Effect of clonidine on the cerebrovascular system in cats.

The effects of clonidine, a potent alpha 2 adrenergic agonist and an imidazole/imidazoline receptor agonist, were examined in the cat cerebrovascular system, measuring cerebral oxygen and carbon dioxide tension (BrPo2, BrPco2) and arterial blood pressure. Intracarotid injection of clonidine (2 micrograms/kg) produced a gradual decrease in systemic blood pressure without initial hypertension, while BrPo2 decreased slightly but significantly. Before and after intracarotid administration of clonidine, cerebrovascular reactivity to carbon dioxide was estimated by changes in BrPo2 and BrPco2 during and after 3 min inhalation of 5% CO2. Clonidine significantly enhanced cerebrovascular reactivity to carbon dioxide. The data suggest that the alpha 2-adrenoceptor and/or imidazoline receptor play an important role in the regulation of cerebral circulation.