Predisposition for venoconstriction in the equine laminar dermis: Implications in equine laminitis

Equine laminitis is a crippling condition associated with a variety of systemic diseases. Although it is apparent that the prodromal stages of laminitis involve microvascular dysfunction, little is known regarding the physiology of this vasculature. The aim of the present study was to determine the relative responses of equine laminar arteries and veins to the vasoconstrictor agonists phenylephrine (1 nM-10 μM), 5-HT (1 nM-10 μM), PGF_2α (1 nM-100 μM), and endothelin-1 (1 pM-1 μM). We have determined that laminar veins were more sensitive, with respect to the concentration of agonist required to initiate a contractile response and to achieve EC₅₀, for all agonists tested. EC₅₀ values, for veins and arteries, respectively, were 84 ± 7 vs. 688 ± 42 nM for phenylephrine, 35 ± 6 vs. 224 ± 13 nM for 5-HT, 496 ± 43 nM vs. 3.0 ± 0.6 μM for PGF_2α, and 467 ± 38 pM vs. 70.6 ± 6.4 nM for endothelin-1. Moreover, when expressed as a percentage of the response to a depolarizing stimulus (80 mM potassium), the maximal contractile response of laminar veins exceeded that for the laminar arteries for each agonist. These results indicate that there may be a predisposition for venoconstriction within the vasculature of the equine digit. While this physiological predisposition for venoconstriction may be important in the regulation of blood flow during exercise, it also may help to explain why laminitis can result from a variety of pathological systemic conditions.