Mechanisms involved in the early increase of serotonin contraction evoked by endotoxin in rat middle cerebral arteries

1 The present study investigated the mechanisms involved in the increased 5-hydroxytryptamine (5-HT) vasoconstriction observed in rat middle cerebral arteries exposed in vitro to lipopolysaccharide (LPS, 10 mug ml(-1)) for 1-5 h. Functional, immunohistochemical and Western blot analysis and superoxide anion measurements by ethidium fluorescence were performed. 2 LPS exposure increased 5-HT (10 muM) vasoconstriction only during the first 4 h. In contrast to control tissue, indomethacin (10 muM), the COX-2 inhibitor NS 398 (10 muM), the TXA(2)/PGH(2) receptor antagonist SQ 29,548 (1 muM) and the TXA(2) synthase inhibitor furegrelate (1 muM) reduced 5-HT contraction of LPS-treated arteries from hour one. The iNOS inhibitor aminoguanidine (0.1 mM) increased 5-HT contraction from hour three of LPS incubation. 3 The superoxide anion scavenger superoxide dismutase (SOD, 100 U ml(-1)) and the H2O2 scavenger catalase (1000 U ml(-1)), as well as the respective inhibitors of NAD(P)H oxidase and xanthine oxidase, apocynin (0.3 mM) and allopurinol (0.3 mM), reduced 5-HT contraction after LPS incubation. LPS induced an increase in superoxide anion levels that was abolished by PEG-SOD. 4 Subthreshold concentrations of the TXA(2) analogue U 46619, xanthine/xanthine oxidase and H2O2 potentiated, whereas those of sodium nitroprusside inhibited, the 5-HT contraction. 5 COX-2 expression was increased at 1 and 5 h of LPS incubation, while that of iNOS, Cu/Zn-SOD and Mn-SOD was only increased after 5 h. All the three vascular layers expressed COX-2 and Cu/Zn-SOD. iNOS expression was detected in the endothelium and adventitia after LPS. 6 In conclusion, increased production of TXA(2) from COX-2, superoxide anion and H2O2 enhanced vasoconstriction to 5-HT during the first few hours of LPS exposure; iNOS and SOD expression counteracted that increase at 5 h. These changes can contribute to the disturbance of cerebral blood flow in endotoxic shock.