Epinephrine synthesis in the adrenal medulla depends on the enzyme PNMT. Why does bilateral adrenalectomy of the cortex, sparing the medulla, reduce medullary epinephrine content?

• A The cortex supplies dopamine needed as the direct substrate for PNMT
• B Medullary chromaffin cells die without cortical trophic innervation
• C Aldosterone maintains the sodium gradient required for tyrosine uptake into chromaffin cells
• D Cortisol reaches the medulla through intra-adrenal portal vessels and induces PNMT expression ✓

Explanation

Phenylethanolamine-N-methyltransferase converts norepinephrine to epinephrine and its synthesis is induced by high local concentrations of glucocorticoids. Cortisol arrives not by systemic circulation but through a corticomedullary portal capillary system draining from cortex to medulla, so removing the cortex starves PNMT of its inducer and shifts chromaffin cells toward norepinephrine production. Dopamine is methylated, not supplied, ruling out option A.

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

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