THE EFFECT OF XYLAZINE ON THE ISOLATED SHEEP TRACHEA

The effect of xylazine on the isolated sheep trachea and its possible interactions with the alpha(2)-adrenergic antagonist, atipamezole, and the anticholinergic agent, atropine, was studied. The mechanical resp onses of the tracheal preparations were recorded after exposing each o ne to cumulatively increasing concentrations of xylazine alone or in t he presence of atipamezole or atropine. Xylazine exerted a concentrati on-dependent contractile effect, with a threshold concentration of 10( -7) M while the maximum activity was produced at a concentration of 10 (-5) M (EC(50) = 2.3 x 10(-7)). This xylazine-induced contractile effe ct was inhibited by atipamezole, but not significantly modified by atr opine. Thus, it is concluded that alpha(2)-adrenoceptors exist in the sheep trachea and it is suggested that alpha(2)-adrenoceptor agonists may act on airways in sheep directly through stimulation of peripheral alpha(2)-adrenergic receptors and indirectly via central alpha(2)-adr energic receptor activation of parasympathetic tone.