The effect of varying CBF and myocardial O 2 delivery on MV O O2 was studied in 23 experiments. In 14 of the experiments an isolated dog heart was perfused, while in the others the heart of a dog, whose systemic circulation was maintained on cardiopulmonary bypass, was studied. The ventricles were kept empty, developed no pressure and performed no external work, while their temperature was held constant. CBF and myocardial O 2 delivery were controlled by pumping blood into the coronary arteries, total coronary venous return was collected from the right side of the heart and MV O O2 , was calculated during a steady state by the Fick principle. Myocardial anoxia was avoided by maintaining the coronary venous O 2 content above 4 vol % and myocardial O 2 extraction below 78%. A comparison of MV O O2 at two levels of CBF (and O 2 delivery) was made in 42 instances, and in 32 of them 11 MV increased substantially as CBF was elevated, or vice versa. The ten exceptions all occurred when O 2 delivery greatly exceeded MV O O2 with O 2 extraction ratios below 35%. The fundamental mechanisms responsible for these findings are not clear, but a number of possible explanations are discussed.

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With the technical assistance of Bever/ee Lehr • The adrenergic neurotransmitter, norepinephrine, is known to be metabolized almost completely prior to its excretion in the urine. 1 The demonstration that 3-methoxy-4-hydroxymandelie acid is a major metabolic product of norepinephrine 2 indicated that two steps were involved in the metabolism of this amine, 0-methylation and deanrination. A number of studies using isotopically labeled catecholamines have been carried out to establish which of these two is of greater importance in the inactivation of norepinephrine. "0 These studies have indicated that O-methylation appears to be the initial step in the metabolism of injected catecholamine and that the 0-methylated amine is then a substrate for deamination.Since the enzymes responsible for both O-methylation and deamination, catechol-Omethyl transferase and monoamine oxidase, are present in tissues innervated by sympathetic nerves, 7 ' 8 a significant amount of the norepinephrine released from the nerve may undergo metabolism before entering the circulation. The metabolic fate of norepinephrine released from the nerve endings may differ from that of injected aud circulating amine, since in the latter instance the role of hepatic metabolism will predominate, and it may be questioned whether the relative activities of the two enzymes are the same in the sympathetic nerves and in the liver. In. fact, it has been indicated that monoamine oxidase may be the more important enzyme in the degradation of norepinephrine locally in the heart and brain of rats. 9In order to assess this problem more diFrom the Cardiology Branch, National Heart Institute, U. S. Public Health Service, Bethesda, Maryland.Received for publication September 27, 1962. 220rectly the metabolism of norepinephrine studied in an isolated perfused canine heart preparation. With this technic it was possible to examine the metabolism of norepinephrine Avhich occurs locally in the heart. After labeling the cardiac norepinephrine store with tritiated norepinephrine, isotopic activity was determined in the coronary venous blood. The appearance of activity in norepinephrine and its O-methylated and deaminated metabolites was examined both during spontaneous release of norepinephrine from the heart and during augmented release induced by tyramine. In addition to demonstrating the predominant role of O-methylation in the canine heart, these studies relate the amount of norepinephrine appearing in coronary venous blood to the total amount released from the nerve ending, during both spontaneous release and release augmented by tyramine. MethodsThe experiments were performed in five isolated canine heart preparations. The experimental and donor dogs underwent bilateral adrenalectomy several days prior to study and were then maintained on daily parenteral cortisone. Adrenaleetomy was performed in order to reduce the excessively elevated levels of norepinephrine and epinephrine found in blood (2-4 fig/L, norepinephrine; 4-6 /ig/L, epinephrine) obtained by exsanguinati...

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