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Role of thromboxane, prostaglandins and leukotrienes in endotoxic and septic shock.

Intravenous bolus endotoxin elicits a marked but transient increase in plasma TxB2 and 6-keto-PGF1α in a large number of species. A smaller, delayed and more prolonged increase in TxB2 and 6-keto-PGF1α are reported in animals with septic shock, i.e., those with fecal peritonitis or cecal ligation. Thromboxane synthetase inhibitors or antagonists attenuate endotoxin-induced acute cardiopulmonary changes, the delayed increase in serum lysosomal enzymes, fibrin/fibrinogen degradation products and the thrombocytopenia in a number of species. While these drugs increase survival of rats or mice following endotoxin they do not alter survival of rats in septic shock. These results support the hypothesis that TxA2 exerts a pathophysiologic effect in shock following bolus endotoxin. In contrast, nonsteroidal antiinflammatory drugs (NSAID) and dietary essential fatty acid deficiency increase survival of rats subjected to endotoxin shock, and survival time in models of septic shock. These results also suggest that some other cyclooxygenase product(s) is involved in septic shock due to fecal peritonitis or cecal ligation. Preliminary experimental studies indicate salutary effects of leukotriene inhibitors and antagonists in endotoxin shock and in models of acute pulmonary injury. Clinical studies have demonstrated elevated plasma TxB2 and 6-keo-PGF1α concentrations in patients with septic shock, and elevated LTD4 in pulmonary edema fluid of patients with the adult respiratory distress syndrome. In view of these clinical and experimental results, clinical trials of NSAID and/or leukotriene inhibitors/antagonists should be considered.

PLASMA THROMBOXANE CONCENTRATIONS ARE RAISED IN PATIENTS DYING WITH SEPTIC SHOCK

Central venous plasma concentrations of thromboxane B2 (TXB2) the stable metabolite of the vasoconstrictor platelet aggregator thromboxane A2, were measured in 12 patients with septic shock. In 8 patients dying with septic shock the concentration of plasma TXB2 (912 +/- 250 pg/ml) was ten times higher than that in 4 survivors of septic shock (92 +/- 25 pg/ml) and 6 controls (91 +/- 18 pg/ml). Prothrombin time and partial thromboplastin time were significantly prolonged in nonsurvivors compared with survivors. Similarly, the alveolar-arterial oxygen gradient was significantly raised in nonsurvivors (233 +/- 39 mm Hg) compared with survivors (112 +/- 47 mm Hg). This study demonstrates that the metabolism of arachidonic acid to thromboxanes is increased in patients dying of septic shock and this raises the possibility that thromboxanes may be involved in the disseminated intravascular coagulation and respiratory distress syndrome associated with severe sepsis.

Studies on the beneficial effects of Aspirin in endotoxic shock: Relationship to inhibition of arachidonic acid metabolism☆

Endotoxic shock is associated with increased metabolism of arachidonic acid into thromboxanes and prostaglandins. This study assessed the effects of varied doses of aspirin, an inhibitor of arachidonic acid metabolism, on Salmonella enteritidis endotoxin-induced mortality, plasma levels of arachidonate metabolites, and other pathophysiologic sequelae in Long-Evans rats. Aspirin in doses of 3.75, 15, and 30 mg/kg given 30 minutes prior to endotoxin challenge significantly (p less than 0.01) improved 24-hour survival rates from 11 percent to approximately 65 percent, but 100 mg/kg afforded no protection. Pretreatment with aspirin (15 or 100 mg/kg) 30 minutes prior to endotoxin also significantly (p less than 0.001) decreased the endotoxin-induced elevations in plasma levels of immunoreactive thromboxane B2, a stable metabolite of thromboxane A2, and immunoreactive 6-keto prostaglandin F1 alpha, a stable metabolite of prostacyclin. Aspirin doses of 15 or 100 mg/kg given 24 hours prior to challenge with endotoxin significantly improved 24-hour survival rates from 17 percent to 42 percent (p less than 0.01) and 44 percent (p less than 0.005), respectively. Pretreatment with an aspirin dose of 15 mg/kg 24 hours prior to challenge with endotoxin significantly (p less than 0.05) inhibited thrombin-induced immunoreactive thromboxane B2 synthesis in platelet-rich plasma (in vitro) and endotoxin-induced immunoreactive 6-keto-prostaglandin F1 alpha and immunoreactive thromboxane B2 synthesis by rat peritoneal macrophages. Although 24-hour pretreatment with aspirin (15 or 100 mg/kg) significantly (p less than 0.001) reduced endotoxin-induced elevations in immunoreactive thromboxane B2, only the 100 mg/kg dose significantly lowered plasma levels of immunoreactive 6-keto prostaglandin F1 alpha. These observations are consistent with the notion that the beneficial effects of aspirin seen in experimental endotoxic shock may be mediated, in part, via inhibition of arachidonic acid metabolism.

Effects of a pyridine derivative thromboxane synthetase inhibitor and its inactive isomer in endotoxic shock in the rat.

1 We investigated the effects of a pyridine derivative thromboxane synthetase inhibitor and its inactive ortho isomer on arachidonic acid metabolism and pathophysiological sequelae of endotoxic shock. In vehicle‐treated rats, 30 min after intravenous S. enteritidis endotoxin (15 mg/kg), plasma iTxB2 (the stable metabolite of thromboxane A2) increased from non‐detectable levels (< 100 pg/ml) to 763 ± 250 pg/ml (n = 10). Plasma i6‐keto‐PGF1α (the stable metabolite of prostacyclin, PGI2) increased to 1850 ± 426 pg/ml, (n = 9) and plasma iPGE increased to 2350 = 560 (n = 5). Pretreatment with the pyridine active (PA) meta isomer (30 mg/kg i.p.) significantly (P < 0.05) suppressed iTxB2 to 390 ± 31 pg/ml (n = 10) although 6‐keto‐PGF1α levels (1294 ± 358 pg/ml, n = 5) and plasma iPGE (2847 ± 1103 pg/ml, n = 5) were not significantly different from the shocked control values. In contrast, pretreatment with, the pyridine inactive (PI) ortho isomer did not significantly affect endotoxin‐induced iTxB2 (1431 ± 194 pg/ml, n = 5) or i6‐keto‐PGF1α synthesis (628 ± 266 pg/ml, n = 5). 2 Pretreatment of rats with the Tx synthetase inhibitor, PA, significantly enhanced (P < 0.05) survival and prevented splanchnic infarction relative to both endotoxin shocked control rats and those pretreated with the PI isomer. 3 Significantly reduced lysosomal labilization, hepatocellular dysfunction and elevations in serum fibrin/fibrinogen degradation products were seen only in groups pretreated with the PA isomer. 4 The beneficial effects of the latter compound in Endotoxic shock thus appear to be due to inhibition of Tx synthesis, since its ortho isomer did not inhibit TxA2 synthesis nor did it protect against endotoxic shock.

Kinin effects on chloride secretion do not require eicosanoid synthesis

1 The actions of bradykinin on colonic epithelia from essential fatty acid‐deficient (EFAD) rats has been examined. Electrogenic chloride secretion as short circuit current (SCC) and release of immunoreactive prostaglandin E2 (iPGE2) and i 6‐keto PGF1α have been measured. 2 Resting release of prostanoids was significantly less in EFAD than in control tissues. Bradykinin, in a maximally effective concentration, produced no increase in prostanoid release in EFAD tissues in contrast to controls, while the SCC response was 55% of that in controls. 3 In EFAD tissues the SCC response to bradykinin was the same whether or not the cyclooxygenase inhibitor piroxicam was present. 4 EFAD tissues were not more sensitive to prostaglandins than control tissues. 5 We conclude that while prostaglandin release contributes to the totality of the response to bradykinin, the latters effect on electrogenic chloride secretion does not require the obligatory production of arachidonic acid metabolites.

The potential role of thromboxane and prostacyclin in endotoxic and septic shock

The potential role of thromboxane (TxA2), a platelet aggregator and vasoconstrictor, and prostacyclin (PGI2) a platelet anti-aggregator and vasodilator, in endotoxic and septic shock was investigated. Early endotoxic shock in the rat is associated with marked elevations of plasma TxB2 (the stable metabolite of TxA2) and lesser increases in plasma 6-keto-PGF1 alpha (the stable metabolite of PGI2). Selective inhibition of TxA2 synthesis by several different chemical classes of Tx synthetase inhibitors was beneficial in endotoxic shock. In contrast, shock induced by acute intra-abdominal sepsis in the rat was characterized by high levels of plasma 6-keto-PGF1 alpha, which exceeded plasma TxA2 six- to eight fold at most time intervals studied. Tx synthetase inhibitors were not protective in this model of acute sepsis, but treatment with fatty acid cyclo-oxygenase inhibitors, an antibiotic (gentamicin), or reduction in arachidonic acid metabolism by essential fatty acid (EFA) deficiency significantly prolonged survival time. An important aspect of the latter study is that decreased arachidonic acid metabolism was an effective adjunct to antibiotic therapy. Conjoint administration of gentamicin in EFA-deficient rats or with indomethacin synergistically improved long-term survival, a result that was not evident with single treatment interventions. In addition to experimental studies, plasma TxB2 levels were measured during clinical sepsis. These studies demonstrated that plasma TxB2 levels were elevated tenfold in patients dying of septic shock compared with septic survivors or nonseptic controls. These composite experimental and clinical observations suggest that arachidonic acid metabolites play a role in the pathogenesis of endotoxic and septic shock.

Macrophage eicosanoid formation is stimulated by platelet arachidonic acid and prostaglandin endoperoxide transfer

The present study was designed to determine whether platelets transfer arachidonic acid or prostaglandin endoperoxide intermediates to macrophages which may be further metabolized into cyclooxygenase products. Adherent peritoneal macrophages were prepared from rats fed either a control diet or an essential fatty acid-deficient diet, and incubated with a suspension of washed rat platelets. Macrophage cyclooxygenase metabolism was inhibited by aspirin. In the presence of a thromboxane synthetase inhibitor, 7-(1-imidazolyl)heptanoic acid, immunoreactive 6-ketoprostaglandin F1 alpha formation was significantly increased 3-fold. Since this increase was greater (P less than 0.01) than that seen with either 7-(1-imidazolyl)heptanoic acid-treated platelets or aspirin-treated macrophages alone, these results indicate that shunting of endoperoxide from platelets to macrophages may have occurred. In further experiments, macrophages from essential fatty acid-deficient rats were substituted for normal macrophages. Essential fatty acid-deficient macrophages, depleted of arachidonic acid, produced only 2% of the amount of eicosanoids compared to macrophages from control rats. When platelets were exposed to aspirin, stimulated with thrombin, and added to essential fatty acid-deficient macrophages, significantly more immunoreactive 6-ketoprostaglandin F1 alpha was formed than in the absence of platelets. This increased macrophage immunoreactive 6-ketoprostaglandin F1 alpha synthesis, therefore, must have occurred from platelet-derived arachidonic acid. These data indicate that in vitro, in the presence of an inhibition of thromboxane synthetase, prostaglandin endoperoxides, as well as arachidonic acid, may be transferred between these two cell types.

Endotoxin Activation of Eicosanoid Production by Macrophages

The specific cellular and molecular mechanisms underlying the syndrome of endotoxemia and sep sis remain to be clearly defined. It is generally ac cepted that the release of inflammatory substances by the host rather than endotoxin itself mediates the pathogenesis of endotoxemia. A plethora of mediators released in response to endotoxin include a variety of cytokines and arachidonic acid (AA) metabolites (commonly designated as the eicosanoids), in addition to other lipid mediators. The eicosanoids derived from either cyclooxygenase or lipoxygenase pathways consist of thromboxanes (TXs), prostaglandins (PGs), and leukotrienes (LTs).

Lead and Cadmium Effect on Host Defense Mechanisms and Toxic Interactions with Bacterial Endotoxin

There is growing concern over the presence of significant quantities of non-biodegradable pollutants, such as lead and cadmium, in the environment (Wessel and Dominski, 1977; Goyer and Rhyne, 1973; Webb, 1979). Widespread environmental contamination with lead and cadmium occurs from many sources ranging from industrial or automobile emissions (Haley, 1968; Beliles, 1975; Cox, 1974) to consumable items and appliances in the home (Chisolm, 1973; Cox, 1974). Understanding of the interaction of these toxic trace metals with biologic systems is thus becoming increasingly important. Such interactions may result from accidental overt intoxication or potential toxic effects of subclinical body burdens of these heavy metals manifested in the presence of costressors. The impact on host defense mechanisms is an important variable in toxicity assessment of these environmental contaminants. Selye et al. (1966) reported that a single injection of lead acetate increased the sensitivity of rats to various endotoxins from gram-negative bacteria by approximately 100,000-fold. This initial observation of an extreme toxic synergism between lead and endotoxin has inspired a number of studies of environmental significance as well as an understanding of the fundamental mechanisms of endotoxic shock. This chapter reviews some toxicologic effects of lead and cadmium on phagocyte function and host resistance to infection and examines potential mechanisms by which these heavy metals render animals hypersensitive to bacterial endotoxin.

Altered Guanine Nucleotide Protein Function: Potential Role in Endotoxin Tolerance

Endotoxin stimulates macrophages to synthesize and release a number of monokines including tumor necrosis factor (TNF), colony stimulating factor, platelet-activating factor, interleukin-1 (IL-1) and arachidonic acid metabolites [1–8]. In recent years arachidonic acid metabolites or eicosanoids, a group of inflammatory mediators, have generated considerable interest [9–14]. Arachidonic acid is esterified in the 2-position of specific cellular phospholipids including phosphatidylcholine (PC), phosphatidyl-ethanolamine (PE), and phosphatidylinositol (PI) [15–19]. The generation of free arachidonic acid from phospholipids is thought to be a rate-limiting step in the formation of arachidonic acid metabolites [20]. Endotoxin-stimulated cells mobilize their phospholipids through at least two major pathways [21]. In the first pathway, phospholipase C cleaves phosphatidylinositol 4,5-biphosphate, yielding inositol-1,4,5-triphosphate (IP3) and 1,2-diacylglycerol, both of which are second messengers [22–25]. IP3 mobilizes cytoplasmic calcium while 1,2-diacylglycerol stimulates protein kinase C [26–28]. A separate diglyceride lipase cleaves arachidonic acid from the diacylglycerol molecule [29, 30]. In the second pathway, phospholipase A2 (PLA2) directly releases arachidonic acid and other unsaturated fatty acids from phospholipids including PC, PE, and PI.