CClonidine is used as an antihypeensive agent but if used as a fast intravenous injection, it can increase the blood pressure due to:: (A) Stimulation of vasomotor center, (B) Release of noradrenaline from adrenergic nerve endings, (C) Agonistic action on vascular Alpha 2 adrenergic receptors, (D) Cardiac stimulation
Answer is C. (Ref.KDT 6/e p548) Alpha 2 receptors are present both presynaptically as well as postsynaptically. Clonidine acts as an a2 agonist and decreases the central sympathetic outflow. However, it can also act on vascular postsynaptic a2 receptors, leading to vasoconstriction and increase in blood pressure. Chances of this reaction is more when clonidine is administered by quick i.v. injection.