A substantial contribution of 5-hydroxytryptamine (5-HT) in the contractile response of bovine digital artery (BDA), as well as short- and longer-term changes in basal reactivity to the amine induced by endothelium removal, have been recently described. The present work investigated the vasomotor reactivity to 5-HT in bovine digital vein (BDV) in order to evaluate possible differential control by this amine in the two sides of vascular bed. Three main differences in BDA and BDV vascular reactivity to 5-HT were observed. (i) 5-HT induces more effective and potent contractile effects in BDV than in BDA; (ii) the endothelium removal induces short-term increased reactivity to 5-HT in BDA but not in BDV; (iii) significant but opposite long-term changes in contractile reactivity to 5-HT were recorded in incubated BDV (hyper-reactivity) and BDA (hypo-reactivity) that had been deprived of the endothelium before incubation. The disclosed differential arterial/venous vasomotor control of 5-HT may contribute to the physiological haemodynamic regulation of the bovine foot as well as to the establishment of poor blood flow leading to laminitis.

Several chronic progressive vascular diseases, such as laminitis, show vasocontractile dysfunction that might evolve into reperfusion injury and/or vessel structural remodelling, which may be traced back to aberrant endothelial function. In the present study, the vasomotor responses of bovine digital veins (BDVs) to 5-hydroxytryptamine (5-HT) were investigated in blood vessels, with and without endothelium present, and in samples deprived of endothelium before or after overnight incubation in tissue culture medium, to evaluate the effects of short- and long-term endothelial damage on vascular smooth muscle (VSM) reactivity. No significant effects were observed in the blood vessels tested immediately after the removal of endothelium. In contrast, a significant increase in VSM reactivity to 5-HT was seen in vessels incubated without endothelium. This long-term change in smooth muscle reactivity was prevented by exposure to the nitric oxide (NO) donor nitroprusside (P < 0.01), suggesting that the long-term lack of inhibitory control exerted by endothelium-derived NO is involved in increased VSM reactivity. The RhoA/ROCK pathway inhibitor fasudil reduced VSM hyper-contractility to ~65% (P < 0.001), the superoxide dismutase-mimetic tempol normalised the vascular response and the non-selective COX-inhibitor indomethacin exerted a moderate inhibitory effect (P < 0.05). Thus, over-activation of the RhoA/ROCK pathway and production of reactive oxygen species could account for VSM hyper-reactivity, triggered by long-term endothelium-deprivation in BDVs, suggesting that these biochemical mechanisms are potential targets for controlling the progressive vasocontractile dysfunction of digital veins in animals affected with laminitis.

The effects of endotoxin on b-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 b-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 b-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 b-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.