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,