Effects of selective α2‐adrenergic receptor agonists on electrical field‐stimulated contractions of isolated bronchi in horses |
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| Authors: | A. Menozzi C. Pozzoli E. Poli C. Colla G. Placenza S. Bertini |
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| Affiliation: | 1. Dipartimento di Scienze Medico‐Veterinarie, Università di Parma, Parma, Italy;2. Dipartimento di Medicina e Chirurgia, Università di Parma, Parma, Italy |
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| Abstract: | We investigated the effects of different selective α2‐adrenergic receptor (AR ) agonists (detomidine, medetomidine, xylazine, and brimonidine) on the contractions of horse‐isolated bronchi induced by electrical field stimulation (EFS ) and by carbachol. No effects were observed on the contraction induced by carbachol, while α2‐AR agonists reduced EFS ‐evoked contractions in a concentration‐related fashion. The rank order of potency (pD 2) was brimonidine (7.40 ± 0.20) >medetomidine (7.09 ± 0.24) >detomidine (6.13 ± 0.55) >xylazine (4.59 ± 0.16). The maximal effects (Emax) were ?56.3% ± 6.3%, ?40.4% ± 6.9%, ?48.6% ± 9.9%, and ?72.7% ± 12.7% for brimonidine, medetomidine, detomidine, and xylazine, respectively. Adrenergic block by guanethidine enhanced the potency (8.10 ± 0.05, 7.30 ± 0.15, 6.83 ± 0.41, and 5.40 ± 0.22) and the efficacy (?95.2% ± 0.7%, ?45.2% ± 11.7%, ?58.5% ± 9.8%, and ?97.9% ± 0.6%) of brimonidine, medetomidine, detomidine, and xylazine, respectively. Selective α2‐AR antagonist, atipamezole, competitively antagonized the inhibition of EFS ‐evoked contractions induced by all agonists except xylazine. These results suggest the existence of presynaptic α2‐AR s on cholinergic neurons, negatively regulating the release of acetylcholine in horse bronchial muscle, and that α2‐AR agonists may be beneficial against vagally mediated bronchoconstriction. |
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