When epinephrine (E) labeled with carbon-14 in the N-methyl group of the side-chain is infused, the radioactivity excreted in the urine represents free and conjugated epinephrine and free and conjugated metanephrine (MN).1-3 Thus, the cumulative per cent of infused radioactivity excreted in the urine represents phenolic amines. These phenolic amines consist of epinephrine and those metabolites of epinephrine which retain the N-methyl group of the side-chain. Those metabolites of the infused epinephrine which have lost the N-methyl-C14 group of the side-chain, via the action of the monoamine oxidase enzyme system (MAO), and excreted in the urine are no longer radioactive. These metabolites represent the phenolic acids and glycols; e.g., 3-methoxy-4-hydroxymandelic acid (VMA), 3,4-dihydroxymandelic acid (DMA), 3-methoxy-4-hydroxyphenylglycol, etc.
Epinephrine has been shown not to pass through the blood-brain barrier to any appreciable extent in laboratory animals.4 If this is true also for man,
RESNICK O, FREEMAN H. Epinephrine Metabolism in Normal and Psychotic Man. Arch Gen Psychiatry. 1962;6(5):388-394. doi:10.1001/archpsyc.1962.01710230056006