepatitis. From the results presented we conclude that the ADPR transferase plays an important role in the development of the galactosamine hepatitis.

Uridylate trapping, induction of UTP deficiency, and stimulation of pyrimidine synthesis de novo by D-galactosone

d-Galactosone (d-lyxo-2-hexosulose) is phosphorylated and metabolized to the uridine diphosphate derivative in AS-30D hepatoma cells and rat liver. These reactions were catalysed in vitro by galactokinase and hexose-1-phosphate uridylyltransferase. Nucleotide analyses by high-performance liquid chromatography and enzymic assays revealed that this galactose analogue interferes with cellular pyrimidine nucleotide metabolism leading to a deficiency of UTP. [(14)C]Uridine labelling of hepatoma cells indicated a division of [(14)C]uridylate from UTP into UDP-galactosone; the latter was formed at a rate of more than 1.7mmolxh(-1)x(kg AS-30D or liver wet wt.)(-1). As a consequence of UTP deficiency, d-galactosone (1mmol/1 or 1mmol/kg body wt.) strongly enhanced the rate of pyrimidine synthesis de novo as evidenced by incorporation of (14)CO(2) into uridylate and by an expansion of the uridylate pool. This resulted in a doubling of the total acid-soluble uridylate pool within 70min in the hepatoma cells and within 110min in rat liver. Combined treatment of hepatoma cells with d-galactosone and N-(phosphonoacetyl)-l-aspartate, an inhibitor of aspartate carbamoyltransferase, prevented the expansion of the uridylate pool and led to a synergistic reduction of UTP to 10% of the content in control cells. Hepatic UTP deficiency was selective with respect to other nucleotide 5'-triphosphates but was associated with reduced contents of UDP-glucose, UDP-glucuronate, and UDP-N-acetylhexosamines. Isolation of the UDP derivative of d-galactosone revealed an extremely alkali-labile UDP-sugar, probably an isomerization product of UDP-galactosone, that was degraded by elimination of UDP with a half-life of 45min at pH7.5 and 37 degrees C. The instability of UDP-galactosone may contribute in vivo to limit the time period of severe uridine phosphate deficiency in addition to the compensatory role of pyrimidine synthesis de novo. During the initial time period, however, d-galactosone is effective as a powerful uridylate-trapping sugar analogue.

Purine catabolism in isolated rat hepatocytes. Influence of coformycin

1. The catabolism of purine nucleotides was investigated by both chemical and radiochemical methods in isolated rat hepatocytes, previously incubated with [(14)C]adenine. The production of allantoin reached 32+/-5nmol/min per g of cells (mean+/-s.e.m.) and as much as 30% of the radioactivity incorporated in the adenine nucleotides was lost after 1h. This rate of degradation is severalfold in excess over values previously reported to occur in the liver in vivo. An explanation for this enhancement of catabolism may be the decrease in the concentration of GTP. 2. In a high-speed supernatant of rat liver, adenosine deaminase was maximally inhibited by 0.1mum-coformycin. The activity of AMP deaminase, measured in the presence of its stimulator ATP in the same preparation, as well as the activity of the partially purified enzyme, measured after addition of its physiological inhibitors GTP and Pi, required 50mum-coformycin for maximal inhibition. 3. The production of allantoin by isolated hepatocytes was not influenced by the addition of 0.1mum-coformycin, but was decreased by concentrations of coformycin that were inhibitory for AMP deaminase. With 50mum-coformycin the production of allantoin was decreased by 85% and the formation of radioactive allantoin from [(14)C]adenine nucleotides was completely suppressed. 4. In the presence of 0.1mum-coformycin or in its absence, the addition of fructose (1mg/ml) to the incubation medium caused a rapid degradation of ATP, without equivalent increase in ADP and AMP, followed by transient increases in IMP and in the rate of production of allantoin; adenosine was not detectable. In the presence of 50mum-coformycin, the fructose-induced breakdown of ATP was not modified, but the depletion of the adenine nucleotide pool proceeded much more slowly and the rate of production of allantoin increased only slightly. No rise in IMP concentration could be detected, but AMP increased manyfold and reached values at which a participation of soluble 5'-nucleotidase in the catabolism of adenine nucleotides is most likely. 5. These results are in agreement with the hypothesis that the formation of allantoin is controlled by AMP deaminase. They constitute further evidence that 5'-nucleotidase is inactive on AMP, unless the concentration of this nucleotide rises to unphysiological values.

Consequences of recurrent phosphate trapping induced by repeated injections of 2-deoxy-D-galactose. Biochemical and morphological studies in rats

2-Deoxy-D-galactose, in a dose of 3 mmol/kg, was administered intraperitoneally twice daily to young rats for periods up to 12 weeks. This dosage schedule resulted in recurrent phosphate trapping predominantly in liver. UTP deficiency was excluded by simultaneous uridine injections. Phosphate trapping was caused by the rapid accumulation of 2-deoxy-D-galactose 1-phosphate and was most pronounced in liver but also demonstrated in small intestine, brain, spleen, and thymus. The marked, although transient, drop in the hepatic content of inorganic phosphate triggered the catabolism of adenine nucleotides and a loss of ATP. Other metabolic pathways affected by phosphate deficiency include glycogenolysis and glycolysis. Increasing with time, repeated doses of the galactose analog led to retardation and arrest of growth, hepatomegaly, and splenomegaly. The average relative liver and spleen weights were elevated 2.5- and 4.5-fold, respectively, after 12 weeks of treatment. Liver damage was indicated by hyperbilirubinaemia and a progressive rise in the activity in plasma of sorbitol dehydrogenase, alkaline phosphatase, and gamma-glutamyltransferase. Examination by light and electron microscopy showed increasing numbers of vacuoles, surrounded by a single membrane, in hepatocytes, sinusoidal endothelial cells, and Kupffer cells. Focal cytoplasmic degeneration in hepatocytes was occasionally indicated by formation of autophagic vacuoles and finger print lysosomes. Hepatocytes of 2-deoxy-D-galactose-treated rats showed a dissociation and fragmentation of the rough endoplasmic reticulum. Sinusoidal endothelial cells and Kupffer cells were markedly enlarged, the latter contained a PAS-positive but amylase resistant substance. Extrahepatic changes included an increased occurrence of vacuolated cells in thymus. Phosphate trapping and its metabolic consequences are common phenomena in the experimental injury induced b 2-deoxy-D-galactose and in some hereditary diseases such as uridylyltransferase deficiency galactosaemia, fructose intolerance and glucose-6-phosphatase deficiency.