Effects of endotoxin and influence of cyclooxygenase-2 on β-adrenergic mediated relaxation in isolated equine digital artery

The effects of endotoxin on β-adrenergic-mediated relaxation were investigated in the equine digital artery (EDA). Possible involvement of cyclooxygenase-2 (COX-2) in endotoxin-induced effects and basal EDA β-adrenoceptor functionality was also evaluated. Endothelium-intact (e+) and/or -denuded (e−) EDA rings were incubated overnight with lipopolysaccharide (LPS), LPS+NS398 (selective COX-2 inhibitor) or NS398 alone. Vessel rings were then mounted in organ baths and relaxant responses to isoproterenol (ISOP) recorded on U44069-induced pre-contraction. Response to ISOP was further evaluated in either incubated or freshly isolated (e−) rings acutely exposed to NS398. Fresh and incubated (e−) EDAs were also analysed for COX-2 expression by Western blotting. LPS caused endothelium-dependent enhancement of β-adrenergic mediated relaxation. NS398 did not reverse endotoxin effects, suggesting that COX-2 did not have a mediating role. In the absence of LPS, NS398 significantly increased ISOP-induced relaxation. This finding, together with immunoblot detection of COX-2 in both fresh and incubated (e−) vessels, revealed the existence of a constitutive COX-2 exerting tonic inhibitory modulation on EDA β-adrenergic-mediated relaxation. The results support the possible role of endotoxin in the vascular disturbances associated with equine laminitis. Moreover, the involvement of COX-2 in the physiological regulation of EDA tone warrants further clinical investigation into the efficacy and safety of selective COX-2 inhibitors on digital circulation in horses.