Uptake and Metabolism of Tritiated Norepinephrine in the Isolated Canine Heart

Using an isolated canine heart preparation, the myocardial norepinephrine pool was labeled by injecting tritiated norepinephrine into the blood perfusing the heart. The extraction of the norepinephrine during a single circulation through the coronary bed was shown to be large (74%). As the isotopic material, which was extracted, was released spontaneously from the norepinephrine pool 75% to 88% of it was metabolized before appearing in the coronary venous blood. The chief metabolite has been demonstrated to be normetanephrine which accounts for 39.0% to 61.7% of the spontaneously released norepinephrine. Because of this it is concluded that catechol-O-methyl transferase is the enzyme primarily responsible for the metabolic inactivation of norepinephrine in the canine heart. When release was increased by the injection of tyramine, more of the released norepinephrine appeared unmetabolized in coronary venous blood, suggesting that the enzymatic process by which norepinephrine is inactivated may be limited. Therefore, the extent to which enzymatic processes contribute to the termination of augmented adrenergic activity in the heart may be questioned.