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

This study reveals how isoxsuprine interacts with adrenoreceptors in the arteries of a horse’s leg, leading to vasodilation. The researchers found that isoxsuprine’s action was largely due alpha-adrenoceptor blockade and had a higher affinity for alpha- over beta-adrenoceptors. The study further suggests that pure beta2-agonists may be more effective for expanding a horse’s arterial bed.

Study and Methodology

• Researchers studied the vasodilatory mechanism of the drug isoxsuprine on isolated equine digital arteries. For the investigation of the drug’s affinity for beta-adrenoceptors, they used fowl caecum preparations.
• Isoxsuprine is known for its vasodilatory action on arterial smooth muscle which was pre-contracted by an alpha-adrenoceptor agonist such as noradrenaline.

Results and Findings

• The study found that the effect of isoxsuprine is due to alpha-adrenoceptor blockade for several reasons. First, cumulative doses of isoxsuprine did not restore the vasoconstricting action of noradrenaline after prolonged exposure. Second, isoxsuprine did not facilitate relaxation in preparations precontracted by PGF2alpha. Third, the drug shifted the dose-response curve of noradrenaline to the right. Lastly, its affinity in these experiments was similar to that in noradrenaline-precontracted preparations but much lower than that of prazosin, a selective alpha1-adrenergic antagonist.
• In fowl caecum preparations, the affinity of isoxsuprine for beta-adrenoceptors was found to be 100 times lower than that of isoprenaline, demonstrating that the drug has more affinity for alpha-adrenoceptors. These preparations have a large beta-adrenoceptor and a negligible alpha-adrenoceptor population involved in the control of smooth muscle motility.
• The data suggest that isoxsuprine’s alpha-mediated effect on horse arterial smooth muscle is because the drug has a higher affinity for alpha-adrenoceptors rather than due to a low concentration or lack of functionality of beta-sites at this site.

Implications

• Based on these results, the study suggests that pure beta2-agonists might be more effective for causing vasodilation of the arterial bed in horses’ legs than isoxsuprine. The use of such agents could thus be more beneficial in veterinary practice when dealing with conditions requiring vasodilation in horses.