In the elucidation of the mechanisms of synaptic transmission, it was discovered the injections of extracts of the adrenal gland into the circulation caused a strong vasoconstriction. Which of the following best describes the likely site of action of these extracts?

Question

BlackTom AI · Accepted Answer

Adrenal extracts contain catecholamines (primarily epinephrine and some norepinephrine) that activate adrenergic receptors on vascular smooth muscle, producing strong vasoconstriction; this implicates action at the post‑ganglionic sympathetic synapses.

1) Relevant knowledge points and concepts
- The adrenal medulla secretes epinephrine and norepinephrine into the bloodstream; these are catecholamines that act on adrenergic receptors.
- Sympathetic post‑ganglionic neurons normally release norepinephrine at neuroeffector junctions, which acts on alpha‑1 adrenergic receptors on vascular smooth muscle to cause vasoconstriction.
- Autonomic ganglia (both sympathetic and parasympathetic) use acetylcholine at nicotinic receptors; parasympathetic post‑ganglionic neurons release acetylcholine at muscarinic receptors, typically producing effects different from vasoconstriction (often vasodilation or decreased heart rate).

2) Step‑by‑step reasoning
- Injection of adrenal extracts into the circulation exposes vascular smooth muscle to circulating catecholamines.
- Circulating catecholamines bind alpha‑1 adrenergic receptors on arterioles and veins, causing smooth muscle contraction and marked vasoconstriction.
- This pharmacologic effect mimics the action of norepinephrine released from sympathetic post‑ganglionic terminals at target organs.
- Because the effect is produced on the vascular smooth muscle receptors (the site of post‑ganglionic sympathetic action), the logical site of action is the post‑ganglionic sympathetic synapse (the neuroeffector junction).

3) Intermediate steps / rationale shown
- Adrenal extract → contains epinephrine/norepinephrine.
- Epinephrine/norepinephrine in blood → bind alpha‑1 receptors on vascular smooth muscle.
- Alpha‑1 activation → increased intracellular Ca2+ in smooth muscle → contraction → vasoconstriction.

4) Why the correct answer is correct
- The extract directly stimulates the same receptors activated by sympathetic post‑ganglionic norepinephrine at the vascular neuroeffector junction; thus the extract’s effect maps to the post‑ganglionic sympathetic site of action.

5) Why other options are incorrect
- Post‑ganglionic parasympathetic synapses: release acetylcholine acting on muscarinic receptors, not adrenergic receptors; they do not mediate strong vasoconstriction.
- Ganglionic plus post‑ganglionic sympathetic synapses: ganglia use nicotinic ACh receptors; circulating catecholamines do not act at these ganglionic nicotinic receptors, so including ganglionic synapses is incorrect.
- Ganglionic and post‑ganglionic parasympathetic synapses: both use cholinergic transmission and do not explain catecholamine‑induced vasoconstriction.

6) Conclusion
The correct site of action is post‑ganglionic synapses in the sympathetic autonomic nervous system, which matches the provided answer: Post-ganglionic synapses in the sympathetic autonomic nervous system.

In the elucidation of the mechanisms of synaptic transmission, it was discovered the injections of extracts of the adrenal gland into the circulation caused a strong vasoconstriction. Which of the following best describes the likely site of action of these extracts?Single choice

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