We tested the hypothesis that gram-negative bacteremia (GNB) and brief (30 min) reductions in the hepatic O2 supply by low-flow ischemia differentially modulate tumor necrosis factor-alpha (TNF-alpha) gene expression owing to sequence-specific activation of cyclooxygenase vs. complement (C) pathways. Buffer-perfused Sprague-Dawley rat livers (n = 82) were studied over 180 min after intraportal 10(9) live E. coli serotype 055:B5 (EC) or 0.9% NaCl (NS) at t = 0. Compared with EC and NS controls receiving constant-flow perfusion, sequential GNB and ischemia/reperfusion (I/R) were studied in EC + 30 I/R and NS + 30 I/R livers, in which 30 min of ischemia (I) beginning 0.5 h after EC or NS was followed by 120 min of reperfusion (R). This sequence was reversed in 30 I/R + EC and 30 I/R + NS groups. Bacterial clearance, bioactive and antigenic TNF-alpha, prostaglandin E2 (PGE2), and hepatic O2 uptake and performance were serially assessed. Venous TNF-alpha increased in EC controls to peak at 155 +/- 29 U/ml after 180 min (P < 0.001 vs. NS controls) as did hepatic TNF-alpha mRNA. Both TNF-alpha transcripts and protein levels were markedly attenuated in EC + 30 I/R (P < 0.001 vs. EC) despite equivalent EC clearance by Kupffer cells. Indomethacin (10(-5) M) decreased I/R-induced PGE2 secretion and restored TNF-alpha to control levels. In contrast, TNF-alpha levels in 30 I/R + EC perfusates exceeded those of EC + 30 I/R livers (P < 0.05) and were indistinguishable from EC controls. Allopurinol pretreatment but not heat inactivation of C or infusion of soluble human complement receptor type 1 inhibited TNF-alpha production in 30 I/R + EC organs. These results identify a novel sequence-dependent interaction whereby hepatic O2 deprivation after GNB downregulates TNF-alpha via generation of cyclooxygenase metabolites, whereas ischemia preceding GNB increases cytokine expression via reactive O2 species but not C activation.