Phenylethanolamine N-methyltransferase (PNMT), the enzyme that converts norepinephrine to epinephrine in the adrenal medulla, depends on which factor for its induction?

• A High local concentration of dopamine in chromaffin cells
• B ACTH acting directly on chromaffin cells to induce PNMT
• C Sympathetic nerve-derived norepinephrine acting on alpha receptors
• D Cortisol delivered from the cortex through the corticomedullary portal system ✓

Explanation

Chromaffin cells are exposed to very high concentrations of cortisol because venous blood draining the adrenal cortex reaches the medulla through a corticomedullary portal system. Cortisol induces PNMT expression, which is why the medulla must sit anatomically inside the cortex to produce epinephrine. ACTH drives cortical steroidogenesis indirectly supporting this, but it is cortisol itself, not ACTH on chromaffin cells, that induces the enzyme.

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

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