In vitro reactivity of digital arteries and veins to vasoconstrictive mediators in healthy horses and in horses with early laminitis
Authors: Baxter G. M. , Laskey R. E. , Tackett R. L. , Moore J. N. , Allen D. 
Published: February 2025

SUMMARY
The in vitro reactivity of vasoconstrictive mediators that are implicated in acute laminitis was determined in palmar and plantar digital arteries and veins obtained from healthy horses and in palmar digital vessels of horses with early laminitis (Obel grade I). To obtain baseline reactivity data, 3 experiments were conducted, using healthy horses: (1) the reactivity of palmar and plantar digital arteries and veins to angiotensin II, norepinephrine, and 5-hydroxytryptamine (serotonin) were compared; (2) the direct effects of bacterial endotoxin on vascular reactivity were assessed; and (3) the reactivity of palmar digital arteries and veins to angiotensin II, norepinephrine, prostaglandin F2α (pgf2α)> serotonin, and a thromboxane-endoperoxide analog (U46619) were determined. The vascular reactivity of these same 5 vasoconstrictors then was determined in horses with early laminitis and was compared with data from healthy (control) horses.
Obel grade-I laminitis was experimentally induced in horses, using carbohydrate overload. Dose responses were conducted for each agent at concentrations between 10−8M and 10−4M. The potency of a drug was defined as the mean effective concentration necessary to induce 50% of maximal contraction (ec50). There were no differences in ec50 concentrations and in maximal contractions between forelimb and hind limb arteries and veins for angiotensin II, norepinephrine, and serotonin. Incubation with endotoxin had no effect on the reactivity of arteries and veins to angiotensin II, norepinephrine, and serotonin.
In healthy horses, serotonin and U46619 were more potent arterial constrictors than were norepinephrine, pgf2α, and angiotensin II. In veins, serotonin, U46619, and angiotensin II were similar in potency, and all were significantly (P < 0.05) more potent than were norepinephrine and pgf2α. Serotonin induced greater arterial constriction than did all other agents tested. There were no differences in the maximal venoconstriction induced by norepinephrine, pgf2α, serotonin, and U46619. Angiotensin II induced the least amount of arterial and venous constriction. Maximal contractions were significantly (P < 0.05) greater for veins than for arteries for all agents evaluated, except for angiotensin II.
In horses with early laminitis, angiotensin II and serotonin were the most potent (smallest ec50 values) constricting agents for the arterial and venous segments and norepinephrine and pgf2α were the least potent. Serotonin and norepinephrine induced significantly (P < 0.05) greater venoconstriction than did the other agents. Angiotensin II induced the least arterial and venous contraction.
For all agonists except angiotensin II, the mean ECgo values for vessels from horses with early laminitis were either similar or greater than those for vessels from control horses. The EC50 values for norepinephrine and the thromboxane analog were significantly (P < 0.05) greater for vessels from horses with early laminitis, compared with those from control horses. The mean maximal contractions for all vasoconstrictors, except angiotensin II, were significantly (P < 0.05) less for vessels from horses with early laminitis. Significantly greater venous-to-arterial maximal contraction ratios were found for norepinephrine and serotonin in horses with laminitis, compared with those ratios in control horses. These data suggested that the digital vasculature of horses with early laminitis was not more sensitive to the vasoconstrictor substances tested and that the vessels were significantly less responsive than was vasculature from the control horses. However, the venous-to-arterial contraction ratios were either the same or significantly (P < 0.05) greater in horses with laminitis.