Thromboxane and isoprostanes as inflammatory and vasoactive mediators in black walnut heartwood extract induced equine laminitis

Inflammation and vascular dysfunction occur concurrently during the prodromal stages of equine laminitis. The aim of this study was to provide insights into the role that thromboxane and isoprostanes may play in the development of black walnut heartwood extract (BWHE)-induced laminitis. Horses were divided into two groups, either control or BWHE-administered horses. Plasma concentrations of thromboxane increased transiently after administration of BWHE and coincided with the nadir in white blood cell counts, whereas plasma concentrations of iso-prostaglandin PGF_2α (iso-PGF_2α) did not change in either group. At 12 h (for the control group) or Obel grade 1 laminitis (for the BWHE group) the horses were euthanized and laminar tissue collected. Laminar arteries and veins were used in functional studies with vasoconstrictor substances and tissue samples were used for the determination of laminar iso-PGF_2α concentrations. Laminar tissue concentrations of iso-PGF_2α were significantly greater in BWHE horses when compared to control horses. In parallel studies concentrations of iso-PGF_2α in laminar tissue samples obtained 1.5 and 3 h after administration of BWHE were indistinguishable from those for control horses at 3 or 12 h after administration of an equal volume of water. Laminar vessel constrictor responses to either a thromboxane mimetic (U46619), iso-prostaglandin PGE₂ (iso-PGE₂) or iso-PGF_2α were determined using small vessel myographs. In some vessels, the effects of putative prostanoid and thromboxane receptor antagonists, SQ 29,548, SC-19220 and AH 6809, upon contractile responses were determined. In control horses, U46619, iso-PGF_2α and iso-PGE₂ more potently and efficaciously constricted laminar veins when compared to laminar arteries. Responses of laminar veins from BWHE horses to iso-PGE₂ were similar to those of laminar veins from control horses, whereas iso-PGF_2α elicited significantly greater responses in laminar veins from BWHE horses when compared to controls. In contrast, responses to U46619 were smaller in laminar veins isolated from BWHE horses when compared to those in laminar veins from control horses. In the presence of SQ 29,548, iso-PGF_2α elicited a small dilation in laminar veins from control horses, which was not apparent in laminar veins from BWHE horses. These results are consistent with both systemic and local inflammatory events occurring during the prodromal stages of BWHE-induced laminitis. Because laminar veins are sensitive to thromboxane and isoprostanes, these substances may act as conduits between the inflammatory and vascular events occurring in laminitis and may be therapeutic targets for this crippling condition.