Tumor necrosis factor is a terminal mediator in galactosamine/endotoxin-induced hepatitis in mice.

Intravenous injection of murine recombinant tumor necrosis factor alpha(TNF-alpha) to male NMRI albino mice in doses greater than 4 micrograms/kg (specific activity 4 x 10(7) U/mg) resulted in a fulminant hepatitis when animals had been sensitized 1 hr before by intraperitoneal administration of 700 mg/kg galactosamine. Liver injury was assessed by measurement of serum transaminases as well as sorbitol dehydrogenase activity 8 hr after administration of TNF-alpha. Pretreatment with either galactosamine or 40 micrograms/kg TNF-alpha alone did not cause hepatitis. Pretreatment of galactosamine/TNF-alpha-injured mice with 800 mg/kg uridine or with 6 mg/kg calmidazolium fully protected the animals, while administration of either verapamil or nifedipine (100 mg/kg, respectively) had no significant effect. The following inhibitors of generation or action of leukotriene D4, which were previously shown to block galactosamine/endotoxin-induced hepatitis in mice, failed to protect against galactosamine/TNF-alpha-induced intoxication: 200 micrograms/kg dexamethasone, 174 mg/kg BW 755 C or 13 x 10 mg/kg FPL 55712. In addition, unlike in the galactosamine/endotoxin model no prevention was achieved by pretreatment of galactosamine/TNF-alpha-injured animals with the following substances blocking the development of an ischemia/reperfusion syndrome: 2 x 100 mg/kg allopurinol, 3.3 x 10(4) U/kg superoxide dismutase, 10(6) U/kg catalase or 10 micrograms/kg iloprost. We conclude from our results that tumor necrosis factor alpha is likely to act as a final mediator of endotoxin action in a sequence of events which includes formation of leukotriene D4 and reactive oxygen species.

Eicosanoids and skin.

A better understanding of the physiological and pathological roles of lipoxygenase metabolites on skin will be obtained with the development of safe and specific 5- and 12-lipoxygenase inhibitors. Lipocortins have been cloned and the results of clinical trials with them should provide further insight into the mechanism(s) of action of steroids on skin diseases.

Non-parenchymal cells as mediators of physiological responses in liver.

Parenchymal cells (hepatocytes) are the sites at which the principal metabolic functions of the liver are located. In the perfused liver, responses (e.g. vasoconstriction and glycogenolysis) to stimulating agents such as zymosan, platelet-activating factor and arachidonic acid, are inhibited by indomethacin and bromophenacyl bromide, inhibitors of cyclo-oxygenase and phospholipase A2, respectively. Since cultured Kupffer and endothelial cells but not hepatocytes, produce eicosanoids, and since eicosanoids and especially prostaglandins induce similar patterns of responses when added directly to the perfused liver, an involvement of these non-parenchymal cells in mediating the above responses is considered likely. We propose that in most situations the responses induced by these stimulating agents are mediated through a combination of pathways that include interaction of the agents directly with hepatocytes or with vasoactive cells (endothelial and/or smooth muscle cells), or interaction of agents initially with non-parenchymal cells to produce and release eicosanoids, which then subsequently interact with hepatocytes or with vasoactive cells.

The role of eicosanoids in paediatrics.

Prostanoids are unsaturated cyclic fatty acids, are synthesized primarily from arachidonic acid, and, like the leukotrienes, belong to the growing family of eicosanoids. As tissue hormones, prostanoids act on specific receptors near their site of synthesis and degradation. Prostanoids operate as modulators and mediators in a large spectrum of physiological processes. They are involved in the regulation of maternal and fetal circulation, patency of the ductus arteriosus, platelet-vessel wall interaction and kidney function. Besides their physiological function in protecting organ perfusion under stress conditions, they are also involved in diseases as described in the hyperprostaglandin E2-syndrome or--together with leukotrienes--in inflammatory processes. More specific pharmacological tools than the nonsteroidal antiinflammatory drugs, such as receptor antagonists, selective synthesis inhibitors, and eicosanoid analogues offer the prospect of enriching our arsenal of pharmacotherapeutic interventions in a variety of diseases. Before active intervention, however, more and specific biochemical analyses are required to identify the pathophysiological role of eicosanoid.

Eicosanoids as pluripotential modulators of pancreatic islet function.

Eicosanoids both negatively and positively modulate glucose-induced insulin secretion. Although the identity of the positive modulator is uncertain, the negative modulator appears to be prostaglandin E2 (PGE2), because 1) glucose stimulates PGE2 synthesis from islet cells; 2) exogenous PGE2 inhibits glucose-induced insulin secretion; 3) inhibition of beta-cell PGE2 synthesis increases glucose-induced insulin secretion, and this increase is reversed by exogenous PGE2; and 4) PGE2 binds to specific beta-cell receptors that are coupled to inhibitory regulatory components of adenylate cyclase whose activation decreases cAMP levels. Other possible regulatory effects of eicosanoids on islet function include modulation of islet blood flow and its immune responsiveness. From these considerations, the perspective is offered that eicosanoids are pluripotential modulators of islet function.

Eicosanoids and hypoxic pulmonary vasoconstriction in normal man.

Pulmonary haemodynamics and blood gas tensions were investigated in eight healthy volunteers, breathing room air and at the 15th min of an acute inspiratory hypoxia (fraction of inspired oxygen, (FIO2), 0.125) before and after administration of ibuprofen, a cyclooxygenase inhibitor, and of dazoxiben, a thromboxane A2 (TxA2) synthetase inhibitor; both drugs either with or without an infusion of prostaglandin E1. Hypoxia decreased arterial oxygen tension (PaO2) to below 50 mmHg in every subject and increased pulmonary vascular resistance by an average of 100-150% from baseline values. Acute and chronic dazoxiben or ibuprofen administration markedly reduced serum thromboxane B2 (TxB2), the stable metabolite of TxA2, but had no effect on pulmonary haemodynamics and blood gas tensions in both normoxic and hypoxic conditions. Prostaglandin E1 given in addition to ibuprofen or to dazoxiben did not inhibit hypoxia-induced increases in pulmonary vascular resistance. The stability of this hypoxic pressor response on repetition of an acute hypoxic exposure was established in six additional healthy subjects. Although obtained on a small number of subjects, these results do not suggest that products of the cyclooxygenase pathway of arachidonic acid metabolism play an important role in modulating normoxic or hypoxic pulmonary vascular tone in man.

Injury to rat hearts produced by an exogenous free radical generating system. Study into the role of arachidonic acid and eicosanoids.

This study was designed to evaluate the effect of an exogenous free radical generating system consisting of purine plus xanthine oxidase on the isolated rat heart and in particular to assess the possible contribution of arachidonic acid or its metabolites to toxicity produced by this drug combination. Purine plus xanthine oxidase produced a time-dependent depression in cardiac contractility which was associated with stimulated release of lactate dehydrogenase (LDH). Electron microscopic analysis revealed a distinct separation of the glycocalyx from the sarcolemmal membrane with no apparent intracellular defects. Purine plus xanthine oxidase was a potent stimulus for 6-keto-prostaglandin F1 alpha (6K-PGF1 alpha) synthesis but leukotriene production was undetectable under any condition. Eicosatetraynoic acid, which totally prevents the metabolism of arachidonic acid, accelerated the loss in force and increased LDH release invoked by purine plus xanthine oxidase, but produced no noticeable change in sarcolemmal ultrastructure. Cyclooxygenase inhibitors produced little influence although pretreatment with either acetylsalicylic acid or ibuprofen decreased contractility toward the end of purine plus xanthine oxidase perfusion. Nordihydroguarietic acid, a purported inhibitor of 5'-lipoxygenase accelerated the loss in force produced by purine plus xanthine oxidase. The nordihydroguarietic acid effects were associated with reduced 6K-PGF1 alpha efflux but LDH release was unaffected. We also examined whether modification of arachidonic acid release through changes in calcium concentration was associated with altered response to purine plus xanthine oxidase. Lowering the calcium concentration to 0.41 mM (from 1.25 mM control) reduced markedly 6K-PGF1 alpha, efflux as well as LDH release. Although the latter is suggestive of protection, hypocalcemic perfusion resulted in a greater loss in force due to free radical generation. Furthermore, cells from these hearts exhibited a greater degree of glycocalyx separation. Increasing the calcium concentration to 2.50 mM produced no further toxic manifestations in the response to purine plus xanthine oxidase, although the release of 6K-PGF1 alpha was increased. Our results suggest complex toxicity induced by an exogenously generated free radical system. The injury produced by this method is restricted to sarcolemmal changes, the latter being dependent on the external calcium concentration. The study further suggests that accumulation of intracellular unesterified arachidonic acid, which may result from peroxidation of membrane lipids, increases tissue injury caused by exogenous free radicals.

A novel concept on the pathogenetic mechanism underlying ischaemic brain oedema: relevance of free radicals and eicosanoids.

A survey on literature reports and our own experimental studies on the pathogenetic mechanisms underlying ischaemic brain oedema is given and a new concept proposed. In regional incomplete ischaemia the lipoxygenase activity is enhanced, presumably caused by an increase of free radicals and hydroperoxides, leading to an enhancement of endothelial Na+, K+-AtPase and increased sodium and water transport from blood to brain. The aggravation of brain oedema and post-ischaemic hypoperfusion following recirculation appears to be mainly due to an activation of the cyclo-oxygenase pathway with release of oxidants from PGG2, which causes non-specific but detrimental damage to the endothelial and parenchymal cells. This new concept may open future perspectives in treatment which are briefly discussed.