Autonomic Nervous Systems
All of the following statements are true about clonidine except
|A||Causes dry mouth|
|B||Prazosin completely antagonizes it's action|
|C||Inhibits sympathetic outflow|
Clonidine is an α2 selective agent. It is a partial agonist with high affinity and high intrinsic activity at α2 receptors esp α2A subtype in brainstem. Stimulation of α2A receptors present mainly postjunctional in medulla causes sympathetic outflow → fall in BP and bradycardia
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 if α dose is missed. While, Prazosin is a selective α antagonist. It causes dilatation of arterioles as well as venules - ↓ peripheral resistance and ↓ mean BP.
Tachycardia does not compensate for this fall in BP because release inhibitory α2 (presynaptic receptors are not blocked.