The effects of alpha 2 adrenoceptor agonists on airway pressure in anaesthetized sheep.

Xylazine and clonidine, given intravenously, cause an increase in airway pressure in the anaesthetized, ventilated sheep. This increase was dose dependent and was not mediated by histamine, nor was it blocked by the alpha 1-adrenoceptor antagonist prazosin. However, the increase was abolished by the alpha 2-adrenoceptor antagonist, idazoxan. When the alpha 2-adrenoceptor agonists were administered into the cerebrospinal fluid by injection into the cisterna magna there was no increase in airway pressure, although a similar dose given peripherally still produced an effect. These findings would indicate that the increase in airway pressure seen in these sheep, following administration of xylazine and clonidine, was mediated by peripherally located alpha 2-adrenoceptors.