Effects of selective α2‐adrenergic receptor agonists on electrical field‐stimulated contractions of isolated bronchi in horses
2018
Menozzi, A. | Pozzoli, C. | Poli, E. | Colla, C. | Placenza, G. | Bertini, S.
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 (pD₂) 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‐ARs 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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