Studies on the Metabolism of Catecholamines

Administered H 3 -epinephrine and H 3 -nor-epinephrine are rapidly and selectively taken up by heart muscle and retained for many hours. The uptake and retention of circulating H 3 -norepinephrine by the heart is greater than that of H 3 -epinephrine. Two minutes after the administration of H 3 -catecholamines large amounts of metanephrine and normetanephrine are found in the heart indicating rapid O-methylation of these hormones during the period of their inactivation. H 3 -epinephrine is more readily O-methylated than H 3 -norepinephrine. Enzyme inhibition studies indicate that catechol-O-methyl transferase is the enzyme mainly concerned with the metabolism of catecholamines in the heart. Reserpine, cholorpromazine, imipramine, amphetamine, cocaine, and phenoxybenzamine markedly inhibit the uptake of circulating H 3 -norepinephrine by the heart.