Chromaffin cells of the adrenal medulla secrete predominantly epinephrine rather than norepinephrine. Which feature of adrenomedullary anatomy and biochemistry accounts for this?

• A The medulla lacks dopamine beta-hydroxylase, so norepinephrine cannot be synthesized there
• B Sympathetic preganglionic fibers release acetylcholine only onto epinephrine-producing cells
• C Cortisol carried from the cortex by an intra-adrenal portal vascular system induces phenylethanolamine-N-methyltransferase in chromaffin cells ✓
• D High local concentrations of ACTH directly methylate norepinephrine to form epinephrine

Explanation

Phenylethanolamine-N-methyltransferase, the enzyme that methylates norepinephrine to epinephrine, is induced by high local cortisol. Venous blood draining the zona fasciculata and reticularis flows through an intra-adrenal portal system into the medulla, exposing chromaffin cells to very high steroid concentrations. All chromaffin cells receive cholinergic innervation, and they do contain dopamine beta-hydroxylase, so neither innervation nor enzyme absence explains the epinephrine predominance.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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