The role of prostanoids and nitric oxide in endotoxin-induced hyporesponsiveness of equine digital blood vessels

Endotoxin has been implicated in the pathophysiology of acute laminitis, Th e aim of this study was to examine the direct effects of endotoxin on isola ted equine digital blood vessels. Equine digital veins (EDV), incubated in KrebsHenseleit solution containing lipopolysaccharide (LPS) (1 mu/ml) becam e hyporesponsive to 5-HT after 16 h, Cycloheximide and ibuprofen blocked th is effect of LPS and increased the maximum response obtained to 5-HT when c ompared to control vessels. L-nitroarginine methyl ester (L-NAME) reversed the hyporesponsiveness caused by LPS. Vessels maintained in culture medium containing LPS also became hyporesponsive to 5-HT, an effect which was comp letely prevented by ibuprofen but only partially reversed by L-NAME. Measurements were made of 6-keto PGF(1 alpha) and nitrite production by seg ments of equine digital artery and vein in culture medium alone or co-cultu red with peripheral blood leucocytes, LPS did not stimulate nitrite product ion from vessel segments but increased nitrite release from leucocytes, an effect which was inhibited by cycloheximide and L-NAME. Lipopolysaccharide increased 6-keto PGF(1 alpha) production by blood vessels, an effect which was inhibited by cycloheximide and ibuprofen but not L-NAME. No synergistic effect on release of nitrite or 6-keto PGF(1 alpha) was noted in co-cultur es of blood vessels and leucocytes, These data suggest that induction of cyclo-oxygenase by LPS was a major cau se of hyporesponsiveness of digital blood vessels to 5-HT Release of nitric oxide was not detectable in LPS stimulated blood vessels maintained in cul ture even in the presence of activated leucocytes yet L-NAME did protect ag ainst LPS-induced hyporesponsiveness indicating nitric oxide synthase induc tion may play some role in the effect of LPS. These findings are important in furthering our understanding of the pathophysiological mechanisms underl ying the vascular changes which occur in acute laminitis.