Competitive antagonism of which of the following occurs at ß receptor sites?
A. Catecholamines
B. Adrenergic sites
C. Acetylcholine
D. Norepinephrine
Correct Answer: A. Catecholamines
Catecholamine receptor sites are blocked by the action of ß-blocking agents. Both epinephrine and norepinephrine modulate metabolism to increase blood glucose levels by stimulating glycogenolysis in the liver (via beta-2 receptors), increased glucagon secretion (via beta-2 receptors) and decreased insulin secretion (via alpha-2 receptors) from the pancreas, and lipolysis in adipose tissue (via beta-3 receptors).
Option B: Adrenergic sites may be blocked, but the more appropriate response is catecholamine receptors. The adrenergic receptors linked to blood vessels have an especially high affinity for norepinephrine relative to the other amines.
Option C: Acetylcholine is not affected by ß blockers. After an external stimulus triggers the body’s stress response, the pituitary-adrenal axis and sympathetic division of the autonomic nervous system is activated. Glucocorticoids production increases in the adrenal cortex, and acetylcholine (Ach) is released from sympathetic splanchnic nerves. Ach binds to nicotinic receptors located on the membrane of chromaffin cells in the adrenal medulla. These receptors promote exocytosis of catecholamine-filled vesicles for transport in the bloodstream.
Option D: Norepinephrine is a catecholamine. Dopamine, norepinephrine, and epinephrine are physiologically active molecules known as catecholamines. Catecholamines act both as neurotransmitters and hormones vital to the maintenance of homeostasis through the autonomic nervous system.