Figure 4. Sites of hemodynamic actions of alpha2-adrenergic agonists. alpha2-Adrenergic agonists produce sympatholysis and reduced blood pressure by actions in the periphery, brainstem, and spinal cord, effects opposed by direct vasoconstriction from alpha2-adrenergic agonists in the periphery (left). As a result of vasoconstriction from circulating clonidine concentrations, the dose response for epidural clonidine on blood pressure in humans is U shaped (right, above). As a result of the spinal sympatholytic site of action, epidural clonidine reduces blood pressure more when injected in the thoracic than in the cervical or lumbar space (right, below). [49]