Chromaffin cells of the adrenal medulla convert norepinephrine to epinephrine using phenylethanolamine-N-methyltransferase (PNMT). The physiologic basis for high PNMT activity in these cells is:

• A Autonomous expression of PNMT driven by splanchnic nerve firing
• B Local synthesis of cortisol within chromaffin cells themselves
• C Direct stimulation of chromaffin cells by ACTH from the systemic circulation
• D Delivery of cortisol at high concentration through the corticomedullary portal venous system ✓

Explanation

PNMT is induced by glucocorticoids, and chromaffin cells sit downstream of the zona fasciculata in the corticomedullary portal venous system, exposing them to cortisol concentrations far higher than those in systemic blood. This anatomical arrangement explains why medullary epinephrine output falls in primary adrenal insufficiency. Systemic ACTH can maintain some medullary trophism but does not reproduce the local steroid concentration needed for maximal PNMT induction.

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

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