Endotoxin priming of the cyclooxygenase-2-thromboxane axis in isolated ratlungs

Enhanced prostanoid generation has been implicated in vascular abnormalitie s occurring during endotoxemia and sepsis, and the lung is particularly pro ne to such events. Prostanoids are generated from arachidonic acid (AA) via cyclooxygenase (COX)-1 or -2, both isoenzymes recently demonstrated to be expressed in different lung cell types. Upregulation of COX may underlie th e phenomenon that endotoxin [lipopolysaccharide (LPS)]-exposed lungs show m arkedly enhanced vasoconstrictor responses to secondarily applied stimuli ( priming). Isolated rat lungs were perfused with a physiological salt buffer solution in the absence and presence of 1.5% rat plasma and exposed to dif ferent concentrations of LPS (1,000 or 10,000 ng/ml) during a 2-h priming p eriod. No change in physiological variables was noted during this period, a lthough enhanced baseline liberation of both thromboxane (Tx) A(2) and PGI( 2) as well as of tumor necrosis factor (TNF)-alpha was evident compared wit h that in control lungs in the absence of LPS. LPS priming caused a signifi cant elevation in AA-induced pulmonary arterial pressure, ventilation press ure, and lung weight gain. Concomitant increased levels of TxA(2) were foun d in the buffer perfusate. All changes were largely suppressed by three sel ective, structurally unrelated COX-2 inhibitors (NS-398, DUP-697, and SC-23 6) in both buffer- and buffer-plasma-perfused lungs. Anti-TNF-alpha neutral izing antibodies were ineffective under conditions of buffer perfusion. In the presence of plasma components, manyfold augmented TNF-alpha generation was noted, and anti-TNF-alpha antibodies significantly suppressed the incre ase in ventilation pressure but not in the vascular pressor response and lu ng edema formation. We conclude that the propensity of LPS-primed lungs to respond with enhanced vasoconstriction, edema formation, and bronchoconstri ction to a secondarily applied stimulus proceeds nearly exclusively via COX -2 and increased Tx formation, with TNF-alpha generation being involved in the change in bronchomotor reactivity in the presence of plasma constituent s. In context with recent immunohistological investigations, LPS-induced up regulation of the COX-2-thromboxane synthase axis in vascular and bronchial smooth muscle cells is suggested to underlie these events.