Concentrations of labeled norepinephrine and its metabolites were measured under steady-state conditions in the rat heart. From these values, rate constants of a simplified model of norepinephrine metabolism were calculated. It was found that 90 per cent of norepinephrine and 66 per cent of normetanephrine were metabolized by monamine oxidase, while 10 per cent of norepinephrine and 27 per cent of deaminated metabolites were metabolized by catechol-O-methyltransferase. The release of unchanged norepinephrine from the heart was smaller than the uptake of labeled norepinephrine from the circulation into the heart. These results indicate a significant role of peripheral nerve endings for the sympathetic system in the heart and a possible source of error for measurement of norepinephrine turnover in the heart.
