Affinity of isoxsuprine for adrenoreceptors in equine digital artery and implications for vasodilatory action.

Summary

# Editorial Summary: Isoxsuprine's Vasodilatory Mechanism in Equine Digital Arteries Isoxsuprine has long been used to improve peripheral circulation in horses, but its precise pharmacological action remained unclear—particularly whether its vasodilatory effects arose from beta-adrenergic stimulation or alpha-adrenergic blockade. Belloli and colleagues investigated this question using isolated equine digital arteries and comparative receptor binding studies, demonstrating that isoxsuprine acts primarily as an alpha-adrenergic antagonist with relatively weak beta-adrenergic affinity (100 times lower than isoprenaline). The drug proved highly effective at blocking noradrenaline-induced vasoconstriction (pK_B = 6.90) but showed no relaxant effect when arteries were pre-contracted using prostaglandin F2α, confirming alpha-blockade rather than direct smooth muscle relaxation or beta-mediated vasodilation. Clinically, this finding suggests that practitioners seeking true beta-mediated arterial vasodilation in equine limbs may achieve more reliable results with selective beta-2 agonists rather than isoxsuprine, which functions fundamentally differently from how it was often theoretically positioned.

Read the full abstract on PubMed

Key Findings

• •Isoxsuprine acts as an alpha-adrenoceptor antagonist (pK(B) = 6.90) rather than beta-adrenoceptor agonist in equine digital arteries
• •Isoxsuprine's affinity for beta-adrenoceptors is 100 times lower than isoprenaline and 14 times lower than prazosin for alpha-1 sites
• •Isoxsuprine produces dose-dependent vasodilation of noradrenaline-precontracted arteries but fails to relax PGF2α-precontracted preparations
• •Pure beta-2 agonists are suggested as superior alternatives to isoxsuprine for achieving arterial vasodilation in equine limbs

Conditions Studied