Reasons for performing study: Endothelin-1 (ET-1) may be a key mediator in the pathogenesis of laminitis, but endothelin-mediated responses in the venous microcirculation of the equine foot have yet to be fully characterised.
Objectives: To characterise the response of equine laminar veins to ET-1 and evaluate the ET-1 receptor subtypes that mediate this response.
Methods: Small veins (150–500 μm) draining the equine digital laminae from healthy horses and ponies subjected to euthanasia at an abattoir were investigated using wire myography. Concentration response curves were constructed for ET-1 in the presence of ETA (BQ123) and ETB (BQ788) receptor antagonists, and L-NAME, a nitric oxide synthase blocker. The selective ETB receptor agonist BQ3020 was investigated alone and following incubation with L-NAME, with or without BQ788.
Results: Endothelin-1 contraction of laminar veins was significantly inhibited by BQ123 but not by BQ788. In the presence of L-NAME, sensitivity of laminar veins to ET-1 was enhanced 4-fold, and further addition of BQ788 did not alter this increased sensitivity. BQ3020 induced no venoconstriction; however, in the presence of L-NAME, it caused contraction of veins with approximately 30% of the efficacy of ET-1. The action of BQ3020 in the presence of L-NAME was abolished by BQ788.
Conclusions: Both ETA and ETB receptors are involved in the net tonic response to ET-1 in normal laminar veins. A population of ETB receptors may be present on the vascular endothelium and on smooth muscle of laminar veins, and the action of ET-1 at these 2 sites is likely to be approximately equal and opposite.
Potential relevance: Our results clarify the function of the ET-1 receptor subtypes in laminar veins from healthy horses. Further study of ET-1 receptors in laminitic horses is therefore warranted.