In vitro responses of equine digital vessels to dopamine and fenoldopam.
Authors: Baxter, Moore, Tackett
Journal: Equine veterinary journal
Summary
# Editorial Summary Dopamine's effects on equine digital circulation have remained poorly characterised until this in vitro investigation, which examined isolated palmar and plantar digital arteries and veins harvested from healthy anaesthetised horses. Vascular segments were suspended in tissue baths and exposed to escalating dopamine concentrations (10⁻⁸ to 10⁻⁴ M), with tension changes recorded; prazosin (an alpha-1 adrenoceptor antagonist) was used to elucidate the mechanism of action, whilst fenoldopam's vasodilatory potential was tested on noradrenaline-preconstricted preparations. Dopamine induced marked vasoconstriction in both arteries and veins, though only at supraphysiologically high molar concentrations, with no differential reactivity between palmar and plantar vessels; critically, prazosin significantly reduced venous sensitivity to dopamine (elevating EC₅₀ values) but failed to affect arterial responses or maximal contractions in either vessel type, indicating that alpha-1 adrenoceptor blockade accounts for only partial attenuation of dopamine-induced constriction. Fenoldopam, a selective dopamine-1 receptor agonist, produced negligible vasodilatory effects in either arterial or venous preparations. These findings suggest that whilst dopamine causes digital vasoconstriction through partially alpha-1 adrenoceptor-mediated pathways, additional receptor mechanisms are involved—a distinction relevant to clinicians considering dopamine infusions in cases of equine digital or systemic hypotension, as the drug's digital vasoconstrictive properties may compromise already-compromised laminar perfusion in conditions such as laminitis.
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Practical Takeaways
- •Dopamine's vasoconstrictive effects on equine digital vessels occur at pharmacologically high concentrations and are partially alpha-1 mediated, suggesting variable clinical utility for digital perfusion
- •Fenoldopam's poor vasodilatory response in equine digital vessels makes it an ineffective option for improving digital blood flow in horses
- •The differential response between arteries and veins to prazosin suggests distinct adrenergic receptor distributions in equine digital vasculature that may influence drug selection for circulatory support
Key Findings
- •Dopamine produced intense constriction in equine digital arteries and veins only at very high molar concentrations (10⁻⁸ to 10⁻⁴M)
- •Prazosin significantly decreased venous sensitivity to dopamine (increased EC50) but had no effect on arterial responses or maximal contractions
- •Dopamine-induced constriction is only partially mediated by alpha-1 adrenoceptors in equine digital veins
- •Fenoldopam produced very little relaxation of either arteries or veins despite being a DA-1 receptor agonist