The adrenal medulla secretes which of the following in the greatest quantity?

• A. Metanephrine
• B. Norepinephrine
• C. Epinephrine
• D. Dopamine

Answer: (C)

Epinephrine is the main product released by the adrenal medulla. The chromaffin cells there store and secrete catecholamines directly into the bloodstream, and most of what they release is epinephrine (adrenaline), with a smaller amount of norepinephrine and only trace amounts of dopamine. The reason epinephrine dominates is that norepinephrine in these cells is largely converted to epinephrine by the enzyme PNMT, and the activity of PNMT is upregulated by cortisol from the adjacent adrenal cortex. This enzymatic drive, plus the rapid release system of the adrenal medulla, results in epinephrine comprising the majority of the secreted catecholamines.

Metanephrine is a metabolite formed after secretion, not the primary secreted product, which is why it isn’t produced in the greatest quantity.