Autonomic Nervous Systems
The difference between uses of a selective alpha-1 blocker to using a non-selective alpha-blocker is:
|A||Less reflex tachycardia|
|B||Decreased postural hypotension|
a. Alpha-adrenergic receptors mediate many of the important actions of endogenous catecholamines.
b. Responses of particular clinical relevance include alpha-1 receptor mediated contraction of arterial and venous smooth muscles.
c. Alpha-2 receptors are involved in suppressing sympathetic output, increasing vagal tone, facilitating platelet aggregation, inhibiting the release of norepinephrine and acetylcholine from nerve endings, and regulating metabolic effects.
d. These effects include suppression of insulin secretion and inhibition of lipolysis.
e. Blockade of alpha-1 adrenergic receptors inhibits vasoconstriction induced by endogenous catecholamines; vasodilatation may occur in both arteriolar resistance vessels and veins. The result is a fall in blood pressure due to decreased peripheral resistance.
f. For most alpha-adrenergic antagonists, the fall in blood pressure is opposed by baroreceptor reflexes that cause increase in heart rate and cardiac output, as well as fluid retention. These reflexes are exaggerated if the antagonist also blocks alpha-2 receptors.