All of the following statements are true about clonidine except:
|B||Causes dry mouth|
|C||Prazosin completely antagonizes it's action|
|D||Inhibits sympathetic outflow|
a. Clonidine is an α2 selective agent. It is a partial agonist with high affinity and high intrinsic activity at α2 receptors especially α2A subtype in brainstem. Stimulation of α2A receptors present mainly post-junctional in medulla causes sympathetic outflow → fall in BP and bradycardia
b. Adverse effects: Sedation, mental depression, dryness of mouth, nose and eyes, impotence, salt and water retention, bradycardia (due to ↓, sympathetic tone) constipation, rebound ↑ in BP ifα dose is missed. While, Prazosin is a selective αantagonist. It causes dilatation of arterioles as well as venules - ↓ peripheral resistance and ↓ mean BP.
c. Tachycardia does not compensate for this fall in BP because release inhibitory α2 (presynaptic. receptors are not blocked.