Mediators and vascular effects in response to endotoxin

Recent experimental findings indicate that endotoxin (i.e. lipopolysaccharide) interacts with specific membrane receptors localized to mononuclear phagocytic cells and neutrophils. Binding of endotoxin to these cells, together with endotoxin-induced activation of host vascular endothelium, initiates a series of signal transduction events that culminate in release of numerous biochemical mediators. The latter include cytokines, platelet-activating factor, thromboxane A2, prostaglandins, leukotrienes, nitric oxide, proteases, toxic O2 radicals, and vasoactive amines. These mediators orchestrate complex biological interactions and amplification signals that lead to cardiopulmonary dysfunction and multi-organ failure within 4-6 h of experimental infusion of endotoxin into animals. The pathophysiological changes include decreased cardiac output, systemic hypotension, decreased blood flow and O2 delivery to tissues, intense pulmonary vasoconstriction and hypertension, bronchoconstriction, increased permeability, pulmonary oedema, ventilation-to-perfusion inequalities, hypoxaemia, and haemoconcentration. Metabolic alterations include increased blood lactate and pyruvate, metabolic acidosis, hyperkalaemia and hypoglycaemia. Potential therapeutic modalities for treatment of endotoxaemia/septic shock include specific antagonists directed against lipopolysaccharide, cytokine, and platelet-activating factor receptors, monoclonal antibodies directed against cytokines and lipid A/core polysaccharides of endotoxin, antiproteases, and agents that block release of toxic O2 and arachidonic acid metabolites.

Effect of 5-lipoxygenase and cyclooxygenase blockade on porcine hemodynamics during continuous infusion of platelet-activating factor

We hypothesized that 5-lipoxygenase and cyclooxygenase products might be mediators of cardiopulmonary and systemic vascular effects induced by a 4 h continuous infusion of platelet-activating factor (PAF, 10 ng/kg/min) in anesthetized pigs. Indomethacin (cyclooxygenase inhibitor) potentiated and CGS 8515 (5-lipoxygenase inhibitor) attenuated PAF-induced increases in total peripheral resistance (TPR) from 2.5 to 4 h. However, the 5-lipoxygenase inhibitor failed to modify pulmonary vasoconstriction and hypertension caused by PAF. Except for a delay in onset (approximately 44 s) and rate of development of pulmonary hypertension during the initial 10 min of PAF infusion, the pulmonary hemodynamic changes were also not attenuated by indomethacin. On the other hand, at 4 h, the PAF-induced pulmonary hypertension and systemic vasoconstriction were completely or partially reversed, respectively, by WEB 2086 (PAF receptor antagonist). The PAF-induced increases in plasma thromboxane B2 (TXB2) were blocked by indomethacin but not by CGS 8515, and at 4 h the 5-lipoxygenase inhibitor potentiated the levels of TXB2 in pigs treated with PAF. The plasma concentrations of 6-keto-PGF1 alpha and leukotriene B4 (LTB4) were not modified by PAF or CGS 8515 + PAF. We conclude that PAF-induced increases in TPR (2.5-4 h) are potentiated by indomethacin and are dependent on 5-lipoxygenase products other than LTB4. Although the early pulmonary vascular response (< 10 min) to PAF is dependent on cyclooxygenase products, the sustained response (after 10 min) cannot be explained by either 5-lipoxygenase or cyclooxygenase products but may be mediated directly by PAF receptors.